Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: wsscott on November 22, 2024, 12:58:51 pm
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A buddy was recently given this amp from the early 1960's, and we opened it up to see how it looks. What a mess. No power cord, the reverb tank cord is cut off, no Output transformer, a very rusty chassis, some wiring broken loose from solder points, spliced wiring in several connections, and only 2 of the 4 can filter cap nodes are wired into the circuit with the other 2 never being touched and it appears to be a relatively new cap can. This is just based on an initial inspection. Who knows what the values of the resistors and caps are!
So where would you start? Throw it out, or start down the rabbit hole.
If the rabbit hole, would you just take it outside the house, plug it into a Variac and dim bulb tester with no tubes installed, flip the power switch and see what happens? Or is there a wiser approach? I guess my initial concern is whether the PT is any good.
Thanks.
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Here's a photo.
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If someone brought that into my shop, I'd turn down the repair. Massive hackjob by whomever had there hands in that amp previously.
If it's a "labor of love" and the cabinet is good, rebuild the circuit!
FYI, Mercury Magnetics makes some very exacting Ampeg replacement transformers. You may have to call them to get one though.
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I agree with you wholeheartedly! It's more of a project/challenge for a friend.
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Funny, I was going to say, that's not a mess, its a challenge.
If a hobby, the process is fun, no need to tally the time expense. If it were in my hands, I would:
Make sure the friend will pay for the parts.
Test the PT as you suggested. If not familiar, you should read up on safe procedures.
I would leave the reverb out of the circuit until I got the basic amp working.
If its as bad as it looks, I'd document the connections and free-up and remove the board.
Give everything a good cleaning - it sucks to work on filthy electronics.
Obtain an OT. There are much cheaper options (Hammond for one) than MM.
Go over the schematic checking it against your circuit.
Replace all electrolytic caps. Test the others.
Replace any resistors that are wildly out of spec.
Clean and test all the tube sockets.
Clean and test the pots. reinstall and rewire the board.
Obtain a spare set of tubes.
Once the basic amp is working well, tackle the reverb and trem.
I've worked on worse - electrocuted mouse in the chassis, teeth still on the B+ wire :icon_biggrin:
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I'm curious about the wiring on the cap can. It has two 40uf nodes and two 20uf nodes. Neither of the 20uf nodes are wired to anything. Any idea why? The circuit has 6 nodes in it, ie. A-F. Why use a can like this if you only need/want 2 caps?
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Do you know which model this is the? 7591 or 6V6. 7591 model has solid state rectifier.
Figuring out why all the caps in the can aren't used is part of the journey in restoring the amp. You have the amp in front of you and can diagnose it.
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The Power Transformer checks out fine. I unwired it from the circuit, then tested the windings for continuity and shorts to case and that was fine, so then I hooked up the Variac and tested the windings for proper Secondary voltages. All read perfect. So one good thing. Hopefully more.
The amp is missing its Output Transformer. It wasn't attached to the chassis, but there are holes with grommets on the PT side of the chassis where the wiring to/from the OT would go. Do you know where the OT would have been mounted? On the speaker, or someplace on the cabinet?
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Its the 6V6 model since it has a tube rectifier.
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Here's a pic I would say the OT was mounted on the chassis.. This is the 7591 model though it should be very similar. Notice the reverb pan was mounted vertical on a side panel.
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Here's the 6V6 version with tube recto.. you probably could find close ups of one of these if you look around on the net.
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try this for the 3rd time...
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Pics of my '62 Reverberocket. Still needs restoration. Pics show OT mounting. Hope this helps.
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Brian-Thanks. Now I see the 2 empty mounting holes on the chassis. Great!
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Good news on the PT.
There are a lot of muddy schematics for this model out there. But Joe Piazza drew up really clear schematics, and the attached is likely your amp. With the 6V6s you will have your choice of many OTs.
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I've seen this schematic before and it doesn't match my amp. My amp has 7 tubes, not 6, and the 100K plate resistors shown in the schematic are both on V2, where mine has 1 on V2 and the other one on V3. So mine is still a bastardized build from the factory.
It's interesting that the 6V6 builds are supposed to have the tube rectifier, and the 7591 have the bridge rectifier. The Ampeg tube tag in the cabinet shows the 6V6's scratched through with ink, and handwritten above is 7591. My buddy who was given the amp in this condition said it didn't have any tubes in it.
So I've got to see if the pinouts on the 6V6's and the 7591's are the same. If not, then I've got to figure out another mystery in addition to the cap can that never had 2 of its nodes used!
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They have different pinouts..
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I just pulled down the spec sheets. Yes, not interchangeable. Okay, now I need to see how this amp is wired--probably save this task for tomorrow.
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bmccowan-You're right about Joe's schematic. I forgot that V7 is the rectifier tube. So that schematic has the right tubes and it looks like my amp is wired for 6V6's. What's left to resolve is the conflict with the wiring of the 100K Plate resistors connecting to filter nodes E and F. My amp has them connecting to different tubes while Joe's schematic has them both connecting to the Plates on V2. I'll leave it be until tomorrow. Thanks again for your help on this.
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Have you seen this thread. Might be of some help as it documents an information search on the model.https://www.tdpri.com/threads/ampeg-reverberocket-r-12-r-diy-project.1102984/ (https://www.tdpri.com/threads/ampeg-reverberocket-r-12-r-diy-project.1102984/)
Those minor changes in plate resistors are pretty common. Sometime intentional, sometimes just a lunchtime joint.
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You're right about Joe's schematic.
His re-draw of Ampeg's SET (super echo twin) made a world of difference for a repair I did.
while it was a nightmare crawling around inside the chassis, once I plugged in for a test run, you will forget all the sailor talk you had,
she's melted butter to whatever "sound" you play.
I've fixed 1/2 dozen fender twins n supers, they are like cold hands, sterile, compared to the sound of the Ampeg.
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... I've got to figure out another mystery in addition to the cap can that never had 2 of its nodes used!
The attachment to this post (https://el34world.com/Forum/index.php?topic=18071.msg183383#msg183383) is a layout diagram for the 1962 Reverberocket with 6V6s.
You may need some time to get used to how it presents things, but it will show "what parts should be where, connected to what" well enough.
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FWIW
the SET i worked on had one side blown, bad tube, bad OT, couple other pieces;
owner had a donor 6V6 amp, so I stitched in a 6V6 "side" one one channel, left other side 7591 based
this is my 6V6 side schematic
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The wiring in this amp seems under-gauge with poor insulation. Is that just me worrying, or is that something that should be changed. The Heater filament 6.5V wiring isn't even twisted once it leaves the PT. I really do want to change that even though it may not be necessary. Any thoughts on adding a terminal board to run the lead from the PT to, and then wire off of it to the pilot light and another separate lead to the heaters?
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The wiring in this amp seems under-gauge with poor insulation. Is that just me worrying, or is that something that should be changed.
:dontknow: Not enough information.
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The wiring in this amp seems under-gauge with poor insulation. . . . Any thoughts on adding a terminal board?
Some thoughts: Many builders worry about wire size, but in reality 22AWG wires are fine throughout most of our tube amplifiers, even the heater circuit, as the ampacity is in the range 7 Amps.
Your wiring does look the worse for wear, though I bet the insulation itself would be fine. The splicing and prior repairs would be a big concern for me.
Nothing wrong with adding a terminal board, but not sure if your carcass (chassis) has the space. I've used these insulated turrets https://www.ebay.com/itm/362043934117 (https://www.ebay.com/itm/362043934117) for this purpose from time to time. They work well, and take up a little less room than a board would.
Looks like a fun project!
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Thanks. I guess the wire gauges in question are no smaller than the wire on a cap or resistor and that works fine. So I think I'll focus on the ones with bad insulation or splices. Looks like the terminal board won't be feasible. So I'll just run new wire for the 6.5V heaters. Some amp builders I"ve seen on Youtube don't like to run that heater wire directly to the pilot light, and then from there to the heaters. They connect elsewhere, like to a terminal board. I don't know what's the issue. Do you?
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The mystery of the cap can is developing. The can appears not to be original. It is a CE with setup as 40/40/20/20 at 525 VDC. The two 20 nodes appear never to have been used. Maybe the tech just gave up when he got to that point. But what's interesting is that each of the 40 nodes have 2 connections to them going to different parts of the circuit. The schematic of Joe's shows there are six 20uf caps in the circuit. So maybe the prior tech was using the 40uf nodes as dual purpose to cover 2 nodes each. Does that make any sense?
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The mystery of the cap can is developing. The can appears not to be original. It is a CE with setup as 40/40/20/20 at 525 VDC. The two 20 nodes appear never to have been used. Maybe the tech just gave up when he got to that point. But what's interesting is that each of the 40 nodes have 2 connections to them going to different parts of the circuit. The schematic of Joe's shows there are six 20uf caps in the circuit. So maybe the prior tech was using the 40uf nodes as dual purpose to cover 2 nodes each. Does that make any sense?
Agree - certainly not original. If you look at the layout that HotBluePlates referenced you can see how Ampeg implemented the 6 20uf caps. a 4x20 can and two axial 20uf caps. Those components are all readily available.
Did how the last "tech" wired it make sense? Well, no. As you work on this amp, I suggest not trusting that any of the previous work was done correctly. Instead follow proper procedure. You don't want to blow anything up, especially yourself.
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I think I'm going to loose my mind on this one!
Ok, so V-1 is the Tremolo stage
V-2 is the Pre-amp Gain stage
V-3 is the Reverb
V-4 is the PI
V-5 & V-6 are Outputs
The Problem:
V-2A is wired for the PreAmp, but V2B is wired for V3A Reverb.
V-2B is wired for V3A Reverb and V3B is wired for Reverb.
So 1/2 of V-2 is wired like 1/2 of V-3; and 1/2 of V-3 is wired like 1/2 of V-2.
Even though they're both 6SN7's, the connections are not the same. For example, there's a 22K resistor between V-2P's 3 & 6; but V-3P's 3 & 6 are not connected together.
Plus the schematic may show a connection on P-2, but it's wired to P-5, etc. Ok, they're both Plates, but still.
And it looks like it's factory wiring. How could this possibly be? How could it have ever worked properly. Maybe it didn't. Who knows.
Pardon my venting. IF it were 5 o'clock it would be cocktail time!
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It's 5:37 in London - so you are safe.
1/2 or V1 is the tremolo stage - other than that, yes that's what the schematic shows.
I'm having a hard time following the rest of your comments - but I understand you are venting.
My suggestion is that you decide which schematic best matches your amp - hopefully the Joe Piazza version as it's clear. And then follow it.
You likely need to approach this project as a rebuild rather than a repair.
I sympathize with your frustration - not that it helps, eh?
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1/2 of V1 is the input preamp and the other half is the tremolo oscillator. V2A is another preamp gain stage but it has two outputs. The plate output is connected to the grid of V3A which is the reverb driver. And the cathode output is connected to the cathode of V2B mixer.
Ampeg has a different way of mixing the dry and wet reverb signals together, but the circuit works just fine on the schematic. That 22K resistor between V2 cathodes is the dry signal input to V2B mixer. The reverb recovery signal is on the grid of V2B and the combined wet/dry signal leaves V2B plate headed to V4 paraphase inverter. Tricky to look at for the first time but simple once you understand.
If you wire the amp according to the schematic it will work properly.
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Thanks guys. London is a great city. Maybe it will be a G & T tonight! Yes I"m going to use Joe's schematic and looks like its going to be a rebuild.
SEL49, yes my amp has the first 100K connected at V2-Pin2, but the other 100K that's shown on V2-Pin 5 on the schematic is connected to V3-Pin5. And of course some of the other components have been switch around between those 2 tubes. And on my amp, that 22K connects between the cathode on V2 and the cathode on V3. So that connection is not between cathodes on the same tube
No telling what else I'll find. I don't think this will be done by Thanksgiving--but maybe Christmas!
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Maybe you should be looking at this schematic (https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_r12rb.pdf)... It's a bit different than the Piazza schematic.
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That schematic is for a SS rectified version with much higher value filter caps. But the amp in your hands could easily be a transition model. Ampeg and everyone else did that, including Fender.
The important thing is to pick the closest one and follow it. If you try to follow multiple schematics you may be looking at April Fools Day, let alone Christmas.
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I'll check it out, but I feel pretty comfortable with Joe Piazza's schematic.
I also found these 2 YouTube videos. Both are very good, and the one from Psionic Audio is exactly what I need. I actually surprised myself in that I was going to start out doing exactly what he did, ie. cleaning up the area around the PT, cap can, and resistors. Fingers crossed.
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Lyle is meticulous - so good find. Notice he mentions that the amp on his bench matches none of the schematics, including the JP. As he says, "what the Hell, Ampeg."
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"what the Hell, Ampeg."
That's what happens when you hire the engineers Gibson fired :icon_biggrin:
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You guys are great!
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:laugh:
naw, just crazy from too many years crawling around tangled wires.
while you're tackle'n this one, keep your eyes n ears open for it's big-bother the SET.
Ampeg managed to put 2 of your amps in the same chassis :icon_biggrin:
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The mystery of the cap can is developing. The can appears not to be original. It is a CE with setup as 40/40/20/20 at 525 VDC. ...
Agree - certainly not original. If you look at the layout that HotBluePlates referenced you can see how Ampeg implemented the 6 20uf caps. a 4x20 can and two axial 20uf caps. ...
Below is an original Reverberocket.
The silver in the bottom-right corner is the 4-section can capacitor.
The brown tube on the right-edge is a 2-section can capacitor.
There are your 6 filter caps.
(https://i.imgur.com/LRx4bNT.jpeg)
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Georgeous!! Is that how it was when it was brand new, or is that its condition now? If the latter, I assume there was a restoration? Anything of interest develop in the course of that? If that's yours, lucky guy. Thanks for sharing.
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I was looking at the Layout for this amp that's referenced earlier in this topic, and I noticed that the two 20uf electrolytic caps C5 and C6 are shown has having their + leads connected to the GND terminals on the Cap Can. Is this correct? Shouldn't they be connecting to that point with their - leads?
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I was looking at the Layout for this amp that's referenced earlier in this topic, and I noticed that the two 20uf electrolytic caps C5 and C6 are shown has having their + leads connected to the GND terminals on the Cap Can. Is this correct? Shouldn't they be connecting to that point with their - leads?
It would really help the guys greatly if you post the drawing so they don't have to look for it. :think1:
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Shouldn't they be connecting to that point with their - leads?
yes
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I was wondering, since I'm working right now on the power supply side of the amp, would it be worthwhile putting a fuse or diode in the circuit to protect the PT from something going wrong with the rectifier? I don't have much room to work with. If so, how would you recommend wiring it into the circuit? Thanks. Oh, and Happy Thanksgiving to everyone.
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Well I got the wiring finished on the Power Supply section of the amp. I wired in new green 6V filament heater wires from the pilot light to the V6 Power Tube, and then ran a colored red & green twisted pair from V6 to V5 and to V4 filaments. I left the remaining filament wiring on V3-V1 in place since it tested ok. I added a power cord, after testing the PT and it showed everything was fine, I wired it into the circuit. Got the can Filter Cap A (40uf) node and the B (40uf) node wired into circuit. Installed a new 250 ohm bias cap. The 6V6's are partially wired, and I'm waiting on a OT. With no tubes installed, I plugged it into a dim bulb current limiter which is connected to my Variac and brought it up to full voltage, and no shorts. Then I shut it off and installed the 5Y3 rectifier tube, and turned it back on. No shorts or issues. It is putting out about 450-475 VDC. I checked the voltages at the can A and B nodes and through the rest of the Power Supply part of the circuit, and everything is where it should be. The only thing I noticed was when I turned the power on after installing the rectifier tube I only got high voltage on one leg of the cathode, but after a couple of minutes it was reading equal voltage on both legs of the cathode pins 2 & 8. Maybe it just took some time to charge up the cap node.
So for the moment, I'm waiting for parts to come in. As I said, an OT, a set of tubes, a few electrolytics, and some coupling caps are the main items. So maybe by next weekend some more progress will have been made.
I'm still struggling with figuring out the wiring for the rest of the circuit. I can't figure out what the amp builder was doing. There are some tubes that are wired in reverse, ie. the wiring on A and B nodes of the tube are flipped. Some wiring that is shown in the schematic as being wired on Nodes A and B of just 1 tube, are split between 2 tubes, with it wired to A node on tube 1 and B node on tube 2, or maybe A node on tube 2. So it's really hard for me to trace things. I guess I'll just assume it's wired correctly since the wiring looks original and it must have worked since someone bought it.
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I can't figure out what the amp builder was doing.
:laugh:
See reply #37
consider the times in which an amp was built
Vietnam, Hippies just getting started, guys avoiding spending a few years in the jungle, Collège became a social construct rather than an educational one, sex, drugs, early rock n roll
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Yea, those were some really great times. Plus the best music ever! I never did drugs other than alcohol, and I had a high draft number so I didn't see the jungles but I respect those who did. No one can appreciate those times other than those of us who lived it.
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I decided to replace the 6 coupling caps and started working on it today since I had replacements on hand. The ones on the board are covered in glue of some kind so it's hard to read them, but the schematic shows 0.02 uf's for all 6. I removed 3 of the old ones, and ALL 6 of them are 0.1 uf's--no not 0.01uf!! Surprise, Surprise! So I replaced them with the 0.02's.
Who know how this will affect anything.
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Georgeous!! Is that how it was when it was brand new, or is that its condition now? ...
Both. It's a mint 1962 Reverberocket I got a few years back.
So original it still has all Ampeg-branded Sylvania tubes, and the cardboard box that carried the amp from Ampeg to the music store that sold it. The store wrote the list price of $189.50 on the side of the box.
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I want to add a 1/4" speaker output jack to the chassis, and the only spot that looks good is between V4 (PI) and V5 (Output) tubes. Is there any problem with putting it in that location, ie. hum, hiss, noise, etc.?
Also, any thoughts on adding 2 RCA jacks to the chassis for the Reverb IN/OUT, and a RCA or 1/4" jack for the footswitch(s) controlling them?
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You are making progress!
I do not believe in drilling holes in vintage amp chasses, even if they are a mess and non-original.
If you differ with the above, my next semi-rant is to get it working well first before making any mods. If you mix mods and repairs you will have a harder time chasing down gremlins.
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I agree with you about the mods. Get it working first. I don't know what my buddy who owns the amp thinks about cutting holes. That will be his call. He's said he wants the speaker jack, and I don't think he'll change his mind for originality purposes.
So given that a "change is gonna come", any thoughts on my original question?
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That's a typical location for a speaker jack - so should be ok.
But an option to get the functionality without drilling a hole is a cable end female jack:
https://www.parts-express.com/Switchcraft-128-1-4-Mono-Cable-Mount-Jack-Female-with-Screw-Terminals-093-1009?quantity=1&utm_source=google&utm_medium=cpc&utm_campaign=21081298514&utm_content=162693410634&gadid=692832564687&gad_source=1&gclid=Cj0KCQiA3sq6BhD2ARIsAJ8MRwX-r8xbAbbOuUHZ_Wzc0qinnrYn48aTOjdAOxM_5kH31M8FHbUICZoaAvSwEALw_wcB
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Good option. I’ve never seen that type of jack. Thanks.
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That's a typical location for a speaker jack - so should be ok.
Doesn't look like there's enough room.
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My parts order came in today, which includes the new OT that was missing from the amp! Can you tell me how to wire in the new OT so that it doesn't get that loud, horrible, squeal when the amp is turned on? Thanks.
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Can you tell me
:laugh:
got a silver dollar?
IF.. the original OT had Phase marks, AND they were indicated on the schematic AND your new OT has phase marks, you might have a 75% change of good guess
50 50 90
if there's a 50 50 chance, 90% of the time you choose wrong, so choose, then do the opposite :icon_biggrin:
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There is a YouTube for everything. This one describes the approach Shooter describes:
https://www.youtube.com/watch?v=rMz7JBRbmNo (https://www.youtube.com/watch?v=rMz7JBRbmNo)
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Can you tell me how to wire in the new OT so that it doesn't get that loud, horrible, squeal when the amp is turned on?
Just leave the OT plate leads long, until you know for sure you have the phasing right.
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IIRC swapping speaker wires works while testing
the squeal is good, improves reaction time, tests hearing, tests patience, self control, self loathing.... :icon_biggrin:
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Glad to know I’m not missing something. I thought there might be a way to do it correctly and not just by chance! So, I guess it’s better to switch the plate leads vs. the speaker leads so that the + & - leads on the speakers match up.
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yes at the speaker, but at the OT's output, which is + :icon_biggrin:
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Does it go out of phase because the wires on the OT primary are reversed from those on the Secondary when the Secondary is wired to the speaker?
For Example:
Assume you have Red at the top of the Primary, then a Yellow center tap, and then a Green lead at the bottom of the Primary winding.
And on the Secondary you have a Black lead at the top, and a Brown lead at the bottom.
How do you wire the secondary to the speaker to avoid squeal? Black to + and Brown to - terminals?
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Sorry, assume that the Black lead is the COM, and the Brown lead is 8 ohms.
I just checked the OT spec sheet.
So given this info would it be wired Black to - speaker terminal, and Brown to + terminal?
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Yes.
Can you post that spec sheet?
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Here's the spec sheet for the OT.
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see he 2 black dots at the top of the OT near the "coil", those are phase marks, if you put a positive wave on the input wire with the black dot, you will be "in phase" at the dot wire output.
IF your original had those dots, you would wire "dot wire" to "dot wire" with the new OT and it should be "phased" properly.
in the end, it's not worth your brain energy, if it squeals, simply swap either the inputs or outputs, walla, on to the next thing
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Thanks. I've learned something even if I may never use it, at least I'll know what those dots mean.
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I'm starting to wire up the filter caps. This amp's can cap was replaced with a 40/40/20/20, and only the two 40uf nodes were ever used. The circuit's schematic is designed for SIX 20uf cap nodes. So I guess whoever did the work just doubled up or something with the two larger nodes in the can.
I'm wiring it to use all 4 of those nodes, and then adding two 20uf caps. I'm trying to get the voltage supply to the circuit to have some resemblance to the schematic.
So my question is there anyway I could be overloading the circuit by doing it the way I want to, since the rats nest of wiring doesn't follow any schematic and many of the components are not even in the location shown on the schematic. I don't know what "re-wiring" was done, and its gives me a headache trying to figure it out. I don't even know if it was operating before it was cannablized for parts.
Thanks.
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wire it per the schematic you're following, a 40Uf cap is an "Improvement" to the 20uF
make SURE you don't "leave behind" a stray wire from days gone by as your rebuild each section.
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Here's the datasheet for a 5y3.
It doesn't have a maximum rating for capacitance, but the typical value is stated as 10Uf.
Do with that what you will. That would be the only place in the amp where adding extra filter capacitance could cause damage.
https://frank.pocnet.net/sheets/049/5/5Y3G.pdf
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In this circuit, there's no cap before the rectifier but only after its 5VAC filament supply. So I wouldn't think it would be an issue.
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In this circuit, there's no cap before the rectifier but only after its 5VAC filament supply. So I wouldn't think it would be an issue.
I'm not sure what you mean by that, but its that first filter cap, the reservoir cap, that is the issue. One can go down lengthy winding rabbit holes on this subject, but from a practice standpoint, many consider 32uf to be the max reservoir cap value for the 5y3, but many, many amps are operating happily with 40uf in that position.
Related - since you do not know what made this amp fail, and you are uncertain of the wiring, you should be careful on testing and startup. Following a known standard procedure will protect your $ and time investment.
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In this circuit, there's no cap before the rectifier but only after its 5VAC filament supply. So I wouldn't think it would be an issue.
There tied together. The 5acv goes to the rectifiers heater/cathode, depending on if the rectifier tube is directly or indirectly heated. (5Y3 is directly heated) So the 5acv rectifier heater also has the full rectifier dcv on it.
So it is an issue.
https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_reverb_rocket_12r.pdf (https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_reverb_rocket_12r.pdf)
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This amp and the schematic show 20uf caps for nodes A, B, C, D, E and F. The can that's in this amp is 40, 40, 20, 20 and then there will be two additional 20uf caps added. In tracing the B+ wiring, I notice that some of the plates, ie. pins 2 and 5, in some of the tubes might have B+ coming to them from nodes E and F, but on the schematic the B+ is coming from nodes D and F. And maybe the plate resistor is lower or higher than in the schematic. Sometimes it's consistent with the schematic.
My question is does it matter which nodes the plates are wired to as long as they're 20uf caps supplying the B+?
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does it matter which nodes the plates are wired to
short answer, Yes
longer "reasoning";
You want the most "sensitive" tubes further down the line, that way the PS "compounded" filtering provides the cleanest VDC.
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I wondered if anyone could tell me why this circuit has a 100uf/25V cap on the cathode to V1 going to ground in parallel with a 2.2K resistor? I think this is part of the tremolo circuit. It just seems like that's a big cap even though small in Voltage and physical size.
What's wired in the circuit located in this area, but of course it doesn't connect to the cathode, is a 100uf 325V cap that's a monster. I'm guessing it was being used for one of the filter nodes. But there's no cap at all coming off of the cathode on V1 like there should be. Its got the cathode resistor connected but no cap.
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I wondered if anyone could tell me why this circuit has a 100uf/25V cap on the cathode to V1 going to ground in parallel with a 2.2K resistor? I think this is part of the tremolo circuit. It just seems like that's a big cap even though small in Voltage and physical size.
It's not part of the trem.
Read this from HBP on the large cap value for killing humm.
https://el34world.com/Forum/index.php?topic=32255.msg356823#msg356823
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Interesting. 250uf is a big one.
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250uf is a big one.
:laugh:
20 Farads is a big one, 10 thousand mikes (10,000uF) is a normal on computer PS's
250uF is the lazy engineers "fix" in tube amps.
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I got the wiring finished, but not without some frustration with the schematic. There's an error! Surprise, surprise.
I was getting weird voltage readings on V-4, the PI 6SL7, for the plate voltages. Pin 5 was reading ok. But Pin 1, as shown in the schematic, was showing only mv and Pin 2, the grid, was showing high voltages. It was driving me crazy tracing the circuit to try and figure out what was going on. Then I looked at V1, also a 6SL7, and noticed that the numbering on the Pins for the 2 tubes was different!
Long story short, Joe Piazza mis-labeled the PIN numbers for the first half of V4 and reversed the numbers for the plate and grid. The plate should be numbered "2" and the grid "1".
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I've only had the rectifier tube installed and powered on with my Variac and current limiter. No glowing bulb. Voltages are high but as expected.
So I'm planning on installing all the tubes this afternoon and firing it up.
Any suggestions on startup procedure for this one? Any thoughts on what the bias setting should be?
Thanks.
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Any thoughts on what the bias setting should be?
looks to be self biased, PA tubes have a shared common cathode resistor
put your meter across that R, set for VDC, use a 2nd meter to monitor VDC at the PA B+ tap
once the heaters are up and the tubes are drawing current, BOTH meters should "stabilize"
IF the B+ keeps going down and the cathode keep creeping up, POWER OFF
IF they do stabilize at something close to "expectations", use those readings, the R value, crank out some math for tube current
NOTE that R is shared!! so it's the current for BOTH tubes
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Should I start out by just installing the Output Tubes and see how it goes, ie. smoke, sparks, blown fuse? Or put in all the tubes and let her rip?
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Should I start out by just installing the Output Tubes and see how it goes, ie. smoke, sparks, blown fuse? Or put in all the tubes and let her rip?
Put all the tubes in, that's what the bulb limiter is for.
If the bulb stays bright, start pulling tubes 1 at a time until the bulb goes back dim. There's your trouble spot/circuit.
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Ok, I put in the tubes and turned it on, and nothing blew up!
Low glow on the light bulb limiter (LBL). Variac slowly increased to 120VAC.
Here are initial main observations after plugging amp directly into Variac and LBL removed from circuit:
V-7 Rectifier-Pins 2 & 8 read 375 VDC. Perfect. Pins 4 & 6 read 53/46 mv respectively!! When I measured the voltage there with only the Rectifier installed all the pins read about 499 VDC, but of course no load. So 2 & 8 go to proper voltages with load applied with the other tubes, but 4 & 6 have essentially no voltage.
V-1 is not getting warm, and no glow. It reads Pin2 326VDC, and Pin5 290 VDC.
I assume its not drawing any current for some reason.
V-3 voltages on P2/P5 are 140/19. So that's squirrely.
And of course there is no guitar output when its plugged into any of the 3 jacks and played. Just hiss.
So now the sleuthing begins.
Thoughts on these issues? Thanks.
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no glow
filaments, wiring, tube
So that's squirrely.
Tube, tube R's n C's, wiring..
happy hunting, one problem, one solution typically works best
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These are all new tubes, except I am using the old rectifier. So I will change that and see if it makes a difference. I feel like that old tube is okay since I am getting proper voltages on Pins 2 & 8, and high voltage on V-1.
I had previously checked the filament wiring on all the sockets and they were good.
I wonder about the 100uf cap on V-1 Pin 6 cathode that's coupled with a 2.2K resistor. I'm confused on the grounding of the resistor, and what that cap's negative lead is connected to. I have them wired to the same lead going to chassis ground.
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and no glow.
no-glow = no filament voltage or open filament in tube
see if the tube is getting warm, maybe the glow indicator in your eyes is getting on in age :icon_biggrin:
I'd leave the rec tube if volts are good
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I checked the continuity on V-1's 6VAC filament wires and it has continuity back through all of the tube sockets and to the pilot lamp where the 6VAC secondary connects.
Tomorrow I'll switch the V-1 pre-amp/tremolo tube with the V-4 PI tube and see if that makes a difference.
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.... Pins 4 & 6 read 53/46 mv respectively!! .... So 2 & 8 go to proper voltages with load applied with the other tubes, but 4 & 6 have essentially no voltage.
Rectifier tube pins 4 & 6 are ACV. PT only puts out ACV.
When I measured the voltage there with only the Rectifier installed all the pins read about 499 VDC, but of course no load.
You don't really need to measure that with only the rectifier tube in. And you don't really want to do that because as you see now you get 499dcv and that is on the B+ filter caps. Are your filter caps rated for 450v or 500v?
V-1 is not getting warm, and no glow. It reads Pin2 326VDC, and Pin5 290 VDC.
Some small bottle tubes it's pretty hard to see the glow from their filaments, others very easy to see the glow. Turn out the lights in the room, then you can see if their lit.
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Good point on the filter caps. They're 525 volts. Thankfully.
I switched the V1 and V4 tubes, both 6SL7's, and I also checked the voltages at all of the filament socket pins for V1-V4.
V1 and V4 both have a very very tiny glow, much lower than the 6SN7 tubes. It can hardly be seen even with the lights off. And the voltages read 6.3VAC on each tube socket.
So the filament wiring and filament voltage supply to the socket is good, and the tubes are good, but the problem persists.
If current is not being drawn properly on those tubes, would that affect the "glow"? If so, what would be affecting the current draw?
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Good point on the filter caps. They're 525 volts. Thankfully.
E- caps can take some over voltage for a little while, but your still stressing them.
Your being over cautious if you 1st tested the rebuild of that amp with just the rectifier tube. Not only do you have a bulb limiter but your also using a variac with it.You can do that but many guys would just use the bulb limiter by itself.
With both the limiter and the variac, even with just the bulb limiter, you could have put all the tubes in and fired it up. Bulb stays lit, just turn it off. Then go back and start pulling 1 tube at a time until that bulb stays dim. Pull a power tube, turn the amp on, look at the bulb, stays bright, turn the amp off, with that 1st tube still out pull the next power tube, etc. When the bulb dims back down again, you found the tube/circuit that's the problem. Or, it might be a direct short in your wiring somewhere else. :dontknow:
If current is not being drawn properly on those tubes, would that affect the "glow"?
No, if the tube is drawing normal current or not, that won't change the filaments glow.
Some tubes are just hard to see the tip/end of the filament because of the way the tube was made.
If so, what would be affecting the current draw?
Probably mis-wired, or bad tube. Check the dcv's at all pins, you know this by now. What do you get?
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Here are initial main observations after plugging amp directly into Variac and LBL removed from circuit:
re-test volts with amp directly into the wall, PA tube cathode VDC and B+ Tap that feeds the PA tubes, if they fall within "normal"
then start at V1 vdc IF outside norm, fix, add AC signal, get it into and out of V1...this is the fun part, enjoy the process, and DON't add bugs as you progress :icon_biggrin:
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Shooter-I cleaned the tube sockets, then turned it on to test the voltages.
The voltages at all of 6 of the filter cap nodes seem fine.
A 388, B 353, C 304, D 228, E 202, and F 166 VDC.
The rectifier Pins 2 & 8 that feed HT to the circuit read 388 VDC.
The plates (Pin 3) on the power tubes V5 and V6, read 379VDC.
The cathodes (Pin 8) on the power tubes, read 23VDC. That is also where the 6VAC center tap connects on V6. It is then jumpered to V5's Pin 8. I've also got a wire going from V6's Pin 8, to the + side of the 50uf bias cap and 250 ohm 10 Watt bias resistor.
Is the cathodes' reading of 23VDC a "normal" reading? I don't know what is.
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I had turned off the amp after these tests for about 20 minutes, and just turned it back on. When I did I noticed that the V3 tube's filament became very bright for a few seconds, and then went back to a normal glow. All of the tubes are very sensitive to being even lightly touched, and V3 particularly so.
I would like to say that V3 is for the reverb, but the way this thing is wired I think 1/2 of this tube is for either the driver or recovery stage of the reverb, and 1/2 of V2 covers whatever stage V3 doesn't cover.
So I guess removing V3 for the moment doesn't work.
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needs double checking;
23VDC / 250 ohms = 92mA /2 = 46mA per tube
379 - 23 = 356vdc * 46mA ~= 16.38W per tube
If the tubes are 6V6 that's pretty hot, 6L6 about right.
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If I changed that resistor to a 750 ohm, or maybe a 1K 10W, that would get the bias down to almost 12 watts or lower. Correct?
Should I make that change? That’s a big difference isn’t it?
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Sorry I meant 700 ohm not 1K.
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i like moving in slow increments 330, 390, 430, 470
as it sits it's about 2.1W so 5W is fine
ballpark math says a 390 ohm will get you close enough to find n fix everything else
You want a "target" of about 90% tube dissipation, since idling is as many watts as it will draw, as you beat on the strings power will drop slightly in a self biased amp.
then you tweak for tonal happiness :icon_biggrin:
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Sorry, I don't understand your comment "as it sits it's about 2.1W so 5W is fine". Thanks.
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Oh, I got you. You mean I don't need a 10W resistor, but a 5 W resistor would work just fine. Good. I've got a selection of those to work with.
Should I change this resistor before trying to resolve the other issue of no output sound?
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I would, shoot for anything <14W per tube, use the math I posted to check. once you change the resistor both cathode and plate volts will change, so re-measure each time
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Thanks. The smallest resistor I have on hand is a 470 ohm. So that’s where I’ll start. Can I attach the cap after I find the correct resistor or should I do it each time I change the resistor?
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yes
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Ok, so with a 470 ohm resistor, actual reading is 475 ohms, I get the following:
Pin 8=28.9VDC
Pin 3=405VDC
29/475=0.06105. 61ma/2= 30.5ma
405-29=376 x 30.5= 11.47 W/tube
The only other resistor I had was a 680/667 and it gave me a COOL reading of 9.24 W/tube
So should I settle for the 475 ohm bias resistor, or order a few more in between?
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slap the cap on and check the box, you're on to bigger fish
All of the tubes are very sensitive to being even lightly touched,
that's a indication the sockets need re-tensioning
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Ok, here are voltages after the bias resistor was changed.
Filter Cap Nodes 1-6 respectively: 408 371 325 241 213 174. Increase for each node of about 7%
Filaments on all tubes 6.5VAC
Rectifier Pins 2 & 8 read 408
Pins 2 and 5 for V1 thru V4 respectively: 364/320; 159/186; 154/21; 240/154. So V3 is suspicious.
Pins 3, 4 and 5 for V5 and V6 are the same respectively: 401 376 28.5
Ok, so I've got V3 acting weird with the filament flash on just one filament at startup; and the weird V3, Pin 5 Voltage
reading.
Still no output from the input jack with guitar plugged in.
So do I try to trace the input signal, or do something else?
Thanks Shooter for your help.
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These are all new tubes
you bought new tubes or "they look like new tubes"?
you need the DC volts correct before moving on to AC
so it's tedium time, verify wiring, correct component values, solder connections...etal.
hunt up Dougs "how to troubleshoot" procedure where you trace the circuit, check the parts, highlight....
I typically get a 2X photo-copy of the schematic, pack a bowl of herbs, put on music, KEEP POWER OFF while in this state, but the coloring is fun :icon_biggrin:
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My buddy whose amp it is, bought all new tubes for the project.
I did a test on that 6SN7 in the V3 position that seems to have an issue with one of the filaments. I swapped its position with V2. The problem went with the tube to V2. So that tube may have been defective. I don’t think it should be doing that even if it calms down after the initial turn on surge of power.
The swapped tube doesn't have any issue with the V3 position.
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I also checked the Voltages on Pin 5 of V3 and V2, and the low voltage on Pin 5 stayed with the socket for V3. So it's not the tube causing the low voltage. The voltage issue is with the socket. Pin 5 of the 6SN7 that I moved to the V2 position showed proper voltage.
Just one more thing.
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Pins 2 and 5 for V1 thru V4 respectively: 364/320; 159/186; 154/21; 240/154. So V3 is suspicious.
remeasure V1 - V4 and post like this
V1p 555vdc V1k 22.5vdc
V2........
V3
V4
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Cathode voltage is just as important as plate voltage when taking measurements.
Seeing as you have 2 triodes drawing from node F, and the current to that node is 1.7 mA, I wouldn't suspect a short at the moment, but perhaps the 47k resistor has opened up, meaning V3 isn't drawing current. But if that's the case, what caused the resistor to open up could have been a short in either the cathode resistor, the bypass cap, or the coupling cap. Or, maybe it just got tired. It's the voltages that tell the story.
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As I've said, the wiring in this amp doesn't match up with any schematic.
The V3 Pin 5 plate is fed from Node F at 174VDC. It goes through a 100K (actual 103K) resistor, NOT the 47K as shown on the schematic. So maybe this is what's cutting down the voltage so much.
I don't know enough to know how to calculate the voltage drop, but maybe you do. If so, what would the resultant voltage be at Pin 5?
Thanks.
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don't worry "what" just measure V1-V4 plate AND cathode VDC, also take the time to MARK-up a schematic so we in the cheap seats can have useful knowledge.
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I haven't had a chance to MARK up the schematic, but here are the plate and cathode voltages for V1a/b through V4a/b
V1ap 364vdc V1bp 320vdc
V1ak 14.6vdc V1bk 7.67vdc
V2ap 159vdc V2bp 186vdc
V2ak 7.92vdc V2bk 8.05vdc
V3ap 154vdc V3bp 21vdc
V3ak 6.93vdc V3bk 5.41vdc
V4ap 240vdc V4bp 154vdc
V4ak 2.58vdc V4bk 2.18vdc
Let me say this about V3 and the schematic, the wiring is totally different. I would comment also that the V3bp is fed by Node F at 177vdc. It connects to a 100k resistor and reads 177vdc going into the resistor, and 20.05vdc coming out of it, which then connects to pin 5.
So that resistor which is marked as 100K and reads 103K is where the voltage is lost. Connected to the plate side of that resistor is a .005uf cap which leads up to the "Dimension" pot for the Echo. The schematic doesn't show components with these values.
I hope this helps you understand what I'm dealing with. HaHa!
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is the cap I have circled in the circuit?
IF yes, lift the "left side" where it connects to the coupling cap and pot, THEN re-measure plate vdc
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No, there's no other cap, no 680pf or 47K resistor.
Here's a photo with my red line showing the path.
Node F comes in at the top of the board on the blue wire, connecting to a 100K resistor, then down to the eyelet, where a green wire runs to the Plate (Pin 5) of V3. At that eyelet there's a pink 0.005uf cap connected that goes up to an eyelet at the top of the board, and then a green wire runs from there directly to the outside, left tab of the "15K" (actually is a 25K) "Dimension" pot that controls the Echo.
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:laugh:
which leads right to the 680pF cap strapped on the pot
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Sorry, but no cigar. That cap is not connected to the lead going to the pot. The green lead is connected to the pot on the far right in the photo, and that cap is on the one to the left of it. The tone pot.
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the TS doesn't indicate a 680pF so there's another schematic "update" then
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Easier said than done. I think I've learned from watching a lot of Youtube videos on Ampegs, and this model in particular, that you never know what you've got and the schematic is a "starting point", but quality control seems like it might have been lacking. I get a headache when I'm trying to trace the wiring and compare it to the schematic. It probably would have been easier for me to just to start from an empty eyelet board and build it from scratch. But then it wouldn't be "authentic"!! And of course, what schematic could I follow to build it??????
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I get a headache
see previous post on tracing schematics, herbs, music, think of it as where's waldo :icon_biggrin:
The math on V3's volts seem good
156vdc / 100K = 1.5mA
5.41vdc /3.3K = 1.6mA
so any smart kids wanna help guess if this is a herring or problem
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It's been a long time since I was a kid, OR smart, but without some sort of a schematic, there isn't much anyone on this side of the internet can do.
By the voltages, all tubes are drawing current, by the schematic, the nodes don't draw proper current compared to voltages at the tubes. Probably the dropping resistors aren't what is drawn, maybe cathode resistors aren't what's drawn, definitely plate resistors aren't what's drawn.
We do know there isn't a signal, so unless somebody is going to trace the circuit, perhaps the best option would be to go through with a listening amp and find where you lose signal.
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I think I can trace a signal. I've got a simple small digital OSC. Is that the best way to do it, and input a 1K signal with a dummy load on the output jack? Thanks.
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Yes, just be careful around high voltages with your scope, IE, measure at the 0V side of the coupling caps.
I'd also, before hooking your scope up, just check resistance to ground at your cathodes, and compare to known, by reading the bands, values of your cathode resistors. Maybe you have a bypass cap that's shorted. While you do this, print out the schematic, and mark the values as you go. Like Shooter said, sometimes it's fun to colour.
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You do have SOME noise at your speaker, correct?
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Thanks. Yes the noise I have is the crackling of the tubes in the sockets.
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Right, I think I recall somebody suggesting to retension, or clean something along the way. Anyways, you'll be fine. Just remember, Shooter is the best with scopes on this forum. If there's a guy who can get a result from an oscilloscope, it's Shooter.
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I found this Voltage chart for the amp last week and meant to post it, but somehow got distracted. It was prepared by someone with the amp and he shows his voltages vs. voltages he managed to find on an Ampeg schematic(s).
As you can see, the V3 Pin 5 voltage is 109vdc and Pin 6 is 4.4vdc.
Mine is 21vdc and 5.41vdc.
Its also like these readings for my tubes are "flipped" with his readings for V3 and V4.
Regardless, V3 Pin 5 is certainly not close to my reading of 21vdc. So probably not a "red herring", but still "fishy"!
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V1, V2, V3 etc are just identifiers. Typically manufacturers keep tube locations and functions consistent. Key word; typically. To troubleshoot using measured voltages, you need to know the function and connections of each one of those tubes. You have identified several inconsistencies. PIA, I know, but you need to track those connections, even if it does make your brain hurt. That's how to resolve your comment on V3&4 being flipped, because they might be.
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my reading of 21vdc
because you are having so much fun....
Re-measure the plate BUT use a couple different "ground points"
1st use the "bottom" of V3 cathode R (3.3k) on Joe's schematic
2nd Chassis
3rd the NEG side of the Cap that feeds that Tap (F if I recall)
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Thanks guys. Will re-check with different ground points. I thought I might just go ahead and replace that 100K resistor. Maybe something is going on with it just when voltage is applied that causes it to act differently when I check its value without voltage.
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Scooter-I checked the Plate voltage again using the 3 different ground points, and they all read right around 20vdc.
So I guess I'll trace the signal before I do anything else with the circuit, ie. like changing the 100K resistor, and see what results tracing yields.
AINewman: I checked the cathode resistors, and their values from cathode to ground read the same as their resistor bands. So no short. However, although the cathode resistor for the plate in question reads 3.3K which is consistent with the schematic, the cathode resistor for the first half of the tube reads 5.6K and the schematic shows 330 ohms. No where on the schematic I'm using is there a 5.6K cathode resistor. But the plate voltage is ok. So who knows???
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Ok, I connected the scope and checked out all of the Grids on the 4 tubes. Some of the images were jumpy, and I think that's probably a lot to do with needed the sockets tensioned. But I did get wave signals, at least at first, on all of the grids, a couple with beautiful sine waves, and others distorted and really jumpy.
I did save pictures of some of these images on the scope.
The grid on V1, P4 was clean smooth, and pretty but that's where the signal comes in from the input jack.
It's P1 grid was rough and jumpy but still there.
I got signal on the V2 grids but not pretty and steady.
Then on V3, I originally got sine waves on both of the grids, but when I checked again later I couldn't get any signal.
Finally on V4, I got signal on both grids, and it was pretty clean but got a little jumpy.
Finally I checked the grids on the Output tubes. And unbelievably I was getting a Sine Wave with its amplitude changing as I turned the volume and tone pot! This was using the 1K test signal.
So I figured if there's a sine wave on the grids of the output tubes, I must be getting something at the speaker. So I unplugged the dummy load, plugged in the speaker, and I could hear the 1K signal through the speaker and its volume and tone varied with the volume and tone pots.
So I said well let's see if it will play a guitar sound. I plugged it in an VOILA, I'm now getting sound from the guitar through the test speaker. Its volume is still pretty low even at the volume pot's highest setting, but signal is coming through.
So some great progress!! Now to figure out how to get some proper volume. I'll let my buddy work on the sockets over Christmas and maybe that will improve things.
Any idea what could have happened?? Thanks.
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Any idea what
:laugh:
Mine is not to wonder why, mine is to fix shit and go play
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Shooter--You just have no curiosity about you!!
Problem its not loud enough to be heard 20 feet away!
So I've still got to resolve that V3 plate issue, and also get some input from you guys on the forum on what to pursue on the volume.
Maybe its these weird component values that deviate from the schematic that's causing all the headache!
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:laugh:
I watched it kill to many cats
when you troubleshoot for a living, after the "honeymoon phase" it's just become the most efficient, competent, you can, then go live the life you want, mine was mostly stuck to a cliff-face 200 ft up, thinking Huston, we might be in trouble.
once you have the socket issue solved, the amp is stable, not crackling, popping etal then you can scope out each section, measuring input V output of that section to see where gain is made and gain is sucked out.
if it's still set up for scoping;
set all TS to 5, gain n volume to 8, measure/adjust your input signal at the input jack typical would be ~~ 100-140mVac, then measure the speaker + Typical ~~~ 8-10Vac
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Will give it a try. Is the input signal value you mention Vrms or Vpp?
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So I've still got to resolve that V3 plate issue,
This is the tube that the plate problem goes with when you put it in a different socket? It follows the tube?
If so, just buy another tube.
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rms or pp, dealers choice but DO NOT mix n match AND state which. I came from the RF world, everything was done rms. The audio world likes PP because it sounds "bigger" and they can charge for the "extra power" :icon_biggrin:
aside;
wife n I were chill'n on the couch after about 20 years married she says "I never once worried you'd leave me for another woman, there isn't a woman alive that you could love more than them damn machines you fix"
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Lucky for her there are not a lot of women with a bunch of broken tube amps.
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Willabe-no, the brightly glowing filament at startup follows the tube, but the low plate voltage stays on V3 Pin 5 plate. I'm sending the tube back to Antique Electronics Supply tomorrow for replacement since I just got the JJ from them a week or so ago.
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tube amps are just a hobby, she was referring to the $7 million worth of MRI's I was responsible for, the CT scanners before that. She is one of the amazing Techs I dealt with, I could leave a meter on-site and she would understand my instructions, measure what I needed to know, at the other end if I got a call from a sketchy Tech; "Just power down and go to lunch or something, DON'T touch anything!"
now I specialize in landscaping n gardening, the closest I get to electronics is making SURE the electric fence is powered down before entering the garden :icon_biggrin:
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Willabe-no, the brightly glowing filament at startup follows the tube, .....
That's common enough, not a problem. At turn on in rush of current hits the filament causing that brighter glow for a second or 2. Because the filament is cold, it has low resistance, so draws a lot more current, and this causes the brighter glow. A few seconds, as filament gets hot it's resistance increases, and the filaments glow lessens. This is how/why a light bulb limiter works.
Probably why your seeing it on 1 of the filaments in this 1 tube, cathode sleeve is a little low, so filament is sticking out the end of the K sleeve some. Probably nothing to stress about. We've had guys see this before, post about it and the tube was fine.
..... but the low plate voltage stays on V3 Pin 5 plate.
Maybe you have done this already and I missed it but, with that socket, measure the the dcv on the sockets plate pin, K pin and grid pin.
If the plate dcv is much higher, looking normal, then it's got to be that tube, if not, if it's still looking low, then you trace backwards from plate pin to B+ supply node with your meter. Measure other side of plate R, then measure at the filter cap + for that B+ node. You measure both because it could be a solder joint, could be the wire. It can also be the plate pin in the socket.
This shouldn't be that hard to find what is causing this low plate dcv.
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Willabe-my post #117 comments about the subject voltages. The subject plate, ie. 20vdc, connects to Node F carrying 177vdc through a 100K resistor. There's 177vdc going into the resistor, but only 20vdc coming out and making it to the subject plate. So the resistor reads good, but it's losing a lot of voltage at that point. Why is that? Could the resistor read properly but something else in the resistor be causing the drop? I don't know enough about this stuff to even guess. Or is the Node F voltage of 177vdc too low? Based on another guys Voltage Chart that I posted earlier, my Node F voltage seems fine.
So what's pulling the voltage down?
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So what's pulling the voltage down?
the easy answer is high current, but the math says there's only ~~1.5mA
another possibility is "the method of measuring" that seems to be eliminated
the tube not drawing "proper" current seems to be eliminated by multiple tubes causing the same result.
once you eliminate all other possibilities, the problem will be come obvious :icon_biggrin:
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Shooter, and the answer is??? Nothing is wrong? If so, I'm skeptical, but what do I know. Its my gut.
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Shooter-I was tracing the signal through the schematic that we've been using, and I can't see how a "clean" signal gets to the PI without going through the Reverb circuit which is shown as wired through V3. Is the footswitch the answer? Ie. if the footswitch is closed then the signal goes to Ground, and not into the grid of V3, and that ground sends the signal on to the bottom of the Dimension pot, and then out the Center lug on the Dimension pot to the PI?
Can you tell me how the signal would flow? Thanks.
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No, I'm sorry, I misspoke. The signal goes into the grid of V3 and out the Plate, but if the footswitch is closed the signal doesn't go on through the Reverb Tank, the signal goes to ground, and then on to the bottom of the Dimension pot, etc.
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Shooter, I checked the Output voltage at the speaker. It reads 2.25Vac. I had a 1K signal at 135Vrms input at the input jack. I turned the Tone pot to midway, and the Volume pot to about 8.
No tube is in the V3 spot since I have packed it up for return. I should add that the Footswitch which is wired into the circuit and not connected with a RCA jack, was cut off from the circuit board--literally cut off. So there's just a dangling footswitch wire. Would that turn the Reverb ON?
I had a dummy load in the speaker output jack. I connected the + lead of the DMM to the + lug on the output jack, and the - lead to chassis to get the reading of 2.25 VAC.
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once you eliminate all other possibilities, the problem will be come obvious
keep going, you'll eventually get there :icon_biggrin:
once the sockets have been eliminated you're one step closer
I would "rebuild" that circuit, new R's n C's for grid, plate, cathode and next stage coupling. $3's well spent.
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May be helpful...
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SEL49-Thanks so much. I had everything but the cross-over through the 22K between the cathodes.
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once you eliminate all other possibilities, the problem will be come obvious
Shooter, that's one of the best quotes I've seen in a while. :icon_biggrin: I'm saving that for re-use.
wsscott, I see that you plan to have the sockets re-tensioned over Christmas, as you have identified that they are not making solid contact. To me, that's like working on an amp with a bunch of loose connections. You can only get so far until you fix those. You may have reached that point.
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best quotes
I stole it myself, might be a Sherlock Holmes variation
I used it a lot talking with other Field engineers "Once you eliminate all the good parts, only the bad parts remain"
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Did you ever check for DC voltage at the grids?
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.... There's 177vdc going into the resistor, but only 20vdc coming out and making it to the subject plate.
That's with the tube in, yes?
Take the tube out and measure the dcv at the tube socket. It should now read the same dcv on both sides of that plate R. Does it?
So the resistor reads good, but it's losing a lot of voltage at that point. Why is that? Could the resistor read properly but something else in the resistor be causing the drop?
I'd change that R, if you haven't yet.
Yes, the R could be bad internally. Sometimes 1 of the leads is not connected fully to the R's body, you can't see it, R can be cracked inside, you can't see it.
Also, it could be a bad solder joint, bad resistor, bad tube or tube biased wrong, so it's drawing too much current.
It's dropping 157dcv across that plate R, while only drawing a mA or so of current, can't be. Like shooter wrote. The more current you pull/draw through a R the more voltage gets dropped across it. Pretty common for a tubes plate R to drop 1/2 or so the the dcv across it.
Or is the Node F voltage of 177vdc too low?
The nodes dcv is fine. That's not the problem.
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Thanks everyone for the input. I did replace that .005uf cap that ties onto the 100K resistor, but haven't replaced the resistor yet. I delivered the amp this morning to my buddy who was given the amp, and he's going to work on the tube sockets. So I'm out of business until after Christmas. When I get it back I'll follow up with these leads and let you know what I find.
Best.
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best quotes
I stole it myself, might be a Sherlock Holmes variation
I used it a lot talking with other Field engineers "Once you eliminate all the good parts, only the bad parts remain"
Similar to a quote that has been attributed to several sculptors including Rodin and Michelangelo - "the sculpture is already in the stone, I just remove the parts I don't need."
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I don't have the amp, but I think I found a simple solution to the wiring problem. All I need to do is to swap the jumper wires connecting the P5 plates to the circuit board on V2 and V3, and the same thing for the P6 cathodes. I'll have to wait until I get the amp back, but since half of V2 should be on V3 and vice versa, that's the solution. I probably need to swap the P4 grid wiring to the circuit board too.
If this is correct, it's my not being able to see the forest for the trees. Fingers crossed.
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I don't have the amp, but I think I found a simple solution to the wiring problem. All I need to do is to swap the jumper wires connecting the P5 plates to the circuit board on V2 and V3, and the same thing for the P6 cathodes. I'll have to wait until I get the amp back, but since half of V2 should be on V3 and vice versa, that's the solution. I probably need to swap the P4 grid wiring to the circuit board too.
What are you talking about?????
And what does P in P4, P5, P6 stand for?
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What are you talking about?????
:laugh:
yep, i even broke out the Joe schematic, went back to my novel
pics are worth a thousand words, just use paint or some simple pic editor and indicate what you're saying
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Got into the eggnog? :icon_biggrin:
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If V stands for a Valve, then P stands for a Pin of the Valve. Easey Peasey.
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If V stands for a Valve, then P stands for a Pin of the Valve. Easey Peasey.
No not easy, no one uses that.
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"P" is typically "plug" "J" jack so something like J15 >> P15 would be jack 15 to plug 15. P15-3 would be plug 15 pin 3
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Besides the Eggnog :icon_biggrin:
but I think I found a simple solution to the wiring problem
From my perspective this project has been harder than needed because its not clear if the approach is to repair as built, or rebuild according to the schematic you are using. Yes, Ampeg has been wildly inconsistent with this circuit, producing various versions. So you could stick with what they built, or rewire the amp according to a verified schematic of one of the circuit versions. Trying to do both at the same time would drive me to the eggnog and probably some dry refreshment too.
As Shooter, and others, me included, have suggested. Use that schematic as a guide to sketch out what you want to do and Shooter and Willabe will put away the novels and eggnog and help get this beast making music again.
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bmccowan-yea the problem I've been having is that I'd want to repair it as built, but I can't tell how it was built because of the problems with the schematic, and at least 2 prior amp techs that I have recently learned had their fingers in the amp modifying it to make it sound better, and the last one gave up leaving the amp inoperable.
So that's the struggle.
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I see that its a struggle.
I would forget about what the last two "amp techs" did to the amp.
A month ago several of us recommended that instead of trying to repair what was there, you decide on a schematic and follow it.
You decided to do that, using the Joe P schematic - good choice even if it has a couple minor errors.
First step was to go through the amp, with the schematic and document all the differences. The decision to skip that is haunting the project IMO.
Yes, Ampeg did not consistently follow a schematic, even swapping tube positions on the chassis. It was their company and they took liberties, never suspecting that all these years later we wingnuts would be trying to decipher what they did.
You are not far away from having this working.
When you get it back from your buddy, I still suggest you go through the circuit and mark up the differences with the schematic you are using. That will make finishing it much easier for you and others.
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I started that process with Joe's schematic, but there were so many differences, I couldn't write them on the printout of the schematic and still read them.
What's the best way to do what you recommend? Some of the differences are values of components, and some of the differences are how components are wired. So printing out the schematic and marking it up is not so easy to then work from.
I wish I could somehow get Joe's schematic into a piece of software, like DIYLC or something, and work from that.
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I wish I could somehow get Joe's schematic into a piece of software, like DIYLC or something, and work from that.
I'm pretty sure you can and Doug shows how to do that in that DIYLC section.
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I started this, then you went off sideways n I couldn't keep up, I just used paint from MS.
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Shooter-thanks for your work. I use a MAC so Paint isn't an option but I don't know to draw circuits like that in any software. I've used DIYLC before but I can't get it to work on my MAC running Sequoia 15.1.1. I'll see if I can figure something out over the holidays.
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I'm getting a strong feeling of Deja-vu all over again.
On Mac you can use J-schem or ExpressSch (I don't recall which one works with Mac) and work from existing schematics in .sch format.
As with DiY Layout - the instructions for download and use are all here on Doug's site. Its easy to learn, especially if starting from existing schematics.
Attached is a Reverbrocket schematic in .sch and a copy jpg - not drawn by me. Is it accurate? Accurate enough to work with. And you will learn a lot about schematics.
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Thanks.
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Sorry, I hit the Post button to fast.
I appreciate the info on the schematic drawing issue, but I don't want to take up your time fooling with that. I'd much prefer your time/input when I need it on getting the amp working. I'll get this amp figured out and working eventually.
Best.
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Or... use the original schematic that was pasted inside the back panel:
https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_r12r_reverb_rocket.pdf
I didn't check every part on the layout I linked before, but the parts I did check matched the original schematic above.
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None pasted or to be found. Not under the foil.
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None pasted or to be found. Not under the foil.
You don't have 1 but, the link HBP's posted IS a copy of the original schematic that was pasted inside the back panel.
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Or... use the original schematic that was pasted inside the back panel:
https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_r12r_reverb_rocket.pdf (https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_r12r_reverb_rocket.pdf)
I didn't check every part on the layout I linked before, but the parts I did check matched the original schematic above.
None pasted or to be found. Not under the foil.
You don't have 1 but, the link HBP's posted IS a copy of the original schematic that was pasted inside the back panel.
Right. The linked schematic is slightly different from the one pasted in my amp (below), but only because my amp pre-dates the move to NJ. All the parts/connections are the same.
(https://i.imgur.com/yXjMlvM.jpeg)
(https://i.imgur.com/Wddt6dd.jpeg)
(https://i.imgur.com/406iLRw.jpeg)
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HotBluePlates--In the schematic attached in your post above, it shows a 25/25 electrolytic cap coming off V1 Pin 3 Cathode with the Negative side of the cap going to ground. Seems correct. But in the Layout of the schematic shown in Post 43, on the far left side of the layout at Eyelet 2, it shows the Positive side of that cap going to Ground at the Guitar Jack. I assume that is a "typo", but just thought I'd check with you.
I ask in part because in my amp there's a short, 1.5", black wire connected at one end to a Ground on the Input Jack for the Guitar, and the other end is left FLYING in the air. So I'm trying to figure out where that should attach, or if it should be removed.
I haven't gotten the amp back yet from my buddy, but I'm trying to keep occupied until it's returned. I don't know if that could be affecting the low output volume.
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other end is left FLYING in the air.
normal
shielded wire is only grounded at one end, otherwise it creates a "ground loop"
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Yes, but this isn't shielded wire. Its just a short piece of single core wire that's connected at one end on the jack and its not connected to anything on the other end.
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If I'm understanding this, you did all that troubleshooting while you had a disconnected ground wire?
I think you need to look at that layout and compare it with the schematic you are following. I suspect the layout has an error - it may show that electrolytic cap backwards. I further suspect that eyelet two connects one of the V1 cathodes to your input jack ground. But you need to take a close look at the schematic to make sure. And then I would make sure that you have no more "flying in space" leads before you try to get the amp sounding right.
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I understand your point about the "disconnected ground wire", but since this amp had a couple of wires like this which appeared to be "add-ons" since they're not in the schematic or layout, like this wire which is also of a different gauge than other wires in the amp which appear to be original, it's kind of hard to figure out what its for and if it should be there in the first place.
I'll hopefully know more when the amp comes back.
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one of the "tricks" to making a wire "shielded", without shielded wire, is to take a standard solid strand wire, ground one end and wrap it around the wire to be shielded.
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HotBluePlates--In the schematic attached in your post above, it shows a 25/25 electrolytic cap coming off V1 Pin 3 Cathode with the Negative side of the cap going to ground. Seems correct. But in the Layout of the schematic shown in Post 43, on the far left side of the layout at Eyelet 2, it shows the Positive side of that cap going to Ground at the Guitar Jack. I assume that is a "typo", but just thought I'd check with you.
It's a typo. Unfortunately, the draftsman is deceased & someone will need to edit then re-upload the layout.
I ask in part because in my amp there's a short, 1.5", black wire connected at one end to a Ground on the Input Jack for the Guitar, and the other end is left FLYING in the air. So I'm trying to figure out where that should attach, or if it should be removed.
It should be attached as the layout shows, at Eyelet 2, marked "Guitar Jack Ground."
My amp has the Black wire from the jack connected to Eyelet 2, along with a Green wire that runs over to Eyelet 4 to make the Ground connection.
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Progress!!-I got the amp back yesterday, and the first thing I did was to connect Jack 1 ground to Eyelet 2 of the circuit board, getting it a ground for the rest of the preamp. In addition the 25/25 cathode cap was connected to Eyelet 1 and it should have been on Eyelet 2, so I switched the negative end of the cap to that Eyelet.. Finally, the 1.2K resistor (at Eyelet 1) coming off the cathode and going to the footswitch per the schematic was installed with a 2.2K resistor and I replaced it with a 1.5K. I also replaced a couple of other old caps. There are a few original caps left in the amp as well as 1 old small electrolytic.
So I powered it on, and plugged in my guitar. With all the pots at 0, it's dead quiet. The footswitch cable was cut off from the circuit board, so nothing is connected there, and there is no reverb tank plugged in.
When I turn up the volume I get strong, clean guitar sound; however there's a lot of hiss and crackling in the background. I have enough clean sound and volume at 2-3 on the volume pot that I haven't needed to turn it up higher. Tightening up the tube sockets seemed to have cleared up the sensitivity I was having in the beginning with the tubes as one source of the noise, but I was getting minimal output volume even at full volume on the pot
So fixing the ground to the Input Jack was the big fix so far. The guitar sound is really strong, and clean but for the hiss, etc. The tone pot also is responsive.
So now I need to figure out what's causing the noise. There's no hum that I can tell. And there's no distortion in the output sound. So any help on this issue is greatly appreciated.
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hiss and crackling
Hiss is typically associated with old carbon Resistors, usually at the plates
Crackling is typically associated with loose solder joints, cracked foil on PCBS, sloppy sockets...something is "making n breaking" contact.
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I'm working on the hiss issues and replaced the 25uf cap connection the cathodes of V4 Phase inverter. That cathode on Pin 3 connects to a 220 ohm resistor, and also a 10K resistor that goes off and connects to the 8 ohm secondary winding of the Output Transformer, according to the schematic.
The Layout referenced above shows that 10K resistor but its shown like this 10K(1K). What does that mean?
On the circuit board in the amp, the 10K connects from Pin 3 Cathode on the V4 PI tube socket to a circuit board eyelet. There is nothing else connected at that eyelet. It seems like either (1) the 8 ohm lead from the secondary should connect there with another wire then connecting from that eyelet to the Output Jack, or (2) I should connect a new lead from that eyelet where the 10K resistor is also connected and then run that new lead to the Output Jack lug where I currently have the 8 ohm secondary winding presently connected.
Is this correct, and which wiring scheme is better?
I can only assume that whoever removed the Output Transformer before my buddy got the amp, also removed that wiring at the eyelet.
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The Layout referenced above shows that 10K resistor but its shown like this 10K(1K). What does that mean?
Layout referenced where?
You have 192 reply's! You need to keep the layout and schematic in the new post your asking a question about so we don't have to search for them through 4 pages of 192 reply's. :BangHead: :cussing: :l2:
I'm working on the hiss issues and replaced the 25uf cap connection the cathodes of V4 Phase inverter. That cathode on Pin 3 connects to a 220 ohm resistor, and also a 10K resistor that goes off and connects to the 8 ohm secondary winding of the Output Transformer, according to the schematic.
On the circuit board in the amp, the 10K connects from Pin 3 Cathode on the V4 PI tube socket to a circuit board eyelet. There is nothing else connected at that eyelet. It seems like either (1) the 8 ohm lead from the secondary should connect there with another wire then connecting from that eyelet to the Output Jack, or (2) I should connect a new lead from that eyelet where the 10K resistor is also connected and then run that new lead to the Output Jack lug where I currently have the 8 ohm secondary winding presently connected.
Is this correct, and which wiring scheme is better?
I'm not sure on your wording, but I think if I understand you, their both electrically the same.
Look at the schematic and look at the layout drawing, the 220R gets connected on 1 end to the 8 ohm OT secondary and the other end to V4 Pin 3.
Your making this over complicated.
I can only assume that whoever removed the Output Transformer before my buddy got the amp, also removed that wiring at the eyelet.
Yes, probably. They might have wanted more distortion, so they disconnected the NFB loop.
Just re- connect it.
The link for the layout that HBP posted is in reply #23. And SEL49 posted a dry/wet signal hi-lited schematic back in reply 154.
And from now on they both can be found here in reply # 193.
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I didn't re-post the Layout, referenced at Post 43, because one of the moderators said I should just reference or ID a link and not re-post because posting takes up Server space.
Any comment on why the Layout refers to the resistor as a "10K(1K)"?
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HotBluePlates-How does your amp wire the Output Transformer secondary to connect to the speaker?
My amp doesn't have an Output Jack for a speaker connection, and there's no empty hole in the chassis for a jack, and there's no terminal strip where some wiring could occur.
Right now I just have the Secondary wired directly to a 1/4" jack, and will ultimately stabilize it in the cabinet in some way.
Thanks.
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I didn't re-post the Layout, referenced at Post 43, because one of the moderators said I should just reference or ID a link and not re-post because posting takes up Server space.
Where at 193 reply's! Nobody knows the link is all the way back at reply #43! Except maybe you! :laugh:
So, when you ask a new question that the guys need to look at the layout and the schematic, tell them what the reply # is where they can find them.
Any comment on why the Layout refers to the resistor as a "10K(1K)"?
I have no idea why, but I wouldn't worry about it. Just go with the schematic's 10K.
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HotBluePlates-How does your amp wire the Output Transformer secondary to connect to the speaker?
My amp doesn't have an Output Jack for a speaker connection, and there's no empty hole in the chassis for a jack, and there's no terminal strip where some wiring could occur.
Right now I just have the Secondary wired directly to a 1/4" jack, and will ultimately stabilize it in the cabinet in some way.
Look at the picture in reply #11. (And #10)
It sure looks like Ampeg didn't use a 1/4" speaker jack mounted in the chassis. They just ran the OT secondary fly leads directly to the speaker.
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When that amp was new Ampeg connected the OT secondary directly to the speaker using crimp-on quick disconnect terminals. The OT-151A was a special transformer that was used in several of the 15W to 25W combos during that time period. The OT only had a single 8Ω secondary winding, but there were two black leads and two green leads connected as shown in the pic. One green and black lead went directly to the speaker without ever passing through the chassis. The other green and black wires passed through the chassis and connected to the board for NFB.
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Thanks. That's how I'm doing it but using a jack instead of the crimping method.
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PS... The footswitch was also hardwired (no jack)
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Yes, I have the footswitch but it was cut off of the terminal board!
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Reverb will work without the FS, but Tremolo requires a FS to be wired to the board in order to function.
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SEL49-So when the tremolo circuit's 1.2K cathode resistor goes to Chassis Ground when the footswitch is closed, the Tremolo function is then running?
If so, I could just ground the wire connecting to that resistor on the circuit board, and I wouldn't need a footswitch for it. It would be "on" all the time. I could just turn it "on/off" by controlling the Speed and Intensity pots?
And if the footswitch is only needed to turn the Reverb OFF, I wouldn't need it at all. I wouldn't have to re-wire anything for it. I could just control the amount of the Reverb by adjusting the "Dimension" pot?
I ask these questions because my buddy who owns the amp would like to do away with the footswitch and the cabling, just to "neaten" things up.
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All correct
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Still working on the hiss problem.
I've replaced the carbon resistors on the plates and the guitar input jack with no effect.
When I remove V1 the hiss is gone, regardless of whether a guitar is plugged in. Of course there's no signal to process to be sent on to the Output tubes.
Same thing if I put V1 back in and remove V2.
There's no DC on either the Volume or Tone pots. I have put in new caps connecting these pots to their plates.
I measured the 1M Volume pot and it reads about 1.35M. The Tone pot reads slightly below its value of 500K.
Could the Volume pot be causing the hiss? The volume of the hiss and the guitar increase or decrease in turning the Volume pot. Tone becomes more treble or bass, but volume of hiss/guitar signal doesn't change other than the pitch.
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Try grounding the grid of V1, (labeled pin 4 on your schematic), see if that helps with the hiss.
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No, still have hiss. I clipped Pin 4, grid, at the tube and clipped the other end to the chassis, and also tried it clipped at the Input jack, but still hiss.
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a scope is your friend
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I have a small digital scope, but I"m not very experienced in using it to know what to look for. I guess now is the time to get some more experience.
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Or use a listening amp.
Scroll down to find it. A lot of guys use them here.
https://el34world.com/Hoffman/tools.htm (https://el34world.com/Hoffman/tools.htm)
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You could try switching V1 with the PI, see if that helps.
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When the scope is in AC mode, 1khz signal, 135mVrms, I get a nice sine wave through V1 Pin 4 grid, and through the wipers of the Volume and Tone pots. The signal is 135mVrms at the input jack, 79mVrms after the input jack resistors, and 2.25Vrms--So a lot of gain.
There is hiss in the background that's also loud.
When I disconnect the scope's probe, leave it's negative lead attached to the chassis, and switch it to DC coupling, with no signal input at the input jack, I pick up a signal on the scope of about 15khz and 25mVrms. That signal is jumpy and fuzzy looking. Not a nice sine wave. So I assume that's the Hiss.
If I move the scope's probe away from the chassis, with its ground lead still connected to the chassis, that signal disappears or is smaller on the scope.
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of about 15khz and 25mVrms.
That should be visible in AC coupled mode with the scope set up to "view" that frequency. Start to the wiper to the VOL pot after V1a.
Don't inject a signal yet, just see if you can get "eyes on". 25mV will be close to the scope "baseline noise" so you might have to spend quality time looking in the weeds for a "coherent signal"
EDIT: 15khz might be ambient noise in your environment, LED lights, wall warts, so don't go chasin CCP technology :icon_biggrin:
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SEL49 can you help me with the wiring of this OT?
After I added the 10K resistor, and turned it on to test, my 8 ohm dummy load, which is a 3W 8ohm resistor wired onto a 1/4" jack, that I plug into the Output Jack where the Black and Brown secondary wires are connected, got really hot. So I turned it off. It hadn't done that previously. So I don't know if I have improperly wired in that 10K resistor.
I connected one end of the 10K to V4 Pin 3 cathode where one end of the 220 ohm resistor is also connected, and I connected the other end of the 10K resistor to an empty eyelet. Then I ran a new lead from that eyelet to the Output Jack tip lug where the Brown OT secondary is also connected directly from the OT.
Is that correct? Or does the 10K need to be wired "in-line" with the Brown secondary before it is connected to the lug on the Output Jack tip.
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The 10K is connected properly. Swap the brown and black leads on the speaker jack. Any better?
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After I added the 10K resistor, and turned it on to test, my 8 ohm dummy load, which is a 3W 8ohm resistor wired onto a 1/4" jack, that I plug into the Output Jack where the Black and Brown secondary wires are connected, got really hot. So I turned it off. It hadn't done that previously.
That's at least a 15w amp and you are using a 3w R for the dummy load?
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True, but I haven't gotten past 1 or 2 on the Volume pot, so I assumed it would be okay for low volume, short test periods.
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The 10K is connected properly. Swap the brown and black leads on the speaker jack. Any better?
Is the OT in this amp original with the 2 black secondaries and 2 brown secondaries?
Or is this a replacement OT with just single +/- fly leads?
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And why are you using a dummy load to test if you have the NFB hooked up correctly?
You need to hear what it sounds like.
The dummy load wont protect the OT from a short, use your LBL if that's what your worried about.
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True, but I haven't gotten past 1 or 2 on the Volume pot, so I assumed it would be okay for low volume, short test periods.
What tests are you doing with the volume on 1 or 2?
If your using it to look at the signal with a scope, turning the volume pot only to 1 or 2 is not going to give you enough signal to see much of anything if something is wrong.
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It is not the original OT. The OT had been removed from the amp by someone before my buddy was given the amp, so a new one was purchased with just the 2 secondary leads.
I had the dummy load in when I was injecting a signal to trace it through the circuit and see where the signal went crazy, hoping it would be an indicator of the source of the hiss. I got as far as the tone pot and the dummy load resistor heated up so I removed the dummy load and connected it to the speaker.
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Ok, then try what SEL49 suggested.
The 10K is connected properly. Swap the brown and black leads on the speaker jack. Any better?
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So yesterday I spent some time replacing carbon comp resistors, and replacing them with metal film of equivalent or near equivalent values. I had most of the values I needed but there were a few that were a little lower or higher. The 5.6M at the input jack was replaced with a 4.7M since I didn't have a larger value.
I've noticed that the hiss seems less, but also the gain is much lower now. Maybe that's because the gain is less. I can turn it all the way up and the volume is about where it was at a 2 before these changes. Although the hiss is less on Input Jack 1, it's almost negligible on Input Jacks 2 and 3.
Someone had a 25/50 cap installed across the 22K that connects with the 3.3K and 2.2K resistors on the V2, Pins 3 and 6 cathodes. And instead of the 22K connecting to a 2.2K on one end, a 5.6K had been installed. Neither of these are on any schematic. So I removed them, and replaced with values shown on the schematic.
I'll check the values on the resistors I replaced to see if I misread something.
I used the guitar at the input for a signal level comparison and although the hiss is lower so is the volume.
I didn't change any wiring yet on the output jack to the 8 ohm speaker. I did check the AC voltage on that jack and it's reading 14.5 VAC. Does that sound correct?
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I'll check the values on the resistors I replaced to see if I misread something.
if you did this step 1st, you'd already know :icon_biggrin:
always ohm ANY and ALL resistors 1st, the paint colors n schemes of the modern components are NOT from our 64 Crayola box!
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I checked, and I had swapped the positions of the 3.3K and the 2.2K. Their bands and readings are correct, just in the wrong spot.
Shooter-any thoughts on the Output Speaker Jack AC voltage of 14.5 VAC? Is it too high/low/or just right? Like the 3 bears.
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Shooter-any thoughts on the Output Speaker Jack AC voltage of 14.5 VAC? Is it too high/low/or just right? Like the 3 bears.
With no input signal to the amp there should be zero volts at the speaker jack. The speaker jack must be mounted to the chassis to provide a ground to one side of the OT secondary. Alternately, if the speaker jack is just dangling in the air, you must connect a wire between the jack ground lug and chassis ground. Grounding one side of the OT secondary will likely make that 14.5V meter reading disappear.
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Boy do I feel dumb! So I could do it as you suggest, or just wire the speaker terminals directly to the 2 secondary wires? I now assume that’s how it was originally done—as shown in the schematic.
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you must connect a wire between the jack ground lug and chassis ground.
even if you hardwire the speaker
Math: 14.5vac ^2 / 8ohms = 26W ..... When it's actual music and sounds like it "should" :icon_biggrin:
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If I decide to mount the jack to the chassis, there’s an existing hole next to V1. Is that location ok or is it to close to the preamp section so as to cause noise?
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Don't put the speaker jack near V1!
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^^^^^^^^^^
you want large signal wires and sensitive signal wires as FAR apart as possible, Look up "Inductive coupling" "signal cross coupling" for your evening reading as you sip a fine single malt n unwind the day :icon_biggrin:
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Since the amp I'm dealing with didn't have an OT when my buddy was given the amp, and no output jack on the chassis, does anyone know how this amp was physically wired? Was the speaker wired directly to the OT, and a wire added to the OT COM side that then connected to the chassis somehow?
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Have you forgotten reply #198 already?
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SEL49-I remember the post, but since my OT only has 1 black and 1 brown wire, I'm trying to figure out the best place to connect the ground wire to the chassis. I just wanted to "see" how the Ampeg came wired from the factory.
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2 ideas come to mind, both without a 1/4" output jack.
1. Run the OT secondary pair of wires directly to the speaker and solder them in place on the speaker tabs along with a 2nd pair of wires that go back into the chassis. The 2nd pair of wires you run into the hole in the chassis that Ampeg used. You might need to put a rubber gromit in that hole or enlarge that hole a little so you can get a gromit in it. Attach those 2 wires that you brought back in to the solder eyelets/turrets on the board. Those 2 wires are for the -FB loop.
2. Run the OT secondary wires into the chassis through the hole that Ampeg ran them through, might need a gromit. Attach them to the board where they go for the -FB loop. And run a 2nd pair of wires from those 2 solder eyelets/turrets back out through that same chassis hole to the speaker and solder them in place.
Either way, you will need 2 pairs of wires to get the OT secondary hooked up to the speaker and the -FB loop.
And you have to run them through the chassis hole that Ampeg used. That chassis has no room for a chassis 1/4" jack.
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It shows in this layout drawing where to connect the OT 8 ohm secondary -FB loop wire to the eyelet/turret board and where the 10K -FB loop R gets placed/hooked up.
Just need to solder the chassis OT secondary ground wire (2nd OT ground wire to eyelet board, 1st OT ground wire to the speaker) to ground inside the chassis.
https://el34world.com/Forum/index.php?action=dlattach;topic=18071.0;attach=48337 (https://el34world.com/Forum/index.php?action=dlattach;topic=18071.0;attach=48337)
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Willabe-Thanks. Yes I used that Layout in wiring the OT. First, I connected the 2 secondary wires from the OT to the lugs on a 1/4" jack to be connected to the speaker. Then I ran the 10K Resistor to that eyelet from V4, Pin 3 cathode. Then I ran a new wire from there to the 8 ohm lead on the Speaker Jack. Finally, just today, I ran a new ground lead from the Ground lug on the Speaker Jack to a ground on a terminal strip inside the chassis.
So I think I did it correctly. Please tell me if something is wrong with it. It now shows no VAC on the Speaker Jack when there is no input signal, and it shows VAC when there is an input signal. The voltage increases as I turn up the Volume. I stopped when it read about 6VAC on the Speaker Jack because it was really loud.
But I've still got the hiss.
The schematic shows the Input Jacks as 1-Guitar, 2-Accordion, and 3-Mic. The amp is wired with the 470K resistor on the Guitar Input jack, and not the Middle Accordion Input jack. The 5.6M is also wired to the Guitar Input jack and of course Pin 4, Grid.
The hiss is worse on the Guitar Jack, and much less on the other 2 jacks. The hiss is there regardless of whether or not the guitar is plugged in.
If I remove V2 then there’s no sound at all because the signal stops and can’t get to the Phase Inverter for the final processing.
Half of V1 is for the Pre-amp, and the other half is for the Tremolo circuit. The hiss isn’t affected by my turning either the Tremolo Speed or Intensity pots. And of course there’s no Tremolo. So my guess is that the Tremolo circuit is not causing the hiss.
I have replaced all the resistors and caps from the Input jacks through to V3. And I’ve replaced 90% of them through the end of the circuit board.
Because the hiss is less at Inputs 2 and 3 (accordion and mic) its my guess its something either with the jack wiring to the circuit board, or the Volume pot. The jacks have been cleaned as well as the chassis where they attach. The resistors on the input jacks have been replaced. As stated before I replaced the 5.6M with a a 4.7M since I didn’t have the larger resistor on hand.
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Yes I used that Layout in wiring the OT. First, I connected the 2 secondary wires from the OT to the lugs on a 1/4" jack to be connected to the speaker. Then I ran the 10K Resistor to that eyelet from V4, Pin 3 cathode. Then I ran a new wire from there to the 8 ohm lead on the Speaker Jack. Finally, just today, I ran a new ground lead from the Ground lug on the Speaker Jack to a ground on a terminal strip inside the chassis.
Only 1 thing, how are you going to keep the 1/4" jack and 1/4" plug from swinging/flopping around? Did you fasten them somewhere?
So yesterday I spent some time replacing carbon comp resistors, .....
I've noticed that the hiss seems less, but also the gain is much lower now.
I stopped when it read about 6VAC on the Speaker Jack because it was really loud.
So is the amp back to full volume again?
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Yes, I will stabilize the speaker jack.
Yes, the amp is back to full volume.
Is that 5.6M resistor or that 470K resistor at the input jack a potential cause for the hiss? Why are they both so large?
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Yes, I will stabilize the speaker jack.
You really didn't need that 1/4" speaker jack, extra work without a need.
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Is that 5.6M resistor or that 470K resistor at the input jack a potential cause for the hiss? Why are they both so large?
Just change them to MF(metal film), then you'll know if they were a problem.
They used those values to set the bias where Ampeg wanted it that 6SL7 tube.
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I already changed them to metal film. Still the hiss.
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I just noticed that the ground lead in the shielded cable connecting V1 P4 grid to the Input jack is not grounded on either end. It's just the shielded lead that is connected to the tube socket and then to the input jack. Could this cause the hiss?
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NO INPUT.....GROUND pin 4 of V1 with a gator clip, did the hiss go away?
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YESSSSSSSS! NO HISS!!
Okay, so I'll connect the ground shield of that wire to the chassis ground up at the input jack and leave the other end at the tube socket cut off and not connected.
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Ok, so connecting the ground shield doesn't work. So that's not the solution.
So Shooter what is the solution?
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replace the input jack with a proper one that GROUNDS the input with nothing plugged in, maybe the shielded wire also if it's been solered to death over the years
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The jack is a grounding Switchcraft type jack with the grounding tongue on the tip lug.
With nothing plugged in the jack, if I check continuity between a chassis ground point and the tip lug on the jack there is continuity. If I put in a plug there is no continuity to ground. Seems proper?
If I plug in a guitar with the alligator clip connected from Pin 4 to Chassis Ground, I get no sound at all. No guitar, no hiss, nothing.
But if I remove the jumper, I get sound and hiss.
Is that jack wired wrong? I disconnected its connection to the other 2 jacks just to eliminate them from the circuit. I thought let me get 1 input jack working right, and then I can deal with the others.
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So Shooter, is your analysis that the jack is leaking since the circuit is quiet when Pin 4 is jumped to ground, but its not quiet when I rely just on the jack? So even though there's continuity with chassis ground, there's still a bad connection in the jack itself that's causing this problem?
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by grounding PIN 4 you are killing ANY input to V1 so you won't have guitar!
NO SIGNAL......remove the jumper on pin 4 and use it to jumper the VOL pot wiper to chassis....Got hiss?
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It goes dead quiet just like grounding Pin 4. No sound. When I remove the ground on the jumper it has hiss.
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NO SIGNAL......Pull V1......Hiss?
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With V1 pulled, and no jumper between chassis and Volume Pot, No Hiss and very minor hum.
With jumper in place, then no sound. No Hum, No Hiss, nothing.
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NO SIGNAL.....put V1 back in, put Jumper from V1 Pin 1 to ground.....Hiss?
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Yes, Hiss.
The ground shield in the shielded cable from V1 to the Input Jack is disconnected.
But even if I connect it to chassis ground with a jumper, the hiss remains.
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ok, my hand is getting all sweaty n clammy, read over those last few posts while looking at the schematic n tell me what you'd do next :icon_biggrin:
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NO SIGNAL.....put V1 back in, put Jumper from V1 Pin 1 to ground.....Hiss?
V1 pin 1 is the TREMOLO circuit. Maybe you meant to say V2 pin 1?
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Well I would swap this tube with the 6SL7 at the PI. I think I've already done that. But I swapped tubes and still has the same effect. With jumper on V1 Pin 1 to GND there is Hiss regardless of whether the jumper is connected to ground or not.
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If I jumper V2, Pin 1 to GND, only extremely little hiss when Volume turned all the way up. When jumper is off the GND, then it has Hiss.
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When you grounded pin 4 of V1 and it stopped the hiss, you isolated the noise to 8 different components, all between the input jacks and the 1st gain stage of the amp. (I'm not sure why this didn't work when you did the same thing 2 pages ago, but anyways.)
Maybe some good high rez pictures of the input jacks and 1st gain stage might help.
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V1 pin 1
that told me there was no "cross talk" in V1, in other words the trem circuit wasn't influencing the 1st gain stage.
You "killed" the hiss at V1A, both into and out of, so V1A has a problem, socket, components, wire dress, solder connections, ya-de-da.
re-build V1A from the input jack to the output of the TS/VOL, you can do it Johnny Cash one piece at a time, or go nuclear and burn it all down n start new
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Did you try new tubes?
Tube could be noisy.
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Thanks guys. Since I swapped tubes and everything is the same, I would say the tube is not the problem.
I"ve replaced all the components of V1A except for the 25/25 cap which appears to be relatively new and modern.
We've eliminated the V1B section.
So my gut tells me it's in the jacks or the Volume or Tone Pot. The Volume pot is a 1M, but it reads 1.35M. I don't know if that's the cause. The Tone pot is a 500K, and it reads right at 500.
I've only got the middle input jack (as shown on the schematic), but it's the first Input Jack physically on the chassis, connected to the circuit. I disconnected the other 2 jacks to eliminate them from the issue.
The wiring on the jack looks squirrely to me, and I've attached a photo of the one active input jack. The jacks are all shorting jacks, and the schematic shows just standard jacks. So I'm a bit confused on wiring them.
The two resistors attached to the active jack are a 4.7M vs. the 5.6M as shown on the schematic, and a 470K.
I'm also attaching another photo showing the wiring on the other 2 jacks. The dangling red wire is what I disconnected and it had been soldered at the junction of the 4.7M, the 470K, and the shielded lead going to V1 Pin 4. The black wire is on a jumper between the 2 lugs (which I think are the sleeve and switch) on the active input jack, and it connects to a ground eyelet on the circuit board.
Thanks for your help.
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SUCCESS!! I did it. I re-wired that first jack. I realized that the 470K had been wired to the Pin 4 lead with the 5.6M resistor, and the jack needed the 47K resistor added to make it work. So I wired the 470K from the Tip lug to the ground jumper between the sleeve and the switch lugs. Then added a 47K to the Tip lug and connected it to the 5.6M and the Pin 4 lead. Hiss is now barely there. Maybe a new Volume Pot would clean that up. Or maybe I should connect the GND shield from Pin 4 to a GND somewhere. At the jack or somewhere else?
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Thanks fellas so much for all of your help and patience with me. I couldn't have gotten this thing working without your guidance. I guess one very important thing I learned in this process is that if an amp is a mess when you get it to fix, and nothing matches up with a schematic, then don't trust anything about the circuit or the work that had been previously done. Not even the simplest things-like the wiring of a jack. There were a lot of issues in getting this amp up and running again. I still have the Tremolo and Reverb to work on, but since it's all been re-wired according to the schematic, and it has all new components, maybe this part will be relatively simple. Famous last words!
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I should connect the GND shield from Pin 4 to a GND somewhere. At the jack or somewhere else?
a shielded cable is grounded at one end only and typically at "the source" of the "shielded signal" so in this case the shield should ground at the input jack, since that is the source of the signal
V1A is a hi-gain tube, I personally don't like the 1st stage to be hi-gain, I prefer higher signal to noise ratio. so there will always be some hiss, my "It's ok" level; with the amp VOL at 5, guitar VOL at 0 I start strumming and rolling the guitar VOL up, If the hiss is gone when guitar is at 1, it's normal, even 2 if it's a larger watt amp. Then if the owner want better, you pay more for better :icon_biggrin:
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I'm glad you got that straightened out. You are right that Shooter, Willabe and some others showed a lot of patience walking you through troubleshooting.
I am commenting on your "lesson learned" in hopes that it helps the rest of the process and others reach success with less pain.
I guess one very important thing I learned in this process is that if an amp is a mess when you get it to fix, and nothing matches up with a schematic, then don't trust anything about the circuit or the work that had been previously done. Not even the simplest things-like the wiring of a jack.
I think that is right, but it does not offer up an amended approach. From the beginning, multiple folks recommended and/or requested that you take the schematic closest to the circuit you had and go through the chassis marking up the changes on the schematic. You resisted doing that, and I think it cost you a lot of aggravation. The problem with the wiring of the input jack would have easily been found with that process. I think you said that dealing with tracing the circuit made your head hurt. I have to think your head hurt regardless.
You still have a ways to go, and I suggest you reconsider your process before Doug runs out of bandwidth :icon_biggrin:
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Fair point.