Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Mjcopp on November 15, 2017, 12:44:08 pm
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Hey guys. Welcome to my nightmare. This is my first time build and first time electronics endeavor. After studying for 5 months I decided on the Supro Tremo-Verb per Uncle Dougs’ 8 part youtube video series. When I first powered up, the two in-series, 330 ohm/2w cb resistors burned up and the cb cap exploded. Yikes! Not what I expected. Since then, I have been relentlessly researching and tinkering in this amp, (for the past 3 months) hoping to accomplish this on my own, but I humbly admit I need help!!! I have read over 100 threads but can’t find the answers. Please fellas, if you can help me get this amp going, I will be most grateful.
The problems are as follows:
Very low volume, only 1-2 watts power. (Should be around 15w)
Muddy, distorted tone.
No Tremelo.
Link to schematic:
https://m.box.com/shared_item/https%3A%2F%2Fapp.box.com%2Fs%2Fu945p1s2dv7ffrok96ux52lseya2rjv4
I have traced out my wiring per the schematic a dozen times and can find no mistakes presently. I truly hope someone else can!
Voltage measurements: (schematic specs in parenthesis)
V6 5y3 nos
Pins 2&8: 379vdc
Pins 4&6: 339vac
V5 5881 new tung sol
Plate 373 (360)
Screen 370 (350)
Grid .164 (0)
Cathode 30 (17)
V4a 12ax7 nos reverb recovery
Plate 170 (120)
Grid 0
Cathode 1.08 (1.2)
V4b driver for power tube
Plate 180 (140)
Grid 0
Cathode 1.75 (1.6)
V3a 12ax7 nos reverb driver
Plate pin1 366 (350)
Grid pin2 .03 (0)
Cathode pin3 3.7 (1.2)
V3b
Plate pin6 365 (350)
Grid pin7 .42 (0)
Cathode pin8 4.08 (2.4)
V2a 12ax7 (Marshall) tremelo
Plate pin1 87-140 (140) oscillating?
Grid pin2 1.2 (?)
Cathode pin3 .3 (.4)
V2b
Plate pin6 300 (240)
Grid pin7 0
Cathode pin8 2.4 (2)
V1 12ax7 nos
Plate 231 (180)
Grid 0
Cathode 2.4 (2)
All heater filaments: 6.5 vac
B+1: 375 (360)
B+2: 366 (350)
B+3: 300 (240)
V5 Plate to cathode: 340
Cathode bias resistor: 680 ohms
Bias voltage: 30v
Plate current V5: 45.2mA
Plate dissipation: 17 watts
Plate current V1: .3mA
Plate current V4: .4mA
Pot resistances: (viewed from back of pot)
Vol R lug-Wiper: 0-497ohms L-W: 0-285
Tone R-W: 0-465 L-W: 0-465
Trem Int R-W: 0-0 L-W: 0-256
Trem Spd R-W: 0-425 L-W: 0-458
Rev R-W: 0-90 L-W: 0-81
No Dc voltage on pots except Trem Spd showing 7-14vdc oscillating b/t L lug - ground, and Wiper – ground.
Sidenote: Uncle Doug’s final measurements with 5y3, 5881 & 656 ohm cbr were:
Plate current: 45.4 mA
C-P voltage: 333v
Plate dissipation: 15.1 watts
Calculated bias voltage of 29.6vdc
His build sounds real nice. He claims it needed no adjustments, just wired it up and Wa-lah! Beautiful tone!
Other tidbits...
NOS tubes newly purchased from kcanos tubes. 5881 tung sol brand new. Marshall 12ax7 pulled from another amp, (good tube) used in place of 12at7 reverb driver. I had a 12au7 in v2 originally, but thought it might draw too much current for the P.T. to handle? All 12ax7s have been interchanged multiple times with no changes. Light bulb limiter stays dim with 250watt bulb.
V1 pins 1&2 pop nice and loud when probed. V4 pins 6&7 pop. V5 power tube grid pin5 does not pop at all when probed. Checked all R values (within spec). Checked all ground connections for continuity, (good). P.T. voltages test good. P.T. resistance between HVac leads to CTap (red wires to rect plates) tested 174 & 185 ohms. Replaced O.T. with a new Weber WSE15, (5k ohms). Speaker is a new Weber 12F150.
I also shorted the tremelo footswitch jack because the footswitch I purchased was defective. Everything else is per the schematic. The biggest difference I took the liberty of changing is to use a turret board instead of strictly pt. to pt. The board was pretty at first, but after 3 months of toying with it, and changing to a star grounding scheme I hadn’t originally planned for, it is now quite nasty. You will laugh (or cry?)when you see it. No squeals, hiss or hum… just quiet, warble-less, muddy distortion.
I do not have an oscilloscope, signal generator or variac, just a DMM.
I will try to include some pics.
Please let me know what you guys think?
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On quick review, most of those voltages 'seem' okay, but I'd definitely like to see pictures. What you're explaining seems to me that a few things may be going on. 1. somehow the signal is finding a path partway at least to ground more than on down the line. You'd likely need an oscilloscope to track down where it is, and isn't. Also, there could be connections that aren't perfectly soldered, I'd use a wooden or plastic chopstick to tap components all over the signal chain and see if things suddenly change. If so you should reflow the solder around those connections.
~Phil
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Thanks Phil, I'll try to get pics up later today.
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Pics:
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pics 3, 4
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Pics 5, 6
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Cab
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Panel
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First, the pics are a bit dark and hard to see on the top down view but form some of the side angles I think I noted something wrong. This is just one thing I've noticed. I'm attaching a pic to show a red line on a lead that I think is the tube cathode connection to a board and the resistor/capacitor. The capacitor here and on all the other locations seems 'backwards' Normally the positive side of the capacitor has the dip around the top (dimple some call it). On yours it appears away from the tube. I've drawn a red line from the socket on the wire to the connection I "think" is the cap. That cap, if that lead comes from the socket straight to the cap for the cathode, is definitely upside down, and others may be the same. The + end connects to the socket, and the - end connects to ground directly.
They all may be in backwards. This would be a major problem. Also it does look like a lot of the solder connections are 'grey' instead of shiny and aren't well flowed to create a solid joint.
let me know about those electrolytic caps first.
~Phil
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In fact here's a pic from one of my amps before I put the board inside, I've put a red + at the positive top side and a - at the bottom and even circled on the cap where the + is to show that end is positive. That's the side that connects to the tube pin.
~Phil
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Could we see your board layout to compare to the schematic?
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Phil, I had all the caps oriented as you suggested. But I read on Aikens' site that the cathode bypass caps banded end, (which I assumed is positive) goes to ground. Did I mis-understand this? Anyway, I just flipped the caps yesterday, with no change. I'm starting to think I should buy a new board and turrets and start over.
Sluckey, I will re-draw my layout and post it soon. Thanks guys.
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Those definitely matter, the dented end is positive and goes towards the tube sockets, the only time they can sometimes be 'backwards' is in bias circuits. They may have been damaged. As for the turret board, it can't easily be messed up too much. If you want to 'start over' you could remove it, clean all the solder off the turrets with some desoldering braid and try new wires and maybe test all resistors for good values, and replace any caps taht seem to maybe be damaged in some way or don't test for their value. I think you likely can get it as is if you're just methodical about getting good solder connections etc.
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Layout diagram
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Well that certainly explains why that cathode cap exploded! Every bypass cap on the board in your pics is backwards and needs to be installed properly. They are marked to show the polarity.
I've read a lot of stuff on Aiken's site but don't recall that statement.
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^ I'm thinking MJ might have been reading this: http://www.aikenamps.com/index.php/where-to-connect-the-outside-foil-on-capacitors and mistakenly thinking that Aiken was talking about electrolytic capacitors instead of non-electrolytics.
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That's what I thought too. However, the article starts with these words "Some non-electrolytic capacitors have a banded end" and never mentions "cathode bypass caps banded end". The caps on Mjcopp's board have a light blue stripe that has several negative symbols inside arrows that point to the negative end of the cap.
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Ok, I'll flip the caps and report back.
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Ok, I'll flip the caps and report back.
Most of those caps are probably not damaged because they have only had a couple volts on them. But that 50µF that's connected to the cathode of the output tube has had 30V on it. It's likely damaged. One cap has already exploded! I would throw that cap away and get a fresh one. The amp will work without that cap if you don't have a replacement on hand but it won't be as loud as it would be with the cap.
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Yeah I mentioned that earlier too, they're likely damaged and you'll need to replace them. If you put them in the right way and they still seem to 'work' it may be degraded performance and add a lot of noise or just blow up early.
~Phil
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I ordered new caps from Doug. When they arrive, I'll install and let you know the results. Thanks guys.
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Well fellas, I replaced all the electrolytic caps as directed with absolutely no changes in sound or voltage readings. Still same muddy, distorted low volume and no tremelo. I re-flowed most of the solder joints as well. Any ideas?
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New updated pictures would be great, also, as mentioned a bit more light so the innards are clearer. Muddy sound to me makes me think that possibly the tone pot is wired wrong in some way so it's always at max cut, but not sure. If we can see pictures, we could find something else that may look odd?
~Phil
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Updated pics:
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Pots right to left: vol, tone, trem intens, trem spd, reverb
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Is your shielded wire grounded? Looks like your soldering iron isn't hot enough, or maybe using to thick guage of solder.
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"I also shorted the tremelo footswitch jack because the footswitch I purchased was defective. Everything else is per the schematic. The biggest difference I took the liberty of changing is to use a turret board instead of strictly pt. to pt. "
Is the hand-drawn layout your own work, or someone else's, or someone else's VERIFIED layout?
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I'm looking at the pictures on a phone but it looks like your red wires going to your rectifier doesn't have any solder. It also looks like your 2 wires on your fuse holder are connected with solder flow. I'm not sure how your filament wiring is done either. Are you going directly to a tube socket and then to the lamp? Are you using 100 ohm resistors for a tap? Looks like a lot of your solder joints are black. Like I said, I'm away from my computer and looking on a phone but that's a few things I see.
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Layout is my own, based on Valco schematic and Uncle dougs pt to pt layout diagram.
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Shielded wires are grounded at one end. Fuse holder is good. Rect red wires have solder. I started with a 25 watt iron, recently switched to 40 watt.
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Couple questions:
Why no pop or noise when probing grid of power tube? Incorrect bias choking off tube? Problem between power tube and O.T.?
Why is B+3 voltage 60 vdc above spec? 12ax7s not drawing enough current? Why?
(V1-1: .3mA, V2-1: .5mA, V4-6: .4mA)
Why zero resistance on tremelo intensity pot right lug to wiper? (Left lug to wiper has 0-256 ohms).
Does tone control look like its wired properly?
Thanks for any comments guys!
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You don't still have amp connected to light bulb limiter, right?
What are B+ voltages right at power supply caps with only rectifier tube in?
Are you sure whatever the two resistors are between B+2 and 3 are giving you 15k ohms?
Is it possible you cooked trem intensity pot trying to solder to back of casing? Have a spare one?
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Why no pop or noise when probing grid of power tube? Incorrect bias choking off tube? Problem between power tube and O.T.?
Sometimes you get a pop, sometimes you don't. I would not be concerned.
Why is B+3 voltage 60 vdc above spec? 12ax7s not drawing enough current? Why?
There seems to be some inconsistencies among the voltages on U. Doug's schematic. Using cathode voltages and cathode resistance for all tubes that node B3 feeds, I calculated a total of 4.3mA current drawn from node B3. But if you calculate node B3 current using the B3 and B2 voltage numbers and the 15K resistance, you will get 7.3mA. So where is the extra 3mA going? I doubt that 10µF filter cap is leaking 3mA, but who can say. I think B3 is probably higher than what's shown on the schematic. At any rate, it's not a problem.
Why zero resistance on tremelo intensity pot right lug to wiper? (Left lug to wiper has 0-256 ohms).
Maybe the right lug is shorted to the wiper. But that's not a problem. I would have soldered a jumper between that vacant lug and the wiper anyway. Same for the tone and speed pots.
Does tone control look like its wired properly?
yes
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Drew - not plugged into light bulb limiter.
B+ is 483vdc at nodes 1, 2, 3 and 481vdc at node 4.
Resistors at B+2-3 is 14.85 ohms.
Yes, I could have burned the pot on my initial wiring attempt, but Sluckey says its not a problem?
Sluckey - if none of these issues are a problem, what do you suspect might be causing the lack of power/volume, muddy distortion and no tremelo?
Thanks guys
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I suspect wiring errors or poor soldering. Let's divide and conquer. The low volume and muddy distortion are probably related. Let's tackle the tremolo first.
The changing voltages listed for V2a indicate the trem oscillator is likely working. But the trem signal is not getting through V2b to V1. One of your pics shows a green/yellow wire jumper across the footswitch jack. That will kill the trem signal. Remove that jumper and see if the trem doesn't start working.
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I don't believe it! I have tremelo! I don't understand why, but at this point I don't care. It did not work without the jumper and the footswitch now works as well. Thank you Sluckey! Whats next brother? The volume and tone is still the same. But man, you have given me hope!
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I've been reading all the process and I don't recall anybody mentioning double checking the tubes or trying replacement tubes one at a time. A tube could have been effected in the big blow up.
I can understand lack of experience in soldering because I've certainly been there but a lot of those solder joints look dull and not shinny but---I think it was said the joints have been re-flowed.
Mjcopp---I feel your frustration and can tell it's been a long hard road. I have had projects just as tough and seemed impossible to get working but these guys here pulled me through every time and they will you too. So just hang in there and be patient. That thing will be singing like a bird before long and you'll be grinning like a mule eating barbwire. Platefire
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That jumper wire was the equivalent of having the tremolo footswitch permanently turned off or having the Intensity pot always set to zero. Totally killed the tremolo signal trying to get to the grid of V2b. Why didn't it work before you put the jumper on the footswitch jack? Most likely because of all those cathode bypass caps that were installed backwards.
What's next? Tell me what output transformer you have? What speaker? Then do a complete new set of voltage readings, B1, B2, B3, and all tube pins even if the reading is zero. There should be a lot of changes now that the cathode caps are installed properly. Hopefully the voltage readings will give us a hint. And describe the sound your amp is making with a little more detail. For example, is it loud enough that you can't hear someone tanking in a normal voice if the amp is cranked? Does the amp sound clean and tone control works at low volume? Those kind of details.
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...grinning like a mule eating barbwire.
Yeow! That just sounds painful!
In the deep South we would have said "grinning like a billy goat (or a jackass, depending on the company you're with) eating briars". Now I've actually seen that. They will move their lips to avoid the briars touching a tender spot. They show a lot of teeth. It really looks like they are grinning. :icon_biggrin:
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Correction! I think your right! Think the saying is "Grinning like a mule eating Saw Briers". No mule in his right mind would eat Barbwire! Or when someone is puzzled or confused we say "He's Looking like a Calf staring at a new Gate"!
Any Rate, I know Mjcopp will be happy when it's all said and done! If it were me, I would also get out my MM and start doing continuity checks from one point to the other. One lousy solder joint can gum up the whole works. Platefire
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"He's Looking like a Calf staring at a new Gate"!
Don't remember that one but I certainly get it. :laugh:
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Resistors at B+2-3 is 14.85 ohms.
You want 15000 ohms. Did you leave out the "k" or is it really this low?
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He's been leaving out the "K" in most of his resistance readings.
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Sorry for leaving out the "K". I will do as suggested and post tomorro. I understand if folks are busy with the holiday. I'm very thankful for your help. Until then, Happy Thanksgiving.
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Yeow! That just sounds painful!
we had a pet goat growing up, ate a $1000 worth of the rich guys pines n flowers so my dad took it to a friends farm, it died from eating nails mixed in with food stuff on the barn floor :think1:
I still keep askin my wife for a replacement :icon_biggrin:
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I'll be around all day on Thursday.
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Hey guys, here's the latest update:
I have replaced the power tube. I have not replaced the 5y3. I have not replaced the 12ax7s, but interchanged them in all the sockets with no changes. I performed a continuity check thru entire amp and found no problems. The O.T. is a Weber WSE15. The speaker is a Weber 12F150, 8 ohm/25 watt, both brand new. The volume, when cranked is low enough to carry on a normal conversation. Thru a fender tele, bridge pup with tone at max treble plugged in high input, vol at 9:00 & tone at max treble Amp sounds slightly distorted and muddy. With tone turned to max bass - no distortion (and much darker tone). So yes tone works at low vol. Thru low input, same results with vol. Turned to 10:00.
Voltages:
B+1: 379v
B+2: 368v
B+3: 300v
V1-1: 216v
V1-2: .6mv
V1-3: 2.3v
V2-1: 141-173v
V2-2: 1v
V2-3: .45v (oscillating)
V2-6: 300v
V2-7: 6mv
V2-8: 2.3v
V3-1: 366v
V3-2: 1v
V3-3: 4.2v
V3-6: 365v
V3-7: .9v (osc.)
V3-8: 2.3v
V4-1: 155v
V4-2: .1mv
V4-3: 1.18v
V4-6: 168v
V4-7: 2.2-2.8v (osc.)
V4-8: 2v
V5-3: 365v
V5-4: 371v
V5-5: 75-108mv (osc.)
V5-8: 31v
V6-2: 378vdc
V6-4: 378vdc
V6-6: 337vac
V6-8: 337vac
All voltages measured between ground and filament connections/lugs: 3.25vac
Thanks again everyone for your help and encouragement!
I'll keep an eye out for any barbed wire in Grandma's stuffing.
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You show different voltages on V3 pins 3 and 8. Those pins should be connected together and have the same voltage. Recheck these pins.
V4-7: 2.2-2.8v (osc.)
This concerns me. V4-7 should be zero volts. Check it again. In addition, I want you to check resistance between V4-7 and chassis with power off. Turn the reverb pot to zero and put one probe directly on pin7 at the socket. What have you? Don't forget the "K".
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Hey Sluckey, I measured as you requested. Then I noticed my battery was dying on my dmm. So I turned the tremelo off, kept reverb at zero and triple checked all voltages and will post any changes below.
The voltages on V3 pins 3 & 8 are indeed the same, (3.48v).
V4 pin 7 showed 0v and 130k ohms.
The only other changes:
V2-2: oscillating between 1.0 & 1.6v
V3 pins 2 & 7: .15v
V5-5 oscillates between 50-70 mv
I mis-labeled pins on V6, as I'm sure you realized.
V6 pins 2 & 8: 378vdc
V6 pins 4 & 6: 337vac
I hope this helps, and sorry for the bad readings.
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BTW-Happy Thanksgiving Ya'll
Somebody had mentioned in a previous post I think "Chop Sticking it". Not sure your familiar with this form of trouble shooting??? but it consist of having your amp chassis circuit exposed while live/hot/powered up and hooked up to a speaker. The wood chop stick is to allow you to probe the circuit without getting electrocuted---as long as you use a dry chop stick:>)
When you are probing your circuit touching solder connections, wires or whatever in the circuit might be suspect. If you were to touch a certain point of the amp and volume all of a sudden picks up from the low volume your currently experiencing, that is a possible indicator of the problem area to check out closer. It has worked for me several times in the past when a problem was hard to locate. I think it's worth a try. Just be very careful. Probe with the chop stick with one hand and keep your other hand free off the chassis. I usually plug a guitar in while I'm doing this because if you got a shorting to ground input jack un-plugged, your input will be dead without a guitar plugged in. I think worth a shot! Platefire
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Take a few resistance readings. One probe on chassis ground, the other probe directly on the tube pin...
V1 pin 3
V4 pin 8
V5 pin 5
V5 pin 8
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Hey Platefire, I went thru the amp with the stick...no change. I did find a somewhat loose connection on the right lug of the volume pot but the vol. Simply decreased/cut-out when I moved the lug. Nothing increased the vol. At this point I'm wondering if I should buy new 12ax7s or gut the chassis and start over with fresh solder and wires? My problem now is I'm absolutely broke and spending more money on this project at this time is impossible. So either I fix what I have or it sits on my kitchen table for another 3 months. Could it be the tubes? Could it be the power transformer which is only rated for 70mA at 391v? Could it be the the cathode bias resister is incorrect? Could it be a filter cap(s)?
What say ye?
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Hey Sluckey, just noticed your post.
Resistance readings:
V1-3: 991ohms
V4-8: 4.37k
V5-5: 470k
V5-8: 677
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Well if you could pin it down to the preamp or the power amp, would narrow the search down. Then you would know where to concentrate on. You would have to be sure that you were doing it safe, but if you could make a jumper with an old guitar cord with a 1/4 jack on one end and alligator clips on the other end and connect the output of the preamp & Tone stack to another working amp. If you got good sound to the other amp out of your preamp, then you would know it was in your power amp section and you preamp section is good, or if the problem remained, then the problem probably was in your preamp. The output of your tone stack prior to the Phase Inverter would be a good point to tap into. Doug at this site also has info on building listening devices for tracing problems you might want to check out.
So once you got your test amp and connection cord ready, you could check after the tone stack and still got weak sound, go to prior to the tone stack and check. You could even use your test cord to take a pre-amp out of another amp/preamp and insert into the input of the phase inverter and check the power amp. You would just have to be super careful to attached you alligator clip to the grid where DC voltage is blocked by a coupling cap where only very low AC voltage exist. Do that by checking with your MM first.
Just another idea, trying to be helpful! If you decide to try that, I know the others will pitch in and cover stuff I probably missed in the process. You'll find it if you hang in there! Believe me, I've been there to the point where I would find myself laying awake a night re-thinking the circuit trying to figure it out. It does do you good to take a little break from it, go do something else you enjoy and come back with a fresh perspective. Platefire
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Thanks for the ideas Platefire. I'll think about it. Unfortunately , I don't have another tube amp and have no access to one. It is a clever idea and I will look into Dougs info on listening devices. I do understand the need to walk away once in awhile, and I have been doing that quite a bit lately. I have to tell you, I have taken on many projects with zero prior experience...just studied the project on my own and got her done. I never took an engine apart in my life until 5 years ago...didnt even know how a carb worked...but I bought an old 1957 evinrude 10hp outboard, took it apart down to the last nut and bolt, cleaned it all, fixed what was needed, put it back together and it started and ran like new! Since then, Ive done the same with a dozen others... always problems and challenges, but always a solution. Electronics has always been a mystery to me as well and I really want to conquer my ignorance of it. So I will be patient, I will never give up, I will get this amp to work someday. Perhaps I simply need to invest in extra tubes, an oscilloscope, etc...for that to happen?
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As a temporary test I want you to bypass V1 and connect the input jacks directly to V4b grid.
There's a short yellow wire connected between V4-7 and the board. Disconnect that wire from the board and stand it up so it cannot touch anything. Now solder a wire to the free end of that dangling yellow wire. Connect the other end of that wire to the junction of the two 68K resistors at the very end of the board. Don't worry about the shielded cable that's also connected to the two 68K resistors. This is the blue wire shown on my attached pic.
Now remove V1, V2, and V3 and lay them aside. We only want V4, V5, and V6 to be plugged into the chassis. Now plug your guitar into the high gain input and tell me what you hear. It's not gonna be really loud and that's OK, but describe how loud it is. And also describe any distortion you may hear.
What say you?
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Hey Platefire, I went thru the amp with the stick...no change. I did find a somewhat loose connection on the right lug of the volume pot but the vol. Simply decreased/cut-out when I moved the lug. Nothing increased the vol.
Did you re-solder that connection? I'd make sure it's solid and maybe reflow other connections on the pots to be sure-- even if moving those connections didn't change enough to suggest they are your "main" problem.
In my experience a bad tube will USUALLY reveal itself with weird DC behavior. Yes, overall operation of a tube is more than you can measure with a simple DMM but if it draws current and biases correctly (which you can measure) you're likely good... I had a 12AU7 which was passing low-volume signal. Well, it turns out it was pulling excess current which could be determined by a low plate voltage, and a burnt plate resistor. Considering your voltages look reasonable I think it's not a bad tube.
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I say...after following your instructions, Sluckey, I notice that the volume has decreased, but the distortion is still present when picking the low E and A strings, two or more notes at a time (even picking soft), or picking with any force. It does seem less distorted picking high strings soft, but overall it seems like a lower volume with the same crappy tone. I also notice a double note or echo when plucking low open E or A. But yes, strumming an E chord...lots of distortion and muddy tone. Thanks for the help. I cant wait to see what you have up your sleeve next?
92volts, yes I did re-solder the connection. Thanks.
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So, it appears that the problem is somewhere in the V4b and V5 circuit. Leave it connected as is until you fix it. Ain't much left, 4 resistors, 3 caps, couple tubes. Change tubes first since they are easy. What speaker are you using? Do you know that it's good?
I can't be certain but it sure looks like you have a white wire connected to V5 pin 6? If so, that wire should be connected to pin 5. Check that.
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Verify that V4 pin 4 and 5 are connected together.
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Sluckey, the speaker, (Weber 12F150 8ohm/25 watt/5k imp.) is great. Just tested it in a Marshall AVT 20...loud and clear! I did already replace the V5 tube with a brand new tung sol 5881 two weeks ago. I interchanged all 4 12ax7s in V4 after bypassing V1 (no change). I doubt they're all bad. Should I try a new 12ax7 anyway? I have not replaced the 5y3...should I order one?
White wire is connected to V5 pin 5.
V5 pins 4-5 are connected and show continuity as well as 6.4 vac
Thanks.
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I did already replace the V5 tube with a brand new tung sol 5881 two weeks ago.
But have you tried another tube since you installed that cathode bypass cap correctly three days ago?
What did you do with the extra wires on the OT?
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No I have not changed the V5 tube since replacing the caps. Do you think the tube could be damaged from the cathode bypass cap being installed backwards? The extra O.T. wires - I placed wire nuts over the end of each wire and pushed them under the circuit board.
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You need to eliminate the tube as a problem. If it's not the problem you'll have a spare. I would use shrink tubing or electrical tape rather than wire nuts due to vibration.
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Ok, I'll order a tube and report back when its installed.
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Hey guys...with the preamp bypass still connected, I replaced the power tube with a known working tube and noticed no change. I also tested it without the bypass and the volume was still incredibly weak, muddy and distorted. Sluckey, would you suggest I now try replacing the remaining 4 resistors and 3 caps? Is it possible the filter caps were damaged? I've read so many warnings and protocols to drain the filter caps before working in the chassis, yet every single time I shut off the switch or unplug the amp, the caps immediately lose all voltage and never read more tha 1-5vdc. Is this typical or indicative of a problem?
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I think your problem is most likely a wiring error or poor soldering, especially with the grounds. But I'm out of gas and my eyes are bleary from looking at your pics. There's nothing else I can do without having the amp on my bench.
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with the preamp bypass still connected
configure it "bypassed", find an old CD player that has volume, jack it in, play music, full volume, then dial back to "clean". If you can't get reasonably clean music, I'd rebuild V4b and V5.
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Long shot, but...
Bad speaker cable? Speaker jack wiring looks OK in pick but hard to tell - maybe double check that too.
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Long shot, but...
we're into the long shot area now :icon_biggrin:
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Wow! I've been out of pocket about a week fully expecting when I got
Back here this to be up and running. I wish I could lay hands on it!
Love to troubleshoot, nice challenge. Something simple I'm sure--just
finding it? Platefire
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I did a quick look, might be wrong, V4b shows a gain in the ballpark of 50?
If so, your bias at 30ish, a 1 Vrms in should yield something close to max clean drive into the 5881, and a 6V6 should be screaming QIUT!!!!!! :icon_biggrin:
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I going to rebuild V4b and V5 and may re-wire the grounding scheme...we'll see what happens?
Does anyone have a answer to my question regarding the filter caps? I've asked twice and no response, but I am curious....
Is it possible the filter caps were damaged? I've read so many warnings and protocols to drain the filter caps before working in the chassis, yet every single time I shut off the switch or unplug the amp, the caps immediately lose all voltage and never read more tha 1-5vdc. Is this typical or indicative of a problem?
Thanks.
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the caps immediately lose all voltage
I don't see bleeders in the schematic so; with power off, OHM from B3 tap to ground, should be many thousands, charging to many, many thousands of OHMS
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I going to rebuild V4b and V5 and may re-wire the grounding scheme...we'll see what happens?
Does anyone have a answer to my question regarding the filter caps? I've asked twice and no response, but I am curious....
Is it possible the filter caps were damaged? I've read so many warnings and protocols to drain the filter caps before working in the chassis, yet every single time I shut off the switch or unplug the amp, the caps immediately lose all voltage and never read more tha 1-5vdc. Is this typical or indicative of a problem?
Thanks.
The only way to know would be to swap them out. They should discharge pretty quickly if the tubes were warm and conducting well. The way they can keep charge is if you turn the amp on, and shut it off pretty quickly with no bleeder resistors (I didn't see any on your schematic, if I recall). Did you replace them with the entire group? You'd had them upside down everywhere else, not sure if those were as well?
~Phil
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I doubt the filter caps are damaged unless they were installed backwards at some time, in which case I would expect some ot them would have exploded.
Your amp has a cathode biased output tube. This puts a heavy load on the power supply and will quickly drain the filter caps when you turn it off. This is normal and nothing to worry about. If you want to see the filter caps hold a charge when you turn it off, just remove all tubes except the 5Y3. This removes the load from the power supply. Now the caps will hold a charge a long time. If not, then you have faulty caps or a wiring error.
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Also I was just looking at the filters again, and not sure I'm fully understanding the wiring. I'm attaching a screenshot of the last filter and how it's connected. Two questions:
1. it loos like you have two resistors there, was that due to not having 1 of the right values so you just jumpered two? (it's not a big deal, and not uncommon, just wasn't on the schematic).
2. This looks to me like the positive side of the capacitor I've highlighted has two black wires coming off of it, one goes to the 270k resistor that's intentional per the schematic, but the other seems to go over to ground? I can't quite make out what that's actually doing? if so, I'd think the amp couldn't work as all B+ is dumped directly to ground, but just not sure what I'm seeing. (This is part of the reason that Sluckey I think is indicating his eyes hurt, There are a ton of wires going on there and the images are still quite unclear, blurry and dark, if you could put the amp in a very brightly lit room, have someone with a high resolution camera loan it to you, or phone camera etc, and take some more close up shots of each area it may help.
Also I'm attaching a picture of a completed amp I did with the solder blobs on the turrets, to show you what generally well soldered connections should look like. Most of yours seem like there's almost no solder in the turret hole. This could be causing serious problems as well. (don't get me wrong, mine used to look the same way, it takes time soldering to get better, and this pic isn't my best work either, I get better each time I solder)
~Phil
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one goes to the 270k resistor that's intentional per the schematic, but the other seems to go over to ground?
That's not ground. It's labeled as "B3" on the layout. Voltage has been checked at that point and is OK.
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I get that it's B3, and it's not the wire over to the one that I saw, it's the other black wire running away that I can't quite make out. But again, I'm sure you're right because of the voltages, otherwise it would be shorting B+ to ground directly and boom.
Just can't see in that image at all.
~Phil
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Yeah, you can't tell much in that pic. But there is another close-up pic that shows those wires much better. They are really blue, not black.
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Well gentleman, I have returned once again with the amp build from hell. After rebuilding V4b and V5, (removed all components & old solder then re-wired with new grounding scheme) I am back where I started - no change at all in sound or voltage readings. Very low volume and muddy tone with much distortion. I totally understand if you guys want to pass on this one...the wiring is a pain in the rear to follow, the pics aren't very clear, etc... But I did my best to take some new pics if anyone is interested.
At this point all I can think to try is replacing the power transformer, rectifier, filter caps, pots or maybe then, rewiring everything exactly like uncle Doug did (strict point to point with no circuit board). I hate the thought of spending more money shot-gunning, but don't know what else to do?
If the problem is strictlty confined to the power amp section, (V4b-V5) as Sluckey determined, then what is next other than replacing possibly bad components?
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Show us the new pics. Maybe something will show up. Check your speaker jack.
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I'm trying to post pics, but they are all barely over the max image file size. I'll keep trying.
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If you can't get the file size down then upload then somewhere on the cloud and just post a link pointing to them.
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Circuit board
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Control side
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V1&V2
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V3 & V4
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V5
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I didn't find any new clues in your new pics.
I see a lot of soldering iron scars on your wiring, including the shielded cable. The center conductor insulation on the type shielded cable you have is very easily melted. I would replace all that shielded wire with fresh regular single conductor wire just to eliminate defective shielded wire from the suspect list.
Still high on my suspect list is wiring errors and poor soldering workmanship. What kind of solder are you using? And what kind of soldering iron? I can't say that either is the source of your problem but I will never be able to eliminate either from the suspect list just by looking at your pics. Take a look at the pic in the following link. Look closely at the soldering connections, not just on the board, but on the sockets, jacks, pots, etc. Not saying the wiring and soldering workmanship are the best example in the world, but if your soldering and wiring looked like this, they would not even be on my suspect list at all.
http://sluckeyamps.com/VAC15/big_guts.jpg
At this point, I would need to have this amp on my bench to figure out what's wrong. I will continue to follow this thread and will comment if I see or read something that may provide another clue.
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Sluckey, that layout is beautiful. I am presently gutting my board, will order new parts and design a layout based on your style. When all is done I'll post a pic. I am using a Weller 40 watt iron but the solder is a 20 year old roll. I'll buy a new one. Thanks again.
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Solder age is not a factor. All my solder is at least 25 years old. But the type of solder is very important. You need lead/tin with a rosin core. 60/40 or 63/37 are common. Don't use any plumber's solder, especially acid core.
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Hey guys, I rebuilt my circuit board and re-wired everything with proper solder and cleaner layout. (See pics below.) Unfortunately no improvement. Wow! WTF AM I MISSING? Still low volume, muddy tone and nasty distortion. At high volume, tremelo and reverb now thump and squeal. Ok, so I bypassed the preamp, connecting input directly to V4-7 grid as Sluckey suggested, with same result - quieter but stil distortion and bad tone. So if problem lies with power section, then where is it?
All wiring and solder redone. Input and speaker output jacks isolated with fiber bushings. New Power tube tests good. Speaker tests good. V4 12ax7 good. All resistors replaced with new. All filter and non-electrolytic caps replaced with new. Output transformer replaced with new. The only components that have not been replaced are the following:
5Y3 tube
Power transformer
Power Switch
Pilot light
Fuse holder
Input jacks
Pots
Sockets
Latest voltage measurements: (schematic specs in parenthesis)
B+1: 380 (360) vdc
B+2: 371 (350)
B+3: 303 (240)
V1-1: 221 (180)
V1-2: 0
V1-3: 2.3 (2)
V2-1: 168-175 (140)
V2-2: -1 to -1.6
V2-3: .5 to .6 (.4)
V2-6: 297 (240)
V2-7: 0
V2-8: 2.28 (2)
V3-1&6: 367 (350)
V3-2&7: .035
V3-3&8: .45 (1.2)
V4-1: 169 (120)
V4-2: 0
V4-3: 1.04 (1.2)
V4-6: 175 (140)
V4-7: .13
V4-8: 1.8 (1.6)
V5-3: 374 (360)
V5-4: 371 (350)
V5-5: .37
V5-8: 31
V6-2&8: 379 vdc
V6-4&6: 337 vdc
All heater voltages: 6.5 vac
All ground connections test good for continuity.
Schematic check against physical layout.
Could the 5y3 cause this? The PT? Improper ground somewwhere? Do footswitch jacks need to be isolated with fiber bushings?
(680 ohm Cathode bias resistor was removed when images taken.)
Thanks again for any advice.
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Pics
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Pics
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Pics
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Pics
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Link to schematic:
https://m.box.com/shared_item/https%3A%2F%2Fapp.box.com%2Fs%2Fu945p1s2dv7ffrok96ux52lseya2rjv4
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That looks a lot better than the first set of pics. I still can't see the problem from looking at the pics. The only voltage that raised an eyebrow is V5 pin 8. You have roughly twice the voltage that is shown on the schematic. Could be you are using a 6L6? Try a 6V6 instead.
There are three things that all must be in agreement... The schematic, the layout, and the actual build. I suspect one of those three does not agree. I'll assume the schematic was proven by U. Doug. So, that leaves the layout and actual build. Is it possible that your layout does not agree with the schematic? Did you draw the layout based on the schematic?
Of course it could be a faulty component, but I don't suspect any of the components you have not already replaced. At what time did you replace the OT? Was that before or after you fixed all the caps that were installed backwards?
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Hey Sluckey, yes I drew the layout based on the schematic. Ive checked it multiple times. It is possible Ive made a mistake, but U. Doug didnt use a board, just terminal strips and it looked very confusing. I am using a 6l6. U. Doug designed the amp for either 6l6 or 6v6. I thought the 6l6 had a richer tone.
The v5 pin 8 voltage is interesting. The original Supro schematic shows a 6v6 with a cathode voltage of 17 and a 330 ohm resistor. U. Dougs data showed using a 5881 tube, 5y3 rectifier and a 654 ohm bias resistor: a plate current of 45.4mA, a cathode to plate voltage of 333v, and a plate dissipation of 15.1 watts. If my calculation is correct, that is a bias voltage of 29.6v. With a 6v6, 5y3 and 756ohm resistor he got 349v, 34.3mA, 11.96watts and I calculate a bias voltage of 25.6v He wired it up and both sounded great, but yeah, Ive tried to get the bias voltage closer to 17v, but the current jumps into the 90mA range. (And still sounds like crap)
The OT was replaced before I replaced the caps. I probably only ran the amp for a total of 10 minutes with the caps in backwards. (1-2 minutes at a time.) How could this really damage the OT? Just curious.
Spending more money on components that dont remedy the problem is becoming tiresome, ( and I believe my wife will conspire some evil deed against me if I do. So maybe a new OT, 6v6, or layout/wiring mistake I dont recognize?
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In your first post you said "When I first powered up, the two in-series, 330 ohm/2w cb resistors burned up and the cb cap exploded." When that happened the tube and OT were also subjected to extreme current and likely damaged. Was your current OT in the circuit when this happened? If so, it's likely damaged.
When and why did you change the OT?
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Thats exactly why I replaced the original OT, (after the burned resistors and blown cap). The current OT hasnt been subjected to anything other than the backward caps.
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I have an OT from an Epiphone Valve Jr on my junk pile that you can have for the cost of postage. 5W, 7.5k ohm primary; 4/8/16 taps. You'd want to use it with a 6V6.
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I already started putting parts on the side for this, I love Uncle Doug. I watch his vids and write his tutorial material down. I can understand what he is talking about the way he explains. I havent gotten a chance to read this thread yet but I hope you figure or have figured out your problem. Doug is old school with point to point and tag board. It took me a few to notice his grounding on the layout, certain tabs on his strips are grounding to chassis and thats where he lands the ground.
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By all means, take up Drew's offer on that OT. Even if it's not what you want to end up with it will tell you if your OT is the problem. Part substitution is a valid method of troubleshooting, especially with an elusive problem such as you have.
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Thanks Drew for the offer. I appreciate that. Message me with your info and we can work out the details.
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Drews' little OT arrived in the mail today. I wired it up and ....well boys, my amp now has tremendous volume! Thanks Drew! I had my doubts but you called it Sluckey. (I still cant believe I fried 2 OTs, thats just embarrassing.) The tone is ok but not great and break up occurs at very low volume 9-10:00.
Now I have a couple questions. I should now buy an appropriate OT rated for 15 watts SE and 5k ohm primary and install it before tweaking anything else, correct? In otherwords, I wont know if I need to adjust the bias until the proper OT has been installed? Drews OT is a 5watt with 7.5k ohm primary. Can I go ahead and try to adjust the cathode bias resistor value while I wait for a new OT to arrive, or just leave it alone until its installed?
Man, I tell ya, if this didnt work, I was ready to throw in the towel.
Thanks again guys. You have no idea what a relief this is.
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I don't think you need to adjust the bias. Those resistor values already have the tubes throttled back a bit. Just put a 6V6 in the socket and play it until you get the bigger OT. That little OT probably won't be happy with a 6L6 though.
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Greetings my ampaholic friends! Well, I have her buttoned up and ready to play. The tone is not quite what I expected, but very nice nonetheless. Quite sparkily and chimey, but not much for bass. I suppose this is inherent in the nature of SE design? I changed a few caps from .01uf to .022uf which helped a little. I also added a 250pf bright cap to the volume pot which really brought out the treble. Had an issue with tremelo motorboating esp. at high vol/intensity, but moving some wires calmed it down. Overall, it sounds beautiful to my ears... The only issue I would love to remedy is the lack of clean headroom. Clipping/overdrive starts at 9:30 on vol knob and sounds awesome when dimed...very crunchy. I tested v5 cbrs from 220 to 750ohms and settled on a 400ohm cbr that gave B+1: 354v, C/Pv: 340v, C bias vdrop: 25v, P current: 57mA, and P dissipation of 19.5 watts. This 400ohm cbr value gave the best tone and most clean headroom I could get. I wonder if replacing the v1 12ax7 with a 12aT7 would give me a bit more clean HR? Any thoughts?
Otherwise, I love the amp, I learned way more than I ever planned to and I cant wait to build another. I want to thank you Sluckey and Drew and all who helped a total rookie struggle through the most challenging project I've ever dealt with. You must tire of hearing it, but I never would have succeeded without your help! God bless you all.
Mike C.
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Are these things even supposed to have clean headroom? I don't know much about them other than the Jimmy Page connection.
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Are these things even supposed to have clean headroom? I don't know much about them other than the Jimmy Page connection.
I would doubt it, they're not designed to be a massive 'amplifier' in the preamp sense, more about slightly more than unity gain with all that tremolo and reverby goodness. I'm sure that pegging them does provide some great drive, or even just at their max output but still clean drives the input of a guitar amp pretty heavily.
Like a tube gain pedal in a reverb/tremolo box :P
~Phil
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So... any ideas on how to increase cleans and calm down break-up?
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Don't expect too much from that cheap Supro. Build a Fender AB763.
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^ or perhaps consider one of the clean-oriented Dumbles/Dumble-derivatives documented over on ampgarage.com, if you want something less ordinary than a Fender.
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One of dumble's common mods was to do a LNFB on a triode to increase clean headroom. To be more sure of where the break up was starting, you'd want to use a scope with a sine input, but, at a minimum, you could try on the last tube in the chain before the 6V6 and get a couple 22M resistors and some specific capacitor, like a .047uF and put them from the anode back to the grid, with Andode -> 22M, -> .047 uF -> 22M -> Grid
There's other variants, but that one is a smallish one. You may want to try various types of capacitors there, but it will basically clean up that stage and increase headroom and reduce distortion. On the tweedle dee deluxe he just used a 3.3M into a 2.2nF (.0022) cap as well. you can toy with different values pretty easily by installing a 3 prong terminal strip, connect and use a few alligator clip leads etc, to connect a few differing resistor and cap values to get in the ballpark of what 'works' but doesn't impact the tone significantly. Basically more resistance means less feedback and less impact, but there is also a specific frequency range targeted by the cap as well, so changing that will adjust what range is 'cleaned up' as well.
~Phil
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Thanks for the suggestions. I will experiment a bit with the nfb idea. Another problem I just noticed is a nasty speaker buzz only on certain notes - C & C#. Called Weber and was told it is invariably in the circuit. Any ideas where to check first?
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Thanks for the suggestions. I will experiment a bit with the nfb idea. Another problem I just noticed is a nasty speaker buzz only on certain notes - C & C#. Called Weber and was told it is invariably in the circuit. Any ideas where to check first?
That's one of those areas that can be very tricky to fix, sometimes it is a vibration in the cabinet, sometimes it is a tube that's a bit microphonic. Try touching the tubes (careful they can burn, maybe with a glove) while playing the note to see if it goes away. also try putting general weight on the cabinet to see if it goes away.
If not, it could also be not enough filtering, or other types of distortion. Or just 'farty' due to the capacitors not being quite tuned for the specific setup you're using? I think to guess more it may be better to hear the sound it's making in an audio clip.
~Phil
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Well...I determined the source of the "speaker buzz". It is the back panel chassis cover! Take panel off - no buzz, put panel back on - buzz. I tried playing around with the screws, inserting rubber washers to no avail. I'm beginning to think the panel is possibly reflecting the c-d note frequencies back into the chassis/speaker causing the buzz. I really dont know. But it apparently doesn't matter how the panel is fastened... if its on, there is buzz. Any advice would be , as always, greatly appreciated. Still have'nt gotten around to the nfb mod. Thanks again Phil for the help!
Mike C.
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Are you talking about a metel cover that attaches to the chassis or a wooden cover that attaches to the back of the cab? Show us a pic?
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Well...I determined the source of the "speaker buzz". It is the back panel chassis cover! Take panel off - no buzz, put panel back on - buzz. I tried playing around with the screws, inserting rubber washers to no avail. I'm beginning to think the panel is possibly reflecting the c-d note frequencies back into the chassis/speaker causing the buzz. I really dont know. But it apparently doesn't matter how the panel is fastened... if its on, there is buzz. Any advice would be , as always, greatly appreciated. Still have'nt gotten around to the nfb mod. Thanks again Phil for the help!
Mike C.
You could try adding some of that hvac metal tape to do double duty, act as shielding for the chassis, and it tends to help mute some of that kind of thing, or if not, any kind of tape on the back side may help break any vibrations building in.
~Phil