Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Diverted on April 13, 2018, 08:01:59 am
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Hi all,
I'm getting ready to start building an amp for my stepdaughter, who is TINY and does a lot of gigging locally. She physically won't be able to lug a heavy amp around much (I know, I built her one which she couldn't handle), so she wants something as small as possible.
I'm settling on either a one channel 5E3 or Princeton 5F2, each of which would have a one-tube reverb (hopefully). If I can't find a good one-tube reverb schematic I'll just use a regular Princeton Reverb 1164 and pull the tremolo. It will have a 10" speaker.
Anyway, the main thing I need is advice on finding a smaller reverb tank. Most of the tanks I see are 16.75" long. This is too long as it would require the cabinet to be a minimum of 18" wide. I'm hoping for a few inches less, at least.
Are there any short reverb tanks available that would work with the standard Fender reverb transformer Hoffmann sells, and that I could just sub in in place of a 16.75" tank?
Many thanks!
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https://www.tubesandmore.com/products/reverb-tank-accutronics-8ab3c1b
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Thank you for getting back so quickly.
Specs to the short tank you mentioned are very close to the tanks normally more commonly used and sold here on this site (10 ohm vs 8 ohm input impedance, 2575 vs 2250 ohm output impedance). Just a few questions, if anyone has any insight:
Will these changes make an appreciable difference in the level or intensity of the signal?
Will I need to incorporate a dwell control?
Any ideas from anyone on whether a one-tube reverb circuit will work with this tank?
Thanks again!
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Thank you for getting back so quickly.
Specs to the short tank you mentioned are very close to the tanks normally more commonly used and sold here on this site (10 ohm vs 8 ohm input impedance, 2575 vs 2250 ohm output impedance). Just a few questions, if anyone has any insight:
1) Will these changes make an appreciable difference in the level or intensity of the signal?
2) Will I need to incorporate a dwell control?
3) Any ideas from anyone on whether a one-tube reverb circuit will work with this tank?
Thanks again!
1) No
2) Only if you want one. It will control the Reverb "send" level.
3) Yes. Search site for 'one tube reverb'. Plenty of info available.
http://el34world.com/Forum/index.php?topic=7957.0
EDIT: You'll want to choose the appropriate tank specs once you settle on the version of reverb you will use.
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Just to clean this up a bit..
When you asked for a replacement to the tank Hoffman sells I linked you to a 9 1/4" "short" tank with similar specs.
You might not want to use that tank with a one tube reverb circuit because the specs are different than the ones that others have used for a one tube reverb circuit. Once you determine which version you want to use you can order the appropriate tank by using this guide:
https://www.tubesandmore.com/tech-corner/accutronics-products-and-specifications
The first digit in the tank part number determines the length of the tank. So, the 8AB3C1B tells us that it is a short tank.
So if you see a guy using a 4AB3C1B which is a long tank, you can substitute the first number for an 8 and get similar tank specs in a "short" package.
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Thanks Silvergun.
The more I read about one-tube circuits, the less I want to do one. I can do a standard 1 1/2 tube circuit but just wanted to try to save a little space in the chassis, which is going to be small. I'll probably end up going with the Princeton circuit; glad to hear I can use that short take without difficult. Thanks for the help!
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Here's a related question:
So, how about thoughts on this? Since I'm trying to keep the tube count low, how about removing one of the channels from the 5E3 circuit? That would free up one triode in the preamp stage, which I could then use for part of the reverb circuit. Then I would just add a 12AT7 to perform the rest of the reverb circuit?
http://schematicheaven.net/fenderamps/deluxe_5e3_schem.pdf
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So, I have rudimentary layout and photoshop skills but tried to cobble something together. Can anyone critique this layout?
Many thanks to Rob Robinette; I used his reverb circuit layout in this.
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That looks pretty good.
If you want to take the tube count down even further, sub in a SS rectifier.
I'd try to find a "stand-up" style power transformer to help with space also....something like this:
http://www.classictone.net/NOS17375.pdf
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Great, thanks.
I was thinking about doing that to save room. This should be a lot of fun! Have all parts on hand except reverb spring and tranformer. Will order them and try to get a good start on it this weekend. Will post pix as I go. I appreciate all the help.
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Here's a little update.
Started putting the amp together. I'm building this from parts I have laying around (as you can tell from the orange extension cord I harvested for power :laugh:).
Chassis is a 15" aluminum box I bought a while ago but never used. The cabinet will have to be about 18" wide to accommodate this chassis ... not too bad with a 10' speaker. She should be able to manage I hope!
Power and output transformers from a Hammond AO-43 organ amplifier that ran a pair of 6BQ5s. So the output transformer should be fine. Power transformer should be good too. From what I've read HV secondary is 320-0-320@140ma and 6.3v winding is center-tapped, so no need for 100 ohm resistors. Toying with elevating the 6.3v center tap to the output cathode. We'll see.
I found some really cool phenolic terminal boards at a radio show a few months ago in Nashua NH. Figured now's a good time to use them as I'm going to do some experimenting with parts placement and they'll allow me to move things around to get a layout I like before I solder them up. Tube lineup left-right in pix is rectifier/6v6/6v6/12ax7/12at7/12ax7 (V1/V2/V3/V4/V5/V6).
Half of V6 is preamp, other half is reverb. V5 is the reverb driver. V4 phase inverter.
A few questions:
The control panel is not set in stone. Haven't 100 percent settled on order of functions yet but for the time being it is (from left to right): fuse/lamp/power/negative feedback switch/reverb/tone/volume/master/line 1/line2
Does anyone have an opinion on whether I should move any of the functions on the panel? For instance, should I move the reverb farther away from the AC?
Thanks for the help!http://el34world.com/Forum/index.php?action=post;topic=23273.0;last_msg=249980#
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Thanks for the suggestions on the reverb spring. Have a 9" tank on order.
In the meantime I've been laying out the circuit. Any suggestions/critiques with respect to lead dress? The reverb circuit is right in the middle; the large switch to the left of the first pot is a three-way switched feedback switch. Do you think I should use shielded wire in any spots?
Thanks!
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Just finishing up the build. What a great amp! Numbers are right where they should be, it's quiet and the reverb is ridiculous. Thanks for all the help everyone.
If anyone has any interest in the final schematic/layout scheme/transformer info etc I came up with, let me know.
Note that in the pic I'm using a large reverb spring. The small spring I ordered is in the mail.
Ted
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Good looking build! :thumbsup:
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Yeah, nice job
I'm glad to hear that it worked out by cutting and pasting the reverb into the schematic
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It did. Had to change some resistors in the power rail to get things back to where they should be. And a few other things. But it seems to be locked in. Not as much gain as the 5E3 so not super super loud, but it stays a lot cleaner. The reverb circuit must be cutting that gain some.
I'm wondering if I could increase clean (no reverb) gain with a SPDT switch. One half jumpering across the 3.3m/10pf between preamp volume and PI input grid, the other grounding out the reverb circuit ala foot switch?
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So, I have questions :laugh:
The amp sounds great but the more I play it, the less I like it. The gain is too low, and I'm not getting the volume and breakup out of it that I would like, even dimed. Probably due to the 3.3M/10pf network that runs between the volume out and the PI killing the gain?
Tight as it is I have just enough room on the chassis to install one more nine-pin tube, so I would like to add a gain stage between the volume output and the volume side of that 3.3m/10pf junction.
Would it be as easy as running the volume output into the new gain stage input grid, then running the plate to the 3.3m/10pf point?
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You would want to put the extra gain stage between the 3.3M and the PI input. Look at the Deluxe Reverb AB763 schematic.
But, before you add another tube, if you can live with less reverb intensity, replace the 3.3M with a much smaller resistor. I've seen tubenit go as low as 100K. Just start by tack soldering a smaller resistor across the 3.3M. Use many different values until you are satisfied with the gain and the reverb effect. Smaller resistors will give more gain and less reverb.
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Huh, after the reverb circuit? Interesting. I was looking at the AA1164 circuit which has it before (unless I'm looking at it wrong). Thanks!
Yes I think I can definitely live with a smaller resistor than 3.3m. The amount of reverb at max is massive, so I think I can drop it down a notch.
If not, I'll look at a gain stage.
Also, if you look at my last post, please: Am I missing a coupling cap in the bottom drawing?
Many thanks.
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Huh, after the reverb circuit? Interesting. I was looking at the AA1164 circuit which has it before
AB763 puts it after, PR puts it before. Your circuit should work.
Am I missing a coupling cap in the bottom drawing?
Yes. Also missing a plate load resistor.
Give the resistor a try. Easy to do, especially on that neat layout.
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Also, if you look at my last post, please: Am I missing a coupling cap in the bottom drawing?
In the bottom drawing it appears as though all you have done is move the reverb circuit to the other side of the existing gain stage.
All I did is add the cap that was there on your original schematic.
Try sluckey's resistor advice first, and then if you still need another gain stage go back and listen to sluckey again. :icon_biggrin:
If you do add another stage you will probably want to add a pot or a fixed voltage divider between that stage and the PI.
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Thank you for the help. Making progress :)
I alligator clipped a bunch of different resistors in to try to find a good balance, all the way from 1M down to 100K. Obviously the 100K gave the most jump in gain and was close to where I want it, but it muted the reverb too much.
So I think I'm going to try an extra gain stage after all.
Can you critique my updated schematic showing the new gain stage after the 3.3m/10pf network?
Sorry if the questions I have are elementary; they're not for me, having never done this before!
And could you comment on the voltage divider/pot? I definitely don't have room for another pot so if I can accomplish it with a few resistors that would be preferable.
Thank you!
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Update: WOW.
So, I just put in that second gain stage. Put it before the 3.33/10pf network.
The change in the amp is unreal. I didn't know how much adding one gain stage would increase the volume until I did it. The volume is pretty crazy! Completely different amp.
It was so much more gainy with a 12AX7 in the extra stage I added that I switched it out for a 12AU7. I also swapped out V1, half of which is the input triode and the other half the reverb recovery triode, for a 12AY7. It was a 12AX7. That cut down the gain a little bit more, and it's starting to sound sweet.
A few questions:
1. Will running a 12AY7 in V1 cause issues with the reverb recovery circuit? The schematic calls for a 12AX7 for the recovery unit but the 12AY7 is what the 5E3 calls for in the input stage.
2. Will running a 12AU7 in the new gain stage cause any issues?
3. Finally, would I be able to reduce gain a little more by swapping out a lower gain tube in the phase inverter slot? It's currently a 12AX7 but I don't know if it needs to be a 12ax7.
Also, I would like to post voltages later today and have experienced eyes take a look at them.
Many thanks! I am learning as I go here and the help's invaluable.
Ted
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Back again with some voltages. Some look great, some are low.
Also, I reconfigured the sockets so VI (12AY7) serves both preamp stages.
V2 is the 12AX7 reverb driver, V3 (half of a 12AX7) is the reverb recovery, and V4 (PI) is 12AX7, unchanged.
Amp's running a 5Y3, first filter cap upped to 40uf.
Anyway, here are some voltages. Looks like I may have to reduce the value of the resistor between the second and third filter cap to bring up the voltages on the B+ rail? Advice welcomed!
B+: 386
B+1: 332
B+2: 227
6V6GT plates: 380
Cathode resistor (400 ohms) 24
V1 (12AY7. First half is first gain stage, second half second gain stage)
Pin 1 plate: 108
Pin 2 plate: 118
Cathodes at about 2
V2 (12AX7 reverb driver)
Plates: 146
Cathodes: 1.4
V3 (12AX7 reverb recovery)
Plate 1: 146
Cathode 3: 1.3
V4 (12AX7 PI)
Plate 1: 146
Plate 2: 189
Cathode 1: 1
Cathode 2: 46
These numbers look low, and it looks like the amp's dissipating at about 85 percent so maybe reduce the cathode resistor to 350 and see what happens? I would like to get between 95-100 percent so I'll close in on something.
Any insight on these numbers, please let me know! Thank you.
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What gauge wire did you use for the heaters?
If that's 22 gauge wire that's not large enough for the 6V6's plus the preamp tubes.
You can leave the preamp tube heater wires but I'd change the wires going to the pilot light and the 6V6's, they should be a larger gauge.
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> 22 gauge wire that's not large enough for the 6V6's plus the preamp tubes.
#22 is fine for 1.5 Amps:
http://www.hca.hitachi-cable.com/products/hca/faq/data/Current-Carrying-Capacity_Jan2012.pdf
https://www.powerstream.com/Wire_Size.htm
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Hi again,
Strange problems here, could use some advice. Voltages suddenly way too high and can't figure it out.
Initially I was getting something like:
B+386
B+1 332
B+227
The PI, Pre and Reverb plate voltages were too low, around 100 to 180V.
So I went to address the low plate voltages. Discovered that I had hooked half the reverb transformer primary to the B+2 rail, so the reverb driver plates (other half of reverb transformer primary) were only getting about 146 volts or so. Removed the transformer primary red lead from B+2 and connected to main B+, around 360v, as that is where it should be according to the specs.
When I did that, all the plate voltages in the B+2 rail suddenly shot up from about 108-150 or so to the B+1 level, around 332.
In other words The positive leads of the second and third filter caps each had about 332 on them, despite a 22K ohm resistor between them. THe resistor is good (I tried several). There should be 227v on the far side of that third filter cap.
So just to assess what was going on I put the red reverb transformer primary lead back to where it was originally (B+2). Now though the B+2 voltage remains at B+1 level, regardless of where I have the trnasfomer plugged in.
I have gone over the power rail a ton of times this morning, checking resistances and for continuity. I'm not finding anything out of the ordinary.
For example from the plates fed by 100K ohm resistors I'm getting about 127K resistance from the B+ node, right where it hsould be. From plate fed by 56K resistor I'm getting about 83K, etc.
I'm at a loss as to why I'm getting no voltage drop between B+1 and B+2. Without that 22K resistor in place there is no continuity between the second and third filter cap, and the third filter cap has no voltage across it.
Scratching my head here!
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Sounds like you may have blown the reverb transformer. Measure resistance between the red wire and blue wire. What do you get?
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Reverb transformer appears to be good. Getting about 1K across red and blue and a few ohms from green to ground..
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Check voltages on both output tubes pins 3, 4, and 8. What have you. I'm thinking you may have accidently disconnected the OT center tap during this adventure.
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Grids are seeing about 345, plates about 362. OT center tap going to B+ as it should.
Getting the resistance I would expect (100 ohms or so) from 6V61 plate to B+, 6V62 to B+ and 6V6 plate 1 to 2 (the sum of the two halves).
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What about pin 8???
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What about pin 8???
Oh, sorry. I'm getting right about 23v across the cathode resistor. Right where it should be.
The whole rail looks to be wired correctly but this 0 voltage drop between C2 and C3 is ?????
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Where is the reverb transformer red wire connected right now?
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Reverb red wire was connected to B+2 initially. Right now I have it on B+ because it needs about 350+vdc, correct?
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Connect the reverb transformer red wire to the junction of your 5K and 22K dropping resistors, same place that the 6V6 pins 4 connect. I think you are calling this "B+1". That's the standard place to connect it. The reverb driver will work fine with the voltage at B+1 and it will really benefit from the additional filtering.
I think I finally understand your problem and it turns out to be no problem at all. You originally had the reverb driver connected to node B+2. That's wrong. The reverb driver is just a little single ended power amp. It draws a lot of current just like any power amp. You can actually connect a speaker to the reverb send jack and play your guitar through that speaker. Anyhow, since the reverb driver needs so much current it caused a big voltage drop across that 22K dropping resistor when you had it connected to B+2. That caused the voltages at B+2 (and all the preamp plates and PI plates) to be much lower than normal. As soon as you disconnected the reverb driver from B+2, the current through the 22K decreased significantly and allowed the voltages at B+2 to increase to what they should have been all along.
Bottom line, connect the RT red wire to node B+1, recheck all voltages and play on. Everything is fine.
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Well the amp appears to be fixed. I don't know what I did but ...
When I started having these voltage issues and couldn't find the source I started questioning everything, including the Illinois filter caps. So I pulled out the power supply board and replaced it with a JJ 500v can. Once I did I hooked it back up exactly as it was: 6V6 grids to C2, Rectifier and output transformer center tap to rectifer, and C3 to the B+2 rail.
Flipped the switch and the voltages are all normal now
I can't understand what the problem with the power supply board was because I ran all kinds of continuity and resistance checks, and everything around the filters checked out. But obviously something was up because with the new cap can everything's back to normal.
356V on B+
320 on B+1
248 on B+2.
Red wire of reverb transformer is seeing 356 (B+).
You might notice that overall numbers are lower than htey were before. I'm running it at around 115vac at the moment; earlier voltages were taken from house mains.
Very, very weird.
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Updated voltages: The preamp, PI tubes etc are still low on plate voltage so I'm going to reduce the value of some of the resistors and try to bring them back up to spec. Here's everything:
B+ 378
B+1 333
B+2 248
6V6 plates: 371
K: 22 (330 ohm K resistor)
Preamp: (12AY7)
Plate: 115
K: 1.8
Second gain stage (12AY7)
Plate: 125
K: 2
Reverb recovery (1/2 12AX7)
Plate: 154
K: 1.2
Reverb driver (12AT7)
Plates: 374
Ks: 7.5
Phase inverter (12AX7)
Plate 1: 155
Plate 2: 197
K1: -
K2: 45
Revert transformer red wire (primary): 377
Apart from some of the numbers being low, is there anything here that appears glaring to anyone?
Thanks for helping me through my red herring! Still don't know what that was about :dontknow:
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Your voltages are fine. Your numbers ain't low. Don't mess them up! Look at my 5E3 and compare voltages...
http://sluckeyamps.com/5e3/5e3.pdf
The only thing I suggest you do is MOVE THE REVERB TRANNY RED WIRE TO NODE B+1. It does not need to be on that very noisy first B+ cap. The reverb driver is a single end amp and has no hum cancellation characteristics like the push/pull 6V6s, so it needs the extra filtering.
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UGH the issue is back.
After swapping out the power supply the amp worked for a little while, so I took voltages and noted them. Turned off the amp, went and had a bite to eat, came back and same old issue: No drop in B+2 to B+3. Amp's quiet, not a sound.
Checked all transformer windings, all are good. The reverb transformer red wire is connected directly to B+ (high voltage/rectifer pin 8). the lower voltage miniature tubes are connected to B+3.
Ugh. I'm going to start looking again, but I just can't see anything. Anyone see anything amiss here?
For reference:
On the cap can: Main B+ at bottom, red to center tap of output transformer. White to Reverb transformer red wire. Blue to the lower voltage rail. Yellow to 6V6 grids.
Sluckey, once the issue is sorted out I'll move the transformer to B+1. Thank you!
EDIT: Adding that I also have used four different sets of tubes, rectifier all the way to V1. All test good. Speaker VC tests good too, as does output transformer. All windings appear to be OK.
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#22 is fine for 1.5 Amps:
But; 2x6V6 + 4x12_ _7 + panel light = 6V6= .45A, 12_ _7= .3A, light= .15A =
2x6V6= .45A+.45A=.9A, 4x12_ _7= .3Ax4=1.2A
.9A+1.2A+.15A= 2.25A total heater load.
Is my math wrong? :dontknow:
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Hi all,
I traced the whole schematic and it's all wired correctly, far as I can tell. Sorry about the yellow lines; they're my tracings. Some circuit changes are noted on the schematic:
Here's something though:
Cathodes on reverb recovery tube (12AT7) share a 22uf cap and 2.2k resistor to ground.
When I measure from pin 8 or 3 to ground WITH TUBE IN I'm getting just a few ohms.
When I measure from pin 8 or 3 to ground WITH TUBE PULLED I'm getting what I would expect to see, 2.2K.
When I pull the 12AT7 and sub in a 12AX7 I'm getting 2.2K.
Do I have an internal short in the 12AT7?
And could this account for the issues?
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#22 is fine for 1.5 Amps:
But; 2x6V6 + 4x12_ _7 + panel light = 6V6= .45A, 12_ _7= .3A, light= .15A =
2x6V6= .45A+.45A=.9A, 4x12_ _7= .3Ax4=1.2A
.9A+1.2A+.15A= 2.25A total heater load.
Is my math wrong? :dontknow:
Thanks Willabe. Don't want to sidetrack the main vibe of the thread too much. However as to 22g heater wires in this amp, I've used it before in other similar circuits. Never had a problem with it. Hopefully after all the fixes are in for the amp some people can chime in on the gauge issue. Thanks!
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Nothing wrong with the math. But 22awg is good for 7A inside a chassis.
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Found the issue, and I'm pretty sure it's the REAL issue :BangHead: :BangHead:
Found a tiny little sliver of metal underneath the 12AY tube socket, amidst the pins. The heater wires were wound so tight there it was stuck underneath and very hard to spot.. Anyway it looks like it was making intermittent contact with the first grid. I have to thank Willabee for bringing up the heater gauge because I found it when I was running an AC check on the 12AY heater pins LOL. I was worried the current had opened one of the leads on that last tube or somewhere earlier.
How's that for fortuitous! Thanks Willabe :headbang:
Once I pulled it out the amp appears to be stable and sounds killer :)
I am holding my breath to see if another bizarro issue comes up. But for now it's ALL GOOD.
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Nothing wrong with the math. But 22awg is good for 7A inside a chassis.
Because the wire runs are so short?
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That's part of it. But mainly I trust the charts PRR linked to. :wink:
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I've never had any problems with 22g. I've never built anything more powerful than say something like a Fender 5E5A so there's not a ton of current draw.
I've got 20ag wire as well. If I move up to something like a 50W 1164 reverb circuit, would that work or should I go thicker, like 18G?
Thanks again for all the help on the 5E3, sorry it turned into a rabbit hole.
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> Is my math wrong?
No. My math was wrong. Sorry.
Still have ample clearance for your correct numbers.
Never know when an objection will lead to an answer, as in this case where a gauge ? led to an undiscovered scrap of metal.
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> Because the wire runs are so short?
Because of a lot of factors.
Let's take #12, which we "know" from house-wiring is "good for 20A".
That's in cable or conduit where *two* conductors both carry 20A, so the heat is mildly concentrated.
Well actually under NEC assumptions #12 is OK to 25A, but it will run warmer than the terminals on some devices (outlets, switches) are rated, so we are only allowed 20A in nearly all practical cases.
Note that NEC house wire Ampacity tables do NOT dictate wire length or voltage drop. Another section tells you to be good, and that the final goal is "satisfactory service", but leaves that figuring to the designer. This works in practice because #12 @ 20A can run about 35-40 feet without large voltage drop, and that's typically about as far as circuits run.
Look at the PowerStream (https://www.powerstream.com/Wire_Size.htm) table for #12:
Maximum amps for chassis wiring 41A
Maximum amps for power transmission 9A
#12 Copper will actually carry 235 Amps (https://www.powerstream.com/wire-fusing-currents.htm) when it melts. Insulation will fail first. Inside a chassis, wiring is usually done so as not to *rely* on insulation, line we must in cable/conduit. And even if fire starts, it won't burn-down the house like wall-wiring does.
Their "9A" for "transmission", they admit is very very conservative. It is 700cm/A which is approaching "transformer" design where a near-infinite number of current conductors are bunched together.
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Good info on heater wirings. Thank you. And also: I am not a fan of PVC or teflon wire coating and so for regular wiring I will never use any other wire other than Hoffman's awesome cloth solid core. It's a million times better than any other cloth-clad wire I've found. Pick some up if you haven't tried it yet!
ANYWAY, so, the plot thickens and I could use some advice. I think this will either be a very complicated or extremely quick question/answer:
When I first asked my stepdaughter what she wanted out of the amp and if she wanted one channel or two, she said just one would be fine. Okie dokie, that's what I did.
She came over last night to play it and while she LOVES the amp, she now says "I wish it had two channels, one for guitar and one for my vocals, each with their own volume control." D'oh! Why didn't you say that before? That's a 23-year-old for you :)
Anyway, adding a second reverb channel should be easy as I have an extra triode that is currently not being used. So I can do that by running that second channel to where the first goes (input grid of second gain stage) and use switching input jacks on each channel to ground out the unused one when both aren't in use.
HOWEVER she also asked if I could make the reverb on each channel switchable; that is, having the ability to turn off reverb on either channel while still having it go to the other one. That way she can have dry vocals and wet guitar, or wet vocals and dry guitar, etc.
I don't see how I can do this the way the amp is built, but I'm wondering if more experienced people could see a way. To me it looks like reverb is either on or off, no in between. Am I right?
Would moving the second gain stage post-reverb allow any possibilities?
I assume she will ask me to change some other fundamental part of the amp next time I see her :laugh: but for now I want to sort this one out.
Thanks!
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Adding another channel is easy enough. And having reverb on both channels is easy. But being able to switch reverb on and off independently on each channel becomes quite involved.
Sometimes it's OK to just say no.
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Those were my thoughts. It looked like it was going to be a rabbit hole.
Two reverb channels it is!
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The never-ending questions continue :laugh:
I am trying to close in on a better circuit and could use some advice. The amp is playing well but is very gainy and quite loud. I'm worried I'm overloading the gain stages as I'm getting very, very little clean headroom. Amp starts breaking up almost immediately and I would like to clean it up and allow me to turn the volume up past 2 or 3 without it getting too loud.
I've made some changes in red to the schematic, including a 470K grid stopper resistor, smaller bypass cap on a gain stage, smaller coupling caps etc. Most of the changes were recommended by Rob Robinette and others, and from just fiddling around. They help but I need cleaner!
Earlier on in the thread someoen mentioned a voltage dropping circuit to dump some of the signal off the input to ground. I don't know how to go about accomplishing this.
Looking at the schematic, what would you do to reduce gain in this amplifier and make it run cleaner? Would you try anything else apart from that voltage dropping idea? Just looking for options.
And another very simplistic question as I've never fiddled extensively with gain, etc.: Will having such "hot" gain in this amp lead to problems with reliability, tube life, etc. ?
My main hope is to get the amp running clean and smooth. Thanks!
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Here's one way.
I edited your schematic and inserted a voltage divider. As it is now it will reduce the signal level by 50% at that point.
It doesn't 'have to' go there, but it could.
Besides that, you could put the additional gain stage where sluckey had suggested and use a 12AU7 to reduce gain.
Also try getting rid of the bypass cap on the added stage altogether.
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The 3.3Meg:560K reverb mixer.... isn't the lower leg on a Fender not 560K but a value like 100K or 50K?
That would do the same as Silvergun's added divider, but simpler. And you can start by tacking 330k 220K 150K etc across the existing 560K until the thing is less un-tamed.
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Thanks for the suggestions, Sluckey and Silvergun. It looks like that lower leg resistor is 470K. I'm going to leave it as is.
I got some help on the issues from Rob Robinette, who suggested a voltage divider different than Sluckey's, but still as effective. Was going to try both but I'm just going to leave it as is for the moment. The divider is a 680K resistor into the wiper of the volume control. The resistor works to send a good portion of the signal to ground.
I tried a few different values and settled on the 680K that Rob suggested. What a difference. The amp sounds outright fantastic. Breakup around 7 on the dial and clear, sweet tone below that. Prior to this I couldn't turn volume up past 2 or so without hardcore overdrive. It's a different amp.
Anyway with all this shuffling and swapping of parts the amp is a tangle underneath. I may build another one for my stepdaughter keeping the same schematic but an updated and cleaned up layout for her. Then I'll hang on to this one :)
Again, many thanks for all the help. Here's an updated schematic of where the amp's at right now.
Thank you!
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Hopefully this will be the last bug to sort out!
I rewired the amp for a second channel with independent tone control. Had one triode left and stepdaughter asked for it :)
Anyway the amp has an issue I'd like to try to sort out:
It has the Rob Robinette feedback switch, which has 1. negative feedback with 1500 ohm PI bypass resistor in circuit; 2: No negative feedback but 1500 PI bybass resistor in circuit; 3. Normal 5E3 mode with no negative feedback and bypass cap/resistor in place.
When in negative feedback mode the reverb pot performs smoothly, with effect gradually increasing as you go up. At full volume the amp is loud and reverb is full and thick.
In 1500 PI bypass resistor mode (no negative feedback) the reverb becomes faint/shrill and the volume reduces at about 6 or 7 on the reverb dial.
In bypass resistor/cap mode, the reverb cuts out pretty much and volume drops significantly around 4 on the dial. The reverb comes and goes after that point, cutting in and out for lack of a better word up to the full range of the dial.
Switch is wired correctly. Any ideas? All solder joint checked etc.
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Maybe the NFB phase is wrong. Swap the OT primary plate leads and report back.
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I switched it some time ago; was getting a squeal in one position (forget which) so reversed the phase. That cleared it up. Just for the heck of it I’ll switch back after work tonight and see what happens 👍 thabks.
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Output primaries are in phase. Reversed and got the squeal, so switched back.
Problem persists. Very strange, I'll have to post a video. Who knows, with the rat's nest it could be a lead dress issue. I've changed the wiring so many times it's getting a bit sloppy in there. What do you think?
Any other ideas? Thanks!
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Hopefully this will be the last bug to sort out!
I rewired the amp for a second channel with independent tone control. Had one triode left and stepdaughter asked for it :)
Anyway the amp has an issue I'd like to try to sort out:
It has the Rob Robinette feedback switch, which has 1. negative feedback with 1500 ohm PI bypass resistor in circuit; 2: No negative feedback but 1500 PI bybass resistor in circuit; 3. Normal 5E3 mode with no negative feedback and bypass cap/resistor in place.
When in negative feedback mode the reverb pot performs smoothly, with effect gradually increasing as you go up. At full volume the amp is loud and reverb is full and thick.
In 1500 PI bypass resistor mode (no negative feedback) the reverb becomes faint/shrill and the volume reduces at about 6 or 7 on the reverb dial.
In bypass resistor/cap mode, the reverb cuts out pretty much and volume drops significantly around 4 on the dial. The reverb comes and goes after that point, cutting in and out for lack of a better word up to the full range of the dial.
Switch is wired correctly. Any ideas? All solder joint checked etc.
This all sounds much more like a circuit mistake or mis-wiring issue than a lead dress issue.
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I agree. Going over it, but nothing yet. Will update if something changes. Hoping!
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Well I've isolated the problem to the reverb circuit but am not having much luck finding errors. I am sure they're there though :laugh:
With reverb on (foot switch not grounded) it seems like the dry signal is getting drowned out. Volume can't be control by either channel volume knobs. However the reverb knob increases the amp's volume as you turn it up. So the wet signal is getting through but not the dry.
With the reverb input grounded, I obviously get no reverb but both channels behave as they should. With reverb grounded out also the feedback switch works as it should.
I'm going to have to keep retracing this issue. It's pretty baffling but at least I know where to look.
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Found the issue, wired the 470K ohm resistor off the reverb wiper out of position. Put that back in place and the problem was mostly fixed; played with the lead dress a bit and that didn't hurt. The only thing I'd say now is that in the feedback switch's stock fender position, with volume/tone/reverb dimed, I'm getting some feedback that only subsides when I turn down the reverb. It's pretty minor but there.