Stout tone control issue

[jlaj]

Hey Everyone, I’m new to this board but I’ve been watching and reading for years. At any rate, I just finished building a Stout in an old Gibson GA-15rvt that I previously built an EL-84 based 5E3 circuit in.  When I turn the tone control past 3 o’clock or roughly 75%, the volume drops out and becomes nasally.  I thought I might have had a bad pot and replaced it and it still does the same thing.  Does anyone have any idea what might be going on?

[sluckey]

Wrong value caps or resistor, or wiring error.

[PRR]

> 75%, the volume drops out and becomes nasally.

Oscillation. Yes, can be wrong value/connection. But can also be layout. Picture?

[jlaj]

Thanks for ideas.  I’ve re-traced everything and it all seems to jive. All the values look to be correct. Here is a picture for reference.

[PRR]

High-level lines to power tube grids should be FAR from low-level lines to tone pots.

[jlaj]

Ah yes, that makes sense. I never even considered that. I was trying to make due with the original board to see if I could get it to work.  I may add a little terminal strip then to relocate the caps and lines, or simply put a new board in altogether since there’s still lots of room in that chassis. I’ll post an update when I get around to it this weekend.

Thanks.

[PRR]

Just push them further apart an see/hear if it makes any change.

[sluckey]

Try this... Disconnect the ground wire from the pot wiper and connect a 120Ω resistor between the wiper and ground. This is a common fix for oscillation on the 18W normal channel tone control whis is the same as the stout tone control.

[jlaj]

I’ve pushed them apart and no change so I’ll try adding the 120Ω after work tonight.

[jlaj]

Ok so I tried adding the 120 ohm resistor - no change; tried changing the 0.01uF cap going to the pot - no change, changed the volume pot as it was an OLD centralab pot from 65’ or 67’ that ended up having a bad ground - still no change.

I’m stumped now and am even considering trying the TMB tone stack as another option (and also because with a humbucker in the neck position it’s pretty thick).

[sluckey]

Disconnect the negative feedback wire. Any better?

[jlaj]

Maybe my ears are playing tricks on me but it might help a little. The volume isn’t dropping out now as much as the bass being filtered out.

[sluckey]

Reconnect the negative feedback wire and swap the OT primary plate leads. Any better?

[jlaj]

Hmm ok. I’ll try that. This part of the amp is still as delivered from the factory in the 60’s lol.  It’s using the tiny little OT that it came with. But, I’ve got an 18 watt OT that I bought from Doug a few years that I have laying around that i may install. I was gonna see what I could get out of this little monster with the stock iron first.  This has me getting kind of excited.  On another note, when I turn the volume pot right down, I still get some bleed through which leads me to thinking bad ground...maybe that’s the root cause?

[jlaj]

Quick question pertaining to this issue, if the D+ was actually coming from C+ without going through the 4.7k dropping resistor, would that cause the first stage to run too hot? I noticed that that was the case, and it was actually a 10k from the previous circuit that was in there so I changed it to a 4.7k and rewired the C+ and the D+.  Seems ok now...

[sluckey]

Reconnect the negative feedback wire and swap the OT primary plate leads. Any better?

[jlaj]

i didn’t swap the OT leads yet as I’m pretty sure tapping off the C+ instead the D+ could have a major effect.  After rewiring everything the way it seems like it should be, it seems to be ok. I would almost describe the tone when passing 3 o’clock as a bass cut.  I’ve got more experience with the tweed tonestack so maybe I’m used to that and the 18 watt style tonestack responds differently?

[sluckey]

i didn’t swap the OT leads yet

What are you waiting for?

This part of the amp is still as delivered from the factory in the 60’s lol.  It’s using the tiny little OT that it came with.

So what. Neither the original GA15RVT nor your 5E3 build used NFB (negative feedback) so it didn't matter which OT wire connected to which tube. But the Stout circuit does use NFB and now it is very important to have the OT plate wires connected correctly. You must verify they are correct. There's a 50/50 chance they are right or wrong. The easy way to know is to swap the plate leads. This is something that must be verified on any new build that uses NFB.

So, just do it. It may cure your quirky problem. Or it may make things a lot worse. But now you will know which way is right.

Keep in mind that your problem may not be related to wrong NFB wiring. But you must rule out that possibility. Probably take less time to verify than it takes to read this post. 

[jlaj]

I get what you’re saying about design difference not using NFB, but will that have as much difference as using the C+ vs the D+? I’ll still swap the OT leads in the morning to confirm, but I’m just curious now because tapping off the C+ would cause V1 to run at a higher voltage wouldn’t it? Btw, can’t check voltages at the moment as my really old meter died and I’m waiting for the new one to arrive otherwise I’d have checked it before. 

[jlaj]

So I swapped the plate leads and they were definitely correct the first time as it squealed like a stuck pig as soon as I fired it up. (I remember having to do that on my very first 5E3 build back in ‘05 or 06’). Since that’s not the problem and everything is confirmed to wired correctly, I’m thinking it’s just a quirk of this tonestack. The amp still sounds amazing and so maybe it’s just one of those things I’ll have to live with?

[sluckey]

(I remember having to do that on my very first 5E3 build back in ‘05 or 06’).

5E3 does not use NFB. OT wires make no difference in that amp.

But now you know for sure that the NFB phase is correct in your Stout. Checking this is always recommended with a new build that uses NFB. Most times having it wrong produces squealing, howling, or other similar obvious symptoms. But depending on the amount of NFB used in a particular circuit, sometimes the symptoms are very subtle. A few years back a member built an amp with NFB. He played it for about a year then decided to do a simple mod. That's when he discovered the NFB phase was wrong all this time. He had just thought the amp was supposed to sound gainy and unrefined. On certain notes he could get a very good sustain (positive feedback) that he liked. But it only happened on a few notes from a certain guitar.

[jlaj]

You’re right about the 5E3 not having any NFB, now that I think of it, it was a JTM 45 build i did for a buddy. So the NFB issue aside, is this now just a quirk with this tonestack? Because as I listen to it more closely, the volume definitely isn’t dropping out, but I would describe it as the body, or the thickness of the toned attenuating and the top end still remaining.  It just seems strange is all.  Like I said previously, maybe I’m used to the tweed tonestack and this might just need some getting used to because this amp definitely sounds really good.

[sluckey]

The only issue I'm aware of with that tone control is the 100Ω on the pot wiper fix for oscillation. And this issue is not universal, only seems to affect some amps. I had to add the resistor to fix mine. Ceriatone has incorporated the resistor into all their 18W amps.

     http://www.ceriatone.com/ceriatone/wp-content/uploads/2016/08/18Wceriatone.jpg

I've see quite a few Stout builds on this forum and you are the first to mention an issue with the tone control. However, the tone control from the 18W trem channel is more popular. I suggest you make the simple change to your Stout to see how you like it. Look at V1 on this schematic...

     http://sluckeyamps.com/dual_lite/dual_lite.pdf

[jlaj]

Thanks Sluckey. Im gonna give that a try, but only after I try the TMB tonestack. The stout circuit has a lot of gain on tap so I’m not too concerned about the TMB loading it down.  I’ve noticed that between a 1x12 open back and a closed back 2x12 there is quite a different response so having ability to tailor the amp to the cab would be a big advantage. 

On another note, what are your thoughts on the effect of D+/C+ mis-wiring I had?

[sluckey]

On another note, what are your thoughts on the effect of D+/C+ mis-wiring I had?

Lot's of weird things can happen when too many stages share the same power supply node. That probably was the cause of the signal bleed through even with the volume set to zero. You have that properly sorted now, right?

[shooter]

tailor the amp to the cab

 

most of the builds I've followed n done, do the opposite, we spend lots of time matching the cab to the amp.  I typically run 3-5 speakers and 2-3 different cab types till I find the "best match"

the difference between tap C & tap D is mostly about additional filtering, If you swapped tap A with D, that might be an issue

[jlaj]

You have that properly sorted now, right?

This is sorted and wired up properly.

[jlaj]

Shooter, I guess I should have worded the statement better as in just being able to tailor the sound to a Humbucker or single coil because the most part, this amp will be used as an open back 1x12 with the greenback I put in it.  I guess I’m just used to the AB763 I rebuilt in my deluxe reverb last year. That thing will take anything I throw at it...just have to turn the bass down a touch if it’s too deep. And, I know, a deluxe reverb is totally a different beast than an 18 watt based amp. But I did swap different coupling caps in my first 5E3 build and it handled ‘buckers and single coils both beautifully.  I guess I’m just in new territory with a totally differently voiced amp. Will just take some getting used to I suppose.

[jlaj]

Quick update to close this topic off. I swapped in the TMB tonestack and I can say that I’ve nailed it. The overall volume is more usable now, and the tone is right on now for a Les Paul or a strat. I’m very pleased.

Sluckey, thanks for all the insight. Without it I’d likely still be scratching my head.

This forum has always been a great resource , I wish now that I would have joined years ago.