18w Stout - oscillation?

[shaun]

Heyo. I am trying to finish up an 18w Stout that's been half done for a while. It's seems pretty much in the ballpark regarding most voltages, except that there is way too much break up too early, even with two 12AT7's in V1 & 2.

I added 25uF electrolytics to pins 3 & 8 of V1, and I know this can increase gain. The one I added to pin 3 warmed up the sound, and so did the one added to pin 8, except that when the pin 8 was added, I started getting squeal and what I'm pretty sure is some sort of oscillation when volume and tone are turned up passed about 70%. If I remove that cap from pin 8, the squeal and oscillation - or whatever it is - disappears.

The math for the EL84's plate dissipation seems good: 11.24w per tube.  I tried swapping the OT leads every which way, but to no avail. There is no NFB in the design.

Here are the voltages:

Rec plates:  315 / 312
Node A: 349
Node B: 346
Node C: 301
Node D: 289

EL84 plates: 340 / 342      pins 3: 13.13v  (Layout suggests 11.3)

Inverter plates (V2 12AT7) - pin 1: 204     pin 6: 185     pin 3&8: 100v  (Layout suggests 75)

V1 12AT7 -  pin 1: 166    pin 6: 167    pin 3: 2.67      pin 8: 2.67 (Layout suggests 1.9)

At 2.67v, the cathodes values on V1 are higher than the 1.9V recommended on the layout diagram. I'm guessing V1 dissipation is too high, and maybe V2 as well. I considered raising the Rc value on V1 from 2.2k, and tried a swap-out with a 4.7k, but didn't like the tone shift, so I put the 2.2k back in.

Maybe some else has had the same issue. In any case, thanks for any help. Always much appreciated.

[PRR]

> way too much break up too early

If I understand the "hot switch", your build is permanently "HOT". Maybe that is too much? Maybe worth a switch?

That IS a lot of gain for a guitar amp.

I'm also unsure about the far side of the cap on V2 pin 7. We often see this to ground, not to cathode node. I dunno if this is Hoffman Magic or a transcription error.

[sluckey]

I'm also unsure about the far side of the cap on V2 pin 7. We often see this to ground, not to cathode node. I dunno if this is Hoffman Magic or a transcription error.

That older schematic is slightly different from the updated Stout which has NFB connected to that cap.

     https://el34world.com/Hoffman/files/Hoffman_Stout.pdf

[shaun]

Thanks for the info. I agree, PRR, my build just seems a bit too hot all around. Thanks for the updated schematic, Sluckey. Much appreciated. Not sure which direction to take yet, but more info is always great.

[PRR]

> That older schematic is slightly different

More than slightly. The NFB will reduce gain and rudeness enough to notice. But also one has a simple tone knob with low loss, the other has the full TMB tonestack with significant loss. And the one with .022 direct parallel with 470k (may as well be a short) seems unbalanced to me.

[shaun]

And the one with .022 direct parallel with 470k (may as well be a short) seems unbalanced to me.

Very interesting. I will re-wire for the newer version. Thanks!

[sluckey]

the other has the full TMB tonestack with significant loss. And the one with .022 direct parallel with 470k (may as well be a short) seems unbalanced to me.

You stopped at the first schematic. If you keep flipping pages in that pdf you will come to the newer version with the simple tone control and NFB.

[PRR]

> You stopped at the first schematic.

So I can't count past 1. That's been shown before.

The NFB and cap connection should help a lot.

> way too much break up too early
> squeal and oscillation

Any "good" guitar amp can squeal(*) if outputs and inputs are close together. Even internal nodes must be laid-out to avoid sneakage. Wires from pots to circuit just beg for sneakage.

(*) There's gain of "only" 500-1000 from input to speaker, but 10,000 from input to OT primary. It does not take much sneakage to scream, if you amplify 10,000:1.

[jlaj]

To quote Sluckey from my recent Stout Tone Control issue post, try swapping the leads from from the OT to the pins of the output tubes.  That may be where you’re getting the squeal from. Also, I can speak from very recent experience, the difference between the single tone vs the TMB tonestack is very significant. There is so much gain on tap with these stouts that you can afford to go with TMB and still have more than enough gain.

[sluckey]

Show us some hi-rez pics of your build. There may be some clues that we can spot.

[shaun]

Thanks, jlaj, I did a lot of swapping - first thing I tried. I feel it's got way more gain that it can cope with, at least in my build. Adding that second bypass cap seems to allow a little too much gain; the problems stop when I remove the cap. So I'm thinking I'll try and tone the whole amp down a little. I'll tinker with it some more and then post some pics.
Thank you!

[sluckey]

Take a look at the last page of this pdf. Hoffman shows how to make that extra gain stage switchable and also suggests a simple mod to provide a gain pot. Could be just what you want...

     https://el34world.com/Hoffman/files/Hoffman_Stout.pdf

[shaun]

I hesitate to post pics because, while this Stout build is not as messy as the AC-15R Sluckey helped me with, it was my first build and I did not use a board at all. So it's pretty difficult to decipher. From here on in, I swear I'll build boards!

However, I rewired the .022 bypass cap on the inverter to connect AFTER the 47k resistor instead of before, which didn't change anything much. And I have not made the gain switch mod because I seem to have a bigger problem. The preamp stage is very microphonic (I have good 12AT7s in V1&2). This leads me to believe the problem is in the V1&2 sections. I should mention that I used a .02 coupling cap off pin 1 of V1, into a simple 18w Trem single tone pot. Then back to pin 7 of V1.

Pins 2&7 both read 70v (schem suggests 55), and are extremely sensitive to my gauge probe, even with the vol turned all the way down - kinda blowing out my bench speaker. Plus, I'm getting a sort of general white noise now.

Inverter plates (V2 12AT7) - pin 1: 207     pin 6: 189     pin 3&8: 98v  (schem suggests 75).

I've checked and re-checked my wiring - going in circles, as usual. I'm tempted to build a different front end, but I'd like to learn something from this first if possible.

Anyway, thanks as usual.

[sluckey]

Don't be too concerned about the voltage readings on V2, especially the grids. The grid voltage readings are very dependant on which meter you use.

I would put 12AX7s back in and also install the gain switch, or just totally bypass the second gain stage. I really think you will like the results.

[shaun]

Okay. I'll give that a try. Thank you!

[sluckey]

Here's another suggestion... Rewire the Stout hi-gain preamp to the 18W Lite IIB preamp. I have the Lite IIB preamp in my Dual Lite amp. Really like it. I think you will too.

[shaun]

Aha! That looks like it will fit the bill nicely. I'll go ahead and do that and let you know how it goes. Thanks, Sluckey!

[shaun]

The suggestion you made, Sluckey, worked out perfectly. I'm very pleased with this build - I've rebuilt it maybe fifteen times in two years, but this is the keeper! Many thanks. Here's a link if interested:

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