"Thuddy" background noise during pick attack

[wizard333]

Specifically, fast picking on high notes, say anything about 12th fret on the B string. The sound is almost like a kick drum playing in the background along with the pick attack, and not quietly, its nearly as loud as the note. Only on gain channel, no such effect no clean channel, so I think I can rule out power section issues.

It's not the same thing you get with over-filtered tubes or solid state amps, but somewhat along those lines, and far more pronounced.

My first thought was too much low-end early in the pre-amp, so I tried switching in smaller (and much smaller) coupling caps, no joy. Also tried bypassing the Rk on the stage that drives the tone stack with caps of varying sizes, all the way to 22uf, in case it was an issue with too high impedance of a signal driving the tone stack, also, no joy.

Anyone heard something similar and have other ideas what might be causing it? I've not encountered anything quite like it before.

[wizard333]

Its actually worse the higher the note; above an E5, when fast picking runs or tremolo picking, it's probably louder than the note. Not so noticeable when playing slower. Also gets worse as the gain knob goes up.

Koch Twintone II schematics 1434-xxxx.pdf

[pdf64]

I wonder if a bad HT cap may be allowing a positive feedback loop to develop?
Try bypassing each in turn and check for any change.

[wizard333]

Welp I finally figured it out. Damnedest thing.

It was the pre-amp tubes. Not that they were bad, it was the BRAND. Every JJ ECC83 I put in there did the same, but I had some tung sols and Tads laying around, put those in, it shut up.

Never seen anything like that. I've been using JJs in anything from Fenders to vintage Marshall type amps to Mesas for a long time with no issues. This amp has a lot of gain, but I haven't seen the same issues with Mesas and JJs. For whatever reason, as the gain goes up on this thing, with JJs installed, it does that weird thump thing on the pick attack high on the neck on high strings. The preamp tubes are not biased oddly either, so I'm not sure why there are issues with this amp and not with Mesas when using JJs.

[Leo_Gnardo]

Welp I finally figured it out. Damnedest thing.

It was the pre-amp tubes. Not that they were bad, it was the BRAND. Every JJ ECC83 I put in there did the same, but I had some tung sols and Tads laying around, put those in, it shut up.

Never seen anything like that. I've been using JJs in anything from Fenders to vintage Marshall type amps to Mesas for a long time with no issues. This amp has a lot of gain, but I haven't seen the same issues with Mesas and JJs. For whatever reason, as the gain goes up on this thing, with JJs installed, it does that weird thump thing on the pick attack high on the neck on high strings. The preamp tubes are not biased oddly either, so I'm not sure why there are issues with this amp and not with Mesas when using JJs.

The only thing that looks unusual, at first blink, is Koch's referencing the cathode of the output drive/inverter tube to a negative voltage AKA "long-tail". This is sometimes done in hi fi amps, but this is the first instance I've seen in a guitar amp.

I've also been getting along just fine with JJ 12AX7A for a few years now. I like the way they "fade to black" far as noise goes. Some hum a bit, if it's a problem, just try a couple until I find one that doesn't. Seems they've upped their game in the last couple years.

[wizard333]

Look at the schematic for a Mesa DC3; same thing. Fixed bias PI. I've used JJs in those before with no issues though.

[AmpFix]

Welp I finally figured it out. Damnedest thing.

It was the pre-amp tubes. Not that they were bad, it was the BRAND. Every JJ ECC83 I put in there did the same, but I had some tung sols and Tads laying around, put those in, it shut up.

Never seen anything like that. I've been using JJs in anything from Fenders to vintage Marshall type amps to Mesas for a long time with no issues. This amp has a lot of gain, but I haven't seen the same issues with Mesas and JJs. For whatever reason, as the gain goes up on this thing, with JJs installed, it does that weird thump thing on the pick attack high on the neck on high strings. The preamp tubes are not biased oddly either, so I'm not sure why there are issues with this amp and not with Mesas when using JJs.

What are the cathode bypass caps values on the preamp tubes in that amp?

[wizard333]

.68uf stage 1, .47uf stage 2 (ch3 only), last two stages un-bypassed, although I did add a 4.7uf non-polar on the last stage to lower impedance through the tone stack.

[AmpFix]

.68uf stage 1, .47uf stage 2 (ch3 only), last two stages un-bypassed, although I did add a 4.7uf non-polar on the last stage to lower impedance through the tone stack.

All of that seems reasonable.

I'm just surprised that the JJ's gave you trouble like that. I tried them once but didn't really like them. I went back to Tung Sols, Shuguang, GT and various NOS tubes.

[wizard333]

Well as it turns out, the JJs were causing it only by their propensity to kick out more low end than other tubes. Seems to be an overall issue with low end.

[wizard333]

Dropping low end throughout the preamp seemed to help (MUCH smaller coupling caps), but some guitars will still get this effect if they have more low end. A strat may not do it, but a LP will, or some strats that are just thumpier than others. Heavier strings (more fundamental) will do it more than lighter; 11s may or may not produce the effect, 13s will.

Anyone have any other ideas? It's a bitch to try to see on a scope since a signal generator or decaying note won't cause it, only tremolo/speed picking on higher notes. Seems like some sort of odd clipping/blocking or feedback issue, I just can't think of what. Never heard anything like it before.

[wizard333]

Bump. Anyone heard anything like this before?

[MarkusBass]

Bump. Anyone heard anything like this before?

Is there any recording of the problem? From your description it may be caused by blocking distortion. I would decrease the capacitors in cathodes of input tubes (22uF is way to much - this is value for bass guitar) and decrease C41 and C42 in the power amp. Try 22nF instead of 100nF.

BTW, blocking distortion can be seen on oscilloscope if you feed the amp with burst signal (which you can generate on a computer or find on the net).

Mark

[wizard333]

The stock coupling caps have all been replaced with much smaller coupling caps, but the input stage of the gain channel Ck was never 22uf; if you look at the schematic it was/is .68uf. C41/42 have already been changed to .022uf.

As noted in the OP, caps have already been adjusted. I changed coupling caps to .0056uf c8, .0022uf C13, .022uf C16 as the stock .047ufs are huge for a high gain channel, but the problem persists, it just doesn't occur on as many guitars as it did before.

[R.G.]

This is one of those problems where getting out your oscilloscope will tell you more than speculating on the internet for a century or easter-egging every part in the amp.