Too Much Compression - Heavy Squish - Parallel 12ay7 Tweed Style

[chinrest]

The Amp:
Parallel 6v6 single-ended output
Tweed style circuit except for:
Parallel 12ay7 normal channel (100K plateR, 820/25 cathode, .1uf coupling)
12ax7 Bright Channel (220K plateR, 2k2/.68 cathode, .022uf coupling)

The Problem:
With the Normal channel cranked full, a hard-hit chord will compress like a compressor set with low threshold and slow release. The tone is not effected, it just squashes to death. This does not occur on the Bright Channel.

The B+ rail uses 40/16/16 filtering, a 5K dropping R after the first node, 470 screen R on the 6V6, 22K dropping R after the second node. I'm not sure where to look first—the dropping/screen resistor values, or the Normal Channel plate/cathode/coupling values.

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This should probably be in the Troubleshooting forum

[Chuck H]

The description of your amp only raises one eyebrow. A schematic would help. It's possible that there is enough LF in your normal channel that the power sullply is taxed to reproduce it. Resulting in a lot of sag. Other than that I would think something is actually wrong with the amp that needs repair.

[chinrest]

Thanks for your reply, Chuck. I'll try lowering that normal channel coupling cap--see if it helps. The amp otherwise runs perfectly. It only squashes when pounding the normal channel. Here's the scheme:

[pdf64]

I would look to roll off the extreme low frequencies, eg fit a 100k or 220k resistor from the normal channel volume control wiper to ground, and reduce the 0.1uF cap coupling to the power tube grids to 0.022uF.
Pete

[chinrest]

Pete,

Do you know any circuits off hand, that use a resistor from volume wiper to ground, as you suggest? Also, do you see a benefit to lowering the 2nd stage coupling cap, over the 1st stage? Lower both?

-Thanks

[MWJB]

Plate voltage of 50vdc on the 12AY7 is very low indeed, stick another 100K in parallel with the 12AY7 plate resistor see if that helps...and/or try a larger cathode resistor for the 12AY7.

[pdf64]

'Do you know any circuits off hand, that use a resistor from volume wiper to ground, as you suggest'

None exactly like that, but that's may be due to the volume controls in your scheme being arranged 'backwards', like a 5E3.
It causes the vol control to also act as a variable high pass filter, such that the higher the vol setting, the more the low end bandwidth increases, so as to potentially allow excessive subsonics.
The normal method of arranging volume controls (see Brown Deluxe 6G3 etc) provides a more constant high pass frequency, which will increase slightly (due to parallel circuit loading) as the vol setting is increased.

'do you see a benefit to lowering the 2nd stage coupling cap, over the 1st stage? Lower both?'

My feeling is that it's good practice to limit the bandwidth at each stage. The old rule of thumb was that C (in uF) x R (k ohms) ~ 10 for full range low frequency (equates to a high pass break point of 16Hz). For guitar use, that should be reduced to ~3.
Pete

[chinrest]

Plate voltage of 50vdc on the 12AY7 is very low indeed, stick another 100K in parallel with the 12AY7 plate resistor see if that helps...and/or try a larger cathode resistor for the 12AY7.

I was never absolutely sure about this. But looking at a Matchless schematic that has a parallel triode in the first stage, it shows a 220K plate R and 150VDC.

[chinrest]

My feeling is that it's good practice to limit the bandwidth at each stage. The old rule of thumb was that C (in uF) x R (k ohms) ~ 10 for full range low frequency (equates to a high pass break point of 16Hz). For guitar use, that should be reduced to ~3.
Pete

I did lower the coupling cap that feeds the power tube grids to .022uf. It lost a little bit of 'meat' but it's not squashing at full throttle any more. I may go ahead and change the first stage coupling cap to .002uf as well.

I'm still finding conflicting information about the "proper" plate/cathode resistors for a parallel triode like this. I do know that "if it sounds good, it is good." I may try the Matchless-style parallel triode to see if I like it better.

[Chuck H]

Gotta get that plate voltage up. In fact, something may be wrong! Please measure the actual resistance from cathode to ground on that triode with only 50V. Also measure the voltage on that pin. I think you may have an excess current situation.

[MWJB]

150vdc or 50vdc? Your schem appears to say "+50vdc"?

150vdc on a 12AY7 would be fine.

[chinrest]

I found this over on el34world:

For 12ax7 triode gain stage, typical Fender values are 1500 Ohm cathode R and 100K Ohm plate resistor.* Let's say it's 1mA and plate supply is 250V.* So, Vk=1.5V and Vp=150V.

If you parallel two triode stages, halve Rk, but don't halve Rp, you'll have 2mA total bias current (assuming similar transconductance between V1a and V1b) flowing through a 750 Ohm Rk.* So, Vk=1.5V; however, now Vp is (250-2mA*100K)=50V.* So you have dramatically shifted the quiescent plate voltage and with this asymmetric bias point, you'll have less headroom in this gain stage and a different 'feel' in the stage because of reduction in Vp.

Maybe you like this.*

Paralleling triode stages effectively doubles the transconductance of the stage - you get more stage voltage gain, but there are many factors that affect gain.* Consider two separate gain stages, biased the same with same load resistance...* You capacitor couple them into a summing network and end up with two times the signal, right? -38super

[Chuck H]

So does that mean 50VDC or 150VDC???

[MWJB]

I found this over on el34world:

Several of the assertions are wrong in the piece you lifted from that post, but essentially the point is when paralleling triodes you need to halve both plate & cathode resistor values, compared to one triode, to get "normal operation".

A 250v B+ supply on the shared 47/56K plate resistor, with a shared 820ohm cathode will give ~120vdc plate voltage with a 12AY7. That's about as low as I would go with a guitar amp, only lower for harp & applications where you are deliberatey looking to reduce gain & a darker tone.

[chinrest]

So does that mean 50VDC or 150VDC???

Most certainly mine is 50vdc.