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

[Diablo]

Most certainly mine is 50vdc.

So what voltage do you measure on the other side of the 100K plate resistor? The supply side of the rail? I'll guess it's too low.

[Chuck H]

Glad to have that cleared up. I also said:

Please measure the actual resistance from cathode to ground on that triode with only 50V. Also measure the voltage on that pin.

Your sort of jumping around without landing on solutions for anything. You move on to a different issue as we try to help with what you asked us to address!?! 50V on the plates of the 12ay7 is a problem that needs fixing BEFORE the coupling caps are adjusted.

[chinrest]

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".

I long to be "normal"!!!!! Although, I may try the slightly abnormal Matchless style parallel triode: 220k Rp and 1k5 Rk. The current set up is quite dark, especially with no bright cap.

Thanks all, for your help in solving the original issue, and continuing the parallel triode plate voltage conversation. We've added more fodder to search engine results. Hopefully it will be helpful to someone else.

Cheers

[chinrest]

Glad to have that cleared up. I also said:



Your sort of jumping around without landing on solutions for anything. You move on to a different issue as we try to help with what you asked us to address!?! 50V on the plates of the 12ay7 is a problem that needs fixing BEFORE the coupling caps are adjusted.

I think it was established that a 100k Rp and 820 Rk on a dual triode with plates/cathodes/grids jumpered, will give about half the nominal voltage. Do you think this awkward bias would cause excessive compression? Or are these issues unrelated.

The B+ rail reads ~240vdc after the last filter stage. V2 is operating with "normal" voltages.

[MWJB]

The lower the plate voltage the more highs & dynamics are cut back, your 12AY7 channel is mushy because the plate voltage is too low. Do as I said and correct the shared plate resistor value (~50K) and you will see a shade over 100vdc...I'd want a little more for guitar (I even aim for 80-90vdc for harp) & change the 22K in the B+ rail to 10K...but obviously this will have a mild knock on effect to the 12AX7 channel.

EDIT: Apologies Chinrest, I typed this before seeing your post of 02:49PM...I didn't get any notification of that one for some reason.

[chinrest]

Here's the latest schematic/voltages for the modifications I did.


Everything is behaving nicely, and it sounds good. It still collapses a bit when hitting that parallel 12ay7 full throttle with p90s. The best description I've read of this type of compression was something like: "you hit a hard power chord, and the amp goes 'buh-WOW!'" I notice it most when a reverb pedal is in front of the amp. The 'buh-WOW' is like 'chord-REVERB.' The compression was lessened considerably after lowering that coupling cap, and my work might be done. This channel serves very well as a thickener for the bright channel (jumpered inputs).

Thanks everyone for your help and comments.

[MWJB]

At full throttle with p90's you might find a 12AU7 (don't alter any resistor values, just sub the tube in) lessens this compression. Alternatively, a larger cathode resistor for the 12AY7, say 1K to 1.2K?

[Diablo]

Maybe you're getting some compression at full throttle by dropping screen voltage through that first 5K resistor on the B+ string? You could sub in a smaller value resistor there, maybe 1.2K and see what happens.

[chinrest]

Maybe you're getting some compression at full throttle by dropping screen voltage through that first 5K resistor on the B+ string? You could sub in a smaller value resistor there, maybe 1.2K and see what happens.

Tweak, tweak, tweak, tweak... Just when you think you're done...

The 5K dropping resistor/screen voltage was my original suspect, but I ignored it. Today I swapped in a 1k5. There's no more death-squash, and the preamp voltages are a bit hotter. The sound changed a wee bit--no better, no worse--still subjectively nice.

While the iron was hot, and my tweak was tweaking, I snuck in a 470p bright cap on the normal channel. I think I like having two distinctly different channels, rather than using my hard earned parallel stage as a thickener for the bright channel. I have the Tele in the fat channel, and the ES335 in the bright channel. Done?

[Diablo]

Glad the 1K5 dropping resistor fixed your death-squash. One of my amps has a big screen dropping resistor, and I have the same kind of super compressor thing going at high volumes.