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long tail PI and compression/brow sound

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danieleb:
in these days I was trying modifications to my JC18, having very large grid stoppers (100K) compared to the common ones on EL84, I tried to reduce the values ​​of the long tail resistors of the PI (r16 1R->550R and r17  27K->17K) and the sound change a lot, less compression and more attak at rehearsals levels.

Could be a trick (in opposite way) to have a brownish sound in an EL34 amp?

Helmholtz:
Fine, if you like the result.
Technically your changes lower the PI headroom, causing early PI distortion and probably reduced clean output.

Showitevent:
Hi,

for my understanding most EL84 amps dont work too well having more reasonable Gridstoppers like you would find in EL34 amps.

The compression you feel is most likely the loss of some treble content with high value Gridstoppers. The EL84 tend to overdrive relatively quick and harsh when they start to draw current (just my subjective feeling) and more or less sound broken easily than any usable - its a matter of finding the sweet spot here.

EDIT: In case of self biased amps this also depends on the loadline that the Output transformer draws to the Tubes which i would assume is nearly always relatively steep.

If another amp reacts the same way on this change, depends on its guts and strongly on the type of tubes, manufacturer, voltages, e.g....
A more brownish sound is also not necessarily a question of the Poweramp alone. There are various variables to consider from the input to negaitve feedback. But i find it true that "brown" mostly finds place in the backend of the amp.  ::)

Geronimo

Helmholtz:
The EL84 has an effective input capacitance (including Miller effect) around 20pF.
Even if we assume twice that value (taking into account socket and wiring capacitances), the treble loss with an 100k grid stopper will not start below 40kHz.
I rather think that compression is caused by the signal voltage drop across the grid stopper with beginning EL84 grid conduction.

Showitevent:

--- Zitat von: Helmholtz am 29.01.2024 17:22 ---The EL84 has an effective input capacitance (including Miller effect) around 20pF.
Even if we assume twice that value (taking into account socket and wiring capacitances), the treble loss with an 100k grid stopper will not start below 40kHz.
I rather think that compression is caused by the signal voltage drop across the grid stopper with beginning EL84 grid conduction.

--- Ende Zitat ---

Im not speaking about miller capacitance. I also said that there is a subjective feeling of losing treble content due to drive. This may be drive current or miller capacitance or sideeffects that come in when pushing the tubes harder.

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