Fuzz pedal (blocking distortion) makes one EL34 shows early signs of red plating in 50 watt

[Gtr0]

The mark space ratio at the power tubes looks a bit uneven. That can be a characteristic of the way that the preceding stages overdrive, or maybe originating with the input signal (the way the fuzz operates).
What are the waveforms at the input, and going into the phase splitter?
What does the HT Vdc sag down to at the plate (OT CT) and g2 (after the choke) nodes when the amp is being overdriven?
A potential issue being that modern PTs can be too stiff, insufficient degree inbuilt protecting resistance carried over from tube rectifier era designs.

I missed the line about the signal at input and PI input...

the input is 500Hz at 500mVpp...


before the .022 at the PI input...


and on the tube side of the .022... there should be positive vdc here? I thought not and forgot to mention earlier... it doesn't make sense to me here.....

[Gtr0]

Sorry Herr Helmholtz It seems I failed to mention my input source in detail. Too many things to explain. sorry

[Helmholtz]

not sure if you missed it? I said I was using 500mV 500Hz input signal. The whole fuzz pedal thing was while playing... it is when it starts to red plate with a fuzz in front.

I understood you fed the fuzz input with your signal.
So the pictures were taken with 500Hz sine at the amp's input?
Anyway you were massively overdriving the power tube grids. So a little redplating is no surprize.
Also make sure you connect the correct speaker/load impedance. Lower than nominal load impedance increases plate dissipation and redplating.

[Gtr0]

I understood you fed the fuzz input with your signal.
So the pictures were taken with 500Hz sine at the amp's input?
Anyway you were massively overdriving the power tube grids. So a little redplating is no surprize.
Also make sure you connect the correct speaker/load impedance. Lower than nominal load impedance increases plate dissipation and redplating.

All photos were taken with that same input signal of 500Hz.

And I just verified that I do have the correct taps wired for impedance matching. At least according to the color codes I received on the data sheet.

Bummer... it sounds very good as well. Hope I can fix it and that it will sound similar

I triple and quadruple checked values and wiring against the schematic as well as the other amps I have built. this is the third or fourth similar circuit within a few months I have put together - at this point I can wire it up without using the schematic... not to say that I didn't fudge something up!! but I certainly can not find it!

[Helmholtz]

The PI input grid should sit at around +40VDC. Your picture looks more like 160VDC.
Please post PI plate (pins 1,6) and cathode DC voltages (pins 3,8) as well as PI supply voltage.

[Gtr0]

The PI input grid should sit at around +40VDC. Your picture looks more like 160VDC.
Please post PI plate (pins 1,6) and cathode DC voltages (pins 3,8) as well as PI supply voltage.

All VDC to Ground...

Pin 1 - 305 VDC
Pin 2 - 121 VDC
Pin 3 - 187 VDC (shared cathode)
Pin 6 - 299 VDC
Pin 7 - 130 VDC
Pin 8 - 187 VDC (shared)

Thank you for your help!

BTW I have changed pre tubes at least 3 times as well as power tubes... I just picked up a new set of Groove Tube overpriced JJ E34L at my local shop but won't put them in quite yet ;-)

[Helmholtz]

Cathode voltages are way too high, which also lifts grid voltages.
Check the resistors in the cathode tail including presence pot. I expect some wrong (high value) cathode resistor - or wrong wiring of the 1M grid resistors.

And please also post the supply voltage (at other end of plate resistors).

[Gtr0]

For the sake of being thorough :-)

B+ Nodes:

Node 1 - Reservoir - 460 VDC (before choke)
Node 2 - Screens - 457 VDC (after choke)
Node 3 - PI - 383 VDC
Node 4 - v2 - 326 VDC
Node 5 - v1 - 308 VDC

SOME schematics have 2x 10kΩ resistors for dropping before the PI node, some have 1x 10kΩ - I used 1x 10kΩ here was in my JTM amps... bad move?

[Gtr0]

Cathode voltages are way too high, which also lifts grid voltages.
Check the resistors in the cathode tail including presence pot. I expect some wrong (high value) cathode resistor - or wrong wiring of the 1M grid resistors.

checking now - thanks!

[Gtr0]

okay...
cathode r - 469.7Ω
tail r - 9.93kΩ
pres pot - 5.369kΩ
grid r 1 (pin 2) - 1.041MΩ
grid r 2 (pin 7) - 1.044MΩ

and for fun...
Plate r 1 - 85.0kΩ
Plate r 2 - 103.2kΩ

I am looking over the wiring here again...

just fyi - I placed a .68uF in lieu of the .1uF on the presence, but I always place it before the pot's input to keep DC off of the pot.

[Helmholtz]

What total DCR do you measure between cathodes and ground?

[Gtr0]

this is the best image I can get...
NFB 100k lurking just under the .1uF on the PI (it's red wire going to 8ohm tap on output trans). The 680nF from the tail goes to the presence pot input, whose output is connected to ground.

[Gtr0]

What total DCR do you measure between cathodes and ground?

109.6kΩ

I get a 100kΩ reading across that 680nF.. is that appropriate? but I assume this 109.6k reading is going through NFB to ground at the tranny?

[Helmholtz]

109.6kΩ

I get a 100kΩ reading across that 680nF.. is that appropriate? but I assume this 109.6k reading is going through NFB to ground at the tranny?

Here is your problem. You need to have around 16k.

Post post a drawing of your presence mod.

Look at the schematic of a 2203. If your presence pot is not in the DC path you need an extra 4.7k resistor.

[Gtr0]

here it is... sorry for the messy writing, it was on my lap looking at the circuit...