JCM800 2204 clone renovation

[IvanMan]

Hello everybody.
I the last couple of week I worked hard to renovate a JCM800 2204 50W clone I bought some years ago.
It was modificated a lot by the previous owner and I'd like to restore the original Marshall components (my bible was the schematic of Mark Huss JCM800 Lead Series 2204).
I substituted the EL34 and also some coupling capacitors that seem to have small leakages (not 0 Vdc on the output to the following stage).
Also a new 50+50uF (the first in the chain) was introduced because a different value was present.
All the voltages are similar the the Mark's schematic with 5% less, but starting from B+.
I've a couple of questions about the theory of this Amp:
1) I substituted the 22n coupling capacitor of V1a (pin 1) because I measured around 700mVDC on the output part (should be near 0, if I'm not wrong). But also with the new cap the value doesn't change. The other couple capacitors have generally not more than 10-15mVDC.
2) On many schematics and voltage charts the voltage on pin 1 & 6 of the phase inverter are similar (253 and 251VDC on Mark's schematic). In my 2204 I've 215-220VDC on pin 1 and 255VDC on pin 6.
3) The bias current is around 30mA for one valve and 32mA for the other. The first one seems I little more red than the second when playing.
What your opinion? Do you think it's all regular or can you suggest something more to check?
Sorry for the basic questions and thanks in advance for your help.
Ivan

[pdf64]

1/ maybe the ‘6,7,8’ section of your V1 has excessive grid current? Does the voltage vary with the gain control setting?
2&3/ maybe try some different tubes in V3, perhaps even a ‘balanced’ one?
Have you checked the plate resistor values?
How about if the power tubes are swapped over; does the red plating follow the tube or stay with the socket?

[IvanMan]

I worked a couple of hour of issue 2), but with no success.
I changed the tube with others, but the different voltage remain between pin 1&6 --> pin 1 vary from 210 to 220VDC and pin 6 vary from 250 to 260VDC with different tubes. With different tubes when pin 1 decrease it seems that pin 6 increase.
Grid voltage was 31VDC of both pin 2&7.
I disconnected the plate resistors: they are 82K and 100k.

I'm going crazy!

[IvanMan]

Checked also both grid and cathode resistors: they are ok.
I tried without the power tubes. Here what’s happen:
- Pin1 = 230Vdc – Pin6 = 255Vdc (a little better)
- Pin 2 & 7 are not more exactly equal  Pin2= 32Vdc – Pin 7 = 33.5Vdc
Suggest something?

[sluckey]

You can't accurately measure the grid voltages on a LTP PI. The grid impedance is much greater than the resistors would indicate. This is due to the bootstrap grid bias circuit. Your meter input resistance will load the voltage and give an inaccurate reading. Five different meters would probably give you five different readings. I prefer to measure the voltage at the bottom side of the grid resistors (where they join in with the cathode resistors). You can get an accurate reading there.

How does the amp sound? Does it have any issues other than the difference in PI plate voltages?

[IvanMan]

The amp sounds good as tone, but seems not reach the full power, the sound seems compressed after the volume raise after 12.

Looking at the phase inverter issue I replaced the 100n cap on pin 7, connected to the NFB.
And to be sure I replaced also the 2 plate resistor 82k and 100k and try to unsolder the small 47p disk cap between plate resistors.
But nothing change!

At the end all: power tubes relaced, PI coupling cap has been replaced, the PI plate resistor too, the grid and cathode resistors checked, the PI tube swapped with others from the same amp and from an other amp.

In your opinion:
- can this PI measures generate the "not full power issue"?
- can I check something other?
- what is involved to generate a balance between the 2 PI plate voltages, considering the different values of plate resistors? I'm courious to learn: the power side seems symmetric; the cathode too (connected together); it seems only somethings coming either from Master Volume or NFB, but not VDC because I changed both the coupling caps. Can I verify something with the scope? I have it.

Thanks again for your help. You all are so patient with a newbie.
Ivan

[IvanMan]

In addition I tried to apply a sine wave (1kHz) to the Low Input and look what happens to the PI plate pins:
- the VDC voltages drop around 30vdc on both pins (pin 1 goes to 200Vdc and pin 6 goes to 230VDC)
- on the scope the signal wave is around 20% higher on pin 6 than pin 1

Hope this helps.

[IvanMan]

Sorry for the wrong information but the voltage on pin 1&6 of the PI are dropped also without any input signal. Now they are 200Vdc and 215Vdc.
And when I touch the pin 6 of the PI with the scope probe the volume in output (I have a speaket connected) raise up.

At this point only confusion

[pompeiisneaks]

That may mean you've not got a good path to ground in the tube you're probing, (PI) I've had this exact problem several times and forgot that I'd missed a connection to ground, in some cases the PI finds a path but it's way higher resistance so it's super starved for current. Once you double check the ground path, that may be the problem.

~Phil

[IvanMan]

I spent few our in checking the connection to ground, including desoldering and resolder the Presence part, changing the 100n cap.
The situation doesn't change. On the PI I've 217Vdc on the pin1 and 240Vdc on pin 6.
One thing I noted are the Vdc voltage on the NFL line: -2Vdc before the 100k resistor and 14.9Vdc after.
All Others voltages in my amp are 5% less than the Mark Huss schematic, but in this case the 14.9 in Greater (10%) than the 13.5 indicated by MH.
May be this influence the balance of the PI? Can I check something more?
Thanks in advance for your help.

[IvanMan]

Hello everybody.
For your information and to close this thread: all my issues were due to a conductive board.
In the last times I worked a lot on the issue of the coupling cap that had a strange 700mV on the side were near 0V is expected.
Replaced it many times, during operation I try to measure the turret without anything connected and I found a voltage (around 12Vdc).
Going ahead I find that a couple of other free turrets have a positive voltage.
At the end the conclusion was that the board itself were conductive!
In the attached pictures you can see the board modified with my homemade insulated turrets applied to HV nodes.

20180923_195327.jpg

This solve the problem of the voltage after coupling cap of V1a and ALSO the issue of the not well balanced PI (on pin 1&6 now I've 252 and 257V).