Really unbalanced phase inverter?

[slidincharlie (Carlo P)]

Hi guys,
I have another problem in my old Farfisa amp.
The phase inverter shows 70 Vdc on one plate (pin 1) and 200 Vdc on the other (pin 6).
The two EF86 tubes behind this p.i. give 258 and 255 Vdc off their plates, respectively.
Now I'm a total moron when it comes to electronics... but I thought that the two plates in a phase inverter should give equal voltages, shouldn't they?
I have tried two different new 12ax7's in the p.i. position but the result doesn't change.
Voltages on both 6L6's are fine.
TIA.
Carlo

[NZaudio]

Bongiorno Carlo,
greetings from Down Under.I am originally from Europe but live here since 14 years.
you are measuring the wrong things.
Your phase inverter is V5B, pin 6,7 and 8
Voltages on pin 6 and 8 should be almost equal.They are determined by R707 and R703 together with R702. Both tubes share R702. Sometimes the DC decoupling caps (C704 and C705) make a bit trouble.But if you have almost the same voltage at pin 6 and pin 8, you are right and safe. What is wrong with the amp?

[pdf64]

Yes, the phase splitter is a cathodyne type but uses grid leak biasing, see How to design valve guitar amplifiers for detail on how these circuits operate (the triode gain stage for grid leak bias).
Just to expand on NZaudio's comment, the equal voltages at pins 6 (plate) & 8 (cathode) are the ac signal voltage; the Vdc will be rather different.
Pete

[NZaudio]

Thanks PDF, DC could be around a third of the supply voltage

[Leo_Gnardo]

Slightly different plate V's on your EF86's, I'd say no big deal. OTOH that's some fancy phase inverter! The designers had maybe too many espressos when they thought that one up. Your output tube grids are being driven from the plate and cathode of the final triode. The previous triode is being used as a booster/signal conditioner so wouldn't worry what the plate voltage is with respect to the final triode. Since its plate voltage is low, it may also act as a limiter/compressor. In fact its plate voltage is derived from the cathode voltage of the final triode so we expect it to be low. 70V seems about right.

I'd suspect possibly leaky interstage caps in an amp this age. Also high value resistors may not be what you expect them to be. I see a 10M and a couple 6M8.

[slidincharlie (Carlo P)]

Here I am. Sorry for the delay guys. Thanks for your feedback.
I have checked the 47 nF (0.047 uF) coupling caps from the phase inverter to the power tubes, reading Vdc on their lifted distal leg. Actually I don't have a stable reading: the DMM jumps from low to negative values in the tens of dc mV. I have connected two other caps (a used and a new one) to B+ and got the same 'dancing' reading. Now I don't know if replace the old caps or not...
The 10M resistor is fine, I'll have to check the two 6M8's in the preamp chassis.

What is wrong with the amp?

Greetings from the Old Continent NZaudio! The main trouble with this old and sick amp is that it has a very low volume, no matter what tubes I put in it. Channel I is almost silent and the others do not sound as loud as a 20 watter for sure. I have gone through a general overhaul (cap job, grounded power cord, resistor check and the like) to no avail.

[tubeswell]

What are your voltages like thru-out the amp Charlie?

[slidincharlie (Carlo P)]

What are your voltages like thru-out the amp Charlie?

I'll write down the most 'abnormal' readings:
V1 and V2: cathodes 1.6 and 1.5 on V1, 2.3 and 1.5 on V2.
V3 and V4: 255 Vdc instead of 200 on pin 6.
V5: 70 Vdc on pin 1, 200 Vdc on pin 6 (instead of 240), 90 Vdc on pin 8. The schematic doesn't say anything about pin 1 and 8.
Power tubes and rectifier: on specs.

[Bruce / Mission Amps]

Put something like a 1meg resistor or two in series to ground at the end of each of those lifted caps and see if you still get a DC reading.. or at least a steady reading of something.


Oops didn't see it was cathode biased... the grid load resistors are grounded so you should only be reading a tiny mvdc or so off those coupling caps when connected and no power tubes installed.

[slidincharlie (Carlo P)]

(...) Oops didn't see it was cathode biased... the grid load resistors are grounded so you should only be reading a tiny mvdc or so off those coupling caps when connected and no power tubes installed.

Yes, I see almost 0 mVdc with the power tubes off.
Now I need your help guys... I'm not sure of which are the coupling caps in the preamp section. Old Fender circuits are easier to read for a non-tech like me...
Should I check C108 on channel I, C208 on channel II? Any other?

[52 Bill]

... I'm not sure of which are the coupling caps in the preamp section. Old Fender circuits are easier to read for a non-tech like me...

On this amp, all of the preamp circuits drive from the tube plates so, V1 C103 and C206, V6 C606, V4 C602 and V2 C302.

[slidincharlie (Carlo P)]

On this amp, all of the preamp circuits drive from the tube plates so, V1 C103 and C206, V6 C606, V4 C602 and V2 C302.

Back I am, after a long while.
I have checked for leakage all the caps from the preamp tube plates, and they checked out fine. No leakage.
So now I'll have to troubleshoot why this amp sound so damn low... The volume is really low: it starts almost totally mute at the first jack on the first stage and progressively rises until the second jack on the last stage, where it still sounds like a 4-5 watt amp.
I have tried different known good tubes to no avail.
I'll probably start a new thread on this topic as soon as I have a chance...

[pecorporation]

Have you inserted a signal at the ORGAN input jack? Seems like a good place to determine whether the problem lies in the preamp or power amp stage.

[slidincharlie (Carlo P)]

Have you inserted a signal at the ORGAN input jack? Seems like a good place to determine whether the problem lies in the preamp or power amp stage.

Thanks for the tip.
Inserting the guitar cord into the Organ input still gives me a weak sound: nice, clean but very low volume. My silverface Fender Champ (7W?) used to play much louder.