high plate voltage on one side of a 12ax7

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samgraysound

Well-known member
Joined
Jun 9, 2014
Messages
284
Location
Olympia, WA
Hey all,

I'm working on a Telefunken Hymnus Hifi Stereo. Its a console radio/amplifier. Not impressed with the build quality at all, huge rats nest.

It was blowing fuses. Found an incorrect tube in one of the sockets, pentode in a 12ax7 slot.  Replaced that, filter caps, cathode caps, and the bridge rectifier. It is no longer blowing fuses although the amp draw still seems high to me. About 1A idling, with a 1.25A fuse.

I've been checking voltages through the amp. They are pretty high throughout like +25-30%. Which makes sense because it was designed for 110, and the new 1N4004s for the rectifier are probably putting out a higher voltage then what they replaced.

But one plate voltage in particular is just way high. Its V7, a 12ax7, setup in stereo, so one triode is the L and one is the R, with the exact same circuit for both. The schematic says plate voltage of 78v. One side is 140v, one is 260v!

Plate resistors tested the same but I replaced the one on the problem side anyway. Replaced the plate coupling cap. Replaced the cathode cap on both sides, they have the same resistance cathode to ground. 10M resistor grid to ground, reads as open on my multi-meter on both sides. Tried a different tube. What else is there?

I've been working off the schematics here(the 2nd page of schems is the relevant one): https://www.radiomuseum.org/r/telefunken_hymnus_hi_fi_stereo_2004.html

It is both blurry and in german.

They tube complement and general layout is the same, but there is a number of differences from the schematic. on V7 (the tube in question) plate resistors are 100k instead of 220k, and there is a cathode resistor and cap instead of the cathode being connected directly to ground, 1.2k and 47uf respectively.

What else should I try?

Sam




 
First reduce B+/HT by putting resistors in series with the 1N4007's. Next check grid bias in relation to cathode, for an ECC83 it should be around a volt or more, anything less and it's running too hot. If you still have trouble check the valve base, give it a clean and check voltages at the pins, with the valve out.

Andy.
 
I haven't seem the diagram, but I would check your  connections on the cathode resistor & grid to ground resistor.  Also check the cathode resistor is not open circuit.  If the cathode resistor isn't connected to ground there is no current flow so the anode will sit up to h.t voltage.
 
> can't seem to view the schematic

You need to be logged-in there.

Here's the snip (attached).

Volume control, stage in question, SE EL84 with 820K grid resistor. So signal level into this stage is very low. A 10Meg grid resistor will bias-up a 12AX7 perfectly well. Well, good enough for 95% of all table radios. And the Hymnus is just a de-luxe console-case dual-channel many-switch giant radio/phono.

Finding 260V on plate, when B+ should be 170V, tells me this tube-section is dead; and there are other problems also. Change tube. Check cathode-to-ground connection (there is NOT a cathode R-C on this stage, per the linked schematic). Tack another Meg+ resistor across the "10Meg" and see if plate voltage changes any (suggesting the "10Meg" has gone open).
 

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ruffrecords said:
I can't seem to view the schematic via the link you posted. Can you post the actual schematic?

Cheers

Ian

Here you go, hopefuly radiomuseum wont be mad at me.
 

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PRR said:
> can't seem to view the schematic

You need to be logged-in there.

Here's the snip (attached).

Volume control, stage in question, SE EL84 with 820K grid resistor. So signal level into this stage is very low. A 10Meg grid resistor will bias-up a 12AX7 perfectly well. Well, good enough for 95% of all table radios. And the Hymnus is just a de-luxe console-case dual-channel many-switch giant radio/phono.

Finding 260V on plate, when B+ should be 170V, tells me this tube-section is dead; and there are other problems also. Change tube. Check cathode-to-ground connection (there is NOT a cathode R-C on this stage, per the linked schematic). Tack another Meg+ resistor across the "10Meg" and see if plate voltage changes any (suggesting the "10Meg" has gone open).

That's what I'm saying. The actual components are not matching the schematic. The general circuit is the same but there is a number of differences.

On this stage:
Plate resistor is 100k instead of 220k
Grid resistor is 1M instead of 10M
Cathode has a 1.2k resistor and 47uf cap instead of being grounded.

I have now replaced all these components, as well as the grid and plate coupling caps. I replaced the tube again, and a bunch unrelated paper and wax caps started melting...

Sam
 
He mentioned 100k in first post....maybe typo??

Was this thing modded for something else??? Pentode swap and the different component values with the added cathode resistor/cap???seems weird...

Wouldn't a cathode resistor still go to ground if there was one added???
 
scott2000 said:
He mentioned 100k in first post....maybe typo??

Was this thing modded for something else??? Pentode swap and the different component values with the added cathode resistor/cap???seems weird...

Wouldn't a cathode resistor still go to ground if there was one added???

Yes the cathode resistor still goes to ground. That connection is checked.

I'm not sure if it was modded. All the electrolytics seem to be original. It looks like it was serviced at some point, at least one wax cap was replaced with an orange drop.

 
So yesterday a short happened and all the wax caps started melting. Blew a fuse. I was trying to isolate it this morning and now its disappeared and back to "normal"





 
Voltage readings today:

"Bad" side

Plate - 307.5VDC
Grid - 0VDC
Cathode - 0.012VDC

"Good" side

Plate - 209VDC
Grid - 0.033VDC
Cathode - 1.139VDC

Going to test the cathode connection from the other side of the tube socket.

Sam
 
samgraysound said:
yes. everything was tested. although at this point, I have replaced everything on the "bad" side anyway.
Grid and cathode are the same voltage, so tube should be full-on like a diode, with lots of current through the plate resistor, and a low voltage at the plate.

This indicates a bad tube, but you already tried a known working tube.

You are probably not actually checking the tube pin voltages, you are checking the socket voltages, and assuming that they are the same. If you can get in there, can you get to the end of the actual pins? Measure between a pin and it's socket, looking for any voltage. Sockets do go bad, loss of spring tension, corrosion, etc..

Just a longshot.

Gene





 

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