Fixing 60's German tube amp

[bandit88]

Across R1004 I get 2.4v.
Across R903, and 1003 I get 1.2v
Both R901 and 1001 have the correct ohms.

I am using just a standard digital multi-meter.

Thanks

 

[Kay Pirinha]

Disconnect C905 for a try. What's the cathode voltage reading now? And what's the plate voltage reading (EZ80's cathode, pin 3)?

Best regards!

 

[Ketje]

Across R1004 I get 2.4v.
Across R903, and 1003 I get 1.2v
Thanks

Now, do the same measurements without the output tube.
If there is still voltage on R903 and/or R1003 bad C903,C1003 or dirty socket.
No voltage means tube is the cause.
Remains curious, two sides the same error voltage
Mona

 

[bandit88]

With the tube removed I get 0v on R903 and R1003. C903 and C1003 have been replaced and are brand new.

Can anyone else confirm this means bad tube? Otherwise I will test with C905 disconnected.

Thanks

 

[Kay Pirinha]

Probably. I'd also lift one leg of C905 and measure again with the ELL80 inserted.

If cathode voltage remains the same, C905 is not the culprit - though being suspicious just from experience. If voltage reading without it is higher, this designs a bad C905.

Best regards!

 

[U130421]

Measure the -14V before and after bad sound. It is made from the output cathode currents and vdr +47ohms. The vdr temperature dependent resistor regulates the -14v for a range of current. You can replace the 47+vdr by a 1W 15v zener.

 

[bandit88]

I replaced the ELL80 with two EL95s and an adapter, works perfect now and sounds better than I would have ever imagined.

Thanks for all the help! Now to find the next project

 

[pcan]

Glad to see that you fixed it. The ELL80 tube may also be replaced by the comparatively less scarce ECLL800 tube, when the output transformer does not have a ultralinear tap. In most instances no adapters and modifications at all are required, but to be on the safe side pin1 should be disconnected from the circuit (or bent out). I attach the original manufacturer technical note.

 

[Kay Pirinha]

No need to bend out the ECLL800's pin 1, as this might easily lead to cracks in the glass. Just swap the orignal wire from pin 1 to 9 and then connect pins 1 and 7 at the socket. This is what SEL recommends.

Btw, I've successfully replaced two ELL80's in a NORDMENDE stereo console by four EL90/6AQ5's, using similar adaptors as the OP did (same pinout as EL95). Due to the higher heater curent, I had to disconnect the dial lights and replace them by LED's. Additionally, one has to regard that the EL 90 needs more drive as the EL95. In my case, this didn't matter, though.

Best regards!