EMI RS61s - Input signal is faintly audible

[ed rees]

I'm working on a pair of EMI RS61 line amps, encountered a strange issue.  I can faintly hear whatever audio is being ran through the amps. It seems to be coming from the plate choke or output transformer.  Has anyone encountered such a thing?

Tubes are NOS mil-spec. Power supply is of my own design, based on one for a similar EF37A preamp found in the EMI BTR2 tape machines. Delivers ~290VDC under load. 5R4GYA rectifier with choke input LC filtering.  Large PIO caps.

Here are schematics and a response plot, which is 99% identical on both amps. 40 dB pad on the output for testing purposes. They are roughly producing the expected amount of gain (40dB), but obviously something isn't right!

 

[dustbro]

Just at  a quick glance, the resistor feeding HT to VR1 is different on each schematic. One is 47k, the other is 22k .
Is your unit original?  or clone?

 

[PRR]

V2 is missing a screen bypass cap.

40dB pad on output??!

All transformers "make sound", especially >1KHz, especially at high level. I expect an amp to sing on bench-test, though inaudible when potted, or when the same sound also comes through a speaker.

 

[ed rees]

Thanks for the replies fellas. They are original units, probably from EMI Barcelona.

The issue ended up being in my ARTA settings. Somehow they got put back to default, so it was driving the amps way too hard. Explains the distortion and "transformer singing".

I will check the originals and report back which schematic is correct for future builds here.