• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Dead Audio Innovations Series 500

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Audio Innovations Series 500 needs TLC!

Hi there

Hoping someone can advise me on my old AI Series 500 valve amp.

Some years ago, I started a thread about it:


I have come back to it just today.

I replaced the cathode capacitors and the small fuse resistor, but it hasn't made any difference.

The valves are all new and with a line level input I can hear the output but it's very quiet. Both channels are affected.

The source is connected to the tuner input and it is selected with the tape selector in the "source" position (in other words, this bit is as you would expect). Volume and balance have the correct effect (so we can assume they are OK).

The amp is a bit messy inside having been worked on before by a butcher and there are a lot of burned-out looking components, but that said it has always been like this and it used to work well (for many years in fact). The failure incident was preceded by a purple haze around the power valves (the EL34s), which is why I think the first advice was replacement of the cathode capacitors. As you can see from the other thread, I checked various reference voltages and they appear to be OK.

What could affect both channels, any ideas? I have tried also different winding outputs on the transformers (it's got the multi impedance speaker taps) - no change.

I have a set of circuit diagrams for various revisions (not sure which one mine is) and the usual collection of hobbyist tools (plus some experience), but no oscilloscope.

Any advice gratefully received.
Last edited:
Well, I have gone back to basics again.

I measured 0v at the N point on the schematic, and traced it back to the power board, where I discovered that the 3W 10k resistor had burned out and was open circuit. Quick trip to the thrice-accursed Maplin store (oh, how I hate Maplin, but sometimes they come through in a pinch), one wire wound replacement for the princely sum of 58p, swapped it out, power up and the whole thing just sprang to life like Frankenstein's monster. Appropriately enough, I am listening to Kraftwerk's "The Robots" as a test track...

Gosh, this poor old thing has been dead for over 10 years. And now...



I know I am going to sound like a teenager now, but 0h... My.... God...! What a sound! I'd forgotten how sweet an amp it is, and I still have loads of work to do on it as it is still very messy inside - as you can see from the attachment...


  • IMG_1029.jpg
    733.8 KB · Views: 252
Jon, never give it to a butcher for repair again

It came like that, and I have had a good long chat with a specialist over here and he says they were actually built like that at first. Mine's a very early model, after all! So perhaps I can't blame the guy who sold it to me, as he does not appear to have done much to mess it up; it was already messy, right from the factory!

It is now having a rest, as I don't yet have anywhere to put it. Got too much stuff here...
Last edited:
They were made in Weymouth, Dorset by a very good friend of mine. A bit of a mish mash but as you describe it, what a lovelly sound. The build was based on an early Mullard design. The switches however, were upgraded to gold flashed units as time went by, for reliability.
Welcome to my Web site looks after a number of them.

Is that John Snell as in "Snell Loudspeakers"?
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.