issues with a GZ34 and open circuit anode resistors.

[bikerhifinut]

that's about it Richard.

I reckon what i will do as a test/temp fix is to introduce a 4.7uF 630V polyprop LCR that I have a pair of as first cap before the big choke. that will keep the voltage up and also be ok for use with say a 5U4G (miles cheaper than GZ34). The choke and following Electrolytics will be fine for smoothing and reservoir duties. I also will be using proper wirewounds, John C told me last year that those coffin jobs are not really very reliable and i think a better resistor will tolerate a momentary surge much better as I think the problems lie with the initial surge as the first cap charges up.
The present transformer is over specced and has good regulation if a bit noisy. I was looking at making a cover with ventilation holes for the transformers from some sort of wood or metal with damping material stuck on the inside surfaces that might prevent the buzz from being audible at the armchair.

I do have a massive toroid but my feeling now is to make a stereo chassis with it and try and get the best out of these monos for now.

I'm wary of commercial mains transformers now as these are the second batch (different makers) that have been noisy beyond reasonable expectation. It is possible to make silent EI transformers as my leaks and the WD amps testify.

A.

[bikerhifinut]

i think that using cheapo maplins coffin ww resistors is the issue plus the valve is now RIP. I think they can't handle the switch on voltage difference acroos them. It's quality WW from now on in high stress areas.

A.

[bikerhifinut]

further to this:

I calculated the power rating I should have fitted and it was very close to the 7W resistors at 6W. This also suggests i was hammering the anode surge limiters, combined with the fact they were probably inferior quality.

So I am now going to use larger branded jobs from RS online to be as certain as possible that I am actually buying a genuine article.

And i found they have some reasonably priced high ripple polypropylene caps, so will be getting a brace of 20uF at 1000V to use as first reservoir caps. As the original circuit used 16uF there i feel certain that it will work and sound good still.

I was going to do a big off board supply for both amps but the toroid for that job looks like being used for a new power amp project when I get the bits together.

I've got hold of a pair of large 100W rated UL outp[ut transformers. 3.3k A to A which kind of means 2 pairs of parallel EL34 or similar per transformer.

Unless someone here knows of a big valve that will work on that low ish anode resistance and at 400V?

I bet Philip does he seems to have a lot of experience in different out.put valves
A.

[pre65]

Originally Posted by bikerhifinut

further to this:

I calculated the power rating I should have fitted and it was very close to the 7W resistors at 6W. This also suggests i was hammering the anode surge limiters, combined with the fact they were probably inferior quality.

So I am now going to use larger branded jobs from RS online to be as certain as possible that I am actually buying a genuine article.

And i found they have some reasonably priced high ripple polypropylene caps, so will be getting a brace of 20uF at 1000V to use as first reservoir caps. As the original circuit used 16uF there i feel certain that it will work and sound good still.

I was going to do a big off board supply for both amps but the toroid for that job looks like being used for a new power amp project when I get the bits together.

I've got hold of a pair of large 100W rated UL outp[ut transformers. 3.3k A to A which kind of means 2 pairs of parallel EL34 or similar per transformer.

Unless someone here knows of a big valve that will work on that low ish anode resistance and at 400V?

I bet Philip does he seems to have a lot of experience in different out.put valves
A.

Flatterer.

Must admit I've only ever done one proper PP amp and that uses the 832a valve.

I did try a KT66 (trioded) breadboard but it was unsatisfactory.

If the OPT are not what you really want, sell them and get something you can use, or do you really fancy a 100w amp ?

To be honest, something like a GM70 PP amp would be very expensive to build, even if those OPT would work.

[Richard]

300Bs would probably wrk but it'd be a lot of money and effort if you didn't like the result. Given your familiarity with 5-20 I'd be looking at the Kit88 and KEL80 drawings using parallel KT88/6550/EL34s driven by 5687