Fisher 400 output valves current draw

[PaulR]

Whilst I was waiting for the valves I thought I would replace the power diodes. I thought that the usual 1N4007s might be a bit underrated for current so I have sent off for some 1N5408s. Does that seem to be a good choice?

[Diabolical Artificer]

More than adequate Paul. The 1N4007's are ok current wise but are a tad slow.The 1N5408's are also on the slow side, something with a faster switching speed might be better. Something like the UF4007, BA159 or BYV26E

Andy.

[PaulR]

Thanks Andy

I have sent off for the 1N5408s now so I will fit those.

[PaulR]

The valves have arrived, and so has Andy's bias supply.

The voltages are now stable and are as follows

Bias -16.5 V
10 ohm cathode resistors 350mV
Anodes 391V
Screen 351V

The DC voltage at pin 4 V8/V10 is 24.2V
The AC voltage into CR1 is 23.8V
Across R138 4.6V
Junction of R34/R35 16.9V

I hope that I have identified the correct points to test from!

EDIT I have replaced C63 with a 1kV one

[snowman_al]

That is very encouraging and it all falls neatly into place.

24 volts for the pre amp valve heaters is spot on.
0.035A times the HT of 391 volts is 13.7 watts dissipation, that is fine.
(I assume the 4 valves are all about the same voltage across the 10Ω resistors and are well matched.)

If you did want to reduce the dissipation a little more, then increasing R34 (2.2kΩ) to 2.5kΩ or 2.7kΩ will give you more negative bias...

Alan

PS: Should also mention that it is highly recommended to add a 100Ω 1 Watt resistor to each screen grid (pins 1/7) of the 7868s. Screen grid stability resistors, from the Gillespie articles.

[PaulR]

Thank you Al. I looked up the typical operation stats for ultralinear operation and, although the suggested bias is -20v at 400v Ht the other measurements seem ok. I might increase R34 a bit just to calm it down further and I will add the suggested stability resistors to the screens.

Well, are we nearly there then? Many thanks to all who have contributed. I haven't forgotten PJL's suggestion regarding the runaway valve but I will have a play with that outside the set using a new valve socket and my HT power supply. Here's to the next 60 years of life for this amp!

[PaulR]

I managed to snap one of the leads to the new rectifier bridge whilst replacing R34. Another on order!!

[Diabolical Artificer]

That's good news, I presume you managed to squeeze in the tfmr & bias supply.

Andy.

[PaulR]

Actually, Andy, I managed to get those voltages using the original set up completely unmodified. It would mean moving components to fit the separate bias supply as it is so crowded.

[PJL]

That sounds (nearly) like a really good ending. What valves did you buy in the end, are they the Electro-harmonix reproduction ones or original? From what you have said, they sound well matched and working to specification.

[PaulR]

My son, who owns it, bought NOS ones from Watford Valves. The Russian ones get mixed reviews to say the least. These have the thin pins like the Fisher ones and seem to work well. It's a pity about the rectifier but a replacement is due in the middle of next week.

[PaulR]

The new rectifier arrived yesterday and I fitted it together with an increased value for R34. This reduced the bias. Thinking about it wouldn't increasing R34 would make it less negative (I think!). Anyway I have put it back to the 2.2K it originally and all seems well within parameters.

The screens are daisy chained from one another and R41 is connected to the penultimate valve on the chain. It would be rather a faff finding space for and putting in a terminal strip for the resistors and to re connect R41 so I might leave that for another day and enjoy the set for a while.

Once again, many thanks to all who helped. There really wasn't room to fit Andy's kindly supplied bias supply but I will keep it if that is OK as these things often come in useful when you aren't expecting it.

[Diabolical Artificer]

Increasing R34 lowers bias, if we have minus 30v at the top of R35, we'll have 0v at the bottom of R34. The potential divider is roughly 1/3 and 2/3's, so as is you should get -10v ish as is, increasing R34 drops more V across it, lowering bias.

Your welcome to the tfmr and bias supply Paul, as you say it may come in handy.

Andy.

[PaulR]

Just another question regarding this amp. I have replaced the final audio coupling caps and another two paper ones I found. The rest are circular disk ones. I have found similar ones to be reliable but I am not familiar with these American ones. Are they likely to deteriorate at all or should I leave well alone as I am inclined to do?

Thanks

[Diabolical Artificer]

They should be fine, probably ceramic, so unless you have a specific fault leave em be.

Andy.

[PaulR]

Thanks Andy, that is what I thought