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Old 19th Nov 2018, 12:53 am   #1
bikerhifinut
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Default Pleasantly surprised - amp modification

I did a quick and "dirty" rework of my RH84SE EL84 SE amp.

It was always an amazingly good little thing.

But.....

It had a habit of going unstable which I eventually suspected was due to the CCS on the EL84 cathodes. Theoretically a good idea. Mostly it worked. But I'd get motorboating or sulks.

So I opened out the valve bases for octals.

Rewired it with 270R on the c athode from the 6V6 data sheet. 250V on the anode. and plugged in a pair of 6V6.

bunged it all back in a very good stereo system


and wow! unless I want metallica volume it sounds reet nice.

Just a thought, small room sensitive speakers, a couple of watts and audio nirvana. innit?

A.

Last edited by bikerhifinut; 19th Nov 2018 at 12:55 am. Reason: info
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Old 19th Nov 2018, 8:12 am   #2
Diabolical Artificer
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Default Re: Pleasantly surprised.

It's nice when a plan comes together, a pleasant change to the usual one step forward 3 back.

Tother Andy.
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Old 19th Nov 2018, 2:24 pm   #3
G8HQP Dave
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Default Re: Pleasantly surprised - amp modification

CCS as a cathode bias for output valves is theoretically a bad idea, although undeservedly popular. For PP an unbypassed CCS enforces current balance, but it also enforces fixed average current when what you actually want for low distortion is fixed quiescent current. For single-ended, a bypassed CCS can create LF phase problems in addition to fixing the wrong current. The LF phase problems can then encourage motorboating.
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Old 19th Nov 2018, 3:03 pm   #4
merlinmaxwell
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Default Re: Pleasantly surprised - amp modification

As always in engineering simple is the best.
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Old 19th Nov 2018, 3:05 pm   #5
bikerhifinut
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Default Re: Pleasantly surprised - amp modification

Quote:
Originally Posted by G8HQP Dave View Post
CCS as a cathode bias for output valves is theoretically a bad idea, although undeservedly popular. For PP an unbypassed CCS enforces current balance, but it also enforces fixed average current when what you actually want for low distortion is fixed quiescent current. For single-ended, a bypassed CCS can create LF phase problems in addition to fixing the wrong current. The LF phase problems can then encourage motorboating.
As I discovered.
The other phenomenon I discovered was on odd occasions the LM317 wired as Current source wouldn't start to conduct but went into a sort of sulk, usually sorted by switching off and back on again.

It seemed like a great idea to keep the EL84 cathode current steady but it also stops the valve finding its own bias point if the HT isnt quite right.

Anyway, the main thing is its rock solid now, no instability issues and still hum free which is a definite result on a SE amp with AC heaters. I did take great care on twisting and routing the heater wires and referenced to earth using a pseudo centre tap with a couple of resistors to earth.

I will have to get the sig gen out and get the amp on the scope to see what power its putting out at clip, My suspicions are its only giving a couple of watts but for most of the time that's plenty, even with insensitive 86 dB/Watt smallish standmounters. And I think it's going to be well worth another version using some decent size output transformers and a bit extra HT around the 300V mark.
I have attached files of the circuit I used, including the one with CCs and Zener diode feed to screens. Built both but in my opinion it works just as well without the semiconductors.
Oh and the PSU is a simple full wave job using UF4007 into 100uF-10H-100uF and I only used a 4.7uF to decouple the HT supply to the ECC81 but it works fine and dandy.

Its a lot simpler than the Mullard 3-3, you can argue that the Mullard probably squeezes more out but for a simple power amp it does work rather nicely.
A.
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Old 19th Nov 2018, 4:19 pm   #6
G8HQP Dave
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Default Re: Pleasantly surprised - amp modification

People often try to 'improve' a simple SE amp but few changes actually make anything better. In any case, a simple SE amp is not hi-fi whether 'improved' or not so best to keep it simple and enjoy what it does. Real improvements come from a better circuit (e.g. push-pull, significant feedback) and/or a much more expensive OPT (one of the few components where money really can buy performance).
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Old 19th Nov 2018, 10:30 pm   #7
bikerhifinut
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Default Re: Pleasantly surprised - amp modification

Agreed.

It IS surprisingly good for all that, and I rather enjoy it for what it is.
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