Replacing ECL86's in a Hacker AL42

[PJay1966]

Hi All

Hopefully this is a straightforward question, I am restoring a Hacker AL42 and all is going well. One of the original Mullard ECL86's is dead and needs replacing however I only have a Pinnacle ECL86 in my spares box. Is it ok to have the valves mismatched in this way (ie. 1 Mullard and 1 Pinnacle) or should I try and find an identical replacement?

[Edward Huggins]

You can only find an identical second ECL86 unless it is closely matched by Valve Testing it against the first one. Just because the names on the envelopes are different does not mean it may not work in a quite acceptable way. So do try the Mullard and the Pinnacle together. If it sounds OK - no distortion, no additional hum and with no excessive heat - they enjoy. If this was a true Hi Fi amp like the Rogers Cadet, then matching would be more important.

[PJay1966]

Thank you Edward, appreciate the advice.

[TonyDuell]

The service manual for the Hacker GP42, which also covers the AL42 (the AL42 is essentailly the GP42 without a turntable) makes no mention of the ECL86s having to be a matched pair. I would expect it to say if this was important.

In general valve amplifiers with separate cathode resistors for the 2 output valves (as he AL42 has) are tolerant of mis-matched output valves.

As for 'hi-fi' amplfiiers, the manual for my Leak Stereo 20 (normally considered to be hi-f) specifically states that the amplifier does not require the output valves to be matched for it to give the specified performance and that if one valve fails you don't have to replace the pair. Since alternatives (Mullard number, American number, GEC number) are given for all the valves, I would guess there is no reason why the valves there have to come from the same manufacturer.

[Boulevardier]

Similarly, as far as I recall, I don't think there is one mention in the whole Mullard "Circuits for Audio Amplifiers" book of matched valves. Have to admit, I'd never heard of "matching" pairs until semiconductors came along.

Mike

[GrimJosef]

As far as I know Pinnacle didn't make valves. They sourced them from other manufacturers. If you look closely you might find that your Pinnacle one has traces of the Philips/Mullard type/factory/date code on it and is in fact as 'Mullard' as your other one. Equally it could have been made in eastern Europe but still be a decent match for the Mullard.

Electronic matching is a factor in push-pull valve amp performance. Broadly speaking, matching for quiescent anode current influences the hum level and matching for the transconductance has an effect on the level of even harmonic distortion. It was considered important enough that some amps, e.g. the Williamson, had internal adjusters to correct for mismatches in both these respects.

I'm not sure that any of the classic designers ever specified that matching of manufacturers was required though. More recently there have been instances of foreign-produced valves being rebranded with 'classic' type numbers on the basis that the 6P14P-EV is 'more or less' an EL84, for example. I'd be more reluctant to mix-and-match those.

Cheers,

GJ

[PJay1966]

Thanks all I’ve learnt a lot from that post, I appreciate all of the responses.

[toprepairman]

Originally Posted by Edward Huggins

If this was a true Hi Fi amp like the Rogers Cadet, then matching would be more important.

The Rogers Cadet does not qualify as true hifi, (unlike Leak St20 which does).

Each channel uses a common cathode resistor so if you were to put in a good new valve along with a rather tired (but still working) one then there would be quite a mismatch in anode currents due to the new one hogging the bias.

This would result in hum and poorer bass performance. The bass performance is already badly compromised by the mediocre transformers.

In my own Cadet I have managed to get the performance just about up to ST20 standards.

[Edward Huggins]

My reference to the Cadet is due to its use of ECL86's and the fact that it is designed to perform well in a modest Hi Fi system. Apart from Tripletone, there were few, er, "higher-end" PP amps that ever used ECL86's.