AUDIOMAN2 Posted January 15 Posted January 15 I've wondered about this for sometime and the technology behind this being a bit beyond me, I hoped someone here could explain it to me. You see, I have a very old kit version of the original Mullard 5-10 amplifier which I put together in my late teens. I'm now in my early 70's. It has two 6BQ5's (EL84's) in the output stage. Every year or two I pull it out of it's box and hook it up to a single (I only have one) Wharfedale Airedale speaker. It actually sounds very nice when being driven by a mono Acos GP20 tone arm and HGP39 cartridge which in turn is bolted to a Sugden Connoisseur BD-1 spinner. The amplifier has stopped doing what it's supposed to do so I decided to do a refurbishment. You know, caps, dodgy resistors and I also decided to give it a present of new valves. A friend said, "the output valves must be matched". This actually tripled the cost of these two valves which were already a bit exy. I asked why and he couldn't answer. I pointed out that it is a push-pull output stage and in theory the two valves seem to me to not be operation in some sort of unison like the tyres on a car (poor analogy). Can anyone, preferably in language that I might understand, explain the importance of buying said "matched" valves. Thanks and apologies for the long winded intro.
muon* Posted January 15 Posted January 15 (edited) So this is a mono amplifier? Edit: If it is a stereo amp and you were only connecting one channel to a speaker and inputing a stereo signal harm could have come to the output transformers using it like this. Edited January 15 by muon*
frankn Posted January 15 Posted January 15 (edited) Is it like this as cited in the Wiki entry? Millard 5-10 Edited January 15 by frankn
AUDIOMAN2 Posted January 15 Author Posted January 15 In response to muon*, it is a mono amplifier. I thought that might have been clear in my comments but obviously not. In response to frankn, The one in Wikipedia is a stereo version. Mine is mono. It uses 1 x EF86, 1 x 12AX7 (ECC83), 2 x 6BQ5's (EL84) and a 6CA4 (EZ81). Hence the "5-10" name; 5 valves and 10 watts. The kit was manufactured by AEGIS in Melbourne and could be purchased as a kit or fully assembled. The design was straight out of the Mullard handbook. I also happen to have a mono version of the "5-20" which is basically the same design but uses 6CA7's (EL34's) in the output stage and a beefier rectifier in the 5V4G (GZ32) which I built from scratch. 1
muon* Posted January 15 Posted January 15 I wasn't sure if you meant only one amp or only one speaker, I'm easily confused anyway
muon* Posted January 15 Posted January 15 (edited) You can use a unmatched pair, It just might be better if you have a matched pair. Edit: maybe just look for two strong ones of similar measurements on ebay. @xlr8or Edited January 15 by muon* Typo 1
LogicprObe Posted January 15 Posted January 15 Yeah, the closer the better but most PP amps are designed with at least 5% tolerance.
xlr8or Posted January 15 Posted January 15 (edited) It really depends on whether the amp is manually biased or self-biased. If manually biased (i. e, you can adjust the plate current for each tube in push-pull configuration) then mismatched power tubes can be used. If self-biased (i. e., the bias is governed by a cathode biased circuit with no option for adjustment) then using matched tubes in the push-pull configuration is far more critical. Edited January 15 by xlr8or 1 1
LogicprObe Posted January 15 Posted January 15 Yeah........I've got the original Mullard book here somewhere but it's pouring rain, I'm drunk and probably going to bed soon! Given that Arnold is not much older than me, my advice is to stop listening to audiofools and if the amp still works and you like it.... Just keep listening!
aussievintage Posted January 16 Posted January 16 It is desirable that matched valves be used, but mismatched will still function.
aussievintage Posted January 16 Posted January 16 (edited) 16 hours ago, AUDIOMAN2 said: This actually tripled the cost of these two valves which were already a bit exy. It shouldn't - see here https://www.evatco.com.au/electro-harmonix-el84-power-tubes Edited January 16 by aussievintage
trobbins Posted January 17 Posted January 17 (edited) Using 'matched' 6BQ5's gives you more chance that the harmonic distortion levels will be lower - but it is a game of chance as you likely don't have any way to measure harmonic levels, and likely won't notice any difference anyway as the amp uses global feedback. What you should do imho is measure the R18 and R19 resistance values, and the idle operating voltages across them, and closely match the cathode currents (I=V/R), as that will give the output transformer the best chance of achieving low harmonic distortion down towards the bass end (irrespective of the 6BQ5 matching situation). If you did want to make an effort to achieve the lowest harmonic distortion levels, then you'd need a spectrum analyser like a soundcard and software, and either tube swap the ECC83, or the 6BQ5's or both and look for the best outcome, or add an ac balance pot in the PI stage rail and adjust that. Edited January 17 by trobbins
AUDIOMAN2 Posted January 20 Author Posted January 20 On 15/01/2025 at 10:20 PM, muon* said: I think this is a 5-10 schematic. It sure is.
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