John_73
pfm Member
My Mullard NOS CV492 / ECC83 arrived today from Langrex, via eBay.
The packaging isn't in great condition, though I guess with NOS means one can't expect perfection. Obviously different to the one in the advertised photo though. Was a bit more concerned about the measurements noted on the box - does this indicate the two halves within are not well matched? Or the opposite?
Secondly, the code on the value is 83K1. Not the 161 pictured in the eBay ad. Is this something to be bothered about or not?
Total cost inc. postage was £107.50 which is pretty steep (by my standards anyway) to say the least, but wanted a top quality valve for the first position in my Stereo 20.
The packaging isn't in great condition, though I guess with NOS means one can't expect perfection. Obviously different to the one in the advertised photo though. Was a bit more concerned about the measurements noted on the box - does this indicate the two halves within are not well matched? Or the opposite?
Secondly, the code on the value is 83K1. Not the 161 pictured in the eBay ad. Is this something to be bothered about or not?
Total cost inc. postage was £107.50 which is pretty steep (by my standards anyway) to say the least, but wanted a top quality valve for the first position in my Stereo 20.
Last edited:
Tony L
Administrator
Are you sure that is an 8 not a B? The latter would make perfect sense as a factory/date code (Blackburn, Nov 63), but means nothing to me as a type code. In fact I can see ‘I6’above it in your picture, what I can’t see is the next digit, which will be a 1 or a 3. Should be a nice sounding valve as it has copper rods, which from my experience seems to be worth more than any difference between an I61 and I63 type. Mullard ECC83s with nickel rods tend to sound a bit over warm and gutless to me compared to the examples of either I have with copper. No idea why, or whether is is coincidence. It all seems a bit random date-wise too with no clear point of change.
John_73
pfm Member
You're right on both counts Tony! Just had a look at it in the sunlight (we've had some!) and it's indeed a B, and there is a 161 code there too. So I guess I'm good to go after all.
The three ECC83 Mullards that came with the Stereo 20 (late '67 I believe) are all nickel rods, so will be interesting to see if there's much difference.
Is there any reason for concern with the measurements written on the valve box do you think? I thought both sides are meant to be tightly matched with military spec valves?
Tony L
Administrator
Maybe, maybe not. Valve testing is pretty variable and very often doesn’t indicate what the valve behaves like in the amp. If there is an issue you’ll hear it in position 1 of the Stereo 20. Just listen out for any channel imbalance. I suspect it will be fine and may even give a far more balanced reading after being used for some hours.
Resurrecting this old, and as a new Leak Stereo 20 owner, very interesting thread, with a couple of questions .........
I plan to introduce a fuse into the circuit as shown by Will at Radford Revival and move the fusible resistor to connect to the cathode of the rectifier. If I do this is it still relevant to add diodes onto the base of the rectifier? Also, for the fuse I've bought a fast acting 250ma 250 volt glass cartridge type but I note the circuit at that point is carrying circa 300 volts. I couldn't find a fuse of greater than 250 volts on the Farnell website. Is the fuse I've ordered up to the job or will it keep blowing?
Glenn
PS: I'm no electronics engineer, if it's not already apparent.
I plan to introduce a fuse into the circuit as shown by Will at Radford Revival and move the fusible resistor to connect to the cathode of the rectifier. If I do this is it still relevant to add diodes onto the base of the rectifier? Also, for the fuse I've bought a fast acting 250ma 250 volt glass cartridge type but I note the circuit at that point is carrying circa 300 volts. I couldn't find a fuse of greater than 250 volts on the Farnell website. Is the fuse I've ordered up to the job or will it keep blowing?
Glenn
PS: I'm no electronics engineer, if it's not already apparent.
toprepairman
pfm Member
Assuming it's a standard 20mm fuse in which case you'll be fine with that.
Why on earth would you want to fit diodes onto the base of the rectifier? Haven't I said enough times in the past the best performing and sounding ST20 is the one that is kept absolutely standard. Surely the fact that these amplifiers are still around today and working is a testament to their design and construction. Why would you want to mess with that?
I was proposing to follow the lead from page 28 of this thread from a very highly regarded contributor ie:-
.... yes to protect the GZ34. Mullards are probably bomb proof(famous last words), but the Current crop of Chinese and Russian do have a few problems. The extra diodes do help and if there is a flash over or short they stop AC on the electrolytic caps... I do it as a matter of course now. The idea has been round for a long time, I think it started on the Dynaco forum..
I don't know what the likelihood of using Russian or Chinese rectifiers were when the ST20 was designed but it's probably more likely now, and it's not supposed to change the 'voicing' of the amp at all. You could question why a protective fuse was never in the design, relying on a resister getting so hot it would desolder itself (which by all accounts didn't always work anyway leading to more extensive damage).
On a good system you will hear the introduction of those diodes, no question. If you can't get original GZ34s then there are plenty of US made equivalents (5AR4) that will work significantly better than the Chinese rectifiers. I don't necessarily have a problem with the Russian made valves as they are normally good. What I find worrying is that people on forums who are not experts in electronics seem to know more than the original designers. This is then spread about as defacto and in most cases is incorrect. I have seen so many ST20s, TL12s buggered up as a result of malpractice, with the owner then having to spend a lot of cash to get them fixed and put back to original spec.
chiily
PFM Special Builder
The diodes at the base of the rectifier valve I wouldn't see as "malpractice", a bit of insurance. I've tried valve amps with and without and I couldn't hear a difference. I'm not sure what a good system is, but mine's pretty good, not high end.
I believe the quote is from someone pretty well versed in valve amps. He was passing on advice of accepted wisdom from contributors to the UK Vintage Repair and Restoration forum, where he was a valued, knowledgeable contributor.
And I do agree, plenty of people do mangle some amps that don't deserve it.
Radford Revival
Trade: Radford Revival
Another thing to note is switching noise is unlikely as the valve rectifier switches off gradually
Well I didn't fit the diodes in the end but I wanted to replace the white wire-wound 100 ohm, 3 watt resistor (R21) in the circuit for something that looked and fitted better. When I came to de-solder it the leads had been threaded through the holes in the tags and coiled around. There was no way it was going to drop out when the solder melted in a fault situation. In trying to extract one of the leads the board tag broke consequently I ended up following Will's guide and installed a base mounted fuse, relocating the new resistor slightly to connect it directly to the rectifier.
Alex S
carbon based lifeform
My baby’s back
Mike P did a great job in the first place but I always suffered annoying ground loops. GT Graham/Tron has fettled it so it’s still safe but all buzzes are now banished. He changed the RCAs and did a few other things. My resistor prongs are fully open and it’ll just drop out. It meant replacing the big caps so I’ve lost the VW bronze but it sounds great.
Thanks Graham, and thanks again to Mike
Mike P did a great job in the first place but I always suffered annoying ground loops. GT Graham/Tron has fettled it so it’s still safe but all buzzes are now banished. He changed the RCAs and did a few other things. My resistor prongs are fully open and it’ll just drop out. It meant replacing the big caps so I’ve lost the VW bronze but it sounds great.
Thanks Graham, and thanks again to Mike