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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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1st Aug 2019, 6:05 pm | #1 |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
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Trying to choose good subsitute output valves.
I'm working on two amplifiers.
One is a Heathkit ultra-linear design originally using KT66 valves. I received it using 6L6GB outputs, which I'm still using, after restoration. The main question is, are they a proper sub? Secondly, I'm about to start restoring a 60 YO US Pilot stereo power amp. It still has the original Mullard-sourced EL34's. Is the 6L6GC a good sub, or should try to source four EL34's? BTW, I won't buy any more Sovtek valves. A set of matched quads weren't matched at all. Don't trust them! I will check out the net for JJ Tesla valves. As usual, Dave! |
1st Aug 2019, 6:46 pm | #2 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: Trying to chose good subsitute output valves.
What exactly are your objectives with these amplifiers? How are they biased?
Do you want "IT GOES-UP-TO-ELEVEN!!" loud, or do you think you've got 'golden ears' so can hear the difference between 0.1% and 0.01% distortion? [I've never liked 'ultra-linear' working, because it compromises maximum power-output: feeding the screen-grids from a properly-regulated/decoupled DC supply will let you get more out of your valves] Your original valves could still be good, or they could have been murdered over the years. Someone with a tube-tester can tell you if they are still good [the thing to test for is peak-emission: an EL34 should be able to give at least 150mA cathode-emission - make sure any tube-tester can provide enough current to test to this level - many tube-testers only test for slope/gain not max-emission]. |
1st Aug 2019, 6:52 pm | #3 |
Nonode
Join Date: Jun 2013
Location: Surbiton, SW London, UK.
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Re: Trying to chose good subsitute output valves.
I have used 6L6GC for KT66 in an emergency only. KT77 I have not tried.
The RETMA designation for EL34 is 6CA7. I have sourced JJ Tesla valves from a number of UK suppliers without any problems. |
1st Aug 2019, 7:22 pm | #4 |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Trying to chose good subsitute output valves.
6L6's can be subbed for both KT66's and EL34's with a few caveats. The 6L6 draws less heater current than the others so that should not be a problem but you would need to make sure that they were biased correctly and not exceeding any of the 6L6's voltage limitations (EL34's in particular are rated for higher voltages).
You say that you won't buy Sovtek due to receiving a matched set that were not evenly matched? That is no reflection on Sovtek valves, when you buy a matched set of valves the matching is done by the seller so that would be the fault of the seller, not the valve manufacturer. |
2nd Aug 2019, 12:53 am | #5 |
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Re: Trying to chose good subsitute output valves.
I would add that JJ Tesla is same Russian valves. You have been warned!!
Real EL34's will cost stupid money, BUT as already mentioned, if the origionals have not been flogged to death they may still have a few years left. They are after all, a very strong valve. For the KT66 you can use 5661/6L6/ and with new bases 807's or 5B/254 or 5B/255. 807's have top caps connected to the plate so DONT fiddle with the topcaps. 5B/254 also have topcaps, 5B/255 is the same valve without topcaps. The last three valves 807, 5B/254 and 5B/255 can be bought in very high quality military spec for just a few dollars. Bases are less than $5 in ceramic. Ceramic topcaps are also available for about a buck. Joe |
2nd Aug 2019, 7:28 am | #6 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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Re: Trying to chose good subsitute output valves.
It depends on what your HT is and what's on the screen's, a lot of OP valves have a G2/screen limit of around 300v or less, like the 6L6GB which is 270v in class AB1. If you have more than this or the max screen power is exceeded, your valves won't last as long. the 6L6GC has a rated higher G2/,max.
Is the Heathkit fixed bias? Whatever valve is in there check the bias and OP power of each valve. When sellers sell "matched valves" I've not seen one that says what they are matched to. 10%, 20%, 5%?? Matched in what sense, same emission, same Gm. I bought some matched EL34 quads, no way were they matched, so the seller sent me a load more and I matched them myself. Langrex do or did sell "Langrex premium EL34's", I've had no issue's with these, running them well past their 25w max, no idea where they were made but there are only a few valve factories left running, two in Russia or ex Soviet block AFAIK. Andy.
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2nd Aug 2019, 8:27 am | #7 | |
Pentode
Join Date: Oct 2018
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Re: Trying to chose good subsitute output valves.
Quote:
There are currently four main factories in the world producing valves, JJ Electronic, Slovakia; JSC Svetlana, Russia; Xpo-pul (formerly Reflector), Russia; Shuguang, China. There is some more information on the valves that they produce here https://www.amplifiedparts.com/tube-factories All of these companies now produce reasonable quality valves, the Chinese valves had a reputation for being unreliable in the past but the quality has improved in recent years. The main demand for current production valves comes from the major guitar amplifier manufacturers (Marshall, Fender, Vox etc), these companies buy hundreds of thousands of valves every year. They would not risk their reputations by producing unreliable products so demand a certain level of quality from their suppliers. To suggest that all current production valves are rubbish is quite frankly ridiculous. They may not be quite up to the standard of original Mullard valves but I suspect much of this is due to internet hype from the audiofools. |
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2nd Aug 2019, 9:26 am | #8 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Trying to chose good subsitute output valves.
Quote:
As far as matching goes, we can't complain about matching if we run the valves at operating points away from the test point. The nominal test point back in the day for EL34s was with 250V on both the anode and screen grid. Very few power amps run with voltages that low. Valves which were matched at that point might very well not be matched with, say, 350V on the screen and 500V on the anode, or run ultralinear with 400V or so on both electrodes. Cheers, GJ
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2nd Aug 2019, 10:03 am | #9 | |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
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Re: Trying to chose good subsitute output valves.
Quote:
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2nd Aug 2019, 12:00 pm | #10 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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Re: Trying to chose good subsitute output valves.
RFT made some good EL34, many of them appearing with other brand names on them (including Mullard and Marshall).
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2nd Aug 2019, 5:51 pm | #11 | |
Heptode
Join Date: Mar 2017
Location: Cedar Grove, Wisconsin, USA.
Posts: 823
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Re: Trying to chose good subsitute output valves.
Quote:
I'm working on these amps on and off. The Heath U/L is self biased., using a rather clever balancing circuit. The schematic is available on the US Heathkit manual website. At this point, I'm not going to work on the Pilot. I haven't turned it on in about 30 years. I have three decent tube checkers, including a Tripplet laboratory grade type. Dave, an entitled US radio and electronics collector. |
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3rd Aug 2019, 12:20 am | #12 | ||
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
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Re: Trying to chose good subsitute output valves.
Quote:
BUT, when did Slovakia become Un-Russian? Just a few years ago I think. All its technology came from the Russian occupation after 1945. I have had problems with their valves, so am wary. I only offer this information because it was asked for in a general fashion. I answered with my experience. Yes I'm an audiophool, but not when repairing amps that were reasonably well regarded in their time. Joe |
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3rd Aug 2019, 11:03 am | #13 | |
Pentode
Join Date: Oct 2018
Location: Coventry, West Midlands, UK.
Posts: 220
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Re: Trying to chose good subsitute output valves.
Quote:
https://en.wikipedia.org/wiki/Slovakia https://en.wikipedia.org/wiki/Czechoslovakia |
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3rd Aug 2019, 11:28 am | #14 |
Moderator
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Re: Trying to chose good subsitute output valves.
Sufficiently Russian for the Russians to invade the country in 1968 when the Czech government decided to stop toeing the Russian line.
This isn't a forum for political discussions though.
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3rd Aug 2019, 11:36 am | #15 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
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Re: Trying to chose good subsitute output valves.
I believe JJ Electronic inherited most of its technology from the Tesla company which, apparently, was the name given to the earlier (founded 1921) Elektra company when the latter was nationalised in 1946.
It has been suggested https://jacmusic.com/tesla/tesla-jj.html that if there are shortcomings with JJ valves then it's because they aren't as good as the earlier Tesla ones, not because they're worse than them. Some of the Teslas have very strong reputations. I have to say that my experience of the JJ GZ34's, EF806S's and some ECC83 variants has been fine. I've never really thrashed them though. Cheers, GJ
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5th Aug 2019, 11:09 am | #16 |
Heptode
Join Date: Jan 2006
Location: Todmorden, West Yorkshire, UK.
Posts: 870
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Re: Trying to chose good subsitute output valves.
What's the difference between not being as good as and being worse than?
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5th Aug 2019, 11:40 am | #17 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Trying to chose good subsitute output valves.
Good point. My post was confused.
I understood Joe to be suggesting that JJ valves couldn't be good because they came from a Russian (Tesla) background, and even if their standards have climbed since then the Russian base level was so low that they still wouldn't be great. Paul pointed out that Tesla wasn't a Russian company and I added that, for what it's worth, Tesla pre-dated Czechoslovakia's move behind the iron curtain. I was also trying to say that in fact old-stock Tesla valves are not regarded as poor. They're actually rather desirable, and if there is anything below-par about JJ valves (personally I've not seen any sign that there is) then that would be because their standards had fallen relative to the earlier Tesla ones. Cheers, GJ
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5th Aug 2019, 1:25 pm | #18 | |
Nonode
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Location: Bognor Regis, West Sussex, UK.
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Re: Trying to chose good subsitute output valves.
Quote:
Peter |
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5th Aug 2019, 2:56 pm | #19 |
Hexode
Join Date: Aug 2017
Location: Taunton, Somerset, UK.
Posts: 318
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Re: Trying to choose good subsitute output valves.
Disregarding “matched” nomenclature derived from tested values mainly because I do not have access to a tester I am reliant upon what my ears tell me is acceptable or otherwise. I had recently built a Dynakit ST70 and purchased “matched” JJ EL34Ls of Slovak origin. Here, I observed a really nice sound but wondered what a supposed slightly more expensive quad set may sound like; enter a set of “matched” Tung Sol EL34ls of Russian origin.
To all intents and purposes there is very little difference between the two manufacturers. If anything the Russian items provide slightly less base which is what I prefer. |
5th Aug 2019, 3:42 pm | #20 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: Trying to choose good subsitute output valves.
If your EL34s are grossly mismatched for anode current then the first audible sign might be an excess of 100Hz hum from the speaker in the absence of music. There could also be increased second harmonic distortion, but this is quite difficult to hear until it reaches a relatively high level.
Any mismatch in transconductance will, to a large extent, be corrected by the 20dB of global feedback which the ST70 has. There is also, in effect, additional local feedback in the output stage arising from its ultralinear configuration. The effect, intended by the designer, of all this feedback is to reduce the impact of individual valves' characteristic variations on the amp's performance to the greatest extent possible. If you had been able to hear a difference between valves then David Hafler would have been disappointed. The spec on the frequency response is pretty tight - +/-0.5dB from 10Hz to 40kHz at low signal levels and +/-1dB from 20Hz to 20kHz at full power. If you really can hear a fall-off in the bass with some valves but not others then I wonder if the amp might actually have a fault ? Cheers, GJ
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