Lavane
Lunatic Member
That's what I've been told. At $20 a pop the JJ's do sound like they should cost more. The gold lyons will be next.
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The GL KT77. A cure for blocked audio ducts?
- Thread starter futureman
- Start date
I got a 255A recently that came with EL34's even though it is labeled for 6L6. Can you tell me what the mod component value should be so I can check? I assume it's a cathode resistor change.
Thx,
John
Tdogzthmn
Active Member
I recently bought an EL34 amp and am now thinking about options to replace the stock Chinese EL34B tubes that came with the amp.
I've had good experiences in the past with JJ 2a3 tubes in a previous amp so I'm thinking about one of their EL34 variants or one of their related siblings (6550, KT66, KT77)
This is my first time owning and EL34 tube amp so I'm eager to figure out which tube has the most synergy in the amp and my system overall.
Other than the GL KT77, I really like the new issue Tung Sol EL34B. Solid performer.
Progrocker69
New Member
My favorite sounding, new production EL34 variant. Unfortunately, the last two pairs I got from two different, well regarded suppliers... well, one tube from each pair immediately redplated no matter where they were inserted. Quality of construction? QC? Certainly questionable.
It could be the amp design that does not allow kt77 instead of el34.
What amp is this ? Schematics ?
I tried a bunch of different EL34 type tubes, GL KT77, various 6CA7, some quite expensive ones both NOS and new production, and ended up with JJ EL34L tubes in my SE amp.
Progrocker69
New Member
It's a Yarland Chinese EL34 push pull integrated. No schematic, no paperwork. Funny thing is I'm using 3 GL KT77's I bought 4 years ago and they have about 1000 hours on them and they're fine, no redplating. 2 good ones, 2 redplates of the 4 I just bought. Socket connections are tight.
If using fixed bias KT77 req that the grid resistor is less then 250k. Anything above this increases the risk of redplating.
EL34 is much more generous with this. Stay with EL34 to be safe.
Progrocker69
New Member
Thanks Peter, so, it's likely then the tubes that aren't redplating now probably will at some time? What resistor should I substitute? Is it just one or two for cathode bias?
Read https://frank.pocnet.net/sheets/086/k/KT77.pdf for details.
If your amp uses cathode bias and tube loss power is > 28w then the grid resistor should be less then 500k If power loss in the
tube is less then 28w 1 meg could be used.
If your amp uses fixed bias then if the power loss is > 28w 250 k is needed, else 500k would do.
power loss is calculated to be plate power + g2 power ( voltage times current)
The above rules is to prevent the tubes to come inte current avalance aka redplate, breaking these rules does not mean that
every tube will redplate but it means that the risk of redplating is non-zero.
Better is to stay with EL34.
If your amp uses cathode bias and tube loss power is > 28w then the grid resistor should be less then 500k If power loss in the
tube is less then 28w 1 meg could be used.
If your amp uses fixed bias then if the power loss is > 28w 250 k is needed, else 500k would do.
power loss is calculated to be plate power + g2 power ( voltage times current)
The above rules is to prevent the tubes to come inte current avalance aka redplate, breaking these rules does not mean that
every tube will redplate but it means that the risk of redplating is non-zero.
Better is to stay with EL34.
I'm using KT77 in my home brewed circlotron style mono-blocks, 4 per chassis. The amps were originally designed to use the EL34 type, and the amps have a bias adjustment pot for each tube. I noticed that the KT77 and EL34 bias adjustments are quite different from each other.
That may be as much brand as anything. I have 3 sets of EL34, old Mullard, Shugang, and JJ plus a set of EH 6ca7. The JJ and Mullard bias very similar, the Shugang need a lot less negative grid to bias, the EH 6ca7 need a lot more negative. On paper, all a of these tubes are identical.
Even tha same brand will show up a large variation of bias needed. + - 25% is what to expect and this was the same spread observed in the golden days. That's why matching is beneficial.
somewhere I have the notes with the fine points but the EH bias with the pot nearly full travel in one direction, the Shugang full in the other, and the JJ and Mullard reasonably close to the middle. I actually have two sets of Shugang and they are very similar to each other, with a difference small enough to be easily dismissed as production tolerances.
anyway my point was more that its probably not the KT77 to EL34 change, just that tubes can be quite different in practice even when the datasheets say they ought to be identical, especially when changing manufacturer as well.
I do kind of wonder how much thats a case of things don't actually match their datasheets though. Look up a sheet for a modern production tube and quite often you just get a sheet from ~1960 thats for a tube made by a completely unrelated company that just happens to have used the same name, or it may be a modern sheet thats just a direct copy of the old one. If I had a curve tracer and infinite time and money to do it, I'd love to get my hands on a heap of various modern tubes and actually create real datasheets from them to see how they compare to what the mfg claim, and to see what sort of consistency they offer. Seems like an interesting lump of data, but one that would be tedious and expensive to obtain.
anyway my point was more that its probably not the KT77 to EL34 change, just that tubes can be quite different in practice even when the datasheets say they ought to be identical, especially when changing manufacturer as well.
I do kind of wonder how much thats a case of things don't actually match their datasheets though. Look up a sheet for a modern production tube and quite often you just get a sheet from ~1960 thats for a tube made by a completely unrelated company that just happens to have used the same name, or it may be a modern sheet thats just a direct copy of the old one. If I had a curve tracer and infinite time and money to do it, I'd love to get my hands on a heap of various modern tubes and actually create real datasheets from them to see how they compare to what the mfg claim, and to see what sort of consistency they offer. Seems like an interesting lump of data, but one that would be tedious and expensive to obtain.