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In Reply to: Re: Which are the best 7868 tubes? posted by Mechans on April 15, 2007 at 04:56:29:
Once you have installed the EH in your amp you really can't go back to old stock without replacing the sockets - Eli metioned why in his post. He also mentioned the Fisher problem with grid resistors that are simply too high for the EH tube (easily fixed, but it does need fixing).I do have some NOS 7868 on hand right now, all tested and ready to go. But I guarantee if you had the EHs in there the pin fit will be too loose if you got the NOS from me... So tune up your amp to work properly with the new production tubes and you'll be set.
Follow Ups:
Jim: What a bummer I had hoped to go back to NOS one day! I have a Sherwood S5000II it doesn't seem to have a "grid resistor" issue. Are you familiar with this amp does it need to have a grid resistor change like the Fisher?
The S-5500 II uses 150K grid resistors, so you are fine there even if they drifted up 20%.But you need WELL matched tubes (NOS or current production makes no difference) since there is only one bias adjustment for all four tubes. One tube that was running too hot or such could result in an early demise of that particular tube. That may well be what happened. If the tubes you had were distributor matched then I can almost guarantee they weren't well matched.
A leaky coupling cap could be at fault too, if they are original they need to be replaced. Be sure the bias circuit is healthy too.
> > What a bummer I had hoped to go back to NOS one day! < <NOS 7868s are SCARCE and becoming SCARCER, with every passing day. Be VERY grateful New Sensor put the type back into production.
IMO, the pin size issue is minor. Without the new production, many nice units could end up being "parted out".
Check the Sherwood schematic. If the amp uses "fixed" bias and the 7868 grid leak resistors are <= 300 KOhms, you are good to go.
The grid leak resistor value limit is higher for cathode biased amps.
Eli D.
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