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7868 tubes needed for FISHER 400
- Thread starter rudpra
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aeyb701
Member
Probably a safer bet, with the 400's stock fixed bias, to go new EH-made Ting-Sol's by the matched quad. I just ordered some from TubeDepot. I'm told the pins are thicker and will spread a bit the contacts in each pin hole. Small price to pay in my view now.
I recently bought a quad of "matched" used American 7868's on eBay, at least matched in terms of transconductance (a number expressed as "mhos" - opposite of "ohm's" - some number in the thousands) which I was led to believe is a good way to match. WRONG. The current they drew at idle, with bias voltage of around (-) 18 volts, ranged from 13 to 46 milliamperes when measured at the 1/4-watt 10-ohm cathode resistor everyone says to install on each output tube as a way to both measure volts (easy conversion to mA using ohm's law) and act as a fuse should the bias go wonky and the tube red plates, kind of a core melt down in nuclear speak when the excessive current cooks an output transistor.
I digress. To find a matched set of original US tubes you'd have to see the actual current draw figure of each for the same bias, which few sellers provide. Current is the real end point you need to know before you buy, not after you get them.
Good luck.
Jon Archibald
Peterborough Ontario
I recently bought a quad of "matched" used American 7868's on eBay, at least matched in terms of transconductance (a number expressed as "mhos" - opposite of "ohm's" - some number in the thousands) which I was led to believe is a good way to match. WRONG. The current they drew at idle, with bias voltage of around (-) 18 volts, ranged from 13 to 46 milliamperes when measured at the 1/4-watt 10-ohm cathode resistor everyone says to install on each output tube as a way to both measure volts (easy conversion to mA using ohm's law) and act as a fuse should the bias go wonky and the tube red plates, kind of a core melt down in nuclear speak when the excessive current cooks an output transistor.
I digress. To find a matched set of original US tubes you'd have to see the actual current draw figure of each for the same bias, which few sellers provide. Current is the real end point you need to know before you buy, not after you get them.
Good luck.
Jon Archibald
Peterborough Ontario
If I may -- actually, you need both figures, but as you found out, one figure is no guarantee of the other. Having matched cathode current draw under quiescent current conditions is important to minimize output transformer saturation at low frequencies, and keep output tube dissipation levels in check between all four tubes. Tubes that are balanced under this condition have a good "static" balance. However, a good "dynamic" balance is just as important to minimize distortion produced in the push-pull connection. So you need both -- a good static balance which is based on the DC characteristics of a tube, and a good dynamic balance, which is based on the AC characteristics of a tube. A Gm test (the transconductance test aeyb701 mentioned) is an AC based test to aid in dynamically matching tubes.
However, as I mentioned, just because a pair of tubes has a good static balance is no guarantee that they have a good dynamic balance -- and visa versa. Good dynamic balance is no guarantee of good static balance. The better tube vendors like Jim McShane match tubes based on both criteria -- and do so at real world operating voltages. Therefore, tubes purchased from him will have both good static and dynamic balance to produce the lowest overall distortion at all frequencies and power levels. No affiliation -- just a long term satisfied customer.
Dave
However, as I mentioned, just because a pair of tubes has a good static balance is no guarantee that they have a good dynamic balance -- and visa versa. Good dynamic balance is no guarantee of good static balance. The better tube vendors like Jim McShane match tubes based on both criteria -- and do so at real world operating voltages. Therefore, tubes purchased from him will have both good static and dynamic balance to produce the lowest overall distortion at all frequencies and power levels. No affiliation -- just a long term satisfied customer.
Dave
I've never seen one. I have seen 7868 to 7591 adapters, but I don't know if those have the skinny pins. 7591 tubes seem to be less available in old stock anyway.
As for the terrible match on some old stock, I have to agree on that. I've got an NOS set of Sylvania that are just all over the map. Distortion performance is worse than it ought to be and it requires all of the adjustment range I can give them to get the idle currents to line up. I actually had to go back in and fiddle with the values on the bias adjustment board to get enough range to make it all go. To be fair though, the tubes were not sold as matched or tested in any way, it was simply sold as 4 tubes with the same markings and that is indeed what I got.
As for the terrible match on some old stock, I have to agree on that. I've got an NOS set of Sylvania that are just all over the map. Distortion performance is worse than it ought to be and it requires all of the adjustment range I can give them to get the idle currents to line up. I actually had to go back in and fiddle with the values on the bias adjustment board to get enough range to make it all go. To be fair though, the tubes were not sold as matched or tested in any way, it was simply sold as 4 tubes with the same markings and that is indeed what I got.
bhamham
Super Member
Yes, B&K made an adapter for their tube testers (TC-80 I think). It allows a magnoval tube like the EHs to go in the old stock, novar tube socket. They're hard to find but occasionally show up on ebay. Forget using a case with those adapters.
I'd follow Dave's advise and contact Jim McShane (another long term satisfied customer) to see if he has a properly matched set of old stock 7868s, or just make an individual bias board and use a set of matched 7868s for dynamic balance.
aeyb701
Member
Thanks for that information Dave. Does Jim McShane sell NOS or good used matched quads? I'll guess they're expensive, but I would be willing to consider it.
aeyb701
Member
There again is a case for individual output tube bias, such as Dave's bias-balance mod.
Jon A.
aeyb701
Member
That's a novel solution, though I think it would take a while. Dremel grinder? Just be careful of not going too far, weakening the pins or worse, like marring the integrity of the glass or plastic base. Nail file would at least offer greater control.
Jon A.
aeyb701
Member
See rudpra? Always something to learn on AK. Never have thunk it.
That still doesn't really quite make it all work right if the tubes are badly matched. You can make them idle the same, but they still won't match when making music. That Sherwood does have individual adjustments now, and I can just barely get it to balance out at idle.
The 400 does have phase inverter adjustments on it so variations in output tube Gm can be accounted for (within reason) with the right equipment.
Dave
Dave
