• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

a real no-brainer question about 1/2 tubes

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cbutterworth

Member
2006-10-03 11:02 pm
I am sure that I have this right:

When a schematic shows, for example a single channel with 1/2 6SL7 as the driver and 1/2 6AS7 as the output, then the other 1/2 of the tubes could be used for the second channel in a stereo system? In other words, a stereo amp would require only 2 tubes - 1 6SL7 and 1 6AS7?

I think that the answer is "YES" but I am renowned for my blundering.

Charlie
 
A follow up question. I know you can use one of the two triodes only (say you wanted to use the right triode in the right channel and the left triode in the left channel to be able to swap and facilitate triode matching easier). I know you have to still have to heat the whole tube but what is the proper thing to do with the hookup of the second triode that isn't being used? Do you need to put stoppers on it or anything?
 
what is the proper thing to do with the hookup of the second triode that isn't being used?

The common practice is to leave the unused section unconnected. With no place for current to flow, it won't do anything. Plate, cathode, and all grids open, no connection to each other or anything else.

I have seen (rarely) designs where all elements of the unused section are grounded. I would not do this, since there is now a (remote) possibility for unintended current flow.

then the other 1/2 of the tubes could be used for the second channel in a stereo system?

Yes. I have done this and it works well. It has been stated that some compromize in channel seperation may occur when both channels are routed through the same tube. This may happen at the highest frequencies, but I have not seen it. I have found some advantages to doing this even when using several of the same tube in an amplifier, for HK breakdown issues. Say when running an SRPP on 400 volts to drive 300B's.
 
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