Question about installing KT77s in a VH4

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hsnyder

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Hey guys,

Just bought a used VH4. The EL34s that were in it did not survive shipping (or at least some of them didn't), so I've ordered a set of JJ KT77s to replace them, since that's the recommendation these days. I noticed there is a switch inside the back of the amp that reads "6550" on one side and "EL34" on the other - which setting should be used for the KT77s and why? Does that switch change the output transformer primary or something?

By the way I did get the amp fired up with a set of EL34s borrowed from another amp, and man does this thing sound good! I'm very pleased with it.
 
Thanks, got them installed and biased at 35mA each. I noticed that this amp seems to run pretty hot... the front panel (faceplate) of the VH4 is warm to the touch after about 30 minutes... Normal/okay?
 
hsnyder":1eaau1os said:
Thanks, got them installed and biased at 35mA each. I noticed that this amp seems to run pretty hot... the front panel (faceplate) of the VH4 is warm to the touch after about 30 minutes... Normal/okay?
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Biasing any tube to amp is more dependent on the relationship between the specific tube and the plate voltage/cathode current of the specific amp, and the wattage created in this exchange and what level of dissipation you're comfortable with (or in accordance to manufacturers suggested levels). It's hard to say what the tubes themselves ought to be biased at without a better understanding of what the PV is given the particular amp and its inherent circuitry.

Here's one of many: http://www.regiscoyne.com/tech/bias/

Of the amps I've loaded with Genalex KT77s, I've biased them "around" 32.5mA or +/- 1.0mA
Tube dependent in the case of EL34s, I've punched some =C= brand EL34s to 34-36mA

Check the specs on this particular JJ tube, get a read from the VH4 as to PV and go with the recommendations in the aforementioned article's equation. Knowing Peter & Peter, they'll likely recommend 32mA but I'm not going to speak for them on this matter whatsoever.

I'm also not a fan of JJ, just for the record.

Unkle Mo
 
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