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What to build with two 6V6GT's

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  • #16
    You'll see 6v6 based amps with voltages ranging from around 300 to 450 or so volts. This ran the tubes far in excess of designed values but many classic tubes could take that in stride and would function that way for many years.

    Many old transformers assumed that line voltage would sag to around 110 volts by the time it got to the amplifier. When one of these transformers is run on modern line voltages around 122 volts all the voltages in the amp are up by around 10%. This can mean tubes that were already running on the ragged edge may now be pushed over it. Some tubes can take it, some can't. I have a Knight brand transformer from the 60's that now produces filament voltages well over 7 volts AC and plate voltages around AC 410 volts. Once rectified and loaded a little bit it's producing around 550 volts- way too much!

    So...sometimes old transformers aren't very useful. I usually solder an old transformer into an existing chassis and measure voltages to see how it performs with a reasonable filament and plate loads. In the end it may be easier to just build something and if the voltages aren't good replace the transformer with an off-the-shelf unit that has the correct voltages.

    I wouldn't tend to trust old tubes but that's no reason not to build something and see how it works. If plate voltages end up on the high side (and they likely will) you can always use a higher value cathode resistor and larger screen resistors to let the tubes idle a little cooler and avoid excessive screen dissipation when playing loud signals.

    Is this your first build?

    If so a 5E3 Fender is a good starting point. If you get it sounding decent you can always rip out the preamp section later, replacing it with something slightly higher gain. I will still contest that a correctly set up "low gain" amp will yield excellent high gain sounds with the proper pedal in front of it.

    I recently took apart an old integrated TV/phono/radio that had an excellent little z-mount transformer in it. It will make an excellent transformer for lots of small amps- producing around 350 volts DC at about 70ma with solid state rectifiers and 6.58 volts AC on the filaments with about 2.7 amps filament load. This is the opposite of the Knight transformer as the voltages are great for a small project.

    So...with a few tubes hooked up and an output section idling (no input signal, just tubes sitting there doing nothing) what are the plate (DC) and filament (AC) voltages?

    Also- was it a stereo or mono amp? I took apart an old Philco brand stereo that had a pair of 6bq5's but it was two single ended outputs, not one stereo. I made a stereo blackface champ out of it and the results were mediocre. Sometimes old transformers aren't as good as people would have you think.

    How is that for too much information?

    jamie

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    • #17
      Ok, looots of information! This is a mono amp.

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