Fuchs Bj 21 Mk I. Anyone Ever Swap 6v6s For El 34s?

Discussion in 'Other Amps' started by mtm105, Aug 15, 2018.

  1. mtm105

    mtm105 Well-Known Member

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    I have Lucky 7. It is my go-to tone. I also have BJ 21.

    The only thing better would be to double the power amp tubes. Which, in fact, I can with the Black Jack 21.

    6V6s sounds thin, light and brittle. I feel that the Amp can do more oomph!!

    Has anyone ever swapped with EL-34s and re biased with decent results? Am I able to rebias with little knowledge??

    I just LOVE EL 34s in that circuit. LOVE. Or can I just run it with EL-34s for a minute or two without doing damage without rebiasing?

    Mr. Andy is 30 min East on Rt 80 from me. I'll gladly find an excuse to visit shop if I have to.


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  2. printer

    printer Member

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    Well the internet is not being friendly on either the plate voltage or the schematic for the amp. The 6V6's heaters take half the current of an EL34 so the power transformer will have to supply an extra 1.5A, no idea if it can do it. The 34 needs more negative bias, mind you if you do not have enough bias it will operate more towards the Class A side of things. It can probably take it as the plate can take 25W (or more, just took the Class A value) so they should not melt, whether your power transformer likes it is another matter.

    For someone who has a bit of electronic experience (like me) could record the bias voltage and plate voltage of the amp with 6V6's, pop them out and turn the bias all the way cold. Put in the EL34's and clip a volt meter on the heater supply and turn on the amp. If the supply is around 6.3V I would stick another meter on the plate and turn down the bias until I had the same plate voltage as with the 6V6's. I would double check the heater voltage. It should be safe to try the amp with a signal. As I felt more confident I would give it more signal as I kept an eye on the heater voltage and transformer temperature. The tubes might be biased a little cold but they might sound ok. More bias could be given if needed, the important thing is to keep an eye on what the power transformer is doing. It is the main concern.

    Now that is what I would do. But I would also know when something is not quite right and shut it down. Don't think anything quick would go wrong doing the above. Not encouraging a person who does not know enough to do it. But it is something that could be tried. Now let's see if someone has something bad to say about me.
     
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  3. mtm105

    mtm105 Well-Known Member

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    I know for a fact that it can be done. My question is has anyone done it?

    I've read circuit sounds better with EL-34s.
     
  4. printer

    printer Member

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    Oh. From the sound of your first post it seemed like you knew very little in how to go about it and might want to do it.

    Everything sounds better with EL34's. It is a known fact.
     
  5. mtm105

    mtm105 Well-Known Member

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    Right now the 6V6s sounds pretty good. Mr. Andy did gigs with same Amp, likely as a solicitation. Until he built MK II version.

    I look for any excuse to visit his shop. Dude I bought Amp & Cab from has a reputation for doing things with gear. He sold me same Cab, loaded with WGS Green Berets, and sold it as Green Backs. Probably better to just have Amp checked out. On a forum same dude had another/same amp in shop for months.

    I'm looking to replace speakers. Either EVLs or Creams. Not too sure. I've got an ODS I can match up with. However, I pine for WGS Invaders.
     

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