tweed champ 5F1 - cold bias

Discussion in 'Amp Tech Center' started by Jason McG, May 25, 2020.

  1. Jason McG

    Jason McG TDPRI Member

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    hi folks,

    I recently scratch built a 5F1, using the StewMac layout, and while running the numbers on the tube bias calculator, found that maybe it could use a little hotter bias.

    I used the 275V windings on a Hammond 290AX, and all the voltages are very close to the vintage 5F1 spec. B+1 is 335 vs. target of 340, B+2 is 286 (vs. 295 target), and B+3 is 245 (vs. 250 target).

    Per attached, however the plate dissipation is 10.4 watts, and 74.3%.

    Does it make sense to drop the cathode resistor from its current value of 467R to something in the 345R range to drive dissipation up to 100%?

    Amp works fine, but thinking it could be "finer" if it was biassed a little hotter. ie. don't know what you're missing until you hear it sort-of-thing.

    Can't find any threads about a cold bias on a Champ, always seem to be too hot!

    Thanks for the help!

    image.jpg image.jpg
     
  2. jimgchord

    jimgchord Tele-Afflicted

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    I use a 300 ohm for that setup. What rectifier tube are you running?
     
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  3. Jason McG

    Jason McG TDPRI Member

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    its a tung-sol 5y3GT for now, but have a NOS coming I was going to try out as well.
     
  4. jimgchord

    jimgchord Tele-Afflicted

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    If you throw a new production sovtek in there it will get your plate voltage up around 305.
     
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  5. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    The 1Watt rating is what was spec'd for the bias resistor. I would suggest a higher rating.

    As you change the bias, the B+ will change. If you use a different rectifier it is likely the B+ and bias will change.

    I suggest you choose the rectifier you will run in this amp and parallel a 1Watt resistor with the one you have to come up with a desirable bias. Just get a few resistors that will parallel to make the values near the bias range you are interested in. Approximately, with the two resistors paralleled, you will have the equivalent of a 2Watt resistor.
     
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  6. Wally

    Wally Telefied Ad Free Member

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    You must have a power tube there with some cold operational specs....it doesn’t draw much current. One can bias these amps with a change in power tubes...if one has a number of them at hand.
     
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  7. Wally

    Wally Telefied Ad Free Member

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    Duplicate post deleted.
     
  8. King Fan

    King Fan Poster Extraordinaire

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    Along the lines of what Wally said, I’d triple check *all* the variables before I started fixing stuff. His 6V6 swaps idea for instance. Yep, I have a beautiful old Ken-Rad that seems to work fine but biases out a *lot* cooler than the rest of my small collection.

    And you might share a complete voltage table. We’ve seen amps here where the volts weren’t all going where they were supposed to. What’s your 6.3 circuit measure? Is the HV hitting 275? Is the preamp all kosher? I dunno. You do have a lower than target B+ and a nice low HV winding — maybe this is why you get a cold 5F1 when everyone else comes in hot. And in fact, some nominal B+ targets are even higher than the ones you mention. So you may well need to het up the ol' oven, but I’d just make sure before I chopped the extra wood.
     
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  9. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I've built a few Champs & Princetons with that PT and JJ tubes always end up around 340v B+ and within 1-2% of the vintage schematic voltages. I had one I had just built with the 290AX and one from a few years ago with a big ClassicTone model - popular but I can't remember which - and had them side by side, and even played both through the other speakers. The ClassicTone was around 380v B+ and at least 100%, the Hammond amp was around 75% like yours. I honestly couldn't hear enough difference to warrant changing anything. The Hammond was a little fatter sounding, maybe if you listen hard for differences. But, after that, I decided that lower voltage & cooler bias would be better for the tubes, and I couldn't hear enough difference to justify going higher.

    I'll tell you a bonus with your current setup. With mine using that PT and 275v tap, a GZ34 came in at maybe 370v B+ and completely transforms the amp into a tighter, punchier, cleaner amp and it was around 95%. So, for me, I would leave it and enjoy longer tube life and the GZ34 option. Or 5u4 with that PT. 3 safe rectifier options without changing anything.
     
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  10. Jason McG

    Jason McG TDPRI Member

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    thanks, that's what I ended up doing. I started dropping resistor values, but couldn't tell any difference, if anything it just broke up sooner, so I left it as is, and it doesn't seem to lack anything tonally. I'm going to try a GZ34, thanks for the tip!
     
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  11. Jason McG

    Jason McG TDPRI Member

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    dropped in 5AR4/GZ34, and she's happy as a clam at 91%, 12.7 watts!

    great tip, thanks!
     
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