high cathode voltage on champ build (?)

Discussion in 'Shock Brother's DIY Amps' started by Joe Les, Nov 12, 2019.

  1. Joe Les

    Joe Les TDPRI Member

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    A couple of weeks ago I finished a 5f1 champ build but it seems to have a hot bias (13.9 watts according to Rob Robinette's bias calculator). I have been using a JJ 6V6S to handle the higher dissipation, but I would like to get it down to 12 watts so I can use a normal 6V6.

    one thing that seems kind of off is the cathode voltage. I measured it by measuring the voltage across the 470-ohm cathode resistor and got 21.15 volts. Is this higher than it should be? If this voltage was more in the 17 volt range the bias would be perfect - just under 12 watts.

    If this is a normal cathode voltage, I'll just put in a higher value resistor to get the plate dissipation down. If this is a higher than normal cathode voltage, any ideas on why it could be high?

    Also, here are all the biasing-related measurements I have:

    - plate-to-cathode voltage: 327.3 volts

    - cathode resistor value: 470 ohms

    - voltage drop across resistor/cathode voltage: 21.15 volts
     
  2. Mongo Park

    Mongo Park Tele-Holic

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    Wiser minds will know more,but champs run on the hot side normally if the schematic is followed. How does it sound?
    Cheers Ron.
     
  3. Junior Little

    Junior Little Tele-Meister

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    I'm sure others will have different ideas, but if it were my amp I'd put a "regular" 6V6 in that thing and run it.
    I'm afraid to let it be publicly known how high I have seen Champs biased (mine as well as others), but in my observations, the higher biased ones sounded the absolute best.
     
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  4. corliss1

    corliss1 Friend of Leo's

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    Does it sound good? Is the tube redplating? If the answers are yes/no, you're all set. It's normal to see them run over 100% dissipation, sometimes significantly more.
     
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  5. Corvus

    Corvus TDPRI Member

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    Although the cathode resistor is usually given as 470 ohms - a high plate voltage can make this too low. Try something higher - I've often seen 1k used to bring the dissipation down - under 12 watts (which is a maximum). Try another tube - 13.9 watts is too high and the tube won't last long.
     
  6. Joe Les

    Joe Les TDPRI Member

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    The amp sounds good, but with an EH 6V6 it red plated and with an RCA 6V6 GTA the tube got hot very fast, so I turned the amp off.

    My B+ voltages are a little high, Corvus, so maybe that’s the real issue here.
     
  7. Nickfl

    Nickfl Tele-Afflicted

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    The datasheets for 6v6gt tubes say 12 watts plate dissipation, but I've heard a lot of people say that they should be considered a 14 watt tube and lots of people seem to run them that way. YMMV. I usually bias mine up around 12W in a cathode biased amp, but apparently a lot of people go higher. 327v plate to cathode seems close enough to the original to not be an issue. What were the voltages with the non JJ tubes?
     
  8. Joe Les

    Joe Les TDPRI Member

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    for some reason, I didn't write down the plate voltage, but with the EH6V6GT, the voltage drop across the cathode resistor was a little lower, 19.8 volts
     
  9. dan40

    dan40 Tele-Afflicted

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    I can understand the desire to hit the target dissipation of 12 watts, but there are many vintage Champs out there running well over spec with today's wall voltages. Some of them can still be found with their original 6v6 even though they are running a bit hot. Your plate voltage is on the lower side compared to many these days so I wouldn't hesitate to run it with the standard 470ohm resistor. The EH tube that redplated was likely a bit of of spec and pulled an unusual amount of idle current. A different tube will most likely bias up just fine. A single ended class a amp will run the power tube very hot anyways, so don't be alarmed if it seems hotter than other amps you have used.
     
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  10. King Fan

    King Fan Friend of Leo's

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    You're hearing from the smart kids. Your bias is likely fine. If I'm seeing the right numbers, what, maybe 108%. Typical, routine, common, standard, heck, totally desirable. But do re-measure the plate-to-cathode voltage when you change tubes -- it does change.

    As Dan says, a 'got hot quick' RCA is not atypical. IIRC EH are known to have some tubes that don't hold up well. Yes, people sometimes increase the ohms on the cathode resistor, but you don't want to go too far in that direction, since it also pushes more B+ into the preamp...

    For grins, what is your B+? I'd put the RCA back in there and remeasure the P-to-C voltage and cathode R voltage drop...
     
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  11. Joe Les

    Joe Les TDPRI Member

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    Ok, that's good news!

    with the RCA 6V6GTA:

    B+1: 386
    B+2: 338
    B+3: 292

    plate voltage: 363.5
    plate-to-cathode: 345.5
    voltage drop across the cathode resistor: 18.2

    this gives me 12.6 watts / 105%, so pretty much what you said.
     
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  12. corliss1

    corliss1 Friend of Leo's

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    Wait till you do measurements on a black/silverface champ :D
     
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