Just Got My Weber Bias Rite ...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by illinimax, Dec 4, 2004.


  1. illinimax

    illinimax Gold Supporting Member

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    and measured the plate voltage on my '67 Super Reverb - 500 volts. Tried some different 5ar4 tubes and saw similar numbers. I'm a bit concerned as I see the reading should be in 460 VDC range. Any thoughts?
     
  2. illinimax

    illinimax Gold Supporting Member

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    Mine is a '67 (QD date stamp). I tried three different 5ar4 tubes and got vdc levels all around 500, the lowest 495. I went ahead and dropped a few different pairs of 6l6gcs in there and saw some wide swings in current draw as expected. The one pair of 6l6gc (winged c) had the best match current-wise but the plate voltage just slightly over 500.

    One thing I should mention is that the output transformer has been replaced. I just scooped up a real '67 SR output trannie to bring it back to stock and will bring the unit to a local tech. Perhaps the replacement OT is behind the higher readings.
     
  3. Blue Strat

    Blue Strat Member

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    500 is a bit on the high side. What's the bias current on your 6L6GCs? The higher you set the bias the lower the B+ voltage will be. I recommend somewhere in the 35 to 40mA range in you're using GCs.
     
  4. illinimax

    illinimax Gold Supporting Member

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    I'll bump the current draw up from 30ma and see what happens to the plate voltage. Thanks for your help guys!
     
  5. BWilliamson

    BWilliamson Guest

    Could be a number of things:

    PT stock? What's the datecode on the PT?

    Early silverfaces went to a 5UG4 rectifier and the PT's had a bit higher B+. Knowing Fender they could have switched design specs/suppliers at any time and not made mention of it.

    What the voltage coming out of your outlets, if that's abnormally high that could also be causing the higher voltage readings.

    I would take a guess that a tolerance of 10% was quite common between transformers at the the time and still today. Taking the reading from the schem of 460v means a swing of 46 volts either way.

    The voltages on the schem are from one amp and one point in time most likely, lots of variables could have contribulted to those readings. Don't take to much stock in it, more a target and they were throwing hand grenades.

    As long as your biasing your tubes in a safe range you should be alright. Using the bias calculator on the WeberVST BBS and 500v it show 26.6ma for 70% and 34.2 for 90%(Very HOT!). Setting it even lower than 70% wouldn't hurt if the tone is to your liking as there is no hard answer for bias settings. Use your ears and your eyes and make sure the plates are red-plating.
     
  6. Blue Strat

    Blue Strat Member

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    In addition, the clincher is that the power supplyB+ filter capacitors should be rated for at least 500V.

    500V X 40mA = 20 watts which is 67% dissipation for REAL 6L6GCs which have a max dissipation of 30 watts.
     
  7. BWilliamson

    BWilliamson Guest

    I have no illusions of real world tube data, imagine that most of these online calcs are set for "Safest" generic scenario's to cover ones butt. Just used it because it was handy, didn't notice that the post previous was yours or I wouldn't have even brought it up and defered to you.
     
  8. illinimax

    illinimax Gold Supporting Member

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    Good news. Again, thank you guys for helping me out. Setting the bias at 40ma drops the plate voltage down to 478 vdc. The voltage coming out of the wall is 118vac. The transformers aside from the OT are all original. Best of all the amp sounds great. In addition to getting a period correct OT in the next few days, I'm having the guys at Weber recone the CTS alnicos. The amp just had a cap job. Should be good to go soon!
     
  9. drbob1

    drbob1 Silver Supporting Member

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    Since the wall voltage is 8% high, your plate voltage should be 8% high as well, which on a base of 450v is an additional 36v. Your number of 478 is right on the money. If you want to take it a bit easier on the amp, use a variac to run 110v into it...
     

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