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FDP Forum / Amp Mods, Repairs, and Projects / Supersonic 60 Combo Bias Problem

gtto

Thailand

Sep 18th, 2016 04:50 AM   Edit   Profile   Print Topic   Search Topic

I have a Supersonic 60 Combo original version. It had been sitting unused for a few years. I wanted to bring it from the US to Thailand so I had a friend install a Hammond 290 VEX 220/240 transformer. I bought some new tubes and attempted to bias but couldn't get the bias anywhere near hot enough. Plate voltage is 435. yesterday was 425. I don't know if this has to do with the knob positions and channel choice. Today I set everything to the recommendations on the schematic. The highest I can get the bias is 35mA. According to the Weber calculator 70% would be around 48. Update. ...I have been playing for a few hours and rechecked the bias. It is now 33 and 17! Something strange is going on and it is beyond my very basic abilities. The problem is trying to find someone in this area that knows how to fix it. Damn!

blackandblue
Contributing Member
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MT, USA and Jura, FR

Sep 18th, 2016 06:36 AM   Edit   Profile   Print Topic   Search Topic

The plate voltage specified in the schematic is 479 V, so it seems the Hammond transformer is not providing high enough B+ voltage to the plates given the adjustment range of the stock potentiometer that is used to adjust the bias. Solutions could include using a different power transformer such as the Fender export versiion or substituting a bias adjust pot with greater range.

The fluctuations you are observing in bias may be due to heat causing drift in components, for example, the shunt resistors used to measure the bias. It may also signal problems with the power tubes. Even if they are new, a useful check would be to substitute a known good pair.

pdf64

UK

Sep 24th, 2016 03:12 AM   Edit   Profile   Print Topic   Search Topic

Why do you want to idle the power tubes at 70%?
See linked info, the manufacturer's guideline is 33mA cathode current at 478V HT at TP2, ie less than 16 watts.
To maintain the intended conduction angle at your lower VHT, the bias should be adjusted for a proportionately lower current, eg 435/478 x 33 = 30mA.
Strictly speaking, your HT is within the 20% voltage tolerance spec on note 9.
Yes, try new, known-good power tubes. Another cause of the drifting cathode current may be a problem with the power tube's screen grid supply.
Or the battery in your meter!

schematic

(This message was last edited by pdf64 at 05:16 AM, Sep 24th, 2016)

FDP Forum / Amp Mods, Repairs, and Projects / Supersonic 60 Combo Bias Problem




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