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Post subject: Yet Another 59 Bassman Biasing Question
Posted: Sat May 19, 2012 6:31 am
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I am owning a '59 Bassman Reissue LTD, which I have purchased brand new in March 2009.
I have recently acquired JAN Phillips 6L6GWB valves to give them a try, racing that those ones are closer to the original bassman sound.

I have also owned several and currently own a JCM800 Marshall which I can set the bias by myself. Living in Turkey, we don't have professionals around to have the amps checked, so we have to do it ourselves...

Looking at the schematics (fender web site does not have my amp's schematics - and I could not find it on the web), I have had the below values with the stock tubes (GE):
B+ 444VDC
R48 ("BIAS TEST" point marked on PCB) 35mV

Following the replacement of the stock valves with 6L6WGB, I have measured below values:
B+ 435VDC
C- -53VDC
R48 ("BIAS TEST" point marked on PCB) 58mV

The amp features a built-in bias pot, and I can reduce the R48 value to 55mV minimum.

Could it be possible for those of you who know biasing and amp electronics better to help me bias the amp by providing the threshold values I should not exceed for the parameters above? Having the 58mV on R48 - I suspected I can damage the amp and shut it down after playing some riffs. The sound is definitely more organic, the way I like, with 6L6WGBs...

Thank you very much for your time, in advance. I really appreciate your assistance.

Best Regards


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Post subject: Re: Yet Another 59 Bassman Biasing Question
Posted: Sat May 19, 2012 7:58 am
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Hi gunalorsel,

I don't have the schematic, but IIRC, the test point is the reading for both output tubes, so divide the reading by two to get the reading for one tube. The reading at the test point is Cathode current and will be a few mA higher than the actual Plate current, but for this discussion, the Cathode reading is close enough. Since the reading is across a 1 ohm resistor, the mV can be read as mA.

58 mV for both tubes = 29 mV for one tube. This is 29 mA, times the Plate voltage of 435 = 12.6 watts idle. This is approx 50% of a 25 watt tube which is on the cold side, and you could bias up to 70% if you wanted. For 70% idle wattage, increase the reading at the test point to approx 80 mV, then double check the Plate voltage, it will change a little, then recalculate.

Plate volts times Plate current equals Plate idle wattage. Cathode current will be about 2.5 mA higher than the Plate current for each tube, or 5 mA difference for both tubes. Typically 50% to 70% is the accepted range for bias of nearly any tube.

_________________
---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Yet Another 59 Bassman Biasing Question
Posted: Sat May 19, 2012 10:08 am
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Dear Shimmilou,

Thank you very much for the valuable information. I was afraid I am about to make some damage, but I will now bias the amp with the provided information.

Thank you very much, again and again...

Best Regards

Gunal


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