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Post subject: Bias pot on ’59 Bassman LTD maxed out
Posted: Fri Dec 05, 2014 9:08 am
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I’ve searched this forum (and many others) and while the topic of a maxed out bias pot comes up, I haven’t been able to find an answer specific to my situation.

I have a year old ’59 Bassman LTD with a matched pair of new production Tung-Sol 5881’s (maximum plate dissipation 25W). It’s biased to 60mV at the test point which, of course, translates to 30mA per tube. The plate voltage is running 490VDC so that’s right around 60%. No problems with this setup and, along with the 12AY7 / 12AX7 / 12AT7 in the preamp, it sounds great.

Fast forward. I decided to swap out the factory 5AR4 rectifier with a GE NOS 5A4 for more sag. (Can't leave well enough alone.) It dropped the plate voltage (as you’d expect) to 445VDC. Problem is when I tried to re-bias, I discovered that the trim pot now maxes out at 43mV.

Any insight would be appreciated.


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Post subject: Re: Bias pot on ’59 Bassman LTD maxed out
Posted: Sat Dec 06, 2014 11:54 am
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Location: Province de Québec, Canada
43 Mv not bad , you can leave it there;

http://www.webervst.com/tubes1/calcbias.htm

Best is colder ?

I can't find LTD schematic , only Bassman 59 reissue and there is no bias pot .

You can replace a resistor in bias circuit to do what you want with the pot


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Post subject: Re: Bias pot on ’59 Bassman LTD maxed out
Posted: Sat Dec 06, 2014 1:04 pm
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Thanks stratele, but the Weber site is calculating the bias in mA per tube. The test point in a Bassman LTD gives you the combined bias for the pair of tubes across a 1 ohm resistor in mV. So that 43mV at the test point translates to 21.5mA per tube. That's downright chilly! On the plus side, my power tubes will last a long time. :D


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Post subject: Re: Bias pot on ’59 Bassman LTD maxed out
Posted: Sat Dec 06, 2014 1:39 pm
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Da Bluze wrote:
Thanks stratele, but the Weber site is calculating the bias in mA per tube. The test point in a Bassman LTD gives you the combined bias for the pair of tubes across a 1 ohm resistor in mV. So that 43mV at the test point translates to 21.5mA per tube. That's downright chilly! On the plus side, my power tubes will last a long time. :D



Across 1 ohms resistor , mV = mA

60 % is 40 ma at 445 v

43 mA is little more than 60 %. This is so close to 43 mA it make not real difference .

But in theory your tubes life will last shorter at 43 .

Most of Fender amps does not plays better at more than 50 % and this save your tubes.


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Post subject: Re: Bias pot on ’59 Bassman LTD maxed out
Posted: Sun Dec 07, 2014 4:56 am
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Too many are falling for the 70% idle suggestions out there on the net. Experience proves that is nonesense.......and actually sounds worse (not to mention wears out your power tubes faster and runs your pwr tranny too hot).
If you want better sound, set idle between 50-to 60% and roll some v1 or pi tubes.


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