Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: acheld on September 26, 2019, 10:04:46 pm

Title: Blues Junior Bias and Voltage questions
Post by: acheld on September 26, 2019, 10:04:46 pm

Question regarding voltages and bias observations:

I built a Hoffman BJ last year, and love the tone.  I built a new cabinet/chassis this summer, and transferred the guts to the new cab.

I am using a Monotone 780 PT, and an Allen Amps TO20B OT. 

I put some spare Groove Tube EL-84's in place, checked biasing, and was amazed at how different the bias current was between the tubes.  (In honesty, my thought was:  what did I screw up by moving the build to a new chassis??).

I replaced the tubes with others in my stash, and rechecked all the voltages.  No adjustments were made between the runs with different tubes, though each run occurred on a different day.  All voltages were stable at the time of measurement.   See upload.

Is it common that EL-84's would be that far apart in their biasing requirements?   Why the differences in the B+ and plate voltages?   Any idea why Hoffman schematic shows 340V for B+, yet all other measurements that I've seen on the forum are lower?

The amp sounded great with the TubeAmpDoctor EL-84s, so I'm going to stick with them.  Very interested in the experience you guys have with these amps. 

Title: Re: Blues Junior Bias and Voltage questions
Post by: sluckey on September 26, 2019, 10:37:05 pm

Hoffman used an Allen TP24 power transformer in his build. How does that compare to the PT you used? But really... 340, 331, 329, 321 are all the same to me. Only 6% difference between highest and lowest. Don't see a problem.

Title: Re: Blues Junior Bias and Voltage questions
Post by: acheld on September 27, 2019, 09:35:33 am

Thanks.  The PT difference explains it. For comparison:

MojoTone 780
Primary: 120VAC
Secondary: 250VAC  @ 200 mA
Heater:  6.3VAC @ 2A
Bias:  21VAC
-- $60

AllenAmps TP24
Primary: 120VAC
Secondary:  365V DC @ 150mA (as best I can tell, the description is confusing)
Heater:6.3V @ 4A
Bias:  20V at 100mA.
-- $75

Mercury Magnetics FBLJR-P
Primary:  120VAC
Secondary:  262 VDC (amps not spec'd - however, they have very good customer service and will supply specs quickly IME)
6.3V. (amps not spec'd)
22V (amps not spec'd)
-- $149

Hammond 290XX
Primary:  117VAC
Secondary:  260VDC @ 170mA
6.3V @ 3A
20V @ 100mA
-- $160 on Amazon (maybe cheaper elsewhere)