Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: SirElwood on August 12, 2010, 04:22:08 am
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Hi.
I'm building a 5F11 harvard (http://www.ampwares.com/schematics/harvard_5f10.pdf). I have no bias tap on PT (hammond 290AEX tweed deluxe/princeton reverb PT), so I build a princeton reverb style bias supply from Kevin O'Connors book the ultimate tone 2 (see attachment). However, it does not work very well in this amp. I can get only -30 volts whit VR1 and R3 removed and -24 volts with them. This is not enough to get this amp biased properly. Anode voltage on power tubes is a bit high, 350V. All other voltages are spot on with schematic (less than 5% difference). There is no other modifications than bias supply in this amp.
Any ideas? My last idea is to make this amp cathode biased.
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R1-R2 form a voltage divider....adjust that for more raw voltage for the bias system to work with. Increase R2 value for more voltage (100k, 250k?)
Being nit-picky.....Harvard is 5F10 > http://www.el34world.com/charts/Schematics/fender/HARVARD_5F10.pdf
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Get rid of R2 altogether. The BFPR bias supply doesn't have a resistor there, and it is just adding more voltage drop than you need. I would also make C1 10uF 150V - it will charge up faster - given that you are running the bias off the HT winding and you want the bias voltage ASAP when you switch the amp on. If you want more filtering, you can put a Pii filter in (after the 100k limiting resistor and reverse biased diode) using 10uF - 1k to 5k -10uF and then the voltage divider leg (with the pot and series resistor) to ground.
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R1-R2 form a voltage divider....adjust that for more raw voltage for the bias system to work with. Increase R2 value for more voltage (100k, 250k?)
Ok, I'll try this. Thanks. :smiley:
Being nit-picky.....Harvard is 5F10 > http://www.el34world.com/charts/Schematics/fender/HARVARD_5F10.pdf
:embarrassed:
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Go with the Tubeswell suggestion(s).....I'm a cathode bias guy & don't mess with fixed bias much, so my experience is limited. :smiley:
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Go with the Tubeswell suggestion(s).....I'm a cathode bias guy & don't mess with fixed bias much, so my experience is limited. :smiley:
I'm also a cathode bias guy! :smiley: I think this is only my 6th fixed bias amp in four or five years time, so basicly I have no experience on this what so ever.
I have some heavy fixed bias workout tomorrow. :grin:
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Take a look at the "improved" bias circuit shown at the bottom of this page:
http://www.el34world.com/charts/Biascircuits.htm
R1:100K in your circuit sometimes is called the "range resistor" because it establishes the range of adjustment for the bias voltage. Lowering the value from 100K would increase the bias voltage, all other things being equal.
However, you can also tweak the 47K resistor shown in Hoffman's improved circuit (lower right schematic in that link) to adjust the range. Lowering the value of that resistor drops the range and raising it increases the range (in terms of the absolute value of the bias voltage).
Hope this helps,
Chip