Question - supply dumble style bias circuit from B+?

[jlatrace]

My PT has no bias tap. Can I supply power to a D-style bias circuit (ala #124 & #102) using B+ and a dropping resistor? Or do I need to use a bias circuit similar to a JTM 45 circuit?

Thanks in advance, I have spent hours researching the issue today without resolution. The only discussion I found was regarding low verses high Z and uF values.

[ToneMerc]

Similar to a Marshall will work, see document for an minimalist example as reference.

TM

[jlatrace]

Thanks, that is what I thought. Now I have to work up a board to fit it into the chassis.

Can you explain the technical reasons that the D-style board will not work? I am trying to learn (I'm am accountant that starting building amps in 2002 and I was taught by folks like you). Hopefully it will help others with the same question in the future.

Thanks<
Les

[ToneMerc]

Thanks, that is what I thought. Now I have to work up a board to fit it into the chassis.

Can you explain the technical reasons that the D-style board will not work?

Les

Les, a jumper from one of the HV board HT junctions over to the traditional bias tap input should suffice, as it will act as your bias tap. Next change the bias resistor(higher, maybe 180-220K) accordingly.

Also, I assume the PS is not configured as a FWB or doubler correct, just straight FW?

TM

[Jana]

Doing it like a Marshall should work fine if you are FW and not FWB. As ToneMerc said, a 180K to 220K resistor should get you in range. One thing to keep in mind with this arrangement is to take your bias tap from a place that is alway hot--meaning, if you are switching the HV secondary for the standby like in a Marshall, be sure to take your bias tap from the hot side and not the switched side. That way you always have a bias voltage. If you switch the bias voltage with the standby, as did some of the early Marshalls, you have a brief period of no or low bias voltage to the power tubes while the bias circuit is charging up.

Also, if you are using a Marshall style circuit with the higher value resistor mentioned, your bias caps only need to be 10uf. They really shouldn't be larger than that or the charge time for the bias circuit gets a bit long.

[jlatrace]

I just wired it up using the Dumble swtyle bias and a 220k resistor as suggested. Thanks to everyone.