How To get Fixed Bias when you Don't Have a Bias Tap

[Bruce / Mission Amps]

Here, this does work:

[Bruce / Mission Amps]

If he used a FWB block then he did not connect the anodes of the FWB to ground and only used half the diodes.
You can just use the two or three of the axial diodes.

[Tom Phillips]

…Can I just follow the Hoffman layout as Bob says with this PT?

Yes, with the P-TM-P50 power transformer, you should use the stock Hoffman circuit. The P-TM-P50 has a center tapped high voltage secondary designed to be used with a full wave (two diode) rectifier set up. The lack of a bias tap does not require you to switch to a full wave bridge rectifier. Marshall amps typically did not have a dedicated bias tap on the power transformer secondary winding. The bias supply was tapped off one side of the high voltage secondary. That’s exactly what the Hoffman diagram shows. This makes it easy for you. I can only speculate that the power transformer in the amp your boyfriend built was originally designed for a FWB and that’s why he didn’t use the stock Hoffman arrangement.

Max – I believe that the capacitor coupled circuit that Bruce just posted solves the “shorted secondary” problem I was referring to. Basically, try as you might, if you connect two diode bridge rectifiers to a shared transformer winding without using those caps, then ground the positive output of one AND ground the negative output of the other, you will end up with the equivalent of a short across the secondary winding.

Regards,
Tom

[KevinOConnor]

Hi Guys!

I can't say I am a fan of capacitively coupled bias supplies even though conceptually they make sense. The reason I don't like them is that never seem to produce enough voltage to really control the tubes.

I prefer the old method of getting a 6V or 12V transformer and reverse-wiring it to the heaters - there is always a heater winding! The auxiliary transformer can be very small and will put out about 80-160V. It will also be much lower impedance than the capacitive supply, so you can have individual bias pots if you wish.

Bruce, how much voltage do you get with your circuit?

Have fun
Kevin O'Connor

[bob p]

I prefer the old method of getting a 6V or 12V transformer and reverse-wiring it to the heaters - there is always a heater winding! The auxiliary transformer can be very small and will put out about 80-160V. It will also be much lower impedance than the capacitive supply, so you can have individual bias pots if you wish.

Interesting. I've never heard of that trick. I guess that if you stay in the "1" operating subclasses, then additional current drain on the heaters will never be an issue.

[leadfootdriver]

Here, this does work:

I was thinking about trying this. Can I use .022 decoupling caps instead?

[KevinOConnor]

Hi Guys

The capacitive coupled bias supply is easy but not preferred. The result is a high-impedance supply that requires very large coupling caps if you wish to have individual bias pots, or if you wanted to add Power Scaling later.

See the article on our site "Raw Bias Supply Modifications"
Tube Amp Kits, Tube Amp Books, Tube Amplifiers by London Power

The operating class of the output stage does not effect heater load, so has no impact on the use of the little auxiliary transformer connection above - and vice versa.

Have fun
Kevin O'Connor

[mooreamps]

Hi Guys!



I prefer the old method of getting a 6V or 12V transformer and reverse-wiring it to the heaters - there is always a heater winding! The auxiliary transformer can be very small and will put out about 80-160V. It will also be much lower impedance than the capacitive supply, so you can have individual bias pots if you wish.



Have fun
Kevin O'Connor

Of all the different bias circuits I've seen, I think I would vote for this approach...Secondly as Kevin points out, this transformer can be very,very small given the almost no current load on this circuit... Also, given the "almost no current" load part of it, I do the Fender Style single diode half wave rectifier with a 100 uf filter cap... Works just fine and dandy enough...

-g

[loudthud]

Using a small transformer to step-up the heater supply can be disappointing. Here's a related thread:

http://music-electronics-forum.com/t4086/

[tubeswell]

Hey Max,

I think that Bob was trying to illustrate the resistive divider - as best I know Fender never used a cap type - that's why I added my comments.

Rob

Don't know if you can see this where you are Rob, but I was wondering about how you are this morning and then I saw this old thread come up and see you haven't posted here for nearly 2 years. If you are not in the land of the living anymore, then I hope move to the other side went smoothly. Otherwise if you are still around, it would be great to hear from you again. Pete

[leadfootdriver]

'Not getting this to work very well. I have no center tap, so I did the FWB with .047's between the parallel secondarys. I get little volume. I have a master volume feeding the single ended power tube, but my bias voltage would go to zero with the MV off, so I put a decoupler between the bias and MV. That fixed the bias bleed, but I still get little volume.

What's up?

[Dave H]

'Not getting this to work very well. I have no center tap, so I did the FWB with .047's between the parallel secondarys. I get little volume. I have a master volume feeding the single ended power tube, but my bias voltage would go to zero with the MV off, so I put a decoupler between the bias and MV. That fixed the bias bleed, but I still get little volume.

What's up?

It sounds like you have a low impedance bias supply connected directly to the power tube grid. If that's the case the bias supply will be shorting out the signal. Try putting a 220k resistor between the bias supply and grid.

Dave H.

[leadfootdriver]

It sounds like you have a low impedance bias supply connected directly to the power tube grid. If that's the case the bias supply will be shorting out the signal. Try putting a 220k resistor between the bias supply and grid.

Dave H.

YOU are the winner! Thank you sir.

It seems to get even louder with a 470, and a 1M resistor.

Any objections to this?

[Tom Phillips]

You did a good job of interpreting the narrative Dave.
This is a case where a posted schematic would have made the problem much easier to see.
Something to keep in mind for future troubleshooting.
Tom

[mooreamps]

YOU are the winner! Thank you sir.

It seems to get even louder with a 470, and a 1M resistor.

Any objections to this?

The value of 220K comes directly from the data sheet.. If anyone objected, it would be the power tube itself..

-g