AB 200 bass amp bias

[Ran Randle]

Hello,

Having issues getting the proper bias voltage and not sure the best way to build the bias circuit since PT doesn't have a bias tap winding.
I supplied the bias circuit from the HV sec. tap and tried large dropping resistors to get voltage supply to bias circuit in range but get no or very little MV reading from cathode resistor (1mV = 1mA). Will this PT be over worked with this many output tubes? Can I supply the bias with a separate transformer? I have not done that before so unsure about it.
Amp is based off of the Weber AB200 design. They don't sell this kit anymore so had to find suitable replacement transformers which the PT will not cut it current wise perhaps. Amp is passing signal but having problems getting the MV rating up 44mv or so to properly bias the matched sextet of 6550 output tubes. Plate voltage is running at 530VDC with output tubes installed.

Using the Hammond 278cx power transformer 800v sec. 535ma rating

Hammond 1650WA 280 watt output transformer 1,900ct 806ma 4-8-16 ohm 28lbs

[Jazz P Bass]

Can you draw the circuit that you came up with.

The circuit attached should be close.

What voltage do you have after the reverse biased diode (D1)?

[loudthud]

A couple of problems: 1) The primary impedance of the OT is too high for a 530V B+. You will only be able to get 160W or so. 2) The PT is only rated at 6A on the 6.3V winding. This amp needs twice that. 3) With six 50K pots in parallel to set the bias, you will need to burn 5 to 10 Watts or more to derive bias from the HV secondary.

The problem with multiple power transformers is safety. You don't want to run them all through the same fuse because a fault in one of the smaller ones might not blow the fuse and start a fire. If you run separate fuses on each transformer, what happends if the one for the bias supply opens? With no bias those KT88s will jeopardize the OT and/or PT. Things get complicated pretty fast. So now you need a B+ fuse and maybe one for each KT88.

Kevin O'Connor uses that OT in a couple of his projects. He uses a B+ over 600V and what he uses for a PT aint pretty.

[Ran Randle]

Can you draw the circuit that you came up with.

The circuit attached should be close.

What voltage do you have after the reverse biased diode (D1)?

Hi JPB, I will copy section of the bias schematic with differences noted.

Will need to check voltage after the bias diode.

The attached circuit is very close to what I built.

-Thanks

[Jazz P Bass]

Loud Thud seems to have raised some serious issues.
Best to heed his knowledge.

[Ran Randle]

Thanks very much JP and loudthud.

[Ran Randle]

A couple of problems: 1) The primary impedance of the OT is too high for a 530V B+. You will only be able to get 160W or so. 2) The PT is only rated at 6A on the 6.3V winding. This amp needs twice that. 3) With six 50K pots in parallel to set the bias, you will need to burn 5 to 10 Watts or more to derive bias from the HV secondary.

The problem with multiple power transformers is safety. You don't want to run them all through the same fuse because a fault in one of the smaller ones might not blow the fuse and start a fire. If you run separate fuses on each transformer, what happends if the one for the bias supply opens? With no bias those KT88s will jeopardize the OT and/or PT. Things get complicated pretty fast. So now you need a B+ fuse and maybe one for each KT88.

Kevin O'Connor uses that OT in a couple of his projects. He uses a B+ over 600V and what he uses for a PT aint pretty.

Would this PT be suitable?

British Audio Service, Inc. */*Trace Elliot*Transformers

Trace Elliot Power Transformer V6
Trace Elliot Power Transformer used in V6 Bass Amplifiers.
Model #: 73-TRAN-V6-M

I emailed them asking if it would be suitable and one of the reps said: "The honest answer is....I'm not sure. This guy was used for 6 x KT88 and 4 x 12AX7s. I do not have the specs and there were really never any released. However, to this day, I have not seen one failure in the field."

I'm thinking it will be suitable...