AC-15'ish amp with fixed bias

[Super_Reverb]

New member here with a question on a build.

So I have decided I'm going to build a fairly basic amp using a pair of EL-84s, two 12AX7 gain stages, and a diff amp (long tailed pair) PI. Plan is to use only bass cut and top cut type tone controls so that there is no gain suck from the typical F/M/V tone stack.

My question is have you guys built an EL-84 P-P output with fixed bias? My plan is to take unused 5VAC filament winding and rectify with FW bridge to create negative bias for output pair. Peavey Classic 30 does this with 6.3VAC winding and generates more than enough voltage. Classic 30 sound is very Vox like (in spite of the Illinois filter caps) and I like the feel of the fixed bias output.

I have built a few cathode biased amps and would like to see if a fixed bias amp suits me better.

thanks,

rob

[martin manning]

Sounds like it might be cutting it close using the 5VAC winding. Do you know what the plate and screen voltages are going to be on the EL84's? 6.3VAC will get ~8.2VDC max with a half-wave rectifier, and at 250V Va and Vg2 you'll need all of that. A 5VAC winding will only produce about 6.3VDC. A FWB will cost you another diode drop of ~0.7V. The Classic 30 schemo says bias voltage is -14V, taken from a 36VDC supply... Maybe you could use a voltage doubler on your 5VAC winding for the bias supply?

[dynaman]

Fixed bias EL84s are just fine but I don't think you need to resort to that kind of supply for the bias voltage. Like Martin said, it won't deliver the voltage you need. Take a peek at Soldano's Atomic 16 w/EL84s. Basic Marshally stuff. I've done it plenty of times with 6V6s. The bias supply could even be more simplified.

[tubeswell]

Yeah the Classic 30 PT is more like 0-240VAC @200mA for the HT winding, and 28.VAC@1A for the heater winding* but you could probably go to 2 or 3A for a cooler-running PT.

* The pre-amp heaters are run as 12V in series - i.e.: needing 36VDC at a total of 150mA for all three tubes. When you rectify 28VAC with the bridge rectifier you get 39.5VDC - but you lose about 3V through R66 and a bit in the rectifier diodes. To cross check it - the EL84s are run in series at 24VAC for a total of 760mA for all four tubes, but you are losing 4.5V through the 2 x 3R resistors in series with the EL84 heaters, so you must've started with 28.5.)

[KT66]

Just steal the bias from 1 leg of the HT winding. The attached schematic is of an 18W with fixed bias and other mods designed by Ritchie Hall. The values in the bias circuit should get you in the ballpark ( Sorry for the blurriness of it, but you can still make out what's going on i think).

[Super_Reverb]

Thanks for the insight. I did not look closely enough at the Classic 30 schem with its output/pre tube heaters in series.

The Soldano atomic 16 schems I have found are hand drawn showing a bias circuit based on a PT bias tap.

It seems like the simplest way to go is to pull off HT and divide down for my my bias inputs.

rob

[dynaman]

Thanks for the insight. I did not look closely enough at the Classic 30 schem with its output/pre tube heaters in series.

The Soldano atomic 16 schems I have found are hand drawn showing a bias circuit based on a PT bias tap.

It seems like the simplest way to go is to pull off HT and divide down for my my bias inputs.

rob

Oops, you're right. I forgot that particular schem shows an actual winding for bias voltage. Regardless, the principal is still the same. I simply use the same circuit and pull the voltage from one of the HV windings. Just add a 1W dropping resistor before or after the diode and basically do what you said.

The schem posted by KT66 is what I meant by "simplifying" the circuit. It uses one less resistor and cap than what I cited and will work just fine for your project.