PA designs to study which use DC CF for output stage grid drive

[mashaffer]

I am looking for some amplifier designs to study which drive the output tubes by a direct coupled cathode follower. I am interested in fixed bias, self bias, and combination bias. I am intrigued by the possibility of using such drive with cross coupled "garter" bias.

Any good designs that you can recommend for my education?

 

[rayma]

I am looking for some amplifier designs to study which drive the output tubes by a direct coupled cathode follower.

Schematic - McIntosh MC-275 Tube Amplifier @ AmpsLab.com
Lux Audio MB-3045 Power Amplifier (Nov. 1977)
811A 24W SE AMP 2000

 

[multi]

811A AMP 100 WATTS

811A/572B 100WATT AMP.

 

[artosalo]

Heathkit W6M.
It is a 6550 UL PP amplifier with fixed bias. Driver stage is 12BH7 as cathode followers.
It also has an adjustable output impedance (with ~fixed GNFB).

 

[jazbo8]

Norm Koren Class AB2:

 

[Yvesm]

Me:

 

[mashaffer]

Many thanks. Lots of good meat there.

 

[Wavebourn]

Here is mine

 

[Wavebourn]

One more version, based on my design:

 

[Wavebourn]

I myself prefer a source follower loaded on a CCS. MOSFETs have much higher transconductance that tubes. I even designed and ordered some boards for that.

A2 driving boards (for tube amps)

 

[wg_ski]

I’m looking into building something on the large side, probably with 6 6550’s, probably requiring CF drive because of the relatively heavy 15k total load to drive 3 g1’s even if capacitively coupled. Going to direct drive would probably be better, requiring individual triodes so all outputs can be individually bias trimmed. Most suitable devices are dual triodes - sort of a dilemma for driving an odd number of outputs per side. How critical is the matching between driver devices? Trying to figure out the best option - parallel both halves of a dual and use six, use half of one to drive each, with one unit being operated out of phase, or use four with half of one unit being unused?. Really depends on how critical the match is. If I went with a four dual triode option, would the dummy unit need to be DC biased to keep them all the same thermally? 2 x 10mA wouldn’t break the budget with 800 VA of power supply on tap, and another 100VA trafo for the negative supply and there is more than enough heater current to spare.

I suppose the other option is just use mosfets out of the same rail and be done with it, but I wanted to look at an all tube option before writing it off as a bad idea. Power supply and screen regulators would be mosfet anyway, because of the ease of adding current limiting.

 

[Ketje]

Most suitable devices are dual triodes - sort of a dilemma for driving an odd number of outputs per side. How critical is the matching between driver devices? Trying to figure out the best option - parallel both halves of a dual and use six, use half of one to drive each, with one unit being operated out of phase, or use four with half of one unit being unused?

Well how about the 6AC10
Mona

 

[wg_ski]

Two minor problems - first its a little woosy. It would take two Or three sections to get the current high enough to be useful. 5 mA would be the minimum bias I’d want, and 10 would be better. Second, where the hell do you get 12 pin sockets these days?

Is it at least on the dollar list?

 

[kodabmx]

Here is mine

Could the FET be replaced with a triode or pentode?

 

[Wavebourn]

Sure, but I would not do that, because here FETs have advantages, their transconductance is much higher than a best tube could have, so impact on sound is absolutely negligible.