Using a PreAmp tube for PushPull Bedroom Final?

[big_teee]

On another Forum a friend bought a BlackStar HT-5 Head.
It is a very fancy little head Amp.
He wanted to know about the bias on the amplifier, so I finally found a small Web Schematic.
They are using a 12BH7 Double Triode for the Output section.
They have the 2 Triodes wired in a Push Pull configuration, With a Non Adjustable Resistor Diode Cathode Bias.
You can get a reissue EH 12BH7 Tube for 12 Dollars.
Seems like a real novel and interesting way to do a BedRoom amp.
My ? is has this been around a long time and is everyone doing it, or is the a fairly new and novel Concept?
Your Thoughts?
B_T

[Steve Conner]

The first one I remember seeing is Mark Lavelle's Decimator from 1990 something, a simple self-split design.
The Decimator: Description

[Mandopicker]

A quick question...

I have a 125E laying around doing nothing and was curious if anyone had an opinion as to whether it would work well for this project.

Thanks in advance...and thanks Steve for the informative link.

[big_teee]

The first one I remember seeing is Mark Lavelle's Decimator from 1990 something, a simple self-split design.
The Decimator: Description

Very Novel and I bet it sounds good.
I don't understand the Need for the 6AU6A on the Input?
Could we use another 12AX7 or a 12AT7 instead?
With the 12xx7 Tube we could switch in another stage, WDUT?
T

[Steve Conner]

He probably just used a 6AU6 to see what would happen. No reason not to substitute another triode tube. The miniature output stage can pretty much be grafted onto the end of any amp you fancy.

I used the Decimator output stage as a reverb tank driver in one of my big amps. It makes about 0.3W with the 12AU7. I found I could pull out the PI tube, plug the speaker into the reverb send, and get a surprisingly good low-volume cranked tone.

[Gregg]

I've tried numerous designs with preamp tubes as low volume power amps and couldn't like any of them. To my years they're just not it.
Reverb drive transformers sound very thin to me, really nothing interesting.
And don't expect your speaker to produce THAT sound at half Watt. It's just not going to happen.

[jimboyogi]

I have been playing around with using twin triodes in AB1 push-pull.
With a 12BH7, 340V on the plates, 14V bias, into an 8K ohm OT, I was getting up to 3WRMS output.
With an ECC99, 460V plates, 18V bias, 6.6K OT, up to 5WRMS output.

Both sound great IMO, but don't sound their best with pure cathode bias, as they sound a little too muddy/compressed this way, especially the ECC99. Maybe it's just the slower attack of cathode bias in combination with the triode response , but hey, you might like the sound! I also found the overdrive a bit "fizzy" using pure cathode bias. I'd recommend trying fixed bias, or some combination of fixed/cathode to taste. I use combination, and really like the overdrive, nails that grunty power stage compression really well. Still bloody loud through an efficient 12" speaker!

[Steve Conner]

I guess it depends on what your idea of "that sound" is. Getting "that sound" at all volumes is a pure myth. If you ever did get it, you wouldn't recognise it because of the Fletcher Munson curves I keep banging on about. Bearing that in mind, a flea powered amp can be a useful thing to have. I've been playing mine a lot lately.

There is a school of thought that says you shouldn't use triode power tubes because they sound too dull when overdriven. It is true that they have a lower output impedance that damps the speaker more, which is a bad thing for electric guitar. And when I tested the self-split 12AU7, the clipping was notably rounded, implying not a lot of high frequency energy, and maybe a dull sound.

If you subscribe to the pentode school (Blackstar obviously don't) the smallest tube worth bothering with would be something like a 6K6, 6AQ5, EL84, or the like.

[big_teee]

Here is the Schematic of the BlackStar Power Section.
T