SSS 12AU7/12BH7 PI/DCCF Driver

[David Root]

A bit off-topic to the SSSreleaSSSe thread, so I posted this separately. This is the combination HAD used on some of the 150W SSS, most notably I believe SRV's, and the Dumbleland bass amps.

I'm OK with the 12BH7 driver, but been doing some research on alternatives to the 12AU7 PI in conjunction with the DCCF 12BH7 driver.

It appears that the 12BH7 as a long tail PI is capable of a wider voltage swing ie higher headroom than the 12AU7 at lower harmonic distortion levels. I realize this may or may not be necessary or even desirable but it could be beneficial.

Another 12AU7-similar candidate might be the E80CC/6085. Some of the audio guys are reporting improved tone with the E80CC as a PI replacement for the 12AU7.

Interestingly both these tubes have longer plates than the 12AU7, which we know can be useful in a longtail PI.

Anyone have anything to contribute here? I know that it's possible HAD may not have considered the E80CC as it may not have been readily available in the US thirty years ago, but he already used the 12BH7 as a driver, so why not as a PI? Or maybe he did do so? I've never seen any documentation that he used the 12BH7 as a PI.

I also ran across an interesting tip for reducing intermodulation distortion in the long tail PI. It consists of a 150pF cap between the plate and the grid of the PI entrance stage.

[Max]

This is the combination HAD used on some of the 150W SSS, most notably I believe SRV's, and the Dumbleland bass amps.

AFAIK you'll find this combination (12AU7 phase inverter and 12BH7 driver) in most of the Dumbleland Special 150W amps for guitar (the 6267 Dumbleland an exception) and in all the Dumbleland 300SL amps for guitar, too - and not only in the Dumbleland amps for bass.

Have a great sunday!

Max

[Roe]

In case its of interest, here's what Randall Aiken writes about using this type of driver to reduce blocking distortion:

Add a DC-coupled cathode follower between the phase inverter and the grid of the output tubes, with the cathode follower cathode resistor returned to a high negative voltage, and the grid bias applied to the grid of the cathode follower. This effectively isolates the output tube grid circuit from the phase inverter and its associated AC coupling, and provides a very low impedance source for the output stage. This will prevent the output stage from going into grid clamp, and will eliminate the long time constant of the AC coupling. This method has the unfortunate side effect of requiring an extra tube and completely ruining the value of your vintage amp, so it is best used only on new designs, but is highly recommended. You will also get more power out of the output stage because it is now running in class AB2 or class A2 (the "2" suffix indicates grid current flows for a portion of the cycle).

[David Root]

Thanx Roe, I had been to Aiken's site but it was under construction and it wasn't available. I have seen most of what he says elsewhere so I knew what I was getting into, however I didn't make the leap from grid current to realizing that is AB2 operation. Duh! I should have.

That explains why it is so freakin' LOUD!