paraphase

[andrew]

Has anyone experimented with a paraphase PI? I used the values from the paraphase circuit on Valve Wizard's site and patched it in a 2-EL-34 Rocket project (an amp in constant flux) It had a cool grubby feel. I don't know if the values were the best for it - the Supro values are different. I tried an Express preamp with it and it sounded vulgar. Next project up is a 2-EL-34 Spitfire with the paraphase. Thanks and have a safe holiday weekend!

[Phil_S]

I tried it. It was a very bad match for the amp I was building. I was powering a pair of 6SN7's. Apparently, triodes don't need to be pushed. The paraphase gives plenty of voltage swing and gain.

I was building an amp intended for clean headroom and reasoned poorly: Ampeg used the floating paraphase; Ampeg had a reputation for great clean tone; therefore, it is the appropriate PI. Of course Ampeg was driving high power pentodes. Take a look at the B-15 circuit.

BTW, I ended up using a concertina.

[Andy Le Blanc]

Paraphase amps are fun, and have a great tone all to themselves. If you
keep them simple, and don't try to over complicate the circuit. Most older amps
used various forms at on point or another. They have a lot of gain but don't
play well with feedback. And, break up colorfully before they distort, never
quite clean. keep it to a one gain stage pre and use simple high impedance
tone volume. You have a selection to choose from and each kind has it own tone.

[KT66]

I have been experimenting with the Supro paraphase in a Thunderbolt clone I built into an old PA. I put a pot in to adjust how much signal goes to the second half of the PI. When it is all the way down the circuit is stock, as you turn it up it sends more signal to the second half and adds more raunch. Using a 25K pot all the way up, the PI is pretty unbalanced and the signal coming out of the second half of the PI is around 4 times the signal going to the other output tube, The output waveform from the OT doesn't show this much severity though. You could use a more conservative values of a 10K pot and 260K for R35 and it still makes a good "raunch" control. The schematic is posted below. I have a cool idea for a master volume for this type of PI, but I havn't tested it yet, I'll post the results here when I do.

[Andy Le Blanc]

AC and DC balance can make a huge difference with the fixed paraphase.
A pot. in the plate circuit (DC) and one as demonstrated (AC). That this
circuits drawback, and why the trend is any from that type. Bandwidth is
determined by balance which needs to be maintained with changes of
voltage condition and tube drift. But they have a nice tone , great for house rockin blues.