What phase inverter style has the best output to linearity ratio?

[Ray Ivers]

R.G.,

There's nothing I can really add to your last post, except maybe a couple of diagrams as visual aids (JFET's to be subbed for the lower tubes):



Last month's issue of AudioXpress magazine has a tube PA design using current-mirror cathode bias - I'll try and scan/post as soon as I can.

Ray

PS: FWIW, your posts championing the enhancement of tube-amp performance using high-performance semiconductor technology never fail to make my day (not a stroke, just the truth!).

[StevieP]

Current Limiting Diodes

The typical PI cathode resistor arrangement can be replaced by a CLD.
MESA Eng used this technique in the SOB - perhaps others.
The CLD is easy and cheap to implement - essentially a JFET with gate tied to source.
Steve

[Bruce / Mission Amps]

There are some Interstage trannys at Mouser but they aren't to beefy and are actually real close to mic-input trannys used in Telephone circuits. They are also quite noisy unless moved in the exact right spot as they will pick up almost anything and amplify it. It's one of those moving wires around things but the frequency response of the Mousers aren't to good so maybe the old ones would be better if you can find one or maybe you have an old one ? I'm also with you guys on the LTP but then again Carl had some good input for the type of application your using it for.

Andy at MOJO gave me a couple prototype trannys from Heyboer to try ... but the PT-124B at AES for $25 should be good enough Kerry.

Bruce

[Ray Ivers]

Steve,

Thanks for that info - I never looked at the SOB schematic before. Wow, 93V across a 50V part (and that's at idle!) - they really are brave S.O.B.'s.

Ray

[R.G.]

Brave is one word for it. Lucky is another.

I've always disliked CLDs for no particularly good reason. Not available enough in well-specified enough ways, I guess.

With so much voltage to play with, it's easy to put a low voltage current source, like a JFET or a bipolar device, and then use an 800-1000V MOSFET cascode to stand off the voltage. The voltage may get down to under 20V, but it's hard to say it won't go to B+ on some odd combination of transients and start up or shut down. The low voltage JFET and/or bipolar lets me adjust current closely by changing a resistor.

[paulh]

Interstage trannies

I get them at Antique Electronics. I have had real good results using transformers as a phase splitter. You can't get much more linear than that.

[aletheian-alex]

My $.02

My current gigging amp is running a LTPI with a CCS in the tail. The CCS is just a simple BJT cascode with a trimmer to set the current, which is tied to a negative DC supply (about -30v IIRC). I tied in a small "error correcting" network, which is just a fre high impedance resistors from the plates with the junction of them tied into the CCS. Seemed to clean things up a bit.

I ran a bunch of simulation and then built up about a dozen different LTPI circuits and ran real world tests, and as it ended up, that was the one that won out in just about every category. The outputs are nearly identical, the PRSS is better, and it allowed me to crank the current up a bit to set the tube into a sweeter sounding op-point that I had never quite hit with a resistor in the tail. But most importantly, it SOUNDS better... transistor hatred aside.

[Amp Kat]

Maybe Not So Lucky.

The CLD may explain why so many MESA's have the switching problems. When the amp heats up it tends to switch channels or crosstalking. I've seen it in several newer amps and thought it was the optoisolators but the newer models don't use the optos it's all relays so maybe they are giving it up ?

[R.G.]

... But most importantly, it SOUNDS better... transistor hatred aside.

It is hard to give up on early hatreds, just like on early loves. Good on you for the open mind.

[aletheian-alex]

It is hard to give up on early hatreds, just like on early loves. Good on you for the open mind.

Thanx R.G. I just go with what works best.

[Arthur B.]

Since we're talking about types of PIs...

Isodyne

From http://www.bonavolta.ch/hobby/en/aud...it.htm#Isodyne:

Pros:
Very linear
Easy to adjust and stable
Low distortion
High gain
Direct coupled
High amplitude output
Low output impedance
Cons:
The tubes must be paired from a twin tube

[Ray Ivers]

Arthur,

Cool! I hadn't seen that one - direct-coupled, too!

Ray

[Zoe_N_Iain]

(Iain) And there was me commenting on 18W the other day that I hadn't seen a guitar amp with an Isodyne!

Must admit, I like the look of that PI - for some inexplicable reason. And as you say, it's direct coupled.

Zoe's Sound City L-120 is moving up the project queue slowly and we were toying with using an Isodyne in that, just to throw a curve. We want to keep the active tone controls in the preamp, of course. But a change of PI might be fun.

Don't suppose you see an obvious way to fit a variable PI balance control? We're liking the idea of being able to control the balance and, hence, the harmonics generated by the power amp to taste. What's one more knob?

[Steve Conner]

I experimented with the see-saw type of PI using a 12AT7. My implementation of it has an adjustable balance control.

http://scopeboy.com/poweramp.html

I never did figure out whether it sounded better or worse than other designs, though. This amp was designed with high-gain metal sounds in mind. I found that it sounded great at medium volumes for recording. But later when I took it out to jam with a drummer who was hitting pretty hard, I had to turn it right up, and I found that it kind of lost that heavy thumping bottom end you need for metal, and went mushy. (It was plugged into a Marshall 1960A IIRC.)

I think, like other posters have mentioned, the fine details aren't important, you just need more power to hold that metal rhythm guitar sound together at gigging volumes. Amps like the Marshall Mode4 and Randall Warhead used several hundred watts of solid-state power.

I think the most awe inspiring metal sound I ever heard was on a recording of Ken Gilbert's Big-Ass Guitar Amp system, which had (iirc) a 400W tube power amp firing through two 4x12"s fitted with EV speakers and stuffed with that acoustic blanket material they put in hi-fi speakers. I almost felt sorry for the poor drummer and bassist trapped in the room with it.

Of course, the best way to get a heavy thumping rhythm guitar sound is to have the bass player double what you're playing. ;-) A lot of good metal riffs seem to have the rhythm guitar, bass and kick drum stomping out the same thing, and you'll never get a guitar played on its own to sound as heavy and tight as this, no matter what you do to the amp.

[Zoe_N_Iain]

(Iain) I've saved that one for future reference. Cheers, bud.

The simple version we designed for a cathodyne/concertina looks like this:

http://www.music-electronics-forum.c...1&d=1154780181
varicathodyne1.jpg

Seemed to work Ok in the little test bed we built. Haven't tried it in a complete amp, yet.