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

[Macguyver]

I am wondering if there is a phase inverter out there that will stay more linear than the standard long tail pair at high outputs. The kind of music I play, and everyone I do amp stuff for is metal, which the most common requirment being a tight, huge sounding low end. I found that with any version of the long tail pair, I am sacrificing either linearity or power when I try to modify the design. My goal is to have a powamp that will stay transparent at louder volumes, and it doesn't matter to me how many tubes it takes to make it happen, is there any other design that is more up my alley?

[Ray Ivers]

M,

It seems like I'm recommending this one a lot lately, but IMO it's hard to beat the Marshall 1967 (Major) setup, for both simplicity and performance:

http://www.schematicheaven.com/marsh..._lead_200w.pdf

This uses a 12AX7 concertina phase inverter followed by a 12AU7 differential amp. A 12AU7 diff-amp will have a very large voltage swing - much more than is required for full output - so at the normal reduced swing the linearity is improved.

The problem with maintaining good driver-stage performance (which I define as "forcing the output tube grids to do exactly as the driver tells them to do") at high volumes is that the driver output swing on positive signal peaks is up around the 0-volt area, where the grid circuit impedance drops almost instantly to a few thousand ohms; IME you need both direct-coupling and a very low driver Zout to successfully fight this battle. But if your power amp, OT, and PS are all beefy, the Major setup should work just fine; in fact, the whole Major PI/PA/PS circuit (minus the NFB) would work extremely well for bass-heavy metal IMO.

Ray

[Carl / Zwengel Amps]

From your description of they style of music you're working with, linearity is the LAST thing you need to worry about. The good ole schmidt differential ala Fender/Marshall is the defacto go-to driver for this particular tone. Pretty much all the metal amps use this for a reason and as the saying goes...if it aint broke dont fix it! The devil is definetly in the details with this thing. The trick to tight huge low end is not in the the type of design but how you schedule the gain and how you curve the freqiency response of the circuit, particularly in the preamp. Small changes can make huge differences in the character of the amp. And as always, having really good iron in the OPT.

Now, if you're looking to get all your gain and distortion completely from the preamp with a very clean power section a good way to do this is with some sort of differential driver feeding a CF(AT7, AU7, or BH7) coupled to the power tube grids so you can get into AB2. And of course complete overkill on the power output...100-120 watts minimum! The down side here is that it'd be a scratch design so there's really no "canned" circuit you can just stuff in and have it work right.

-Carl

[Shea]

Now, if you're looking to get all your gain and distortion completely from the preamp with a very clean power section a good way to do this is with some sort of differential driver feeding a CF(AT7, AU7, or BH7) coupled to the power tube grids so you can get into AB2. And of course complete overkill on the power output...100-120 watts minimum! The down side here is that it'd be a scratch design so there's really no "canned" circuit you can just stuff in and have it work right.

-Carl

I was thinking an interstage transformer might be good, if he's going to have a completely clean power amp. Maybe that would work nicely with an ultralinear power section and no negative feedback loop.

Shea

[Bruce / Mission Amps]

I bought an interstage transformer to try myself but have not had time and energy to get with it...
love to read what you find.

[DirtyGeorge]

A while back there was a thread about replacing the tail and cathode resistors with a choke. The Vk would then be only the bias voltage above ground instead of raised by the tail resistor voltage drop.

Wouldn't this approach give more voltage swing, all other things being equal? So maybe this PI would drive the PA harder wiht less PI distortion.

DG

[Ray Ivers]

DG,

Sure, chokes can definitely increase your signal swing; using one in the cathode circuit will give you mainly more negative voltage swing (which might come in really handy in a low-mu triode output stage using a high B+ voltage or something similar), and using a CT plate choke (or two separate ones) in the plate circuit would greatly increase your positive swing from the quiescent point.

FWIW, using a constant-current source - either the conventional transistor stage or several constant-current diodes in series - as a diff-amp Rk will give you the extra swing of a cathode choke along with rock-solid DC conditions, no phase shift, and micro-buck expenditure; pretty cool IMO.

Ray

[Shea]

If you have a negative voltage source that can supply enough current for the driver, you can feed a negative voltage through a cathode resistor that has a resistance in the 100's of k's, and reference the grids of the pi to ground. Kind of a faux constant current source. I think it's probably easier and cheaper than using either a choke tail or a true constant current source. I tried it once, and it works well.

Shea

[Guitarist]

Shea, are you referring to a HiWatt style circuit then? Perhaps one could sub in IRF820 instead of the 1/2 tube. On this route is anyone interested, please, in erasing the feedback/presence circuit of the Hiwatt output so something along those lines may be easier to experiment with in keeping with this topic. I personally have little use for for 6KHz.

I was just reading how the Children Of Bodom guitarist uses an Ampeg SVT for his heavy detuned to C tones. "Size matters." Any ideas to improve that Major circuit? Does that 12au7 shove alot of current at the KT88s vs a CF arrangement?

[oc disorder]

similar phase inverter on Aussie Site to maj 200

Just thought you guys may be interested in an Aussie site...
Similar p/i to the Maj 200 in an extinct "Holden " + some info

http://www.ozvalveamps.elands.com/holden.htm

thats all

[Shea]

Shea, are you referring to a HiWatt style circuit then?

No, it's something that Ken Gilbert talked me through, or maybe it was Ray Ivers or RG Keen. Imagine a long tail pair. Instead of having a chain of two or three resistors running from the cathodes to ground, you use a single resistor in the 100's of k's - connect one end to the cathodes and the other to a negative voltage source. When I did this, I think I used a 330k cathode resistor and a negative voltage source that was around -430 volts (I just rectified it off the HT winding and filtered it the same way as the B+ supply). The grids are referenced to ground instead of to a positive voltage. Just run a 500k or 1meg resistor from the main input to ground, and the other grid can be connected directly to ground unless you're using negative feedback. If you are using negative feedback, then you just make a simple voltage divider from the appropriate OT tap to ground, and connect the grid to the junction of the two resistors. With this type of phase inverter, the plate load resistors should be of equal value.

Shea

[Ray Ivers]

Shea,

Was your circuit (excluding NFB if any) something like this?



With the kind of voltages you were using, I don't think you can really get any more linear than this, for these kinds of output signal swings; balance should be outstanding as well. A current source in the tail will get you similar results with less negative voltage, and a McIntosh-style bootstrap will get you more total swing, but neither would be appreciably more linear IMO.

Ray

[Shea]

That's it, Ray. The two resistors marked Rl would be referenced to the bias voltage for the output tubes instead of ground.

Shea

[Amp Kat]

Bruce where ya get that tranny ?

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.

[R.G.]

The suggestions already given are good. I missed this thread early on, or I'd have replied sooner.

The standard diffamp/Schmitt PI is going to have enough linearity and swing if it's done correctly.

With no current source in the cathode circuit, input does not get fully transmitted through the cathodes to the second stage. That's why a negative supply and big resistance is good for this. But rather than making a negative supply, use a current source, as ... Ray?... suggested. I'd use a high voltage MOSFET to ensure longevity. Something like an 800-1000V rated MOSFET. The smallest current rating will be overkill for a PI tube stage. The high impedance of the tail current source will let you use all of the voltage swing available for the tubes.

Triodes don't really "saturate". It's hard to get Vpk less than about 50V. But that leaves you a lot of volts to play with. Most power tubes are driven from cutoff to Vgk=0 with less than 70V, so you could in theory need up to 140V of swing on the plates of the PI. That gets hard by the time you go through the whole voltage budget unless you do have more voltage at the cathodes or a solid state current source.

Two things can help here. One is a current mirror load for the two PI stages. A simple design with a couple of high voltage p-channel MOSFETs or a complicated, difficult, and touchy bipolar design would get you a plate load that would let you use almost the entire B+ for output swing. However, you have to watch that, as 12A?7's are really only 300V rated devices. You could have them arcing over internally. You might have to go to higher voltage rated tubes for the PI if you allow more of the B+ to be available for the PI.

An alternate if you're not listening for and liking triode distortion in the PI is to replace the PI with solid state entirely. You can put in a JFET diffamp which uses some high voltage N-channel MOSFETs to cascode them. The JFETs do the gain, the MOSFETs do the voltage swing. That can get you almost the entire B+ for output voltage swing, and no arcing on the PI tube. This thing is going to be linear far beyond the range of the output tubes and transformer, and probably beyond the range of the tube driving it. The gain will be high, so you'll have to pad the signal driving it down a lot to stay within the few volts' range of the JFET gates. Because the SS PI is linear through a bigger range than the tubes, any possible SS distortion hides inside the nonlinearities of the tubes, and is no audible as such. If you hear and like the distortion of wide range tube swing in the PI, it will be missing because the SS stuff will not be distorting.

As already mentioned, direct drive on the output tube grids will also do a great job of providing more output power range, although it may not be perfectly clean.