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Old 19th May 2007, 10:48 PM   #1
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Location: East central MN
Default Trouble with a paraphase PI

Hey, I recently converted a Bogen K15 into what I thought would be a fairly nice guitar amp. I redid the preamp but the PI was a paraphase type, I'm not too familiar with them and there's not a lot of info out there about them so I just kind of left it the ay it was.

Anyway, it didn't turn out as good as I thought, it sounds great clean but when you play "on the edge" of breakup, you can hear some nasty distortion mixed in. I ran through it with my scope and I saw that the source of the nastiness was indeed the phase splitter stage. There are two preamp stages before it, after the first one there's no breakup at all (as expected) and after the second there's a little smooth, asymmetrical clipping, also as expected. However, when I checked the signal coming from the "first" half of the PI, I noticed that it would reach a certain pont and clip hard, very SS like, and only one side of the wave. This signal goes to one output tube and also the grid of the other half of the PI. When it gets here it "flips" it (as it should) and does the same thing to it, so now both sides of the wave are clipping hard, and this goes to the other output tube. This, as you can imagine, sounds like a mess.

This is the original K15 schematic by the way. Don't bother looking at the preamp, just the two halves of V3 onward.

So waht I want to know is, is this "SS clipping" inherent with this style of phase splitter? I tried a few different tubes (a couple Sylvanias and a Mullard) and they all did the same thing. Can I tweak some values to get it to sound better or am I going to have to switch to a LTP design? I really don't want to do that because this amp is a horribly laid-out mess inside (not my fault, it was a mess from the factory) and I'd rather not go to that extent with this amp unless it's 100% neccesary. I suppose there's a reason why everyone stopped using this design in the 50's though.

The original components are still in the PI stage. What exactly constitutes a "leaky" cap? I checked the coupling caps and they were only letting a few mV get through, is that enough to cause a problem?

Thanks in advance.

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Old 19th May 2007, 11:41 PM   #2
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Join Date: Apr 2004
Location: Monroe Township, NJ

Letting a few mV. thru is leaking, not badly, but still leaking. Replace the coupling caps. with Orange Drops.

The paraphase splitter is out of favor. Things like that usually happen for a reason. What you observe is likely 1 of a number of reasons.

The big advantage of paraphase topology is gain. Less gain is available when differential splitter topology is used. Let's see, if a 12AT7 differential splitter replaces the 12AX7 paraphase splitter, some of the lost gain could be made up by buffering the splitter triodes with DC coupled MOSFET source followers as described in MOSFET Follies. An advantage of differential splitter topology is the very convienent point the non-inverting triode's grid provides for the application of loop NFB. You get a short loop around only 2 stages.

BTW, 200-220 V. on the 'T7 plates and IB = 3.5 mA. sounds good.
Eli D.
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Old 20th May 2007, 12:26 AM   #3
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Location: Finger Lakes, NY
No, that kind of clipping is not a characteristic of paraphase inverters. Most likely it is a consequence of NFB. Just for kicks, you might try changing the NFB connection from the 70V tap to the 25V or 16 ohm tap. The wire is probably soldered right to the speaker terminal on the back of the chassis, which might not make it easy to to get to...

I have used the paraphase inverter and found that it can sound quite good, both for creating music and recreating it. My suspicion is that it fell out of favor because (a) it does not look as elegant on paper as a concertina or LTP, and (b) because it makes it more difficult to apply NFB without squirrely behavior. Anyway, I once built a guitar amp using a paraphase inverter and I thought it sounded very good. It had a nice transition from clean to overdrive that was easy to control and smooth. Just the opposite of what you describe.

As for the coupling caps, a few mV is normal and is the result of a minuscule amount of grid current. Are the grids a few mV positive or negative? Positive means leaky caps, negative means grid current which is harmless.

-- Dave
January 20, 2009: Bush's last day
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Old 20th May 2007, 01:24 AM   #4
PRNDL is offline PRNDL  United States
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Did you try replacing the 12AX7 with a lower gain 12AU7?

Did you measure the voltages at pin 6, which should be 155-160V.
That should leave enough swing before clipping at 300 or 1.5.
(Is the clipping occurring at the top or bottom?)

What did you do for the preamp stage?
The 12AX7 and 6AV6 may have had enough gain for guitar.

Also check pin 8 for 1.5V. If it's off, check C12.
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Old 20th May 2007, 01:28 AM   #5
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I agree that a paraphase splitter could be made to sound OK in a guitar amp, but I'm surprised to see 12AX7 driving 6L6s. I'd expect the splitter to be subject to overload. I think you might be better off with a more rugged tube like 12AT7 or even the (much maligned) 12au7. You'd need to reduce the splitter's plate load resistors from 220k to around 47k and reduce its cathode resistors from 2.2k to around 470Щ. The gain would be reduced singificantly. Alternatively, Eli's idea of buffering the 12AX7 splitter with MOSFETs (e.g. IRF820) could work well, save you from losing gain and give better overdrive behavior of the 6L6s.
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Old 20th May 2007, 02:45 AM   #6
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I'm surprised they used such high plate resistors on the 12AX7 - with 220K load and 470K grid on the next stage. Going to 100K (with cathode resistors halved as well) would give a little more drive.

As already suggested, the feedback may be the problem - remove it (150K resistor) and see how it sounds - should reduce the abrupt transition to clipping.
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Old 20th May 2007, 03:27 AM   #7
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Eli, I'll replace 'em with orange drops then. I don't think I have any laying around of that value but I'll grab some soon. I know the orange drops I used in the preamp don't leak at all.

Dave, I actually wired an SPST switch in the NFB loop, one of the first mods I did actually. I never thought to shut the NFB off when I was testing it on the bench though, I typically keep it on unless I really want to crank it and want the extra distortion. I'll shut it off and check again but if I remember correctly I did hear pretty much the same thing when it was turned off. The voltage I was getting was positive. I guess it's for sure that the old ones need to go then.

PRNDL, I remember checking voltages but I can't remember off hand what pin 6 was. I think it was somewhere in that area though. I'll go check again. Clipping is happening at the top. I'll try a 12AU7 in there tomorrow and see what happens, I wish I had a 12AT7 to try as well. I know the preamp is fine because it's pretty much identical to the one I used in my CHB-10A. Kind of similar to a 5D2 princeton but with no tone control. That preamp driving a single 7868 is the greatest guitar amp I have ever played through. I used the same preamp in this amp hoping to get the same results but the darned PI is in the way

Ray, I believe there was some Fender models that had a 12AX7 driving 6L6s early on. I know the 5D5 Pro did for sure. My amp actually has Tung-Sol 5881s in it right now, once they start to break up it sounds great, but I can still kind of hear the phase splitter's nastiness even when it's dimed.

Tom, I was actually thinking of trying to duplicate the one out of the 5D5 actually. It uses 100k I believe.

Thanks for the ideas guys, I'll check out some of this stuff when I get a chance tomorrow.

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Old 20th May 2007, 01:39 PM   #8
PRNDL is offline PRNDL  United States
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I agree with Ray.

The problem is that there's tons of gain and it's all hitting the phase inverter. If you check the 5D5, the resistors are 100K and 1.5K for the pair. Also, there's only one gain stage from a 12AY7.

Also check the Bandmaster AB763 schematic, since it's similar to the 5D5 with an extra gain stage, which would be the 6AV6 in the K15.
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Old 20th May 2007, 03:36 PM   #9
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I guess I'll stick to SE amps from now on .

Anyway, I checked pin 6 and it was 158VDC, just fine. Pin 8 had 1.5, as expected.

I know the preamp is driving the PI pretty hard. To me it seems like it should be clipping smoothly though, instead of the harsh abrubt clipping that it's doing. If "too much gain" really is the problem, then I will probably either switch to an LTP design or abandon this project altogether and get started building something useful. I'm just not uderstanding why it's clipping the way it is, I don't mind some distortion coming from the PI but it can't sound like this - I've never come across this before and I've also never messed with a paraphase style before so I just kind of assumed the general design must be the reason why it sounds bad.

I'll try a 12AU7 in there the way it is now and see what happens.

Forgot to mention, I shut the NFB off and I do indeed get the same thing happening.
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Old 20th May 2007, 04:16 PM   #10
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Tried a 12AU7 for V3, same result. I also stuck a 12AU7 in V1 just to reduce the overall gain of the preamp, also no change. This has got to be a result of the paraphase design, I've tried five different tubes and all the voltages are in spec - I just don't get it.
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