Odd-ball PI circuit identification request, and headroom question - diyAudio
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Old 28th June 2012, 05:56 PM   #1
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Default Odd-ball PI circuit identification request, and headroom question

Going in the guitar amp direction with this one -

This schematic is modified from an old Bell & Howell projector amp for model 302 - PP 6v6's with a 12AU7 PI. Can anyone identify the PI circuit? It appears closest to "floating paraphase" to me.

My only complaint with this circuit is that I'd like more clean headroom/volume. Driving it with a guitar, I'm getting clipping at about 1/3 of the way up on the volume.

The clipping/distortion itself is pretty musical to me - that keeps me from switching to LTP or some other type of PI. But I'm looking to get a more "drummer friendly" volume from the amp without being slaved to a ton of crunch - idealy I would have a workable "clean" volume and then hit a boost pedal to push it into saturation.

I don't have all the specs of actual voltages and such to provide, but I did send a sine wave at 1v rms through the amp and found that the PI is distorting well before any Preamp stages - I don't even know that the preamp reaches saturation/clipping.

Basically, I really love to tone of this amp, but want more range of use and volume.

What can I change in the PI circuit to get more or to reduce clipping?

Thanks for any input!
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Old 28th June 2012, 05:58 PM   #2
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always helps to post the schematic:
Click the image to open in full size.
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Old 28th June 2012, 06:20 PM   #3
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Quote:
Originally Posted by brilliantblue View Post
always helps to post the schematic:
Click the image to open in full size.
Something about the schematic is not downloadable. Can't see it, just freezes.
Generally, when you reach the limit of available voltage, the amp clips. More headroom = more voltage.
So then, some stage in this amp is running out of available voltage, may be the PI.
More available plate voltage = more headroom before clipping.
Pictures? or a different schematic link?
The idea being that the preamp and PI remain clean, up until the point where the output tubes start to clip. That is the foundation of the Marshall Plexi / super lead amplifiers. The preamp and PI always clean...distortion comes from the output tubes.
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Old 28th June 2012, 08:40 PM   #4
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Default different schematic link

Hey Soundguruman - thanks for the attempt - it could be diyAudio, as I can see the schematic from here (a different computer than the original post) and diyAudio is going super-slow for me right now.

Anyway, a photobucket link: http://i1049.photobucket.com/albums/...-pi-output.gif

and a reattempt at attaching:
Attached Images
File Type: gif 9431-pi-output.gif (53.4 KB, 104 views)
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Old 28th June 2012, 09:09 PM   #5
DF96 is offline DF96  England
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Looks like a floating paraphase. Essentially one triode acts as a normal voltage amplifier, and the second one acts as a unity gain inverter - like the inverting opamp configuration. Sometimes they share a small cathode resistor, but not in this one.

What makes you think it is the PI which is clipping?
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Old 28th June 2012, 10:17 PM   #6
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"what makes you think it is the PI that is clipping"

verified on the 'scope - sine wave at 1v rms was still clearly a sine wave at all preamp stages, but distorted at the grid of the output tubes - just after the PI
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Old 28th June 2012, 10:43 PM   #7
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Quote:
Originally Posted by brilliantblue View Post
Hey Soundguruman - thanks for the attempt - it could be diyAudio, as I can see the schematic from here (a different computer than the original post) and diyAudio is going super-slow for me right now.

Anyway, a photobucket link: http://i1049.photobucket.com/albums/...-pi-output.gif

and a reattempt at attaching:
We really need to see the entire amplifier in the photobucket...BUT
You see the 10K resistor between the screen grid supply and the PI power supply?
That is the dropping resistor that can be decreased, causing the PI supply to go higher in constant voltage...causing the available headroom to be realized.
You can make the swing like, 325 volts, even higher
However, when you increase the voltage you can only increase it so much before the cathode (of any stage) needs re-biasing...
Checking each stage with the o scope, you "could"see that the clipping of the stage is equal in both positive and negative voltage swing of the sine wave. "could" Rebias each cathode to accommodate a higher plate voltage, when you get it up there high enough.
The result being that the sine wave from the PI remains clean up until the output tubes can be over-driven- then you have reached the limit
of B+ voltage available.
Also, there is only so much headroom in the power amp, that won't be increased.
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Old 28th June 2012, 11:17 PM   #8
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Thanks for the suggestions. I thought of lessening the 10k, and of compensating to keep the voltage "downstream" the same by raising the next resistor by the same amount so that I don't have to rebias other stages. I guess I just wondered if there was some obvious other issue that could be tweaked.

I saw "floating paraphase" as an possible description of the circuit, but each example I've found online had an additional stage of 1m resistors between PI and output. that's what struck me as odd-ball about this one. however, i've still a lot to learn.

here's the complete schematic, though it has an error where there is a bridged resistor (actually the output screen resistor) and also ignore the red values in the tone stack, I went with standard bax values for now - oh and the tone stack is receiving signal through a .05 coupling cap instead of either value mentioned. voltages are fairly accurate. i plan to raise the plate voltage on the output section to at least 350 but don't yet have the appropriate cathode resistor to make it safe.

http://i1049.photobucket.com/albums/...onesuggest.gif
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Old 28th June 2012, 11:35 PM   #9
jjman is offline jjman  United States
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If the grids of the output tubes are clipping on the scope on the top of their voltage waves, it may be the output tubes clipping. I think it's called "clamping." I was fooled by it in a 6v6 Fender amp of mine although it's fixed bias, not cathode biased.
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Old 28th June 2012, 11:41 PM   #10
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Quote:
Originally Posted by brilliantblue View Post
"what makes you think it is the PI that is clipping"

verified on the 'scope - sine wave at 1v rms was still clearly a sine wave at all preamp stages, but distorted at the grid of the output tubes - just after the PI
This is actually the normal condition for clipping in a well designed amp. The output tubes' grids conduct above about 0 volts grid-to-cathode, clipping the preceding stage. An interesting test is to pull out the output tubes and see if a larger voltage can then be made at their grid pins.

All good fortune,
Chris
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