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!
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!
always helps to post the schematic:
An externally hosted image should be here but it was not working when we last tested it.
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.
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/s400/jeffersonmueller/9431-pi-output.gif
and a reattempt at attaching:
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/s400/jeffersonmueller/9431-pi-output.gif
and a reattempt at attaching:
Attachments
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?
What makes you think it is the PI which is clipping?
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.
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/s400/jeffersonmueller/09431-schemo-tonesuggest.gif
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/s400/jeffersonmueller/09431-schemo-tonesuggest.gif
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
folks - I owe you an apology
it turns out that the 12au7 was "shot". i had some time away from the amp to clean up my bench, and through tangents got caught up testing tubes - pulled the 12au7 and it was so poor. i had several others around and was able to find a strong one (and discard the weak ones), but I haven't had a chance yet to work with the amp. i'll update with more when I get a chance.
in the meantime, anyone out there with experience on the "floating paraphase"? the web search didn't bring me much information.
it turns out that the 12au7 was "shot". i had some time away from the amp to clean up my bench, and through tangents got caught up testing tubes - pulled the 12au7 and it was so poor. i had several others around and was able to find a strong one (and discard the weak ones), but I haven't had a chance yet to work with the amp. i'll update with more when I get a chance.
in the meantime, anyone out there with experience on the "floating paraphase"? the web search didn't bring me much information.
Merlin's site has good information on it. You can also check out AX84, there were numerous discussion threads on it. Now that you got your tubes sorted, I think you need to take a look at your gain stages...
The first stage has unusual values for a guitar amp, since they are carried over from the projector which had another transistor stage that came before it. You can change them to the more typical 100K and 820 combo a la Fender. I would also recommend that you remove all the filter after the second stage, leaving just the 0.01uF (or 0.02uF) coupling cap. Less parts = cleaner sound
These changes are fairly minor, and will not change the fact you have tons of gain, is this suppose to be a high-gain shredder amp?!
Cheers,
Jaz
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Just a paraphase
Guys, after taking a closer look at the schematic, I think it is just a simple paraphase. I highlighted the two resistors, that form the voltage divider. So the V3's second half just gets 50% of the signal from the first half.
Jaz
Guys, after taking a closer look at the schematic, I think it is just a simple paraphase. I highlighted the two resistors, that form the voltage divider. So the V3's second half just gets 50% of the signal from the first half.
An externally hosted image should be here but it was not working when we last tested it.
Jaz
Sorry, jaz you are so wrong. It is a typical floating paraphase. What you show i s no voltage divider and practicly no signal is lost.You must understand thar the Z in the junction point of the three resistors is very low. The second stage is an anode follower that ideally should have 1x gain.
Big headroom whatever type you use. More headroom than LTP and also 6dB more gain. In our amps we only use the OS(simple?) paraphase as this is what we have found sounding best. Check the GA-40 Les Paul with 12AX7 PI.
Big headroom whatever type you use. More headroom than LTP and also 6dB more gain. In our amps we only use the OS(simple?) paraphase as this is what we have found sounding best. Check the GA-40 Les Paul with 12AX7 PI.
The 390k from its own anode makes it a floating paraphase. The 12AU7 has too little gain to do this properly, which is why they have unequal resistor values.
Glad you have found the problem. I was going to suggest that this was probably a 'repair' matter rather than a 'mod' matter, given the age of the unit. Check the output stage grid and cathode voltages, just to reassure yourself that the 0.02 coupling caps have not gone leaky.
Glad you have found the problem. I was going to suggest that this was probably a 'repair' matter rather than a 'mod' matter, given the age of the unit. Check the output stage grid and cathode voltages, just to reassure yourself that the 0.02 coupling caps have not gone leaky.
thanks for the additional discussion.
this is not a shredder amp! though it has 4 gain stages, two of them are through the modest 12ay7, and only one of all four essentially sees a cathode bypass cap! the cap on the very first cathode I removed because to me it translated into "presence" and I wanted to get this amp away from "hi-fi" and toward guitarland.
however, there is plenty of gain. and now, with a "new" PI valve, it is pretty amazingly "clean". I had several minutes of playing time with it after my last post, and have to say that if I was adjusting component values to reach a mark of more headroom, but with distortion still available when boosted, then I'd consider myself to have overshot the mark! Alas, it seems I enjoy the sound of a 12au7 self-destructing!
by the way, all resistors/caps have been updated. the only thing 50yrs old was the valves! ridiculous of me not to consider updating them . . . my late night test was with a stronger 12ax7 in place as well.
there is an issue that is now brought to light by the greatly improved output - i may post a new thread about it if needed but thought I'd share it here since I have your attention: with Open Circuit (didn't have a shorting jack at the time) I find that the amp is "microphonic" anywhere I tap it. But with an instrument plugged in, this goes away entirely. I've read that 12ay7's are subject to microphony, but this doesn't seem localized. I did not flip the amp over to "chopstick" the components within - probably my next step. But as I mentioned, no microphony at all when an instrument is plugged in. Any thoughts?
this is not a shredder amp! though it has 4 gain stages, two of them are through the modest 12ay7, and only one of all four essentially sees a cathode bypass cap! the cap on the very first cathode I removed because to me it translated into "presence" and I wanted to get this amp away from "hi-fi" and toward guitarland.
however, there is plenty of gain. and now, with a "new" PI valve, it is pretty amazingly "clean". I had several minutes of playing time with it after my last post, and have to say that if I was adjusting component values to reach a mark of more headroom, but with distortion still available when boosted, then I'd consider myself to have overshot the mark! Alas, it seems I enjoy the sound of a 12au7 self-destructing!
by the way, all resistors/caps have been updated. the only thing 50yrs old was the valves! ridiculous of me not to consider updating them . . . my late night test was with a stronger 12ax7 in place as well.
there is an issue that is now brought to light by the greatly improved output - i may post a new thread about it if needed but thought I'd share it here since I have your attention: with Open Circuit (didn't have a shorting jack at the time) I find that the amp is "microphonic" anywhere I tap it. But with an instrument plugged in, this goes away entirely. I've read that 12ay7's are subject to microphony, but this doesn't seem localized. I did not flip the amp over to "chopstick" the components within - probably my next step. But as I mentioned, no microphony at all when an instrument is plugged in. Any thoughts?
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