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Old 29th June 2012, 01:45 AM   #11
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thanks JJ and Chris - those are both good insights into matters I'm not familiar with. I'll investigate further . . .
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Old 29th June 2012, 04:04 AM   #12
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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.
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Old 29th June 2012, 04:49 AM   #13
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Quote:
Originally Posted by brilliantblue View Post
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.
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

Last edited by jazbo8; 29th June 2012 at 05:13 AM. Reason: Add link.
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Old 29th June 2012, 05:35 AM   #14
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Default 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.

Click the image to open in full size.

Jaz
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Old 29th June 2012, 06:23 AM   #15
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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.
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Old 29th June 2012, 09:29 AM   #16
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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.
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Old 29th June 2012, 10:16 AM   #17
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There is also current feedback as the cathode resistor is unbypassed, making the feedback resitors even more unequal. Only with infinite gain the resistors would be equal.
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Old 29th June 2012, 11:49 AM   #18
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Quote:
Originally Posted by revintage View Post
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.
Got it, lots to learn still...

Jaz
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Old 29th June 2012, 12:18 PM   #19
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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?
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Old 29th June 2012, 01:14 PM   #20
DF96 is offline DF96  England
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Is there a grid stopper on the first stage? Is it in the right place: right at the grid pin? Widespread microphony is often a sign of RF oscillation.
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