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Old 29th June 2012, 05:18 PM   #21
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68k grid stopper soldered directly to pin. i did not, however, use carbon comp
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Old 29th June 2012, 05:20 PM   #22
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shielded signal wires except where coupling caps literally jump one pin to the next. as mentioned, no microphony when the circuit is closed, only when open.
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Old 29th June 2012, 09:10 PM   #23
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Should anything but oscillation be other than expected with 4 gainstages? You are begging for it.
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Old 29th June 2012, 09:24 PM   #24
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yeah, that could explain all the plate to plate caps and plate to ground caps and so forth in the schemo. that and the fact that it sat under some projector motor may be partly why it originally used 12vdc heaters.

its a "tight" sounding amp, though, and now with a decent PI driver its loud and "clean". surprisingly balanced. i've often considered converting it to 5e3 or some other schematic, but it sounds good as it is - i collect (though no longer seek out) small amps to acquire the pauper's version of a collectors selection, for both gigging and recording. i'm not really looking to change the sound. in fact, that dying 12au7 sounded awesome for a particular range of material - i'm tempted to keep it on hand to plug in when the material calls for it.

thanks again for all the input. i'll update if something about the PI comes up, but otherwise I'll start a new thread if the microphony proves to be a problem. cheers!
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Old 29th June 2012, 11:38 PM   #25
jjman is offline jjman  United States
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A shorting input jack is mandatory in my mind.
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Old 30th June 2012, 03:56 PM   #26
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Back to the PI -

this second stage has a grid resistor (ground ref) that is not in the "floating paraphase" circuits that I've been reviewing - is this providing the input ground reference? is it also providing ground reference to the output tubes? i'm wondering if it would be better to remove it from pin 7 and put it at the 6v6s pin 5-to ground, at say 470k. or is that ridiculous?
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Old 30th June 2012, 04:34 PM   #27
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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
It could be the coupling cap (.005) going to the grid of 12AU7, pin 2, has DC leakage, upsetting the bias of the phase inverter circuit. Worth checking. Grid to cathode difference is 3.7v, according to schematic, so it should have enough headroom.

Regarding the phase inverter circuit, (paraphase PI is my favorite sounding circuit, great tone) I prefer to run both sections in equal current so the cathode resistors of 3.3K are not really equal as the first tube's cathode is really parallel with the 10K feedback resistor which makes is really 2.48K. Either parallel another 10K resistor to ground on the second tube's cathode or just use one 2.5K resistor. In this case both plate voltage will be equal. Checking AC balance of outputs from both phases can also reduce distortion by adjusting the value of the 390K input resistor. Bear in mind I am only thinking in terms of audio amp but the mismatch might be responsible for the sound or "tone" in a guitar amp. Try it and see what happens.

You also try to add AC balance this way:
Click the image to open in full size.

Last edited by directdriver; 30th June 2012 at 04:57 PM.
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Old 30th June 2012, 05:14 PM   #28
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thanks DirectDriver!

(edited to add: i just saw your pic after writing this post)

I was wondering about possibly using a pot for that balance (390k/240k) - maybe 1meg with 240k resistors from either lug to the wiper (which then heads to pin 7 of 12au7) - my thinking on the resistors is primarily to ensure ~470k input resistance for the 6v6s.

i'll try swapping out the second stage cathode resistor as you recommended. i'm going through checking figures and finding some typos in this schematic and changing values to compensate (most obvious one being the 12ax7 plate voltages - they're flipped) so I'll also be reevaluating the components in the PI in general

what is your opinion on my previous post - should I "free" the input grid of the 2nd stage of the PI from ground reference ( remove 240k pin 7 to ground) and then give the 6v6s the needed ground reference by adding 470k (pin 5 to ground) for each?

Last edited by brilliantblue; 30th June 2012 at 05:17 PM. Reason: to refer to drawing in message body
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Old 30th June 2012, 11:55 PM   #29
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Quote:
Originally Posted by brilliantblue View Post
should I "free" the input grid of the 2nd stage of the PI from ground reference ( remove 240k pin 7 to ground) and then give the 6v6s the needed ground reference by adding 470k (pin 5 to ground) for each?
Either way will work but having extra set of resistor is not necessary. If you want the PI to see 470K, then just increase the input resistor to ground to 470K and add a pot to get AC balance. The second half of the 12AU7 has to see a portion of the first half 12AU7's signal that's equal to its input voltage, basically both halves of 12AU7 need to see equal voltage at their grids to ensure AC balance. Let's say the mu of the first half of 12AU7 is 15 then the second half of 12AU7 needs to be feed 1/15 of amplified signal to be back to unity. Having a pot at the grid makes adjustment much easier and you can even purposely make it imbalance to get the distortion or lack of distortion you want. Adjust-ability comes in handy.

Stock circuit:
Click the image to open in full size.

Since you like to have 470K input resistor at the grid of 6V6, I just tweaked the values to use commonly available resistors. Adjusting the 100K pot is important, most likely the wiper will be at around 30K to ground and try that as the starting point and adjust accordingly. Make sure to measure the AC balance of both outputs of PI.
Click the image to open in full size.
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Old 1st July 2012, 08:47 PM   #30
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I'm not hooked on 470k for the 6v6s, but in the original schematic I saw the series resistance as over the 6v6 spec of .5M max grid resistor - the 240k to ground on pin 7 plus the 390k. I wonder if that was another typo in the original schematic. i can't recall if the component was there or not, and have since learned that its a good idea to verify/modify the schematic during deconstruction when looking to maintain an amps sound but update components.

does 100k on the PI plates seem a bit high to you? would the headroom decrease or increase if I went to say 47k? In preamp stages I've built, the higher the plate resistor the more "nasal" and "pinched" the sound seemed to become - well, particularly getting around 220k and higher on a 12ax7.

I have since realized that I do have a ton of gain here, even when quick swapping the 12ax7 with a 12at7. I guess my night audition must've been with the guitar volume at 5, because this amp starts to break up pretty quick beyond that! I may bring the 220k 12ax7 stage down to 100k. I may just put in another input and jump it to the volume pot (skipping the 12ay7) for variety instead.

thanks for your efforts. hopefully i'll get some soldering time today - refinishing cabinets for now.

Jeff
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