Strange issue, debugging help

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Firestorm
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Re: Strange issue, debugging help

Post by Firestorm »

WRC34 wrote: Wed Apr 19, 2017 11:44 pm Firestorm, pardon my ignorance, but how would I attenuate the post/pre PI signal?
Attenuate just like you would with pre-PI and post-PI masters, just with fixed resistors instead of pots. To look for parasitic problems a gut shoot would help.
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WRC34
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Re: Strange issue, debugging help

Post by WRC34 »

Finally got around to making a crude schematic as well as an updated gut shot. Haven't had a chance to try any other "live" troubleshooting but I want to get back to it and track the problem down.

There are some unconventional values on the schematic. Nothing wildly so but if anyone sees a suspect value of component (specifically any resistor) please feel free to speak up. I thought maybe I could up the grid stopper value going into the EL34s from 4.7K to something more in the 8K range, although I know that is a band-aid fix and doesn't help me learn why the issue is occurring to begin with.

Also, it has been a while so I didn't realize until I got about 1/2 way through that I wasn't using the classic "jagged line" symbol for resistors. Rather, I just wrote in their value in the place they'd be found in the circuit. Please forgive this technical error.
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WRC34
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Re: Strange issue, debugging help

Post by WRC34 »

Just for laughs I swapped out the output tube grid stoppers from 4.7K to 22K with no helpful results!
Andy Le Blanc
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Re: Strange issue, debugging help

Post by Andy Le Blanc »

check the voltage difference between the plates and screens, if the screens are positive with respect to the plate at static condition you can get fizz as signal hits the power side of the rig.

ive experienced a similar condition if the secondary winding leads of the opt are too long or run in proximity to the pre amp wiring
it could be a component layout issue
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didit
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Re: Strange issue, debugging help

Post by didit »

WRC34 wrote: Sat Apr 15, 2017 2:21 am Both preamp tubes have been changed as well (all are known to work well in other amps).
In the other amps are the preamps subject to similar conditions? Do you have a few other 6SJ7s and 6SL7s to test? Did you try tapping preamp tubes (gently) to check for microphonics?

Best .. Ian
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WRC34
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Re: Strange issue, debugging help

Post by WRC34 »

Andy LeBlanc - thanks you for taking a look.

Screens are below plates at idle by about 17VDC I think (plates @ 340VDC, screens @ 323VDC)

If you look at the gut shot photo the OT secondary leads are yellow, green & black going to the speaker output jacks to the left of the IEC plug connector. The green is 8 ohms and the yellow is 16. They're both a fair distance fro any preamp wiring. However, there is a 4 ohm tap which is the brown lead coiled up and sitting between one of the large grey coupling caps and the blue & white cathode bypass cap for the inside output tube. I will move this lead around to see if it makes any difference!

didit - thank you for your question - yes, I have another amplifier that uses 6SJ7 and 6SL7 under similar conditions. In that amp the idle voltages may be a little higher, but not by very much. And I have tried 4 or 5 different 6SJ7s and 6SL7s in different combinations.

This amp does not use negative feedback. Any chance I should increase the tail resistor of the phase inverter to stabilize things? Currently it is 33K with a 620 ohm cathode resistor & 1Meg grid resistors.
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WRC34
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Re: Strange issue, debugging help

Post by WRC34 »

To address Andy LeBlanc's suggestion of the issue possibly being related to component layout, if anyone sees anything suspect in the gut shot posted earlier please let me know.

Here is a gut shot of a different amplifier - fixed bias 40 watt w/6L6s - that uses a cascaded 6SL7 into another 6SL7 ltp phase inverter. I made this amp about a year and a half ago and it's a bit messier in it's layout. I did have to change the components a bit to get it to behave, but the newer (cathode biased EL34) amp is much neater IMO.

Here is a link to a live gig I used the older, fixed bias amp at a week or so ago. It worked & behaved perfectly. The music is fast hardcore so it might sound a little chaotic but the amp was fine.

https://www.youtube.com/watch?v=9ZpP_ScUZts&t=1118s
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Firestorm
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Re: Strange issue, debugging help

Post by Firestorm »

The thing I see that might be an issue is lead from the 100K PI plate is in phase with the input and the tone circuit. I can't make out how close together they are from the overhead shot but it might be worth experimenting there. Maybe that lead shouldn't route back to the front where its signal could couple to the PI input.
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WRC34
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Re: Strange issue, debugging help

Post by WRC34 »

Firestorm - thank you! I think you may have helped me track down an issue in the other amp I posted a gut shot of, so thanks again for your help there as well!!

There's about 3/4" vertical space in height between the white & red wires there. Good eye, but I don't think that was causing the issue.

I did however read about carbon comp resistors becoming faulty over time and creating the "popcorn crackle" sound accompanying notes. Upon extreme close inspection, I discovered a tiny bulge on the otherwise pristine looking 82K plate resistor in the phase inverter. I just swapped it out for another of exact value & testing very quietly with a temporary MV pot clipped into the amp I am no longer hearing the issue!! It's late and I have downstairs neighbors so a full volume test tomorrow will prove more conclusive, but this has been the most encouraging result so far.

Will report back tomorrow!!
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WRC34
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Re: Strange issue, debugging help

Post by WRC34 »

Welp.that was a red herring. At volume the issue remains :/ I will try the other carbon comps, too. Could the bias feed resistors on the output tubes cause this issue if they're aging/faulty?
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WRC34
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Re: Strange issue, debugging help

Post by WRC34 »

I'll tell you guys, the reason this has been frustrating me is that I have built a fair handful of amps in this point-to-point esque style and have never had this much trouble tracking down an issue. My living situation definitely impedes my ability to swiftly diagnose this type of problem because I cannot just swap a component of re-arrange wiring and then fire it up to test. I have to wait until neighbors aren't home, etc. Still, I am generally able to successfully troubleshoot most completed builds within a week or so.

Here are some more photos of yet another amp I just finished re-wiring in this fashion. The old layout was a total mess. This may look messy but is much more thought out. It is pretty cool in that it uses a 6SN7 or 6SL7 to drive a 6SJ7 which then hits another 6SL7 utilized as ltp phase inverter. Power section is fixed bias and can accommodate KT66s, EL34s, 6L6GCs, etc. The PT has multiple secondary windings and a DPDT switch alternates between two of those windings allowing for use of even 6V6s at Deluxe Reverb level voltages (400VDC plates - I know, that is abuse). There is a separate input jack on the back panel which goes directly into the 6SJ7 and skips the front panel input stage for cleaner tones. The amp is more complicated and the wiring layout more complex than the cathode biased EL34 amp this thread was started about, and this thing is dead quiet with zero issues. The red wire from the speaker output jacks returns to ground at the power supply filter caps main ground which I think may assist in the quiet idle? Anyhow, I'd expect this amp to suffer from issues rather than the one that is giving me trouble!! You can see in the first photo how the signal wires are a good distance above the other wiring down below (blue cloth covered wiring).
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tubeswell
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Re: Strange issue, debugging help

Post by tubeswell »

Seeing as how you report the problem as occurring when the amp is under or approaching full-signal conditions, my guess is that the LTP may be too hot-biased. Try increasing the bias resistor to 1k
Last edited by tubeswell on Wed May 03, 2017 7:54 pm, edited 1 time in total.
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Andy Le Blanc
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Re: Strange issue, debugging help

Post by Andy Le Blanc »

Aluminum chassis...

Ive done much the same approach with point to point.
What I learned was to be very conscientious about my grounding.
Aluminum has a higher resistance than steel and doesn't provide the same
shielding, you can suffer stray capasitances that can play hell with your resulting project.
I went with a single point star ground and copper bus.
I use a plastic input jack to keep the input ground off the chassis ground.
The only circut ground being the star point and the opt secondary.
The chassis becomes a sheild other than a ground plain.
This has improved my project results over all even with a steel chassis.
Even if this build is problematic, this is where I would look for resolution on the next build.

Another place to look is providing DC for the heaters.
This can be done by using 12sj7 and 12sl7 in a series string in place
of a chathode resistor for the power tubes.
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martin manning
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Re: Strange issue, debugging help

Post by martin manning »

Resistivity of 1020 steel is ~5x 5005 aluminum.
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WRC34
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Re: Strange issue, debugging help

Post by WRC34 »

tubeswell, thank you - I just installed a 1K resistor in place of the 620R cathode resistor on the phase inverter. No difference :|

Andy, I'll re-think my grounding scheme, thanks for the input!
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