Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: audiosalvage on March 04, 2023, 07:12:57 pm
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Hey all, I've got a princeton build here that gets all farty and buzzy after 3:00 on the volume knob. You can actually hear the double tremolo beat at this point too. I've got the 470R 5 watt grid stoppers , 1.5K screen resistors and 1 meg grid stopper on pin 7 of phase inv. in place. Also tried .047 phase inverter output caps with no avail. Speakers all good, tubes good. What else can I do? or is this normal?
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Hey all, I've got a princeton build here that gets all farty and buzzy after 3:00 on the volume knob. You can actually hear the double tremolo beat at this point too. I've got the 470R 5 watt grid stoppers , 1.5K screen resistors and 1 meg grid stopper on pin 7 of phase inv. in place. Also tried .047 phase inverter output caps with no avail. Speakers all good, tubes good. What else can I do? or is this normal?
FWIW You can go up to 47k on the 6V6 grid stoppers without any guitar hi frequency roll off.
As to what’s causing your issue, it’s a stab in the dark without pictures of your build to check the layout and lead dress and no schematic/information on what mods may have been done to the 6G2 circuit and nothing on the voltages.
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Here are my voltages, a gut shot and the layout i followed.
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Here are my voltages, a gut shot and the layout i followed.
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Hey all, I've got a princeton build here that gets all farty and buzzy after 3:00 on the volume knob. You can actually hear the double tremolo beat at this point too. ...
This is a known issue with driving the cathodyne inverter & output tubes hard (http://www.valvewizard.co.uk/cathodyne.html). Read near the bottom of the linked page for one possible solution.
Separately, if you still get issues with blocking distortion, you can use Paul Ruby's Zener-diode clamp (https://www.18watt.com/storage/18-watter_buzz_info_311.pdf) to minimize output tube grid current. The actual voltage-rating of the zener diodes will depend on the typical bias voltage of your particular amp.
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Here are my voltages, a gut shot and the layout i followed.
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Try shielding the cable from the grid of V1 to the vol pot wiper. The unshielded wire you've got there looks like it's running behind the board in the vicinity of the cathode resistor for the LFO circuit. While its a low voltage low current at the LFO cathode, its still right next to the unshielded pre-amp grid wire (which might be a source of unintended AC coupling from the tremolo into the signal path, or not. Try it and see)
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That first link from HBP says 100k to 1 Meg grid stopper on the cathodine may be needed. I had a similar issue on a different amp and ended up at 220k.
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I'll try these suggestions
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tried the 220k and flipped the two 12ax7s so the wires are shorter also added shielded wire to othe grid to vol pot. No change
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try replacing the 25uF cap on V1 to 5-10uF
try removing the NFB
either move you in the correct direction?
NOTE: using this schematic,
https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_6g2_schem.pdf
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i turned my fender excelsior (basically already a 6g2) into more of a hot-rodded 6g2 with tilt tone control on a push/pull pot to defeat it and the 'paulc mod' on the phase inverter. even all the way up it certainly doesn't fart out/blocking distortion/whatever. so yes i would look into the phase inverter biasing
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I currently have a 4uf cap on v1 and a NFB switch installed. I'll look into this Paul c mod
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You mention hearing the thump of the tremolo. And, I see that you installed a bias pot. I did the same with my 6G2 and found that there was a fine line setting the bias to get good tone and avoid the thump. And I had to change the bias resister value to get in the right range. It appears to me that Fender biased this model pretty cold, probably to avoid that. Have you played with that adjustment?
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tried the 220k and flipped the two 12ax7s so the wires are shorter also added shielded wire to othe grid to vol pot. No change
Did you try 47k grid stoppers on the 6V6 (signal) grids?
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I just rebiased a bit colder after trying out the 650 PT tap (pointless) so we'll see. I have not tried 47k stoppers yet
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NOW were talkin'! I put some 47k grid stoppers in and I'm happy with the results. full vol has a little saturation and backing it off a bit gives a rich, warm edge. With the tremolo on, it can do "Born on the Bayou" perfectly. Time to put her in a cabinet. Thank you all again.
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Nice job Tubeswell. There's nothing better than getting an amp working. The guys on this site are the best.
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couple of scope shots from my last build with your problem. you can find a write-up on the Valve Wizards site