Could use some help troubleshooting fresh Fender 6G3 build

Diverted

Member
Messages
51
Hi all,

I have been piecing together a Fender 6G3 over the past week or so. I fired it up for the first time tonight and I'm having issues:
Bias voltage sits at about 8V. Spec is -26. This amp ties right into the HV secondary so there's no bias tap on the transformer.
The 6V6s are red-plating.
B+ is about where it should be at about 350 or so.
The amp is playing ... tremolo works, everything appears to work but it's breaking up at 1 on the dial. Very distorted, harsh sounding. I'm afraid to turn it on with the screwed up bias and red-plating output tubes.
I have checked, re-checked and re-re-checked my wiring, checking all points on the board, pins etc. As far as I can tell everything's wired properly. But, I am definitely missing something here.
This is my first fixed bias amp build and I could use some help with trouble shooting tips and getting this amp right. Thanks!

Ted
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Diverted

Member
Messages
51
Fixed the problem. Rookie mistake: Diode was reversed.
In my defense though the schematic and layout are both really confusing. Check out the schematic and layout below: Where the diode is shown, it has the cathode (negative) joining with the 100K resistor. However right above it there is a "+" sign. When I was looking over the schematic laying out the bias board, I took that to mean diode polarity. Now I realize the artist must have meant whether the voltage was positive or negative at that point.
Seems like a confusing way to do list it, but like I said this was my first fixed build.
Update on the amp: Holy cow, first impressions are pretty awesome. Super swamped out tremolo, not a peep of noise. Sounds great so far!
deluxe_6g3_schem.gif

deluxe_6g3_layout.gif
 
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fusionbear

exquirentibus veritatem
Platinum Supporting Member
Messages
11,771
That is a cool sounding circuit! Enjoy.... My first thought was the diode was reversed when I saw the thread. Glad you sorted it out...
 

Diverted

Member
Messages
51
Yep. That's how I learn! I'll never do that again, guaranteed. Lucky I didn't have to sacrifice a set of output tubes.
 

Pedro58

Silver Supporting Member
Messages
6,563
I would have made the same mistake. I owned a 6G3 and sold it after building an 18 Watt Marshall (Which, oddly enough, I modded to a 36 Watt circuit that I sent to fusionbear to repair because I could not find a ground buzz! Thanks again, fb!). Very similar sounds when driven, which was how I used them, so the vintage amp was sold. I don't really miss it, but the trem is really nice. When I finally roasted the original speaker, a brown-labeled Oxford, I could never get the same sound out of the recone. And every replacement fell short. Maybe with today's host of speakers, I could find a proper sounding replacement? Anyway, enjoy the amp. It's a great circuit.
 

Diverted

Member
Messages
51
Thanks Pedro. I love it so far. Actually I've been thinking about building an 18W Marshall next. I've got a nice pair of transformers from a Hammond organ that ran push pull 6BQ5s, so I have the building blocks. I may get to it after Christmas. Cheers!

Here's the amp in present form. Waiting on a dual pot for the lar/mar master volume control and I have to wire a trimmer into the bias circuit. But for now it's playing stable, clear and beautifully.
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neteraser

Member
Messages
1,993
So the half of the answer is to correctly put the question. :) Congratulations on your first build!
 

Diverted

Member
Messages
51
Hello again, I'm back.
First off, thanks for the tips on the volume control and everything else. This has been a great learning experience so far and a ton of fun. As for the MVC, I'm building this for a good friend of mine and he asked for a VC. I'll put it in and see if he likes it; if not, I'll take it out and put something else in the hole; maybe a front panel tremolo switch instead of at rear, possibly.

Anyway, I've been going over the numbers and I have some questions. Please bear with me, it's a bit long but I am wondering if some of you with more experience could let me know what you think is going on here. Thank you!

Here's what's up:
The schematic allows for voltages within +/- 20 percent of spec. I am using an older Mullard GZ34, a few 6V6GT pulls that match well together and for the PI and preamp tubes, three good pulled Telefunken 12AX7s. My actual readings are below, with the spec’d values in parenthesis.

Bias -26 (-26). Right on the money.
Note: For the bias board I initially had it wired per stock with a set fixed bias. When I turned it on for the first time I was getting -34V. I have since installed a 10K bias trimmer to help dial it in. When I turned it on initially after installing the pot, the highest I could trim the voltage was -28V. So I removed the 22K resistor on the bias board and replaced it with a 15K. Using that value the trimmer now gets the bias to the -26 spec right in the middle of the pot.

B+ 416V (375 spec)
Second filter cap: 406V (365 spec)
Third filter cap: 365V (325 spec)
Fourth filter cap: 310V (270 spec)

6V61
Plate: 396 (365)
Screen grid: 405 (365)

6V62
Plate: 402 (365)
Screen grid: 405 (365)

12AX71
Plate: 288 (220)
Cathode: 17 (20)
Plate: 271 (225)

12AX72
Plate: 195 (165)
Cathode: 1.4 (1.4)
Plate: 242 (205)
Cathode: 2 (2)

12AX73 (7025)
Plate: 190 (165)
Cathode: 1.6 (1.5)
Plate: 192 (165)

Biasing question
I have never biased a fixed bias amp before. I do not have 1 ohm resistors handy to run from output tube cathode to ground to measure voltage drop. With the numbers above, is it possible to tell whether I’m running the tubes too cold or hot? I could use some clarity on the biasing question, as I need to learn more about the procedure.

Issue
I’m wondering if the tremolo intensity pot doesn’t go quite to 0 at the bottom of its sweep, or whether subbing out the 22K with the 15K/bias pot is causing an issue. Even with the pot turned all the way down, I can hear a slight, faint surging in the background, and the bias voltage fluctuates from about -25.5 to -27 or so. When I ground out the 1M resistor that goes to the tremolo foot switch (I have not installed one yet) the very faint background wawawa/surge/tremolo sound goes away.

Thank you for the help! I would love for some more experienced builders to assess how the amp’s trimmed. It sounds great, FWIW. Nice and quiet, lots of volume and smooth and crisp, with good breakup higher up the volume pot.

Ted
 

Diverted

Member
Messages
51
After switching to a 5Y3, went back to the GZ34. I swapped out the 2W resistors in the B line (1K, 10K) for two 5Ks). This brought the output plate voltages down to 378 and the screens to 350. The phase inverter plate went down to 269 (still 20 percent too high) but the preamp plates are good at 183 and 178.
 
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