Bias issue, 85W AB763 build

[Diverted]

Hi all,

I just finished wiring up a single channel, no tremolo version of a Fender Twin Reverb AB763. I am having no luck biasing anywhere close to where I need to be. I could use some tips on fixing whatever seems to be the issue. This is the first four-6L6 amp I've tried.

OK, the bias supply is putting out anywhere from -65 to -50v bias voltage and for purposes of providing numbers here I biased it at -52(spec). At that bias voltage the output tubes are drawing 13.5, 13.6, 14.6 and 15.1ma. The tubes are new JJs; I also used a set of vintage Sylvanias that test strong on my Hickok tester and bias fine in other amps, so I think I can eliminate bad tubes.
Apart from the bias issue, the amp seems to be functioning more or less normally in every other respect (though there is an underlying hiss/noise floor that I won't really try to address until I fix the bias issue.

Here are my voltages:

Power rail:
A node: 458
B node: 452
C node: 444
D node: 419

Bias voltage: -52

V1A 12AX7
Plate 285
Cathode: 1.9

V1B
Plate 284
Cathode: 1.9

V2 12AT7
Plates: 448
Cathode: 8.9

V3A 12AX7
Plate: 284
Cathode: 2.2

V3B
Plate: 279
Cathode: 2.2

V4A 12AT7
Plate 244
Cathode: 92

V4B
Plate: 265
Cathode: 92

V5 and V6
Plates: 453
Second grids: 451
First grids -53
V5 ma: 13.5
V6 ma: 13.6

V7 and V8
Plates: 453
Second grids: 452
First grids: -54
V7 ma: 14.6
V8 ma: 15

Additional info that may or may not be applicable:

1. Using Classictone transformers and choke for Twin Reverb. Tubes are JJs, 12AX7s and 12AT7s are known good vintage ... RCA, Sylvania, Mullard etc.

2. There are three grounds in the amp:
A. Power ground at one of the transformer lugs, with arms reaching out to a few terminal strip lugs for various stuff. This ground contains all filter cap grounds (except for D node), 100 ohm artificial center tap resistors, bias circuit ground, 1 ohm resistors on the 6L6 cathodes, neutral AC line (green) ground, HV secondary center tap and Faraday shield, etc.
B. Preamp ground: Soldered to input jack, soldered to back of pots. Contains all preamp grounds and the ground for the D node electrolytic.
C. Reverb transformer is grounded at the reverb tank jacks.

3. Electrolytics are:
A node 68/68uf @350V in series with 220K resistors for equivalent 34uf@700V
B node 68/68uf @350V in series with 220K resistors for equivalent 34uf@700V
C node 68/68uf @350V in series with 220K resistors for equivalent 34uf@700V
D node 22uf@500V

3. I used Hoffman's single channel AB763 schematic here, deleting the tremolo.
http://el34world.com/Hoffman/files/Hoffman_AB763_1.pdf

4. I made an eyelet board based on the Fender Twin Reverb but did a few things including taking the plate load resistors off in several spots and running them above V1 and V3 to save space on the board (this is in a Princeton chassis). I shielded the D node wiring to the tubes with a wrap of wire grounded to the preamp ground.

Any red flags? Or what should I try next?

Thank you!

[Diverted]

One other thing to add:

I tried swapping out the bias resistor to 15K (spec 27). The least negative voltage that gave me was -42.
At that level, the tubes were biased around 37ma, with plate voltages dropping to around 410 (50V low).

I am running the amp at 122VAC.

[shooter]

I am running the amp at 122VAC.

at the wall or through a lamp limiter or variac?

your 1st #'s ballparked around ~~6.5W/tube, your next #'s ballparked ~~~ 15W/tube.

are you using a speaker or dummyload?

[Diverted]

I've run it through both the wall and a Sencore PR57 variac/iso transformer. I don't have a dim bulb tester but the Sencore has an ammeter and it's been around or just under an amp or so at idle (if my memory serves me)

I'm using an 8 ohm speaker at the moment as the output transformer on this one has 4 and 8 taps.

Yes, the second one was a little higher but still way low. And the plate voltage dropped as expected, down to 410 or so. That's not right, should be around 450 or so at 40 or so ma.

Thanks!

[Diverted]

That was very, very strange. I fixed the bias issue and it came from a weird place: Standby switch.

I had the switch wired correctly but just checking various voltages I decided to jumper the switch to take one part out of the circuit, however small. With switch jumpered I suddely got 440 on the plates biased around 45ma per tube, with bias voltage aorund -47. Was there some type of internal resistance that was preventing a closed circuit? Weird.
So I took the switch out and for the moment there is no standby in the amp. I'll have to find a better switch; I purchased this one from Hoffman's store; what do you all use for standby's?


Next question: I have a noise floor on this amp that's a little higher than I'd like. Looking at my grounding info above (or any other clues), any suggestions on how to bring it down a little bit? Thanks!

Ted

[shooter]

I took the switch out

that's how I build mine 

[Diverted]

This is the first amp I have ever put a standby in. But with solid state rectifier and really high voltages/current, I decided to go with one just as most people do.
Will I unnecessarily be abusing these tubes if I leave it out? Seems like it would be a good thing to have on startup to keep the tubes from getting hit with DC before the heaters can warm up.
Should I replace? Or leave out?

[pdf64]

...Seems like it would be a good thing to have on startup to keep the tubes from getting hit with DC before the heaters can warm up.

However it seems, I’m not aware of any credible technical info that supports that hypothesis.
The tubes we use are not stressed if their HT and heaters are energised simultaneously.

[shooter]

Should I replace? Or leave out?

It's been debated for 8yrs to a 50/50 tie   

monitor the V at power on, my last amp was 520 for 30sec, then ramped down to 460 operating vdc, No standby 

[PRR]

> I'll have to find a better switch; I purchased this one from Hoffman's store; what do you all use for standby's?

Hoffman's stock is THE switch used in millions of Fenders, also vacuum-cleaners (a worse job).

It is not impossible you got a bad switch.

[Diverted]

Must’ve. Switch in, bad. Switch out, good. I’m fine with it, just glad I figured out the issue. I will probably just leave it out.

Next, just need to eliminate this ever-present white noise.

[sluckey]

It's possible there was a questionable solder joint on the switch. I would test the switch with an ohm meter before throwing it away. Check for actual on/off resistance, not just continuity beep.