biasing parallel single ended cathode biased 6v6

[vampwizzard]

Folks,

Looking for a sanity check on biasing the GA-8 with its paralleled single ended design.

I do this like I'd normally would do a single ended, but with with each 6V6's plate voltage, correct?

Measure the cathode resistor actual value, measure voltage drop across cathode resistor, figure out total current dissipation for BOTH output tubes. Would I use half this current for the rough estimate?

Find plate voltage for each 6v6, calculate bias on each tube.

Alternatively, would I find the common plate voltage headed to both, calculate bias and divide at the end? Total dissipation for the two output tubes?

Schematic of mine attached.

[tubeswell]

Simplest thing to do is measure the voltage (down to say, 1 decimal place) across the 200R resistor, and measure the exact resistance (i.e. down to the nearest ohm) of this resistor, and divider the number of volts by the number of ohms, and then divide that by 2 to get the tube (cathode) current for each tube. Subtract 2mA (from each tube) to allow for screen current - the remainder will be plate current. Then multiply that by the plate-to-cathode voltage to get Pdiss for each tube. There'll be some actual discrepancy because no two tubes are the same, but its good enough for rock'n'roll.

[PRR]

There have been many 2-6V6 amps plans published. Find one with similar plate voltage and steal its cathode resistor. Check for red-plating. Play.

[vampwizzard]

Im a math guy. This is for testing this one against a schematic.

New cathode resistor had a perfect tolerance according to my meter.. which was weird.

R.cathode = 200 Ohms
Voltage drop = 14.43VDC
current = 72.15mA (two tubes) or 36.075 per 6V6
Plate voltage = 264.0 VDC

Dissipation = 9.52W, about 80% of max.

Someone had done work in this amp before I got it so I wanted to make sure they actually serviced it. It's got the original Gibson branded 6v6's in it.. im looking at a pair of vintage RCA 6v6GTA's to replace them. There's either a gremlin or a design flaw in the amp.. it breaks up in a funny way that I havent heard on demo videos etc. Wanted to check bias first before hunting it down.

[shooter]

either a gremlin or a design flaw

IIRC the G1 tie-point to the 220k/470ohm gives one grid a straight shot at signal and the other grid the signal goes through the 470.
give each g1 it's own 470ohm and see if that helps.  maybe even tweak that value as high as ~~5.6k per tube

[PRR]

> 220k/470ohm gives one grid a straight shot at signal and the other grid the signal goes through the 470.

"Through the 470" to what? A grid. Which is almost infinite impedance. What is the loss in a 470/infinity divider?

Vamp, I think your cathode is fine. There's a dozen other nodes here. Which ones have dubious voltages? Which ancient capacitors have shuffled this mortal coil? Is the speaker even roughly matched to the transformer? What is a "funny way"? Benny Hill? Judy Tenuta? Soupy Sales?

[vampwizzard]

The original coupling caps are sprague black beauties.. my .022's are measuring .030 which I'm not all that concerned about as long as they arent passing DC. My B+ seems low.. 264 to me and I was expecting 320 off my schematic. That being said, if its biased to the advertised output with the original tubes and a dead accurate cathode resistor Ill just assume this is normal.

On pause for now. It'll take me a bit to measure the rest of the resistors. The schematic is accurate. Ill write up my voltages and nodes and come back after Ive swapped tubes.

The "weirdness" is that even at one I can slam the strings and get a noticeable distortion in the amp. More like a rattle than true overdrive.

[tubeswell]

Got a new rectifier tube handy?

[vampwizzard]

Got a new rectifier tube handy?

Absolutely! Ive got a couple of modern and at least one vintage 5y3, amongst others. The 12AX7 was newish but I've got a bajillion of those around too.

[tubeswell]

Got a new rectifier tube handy?

Absolutely! Ive got a couple of modern and at least one vintage 5y3, amongst others. The 12AX7 was newish but I've got a bajillion of those around too.

Plug one of them in

[PRR]

...The "weirdness" is that even at one I can slam the strings and get a noticeable distortion in the amp. More like a rattle than true overdrive.

So measure the voltages around that first preamp. Or all small-tube voltages while you are at it.

[tubeswell]

What PRR said - an unconnected/ungrounded filter cap node can produce 'spongy' symptoms when you whack a power chord. If a power supply node is not being properly decoupled, this results in signal feedback between power supply nodes through the High Tension rail. (for instance, what you're looking for is voltage measurements of the HT rail PS nodes that show the same DC voltage on the pre-amp node as the screen supply node etc, which could indicate that the pre-amp supply resistor and the screen supply resistor are just acting as one big joined up plate resistor with taps along it that feed back signal into the other nodes where you don't want signal fed-back to)



I'm thinking that the low overall B+ and reported sponginess could also be a symptom of a worn-out rectifier tube, which plugging in a fresh tube would indicate.

[HotBluePlates]

The original coupling caps are sprague black beauties.. ...

Take a look at the print on those.  If there is red print, they're di-film caps & less likely to leak.  If they have yellow writing, they're paper dielectric and almost certain to leak d.c.  If they're "bumblebee" types with color-coded values it's a 50/50 shot whether they will be leaky.

A tiny bit of leakage can really mess up a preamp tube's bias.

[ALBATROS1234]

+1 on a filter cap being bad or loose. That was my first thought when I started reading this.

[vampwizzard]

Alright! Got a pair of NOS RCA 6V6GTAs and a modern 5Y3 in it. I think I sent someone home with my last NOS 5Y3...

Either way, B+ off the rectifier is 408 vs 350 on the schematic.. probably fine all things considered and will lower when I put an original spec 5Y3 in there.

Plate voltage 282.5VDC
Voltage drop 16.82VDC, 200Ohm measured on the cathode resistor, current of 84.1mA for the pair.. 42.1mA per tube. Dissipation at 11.88W, probably a little lower due to the screens. GTA data sheet puts us at 85% of max (14W)

I made a video of the weird distortion. Almost sounds like ghost notes. This distortion ahappens at every value on the tone and volume knobs. Any time I dig in it does this. Almost sounds like a rattle but I cant find loose parts.

video https://youtu.be/n7dq-UXi6Gg

Ive got to get contact cleaner in the pots and Im probably replacing the 500mmf tone cap. Any other thoughts on what im hearing? Speaker seems intact but Ive not dealt with a bad one before.

Should have the NOS 5Y3 here in a few days.  Thanks y'all.

[tubeswell]

Alright! Got a pair of NOS RCA 6V6GTAs and a modern 5Y3 in it. I think I sent someone home with my last NOS 5Y3...

Either way, B+ off the rectifier is 408 vs 350 on the schematic.. probably fine all things considered and will lower when I put an original spec 5Y3 in there.

Plate voltage 282.5VDC
Voltage drop 16.82VDC, 200Ohm measured on the cathode resistor, current of 84.1mA for the pair.. 42.1mA per tube. Dissipation at 11.88W, probably a little lower due to the screens. GTA data sheet puts us at 85% of max (14W)

I made a video of the weird distortion. Almost sounds like ghost notes. This distortion ahappens at every value on the tone and volume knobs. Any time I dig in it does this. Almost sounds like a rattle but I cant find loose parts.

video https://youtu.be/n7dq-UXi6Gg

Ive got to get contact cleaner in the pots and Im probably replacing the 500mmf tone cap. Any other thoughts on what im hearing? Speaker seems intact but Ive not dealt with a bad one before.

Should have the NOS 5Y3 here in a few days.  Thanks y'all.

Those voltages and Pdiss look fine. I wouldn't bother putting in an old-worn out NOS rectifier tube.  I'd focus on improving lead dress, signal wire shielding and using grid stoppers to reduce parasitics.