Bandmaster Biasing Question

[bootsypratt]

Greetings all, new to the board and pretty much a novice in tube amp work and greetings to Structo from the Reranch.

I've run into something that is confusing me. I have a 68' Bandmaster which I've had about a year. I have recapped it and got some old Sylvania 6L6GC for it and had it biased in the low 40'smA. It sounds great but was looking for a little more headroom and wanted to try the new TungSol 7581's. When I put these tubes in, the bias is way high 70'smA. I changed the resistor across the bias pot to a lower value and the bias reading went even higher? It should, as I lower the resistor value my bias reading should go down, right? But it seems to be the opposite. I put my 6L6's back in and now I have 100k (used to be a 20k) resistor across the pot to get it to the low 40'smA. Also when I turn the bias pot clockwise the mA's do go up.
I am utterly confused, or maybe my thinking was totally backwards last night. Also the amp sounds fine.

Thanks

[xtian]

Welcome! You work at Reranch?

Bias can be confusing! You're applying a negative DC voltage to the grid in order to set the tube's idle current. The greater the negative voltage applied to the grid, the less current flows through the tube.

If you put in new tubes and you cannot lower the idle current enough, you need to apply more negative DC to the grid. E.g., if -40vdc wasn't doing it for you, you need -60vdc, as an illustration.

Now look at the shunt resistor in the bias circuit. As you turn the bias pot toward that 15K (in the original schematic) resistor, you are causing the bias voltage to approach ground, or 0 volts, and your idle current will be INCREASING as bias DC approaches 0.

So, to generate greater negative bias, you need to INCREASE the value of that resistor.

Hope I got that right! Guys?

[Firestorm]

Do you know what model your Bandmaster is? By the AC568 model, these all had bias balance circuits instead of true bias adjust. It might have been modified, though, because the stock shunt resistor would be 15K. As xtian said, a larger shunt resistor makes the negative bias voltage more negative. If the circuit is still bias balance, you may have to roll through several resistor values to get it right.

Tubes are all different even within the same designation, let alone going to 7581s, but the jump up to 70mA seems large.

[bootsypratt]

It's not a balance.
The shunt resistor is the one going across the pot, right?
It was around 50K with my old tubes and I tried to 100k resistors with these new tubes and the bias was still high. I do bias by cathode resistor.

[Firestorm]

Yeah, the one from one leg of the pot to the pot housing. Stock value is 15K so if you can't get the bias negative enough with 50K (!) something is wrong. Maybe the bias cap is shot.

[bootsypratt]

That's what I'm thinking.
Also I wondering too if my meter is on the fritz too.

[Firestorm]

Try fresh batteries. When meters die (I've killed a few) either the voltage/resistance side goes or the current side. The other part will often still work right.

[Leo_Gnardo]

Yeah, the one from one leg of the pot to the pot housing. Stock value is 15K so if you can't get the bias negative enough with 50K (!) something is wrong. Maybe the bias cap is shot.

Thinkin' the same thing, if that bias filter cap is original, or even 20 years old or more, $2 - $3 spent here is cheap insurance.

Also, in Fender bias supplies, there's a resistor in series with the rectifier that may have increased in value. Depending on whether there's a bias tap on the transformer or not, the value is 470R-1000R or 100K. Just had a Princeton come thru last week whose 100K measured 260K - output tubes were headed for melt down. Don't panel-beat your bias voltage into submission by running that grounding R up to 100K. Replace the filter cap and I'll bet things go the way we expect. And do make sure it's "pointing the right way." Yup, I've done that too.

[bootsypratt]

Did replace the bias filter cap, and up graded to 100/100 (I believe that was the value). If it was the wrong way I would have had major problems, right? I have gigged this amp numerous times with no problems.

Where would that resistor be if my case was similar to the Princeton you just worked on?

[Firestorm]

Yeah, the one from one leg of the pot to the pot housing. Stock value is 15K so if you can't get the bias negative enough with 50K (!) something is wrong. Maybe the bias cap is shot.

Thinkin' the same thing, if that bias filter cap is original, or even 20 years old or more, $2 - $3 spent here is cheap insurance.

Also, in Fender bias supplies, there's a resistor in series with the rectifier that may have increased in value. Depending on whether there's a bias tap on the transformer or not, the value is 470R-1000R or 100K. Just had a Princeton come thru last week whose 100K measured 260K - output tubes were headed for melt down. Don't panel-beat your bias voltage into submission by running that grounding R up to 100K. Replace the filter cap and I'll bet things go the way we expect. And do make sure it's "pointing the right way." Yup, I've done that too.

470R or 1K in Bandmaster If it's still the AB763 circuit, the cap is underrated (50V), so I would totally replace that. 100uF/100V. Most important cap in the amp. Clean the hell out of that bias pot, too. Replace it even. The way the bias circuit is designed, if the wiper makes poor contact or opens up, the tubes cook.

[Firestorm]

You posted while I was typing. Good on the 100/100. Positive end to ground. The dropping resistor is on the board, next to the reversed diode.

[bootsypratt]

That's the 470 1w (.47k?)? I can check it's value while still on the board, right?
Not a fan of poking around with 2 metal sticks

So the value of that resistor could have possibly floated upwards?

Thanks guys

[Leo_Gnardo]

That's the 470 1w (.47k?)? I can check it's value while still on the board, right?
Not a fan of poking around with 2 metal sticks

So the value of that resistor could have possibly floated upwards?

Thanks guys

Poke your prods while the amp is powered down & unplugged & you can even discharge the hi voltage just to be xtra safe. And yes you can read it while still connected. Even if its value increased, the more likely failure mode is the old filter cap was failing, becoming conductive, driving the bias voltage towards zero. You have insulated handles on those metal sticks, no? Bright lights, steady hands, concentrate, all will be OK. For an extra measure of safety you can slip some heat shrink on, or wrap electric tape around most of the test prod points, just letting the last 1/8 inch or so show. Doesn't hurt to sharpen 'em either.

Just replaced another one right here, was supposed to be 470, I measured 560. That little of a shift wouldn't make much diff in bias voltage but it worries me that it drifted up, how much further will it go, when, and does it get worse when it heats up? New 470R and no more worries.

Another thing that happens, if an output tube has an internal short, it can send a big hi voltage spike into the bias department. That sort of thing can wipe out any or all of the components including the adjustment pot.

[bootsypratt]

That's the 470 1w (.47k?)? I can check it's value while still on the board, right?
Not a fan of poking around with 2 metal sticks

So the value of that resistor could have possibly floated upwards?

Thanks guys

Poke your prods while the amp is powered down & unplugged & you can even discharge the hi voltage just to be xtra safe. And yes you can read it while still connected. Even if its value increased, the more likely failure mode is the old filter cap was failing, becoming conductive, driving the bias voltage towards zero. You have insulated handles on those metal sticks, no? Bright lights, steady hands, concentrate, all will be OK. For an extra measure of safety you can slip some heat shrink on, or wrap electric tape around most of the test prod points, just letting the last 1/8 inch or so show. Doesn't hurt to sharpen 'em either.

Just replaced another one right here, was supposed to be 470, I measured 560. That little of a shift wouldn't make much diff in bias voltage but it worries me that it drifted up, how much further will it go, when, and does it get worse when it heats up? New 470R and no more worries.

Another thing that happens, if an output tube has an internal short, it can send a big hi voltage spike into the bias department. That sort of thing can wipe out any or all of the components including the adjustment pot.

I replaced the filter caps.
Would a 1/2watt 470 be ok?
Also, the amp makes a decent sounding pop when it's switched off standby to on, is this normal?