Bassman bias balance pot dilemma

[bluesguitar]

I have a '74 Bassman. Because of one 6L6 power tube that was red plateing I installed an old set of tubes to see if the problem was only the tube. My plate voltage is 455 V. If my calculations are correct 70% max plate dissipation is 46mA. The problem is that my tubes now balance out at 58mA which is way to high for safe operation. Without a true bias adjustment pot I can't lower the bias of the tubes. Can anyone shed some light on what the solution to this problem is.

[Reeltarded]

-46 @ 455!??

Try like -27-28.

You know where the bias feed resistor is in your circuit? That needs to be a larger value. What is it now?

http://www.diyguitarist.com/Images/BiasChart-6L6.jpg

^^^ easier than arithmetic

[bluesguitar]

I have a good idea where to find the bias feed resistor, but I'm not exactly sure which one it is. I think you are right though that it has to be changed to a larger value. As it is I'm getting my bias reading from my Amp Head bias probe tool, and if it reads 58mA there is nothing I can do about that. It is what it is. I have to have more resistance so the tubes are getting less current. Correct me if I am wrong.

[rdjones]

I have a good idea where to find the bias feed resistor, but I'm not exactly sure which one it is. I think you are right though that it has to be changed to a larger value. As it is I'm getting my bias reading from my Amp Head bias probe tool, and if it reads 58mA there is nothing I can do about that. It is what it is. I have to have more resistance so the tubes are getting less current. Correct me if I am wrong.

You want more negative voltage to reduce idle current.

There's another trick you can use if you have a mismatched set of tubes.
Try swapping them into the opposite sockets and take the readings again.
The reason this sometimes works is that on one version of the Silverface bias balance circuit one socket is 'preset' and the other is adjustable to set the balance.
Later versions use a true balance adjustment that decreases the bias (less negative) alternately on one socket or the other depending on the direction of rotation.
Before making any circuit changes test the voltage at each output tube socket (pin 5) and measure the range of bias voltage as you turn the adjustment.
This can be done with without output tubes in place to determine the range and if both sockets are capable of being adjusted.

Healthy tubes shouldn't redplate with -45V bias.

rd

[Structo]

If it were me I would change the bias circuit to a adjustable fixed bias.
You can google that.

I know that sounds conflicting but what it means is at idle, you can adjust the voltage which is then fixed at that setting.

[NickC]

I recommend using the bias circuit in the attached article and schematic. I used it in my 1966 AB165 Bassman (actual printed label says AA165, but the circuit inside was AB165).

[bluesguitar]

I've thought about adding a bias adjustment potentiometer to my existing circuit, but couldn't find a supplier who had a surface mount pot. Anyone know of one? As for my dilemma I decided to put some slightly used JJ tubes from another amp in my Bassman and see what the bias reading was. It was 46mA, which was the same as the JJ's with the redplateing tube. I played the amp for over a half hour with no problem. With the problem tube I wouldn't have made it 10 minutes. So, as far as I'm concerned it was the tube that was bad. As for the Groove Tubes that had a bias reading of 58mA I don't know why they bias so much hotter. I know I wouldn't feel comforable having a set in this amp.

[martin manning]

Not sure if this aa371 is your Bassman, but here's how you can add a variable bias to the balance circuit. The red jumper from the diode to the cap is already there. A 10mm round Piher trimmer would be a likely candidate for this.

[Structo]

I like that.

Best of both worlds.

[bluesguitar]

It's back. It took about 45 minutes of playing, but I started to get a static sound like a scratchy pot that became more pervasive when striking my strings. This is the same symptom that I noticed and posted about on August 1st, if you want to check the history. I have since changed the 470 ohm resistors on the power tubes, and changed power tubes. My concern is that since this symptom is identicle to the first symptom that I noticed, and eventually led to one of my power tubes redplating (if this was the cause), that I may end up with another tube redplating. Where should I look next in my troubleshooting?

[rdjones]

It's back. It took about 45 minutes of playing, but I started to get a static sound like a scratchy pot that became more pervasive when striking my strings. This is the same symptom that I noticed and posted about on August 1st, if you want to check the history. I have since changed the 470 ohm resistors on the power tubes, and changed power tubes. My concern is that since this symptom is identicle to the first symptom that I noticed, and eventually led to one of my power tubes redplating (if this was the cause), that I may end up with another tube redplating. Where should I look next in my troubleshooting?

Did you check for bias voltage at both output tube sockets ?

rd

[bluesguitar]

This stuff is keeping me on my toes. I decided to play my princeton reverb to see if that problem was solved, or if i'm just cursed with bad luck. I had static right off the bat, but this time even with my reverb off. I never had that before. So, to make a long story short I discovered that the static (this time) was coming from my guitar pot or pickup or switch. Anyway I switched guitars, plugged in the bassman again, and played for 45 minutes problem free. So, all's well that ends well.

[Structo]

Sometimes in the dry winter months, guitars with plastic pickguards or scratch plates (depending which side of the pond you are on), will build up static electricity.

Stratocaster type guitars particularly.

So when you are picking and your hand is moving around it, you are disrupting that voltage creating the static you heard.

Get one of your wife's anti static dryer sheets (Bounce or similar) and rub it all over your pickguard.

This generally cures it for a while.

[bluesguitar]

Sometimes in the dry winter months, guitars with plastic pickguards or scratch plates (depending which side of the pond you are on), will build up static electricity.

Stratocaster type guitars particularly.

So when you are picking and your hand is moving around it, you are disrupting that voltage creating the static you heard.

Get one of your wife's anti static dryer sheets (Bounce or similar) and rub it all over your pickguard.

This generally cures it for a while.

Thanks for the reminder. I have actually done this to one of my other strats recently and it did the trick. Funny how when you get in a mind set of electronics issues that you fail to think of the possible simple causes and remedies.

[NickC]

re: static buildup on plastic pick-guards

I usually put adhesive copper-foil beneath the pick-guard (I source it from Stew-Mac), making sure it contacts the copper-foil I line the pick-up and control cavities with. The copper-foil is then grounded. That pretty much eliminates the problem.

HTH = Hope this helps