Fender Bassman 50 with Master Volume

[Structo]

Does the heater balance pot do much when you turn it either way?
If not then it could be suspect.
Those are generally 100 ohm pots.

What Gibsonman63 is referring to is the heater wiring should have the same wire go to pin 2 on both power tubes then on to the preamp tubes the same wire should go to pins 4&5.
Basically it means it should stay in phase throughout the tubes.
Since Fender used one color of wire throughout the heater wiring, there is a 50/ 50 chance that it is out of phase.
But that shouldn't cause a very loud hum if any if it is out of phase.

Try to keep all signal carrying wires away from AC power or other wires not carrying signal.

70% bias current would be in the 46ma area. If you take screen current into account then it would measure around 50ma for plate plus screen current.

But I would bias it a bit cooler to extend tube life so at 60% it would be around 40ma.
That does not take into account the screen current which is probably 3-5ma.
So maybe 43-45ma.

[xtian]

Does the heater balance pot do much when you turn it either way?

No, not much, until it's way clockwise, then it suddenly starts buzzing. Worth it to replace the pot, or just use a pair of 100R to ground?

What Gibsonman63 is referring to is the heater wiring should have the same wire go to pin 2 on both power tubes then on to the preamp tubes the same wire should go to pins 4&5.

Yes. One of the early things I checked. Thanks.

I would bias it a bit cooler to extend tube life so at 60% it would be around 40ma. That does not take into account the screen current which is probably 3-5ma. So maybe 43-45ma.

I need instructions on biasing based on the bias adjustment pot. What method do I use?

[xtian]

Still hoping for answers to the two questions in the post just above. Thx. (BTW, I'm using a JJ tube set.)

[Structo]

You could certainly disconnect the hum pot and temporarily install two 100 ohm resistors to see if that is where the problem is.

Do you have 1 ohm resistors from pin 1 to ground?

If not you should install those as that is the easiest way to measure the bias.

If you have to order them try to find the lowest tolerance you can find.
I know Vishay makes them down to .1%

Remove the existing link to ground on pin 1, then solder the 1 ohm resistor from pin 1 to a solder lug.
I see your sockets are attached with machine screws.
I would remove the screws on the back side of the socket and use a screw and lock nut and attach a solder lug on each tube so you can solder the 1 ohm resistor to it.

Then to check the bias you set your meter to read DC volts ( I know volts not amps) and clip the black lead of your meter to chassis ground then touch the red probe to pin 1.

That is your total plate current plus screen current on that tube. If you want to you can measure the voltage drop across the screen resistor and divide that by the resistor value which will give you the screen current.
Subtract that from the current you measure at pin 1.

Now, why do we measure voltage at pin 1 instead of current?
Because due to Ohm's Law, the 1 ohm resistor converts amps to volts.

E/ R = I where the R = 1 so E/ 1 = I
Any number divided by 1 is the number itself.
So any voltage you measure there is a direct reading of current.
Make sense?

Now thinking about your hum problems, you said that one channel at the beginning of this venture hummed a little bit and the other channel was fine.
This tells me either you made a wiring error or there is a faulty component.

If you have been obsessing over the amp it is best to take a break and walk away for a day or two.
Then come back with a fresh mind and carefully check your wiring by comparing it to the layout.

Good luck!

[Cliff Schecht]

I noticed on my Rocket build that the heater balance pot seems very ineffective. Surely I didn't miswire it but I never found any sort of a null point or anything. I don't think elevating the CT by the cathode voltage of the power stage helped much either. My problem might also be that I'm using very unknown tubes which could be both bad and many mA apart - this will also cause lots of hum.

[xtian]

Thanks, Tom. I do have 1% 1ohm resistors, so I'll go that route. I also will replace the balance pot with 100R resistors to see if there's trouble there.

Cliff, I've built three SE amps now, and battled 60Hz hum in all of them, and never had any success with elevated heater voltage, artificial center taps vs. real center taps, or lead dress. Frustrating!

What do you guys think about the idea that the PTs themselves may be mechanically/electrically balanced or not? I mean, some PTs hum loudly all by themselves when plugged into 120vac. I can feel the PT in this Bassman vibrating mechanically, and I can hear it hum, too (not thru the circuit, just mechanically humming, though very quietly).

[Structo]

I have noticed some PT that vibrate and others that don't, I'm not sure what causes it but I haven't found any relationship between that and electronic hum.

Basically there are two types of hum, 60Hz and 120Hz.
If it is 60 then it is related to your heaters.

I'm still thinking there is a possibility of an error in the wiring.

You will have an ah ha moment and see it and fix it.

Because if you have ruled out tubes, outside interference and defective components there isn't much left.

[xtian]

I put in the 1R 1% resistors and biased to 44ma.

Also removed the heater balance bot (tested bad after disconnecting) and cleaned up the heater wiring, paying attention to phase. Added 100R resistors from heaters to ground at the pilot light.

There is no more buzz, so that's solved. But the 60Hz hum is still significant. Maybe I could swap the phase between preamp and power tubes.

There's a lot more white noise on the Bass channel than the Clean channel, but then there's a lot more gain on that channel, too. I put in ear plugs and played my Strat a bit. There's plenty of good sounds available, though I might have detected some ghost notes.

Tom's right--I've got to stop obsessing for a while. I'll take it to AC/DC practice on Monday night and see how it is with a loud band.

[xtian]

How much current can a 6v lantern battery provide? Not enough, apparently. I got one and metered it: 6.5vdc. Then unhooked my 6.3vac power from the heater lines, and clipped in the battery to provide DC heater current. The filaments lit up, but just barely, and I was able to play guitar thru the amp, but at a greatly reduced volume. There was NO 60Hz hum.

So. Should the lantern battery be able to power the heaters of the Bassman's complement of tubes?

Seems like I've isolated the hum to the heater system. Not sure where to go next. The lead dress looks clean. Artificial CT did not help. Can it be solved?

[xtian]

12ax7 = 300ma heater current,
6L6 = 900ma
so total is about 3 amps.

Discharge rate for my lantern battery is about 250 ma.

Duh. Are people talking about some more powerful battery when they suggest this test?

[Structo]

Most likely a grounding issue.
As in you either have a ground in the wrong spot or a faulty connection.

I like the Dumble way where things are grounded where they appear in the circuit.
In other words, power supply filters ground near the AC inlet.

Preamp cathode grounds should be near the input.

[rmroza]

Hey Guys, working on a friends dads 1965 Bassman that was sick. Got it all working again, but he requested a Master Volume. It has this balanced thing going on as this original poster mentioned. I want to add the PPIMV, however with 5 pages, it was not 100% crystal clear how to hook it up as no real good pictures or layout drawn!

Can anyone chime in and confirm how to hook this up and supply good photos and /or layout?? It appears the balanced bias will need to be replaced to a more traditional split-tail design and then add the PPIMV (LarMar) in as normal.

Is this correct?? I'd like to get it to him this weekend without screwing up the bias and burning up his output tubes or something stupid! PLEASE ADVISE and thanks in advance.

[martin manning]

Here's a solution: http://ampgarage.com/forum/viewtopic.ph ... 033#254033

[rmroza]

Thanks Martin. Don't know if I will cnovert to fixed bias (easier) or use your balanced bias then add the LarMar PPIMV, but your direction is greatly appreciated and I'll be able to finish the amp and get it to my buddies dad this weekend!