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**cbarrow7625** · 02-28-2008, 09:05 PM

It could be a million things...

The first thing that pops into my head is that it may be a ground loop that was inadvertently caused when you rebuilt it. Did you put the ground wires back exactly like they were or did you move them around in some way?

You may want to try creating a star ground instead of using the typical Fender brass plate ground. htose things can get corroded over the years and they stop conducting to the chassis as well as they used to. That can cause it.

It also could just be a bad filter cap in the first stage. Did you replace those? Do you have a good way to measure the power supply ripple (a quick n' dirty way is just to measure the B+ using the AC setting on your meter) it should only be well down in the millivots range.

How about the heater hum balance (does this amp have one?)? Is it working? Have you adjusted it? If the filament supply has a center tap you don't need it. You can create one with some 100 ohm / 1-3watt resistors to ground from each leg.

Check into those things & report back.

**Enzo** · 02-29-2008, 12:23 AM

Are your power tubes a matched set? Mismatched power tubes will work just fine, but they won't balance out the hum in the stage.

**joepopp** · 03-03-2008, 06:52 PM

Sorry for the late reply guys. I appreciate the responses.

CBarrow:
I did put all of the ground wires back to the way the were but that brass plate does look a little corroded. I will try a star ground. I use a star on my marshall builds and they are super quiet.

With all of the preamp tubes out it still hums. It's not a very loud hum but humming all the same and louder than it should be.

Where should I take the reading for the B+? I did change the filter caps but I heard these can be bad from the factory.

The amp does have a heater hum balance. It does not seem to do much except crackle in the middle a little. The Hum adjust will decrease the hum when turned all of the way to the right but it seems it is just turning down the output tube power.

I will do some more testing and post here.

Enzo:
Yes, the power tubes are a matched set and I tried different matched sets.

Thanks again guys!

**cbarrow7625** · 03-03-2008, 07:01 PM

Your description makes me real suspicious of that Hum Balance pot. "Crackle in the middle" could be a burned up pot. "Turning down the output tube power"...what? The hum balance pot should have absolutely no effect on anything but the amount of hum. If is is turning down the output tube power then you have something strange going on.

Disconnect the hum balance pot & pull it out of the amp. Check it from end to end & from each end to center with an ohm meter to make sure it actually works. You can either replace it with another pot or replace it with fixed resistors similating the pot at its middle setting.

Try the star ground as well.

**joepopp** · 03-03-2008, 07:17 PM

Cbarrow:

Sorry, I was not looking at the amp when I wrote this this.

There are two screw pot adjustments. One says "hum balance" and the other says "Output tubes" Turning up the one that say "Output tubes" increases the hum, but also seems to increase the power. The "Hum balance" has the least hum when at either end of its rotation. It is a little scratchy in the middle but seems to work.

I noticed something today though. The power tubes are actually vibrating very slightly. I have never witnessed this before. Thanks for the help!

**Euthymia** · 03-03-2008, 09:24 PM

Here's what I would do:

1. Replace that hum balance pot with a pair of 100R 1/2W resistors (just like having the pot set in the center). Sounds like the pot is toast. Just a 100R from each side of the heater to ground.

2. Convert that amp to BF-style global bias and set the bias by current draw.

Then see if it's still humming.

**joepopp** · 03-04-2008, 04:09 PM

I disconnected the hum balance pot and put 2 - 100R to ground. This really did not help much.

The pot labeled Output Tubes (Balance pot) has a resistor on it. Disconnecting this the resistor on this pot solves the hum! I don't want to fry my tubes though.

The amp was modded before I rebuilt it so I am not sure if the layout is even right! There seems to be great differences between mine and others I have seen. Below is a link to a pic. Any advice helpful. I am ready to stick it in a box and send it Torres to get it right.

Pic of modded section

**52 Bill** · 03-04-2008, 04:28 PM

From the photo, it appears to have been "blackfaced", because the driver has what appears to be standard AB763 type values.

What resistor on the balance pot are you talking about? If this has been modded, I would guess that the bias circuit was changed over to a normal variable bias adjust, and may no longer be a balance circuit. Maybe take a photo showing more of the amp.

**joepopp** · 03-04-2008, 04:53 PM

**Mark Black** · 03-04-2008, 05:03 PM

In stock form the resistor on the balance pot is the bias supply pull-down resistor which sets the bias voltage. It can be substituted with a different value to set the idle current draw. Pulling it out would allow the bias voltage to rise considerably in magnitude, making the power tubes run colder. This would lead to reduced current draw and therefore less hum (but at the expense of too-cold operation and crossover distortion).

If the bias circuit has been modified the resistor may be used as a range-limit, setting the minimum amount of bias voltage (and therefore just how hot the power tubes can be made to idle). Pulling it out would have the same effect as above.

Or I suppose the circuit could have been modified to be completely FUBAR....

Good luck!

OK I just saw the pictures which posted while I was composing this. Looks like the hum balance is at this point completely disconnected and the bias control is indeed changed from a balance to bias-set.

**52 Bill** · 03-04-2008, 05:15 PM

From the photos, the bias circuit has been converted to a bias adjust. And as Mark noted, removing the resistor on the back of the pot will cause the bias to rise, shutting off the output tubes.

Have you adjusted the bias on this amp? What is the current draw on the output tubes?

I don't see the bias supply parts. Where does the wire that goes from the bias pot through the chassis go to? What are the voltages on the 3 terminals of the bias pot and what is the B+ on the output tubes?

Upon further review:
I was checking out the photos, and noticed that there was a 470K ohm resistor on one of the output tube sockets. This should be a 470 ohm resistor like on the other socket. This could cause a mis-match in the output stage, thus your slight hum.

Now I'm totally confused!
The first photo showed a phase inverter circuit with blackface values, but the later photos show the original Bassman 70 values. What's going on here?

**Bruce / Mission Amps** · 03-04-2008, 05:38 PM

Originally posted by 52 Bill View Post

I was checking out the photos, and noticed that there was a 470K ohm resistor on one of the output tube sockets. This should be a 470 ohm resistor like on the other socket. This could cause a mis-match in the output stage, thus your slight hum.

BINGO! What a great catch.
I wonder how much less current that one is drawing compared to the one with the correct 470 ohms.

**joepopp** · 03-06-2008, 02:06 PM

What a Bonehead!!!

Thanks guys, I thought I checked everything three times...

I will post results.

Joe

**52 Bill** · 03-06-2008, 04:52 PM

Let us know what happens.

I'd still like to know why one photo showed a modified phase inverter and another one showed original values?

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