1974 Bassman 410

[byobkim]

Did mod 2 on diagram no volume. Reverted back to figure one keep the 220 ohm resister instead of the 100 ohm resisters, but have 10 volt difference between neg voltage on the output tubes.
Is this a problem?
Amp sounds great though.

[Gibsonman63]

If I am not mistaken, you are looking at Gerald Weber's book for this mod and converting from bias balance to adjustable fixed bias.

For the bias balance, you would just adjust the pot until you get the least amount of hum, which should but your power tubes close to the same current draw. Did you measure bias current and plate voltage?

I did this conversion on a Silverface Super Reverb with no problems, but I had to copy the AB763 Schematic to get it right. I don't remember if your amp has two or four power tubes, but you could copy the AB763 schematic for a blackface amp with the same number of power tubes. For me it was a little easier to understand.

I also have a 1973 Bassman 100 with this mod. There is a dropping resistor between the bias voltage supply and the bias adjust pot. I had to increase it to get to the correct bias supply voltage. I used little trial and error, but I am sure there are guys here smart enough to calculate it. I copied the AB763 Twin Reverb schematic for this one.

[martin manning]

Did mod 2 on diagram no volume. Reverted back to figure one keep the 220 ohm resister instead of the 100 ohm resisters, but have 10 volt difference between neg voltage on the output tubes. Is this a problem? Amp sounds great though.

It would help to know exactly which model Bassman you are working on, but ok figure 2 is a mod to replace the bias balance control with a conventional bias circuit where the same voltage is fed to both sides of the PP output stage. If you got no sound* when you did this, then there must have been some error. Reverting back to the bias balance control but keeping the 220k bias feed resistors will make some difference in the sound, but otherwise shouldn't be an issue. The 10V difference you have now is after adjusting the bias balance control for what, minimum hum? If that's what it takes then that says that the power tubes are not well matched, and this voltage difference is required to get the current flowing through each side to be approximately equal.

* Gibsonman63 brings up a good point- the range of the bias control (as shown in figure 2) might need to be dialed in, which is often done by adjusting the value of the resistor on the rectifier board or the resistor from the board to "B" in the sketch.

[byobkim]

Yes, this is Gerald Webers book. This is a 50 watt two power tube amp.
I was thinking the dropping voltage resistor might be the issue. I am not sure what value to put in its place. It is a 3.8k resister now. Would you go to a 5.6k?

I do know the power tubes were a matched set. At least according to Parts Express. Sovtek
By the way the amp sounds tight. Love it. I replaced all of the caps. Used orange drops for the poly caps. Replaced the filter caps and installed a new power transformer. I think the original power transformer was weak. Put in a Magnetic Components transformer.
Other than the 10 volt difference in the bias voltage. I'm concerned that this might affect tube life or output transformer. I am not sure about the plate voltage.

I can send pictures of the other mods I made. I am going to pull the chassis anyway. If anyone is interested.

[Gibsonman63]

The bias voltage is really not the end result it is the plate disipation of the tube that you should more concerned with. I use a bias meter because it was cheap and I am chicken to try the other biasing methods without some one more experienced showing me how first.

But don't panic because it is not a huge deal. If the tube is biased hot, it will shorten tube life. On the other end of the spectrum some people don't like the sound of an underbiased tube.

If you like the sound, it may not be a problem. Shortening tube life may not really be a problem, tube are relatively inexpensive and still last lots and lots of hours.

[byobkim]

I think I will leave it alone and see what happens. You are right worst case I lose the power tube.
Hopefully not while playing.

Going to go play the 1950 Valco.