what the best way to add bias to combo

[echo44]

SO I just bought custom 5o 1979 combo. It is turret board inside.
I checked out this site and there are post about adding a potentiometer
for bias for a DR103 circuit. Not sure what circuit is in my amp. Will this work?
viewtopic.php?f=2&t=1378&hilit=bias

I did measure the voltage from the wall tonight and it was 121 volts. So I plugged the amp into a variac
and reduced the voltage to 110 and it did sound smoother. I currently am using amperex 1957 NOS tubes nickel base.

Another interesting note about this amp is after playing for 30min and stoping I notice a persistent ringing in my ear

[echo44]

oh here is a picture of the insides it looks all original to me?

[OldSchoolDave]

oh here is a picture of the insides it looks all original to me?

I spot two relatively fresh solder joints. Other than that, it does look original.

The resistors on the power tube sockets look charred. Not unusual - they're subjected to a lot of heat. I've had them crumble in my hands before. Should be replaced.

Somewhere here on the board are some pics of bias adjustors members have installed. I swear I did it to at least one of my Hiwatts, but can't find any pics now that I need them .

The first thing to do (after replacing those resistors) is to measure the voltage and current to the power tubes at idle. With that information, it's possible to calculate how much adjustment is necessary.

Be careful when poking around - there are potentially lethal voltages inside.

Dave

[mhuss]

It looks like someone already tried to perk up the bias voltage (the fresh solder joints on the skinny board). I'd measure what the bias is currently before making any further changes. (Carefully!) Measure the voltage at the 22k resistors going to the big tube sockets. It should be around -45 VDC with respect to ground.

It's hard to tell from the picture if the screen resistors on those sockets were overheated or not, because they're dark green to begin with. Compare them to the color of the larger green resistor.

--mark

[echo44]

Thanks

Ok
so I measured the bias voltage at the 22k resistor it was -39
plate voltage = 480

what would be the next step?
I saw a post where someone added a 1meg potentiometer to a 103 circuit. I do have a 1meg potentiometer. However,
I saw somewhere else that with the 504 circuit which is what I think I have to use a 20k would it matter if I used a 1 meg or
others have added "marks board"? Not sure what the best route is or do i need to do anything? I also found I have a 20k potentiometer
as well but it has only two prongs and is rated at 1/4 watt. And one last question looking at the picture I posted

at the point where there is a fresh solder joint someone added a second resistor in parallel is this where I would remove the resistors
and change them to a potentiometer? It currently measures 111k between those parallel resitors

Thanks

[echo44]

Update:

I added a 20K potentiometer biased the amp to -45
sounds great!!

[mhuss]

Glad to hear you figured it out.

--mark