vintageHIWATTconvention.com

Hi,

I'm about to delve into my SA112 further to sort a few small issues - probably a volume pot (real mismatch in channel volume despite tube changes), and the fact it seems to run a bit warm, temperature wise. What's the best method to ascertain bias & set bias?

I'll pop some new power valves in - tempted to go for JJ 6V6's so I can really push it when I gig with it.

Unfortunately, there isn't any way to check the bias in a stock unit that is not mildly dangerous unless you buy one of those plug-in bias measuring tools. Check out the tube amp FAQ for various methods. I generally use the 'measure the OT resistance' method described there.

Is this a 50 or 100? The 50s are much easier to change; the 100s tend to be under-biased to begin with.

--mark

Hi Mark,

It's a 50 watt one. I've been tempted to make up a bias tool - they're just all straight through, but a 1 ohm resister on pin 8, with test points either side right?

Can you point me to the tube FAQ?

Cheers,

Andrew

Yes,
remove the buss wire from lug 8 to ground and replace it with a 1 ohm 2 watt or higher resistor. it makes for a simple way to check the bias.

Mark made a nice diagram of where to put a bias pot in a DR504,

Cheers!

this is really handy too!

http://www.dreamtone.org/Calculate_Bias_Form.htm

Cheers again - I have used the formula when biasing my fenders.

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#howtobias

Use the Google.

--mark

Sorry Mark - I thought you meant an onsite reference

Here's a gutshot...somewhat fuzzy