Thursday, October 17, 2013
Biasing a VHT Pittbull 100w head.
However, after pulling the chassis and inspecting it for damage-always done with a new customer with an amp failure complaint-and measuring voltages I got down to checking the bias, and what I found was a 20ma mismatch and a 65 ma draw on some tubes-about twice the limit.
Finding out information on these amps is not that easy because they're relatively low production and not often seen in these parts. Fact is this is the first one I ever opened up. But nevermind. It seems pretty straightforward and well built, but there's not a lot of information to be had and I understand Fryette is very protective of their trade secrets. A request to their customer service people has, as yet, gone unanswered but if you read their website you'd conclude that biasing an amp is just one step short of launching the Space Shuttle.
Even with the level turned all the way down, it was still waaaaaay too hot and the mismatch wouldn't pass muster.
As I was waiting for a response from Fryette and consulting with a couple of savants, some basic knowledge of bias systems came to my aid. The AC from the power transformer bias winding goes through a single diode where it is rectified into DC, passes through a 47k 1w resistor, and then is sent on to the tube board to be distributed by the bias pot.
It seemed that if I wanted to raise the voltage and thus lower the bias current at the tubes, I'd have to reduce the value of the 47k resistor-in this case to 33k.
Once that was done and the second set of JJ EL34s was installed-the first was far too noisy-bias was at 20 ma on the lowest setting, so I ramped it up to a nice even 31 ma and that's where it stayed.
One point to remember. The standby switch should be in the II position when testing, and make sure the tube selector switch is properly set for the kind of tubes you're using-6L6GC or EL34.
UPDATE: As of November 8 the Fryette people have not responded to my request to supply information that I sent off to them. Not that I expected they would, mind you, but if you're in the market for an amp, you want the company to support it, right? Something to think about.
As usual, what you do with this information is solely your problem and your responsibility, not mine.