Biasing or quiescent current procedure vh140c

[xjbear]

I was hoping I could get some help/clarification on biasing up the old Ampeg vh140c amps. It's a pretty basic complimentary pair push pull amp using only 2 power transistors. I've started replacing the mj15001/002 pairs with mj21193/94 pairs. I own 3 of these and really abuse them til they shut down on me. Lately I've polished the heatsink and mount surfaces and ditched the factory Sil-pads for mica and grease. Hoping these changes will sturdy them up a bit.


Anyway back on topic: what's the best procedure for setting bias? At the moment I've just set them for 5mv across the emitter resistors with a 1v input signal at 100hz. I'd like to get them set for a good and reliable operation. I've got the schems but there's no service procedure that I can find.

Thanks in advance!

[xjbear]

Schems

https://drive.google.com/file/d/1Tqa...w?usp=drivesdk

[Jazz P Bass]

If a 5mv bias has no observable crossover distortion, then leave it there.

Or you could try the 'Enzo Method'.

Crank the bias down.
While observing the overall current draw of the amplifier, turn the bias up until the amp starts to draw current.
Back it off a tad and call it a day.

[xjbear]

Thanks Jazz, that seems like a good idea. What kind of input signal would I use and are we measuring current at the mains input with this setup?

[Jazz P Bass]

You do not apply a signal when setting the bias.

I use a Kill-A-Watt meter at the mains.

[xjbear]

👍👍👍👍

[J M Fahey]

The name itself explains it:

quiescent current

"quiescent" meaning: "static/steady/no change" which implies "no signal", then you measure DC voltage drop with a Multimeter across emitter resistors or power transistor DC current (generally you open a power rail for that, older PCBs had a jumperable pair of pads for that purpose)

*OR*

you use the "dynamic" method where you feed low level signal to a loaded power amp and watch a scope screen for dissapearance of the crossover notch.

Itīs either one or the other method, not both mixed.

[xjbear]

Thanks JM. Definitely, using the scope to verify a current dissipation setting of 5mv, that was set with a signal, will just show crossover if you can even get the trimmer to go that low with a 1v signal on input and knobs cranked. "Without any signal", all knobs up for Max preamp noise, and no cab attached, then set for 5mv across the emitter resistor. That's the process I was using, and need to check it with signal on the scope and see where it's at.
So the mains current draw is quick for sure, how important is it to really get it perfect? As long as it's not cooking at idle of course,

[Jazz P Bass]

"how important is it to really get it perfect?"

If the bias is really cold you will here the raspy, buzzy x over notch.

If it is way too high the transistors do not cool off properly.

[xjbear]

Awesome. I just realised it's a stereo amp so I'll have to just stick with the scope to do it proper. So much for wanting a shortcut lol

[xjbear]

Thing is a trip. On the scope with a signal I get a beautiful sine wave and the notch disappears just perfectly right in the mid point of travel on both channels.
So I then unplugged signal and cab and measured the emitter resistors just to try and get a benchmark mv measurement and there's nothing lol. 4 on 1, .4 on another, 1.4 on one and the last is like .01.
Guess I'll just stick with the scope and forget the rest