Problem with high mAs...

[Bob M.]

Hi,

I was biasing up a Traynor YBA-3A bass amp, using 4x 6N45C russian tubes, by the transformer shunt method and I kept blowing the 200mA protection fuse in my ammeter. I was setting the bias for 50mA per side of the push-pull circuit, as per Traynor's recommendation, but I noticed when I took the amp off of standby, I got very high milliamp readings initially that lowered down to normal in about a second or two. This initial high current, I think, was the culprit in doing in my protection fuse. This amp has a separate bridge rectifier for both B+ and screen supplies and it achieves standby by lifting the ground on both bridge rectifiers. The bias supply is not standby-ed and is activated with the on-off switch, as per most amps. I was thinking of adding some sort of soft start to the onset of 'play mode' from standby. If this high inrush current is bad for my meter, I can't imagine it's good for the amp either. Maybe a thermistor or some other idea to keep that current surge more normalized. And comments or ideas? Has anyone had this problem before?

Thanks in adv,

Bob M.

[nickb]

A wonder if the relatively long time constant on the grid side of the 6K6's 82k+100k=182K x 0.33uF might be responsible for this. When the standby closes the power to the pi is applied and so the ramping voltage on the pi plates will pull the grid of the power tubes up.

Personally, I learned never to use a 200mA meter in a circuit that can source more than that. I use the 10A range here - it's plenty accurate enough.

[Bob M.]

Well, you're right about using the 200mA range on my ammeter in such a high powered amp. Unfortunately, when I use the 2A range or the 10A range, I don't have the resolution that I'd like to see (less digits). Actually, the 200mA range has been fine for almost all MI amps except the most high powered. I've never had a problem with this range with, for example, a Twin Reverb or other 4x 6L6 amps. But your point is well taken.

However, that's not actually what's at issue here. I'm wondering if I should be limiting that inrush current when I switch from standby to play?

Thanks for your reply,

Bob M.

[nickb]

However, that's not actually what's at issue here. I'm wondering if I should be limiting that inrush current when I switch from standby to play?
Bob M.

You'd need to actually measure the surge to find out how big and how long it is. Doing a rough simulation gave me ~250mA for 300ms. Far too small and short to be a concern.