Low voltage readings on new build

MattPete · Post by **MattPete** » Sun Apr 01, 2007 9:33 pm

I just fired up a new Metropoulos 1969 Superlead build, and I'm getting low voltage readings:

pre- and power-amp heaters read 3.0 VAC (not too far from 3.25)
Power amp pin 5 reads -30 VDC (good)

Power amps pins 3, 4, & 6 read 235 VDC (way too low).

Any idea what's causing those low readings on the power amp sockets?

thanks in advance...

MattPete · Post by **MattPete** » Sun Apr 01, 2007 10:32 pm

Turns out I never bridged the two positive terminals under the filter cap nearest the power switch. I must have missed that in Step 9. Doh!

Now I've got 480-482 across the board. Time to call it a night, and try firing it up with the tubes in tomorrow.

Flames1950 · Post by **Flames1950** » Sun Apr 01, 2007 11:46 pm

Best to set that bias voltage at pin 5 of each power tube socket lower than -30 before firing it up with tubes!!! Adjust the bias voltage to as large a negative value as you can get (-50VDC maybe??) to ensure that your tubes will be running cold at first, as opposed to smoking hot.

MattPete · Post by **MattPete** » Mon Apr 02, 2007 10:13 am

Flames1950 wrote:Best to set that bias voltage at pin 5 of each power tube socket lower than -30 before firing it up with tubes!!! Adjust the bias voltage to as large a negative value as you can get (-50VDC maybe??) to ensure that your tubes will be running cold at first, as opposed to smoking hot.

Thanks for the tip!

The one thing I left out was that I had my first experience with a discharging cap. I decided to bridge the two terminals (amp unplugged, of course) with a piece of wire, and as I'm lining the wire up with the terminals -- thankfully gripped with insulated pliers -- a get a loud zap and a bright spark, followed by a rapid heart beat and lots of cursing.

I had checked to make sure that the filter caps were drained, but I never checked that one...