Current draw on new build

[mscaggs]

Guys,

I am a little miffed on this one. I have done a frame off resto on a 67 Bassman. It all went together very nice but when I fired it up on the variac I noticed the amps climbing after 30 seconds or so. I pulled all the tube and no current draw. Then put in some known good 6l6s and the amp and it started climbing again.

Now, this amp did not work when I got her so who knows the history.

The PT and Choke both checked out in another project where I tested them. I have not tested the OT first hand.

Note, screens are new, diodes in the bias control and cap all replaced. No AC shorts I can find. Switches and fuse all check out.

Looking for ideas and I'm sure I will feel really dumb when I find the issue.

Thanks

[xtian]

Current draw SHOULD climb when tubes start conducting. What makes you say it's a fault state?

OT could be suspect, if all else checks out.

What's your experience level? (sorry, don't think we've met yet!)

What does "frame off" restoration mean?

[mscaggs]

Current draw SHOULD climb when tubes start conducting. What makes you say it's a fault state?

OT could be suspect, if all else checks out.

What's your experience level? (sorry, don't think we've met yet!)

What does "frame off" restoration mean?

Hi and thanks,

I do quite a few Bassmans and usually only draw .7Ma at idle. This is climbing to 2 amps and I back off at that point, something is up...

My experience level is non-expert but I do a lot of amps...

www.emike.net/67bassman
this is an example of another 67 I did not long ago

Frame off means I pull them down to the bare chassis and start over

[sluckey]

I do quite a few Bassmans and usually only draw .7Ma at idle. This is climbing to 2 amps and I back off at that point, something is up...

Sounds like you may be referring to AC line current?

[Stevem]

If you want to prove out that the PT is not shorted you can do this check out.
First pull all the tubes out, pull out the lamp and disconnect all of the secondary wires.
Then on the AC input side of the fuse wire in a 1 ohm 5 to 10 watt resistor and hook up your meter across this resistor, and set it to read low ac volts.
Turn the amp on and you should only read .2 to . 4 volts, a shorted PT for this size amp will read .5 to 1 volt.

[sluckey]

...when I fired it up on the variac I noticed the amps climbing after 30 seconds or so. I pulled all the tube and no current draw. Then put in some known good 6l6s and the amp and it started climbing again.

This sounds like the output tubes are running too hot. What voltage do you have on pin 5 of each output tube? You can check this with output tubes pulled and variac set for 120VAC.

[rp]

I have done a frame off resto on a 67 Bassman

Welcome to the forum, really hate to start off this way but why would you tear BF or early SF Bassmans apart and rebuild them? I could see if the amp was pulled out of a flood or had previously been tragically molested but Fender amps are practically indestructible all they need is some basic service when called for and simple maintenance every decade. AFAIC you've destroyed the value and worse the appeal of the amp. I wouldn't want the one in the pict. Best use for it now is to turn it into a 50W Dumble.

It's like when you find a beautiful intact piece of old furniture that someone stripped the finish, charm and patina, over-polished the brass, and made it shiny new, or an old perfect guitar that someone didn't like the normal effects of time on it and refinished it. Please desist and limit yourself to basic service. Apply the anal madness to new builds.

Just to reinforce my argument, the vintage cars that are worth the most now are no longer the factory mint-like rebuilds but old solid models that still have their old paint and upholstery and as many orig parts as possible - the "little old lady from Pasadena" cars. It's going that way for everything vintage.

[Stevem]

If the PT checks ok then open up the bias pot wide and look for over - 54 volts on pin 5 of each output tube, if you get that much then you should not read the PTs primary current draw topping 2 amps.
If that checks then you should be good to set the output bias current as normal.