'74 SFTR rescue

[firemedic]

Howdy again everybody, it's been a while....

I'm in the process of resuscitating a buddy's old SFTR (MV) and I may need some guidance along the way. This amp has been rode hard & hung up wet. Original caps. All original tubes except one of the power tubes was replaced by god knows what kind of tube, with no markings & vaguely resembling a 6L6GC but not really. The faceplate was bent in from physical abuse.

Just to get the ball rolling, I started with the basics and got 4 matched JJ 6L6GCs, and replaced every e-lytic. BTW it has those blue coupling caps. I also replaced the screen resistors. It has a grounded 3-conductor power cord but the ground switch is still wired up stock.

Then I brought up the voltage on a variac & lightbulb current limiter like I was s'posed to. The B+ rail and bias supply did fine w/ no tubes in. 

Then I tubed it up, and when I turned the power on, the limiter light bulb did flash brightly for a moment before dimming. BUT, the filaments were really dim, too, and the heater voltage was @ 2.5VAC. I disconnected and checked voltage on the unloaded PT filament leads and they had exactly 6.3VAC which I'm pretty sure is not right, it should be somewhat higher with no load.

Anyway, that's where I'm at right now. The owner is not in a big hurry which is great, so I'll be troubleshooting for a little while. Luckily I happen to have a TR PT just laying around, but I'd rather not have to use it if it's not necessary.

Any comments or advice are welcome.

[sluckey]

It has a grounded 3-conductor power cord but the ground switch is still wired up stock.

That's fine. Looks like the power cord is stock also.

Then I tubed it up, and when I turned the power on, the limiter light bulb did flash brightly for a moment before dimming. BUT, the filaments were really dim, too, and the heater voltage was @ 2.5VAC.

All voltages will be low while using the lamp limiter. Time to plug it straight into the wall.

[eleventeen]

"Time to plug it straight into the wall."

Make it so!

[firemedic]

Ya see that? That's why this forum is here.

I would have torn out my chest hairs trying to figure this out, and never found the problem. Stand by for results. It still may need work since the fuse was blown when he gave it to me, so we'll see.

Thanks for the common sense infusion guys!

 

[firemedic]

Update: She's up & running!

The stock bias is so cold! 16mA per tube @ 450V. -51v on the grids. At least the tubes will last forever. I'll mod the balance circuit if the owner asks me to, after we discuss the matter.

At first I thought the trem circuit needed work but I just needed to short the footswitch connections.

Lastly, I get 438v on the "top" plate of the LTPI, and 306v on the "bottom".

If it were my amp I'd replace all the plate resistors (or at least the ones that see >350v) and redo the bias circuit. What is the most commonly used method of dropping that bias voltage?



 

[sluckey]

What is the most commonly used method of dropping that bias voltage?

The PT bias tap usually connects to a resistor. That resistor is typically 470Ω, 1KΩ, 1.2KΩ, etc. Increase the value of that resistor to decrease the negative bias voltage on the grids.

[firemedic]

Okay, she's done & wrapped up. That is a loud MF.

It really just needed a cap job. I did replace the 1k bias feed resistor with a 1.5k, which brought the idle up from 14mA to 29mA for each tube, at 448VDC. Still very conservative but much warmer.

When I fired it up, the 7025 in V2 was extremely microphonic so I put it in V5. I figure the owner can replace the preamp tubes himself, on his own time.   

For me, it's just a hobby, but I do love this stuff. I don't even charge anything beyond the cost of getting parts.

Thanks again for letting me borrow the training wheels!