Small Special without a Choke?

[lovetone]

Have you installed 1 ohm cathode resistors? if not install them first.

If so set your meter to measure mv and connect it across one of the resistors as the light bulb lights up you can read the current to see what value you have.

Then do the same with the other, that way we can work out if there is a large current draw where it might be going.

Geoff

[lord preset]

What is the bias voltage at this point?

It's set to the mid value of the bias pot - about -29V.

[lord preset]

Have you installed 1 ohm cathode resistors? if not install them first.

If so set your meter to measure mv and connect it across one of the resistors as the light bulb lights up you can read the current to see what value you have.

Then do the same with the other, that way we can work out if there is a large current draw where it might be going.

Geoff

The 1 ohm resistors are there. I went ahead and took the amp off the light bulb limiter. Plates are at 448v but the minimum bias is way way hot (86mv/77mv). I will need to tweak the bias circuit to get to safe levels.

The thing is, even off the light bulb limiter the amp makes no sound at all - no hum, no crackles when touching screen or plate connections - nada. Also, the plates are lower than expected - the peavey schematic shows 500v plates and 490v screens.

[martin manning]

You're certainly dragging the voltage down with the high current draw. The lack of sound is another problem with another cause. First get the bias dialed in.

[jelle]

+1

[lord preset]

OK, progress. I upped the resistor from the bias pot to ground and now have the tubes biased coldish and have tweaked the dropping string to get reasonable voltages for V1 and V2

V1 215/215
V2(PI) 289/284
V3: 476 Plate / 477 Screen, Cathode: 39.5mv
V4: 476 Plate / 478 Screen, Cathode 32mv

Note that the screens are not lower than the plates. I also noticed that the voltage entering the OT is 481v but the plates are 5v lower, not something I have run across in my limited experience.

And did I mention there is no sound? I thought there would be sound.

[jelle]

Hmmm, maybe the previous owner busted the secondary of the OT? Do you have a scope?

[lord preset]

Hmmm, maybe the previous owner busted the secondary of the OT? Do you have a scope?

I do not. But as I mentioned, the speakers on this combo were completely dead when it came to me so I have always suspected the OT. I have another one coming in a few days that I'm going to try.

[lord preset]

So night before last I got a different OT to try and hooked it up fully expecting that to be the problem , but --- zilch. No sound. I threw in the towel for the night and resolved to pull out my DIY signal tracer the next day to figure out what was not connecting. Yesterday I fired up the amp and injected a test tone into the normal input and before I could put probe to test point the amp made sound. It had miraculously healed itself!

Actually I have no idea what I was doing wrong previously. Most likely in the course of replacing the OT I retouched a bad solder joint but I doubt I will ever know. But I take breaks whenever I can get them so I went back and reconnected the original OT and sure enough it worked fine. I then turned my attention to the new-to-me FET input and adjusted the bias to fairly close to spec.

I now have it in the combo cabinet with some generic Emi speakers. With a new grill and corners it's hard to believe that the cabinet is 38 years old. The Peavey tolex made it through the years almost unscathed. Now that it's all together, it sounds good, has a wide range of tones, but actually has less headroom than I was expecting, probably because of the fairly lightweight Peavey iron.

EDIT: I had a 12AX7 in PI position rather than a 12AT7. Big difference in headroom. Nevermind

[M Fowler]

That's a nice looking build

Mark

[Colossal]

Small Special. Big chassis.

Looks good!

[lord preset]

Thanks. As we all know - It is better to look good than to sound good..

[img:320:180]http://static.oprah.com/images/o2/20140 ... 20x180.jpg[/img]

[drew]

I like your back panel warning, but you might want to add something about not operating the amplifier while in the bathtub.

[lord preset]

Cleepzz:

https://www.youtube.com/watch?v=drIPBZRx1t8