Tube amp troubleshooting...

[Richard Crowley]

My prime suspect would be drying capacitors. Followed by dying firebottles.

[BBlack]

This amp had it's capacitors replaced with fresh new ones last year.

Now what is a 'firebottle'?

[Richard Crowley]

Were ALL the capacitors replaced? Was it done properly? Did your symptoms happen right after the capacitors were replaced. Help us out here. Don't make us play 20 questions.

A "firebottle" is a derisive term for a tube/valve. Have all the tubes been tested and verified as good recently? One of the big problems with tubes is that they have finite lifetimes due to several "wear-out" factors (vs. solid-state). Are the tube pins and tube sockets making contact properly? Another failure point unique to firebottles and exacerbated by the heat.

What is the status of the potentiometers (the troublesome one in particular). Have they been cleaned/replaced/refurbished recently?

[drtechno]

yea, I think rcrowley is correct in thinking all the caps aren't replaced.

the .047 caps that make up the "relaxation oscillator" can cause this.

another area is to look at the 20Uf cap on the 6eu7 (v4 I think from memory) cathode and make sure the cathode's resistor resistance is 2.2K to 2.7k.

also I've seen the control that feeds the oscillator the signal go bad. 250K control I think it is (if memory serves me correctly) .

[BBlack]

This technical talk is rather foreign to me, and I have no gear to test these things.

This did not start to happen right after the caps were replaced. The amp functioned well for months after, with no problems. The tech was a competent guy, as far as I could tell, but as I said I'm not totally experienced as far as in-depth electronic stuff goes. I do recall that he removed one capacitor and replaced it with two that equaled the value of the old one. I still have the removed capacitor, and I could write down the numbers on it if that is of any use.

rcrowley - I will give the pots a good cleaning and see what happens. I cleaned the tube sockets a few days ago, and did notice some of the pins on a few of the tubes weren't exactly straight. When I was at the shop last, I had the tech test all my tubes, and they were all in good or good-enough shape. I don't feel like I've used the amp enough since then to have worn them out, but you never know.


Another problem with this amp is that the built-in footswitch isn't always functional. You have to click the switch over and over and over until the reverb or tremelo engages. And sometimes, it simply doesn't work. Could these two issues be connected in some way?

I appreciate the help! I also realize this fix may be a tad beyond my scope, and that I'll have to take it in. We'll see...

[BBlack]

One thing -

The amp is apart right now, and upon inspection I noticed some whitish powder-looking stuff sitting on the inside chassis surface right below a capacitor that hangs over the particular area. Could this be the result of a dried-out cap?

[Richard Crowley]

That is a strong evidence of a drying-out capacitor.

[BBlack]

Thanks for the help rcrowley.

I think I'll be taking this amp in for repair tomorrow. Will report back with the diagnosis!

[fireberd]

Just a comment on checking tubes. The only real way to check a tube is to replace it with a known good tube. Tube testers can check some parameters but they can't test everything, even the mutual conductance type testers.

I used to work as an amp tech (in Nashville) in the early 70's when almost everything was tube and from experience the only sure way is with another known good tube.