Bogen PA--->Guitar Amp, few questions

[Turbo7MN]

Bias supply:

[Tchefong]

Sorry i dont understand that either. I've never built amp before i dont even know what is a b-.

But the info you got there look like to set a good individual bias for each driver tube but dont talk about EL34. Is that bias gone to work whit those.

By the way the link i sent you also have your amp on ebay just do a search for bogen.

[ethel188]

If you want an internal picture of the same amp I have on ebay here it is. The amp is working.

[Turbo7MN]

Okay, I was looking at some other circuits and got to thinking, why can't I just change the cap and resistor values to change the voltage? This is what's reccomended on the Triode Electronics site for the Quiksilver 8417 amps, why not the Bogens? I think it would be much easier then cramming in more stuff.

I also found a thread on here about a guy having identical problems as me, low volume with what sounds like preamp distortion. It turned out to be a problem with the tone controls, he recapped them and it fixed the problem. I might order some replacement caps for my tone stack too.

how do i figure out what's wrong with this (vintage) amp

Also, an MX-60A popped up on ebay and it just looks like an M60A with extra inputs.

[Turbo7MN]

Although I'm not getting much input here anymore, I'll at least say that I got the tubes tested. The 8417s check out good with similar results. The 7247 is in incredible shape, from how it looked on the tester I think it will outlive me.

Strangely, the 6EU7s tested completely dead, but this can't be the case because they light up and I was getting sound so they obviously can't be dead.

[anatech]

Hi Darren,
I agree. If the filament lights you will get some kind of reading on at least one tube. Test them in circuit by measuring the plate voltage and also the DC voltage across the cathode resistor (to figure out the current draw).

I really wouldn't worry about it unless you have changed the coupling caps and have no/low signal.

-Chris

[ethel188]

The 6eu7's of the m60a amp I just sold had a bad set of 6eu7's that I replaced. A bad tube may still light up. The cause of a bad tube may be the result of leakage. That is why one of my tubes was dead. That may also be the cause of why you had that loud noise when using the amp. On another amp I had the 7247 had leakage and it caused a lot of noise and half the tube was dead and the tube filament was working just fine. I also have some 8417's that were used alot and are almost dead and still fire up. Also did you make sure to test both plates of the tube?

[anatech]

A bad tube may still light up. The cause of a bad tube may be the result of leakage.

True, but the tube tester would still read something.

That's why sometimes it's best to troubleshoot the tube in a known good circuit.

-Chris

[Turbo7MN]

Originally posted by ethel188
The 6eu7's of the m60a amp I just sold had a bad set of 6eu7's that I replaced. A bad tube may still light up. The cause of a bad tube may be the result of leakage. That is why one of my tubes was dead. That may also be the cause of why you had that loud noise when using the amp. On another amp I had the 7247 had leakage and it caused a lot of noise and half the tube was dead and the tube filament was working just fine. I also have some 8417's that were used alot and are almost dead and still fire up. Also did you make sure to test both plates of the tube?

I'm not sure about both plates, the shop guy was running the tester, not me.

The needle was as far the the left (bad) side as it could go for both tubes, due to the strange pinout of the 6EU7 it could have just been a matter of his tester not being set up for them. The "loud noise" I heard sounded awfully like an explosion of some sort, could that be a result of leakage?

On another note, I got adventurous (or stupid, take your pick ) and fired up the amp without tubes. No hum, and and all the voltages were normal. Then I put the tubes back in (stupid, I know) and fired it up again. It's acting exactly like it did before, barely audible hum but low volume. I didn't keep it on long and I don't intend to fire it up again without new caps, but it seems to be alright.

What puzzles me is that if there's no hum, and all the voltages are fine, what could have exploded?

-Darren

[rickl]

Although I'm not getting much input here anymore

I thought we were waiting for your new parts to arrive and get installed .

Strangely, the 6EU7s tested completely dead, but this can't be the case because they light up and I was getting sound so they obviously can't be dead.

I'd rewire the socket for a 12ax7 and use one of your 12ax7 spares.

I've had tubes that light up but are dead. Or at least one triode section. Since you know it is dead, don't use it.

I haven't looked at your schematic but when I was debugging my HK PA amp (with help from this forum) I was able to bypass the tone stack for initial testing.

Once you get your new parts installed, hookup a load (old speaker), short the inputs and measure every pin on every tube.

Post voltages. I like Va for plate voltages, Vk for cathode voltages, Vg1, Vg2, ... for grids. Don't forget to check heater voltages. My HK used the heaters for the driver tubes to bias the output tubes. I pulled the mic driver tubes so the output tubes were not getting biased. This threw me off until more wiser members straightened me out.

We can start looking at suspect voltages (input grids should be zero or less), cathode biased sections should fall within spec, ...

rick