No sound - volume knobs are ineffective

[GlennW]

This is a Lafayette PA645 amp. It uses a 5AR4, (2) 6L6GC, 6AV6, (2) 12AX7. When it's turned on the pilot light and all the tubes light up. There's a faint "it's alive" sound coming from the speaker.

It has four volume knobs: Mic 1, Mic 2, Aux 1&2 (1 to the left, 2 to the right, 0 at 12:00), and Master. Turning those has no effect on the sound.

When a guitar is plugged into the Aux inputs with a 1/4"-to-RCA adapter nothing happens, still the same.

I swapped all the tubes except for the 6AV6 with tubes that are known to work and there was no difference.

Is there a likely common cause for this? OT? Thanks.

[Tom Phillips]

Glenn,
One idea.
The gain of the AUX input channels is likely too low for effective use with guitar pickup signel. Did you try the mic inputs with the master all the way up?
Tom

[GlennW]

Tom, I've played through old Bogens using a RCA adapter with no problem. I don't have an adapter or mic that will fit the mic inputs. Turning the knobs does nothing as far as the sound from the speaker...it's like they're FisherPrice knobs.

I've been reading about testing OT's, but I'm pretty inexperienced at this.

[Enzo]

Why assume it is the OT? You already told us there is a "hello, I'm on" sound. If the OT was screwed to the point of zero sound coing out, your background sound would be gone as well.

What voltages do you find onthe tubes? Got B+ on pins 3 and 4 of both 6L6s? Either there is negative gid bias on the pins 5 or they are at zero and the cathode is a + voltage instead. All that depends on if it is cathode or fixed bias.

Then the other tubes. Got some reasonable plate voltage at each plate? It is present or gone or present and very high. Then look at each cathode. If any of those are at zero voltage, then that tube is not running.

Got all the volts you need on the tubes pins that need them? The use signal tracing and signal injectin to find the trouble. Touch some metal thing to pin 5 of each power tube. Your meter probe - without grounding the other one - should do. Does that make a little more hum out the speaker? Testing the tube for amplification. Of course you could inject a real signal, but in go/no go testing hum is enough for me. Move back through the circuit injecting at each grid. When you pass the bad stage, your injection will no longer work.

Or apply a steady signal to the input jack. Now trace the signal through the stages, using a signal tracer, a scope, or your meter set to AC volts.

[GlennW]

Thanks a lot, Enzo. I'll print out your outline and learn how to read voltages. I have some test equipment and it's time for me to learn how to use it.

I was suspecting the OT from some things I've read, plus it's a common denominator to all the channels. I'm glad to hear that it isn't the problem.

I have no idea what the voltages should be. A. G. Tannenbaum has a Operation/Service manual for this amp and I'll be ordering one.

[GlennW]

Just got done reading Joe Jasniewski's article on converting PA amps into guitar amps again, and it reminded me of something. When using the Aux input on the Bogen amps there was a little switch to change between Mag and Mic. The Lafayette amp doesn't have that switch, so maybe that's part of the problem like Tom said. Just guessing. I'll see if Radio Shack has an adapter for the Mic inputs. I'm still going to learn how to go through it.

http://www.rru.com/~meo/Guitar/Amps/PA2Guitar/

[Tom Phillips]

Just got done reading Joe Jasniewski's article on converting PA amps into guitar amps again, and it reminded me of something. When using the Aux input on the Bogen amps there was a little switch to change between Mag and Mic. The Lafayette amp doesn't have that switch, so maybe that's part of the problem like Tom said. Just guessing. I'll see if Radio Shack has an adapter for the Mic inputs. I'm still going to learn how to go through it. http://www.rru.com/~meo/Guitar/Amps/PA2Guitar/

I suggest that you just convert the mic inputs to 1/4 phone jacks. You will want to do that anyway. No need to buy adaptors unless you are trying to keep the amp all original for some reason.
Do the tests that Enzo listed. When you get the service info that should help to trace the signals.
I don't think the lack of the Mag/Mic switch is your problem. It appears that your amp just doesn't have that feature.
My original comment was based on the fact that the mic inputs are usually much higher gain than Aux inputs and you indicated that you were using the Aux inputs. Again, when you get the service information it will help. These amps are usually very simple. There is a good chance that you will find the problem when you start taking voltage readings.
Good Luck,
Tom

[GlennW]

The adapter didn't change things - didn't expect it to, but was hoping for an easy way out. There's a 1/4" phone jack inside the chassis near the tubes, and it's wired in such a way that it looks like it should work, but it doesn't. I have no intentions of preserving it, I'd like to learn enough to make it a good guitar amp because it has a nice tube layout. Once it's working I'd like to put the input in the front using the hole for the Aux volume.

Thanks for all the help, I ordered the manual and will update if there's any progress or more problems.

[GlennW]

Sometimes I don't don't see what's right in front of me.

I waited a week or two for the schematic people to retuRn my emails and phone call, then I tried someone else who has one and he wanted $38 + s/h, I don't think so. I fooled around with some other amps and got back to this one.

As typical for these things there's a line of screw terminals for speaker connections and two screw terminals marked standby (which weren't connected). I didn't notice them before. I put a jumper between the standby terminals and it works just fine.

[Tom Phillips]

Cool. Thanks for the update. I've made a mental note about the external standby terminals.
Now that it works, it would be a good project just to trace out the circuit and draw our own schematic.

[GlennW]

I've had some Bogens in the past and just assumed that the terminals on the far right were for 70 volt or remote connections.

Making a schematic would be an excellent learning project for me - I might try to do it in sections so I can learn as I go.

[mykey]

Is there a likely common cause for this? OT?

could be a dead preamp tube, does this thing have the right size fuse in it?
if it has an oversized fuse, someone may have stuck it in the to compensate for a shorted (?) does it have a power amp in jack?
will try to find the schematic, or do you know where its posted?

[Enzo]

Mykey, please reread the last sentence of post #9 in this thread.

[aaronp]

I'm new here and this is my first post. I just read through the thread and saw that you figured the problem out...good deal.

Since you figured the problem out, I'll just throw this in as another possible cause of that symptom, just in case someone experiences the same problem in the future:

I know it's not a boutique amp but I had a Peavey Classic 30 that exhibited the same symptoms; the light would go on and there was a faint sign of life, but in general, no sound. The problem ended up being the tube heater circuit. A relatively simple (and inexpensive) fix.
So, there's another diagnostic possibility to keep in your hat, in case the type of problem shows up here, again.

aaron

[mykey]

I had a Peavey Classic 30 that exhibited the same symptoms; the light would go on and there was a faint sign of life, but in general, no sound. The problem ended up being the tube heater circuit. A relatively simple (and inexpensive) fix.

yes good suggestion, the PV uses a diode to ground in the heater circuit so that the heaters are running off 1/2 wave AC. this cuts down the heater noise in the audio path. I recall that once or twice I found those diodes shorted, causing this type of problem.