Mesa Solo 50 Head - Low-ish Output

[mikeboone]

I have a Solo 50 head (series 1 EL34 only) that I received for repair with a few odd circumstances.

First, we powered it up and all it did was hum. I looked in the back and saw suspiciously small output tubes. Yes, some knucklehead put 6V6's in it for some reason.

I opened it up and checked the screen and grid resistors on the output section to make sure nothing was fried. Checked the PT voltages, OK...

There was no high voltage present anywhere... So I traced it to the standby switch. The outbound side was not connected and the wire was taped up and ty-wraped back. Not sure why, but it was. I dont know if this was done before or after the 6V6's were put in though???????

So, I reconnected that. Checked for voltage and shorts, OK... I put a set of EL34's in it and all seemed normal. I turned it on and it plays and sounds, tonewise, like you would expect but only plays half as loud as it should. I compared it to my Rectoverb and it is not as stupid loud. Annoying to my children watching TV, but not Mesa loud.

It does start to sound a bit rough when it is cranked.

I checked all the voltages on all tubes against a schematic that Mesa emailed me for my own amp. All plates and cathodes check very close to schematic. I even checked the B+ on the output tubes with it cranked and me playing to see if the voltage fluctuated. Not much at all, a few volts.

Any thoughts? OT?

[Gtr_tech]

I'd not rule that out since they're on the dinky side and not too durable. Wire in another just to verify.

[acorkos]

make sure the effects loop jacks are clean and gunk-free

[Gtr_tech]

True....plug a cable into the loop (1 cable from the send right to the return) and see if that brings it back.

[mikeboone]

Will do... Thanks.

I did discover that this amp takes 6L6's and EL34's. It is slightly louger with the 6L6's but still not stupid loud.

I will connect the loopback I/O together tonight to see if it makes a difference.

[mikeboone]

Revisiting this amp... I put it on the side for a while and started messing with it again today.

The output is pretty low. I have cleaned, inspected the effects loop jacks and also tried putting a cable connecting them and no difference.

If I run the "slave out" or "effects send" of my other Single Rect to the "effects return" on the suspect amp, it plays out plenty loud so I am certain the power section is OK. Reversing the roles of the amps is not so good. The good amp receive and plays loudly the skittish output of the suspect amp.

So it is fair to say that the issue is somewhere between the input and the effects send but I've had no luck isolating it.

-I have a schematic and checked all the voltages on the preamp tubes and the are in order.
-I have cleaned all the jacks, pots and tube sockets with deoxit.

Any idea where to turn next. This problem cant be all that bad, jusr being difficult to locate thus far.

[g1]

Is this the schematic? http://www.webphix.com/schematic%20h..._rectoverb.pdf

[mikeboone]

I believe it is the correct schematic as this is a Series I. There is a Series I and II Singel Rect. The Series II has a bias switch to change from 6L6 to EL34 and I dont see anything in this schematic for that.

Its my understanding that not much else chaged besides the bias switch in the Series II

[g1]

You said the voltages seem to be good, have you tried other preamp tubes? Otherwise maybe bad solder or a bad relay(s)? Have you tried tapping with a chopstick while passing signal? A scope would be a big help, otherwise for the relays you could try bypass jumpers.

[mikeboone]

I used all the tubes from my other Single Rect. in this one and no change.

I have poked and prodded extensively with the chopstick looking for bad connections to no avail.

Could you give some insight as to how the relays may cause low output? The amp works at volume that you would have to talk to over, but nothing like my other Single Rect.

[Jazz P Bass]

What test equipment do you have?
It sounds as if you are stabbing in the dark.

[mikeboone]

I have a Fluke, a tone generator and a scope but unfortunately the scope is broken. Well thought out stabs in the dark, but stabs in the dark just the same.

[Jazz P Bass]

"Well thought out stabs in the dark, but stabs in the dark just the same. "
I hear ya.
Use the tone generator (100mv, 1K sine wave)& the Fluke to find where you are losing the proper signal amplitude.
I find it helps to look at the plates (Vac) & the other side of the coupling cap (Vac & Vdc) when looking for a problem.
You stated that the voltages checked out. I would still read each tube plate & cathode voltage (Vdc).
Write everything down on the schematic.
Dig in!

[mikeboone]

Thanks JPB... This is good info. I can acutally benchmark against the working Single Rect. How we get blinded... I will "dig in" this week. I also posted a conclusion to a post for a Hughes and Kettner amp that you helped me locate the schematic for.

[52 Bill]

And to answer your earlier post, a relay is nothing more than a mechanical switch that is operated by an electromagnet. If the contacts get dirty or oxidized, they can develop high resistance and cause signal drop outs etc.

You can test the contacts with an ohmmeter.