Mesa Boogie Recto 3ch - NO clean channel

[godmustuseaboogieman]

Hi,

My 3ch recto was fine on Tuesday... very loud and the clean channel was clean. It did blow a fuse (which i replaced with the correct one) I turned it back on to check it was working but didn't play any more that day.
Yesterday it was turned on and was (comparatively) very quiet at the same knob positions. The clean channel (not pushed) is suddenly quite heavily crunched.

Any help? I have electronics experience, my first thought was that the HT has markedly dropped, so first thing I'm going to do is check voltages against the schematic I found online (attached, if I did it correctly)

The issue is the same in vacuum and diode rectifier configs. Diode level is slightly heavier, but I believe that's to be expected any way.

Any help much appreciated.

[Dr Kaos]

Sounds to me like you have a poweramp problem such as a dud 6L6 or two, hence the blown fuse.
Let it run with no signal for a few minutes and then check the temperature of each 6L6 by putting the tops of your fingers a few milimetres away from the glass. Keep an eye out for any 6L6 plates glowing red. Any valves that are too hot are suspect, any valves that are cold are also suspect. If you have a cold tube swap it to another socket and try again. This will tell you if it is the tube or something associated with the particular socket. This amp can be run with just two 6L6s but they need to be in sockets 1 & 4 or 2 & 3.
If you suspect the rectifier tubes, e.g. flashes inside the glass, remove them and set to solid state.

Hope that helps.

Dr Kaos

[godmustuseaboogieman]

Hi DR Kaos and all.

Replaced all 4 Power tubes... no better. So I popped it open (being careful) HT across the main reservoir caps is 430.

I've removed the valve rectifiers and set to diode, just in case. No different.

Anybody got any advice please? Happy to look at this myself.

[g1]

It's not quite clear whether the fault is only the clean channel or all channels, please clarify. You did say it is quieter than before when volumes set the same.
So for now I'll assume there is a problem no matter which channel.
One thing to try would be a patch cord between FX send and return jacks. Any change?
Was that 430V on spongy or bold setting? What are the other power supply voltages (A thru E) shown on the power supply drawing? What is the AC line voltage at your wall outlet?

[godmustuseaboogieman]

Thanks g-one.

Issue is there no matter what channel. My post title was rather stupid. It's just obvious on the clean channel cos ther's no headroom.

The FX loop is parallel, what would you expect a patch cable to do? I'll try it.

Will have a look at the voltages in the morning.

Thanks once again.

It's not quite clear whether the fault is only the clean channel or all channels, please clarify. You did say it is quieter than before when volumes set the same.
So for now I'll assume there is a problem no matter which channel.
One thing to try would be a patch cord between FX send and return jacks. Any change?
Was that 430V on spongy or bold setting? What are the other power supply voltages (A thru E) shown on the power supply drawing? What is the AC line voltage at your wall outlet?

[Dr Kaos]

Did you check the temperature of the tubes as I suggested because it will show up faults other than just the tubes.
The problem could also be with the phase inverter stage, substituting the valve is one step, checking the voltage on the two anodes (plates) is another.
Do you have access to an oscilloscope?

Dr Kaos

[godmustuseaboogieman]

Hi,

I did check the tube temperature, all the 6l6s were evenly hot to the touch. I felt like they did take a long time to get hot (3-4mins), but that could be just 'cos I haven't checked before.

I do have access to a scope.

430V was at point A with the spongy setting. I'll check the other points B-E and the anodes of the PI tonight, didn't have a chance to before work.

Thanks,

[52 Bill]

Why not test to see if the power amp is the problem by inserting a signal into the FX return, bypassing the preamp. Be sure to set the FX mix control to full FX.

If that doesn't work then you should look into the mute circuit for problems.

[godmustuseaboogieman]

OK some feedback

Power supply point voltages are a little low, but not very
A 457 (473 with HT switch open)
B 455
C 420
D 385
E 400

Mains is 250ish (high part of UK)

PI anode voltages are 289 (P6) and 283 (P1) (does that sound right?) I swapped the anode valve for another 12AX7 (JJ) I had lying around. Got similar anode voltages but two of the tubes (1 and 2) began to red plate... Maybe this is the problem area?

Thanks,

[g1]

Sounds like the power tubes may be running hot and pulling down the supply voltages, as well as blowing fuse and red plating.
The coupling cap (.047) between the phase splitter and the tubes you are calling 1 & 2 may be leaky. Both those coupling caps should have around -52VDC on the power tube side. You may find that voltage is going more positive when the tubes start overheating.

[godmustuseaboogieman]

Thanks g-one.

I ran the amp up again and check the PA side of those funky chunky 47n caps. -52 on both. Let the amp warm up, but no difference.

Ran a sine wave signal into the FX return as suggested. Get a clean (ish) signal at the anodes of the PI valves (p6+3), but only about a volt (heavily distorted) measuring across the speaker terminals. I'm thinking 8 ohm speaker into ohm's law gives me sod all output power.

Am I being over simple?

[g1]

Before getting further into this, please verify you have tried a different cab and speaker cable.
When you saw 2 of the power tubes red-plate, were you playing or was that with the amp idling?

[Billy R aka DynaFreak]

Have you checked for screen voltage at pin 4 on all the pwr tubes? A blown fuse followed by distorted clean channel sounds like you may have a tube failure that resulted in a blown screen resistor. This is a very common problem.

[godmustuseaboogieman]

g-one:

Checked with different speakers and cable. When the two tubes started red plating, I wasn't playing, but I'd left the jack cable in, there was a reasonable amount of 50Hz. I guess that probably has a higher RMS than playing?

Billy: That's the 1k5s yeah? So I'm looking for about 52V volts on both sides of that resistor?

[g1]

Billy: That's the 1k5s yeah? So I'm looking for about 52V volts on both sides of that resistor?

No, he was talking about the screen grids which are pin 4 of the power tubes. The screen resistors are the 1K 2watt resistors going to pin4 of each power tube. You should get somewhere around 470V at each power tube pin4.
If it turns out to be all good, I may have to utter the OT word