Mesa Boogie Dual Rectifier Roadster

[carlosraj7]

Hi

Does anyone has the power amp schematic for this amp. I have this amp and they is no sound from the amp. If I plug a monitor amp into the "send" , I get sound from all the channels, which I guess means the preamp is OK. There is totally no sound from the speaker output. There is 0 amp from the tubes. Measuring the voltage from ground to plate, it reads 438V , but the voltage from cathode to plate reads less than 100V. I guess this amp is cathode biased, does it means that there is a problem with the cathode connection, since I am getting less than 100v. (cathode to plate). Please assist.

Thanks

 

[Eevil]

Hi

Does anyone has the power amp schematic for this amp. I have this amp and they is no sound from the amp. If I plug a monitor amp into the "send" , I get sound from all the channels, which I guess means the preamp is OK. There is totally no sound from the speaker output. There is 0 amp from the tubes. Measuring the voltage from ground to plate, it reads 438V , but the voltage from cathode to plate reads less than 100V. I guess this amp is cathode biased, does it means that there is a problem with the cathode connection, since I am getting less than 100v. (cathode to plate). Please assist.

Thanks

Not an expert with the internals of the amp and/or voltages.

Usually I suspect first the return jack of the fx loop which will gather some turd a bit.

• One suspect would be a PI tube. If it goes down completely the power amp seems to be dead.
• Yet another easy to check would be to check fx loop return tube.
• Some pre tubes are cathode followers and some (namely russian origin) tubes will die there. Google will tell about the which positions were CF - my brains have erased that info long ago

Happy debugging and if its some bigger issue some people could chime in.

 

[rarebitusa]

as @Eevil alluded to... For the Roadster check: V3 & V5, this link covers many of the details about the shortened tube life for

Cathode Followers

 

[domct203]

Do you get sound from the speakers if you hard bypass the FX loop? I’m not talking about foot switching the loop off, I’m talking about the hard bypass switch on the back of the amp. Hard bypass will remove V5 from the circuit.

I also recommend checking the cathode follower tubes (V3 & V5) and PI (V6), JJ’s work fine there.

Have you confirmed bias voltage to the power tubes?

Can you measure any current at the plate of the power tubes when the amp is on and out of standby?

 

[Jay Omega]

Hi

Does anyone has the power amp schematic for this amp. I have this amp and they is no sound from the amp. If I plug a monitor amp into the "send" , I get sound from all the channels, which I guess means the preamp is OK. There is totally no sound from the speaker output. There is 0 amp from the tubes. Measuring the voltage from ground to plate, it reads 438V , but the voltage from cathode to plate reads less than 100V. I guess this amp is cathode biased, does it means that there is a problem with the cathode connection, since I am getting less than 100v. (cathode to plate). Please assist.

Thanks

I don't have a schematic but I'd be absolutely shocked if it is cathode biased.

First thing I would do is set your output / solo / send / master pots to noon, bypass your loop in the back, and take out the rectifier tubes and change all channels to the diode / 100W and recheck. If it still reads funky, take out all four power tubes and check again. If all is good, put in the outer power tubes and check again. If still goofy, take out the outer power tubes and put in the inner pair and check again.

 

[bandit2013]

Lots of good points above.

I vote for the hard bypass test first. if that works, it could be half of the V5 tube is bad.

You could also leave the amp with the FX loop on, set all of the mini toggles for each channel to turn on the FX, those are also on the back panel just below the reverb controls but the one to the right of the center switch. setting the FX to on, leave it in active mode and plug your guitar into the FX return jack. If the return/recovery stage V5B is working then you will hear some clean guitar sounds through the speaker. If that does not work it could be the V5 tube or the phase inverter tube V6. The hard bypass will bypass V5 completely and ignore the FX loop, solo and output controls. Note that the channel masters will become the volume control in hard bypass.

The last idea is the turner/MUTE circuit on the footswitch. I would try disconnecting the DIN cable and set the channel on the back panel with the rotary knob to a channel. That mute circuit will disconnect part of the circuit for the phase inverter so it will not make any sound at the speaker so if there is an issue with the DIN cable and the footswitch is stuck in mute, that will do it.

Roadster is control grid biased. I doubt you will find a Mesa that runs a cathode biased or self-biased circuit with resistors for a Class AB power amp. The Mark V90 in 10W mode will do that. I thought there was a Mesa amp that was single ended Class A but could not find it in any of the references I have seen.

The strobe mute circuit to drop out the output when channel changes occur is in front of the FX loop send circuit so that is not the issue as you have indicated the SEND can be used into a monitor and that works. First thought of that as the JFETS can fail after some time. However, there is a relay to used for mute/tuner that disconnects the cathode circuit of the phase inverter from it's circuit that effectively shuts off the output.

Cathode to plate measurement should be around 467VDC Cathode is connected to chassis ground so you can use the chassis for in place of using the terminal on a power tube socket. The control grid should have close to -51VDC in reference to ground. The screen should have around 465VDC in reference to ground.

Are all of the heaters active in the power tubes? If one is dark, double check to make sure it is not a Rectifier tube in the wrong location. If that is the case, what would be in the circuit would be pin 8 and pin 4 (which is the screen connection). You will not have a heater glowing and if the fuse does not blow out.