MB 295 saved from the trash.

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mark1406

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A friend of my brother was going to throw out a Boogie 295 dual power amp. I was shocked to say the least.

Apparently he didn’t want to sell it as he was told it would be unreliable in the future.
I certainly hope this person wasn’t a technician.

I have had a quick look at it and it appears to be blowing main fuses. I don’t think it should be that big a deal fixing it. I think the biggest issue with be getting the circuit and the layout. I have contacted Mesa Boogie and I will see what they say.

I have never set the bias on a simiclass amp, so I will need some advice there.

I had a look on Reverb and these amps are pretty cheap second hand, I think providing you can lift them, they represent a pretty good deal.

Regards

Mark
 

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If it's blowing fuses, it's probably due to bad caps. They're 36 years old, after all.
The bill to replace all those caps will be substantial but worth doing.
I'd check the resistors on the tube sockets to be sure they're all in spec on value. If they are, a new set of caps should cure all ills.

Don't worry about the bias, as long as you're using Mesa tubes in sets.

Reliable? After a full recap job it'll probably go another 20 to 40 years without ever needing anything but occasional tubes if it's used a lot. Those are very reliable amps and anybody who said otherwise is an idiot.
 
If it's blowing fuses, it's probably due to bad caps. They're 36 years old, after all.
The bill to replace all those caps will be substantial but worth doing.
I'd check the resistors on the tube sockets to be sure they're all in spec on value. If they are, a new set of caps should cure all ills.

Don't worry about the bias, as long as you're using Mesa tubes in sets.

Reliable? After a full recap job it'll probably go another 20 to 40 years without ever needing anything but occasional tubes if it's used a lot. Those are very reliable amps and anybody who said otherwise is an idiot.
Hi Woodbutcher

The fault ended up being the bridge rectifier two of the four diodes were short circuit. I was surprised that they were 1N4007’s as they have a 1A rating and most guitar amp use 1A, but in this amp there are two power amps. I replaced the 1N4007’s with 1N5408 which have a 3A rating.

The next part is getting valves and biasing the amp. I have elected not to use Mesa Boogie valves as they aren’t readily available. The 6L6’s are still measuring g fine but the EL-34’s are shot. I assume that the 6L6’s were running quite cool which accounts for their longevity.

As far as I can tell. The EL-34’s have to be under 100% dissipation which is about 50mA or less. The 6L6’s are whatever they are.

What have you encountered?

Regards

Mark
 

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Last edited:
Nice amp!
You can get power tubes that are within Mesa spec from just about any tube venfor these days

Www.thetubestore.com
www.tubedeot.com
www.dougstubes.com

Don’t worry about biasing the amp, just find tubes that are within Mesa’s specs

If that were my amp, I would definitely replace all of the filter caps, as well as any resistors that are directly attached to the power tube sockets and rock the **** out of it for another 20 years!
That amp will outlive all of us with proper maintenance
 
Nice amp!
You can get power tubes that are within Mesa spec from just about any tube venfor these days

Www.thetubestore.com
www.tubedeot.com
www.dougstubes.com

Don’t worry about biasing the amp, just find tubes that are within Mesa’s specs

If that were my amp, I would definitely replace all of the filter caps, as well as any resistors that are directly attached to the power tube sockets and rock the **** out of it for another 20 years!
That amp will outlive all of us with proper maintenance
I tested the caps with my Peak cap tester and they measured good. They are quite an expensive item to replace if there is nothing wrong with them.

I measured all the resistors on the output stage and they were good too. I was surprised as carbon comp resistors typically drift over time.

I’m not sure what you mean by Mesa specs?

I would assume that a set of valves for a simuclass amp would need an average set of EL-34’s and a set of 6L6’s with high transconductance. That way the bias voltage is high which means the EL-34 doesn’t exceed 100% dissipation and 6L6 can set up around 60% dissipation. The bias voltage will be higher so distortion due forward conduction happens much later.

if I’m mistaken with my assumptions please let me know.

https://www.peakelec.co.uk/acatalog/esr70-capacitor-esr-meter.html
Regards

Mark
 
Okay, the saga continues. The output valves seemed fine and the amp wasn’t blowing fuses anymore. I tried plugging a guitar in and I was getting nothing out. I decided to use the iPhone to pump a sine wave into the amp and I got a sound out of it. I thought signal level might be an issue, so I used an OD pedal to give the guitar more amplitude and the signal was quiet and distorting.

I found the plate voltage of the first stage was 7vdc!!!
That’s insanely low, to save a long story, the issue was Mesa uses 400vdc caps in this amp when they should use 715 600v caps.
I found caps in the preamp and phase inverter to be short circuit. I can’t remember the last time I encountered this, it’s so unusual.
I’m amazed Mesa used a part that wasn’t fit for purpose. The same can be said for the rectifier diode, the 1N4007’s are a bit light on in a fault situation, it needs 1N5408 diodes which are good for three amps.
 

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I found caps in the preamp and phase inverter to be short circuit. I can’t remember the last time I encountered this, it’s so unusual.
Very usual for the PI going over voltage at 400V and shorting. Mesa re-spec'd at 630V in the mid-80's. Commonly causes red-plating for one side of power tubes since usually only one coupler blows at a time.

Pre-amp ones not usual but in a dumpster dive at 40 YO not surprising.
 
This is the actual circuit diagram of the “preamp” stages of the 295 dual power amp. The drawings on the internet leave out the presence control. I hope this is of assistance to people needing to repair a 295 amp.
I was quite surprised by the 400v orange drop caps going short circuit. It’s probably a good idea to replace them with 600v cap. The caps will have to leads that are 0.7 to 0.8mm in diameter. The new 716 caps have leads which are 1mm in diameter and they won’t fit the holes in the PCB.

I hope this is of assistance.

Regards

Mark
 

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