Marshall JVM410 transformer underrated?

[greekie]

Hi guys,

I had one of these amps in for a tube change and minor check up. Not really my kind of amp, but in some ways I admire what the Marshall engineers can get out of the limited stuff they have, due to cost cutting, etc.

The thing that got me wondering was the specifications of the power transformer - the HT winding was specced at 350V / 0,5A - that's 175VA total. Is that even enough?! The entire transformer was specced 244VA. The transformers that I buy for my 100W-projcts are specced at 370VA with a HT-winding of 2x 0-360V @ 0,4A - almost the double of this one!

Is this just modern day cost cutting? Will this amp even be able to do 100W clean? I'm without a dummy load at the moment, as I loaned it to a friend.

Jake

[Enzo]

The guitar amp is not a hifi, and it is never asked to produce full powr sine waves over time. SO you can;t take your steady state specs and apply them to music. At least not directly. Unless you are a fog-hornist, your 100 watt amp is not being asked to produce a steady 100 watts of anything. Music is peaks with valleys between. And that is where you get the whole RMS power, program power, etc etc.

My car has a 6000rpm redline on the tach. But no matter how hard I am driving, I never crank it up to 6k and leave it there.

I learned the hard way that my Carver amp wold produce its full 400 watts or whatever it was of music all day long, but if I tried to make it amplify a 100 Hz sine wave at 80% power even, it only took a few minutes to overheat and protect.


The clean question is a good one. CAn it make 100 watts clean? No. But it doubt it can make 15 watts clean. Guitar amps are not hifi amps, they are designed to add their own character to the signal. They are not clean to start with.

I am sure over-specing your PT will produce reliable results. I am not sure that a 150 watt PT spec in a 100 watt amp is necessary.

[greekie]

Thanks Enzo. I actually already knew that... or well, I've read it before anyways! You can sometimes get an idea in your head that blocks out other knowledge and/or reason.

I've been seeing a fair amount of old Marshalls the past 6 months for repair or maintenance, and the power supplies in these old amps always seem to be capable of delivering more power than those in newer models. I mostly build amplifiers for the heavy metal guys, or loud, clean amplifiers. In both cases, you need a solid power supply to get the punch and clarity that those two types of sounds need, which I feel is lacking a bit in this amp, probably due to the amplifier itself.

My thought was something along the lines of power efficiency - e.g. how many watts we put in compared to how many we get out. I read in an old german manual, that efficiency of class A tube amps were along the lines of 10-20%, and class A/B tube amps around 20-50% in peak efficiency. So I would think that in order to deliver 100W, the power supply should be at least 100W * 50% = 200W of power. But as you say, we're not designing hifi-stuff.

Thanks!

Jake

[Enzo]

yeah, but now you are getting into other-wordly stuff.

Having said things earlier, I would also not disagree if someone says a Fender or Peavey transformer is a lot less likely to fail than a Marshall. Of the major brands, I'd have to say the number of Marshall transformers I have replaced is probably about the same as all the others combined. So I think my message is, the transformer doesn;t have to be a large as you think, but the ones in Marshalls are not as sturdy and maybe they ought to be.

And I think a little failure analysis is due. WHen a Marshall xfmr fails, is it from "too much power," or did it fail because it was made without sufficient protection against arcing and overvoltage? Just as an example.

[greekie]

From my experience, it seems like it is mostly failure due to the less-than-optimal quality of the transformer. I got an old Marshall 1959 in last month, where someone had modded it for 4xKT88. The 0,4A extra draw on the heater supply is probably OK, but the guy had changed the input voltage to the 200V setting (we have 230V over here) and hardwired it at that. This meant that the B+ was around 580VDC and the amplifier produced something like 120-125W before going into distortion, where I measured around 180W peak.

What I was amazed by was, that the old Drake transformer had survived this so far. Wall voltage was +15% over spec, and the current drawn from the power supply was about +20% of what I had measured afterwards when we put EL34's back in and took the B+ down about a hundred volts.

Oh, and if you wanna save tubes in an amp like this, install a voltage regulator that keeps the screens at about 400VDC. 425VDC is max for most tubes, some even 400VDC. Add a trimpot to the plate resistors of the phase inverter to balance it, and your tubes will last much longer. YM2C.

Jake