Carvin Nomad 112 Issues

[Perkinsman]

I'm working on my first Nomad and have some questions. I put all new tubes in today and set the bias at 75mA. Vp on the 5 preamp tubes are all 15-20% lower than the schematic values. Voltages in the power supply are also lower, which probably explains the low Vp on the preamp tubes.There are 5 22uf caps in the power supply (C40-C44) but I can not locate C42, C43 or C44 on the board. There is a C83 & C84 but they're not on the schematic, lol. All are 22uf but C40 & C41 are 2200. What am I'm missing here?

[Enzo]

Nothing. They used the same board for more than one model, certain parts differences would be for that. Like C81,82 are in the same spots as C40,41, but the "missing" parts were axial leaded and would lie flat in place of the vertical (radial) C40,41. I would assume what is in there is correct as made, even if not matching the drawing.

I don't worry if the whole thing has lower voltage. But I must ask, are you using a light bulb limiter? If so, that explains your low voltages.

[The Dude]

I believe there are different revisions of that amp. My guess is that your schematic is not the correct revision. Is there something you're trying to fix? "If it ain't broke, don't fix it."

[g1]

You set bias for total current through standby switch?
That is what they specify.
If you set for 75mA idle current at power tube, it would certainly pull down all the voltages.

[alexradium]

I'm working on my first Nomad and have some questions. I put all new tubes in today and set the bias at 75mA. Vp on the 5 preamp tubes are all 15-20% lower than the schematic values. Voltages in the power supply are also lower, which probably explains the low Vp on the preamp tubes.There are 5 22uf caps in the power supply (C40-C44) but I can not locate C42, C43 or C44 on the board. There is a C83 & C84 but they're not on the schematic, lol. All are 22uf but C40 & C41 are 2200. What am I'm missing here?

the power section on that schematic is the exact same of the 100w models MTS and legacy,i bet its less voltage in reality,more like 350/400V on B+ which is more appropriate for el84s,,that explains why the caps have different numberings

[Perkinsman]

You set bias for total current through standby switch?
That is what they specify.
If you set for 75mA idle current at power tube, it would certainly pull down all the voltages.

Yes, I did bias thru standby but that is a good point.

[Perkinsman]

I believe there are different revisions of that amp. My guess is that your schematic is not the correct revision. Is there something you're trying to fix? "If it ain't broke, don't fix it."

Possible but every revision I saw had the same higher voltages and the same power supply.

[g1]

Is heater voltage also low?

[Perkinsman]

Nothing. They used the same board for more than one model, certain parts differences would be for that. Like C81,82 are in the same spots as C40,41, but the "missing" parts were axial leaded and would lie flat in place of the vertical (radial) C40,41. I would assume what is in there is correct as made, even if not matching the drawing.

I don't worry if the whole thing has lower voltage. But I must ask, are you using a light bulb limiter? If so, that explains your low voltages.

Thanks Enzo, I’m glad to learn this. I don’t use a bulb limiter but my isolation variable transformer has a current meter. Doesn’t that accomplish the same thing?

[Enzo]

I myself use a variac and amp meter for testing, but that misses the point. Many people use the bulb, and when you do, all the voltages in the amp wind up low. SO that is why I asked.

[g1]

What is the AC voltage at the actual primary leads of the power transformer?
What is the heater voltage?

[Perkinsman]

I understood what you were asking...I went off point a little because I was hoping you would explain why using a variac with separate voltage & current meters isn't as effective as using the variac and limiter.

[Enzo]

The bulb limiter is something effective and protects the amp. It is easily made with stuff anyone has at little expense. One can make one in fifteen minutes or less. No extra meters to buy or wire up. The bulb limits the current through the amp under load, especially fault mode loads. That said, I don't have one myself. But when we see the need, we can ask a guy to make and use one so we can continue. We could tell him to buy a variac and meters, and wait for them to arrive, sure, but then we have to put the repair on hold. The bulb is automatic, can't accidentally turn it on full as one could with the variac.

The variac requires discipline, I start to turn up my variac, and if current starts to ramp at even 20 volts, that means trouble and back off. yet time after time, guys write in here telling us they could get their variac up to 90 volts before something starts smoking. The variac is not a contest to see how high it can get, it is a tool to indicate trouble. But many people use it that way. It is easy to use a variac wrong, the bulb is hard to abuse. Well I suppose one could use a 500 watt bulb instead of something more useful.

And a guy with neither bulb or variac is usually not that advanced a technician, and the bulb is easier to interpret, at least for a novice.

explain why using a variac with separate voltage & current meters isn't as effective as using the variac and limiter.

I don't believe I said that. I only asked if you were using the bulb, as many many do, because you reported low voltages, and that would be a common reason for it.

[DrGonz78]

For me when I work on solid state amps I reach for the light bulb limiter. The idea of checking voltages even when there is a short somewhere feels empowering. A light bulb limiter is like a condom, easy to use and it does one thing. Honestly I don't use the lightbulb limiter on tube amps and just use variac with ammeter. I think it's good to have both.

G1 had a good question that has not been answered about voltage readings.