MEDIC ! My Carvin just crapped out on me

[JohnW63]

So, I'm still down on current. < insert fav expletive here >. Other than the output transformer primary side being too high of impedance, I'd have to check all the tube circuit resistors and see if any are out of spec enough to drop the current down. That looks like the only things on the far side of the circuit from the transformer. Or, the power transformer isn't providing enough current to begin with. The only replacements I found for those are not cheap.

 

[JohnW63]

All the resistors that are on the cathode or plate side of the tubes that look to be able to have DC current flow check out within spec. They're all pretty close to the listed values. I don't know what the impedance of the output transformer should be and I don't have am accurate way to check it, if I could find out. I guess I could see what the replacement parts are, since the repair video guy replaced them in one clip, but I would be surprised if they list direct resistance.

 

[fronobulax]

Thanks. I like the story even if the days when I understood it in detail are long gone.

 

[JohnW63]

I'm just on the hunt to make it right. I have cash-back awards trying to burn a hole in my credit card pocket, but , I would rather fix my standby amp than get another one that mostly fits the same sonic space.

I would love to know why it failed. It's looked as clean as the day it was new and no components even look like they got hot. Makes me think the issue is in either the power transformer or output transformer. They're just big metal boxes, with all the important stuff hidden inside. I can't find specs on testing either one.

 

[Nuuska]

Your power transformer is ok - it could not provide that voltage, if it was bad.

 

[JohnW63]

Nuuska,

I figured it probably was OK. It looks like I either don't have the current to supply, ( ration of primary to secondary windings in the the power transformer ) which is less likely, or something is too high resistance to allow it to flow at the proper level. I haven't found anything out of spec on the output portion of the circuit, yet.

 

[DThomasC]

I haven't found anything out of spec on the output portion of the circuit, yet.

You're sure the output tubes aren't beat after the abuse they took before you replaced the coupling caps?

 

[JohnW63]

Anything is possible, but, I don't see how they would effect the voltage going in their respective grids. It looks like the bias current that I can't set to spec does go through the preamp tubes, but I don't see a power tube connection.

 

[DThomasC]

Is your concern the grid bias? -15V vs -17 printed on the schematic? That may or may not be an issue, but less negative bias results in more plate current not less. If you could adjust to -17 you would be even farther from 40mA current.

 

[Nuuska]

Anything is possible, but, I don't see how they would effect the voltage going in their respective grids. It looks like the bias current that I can't set to spec does go through the preamp tubes, but I don't see a power tube connection.

NO WAY !!!

 

[JohnW63]

Nuuska,

I'm pretty sure, ..way. If you look at the schematic, points C and B from the line of resistors that are part of the circuit in line with the output transformer connect to the plates of the preamp tubes, go through the tubes and then to ground.

 

[JohnW63]

DTC,

My concern is both. I want to get the voltage listed and current listed to be within spec. Yes, -15 would make it be biased hotter than -17.

 

[Nuuska]

Voltages at points B & C are ANODE voltages - more or less common for all tubes - only reduced for preamp tubes. Nothing to do with power stage bias. If you can not get it biased to lower current - correlates with more negative voltage at viper of P7 - then either your output tubes have something wrong or the bias adjusting circuit is not right.

Did you ever try my suggestion of switching R31 & R32 ? That would shift the P7 voltage point to more negative - so instead having the viper at one end and not enough - it might be enough closer to middle.

That is easily tested - and the parts are there.


EDIT - read post # 129 @ July 16th

 

[DThomasC]

DTC,

My concern is both. I want to get the voltage listed and current listed to be within spec. Yes, -15 would make it be biased hotter than -17.

You might have to try a lot of different tubes to get both at the same time as tubes are all different. That's why they sell matched sets.

The current is what you should aim for. The voltage is just for reference.

And what Nuuska said...

 

[JohnW63]

Nuuska,

Does not the circuit from the preamp tubes not find it's way to QC2 on the output transformer ? This may only be the primary winding, but it gets there non the less. The BIAS current adjustment is measured there, so what current am I adjusting ?

It seems you both are looking at different issues.
"Did you ever try my suggestion of switching R31 & R32 ? That would shift the P7 voltage point to more negative "
and
"The current is what you should aim for. The voltage is just for reference. "

Nuuska,
No, I have not flipped the resistors. My thought is that that is a work around and not a fix of the issue. I want to find out what has gone bad to pull the voltage out of spec. If I can't find the cause, I will consider the work around.

DTC,

Since the voltage and the current are listed in the schematic, how would I know which one is more important than the other ?

and ..

"That's why they sell matched sets. "

Yep, which is why I bought replacements, a few years ago as a matched set.

 

[DThomasC]

Nuuska,

Does not the circuit from the preamp tubes not find it's way to QC2 on the output transformer ? This may only be the primary winding, but it gets there non the less. The BIAS current adjustment is measured there, so what current am I adjusting ?

The AC music signal from the preamp tubes eventually makes its way through the output tubes and then the output transformer, but that's only AC. All of the DC is blocked by the coupling capacitors along the way: C3, C34, C7, C41, C18, C22, and C23.

When you adjust the bias current in the output tubes, all you are measuring is the DC current. The only current that passes through the primary side of the output transformer is the current through the output tubes. When you adjust bias the amp is idle - no AC signal - so it's just the DC current through the output tubes.

The preamp tubes are biased automatically by the resistor in the cathode circuit. Many amps with EL84 output tubes bias them automatically with a cathode resistor too, but you Carvin happens to have adjustable bias for the EL84 output tubes. (Which is ironically called fixed bias, but never mind the terminology.)

DTC,

Since the voltage and the current are listed in the schematic, how would I know which one is more important than the other ?

Because someone experienced with these circuits tells you.

 

[JohnW63]

OK. When I get the current right, I'll let you know what it took to get there.

 

[JohnW63]

I've got 40ma on setting the bias and 15.75vdc at pin 2 on the power tubes. I may test other stuff, down the road, but I think I'll leave it alone, for now.