12ax7 - 334V schem says 185V ???

[Krwkka]

I'm an amateur working on a 10+ yr. old Carvin MTS-3200. The amp came to me sounding weird. Re-tubed it and it sounds good, except that I'm not sure the distortion channel sounds right (never heard one of these before). Distortion is generated by Zener diode clippers. The distortion is very abrupt. There is also a strong low frequency (sounds like a sub-harmonic) tone that is emitted at the early part of a note - when you first pluck the strings. I'm thinking that the distortion problem and the voltage discrepancies might be related.

The schem says I should be seeing 185V on the plates of V2 on both sides of the tube. What I see, however, is 334V on pin 1 and 242V on pin 6. What can be causing this? Also have too-high voltages on V1 and too-low voltages on V4 (see chart below).

The heaters for V1 and V2 are on DC. I'm only seeing -2.3V on pins 4, 5 and 2.3V on 9 (referenced to chassis). Is that correct???

I'm not so much concerned with the other tubes, but V2 voltages look ridiculous to me and that tube is in the distortion channel (where I think- things don't sound right). I've tried subbing in different tubes in V2 and although the voltages changed, they were still high (higher than before, in fact).

HELP!! I don't know what to do next.



Link to schematic: http://www.carvinservice.com/crg/sch...00%20REV-E.pdf

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[Enzo]

Heaters - it is irrelevant what you see with respect to chassis. What voltage is there across the heater itself? ON the other hand, if those numbers mean about 4.6v across the tube heater, well that won;t work well at all.

Look just above the power tubes on the drawing. See the tube heaters? Now see those two "fuselink" resistors? Either one open? Rectifiers D6-D9, any of those open?

The readings you get for V2 are suspicious. The schemastic calls for a 200v drop across the plate resistor, that means 2ma flowing through the tube. That same 2ma flows then through the cathode resistor, 3.3k. SO that means we ought to see about 6.6v at the cathode pin 3. Your numbers show about 0.8ma. Now if those heaters on V1,V2 really are running on 4v, then that would explain the low tube currents.

That would also probably cure the crappy sound.

[Krwkka]

The rectifier diodes and the fuse resistors are fine. I get a reading on the resistor/fuses of .6 ohms. Schem says .56 ohms.

Heater voltages revisited:
4.7 VDC across the heaters - with both 12ax7s in
6.67 VDC from pin 9 to 4 or 5 with both tubes removed.

Does that sound right?


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[Enzo]

No, it sounds like your problem. FInd out why the voltage drops so much. What is dropping across those two resistors?

[Krwkka]

.343 volts across each .56 resistor.

[Enzo]

Well, the numbers add up, but that sure seems awfully low for heaters.

[kvimbo]

KRWKKA, try to check the ripples on C29, C30, C55. It is possible that the capacitors are defect (unusual for 3 in parallel, but who knows), and maybe the drop in the heaters voltage is due to the change at the form of the voltage. Try to see with scope what is really going to the heaters.
Good luck!

[Krwkka]

No ripple problem here. Scope? What's that??? Kidding. Some day I'd like to have one, but to be honest, I really have never felt an urgent need!

Updated voltage chart below.

I did a little comparison measuring on the heaters of a Marshall DSL-50. Similar setup with DC on V1 and V2. On the AC heaters I got 6.37v. On the DC heaters I got 6.78v. Measuring the DC pins with the tubes removed I got 7.9v.

Comparing the two amps, the AC to DC difference on the Marshall is what I would expect - slightly higher number on the DC. On the Carvin, however, the AC heaters show 5.96v which drops to 4.7v on the DC side. Not what I'd expect.

I guess this is what Enzo was looking at. So what can cause this voltage drop? Could the Caps be bad even though the ripple is actually better than on the Marshall? Could the transformer windings be shorted together or something?


EDIT:
I just hooked up the heater circuit to another transformer. I got 7.1v on the AC and 5.6v on the DC. Still a big drop from the AC to the DC. I guess that pretty much puts to bed my concern about the Carvin's transformer output. I don't know that it matters, but I'll mention that the plate transformer I used is a little guy, so I removed all but 3 tubes from the amp when I tested, leaving in V1, 2 and 3.

I don't know where to go from here. I haven't put my meter on the grids yet, but the other sockets on the tube sockets appear to be making good contact between the board and the tubes. I have a 10,000uf cap in my box that I could temporarily substitute for the three 2200uf caps on the DC heater circuit (C29, C30, C55). Would this be a sensible thing to try, or does somebody have a better suggestion?


Link to Schematic





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[Krwkka]

Hmmm.

I've just stumbled onto this post from 2006 which talks about the heaters on the MTS3200:

With EL34's your total current draw is a whopping 7.5 amps! (5 X 300 + 4 X1.5) The EL34's pull more current by themselves than the power supply is rated for. This inturn translates to the voltage dropping on the 12Ax7 filaments, V1 and V2 use DC rectified filaments with inline 56 ohm fuseable links that drop the voltafge even lower. Typical voltage on the V1 and V2 heaters of a MTS with EL34's is 4.3 to 4.8 VDC. Much lower than the +- 10% specification allowed. O the low end the voltage should be 5.67 and on the high end 6.9.

http://www.thegearpage.net/board/archive/index.php/t-174897.html

I have no idea whether the poster is correct on the transformer specs, and even though he is incorrect regarding the resistance value of the fuse (by a factor of 100), this post makes me question whether the amp I'm working actually has a problem. (Who knows, it may be the very same amp!!) Well, it does have a problem -- very low voltage on the DC heaters. I dunno. By the way, I didn't have EL34s installed when I took my voltage readings; the output tubes were Sovtek 5881s and I only had two installed.

Who knows, eh?

[Krwkka]

I just replaced the caps in the DC heater circuit to see what would happen. It made no difference.

I'm wondering if I should try doing it the way Marshall did on the DSL50. Instead of the two 100 ohm resistors to ground, they use two capacitors. Would that help raise the voltage?

Link to the Marshall schematic is below. The heater rectifier circuit is in sections A-3 and B3.

DSL50 Schematic

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[Krwkka]

Latest update (if anybody cares).

I jumpered across the two dropping resistors that lead to the DC section of the filament circuit. I now have 6 volts DC across the heaters in V1 and V2. The AC section is giving me 6.1VAC. I have no idea why the DC is so low after going through the rectifier. I replaced not only the caps but also the diodes - just to see if that was the cause.

The plate voltages on V1 V2 are now a BIT lower:

V1:
Pin 1 212v (was 221: schem says should be 160)
Pin 6 208v (was 226 should also be 160)

V2:
Pin 1 325v (was 334 should be 185)
Pin 6 208v (was 242 should also be 185)

I dunno. I just wish somebody who has worked on one of these would tell me if these numbers are normal for this machine.

Soundwise, everything seems the same. Still have a weird low freq sound when I hit a chord in distortion channel. Wish somebody who knows these machines would tell me if this is normal.

[hasserl]

I've seen this (the low heater voltage on V1 & V2) a number of times on these amps, it is very common. In fact, the heater voltage on all the tubes is on the low end. It's worse when running EL34's than it is when running 6L6's, so if you're running EL34's that would go right along with what I've seen in the past. Take a look at the green wires from the power transformer to the power tube pcb, I've seen those scorched as well. Cutting the ends off and installing new female spade terminals may help if that is the case. You may also see discoloration from heat on the pcb where the male terminals are mounted. It's clear that Carvin has undersized the heater circuit on these amps, not just with the tranny but all the way thru. Some folks have replaced R29 & R30 with wire (you can use spare leads clipped from resistors and caps to do this) to pick up some voltage at V1 & V2.

[Krwkka]

Thanks, R. I did a jumper wire across those resistors and the heater voltage looks good. I have only two 5881s in the amp right now, so this voltage drop is quite puzzling to me.

Do you know if it's normal/common in these amps for the 12ax7 plate voltages to be so far off from the schematic? For example, on V2 pin 1 I have 325v but the schematic says it should be 185v! That's a huge difference! Other plates are pretty far off, too (see my table and link to schem above). I checked all the resistances on each tube (top and bottom) and they all look right.

[sean k]

Have you checked the power supply reistors for the preamp tubes? and if you have are they as the schematic specifies? It's the job of those resistors to drop current and voltage from the higher voltages to the power amp tubes so they get the voltage on the top of the plate resistor you want and therefore the current to be dropped accross that plate resistor.

And so the current draw of the power tubes is going to dictate the power supply resistors for the preamp tubes... Maybe you need higher value dropping resistors for the preamp tubes.

[Krwkka]

Yeah, Sean. I checked all the resistor values and they all agree with the schematic. However, the voltages are a bit high all the way across the power supply.

The oddest thing is that the three preamp tubes in the amp all get their juice from the same power supply node. But the third tube's voltages are just about PERFECT (according to the schem). How can this be??


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