Another Mesa (DC-2) problem

[sluckey]

Edit: there seems to be a very big voltage difference on V1 (pin 1 measured) with and without a tube installed. The schematic calls for 140v if I recall, it is about 144v with a tube in place. Without it was above 260v... I have never noticed/seen this before. Can this be a clue? I am sorry fir all this back and forth, I guess i should take three steps back and try to document and measure everything systematically...

This is not a clue. This is the way this stuff operates.

[Jered]

Have you tried cleaning the preamp tube sockets with a good cleaner?

[goldenGeek]

Have you tried cleaning the preamp tube sockets with a good cleaner?

I have tried with electronic cleaner and pure isopropyl alcohol (I think that's what its called in english).

Edit: I have also tried to replace the caps and resistor in the tone stack without any improvements.

[goldenGeek]

Okay, so I'm still banging my head against the wall, but I've started to slowly make a voltage chart which I will post when its done. As a side note, the power transformer gets extremely hot so it could possibly be something pulling too much current I guess. I dont know how hot a PT should get, but I've never touched a PT this hot before, I'll get hold of a IR-camera and measure but I can not put my hand on it for more than one second after being on for about 15 minutes.

[goldenGeek]

So heres some voltages - I know some values are sticking out, but is it too much off? V1,2,3,4 and V5b is feeding off the same B+/filter cap. V5a, V6,7 and 8 also of a common B+. There is really very little filtering in this amp, only two 220uf caps (in series, which should result in 110uf, right?) and two 30uf caps. Anyway, heres the voltage chart:

DC-2_VoltChart.png

[Phil_S]

It would be interesting to know the temperature. I see the heaters are about 5.8-5.9V. This is within 10% of the 6.3V target, so tubes should operate OK. However, I wonder if this is the source of the heat. Maybe the heater supply is not enough for all 8 tubes. With all the tubes pulled, what is the heater voltage?

This is from Mercury Magnetics website. It suggests anything below 55C is OK.

https://www.mercurymagnetics.com/take-y ... arts-i-ii/
Temperatures below 100°F are generally considered “cold” or “cool” in terms of the operating temperature of a transformer.
Temperatures between 100°F and 110°F are considered “warm.”
Temperatures between 110°F and 120°F are considered “very warm.”
Usually temperatures above 120°F are considered “hot.”
Temperatures above 130°F may cause second or third-degree burns on adult skin.

Transformers designed with high-temperature insulation systems can run safely at temps up to 200°F. But remember, a hot-running transformer is an angry transformer.

Anode and cathode voltages are close enough to schematic values. It might be useful to also know the voltages at the + side of the filter caps. I don't think this is the source of any problems.

I'm trying to recall exactly what you'd like to fix. I think you said there is hiss and hum.

The two main sources of hum are poor filtering and lead dress. Poor filtering may be due to old caps that need replacement, or not enough filtering. For lead dress, I'd look for places where the heater wiring is too close to signal wiring. You say it is 100Hz. For that I'd look at filtering first. You may also have underlying 50Hz hum (heaters) that is masked by the 100Hz.

There can be many things that cause hiss. I'd replace the plate load resistors, one at a time with a 1W or 2W metal film resistors. Hiss can also be caused by bad tubes, bad caps, faulty jacks, and plenty of other things. I'd work on the hum first. It may also fix the hiss.

[goldenGeek]

It would be interesting to know the temperature. I see the heaters are about 5.8-5.9V. This is within 10% of the 6.3V target, so tubes should operate OK. However, I wonder if this is the source of the heat. Maybe the heater supply is not enough for all 8 tubes. With all the tubes pulled, what is the heater voltage?

This is from Mercury Magnetics website. It suggests anything below 55C is OK.

https://www.mercurymagnetics.com/take-y ... arts-i-ii/
Temperatures below 100°F are generally considered “cold” or “cool” in terms of the operating temperature of a transformer.
Temperatures between 100°F and 110°F are considered “warm.”
Temperatures between 110°F and 120°F are considered “very warm.”
Usually temperatures above 120°F are considered “hot.”
Temperatures above 130°F may cause second or third-degree burns on adult skin.

Transformers designed with high-temperature insulation systems can run safely at temps up to 200°F. But remember, a hot-running transformer is an angry transformer.

Anode and cathode voltages are close enough to schematic values. It might be useful to also know the voltages at the + side of the filter caps. I don't think this is the source of any problems.

I'm trying to recall exactly what you'd like to fix. I think you said there is hiss and hum.

The two main sources of hum are poor filtering and lead dress. Poor filtering may be due to old caps that need replacement, or not enough filtering. For lead dress, I'd look for places where the heater wiring is too close to signal wiring. You say it is 100Hz. For that I'd look at filtering first. You may also have underlying 50Hz hum (heaters) that is masked by the 100Hz.

There can be many things that cause hiss. I'd replace the plate load resistors, one at a time with a 1W or 2W metal film resistors. Hiss can also be caused by bad tubes, bad caps, faulty jacks, and plenty of other things. I'd work on the hum first. It may also fix the hiss.

The temperature was right under 50 degrees C (120-ish F?). I did a full cleanup and re-solder on the tube sockets and I think it's a bit cooler. But its not better noise wise. There is almost no hum, but there must be a gigantic waterfall inside that's hissing a lot

As for heater wiring, there is virually none because its all in the PCB. There is a short wired connecton to the power tube PCB but other than that its all on the PCB. Lead dress affect the hum, I know, but there is not much wiring in here and putting the wires in about "original" position make the hum pretty "normal". Input jack is replaced and properly tied to the chassis ground. I've replaced plate load resistors on V1, but I've run out of resistors so I'll have to wait for new ones before doing the rest. I have 1/2W carbon comp resistors, is it worth trying those?

Voltages on the filter caps are about 200V on point C and about 300V on point B in the schematic. I forgot to write it down, but it was right around those values.

What I've discovered yesterday is that there is a leakage between the channels. If I have the clean channel gained up (you hear it on lower gain also, but it is much more prominent when gained up) you get more hiss when gaining up the drive channel also. This also happens if the lead master is all the way down. It is also the case the other way around, on the lead channel, the clean channel is generating hiss when gained up.

This amp is driving me nuts. I'm tempted to just return it and forget about it, but there is something inside that make me wanna figure out whats wrong with it.

[Phil_S]

According to the chart 50C is OK for temperature. I would not try 1/2W cc resistors. Carbon comp resistors are known to be noisy. Get 1W or 2W metal film.

[goldenGeek]

According to the chart 50C is OK for temperature. I would not try 1/2W cc resistors. Carbon comp resistors are known to be noisy. Get 1W or 2W metal film.

I wasted a couple of hours today debugging some more (without getting much further). The temperature kept on raising and the PT was at about 67 degrees Celsius (152-ish F?) after half an hour. I guess this is kind of bad news. I ordered new resistors for plate load and some capacitors yesterday so that'll be here in a few days. I guess I need to keep on digging to find out what's causing the very hot PT and hopefully it will lead me to the hiss as well. WHile having the IR-thermometer out I scanned all components and there's a few hot spots, but I'm not skilled enough to tell if these temperatures are within "normal" values. I'll try to explain below:

Low voltage supplies:
150uf/100v cap - about 73 degrees C (163F)
4 x 100k resistors i parallel - 100 degrees C (212F), decreasing to 65C from the furthest left to the furthest right (seen from the front)
2 x 1k2 in parallel, ca 70 degrees C (158F)
2 x 12V Zener in series, 47 degrees C (116F) on the one tied to -30V, 60 degrees C (140F) on the one tied to 0V (-15V between the two zeners)
Then there's this 2200uf/35V capacitor, but it's 220uf in the schematic... typo in the schematic..?

Bias supply:
120k resistor - 99 degrees C (210F)

[Phil_S]

I don't know if your Norway internet will let you see this link. I am thinking you have a bad transformer. There is a test described in this article:
https://www.premierguitar.com/the-super ... mer-tester

[JD0x0]

See if you can find Paco Casanova's mod. He has a specific mod to remove the hiss from the clean channel. It's a known circuit flaw, but I don't remember the exact specifics of the mod off the top of my head.

Edit: found some info.

Recentely I posted a mod on the Mesa Boogie Studio 22+ amp about how to tame the massive noise! This mod wasn't complete....so I like to add my thoughts here:
First, I got a responce from a fellow member that when I cut the input impedance from 4.3M down to 1M, I would face some treble loss. This is not the case here. For example, if I took a relative high impedance pickup (approx. 16k - Dirty Fingers HB by Gibson), the difference from 4.3M to 1M is very small - 15.94 vs 15.74kOhms - this wouldn't result in a massive treble loss.
The 2nd issue is bigger. The clean preamp tend to distort a lot, when this noisy 3M3 / 47pF arrangement is shortend, just because there is too much signal on V3b, which goes into clipping. V2A and V2B were the typical fender circuit topology with a few slight changes. The easiest solution would be to put that 3M3 /47pF arrangement behind V2b but before the reverb circuit. You can't use the reverb mixing resistor (220k) for this, otherweise you would loose the balance between reverb and dry signal. The solution is to put this filter arrangement right after the coupler cap behind V2B and make it switchable with the 2nd portion of the DPDT relay which is part of my mod.

[goldenGeek]

So I just delivered the amp back to the owner, only charging him for the parts used. I have spent way more hours om this amp than what's sane, but that's another story I guess. The hiss is not, by any means, gone. It's better than when I got it, but it's far from what I'd expect (and want) from an amplifier. The customer says he probably can live with it for now. As far as I have concluded the problem is probably the PCB, it could also be the power transformer (or both) as stated earlier in this thread, but since the hiss is a lot worse in the clean channel I'm leaning against the PCB being bad/conductive. I'll sum up what I have done and how this ended (so far).

-Replaced all tubes, got rid of crackling and popping and it made the amp sound good (again), but not really a big improvement regarding hiss
-"Tube rolling" with other known good tubes, no improvement in hiss
-Installed a bias potentiometer and changed a resistor so that the bias could be lowered and adjusted, it was (like many Mesas with EL84) way over spec, no change in hiss
-Replaced all plate load resistors, no change in hiss.
-Re-soldered and cleaned tube sockets, no change in hiss but it fixed some bad connections that made the signal disappear and/or buzz if one of the tubes was pushed sideways (or by vibration).
-Replaced filter capacitors and some resistors and diodes in the power supply, no change in hiss, but the voltages raised quite a bit. The heaters are now almost perfect at 6.3V, B+ is a bit high though.
-Replaced resistors, diodes and capacitors in the low volt supply, no change in hiss, but there was a few caps way out of spec
-Replaced transistors, LDRs, relays etc in the switching matrix, no change in hiss, but the channel bleed is gone
-Cleaned up a previous mod for serial FX-loop, no change in hiss
-Some other resistors also changed, and I tried to replace the resistors and caps in the tone stack, no change in hiss

See if you can find Paco Casanova's mod. He has a specific mod to remove the hiss from the clean channel. It's a known circuit flaw, but I don't remember the exact specifics of the mod off the top of my head.

That's the studio 22+ which has a different pre amp from what I can tell from the schematic.

I don't know if your Norway internet will let you see this link. I am thinking you have a bad transformer. There is a test described in this article:
https://www.premierguitar.com/the-super ... mer-tester

I never got around to do that test since I do not have a neon bulb at the moment. However since the hiss is only a big problem in the clean channel I suspect the PCB rather than the transformer. But I'm not sure, and with that heat going on it could go bad any time I guess. I told the owner this and he's willing to take the chance.

[TUBEDUDE]

You'll see this amp again.