Help debugging my mesa boogie studio preamp

[Digamma]

Hi All,
This is my first post My new(used) preamp has a problem and I need some help. I have a little experience/knowledge in electronics but mostly non-pratical from Uni. It is not a bad tube as I have re-tubed all positions already. Here are the symptoms:

- day 1, gain dropped out on lead channel temporarily
- day 2, same thing but it stayed like that for a while.
- day 3, changed tubes. Now noticing a lot of intermittent noise(chirping/interference) on lead channel until finally gain dropped out permanently. No more noise. Just and low output with no distortion/gain. While experiencing this interference, I noticed a hissing sound coming from inside the amp.

So my clean channel is fine. My lead channel now is weak and has no gain at all. The lead gain control doesn't even make any difference at all. All tubes light up. I have the schematic.

http://www.tubefreak.com/studio.gif

I'm guessing the problem is around V3 since this is the tube that provides gain on the Lead channel. I'll start by measuring the resistors around v3 specially the plate resistor. I have no idea what else to check.

Any help would be appreciated! Thanks

Miguel

[Don Moose]

Since it got worse after the tube change:
1) try the tubes in something else - sometimes you get a bad one
2) check the soldering around the tube sockets - the stress of insertion/removal may have made an iffy joint actually bad.
3) try the tubes in a different order - help you narrow down which one is bad.

[Digamma]

Hi,
Thanks for the answer, but I have tried more than 2 or 3 tubes in all sockets, and interchanged them. Its still the Lead Drive that doesn't work though.

Anymore ideas?

[cbarrow7625]

Since the rhythm channel works fine, that rules out prettymuch everything in the am except the circuitry around V3 (a & b). All other tubes & circuitry in the amp are used for both channels, V3 is only used with the lead channel.

So, you need to probe all of the pins of V3 and see if the voltages on the tubes look normal (high voltage DC on pins 1 & 6 (plates), just a couple of volts DC on pins 3 & 8 (cathodes), zero volts DC on 2 & 7 (grids) and around 6.3 Volts AC between pins 9&4 and 9&5 (filaments).

If any of those voltages look odd or non-existant then you should look for broken solder joints or burned up components connected directly to the tube socket. If there is DC voltage on the grids, replace the coupling capacitors going to the grid of the stage showing the DC.

If the voltages on the tube look fine, you probably have a broken solder joint or shorted component in the circuit around V3 somewhere.

Assuming the tube voltages are fine, from your description I would check 2 components first: 1.) The lead drive Master volume - just make sure that it is not shorted to ground & that the pot actually changes resistance. 2.) R214 & C6 - check to see if they are shorted for some reason. They would shunt all of your pream signal to ground when the lead channel is switched in if one of them is shorted to ground.

I think the most likely candidates for a problem are those parts or just a bad solder joint on the tube socket of V3.

Good luck.

Chris

[Digamma]

Thanks for the great answer Chris. I just went and did some measuring.

The only 2 things that were out of norm were some values at the tube pins.

So, you need to probe all of the pins of V3 and see if the voltages on the tubes look normal (high voltage DC on pins 1 & 6 (plates), just a couple of volts DC on pins 3 & 8 (cathodes), zero volts DC on 2 & 7 (grids) and around 6.3 Volts AC between pins 9&4 and 9&5 (filaments).

Following your instructions, I measured all the pins. The ones with wrong values were pin 1 (plate) -> 0 volts(instead of High voltage like on the other plate pin6 where I measured 183V DC). And pin 2, 0.5v (Instead of 0V like on the other grid pin 7).

By looking at the schematic, what could be wrong? Given these values?

Thanks a bunch!

Miguel

[cbarrow7625]

Well, most likely, either the plate resistor going to pin 1 is open (burned up?) or there is a bad solder joint on one end or the other of that resistor.

That's going to be the 82.5K ohm resistor. Unplug the amp (very important!!!), remove V3 & measure the resistance of that resistor 3 differnet ways:

1.) Directly across the resistor leads (if you read something colse to 82.5K, then the resistor is fine).

2.) From the power supply side of the resistor to the actual Pin 1 connection on V3 (try to get your ohm meter lead actually into the tube socket where the tube plugs in. That way, if there is a hidden cracked solder joint holding the tube socket in place you'll know it). If you measure nothing (infinity), there is a bad solder joint somewhere between the resistor and pin 1 of the tube socket.

3.) From the plate side of the resitor back to the power supply node it is supposed to connect to. If this is open, the cracked solder joint is on that side of the resistor / pc board.

You can also just try measuring for DC voltage at the top (power supply side ) of the 82.5K ohm resistor to see if DC is even getting to that resistor. If you do that, you can skip step 3 above (if no voltage appears there youcan also skip steps 1 & 2 - you found your problem area).

Good luck. Let us know how it goes.

[Digamma]

Hi Chris, thanks again for your help.
I've just done some more measuring. Voltage is arriving at the resistor. The resistor measures infinity resistence so it is blown I guess. On the other side of the Resistor, the tube side there is 0V. So I guess thats my problem. I hope it is the only one. I read on harmony central that someone blew their plate resistor on the studio preamp by using 5751 instead of normal 12ax7/7025 tubes.

Does the plate resistor often blow in tube amps?

Now I have to go out and buy a resistor, see if that fixes it! If you have any other suggestions, let me know! You have been a great help!

Miguel

[cbarrow7625]

I don't think that is a common problem. Maybe just with that amp. It seems really strange to me. I have never come across a blown plate resistor but I suppose it is possible. Are the resistors really small, like 1/8 watt or 1/4 watt?

Anyway, I hope that is all it is. That's an easy fix.

[Digamma]

Hi again,
I removed the resistor and just under it, it had opened up and didn't look good. I measured it a little better and it was totally off.. So I put a new resistor in, actually 82k instead of the 82.5k on the schematic, which should be close enough. The gain channel now works and sounds the same as I remember it.

I did notice something that worried me though, If I turn on the preamp, and it is on lead channel, and no cable is connected to the outputs I get this strange hissing sound sound from the circuit. I immediately put a cable in and it stopped. So besides that, the preamp seems ok. Not sure if that is normal.

Thanks again!

Miguel

[Digamma]

BTW, the resistors are 1/2 Watt, I think! Some others on the board are larger maybe 1 Watt.

[priize]

hello... i m having trouble with my studio pre. I have no signal whatsoever. ARE these values with the tubes in it? Also are they the same for every tube. I recently changed the sockets so ... im thinking its probly one of them. i did get voltage on my 7 position 1st tube....and nothing on my 3 and 8. is that DC voltage? what could it be? help//....ive taken this amp apart dozens of times hoping id get it right. I think i need your expertice...or advice... thank you

[AtomicMassUnit]

You changed out the sockets? Did you make sure to get all the connections in the same place?