Guitar Amp Issue (Questions about Bias and Tubes)

I'm a little confused by which pin is the cathode pin. Looks like it's pin 8 on the data sheet. Pin 8 is connected to ground (via the multi cap mounting plate ground).

Here is the data sheet:
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6L6GC.pdf

Pin 1 and 6 are not connected.

The only resistor is a 47k in series from pin 5 to the PCB.

Here are my PIN - Ground readings

1(n/c): n/a
2(Heater): 0v
3(Plate): 489v
4(grid 2): 489v
5(grid 1): -51v
6(n/c): n/a
7(Heater): 0v
8(Cathode and Beam plates): 0v

Both tubes were the same.

These are my only two tubes...the preamp section is solid sate.

Make any sense?

Thanks again,

Mike

You can measure the voltages Amimatic is talking about without the tubes in the socket. They will be higher readings than when the tubes are in, but it may save your tubes from getting fried while you sort out the issues.

40/40/20/20 @ 525v is the better choice.

Click to expand...

Yes, Animatic is correct. When I said voltage is more important, that refers to the fact that caps should meet the MINIMUM voltage requirements. If you go over, that is simply over engineering and is fine. Since the 525v multi cap has the same uF ratings, that basically would be a perfect match. The 20/20/20/20 would not have quite as much smoothing as the original multi cap. It might very well work, but why complicate things.

Remember, this amp only has 2 tubes in the whole amp. There are 2 6L6s in the power section. It is SS for the preamp. Animatic is on the right track. He is looking to see that the voltage structure, which determines the power/amplification state of the tubes, is in the right range, thus operating as it should.

I don't think this amp is set up with a 1R ohm measuring resistor for a Cathode voltage reading (tells you how much current the tube is drawing, thus where to set the bias). You can always scope the output, or add two resistors to take some easy readings.

Well there likely is a cathode resister, even if not a 1k also
it is the same thing, divide it by voltage.

With a 1Ω or 1K you can do it in your head.

Basically track the voltages backwards across the caps
40 40 20 20, and corresponding resisters,
and if they all seem stable when it's overheated,
then it's another cause, but heat can effect caps,
and the big ones likely earlier than others,
so it''s a logical place to start.

Best to test the voltages with load, if the tubes aren't red-plating
then they likely won't start. If they are badly they are already dead.

Well there likely is a cathode resister, even if not a 1k also
it is the same thing, divide it by voltage.

Click to expand...

There is no cathode resistor(s). That is pretty common for fixed bias. Here is the schematic he is using. These are new tubes he just put in.

http://schematicheaven.com/newamps/peavey_classic_b.pdf

I always like to make sure the power supply is working before I apply power to the rest of the system. That is simply my method of troubleshooting, divide and conquer. Also, it helps insure you do not cause more problems because of the initial problem. Because of the high voltages being put out by the power supply, it makes sense to make sure all is right before unleashing those voltages to the tubes.

There also are no screen resistors on that schematic. :shock:

Thanks again guys.

I'll give it another go tonight. I'm fairly confident all the voltages are correct while it's working fine (of course). What I need now is to get it over heated and check again...only problem is that it has not over heated the past two nights.

The over heating only seems to happen when the back of the amp enclosure is on (not just with chassis in the enclosure). This seems to point to an external chassis mount component over heating. Either the tubes, multi cap, or transformers (unlikely?).

If I can get it to overheat and fart again, I'm going to get a good look at that multi cap and try to confirm it is or isn't the issue.

Another thing to mention...the reverb sounds like shit. I only mention that in case it maybe in some way related. I'll take a look at the transistors and voltages in the reverb section tonight. Could also just be shitty reverb.

Thanks,

Mike

[quote author="Echo North"]Thanks again guys.

I'll give it another go tonight. I'm fairly confident all the voltages are correct while it's working fine (of course). What I need now is to get it over heated and check again...only problem is that it has not over heated the past two nights.

The over heating only seems to happen when the back of the amp enclosure is on (not just with chassis in the enclosure). This seems to point to an external chassis mount component over heating. Either the tubes, multi cap, or transformers (unlikely?).

If I can get it to overheat and fart again, I'm going to get a good look at that multi cap and try to confirm it is or isn't the issue.

Another thing to mention...the reverb sounds like shit. I only mention that in case it maybe in some way related. I'll take a look at the transistors and voltages in the reverb section tonight. Could also just be shitty reverb.

Thanks,

Mike[/quote]

I've never really liked the sound of amp reverbs so that could be normal by sounds like shit you mean what? You need new springs? Anyway What we know so far is that when it is enclosed with the back on it over heats and starts to fart out. That makes sense as there is real place for the heat dissipate from. If there was a problem with the caps and temp, they would fail and need replacing as does most parts when it is temp related. To me sounds like the heat is causing the board to flex which in turn is causing a solder joint to stop making contact.

Have you tried to touch up all the solder connections?

Ah thanks for the schematic Stanz nice to have a real reference.
Cathode ground .

Sure would be nice to have two 1Ωr sitting there.
But no matter. Leave out that measurement.

Maybe they think the Flameproof 400 5w is their screen resister?

47k grid stoppers, looks like they had some occilation issues.

Well the amp used to run fine at it's normal temp,
so I suspect components or bad joints still.
Ventiung the heat just forestalls finding the culpret,
and that might then be found on a gig.

I would have put one 12AX7 in this design to replace the gain stage
before the tone section, but that's just me.

One easy enough place to look is at the tone pot wiper.
If things are good up to there, then we need to look after.

Actually at any of the pots you can gleen some useful info of what's happening.

Put in a sine and or some music and look / listen to what is coming out.
Put it in input and listen at tone wiper,
put it in tone wiper and listen after.

Well three nights of work and I can't get her to fart.

After an hour or so the amp does break up a little on held out notes...it acts like it's about to start farting...just a little toot...then backs off. Weird. I even tried putting the old tubes in.

I'll work on her over the weekend. I'll bring out the signal generator. I'm also going to take a closer look at the solder joints.

Mike

The only thing worse than buried intermitants
is dead as a rock and no burn marks. YMMV.

Good luck, don't get too frustrated.
Epiphanies come when your relaxed.

Yeah.

I was just thinking (here at work) that the only thing I did between the full fart and the little toot scenarios is move the tube socket pins around a little (while testing them).

I'm going to take a long look at those tube socket connections. In theory, with the back cover on, the heat will dissipate slower. This would cause access heat build up around the tubes and sockets. Perhaps a poorly connected socket would fail or begin to fail in that scenario.

Something to look at anyway.

Thanks again,

Mike

http://www.geofex.com/


this guy has excellent information about tube amps...very well respected in the pedal and amp building crowd.

sorry if this has already been mentioned in this thread, as i havent read all 4 pages yet, but figured it would be a really reference.

the part about the tube amps is to the left.

good luck...

Thanks Lo-Fi!!

I've been to this site or message board but somehow missed the part on tube amp repair.

Very helpful. Thank you!

Mike