Guitar Amp Issue (Questions about Bias and Tubes)

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Echo North

Well-known member
Joined
Jan 21, 2006
Messages
4,451
Location
Seattle, WA
Hi all,

So I'm looking at a guitar amp for someone. It's an old Peavy "The Classic".

The schematic (possibly a different revision) is located here:
http://schematicheaven.com/newamps/peavey_classic_b.pdf

This amp has 2 6L6GC power amp tubes...the pre is solid state. I believe it is suffering from over heating.

Here is the breakdown of things I can confirm:

Issue:
The amp works fine for about 1 hour the fizzles and farts out. The amp needs to rest for a while before it works again. (this is why I am guessing overheating)

Observations:
1. The tubes do not seem to be glowing excessively, although they are quite hot. The problem with these observations is that my experience with tubes is minimal.

2. In both tubes there are two metal circles that come out of the middle...the glass near these metal circles (heatsinks, spacers?) is all black. Both tubes have two big solid black spots on the glass.

3. One of the tubes rattles a little...all the inner connections look fine...there are a couple of small pieces of what looks to be glass rattling around in there.

Measurements (using Fluke DMM):

1. Looking at the schemetic, it indicates a voltage of -55V to the wiper of the bias. My reading is -53.5V. Seems reasonable?

2. The side of the bias trimmer where "bias" is written indicates a reading of -77V. Here is where the amp differs from the schematic. There is a 1k resistor in series before the cap/diode/power transformer. The voltage at the trimmer is -76V. Seems ok?

3. The other trimmer lead indicates a voltage of -25V. However on my unit that lead connects directly to ground (no 22k resistor). Soooo.... I get a reading of 0V (measuring ground against ground)

Any advice? Tips? Clues? Or anything you would like me to try.

Also, a question about safety. I'm being very careful since I understand tube voltage/current is excessive. I understand a lot of injuries occur from getting zapped by caps still holding high voltage after the amp has been powered down. What is a safe amount of time to wait before I can know caps have discharged?

Thanks again!

Mike
 
1. The tubes do not seem to be glowing excessively, although they are quite hot. The problem with these observations is that my experience with tubes is minimal.


The tubes will get quite hot, yes. Taking a few guesses about the output transformer I'd say each tube is idling at around 10W, which is enough to make things toasty to an unprotected hand.


2. In both tubes there are two metal circles that come out of the middle...the glass near these metal circles (heatsinks, spacers?) is all black. Both tubes have two big solid black spots on the glass.

Probably just heat-darkening of the glass envelope through regular use, especially if they're old tubes.


3. One of the tubes rattles a little...all the inner connections look fine...there are a couple of small pieces of what looks to be glass rattling around in there.


Does it make any noise through the speaker if you turn it on and tap the offending tube with a pencil? Can you swap the tubes with a known good pair?


1. Looking at the schemetic, it indicates a voltage of -55V to the wiper of the bias. My reading is -53.5V. Seems reasonable?

2. The side of the bias trimmer where "bias" is written indicates a reading of -77V. Here is where the amp differs from the schematic. There is a 1k resistor in series before the cap/diode/power transformer. The voltage at the trimmer is -76V. Seems ok?


Yep and yep. In fact if you check the bottom-right corner of the 2nd page of that schematic it says that you can expect the measured voltages to vary by as much as +/- 20%, so you're well within spec there.

Is there any way you can isolate the power amp section from the preamp section? It could be that the preamp is dying rather than the tubes.


Also, a question about safety. I'm being very careful since I understand tube voltage/current is excessive. I understand a lot of injuries occur from getting zapped by caps still holding high voltage after the amp has been powered down. What is a safe amount of time to wait before I can know caps have discharged?


The amp doesn't have a standby switch, so turning it off will cause the filter caps to drain via the amplifier's circuitry. If in doubt, check with your voltmeter. If still in doubt, make up a little cap discharging tool using a 100K 1W resistor and some insulated aligator clips. Whenever you want to work on the amp, turn it off and clip the resistor across the main filter cap to ground. Always work with the mains lead unplugged!


Oh and is this circuit what they refer to as "Fixed Bias"?


Yep.
 
Thank you for the help Curtis!

Tapping the tube does not create any noise through the speaker.

I do not have any other tubes on hand however, I may purchase some 6L6GC shortly. The problem is, every time I have a funny acting amp I just buy new tubes and it NEVER has fixed the problem, so I'm trying to break that habit :)

By Isolating the pre amp section do you mean providing power from another source? If so, unfortunately I cannot do that on site.

What I have done this morning is start by going through the schematic and checking voltages. I have just completed the power amp stage and found an anomaly.

On the schematic it shows 180V at the collectors of the 64278 transistors. Both of my transistors read 100V. Following that portion of the circuit, to the other side of the 100K resistors there is a reading of 330V which goes to the large metal can mallory cap. My reading is 230V here.

All other voltages listed on the power amp part of the schematic (page 2) are correct.

Note: These voltages were tested before the amp "farted out". If anyone thinks it would help, I can check voltages after a "fart out". Although this would involve me jamming probes into a live amp which scares the crap out of me :)

Is this an issue with bias? Exactly what does the 50K bias trimmer effect? Which voltage/current?

Thanks again,

Mike
 
I do not have any other tubes on hand however, I may purchase some 6L6GC shortly.


Even if you can pinch a pair from another amp it'd do for the purposes of experimentation.


By Isolating the pre amp section do you mean providing power from another source? If so, unfortunately I cannot do that on site.


Sorry, I meant can you unplug the preamp section audio signals from the power amp section? That way when you have a "fart out" you can separate the two and see which one is farting out when you apply a signal to its input.


On the schematic it shows 180V at the collectors of the 64278 transistors. Both of my transistors read 100V. Following that portion of the circuit, to the other side of the 100K resistors there is a reading of 330V which goes to the large metal can mallory cap. My reading is 230V here.


The voltage itself doesn't sound right, but the ratio of collector voltage vs supply voltage still seems OK (ie, the collectors are sitting at roughly half the supply). Can you measure the resistance of the 68K 1W resistor sitting across the points labelled "+500V" and "+330V" on the can cap? You'll need to disconnect at least one side of the resistor from the amp to get a valid reading. The amp should be unplugged from the mains of course!

Does your Fluke measure capacitance aswell? Measure the capacity of the 20u element of the cap if you can. Again, lift one side of the cap to measure, and make sure it doesn't have any voltage on it before you measure (check with your volt meter first).

Do the transistors look cooked? Any bulges or wierd goop oozing anywhere? Anything else on the circuit look cooked? Have a look at the circuit board (If it's a Peavey I'm assuming it's built on a PCB). Look closely at the solder joints, check for any that look like they are split or cracked (can sometimes look like a hairline ring around the "peak" of the solder joint). Reflow any that look suspect.


Is this an issue with bias? Exactly what does the 50K bias trimmer effect? Which voltage/current?


Probably not, although swapping the tubes would help eliminate them as the cause. The bias trimmer varies the amount of negative bias applied to the grids of the 6L6's, and thus the idle current passing through the tubes with no input signal applied. Making the bias voltage less negative increases idle current. Too little bias (ie, more negative voltage) and the tubes run cooler, but the amp will start suffering from increased cross-over distortion (generally accepted to be not-particularly-nice sounding). More bias (ie less negative voltage) will reduce the effects of crossover distortion, but going too far the other way the tubes will start conducting to the point where they will actually bake themselves to death. Often you can see a tube that is over-conducting by looking for any red hotspots on the plates themselves (not to be confused with the glow from the heaters) - something else you can check with the amp running.
 
Hi Curtis,

the 68K resistor across the chassis mount can cap is a 62K resistor in my unit (again different revision from the schematic I think.) Anyway I completely removed it and it is measuring 100K on my meter!

I checked my meter against other resistors and my meter is fine. Possible problem?

I don't have any 62K 1W resistors laying around (or 4 248k 1/4W for that matter).

Not sure how to measure the capacitance of the 20u element of the can cap....there are four terminals. Which terminal to I measure the 20u terminal against?

Looking at the other things you mentioned right now.

Mike
 
The amp is on one circuit board. No way to disconnect the power amp from the pre-amp.

I removed all the knobs and rolled the board over. Looks fine...ran some continuity tests.

All the transistors look fine.

I found 4 1/4W 270K resistors and made a 67K 1W resistor with them. The original was 62K.

Looking at my readings again:

Transistor collectors:
Schematic: 180V
Broken 62K (reading 100K on meter: 105V
New 67K resistor: 140K

+330 to can cap reading:
Schematic: 330v
Broken 62K (reading 100K on meter: 230V
New 67K resistor: 280K

I may see how if she farts out with those reading tonight.

Either way I will key a proper 62K 1W resistor tomorrow.

Thank you for the help Curtis!

I will let you know the outcome.

Mike
 
the 68K resistor across the chassis mount can cap is a 62K resistor in my unit (again different revision from the schematic I think.) Anyway I completely removed it and it is measuring 100K on my meter!

I checked my meter against other resistors and my meter is fine. Possible problem?


Well it's not helping you get the correct voltage readings, so it definitely wants replacing. 62K 1W or 68K 1W will probably work well enough interchangeably in that location. It may be that the resistor is heating up as the amp is running causing the resistance to vary even further, gradually causing the amp the shit itself. When the amp cools down so does the resistor, and the amp appears to work again from "cold".


Not sure how to measure the capacitance of the 20u element of the can cap....there are four terminals. Which terminal to I measure the 20u terminal against?


One of the four is probably common to all three elements of the cap. It might be labelled "Common" or "Ground" or "Earth" or "Negative" or something similar. Any legends printed on the can that might give you some clues? Sometimes the case itself is the common point. You want to measure across the 20u terminal (whilst disconnected) and the common.

Edit: simultaneous posting. Good luck with the resistor swap :thumb:

Double-edit: just keep an cautious eye on things while your soak-testing the resistor fix. Some of your voltage readings seem a little on the lean side compared to the schematic. There might be another reason why that resistor failed, perhaps due to excess current draw from something downstream of it. Just out of interest can you confirm both the other voltage readings around those transistors and also the surrounding component values?
 
I seem to remember several of this series come through the shop with power tube sections of the boards hopelessly cooked. Required new hand- wired power tube sections to get them going again. This clearly isn't that bad. May have missed it, but did you retouch everything to eliminate cold solder joints? Could be joints shifting with heating. I'd tend to agree with heating effects on some circuit board parts, and not really thinking this is necessarily overheating.

Once saw an amp that had a bit of hot glue around and in a power tube socket. You couldn't see it, and it did exactly what you describe, because the glue would melt after awhile, and ruin the socket contact. Cool it down, and you'd have good continuity again. Took forever to find that one; only by accident after replacing the socket in desperation.
 
Well I was able to create a 61K 1W resistor.

My readings are better:

Collector 165V (should be 180V)
+330 at can cap is now 305V

Both of these are within the +/-20% but it's still unsettling since my other voltage readings have been dead on.

I have confirmed the base and emitter voltages are still accurate on those transistors. All of my other power amp voltages seem fine.

The amp has been running OK so far tonight.

I will examine other component values tomorrow.

Thanks again,

Mike
 
Another though...

You don't see this particular amp very much....so I had to struggle to find a schematic.

As I work my way through it I see more and more differences between this amp and the schematic. It may be possible the collector reading are correct for my amp revision?

Just a thought...still running it...so far so good.

Mike
 
Well I was able to create a 61K 1W resistor.

My readings are better:

Collector 165V (should be 180V)
+330 at can cap is now 305V

Both of these are within the +/-20% but it's still unsettling since my other voltage readings have been dead on.


Probably near enough is close enough now. It could be that your mains voltage is a bit low, which would have an impact on those voltage measurements.


You don't see this particular amp very much....so I had to struggle to find a schematic.

As I work my way through it I see more and more differences between this amp and the schematic. It may be possible the collector reading are correct for my amp revision?


Probably not that uncommon. I have a Mesa Boogie Caliber 50, and the schematic I have found for that has many variations in the component values compared to the actual amp. For all we know the differences may be down to something really simple, like the manufacturer had a bunch of 62K 1W resistors they wanted to get rid of quickly, so they made a variant with that resistor in the power supply!
 
[quote author="Curtis"]

Echo North wrote
2. In both tubes there are two metal circles that come out of the middle...the glass near these metal circles (heatsinks, spacers?) is all black. Both tubes have two big solid black spots on the glass.

Probably just heat-darkening of the glass envelope through regular use, especially if they're old tubes.


[/quote]
The round metal circles are the "Getters" (see Wikipedia) and the black spots are the result of the Getters being heated. It's a quick check to see if the valve has "lost" vacuum, as the black spots will be white!
 
hi,
i think i have the same amp. mine is 1972 2x12". i recently converted it to a tweed bassman as i don't like solid state preamp. the outputtransformer was dead anyway (but i still got the original pcb). i used most of the other parts and now it's just a lovely amp.

did you check the caps in power unit? are they still ok?
 
Thanks for the help all.

Walrus thank you for the info on the "getters', I looked all over to figure out what those were with no luck.

The amp ran for a solid 2 hours last night without farting out.

Tonight I'm going to put is a new 1W resistor and check the readings on the can cap.

I'll also look at the other caps in the power supply.

The problem is...the board components are bent at 90degrees then soldered with about 1/2 pound of solder. It's ridiculous, makes it hard to test components.

Mike
 
******UPDATE******

The amp is still not working right.

I doesn't die anymore, but it gets farty and fuzzy after about an hour. Seems to be overheating of some kind. If I take the amp out of the enclosure and give the tubes room to breath it doesn't fart out ever.

When I put it all together and the tubes are covered by the enclosure and back panel...things get farty.

I have installed brand new tubes and the voltages seem fine.

Any thoughts?

Version of the schematic:
http://schematicheaven.com/newamps/peavey_classic_b.pdf

Mike
 
I hate to rain on the parade, but it's a Peavy. Some people consider that a dirty word. You might want to ask if it is worth continuing to repair an amp that is likely to fail again.

First impression would be heat is causing either a contact or trace to separate. The fact that it is OK when it cools down (meaning outside of the chassis) says that it is not a part failure (possibly), at least something like an FET or transistor that is fried. Farty/fizzy could be power supply caps. Hard to tell without being inside it or hearing what you are describing. Are the caps original?

Or... it is not a heat issue, but pulling the chassis out of the cabinet is helping a loose connection tighten up..... Are you familiar with the chopstick method? The idea is to poke around the amp and move wires WHILE IT IS FIRED UP, but use something long and non-conductive, like a wood chopstick, to see if you can find any loose connections or lead dress issues. Please always have one hand behind your back when poking around a live chassis. The idea is to not have the current go through one hand, then your heart, then out the other hand that is resting on the chassis, which also happens to be ground.


First you say:
If I take the amp out of the enclosure and give the tubes room to breath it doesn't fart out ever.

Then you say:
I have installed brand new tubes and the voltages seem fine.

Probably time to start looking past the tubes as the culprit here.



Although it is best to diagnose the problem and fix it, it seems that heat is an issue. Is there a way to cool it down enough to keep it playing nicely? A fan?

Are the voltages fine when it is hot and sounding like crap? Specifically, from the power supply.

If by farty do you mean hitting a bass note causes signal to die and it is hard to get full signal after that and the sound is "farty, unresponsive" or is it more of a distortion over the regular guitar signal, like a bad fuzz/distortion pedal?

Does the volume decrease when it starts acting bad?

Lastly, you are in a DIY group, have you thought of building a nice ALL tube amp? 18Watt.com has some great, relatively easy, projects. A fender 5E3 (tweed deluxe) clone is a great 1st time kit to build with plenty out there. Volume and tone control, that's it, but great friggin tone. Plenty of kits too, so you don't have to build the whole thing from scratch, not that there is anything wrong with that. :grin: :thumb:
 
> it's a Peavy. Some people consider that a dirty word.

Peavey can make very good amps.

They've also, like many other brands, built stuff with low-bid PCBs and way too many ribbon connectors. That was my first thought, and I hate such repairs.

But this one is different. It is from 1975. Many of the cheap-tricks we find in recent amps had not been invented yet.

OTOH, it IS 30 years old. All electrolytics are suspect. And increased leakage at high temperature on a B+ decoupling cap could drop the B+ until it gets crappy.

And while transistors were mature in 1975, there's some extreme situations here. The tube driver has 160V across the transistors. I'd bet on bad caps first, but hot old transistors are suspect.

I think it will be worth fixing some day. This may not be the time.
 
Thank you for the reply PPR.

Where would be the best place to measure the B+ voltage?
a) on start up when it's functioning fine
b) once it starts to get farty

Is the B+ decoupling cap that big can capacitor hanging off the back by the tubes? Are the larger caps coming off pin 5 the decoupling caps?

This is a friends amp I agreed to look at for the learning experience so I'm up for the work (and learning).

Thank you again,

Mike
 

Latest posts

Back
Top