confusing heater filament circuit

[BrianS]

Howdy fellas,

I've got a Peavey Duel 212 on the bench right now that is giving me headaches.  I am doing some work for a music store and that is where the amp is, so I can't look at right now.  But hopefully my schematic link works so you can take a look at that. Oh, I'll post a pic, too...

The problem with the amp is that the power tubes are not heating up and therefore not conducting.  The preamp section of the amp works fine.  I get good signal at the preamp out and effects send jacks.  The power tubes just stay cold. 

There are burn marks on both ends of a ribbon cable wire that connects one of the filament lines to the power tubes.  I read no AC voltage there when the ribbon cable is connected to the power tube board.  I do read +24VDC, though.  This seemed odd to me, but I read a bit about heater circuits and came to find that sometimes the 6VAC for the heaters is elevated on some DC voltage to reduce noise.  When I disconnect the ribbon cable that sends the filament voltage to the power tubes, I do read 6VAC at the end that is connected to the transformer.  In other words, the transformer windings are fine.

There's a lot going on in this circuit.  R200 & 201 are creating...I think...a "center tap", and C200 is part of that circuit as well.  I did measure both resistors in circuit (with the tubes out) and they both read 100 ohms.  I don't think I checked C200, as I have to desolder every pin on all 4 power tube sockets to even get at any of the components on that board (BAD DESIGN!).

Anyway, I'm looking for some insight from anyone who may have seen or worked on this type of heater circuit.  If you dare, take a look at the heater circuit in the preamp...

Thanks,

Brian

Pic of the ribbon cable that attaches the heater windings to the power tube heater circuit.  Both ends of the cable were burned.  Obviously that wire got really hot!  Still makes a good connection, though.

[jjasilli]

I agree that this design seems overly complicated. 


Working backwards from the power tube page, the power tubes heaters are fed as follows: Pins2 are fed by J13, Pins7 by J14. 
J14 re-appears on Page 1, far right, second row of components connected to a relay.  J13 is just to its left and connects to +23V.  Clearly this is DC and is close to your +24V reading.  So the power tube filaments are supposed to be fed +23VDC, but C200 is rated for only 25VDC.  That's a recipe for failure.  I'm also suspicious of C200, because you should probably not be getting good and stable readings of resistors which are connected to a cap which is functional. 



Assuming the power tube filaments are also to get 6VAC:  Ribbon cables are unreliable.  You do have 6VAC at the the source end.  With the ribbon disconnected from the power tube board, do you have 6VAC at the tube end of the ribbon cable wires?  If YES, then the culprit is probably C200.  If NO, then maybe the cable wire has lost continuity.  Alternatively you could pull the cable and test its continuity. 


Lastly, there may be a lack of continuity in the power tube board traces or solder connections.  Are you getting that +24VDC reading at the power tube socket lugs?

[BrianS]

Yes, I tested the continuity of the cable.  I haven't been at the amp since last Tuesday, but I do believe the cable is good.  I'm sure I would have jumpered it if I had found that it was bad.

Thanks for the insight.  I think I will have to desolder all those pins so I can get at C200.

FWIW, the Peavey tech told me that they changed the 8 amp fuse that is in this circuit (F2) to a 10 amp.  The guy said they could watch the fuse wire heat up and bend during the current inrush.  It would cool back down, but after so many cycles (can't remember how many he said), the fuse would fail.  This particular amp still has the 8 amp fuse, and it is fine (ohmed it out), but I will switch it out to a 10 amp before I am done.

Also, it was funny that the Peavey tech was confused by the circuit as well and had no good advice for me.

Thanks again.

[jjasilli]

I tested the continuity of the cable.    Maybe it's intermittent.

I haven't been at the amp since last Tuesday, but I do believe the cable is good.  I'm sure I would have jumpered it if I had found that it was bad.
Lately I've started making notes of troubleshooting work as I do it.

the Peavey tech told me that they changed the 8 amp fuse that is in this circuit (F2) to a 10 amp. 
This doesn't explain the discolored circuit board.  But this might be a false clue.


it was funny that the Peavey tech was confused by the circuit as well  Looks like they wasted their money patenting the circuit.

[HotBluePlates]

... Anyway, I'm looking for some insight from anyone who may have seen or worked on this type of heater circuit.  If you dare, take a look at the heater circuit in the preamp...

Overall, the amp has one heater system for the output tubes, and a second system for the preamp tubes. Output tubes are fed 6vac, but referenced to 25vdc (with a 25v electrolytic cap to chassis ground).

The preamp tubes have their individual heater sections arranged in series for 12.6v operation (instead of the normal parallel operation at 6.3v). Then, 2 preamp tubes are placed in series; nominally this would be 25.2v operation for the pair of tubes, but drawing 150mA total (instead of 600mA at 6.3v for standard heater wiring). There are 4 preamp tubes, and 2 pairs of tubes, each pair wired in this series manner and named "Fil A" and "Fil B".

For example, the input tube has the heater section of the 1st gain stage with pin 4 connected to circuit ground, and pin 5 for the 2nd gain stage's heater connected to "Fil A." This connection is shorthand for the heater section for the other gain stage receiving input from the 1st gain stage, at pin 4. This heater is then continuous with the heater section for the gain stage which follows, where pin 5 connects to 23v.

So instead of 25.2v, this preamp runs the heaters on 23v, which reduces emission and gain a little but also cuts noise a bit.

The source of 23v is shown down at the bottom left of the 1st page, and is a different, later node than the 25v reference for the output tube heaters. That reference is fed through R81 (4.7kΩ).

... Pins2 are fed by J13, Pins7 by J14. 
J14 re-appears on Page 1, far right, second row of components connected to a relay. ...

Unfortunately, Peavey sucks. "J14" on the power tube page is almost certainly not the same "J14" on the preamp tube page.

Instead, to the left of the phase inverter shown in the bottom-middle of the preamp page, there are connections "To Power Amp Board" with the correct various signal and power connections, with the same physical spacing of 7-of-8 places of the ribbon cable & connectors. Unfortunately, they use entirely unrelated "J-" designators, so there wasn't an obvious way to match them up.

What is interesting is the order of the connections differs between the preamp and power amp boards.   

... I read no AC voltage there when the ribbon cable is connected to the power tube board.  I do read +24VDC, though.  ...  When I disconnect the ribbon cable that sends the filament voltage to the power tubes, I do read 6VAC at the end that is connected to the transformer.

The reading of a.c. would seem to confirm the transformer and fuse are okay. The cap to ground could be bad; the presence of burn marks makes the ribbon cable suspect. I would at least try a different ribbon cable to rule that out (and 8 pin ribbon cable shouldn't be hard to acquire; unfortunately, if you're going to work on these kinds of amps, you'll need spare ribbon cables on hand).

What I don't get is why there isn't a resistor to ground for the d.c. reference voltage. Then again, the 4.7kΩ resistor may be intended to limit fault current to 23v/4.7kΩ = ~5mA, even in the event of the cap becoming a direct short to ground. That would make the burning improbable outside a tube failure. Maybe check the value of R81 between F3 and C55 on the power end of the preamp board. After that, you may be looking for secondary effects of board damage due to a past failure.

[BrianS]

Thanks for the tutorial, HBP! Your efforts do not go unnoticed by me!

Thanks to both of you guys.

I did check R81, but I will double check everything now that I've got some more info.

I will be back at the bench all day tomorrow.  I'll post my findings later in the week.

[jjasilli]

Unfortunately, Peavey sucks. "J14" on the power tube page is almost certainly not the same "J14" on the preamp tube page.
Good catch!


I didn't go into the preamp stuff, because it doesn't seem germane to the fault issue. But great analysis! Now that it's out there, some comments.  The DC filament circuit in the preamp seems somewhat self-defeating, because one leg is connected directly to ground.  I think it should be entirely lifted from ground on a proper 2-wire (or 2 trace circuit).  As-is, it can pick up hum and noise lurking in the ground plane, from which it should be isolated.  Why bother to build a DC filament circuit and then contaminate it?!? (This is just a comment.  Since it works, don't fix it!  )

Re the power amp, it seems that the DC "ground" reference is superfluous, over-complicated, and fault-prone.  The PP circuit is more than adequate to cancel hum & noise in a power amp.  Another possible solution, to put on the back-burner for now, may be to entirely defeat the stock DC & AC supplies to the power tube filaments; and instead run pure 6VAC twisted pairs from the PT to the power tube board or power tube socket lugs.

[HotBluePlates]

The DC filament circuit in the preamp seems somewhat self-defeating, because one leg is connected directly to ground.  I think it should be entirely lifted from ground on a proper 2-wire (or 2 trace circuit).  As-is, it can pick up hum and noise lurking in the ground plane, from which it should be isolated.

I don't see this as a problem.

The heater ground should be at/near other preamp grounds, which will have very small alternating ground currents relative to the size of the heater current. So the heater current swamps any ground current issue. This would be entirely different if the preamp tube heater ground was mixed in among output tube power supply grounds. Additionally, the preamp d.c. heaters reduce the chance of injecting hum into preamp signal circuitry.

Re the power amp, it seems that the DC "ground" reference is superfluous, over-complicated, and fault-prone.  The PP circuit is more than adequate to cancel hum & noise in a power amp.

I agree!

Maybe the hot ticket is remove R81 and C200 to ground, and rely on only the a.c. heaters for the output tubes.

[BrianS]

Well, I replaced C200 (same uf, higher voltage rating), and measured AC voltage on the filament line.  Then I put the tubes in, and measured again...no AC.  After measuring a bunch again I realized what was happening:

The ribbon cable and its connector were fine, but the pin that is soldered to the board had come undone. It was making intermittent connections, mainly when I was pressing on the top of the connector with my meter probes.  I had previously measured the voltage on this pin with the connector removed and had measured voltage there.  The DC voltage was always there, because the one line was good, but the AC would only flow intermittently.

When I stepped back and just thought about the circuit for a minute, I realized that I was not seeing the forest through the trees, so to speak.  So, I took the time to remove the main board and flip it over...



Took the pin out, cleaned it up and then resoldered it in...amp works fine.

Working for shop is great in that you see a bunch of different gear and it keeps the brain working.  The bad part is the pressure to get things done quickly, and, in my case, the less than ideal bench/shop conditions.  I took this amp home to my own shop and solved the problem there.

Anyway, it's done.  Thanks for the insights.

[jjasilli]

 

[eleventeen]

Nice! The nice thing about those types of finds is that in all likelihood you really, really learned something....by that I mean, you will never NOT look for something like that next time and next-next-next time.

[HotBluePlates]

Congrats and great thinking!!