Hot Rod Deluxe Bias Ramping up ... all by itself.

[The Dude]

Re-reading the thread, it's a little odd that your screen resistors are getting so hot. Are you sure you put in 470 ohm and not 47, 4.7, or something else?

[christarak]

Re-reading the thread, it's a little odd that your screen resistors are getting so hot. Are you sure you put in 470 ohm and not 47, 4.7, or something else?

Sorry Dude. I meant to say R78 and R79 (the 470R 5W ones in the 16v zener circuit) are at 120C. Out of habit and desperation I replaced them with 470R 10w resistors last night, standing off the board by a 1/4". Changing them certainly helped, but the current ramp came back unacceptably ... but with less of a severe change.

[christarak]

Yes. That would be a good test.

I have now lifted the PI coupling caps (C26 and C27) and the results were so promising for the first 6 minutes I reached over for my capacitor drawer...and then Bias current started to rise again (34mA to 42mA on V5, with similar on V6) to coincide with the Bias voltage dropping (-52.8 to -50.0 VDC).

I checked again with no 6L6 tubes and there was no drift in Bias voltage.

I'm out of ideas, fellas.

[christarak]

I have now lifted the PI coupling caps (C26 and C27) and the results were so promising for the first 6 minutes I reached over for my capacitor drawer...and then Bias current started to rise again (34mA to 42mA on V5, with similar on V6) to coincide with the Bias voltage dropping (-52.8 to -50.0 VDC).

I checked again with no 6L6 tubes and there was no drift in Bias voltage.

I'm out of ideas, fellas.

[Enzo]

Your old tubes did this, and your new tubes. Have you any other 6L6s, like maybe in some other amp? Try them. Coincidental as it may be. perhaps your new tubes happen to have the same malady. Can't hurt to try.


Any chance on the builder plate on the cab rear it says HR DeLuxe III?

Use some alcohol and scrub the circuit board clean around the power tube sockets and the ribbon cable ends that serve them.


Off the wall thought: what if we have a failing inrush thermistor? As the bias voltage drops, what is the AC voltage doing from the transformer that starts the bias supply? Or to be more direct, measure your mains voltage right at the power transformer primary wires, then see if it slowly drops as the symptom starts.

I assume we are not using a bulb limiter.

[pdf64]

Yes, it could be tubes with excessive g1 current; tubes that have previously red plated may become prone to that.

It's worth checking the values of the 220k and 1k5 resistors in the grid circuit, maybe even try warming them up (eg with a hairdryer) whilst monitoring them.

Note that the 6L6 g1 circuit limiting resistance in fixed bias is 100k http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf
Your problem HRD is >3x that, so issues due to excessive g1 current may be exacerbated!

[mozz]

Were the capacitors in the bias circuit changed yet?

[christarak]

Your old tubes did this, and your new tubes. Have you any other 6L6s, like maybe in some other amp? Try them. Coincidental as it may be. perhaps your new tubes happen to have the same malady. Can't hurt to try.


Any chance on the builder plate on the cab rear it says HR DeLuxe III?

Use some alcohol and scrub the circuit board clean around the power tube sockets and the ribbon cable ends that serve them.


Off the wall thought: what if we have a failing inrush thermistor? As the bias voltage drops, what is the AC voltage doing from the transformer that starts the bias supply? Or to be more direct, measure your mains voltage right at the power transformer primary wires, then see if it slowly drops as the symptom starts.

I assume we are not using a bulb limiter.

Hi Enzo,

The original valves did the same thing. I have tried 2 new and matched sets of TungSol 6L6's.
Its definitely a HR deluxe III.
Funny you would mention cleaning down the circuit board. Both PCB's had an oily film on them, as if they had been sprayed with WD40. The cleaning didn't help the problem. I also read in one of your posts from a few years ago to flush out the bias trim pot. No luck with that either.
I was monitoring the 39VAC that feeds the bias circuit. It stayed constant throughout the bias current ramping. (with 6L6's installed. No ramping with 6L6's pulled out)
No bulb limiter. (I only use it for my initial power up - then go straight to the wall if current draw is OK).

When is it OK to say to the customer "I give up", Enzo? I have put in 12 hours to earn my $60 quote fee.

[Enzo]

It is OK to cave when that is what are left with. If you are making no progress beyond guessing, and the customer is not interested in leaving it with you for a learning experience, then admit you are stumped. The most experienced of us do hit brick walls. When I bail on someone, I have a good idea whether the amp is salvagable, I mean it isn't charcoal. I try to be reassuring by telling them I just can't figure out what it needs, but the next tech might solve it in ten minutes. Like when I do the Jumble puzzle in the paper, and cannot for the life of me figure out one scrambled word. The wife looks over my shoulder and takes 10 seconds to tell me the word. Oh well, I'll get her on something later.

[christarak]

It is OK to cave when that is what are left with. If you are making no progress beyond guessing, and the customer is not interested in leaving it with you for a learning experience, then admit you are stumped. The most experienced of us do hit brick walls. When I bail on someone, I have a good idea whether the amp is salvagable, I mean it isn't charcoal. I try to be reassuring by telling them I just can't figure out what it needs, but the next tech might solve it in ten minutes. Like when I do the Jumble puzzle in the paper, and cannot for the life of me figure out one scrambled word. The wife looks over my shoulder and takes 10 seconds to tell me the word. Oh well, I'll get her on something later.

I feel a bit better about caving if someone as experienced as you gets stumped occasionally. I might put it back together and save my sanity. I might suggest he send to Enzo's wife...she has to start somewhere.

I'll report back with what the customer has to say.

Thank you all so very much. I have learned a lot from you.

Chris

[The Dude]

There's nothing more frustrating. I think anyone who's done this for any amount of time has had moments like this. Often times it helps me to step away from a repair for a while. There have been times I've come back to something after giving my brain a break and a light bulb goes off in the head. If you haven't, take a "vacation day" away from it and try again with a fresh outlook. It often helps. We can sometimes miss the most obvious things when we get tunnel vision.

[Enzo]

There is no one on this earth who doesn't find a question he is unable to answer - yes, within his expertise.

Elsewhere here, I told the tale of a little Laney amp I had been poking at off and on for ten years before I finally found the simple mistake they made when assembling the amp. How it took me that long to see it, I have no idea.

Every home run king in baseball strikes out. Every star NFL quarterback throws interceptions. Ask the engineers on this board if they ever had a circuit not work. You can't expect the sun to shine every day. Learn to have the confidence you have a valid systematic approach to repairs, and that in most cases that will lead you to a successful repair. And the times you strike out? Shake them off and get up for your next at bat.

[Chuck H]

I went through the thread pretty quickly, but...

Are you testing this ramp up phenomenon while running an input signal?

I didn't see where you have tested or replaced the PI coupling caps. This is the first area Dude suggested. Leaky voltage there would skew the bias.

I think you may have specified a particular V# (tube) that is responsible at some point. Does this happen with both tubes then, or only one?

Have you double checked the bias circuit to be sure no one has altered it? It does happen. With a zener based (So, regulated? Didn't look at the schem) supply it may not take much of a miswire to mess things up badly.

Here's my key question... Does the bias ever stabilize, or does the tube (do the tubes) continue to rise in current well into the danger zone unless you shut down?

P.S. Sorry to hear about your pain issue. I'm a blue collar guy with a few repetitive motion and tendon issues. The worst of which is probably a "worn" rotator cuff. I asked the doc "You mean 'torn'? He said "No. You have almost no cartilage left there. You've worn it out." I'm talking about a grown man crying in pain. Just awful. I've learned to "manage" it. Sort of. I hope there's a good solution and a more positive prognosis for your issue.

[The Dude]

......Here's my key question... Does the bias ever stabilize, or does the tube (do the tubes) continue to rise in current well into the danger zone unless you shut down?

That is a key question. It's normal for bias current to rise as tubes warmup and you should of course continue to monitor and adjust until things stabilize. Is it possible we are trying to fix something that's normal? Or, maybe it never stabilizes. It might help to know.

[christarak]

I went through the thread pretty quickly, but...

Are you testing this ramp up phenomenon while running an input signal?

I didn't see where you have tested or replaced the PI coupling caps. This is the first area Dude suggested. Leaky voltage there would skew the bias.

I think you may have specified a particular V# (tube) that is responsible at some point. Does this happen with both tubes then, or only one?

Have you double checked the bias circuit to be sure no one has altered it? It does happen. With a zener based (So, regulated? Didn't look at the schem) supply it may not take much of a miswire to mess things up badly.

Here's my key question... Does the bias ever stabilize, or does the tube (do the tubes) continue to rise in current well into the danger zone unless you shut down?

P.S. Sorry to hear about your pain issue. I'm a blue collar guy with a few repetitive motion and tendon issues. The worst of which is probably a "worn" rotator cuff. I asked the doc "You mean 'torn'? He said "No. You have almost no cartilage left there. You've worn it out." I'm talking about a grown man crying in pain. Just awful. I've learned to "manage" it. Sort of. I hope there's a good solution and a more positive prognosis for your issue.

Hi Chuck,
Thanks for your suggestions. Here I go with answers:

Input Signal: In the beginning did I notice the ramp up with a guitar signal, thats when I realised the problem was more than a new set of tubes and a bias. From that first time onwards, all of my tests have been with no input signal.
PI Caps: I lifted the PI caps as The Dude suggested, but the ramping still occurred.
Valves: It happens with both tubes individually and as a pair.
Alterations: The circuit looks stock to me. The +/- 16v Zener side of of the 39VAC tap works fine (I changed the 470R 5w to 10 w for good measure). There was an inexplicable thin film of "oil" over both boards which I washed off.
Stabilise: I stopped all my tests when either bias current got to 44mA. Once the runaway begins (after 3-5minutes) it accelerates.