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

[The Dude]

Another "outside shot": I have, on a couple of odd occasions seen the flyback diodes leaky under stress. My experience is that it's rare but does happen. They normally just short resulting in a blown fuse, but not always. You could try temporarily lifting one end of each of the flyback diodes (D4&5) and see if you still have the symptoms. If they are breaking down, it may cause excessive tube current and it's easy enough to try.

[Enzo]

Easy to try, but for my money, that would shunt current around the tubes rather than through them.

I am still at the wanting to see if bias voltage is sagging right at the tube socket rather than out in the supply.

[christarak]

Thanks Dude. I'll try it as soon as I get back home.

[christarak]

Easy to try, but for my money, that would shunt current around the tubes rather than through them.

I am still at the wanting to see if bias voltage is sagging right at the tube socket rather than out in the supply.

OK, back from the doc (with a full tear of the Supraspinatus) and this time I attached the Fluke right at pin 5. Frustratingly, the same thing: Bias goes from -50.53v to -46.61 over 10 mins, and I4 goes from 36.44mA to 48.23mA (similar result with I5).

[pdf64]

OK, that's good, there's some cause and effect going on, just need to identify what's at the root of it.
Take at least the power tubes out and repeat the monitoring of terminal 5 voltages, along with monitoring the raw bias supply voltage at its trimmer.

[christarak]

Another "outside shot": I have, on a couple of odd occasions seen the flyback diodes leaky under stress. My experience is that it's rare but does happen. They normally just short resulting in a blown fuse, but not always. You could try temporarily lifting one end of each of the flyback diodes (D4&5) and see if you still have the symptoms. If they are breaking down, it may cause excessive tube current and it's easy enough to try.

Hi Dude,
It seemed like we were having some success after lifting D4 and D5. However, within 9 minutes (measuring at 3 minute intervals) the Vbias dropped from -50v to -48.9 and I4 went from 34mA to 40mA and I5 went from 31mA to 41mA.

So then I did what I automatically do with every Hot Rod. I changed R78 and R79 to the same 470 value but 10w and lifted them off the board, reasoning that if the heat doesn't change the resistance so much it will keep a more stable load on the 39VAC Bias supply. Well...it helped...but the ramp is still there. Over 9 minutes Vbias went from -50.9 to -49.2. I4 went from 28.3mA to 33.2, and I5 went from 25.6mA to 34.4mA. When I switched off the power, the current was still increasing.

What do you think? Disconnect the PI coupling caps C26 and C27?

Have I said that I really appreciate the time and thought that you and Enzo have put into my problem on the other side of the planet?

[christarak]

Thank you pdf64. I appreciate you looking over my problem from the UK.
I did those measurements over the same 9 minute interval. The voltage at the wiper of the Bias pot dropped from -54.61 to -53.72 (2% change). The voltage at pin 5 dropped from -52.25 to -50.57 (3%). Does that give us a clue?
If I delay in responding, its only a time zone thing.
Thanks again
Chris

[pdf64]

Can you provide a link with the correct schematic? Various revisions available and it's best to avoid the risk of referring to the wrong part number.

I think the power tubes should be kept out of the amp until the bias voltage issue is resolved.

To me it looks as if a positive current may be pulling the terminal 5 voltage up towards 0V; could be leaky coupling caps from the LTP plates, or perhaps something that should be insulating, isn't; eg pcb, tube socket body.

[HDSarge]

Do not overlook the bias trimmer. Check the resistance of each leg to the wiper. Run the amp until things are getting hot. Turn off and measure again. Or, just try replacing the trimmer.

[Tom Phillips]

...What do you think? Disconnect the PI coupling caps C26 and C27? ...

Yes. That would be a good test.

[christarak]

Can you provide a link with the correct schematic? Various revisions available and it's best to avoid the risk of referring to the wrong part number.

I think the power tubes should be kept out of the amp until the bias voltage issue is resolved.

To me it looks as if a positive current may be pulling the terminal 5 voltage up towards 0V; could be leaky coupling caps from the LTP plates, or perhaps something that should be insulating, isn't; eg pcb, tube socket body.

Thank you PDF64. This is the best schematic I could find: http://www.thetubestore.com/lib/thet...atic-Rev-B.pdf. This schematic shows the inexplicably high -106VDC at TP41 in the bias circuit, which my amp has. (It was -53VDC in earlier schematic revisions - which sent me on a yet another goose chase for too long.)

I don't seem to have the bias creep when the power tubes are removed.
I will definitely lift the coupling caps to the LTP PI and pray that this removes the culprit.

[christarak]

Thank you, Tom. I appreciate your input in this frustrating problem. I haven't come across leaking caps playing this sort of havoc before.

[christarak]

Thanks HD! I hadn't thought of checking the trimmer. I'll test it out.

[Enzo]

Look on your main board near the input jacks. It should say Fender Musical Instruments or FMIC, or Fender something, and a year. What year is on your board. That will tell us which revision you actualy have.

[christarak]

Look on your main board near the input jacks. It should say Fender Musical Instruments or FMIC, or Fender something, and a year. What year is on your board. That will tell us which revision you actualy have.

Hi Enzo, the p/n is 0059781000 Revision B 2009. Thanks mate