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Old 5th March 2009, 05:54 PM   #1
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Default Runaway bias on Hot Rod Deville

Hi all,

been repairing a mid-nineties Fender Hot Rod Deville - schematic available here. When I bought it, it was suffering from extreme volume loss & distortion, which I eventually tracked down to the power section, specifically a plate load resistor that had come open. Replaced it & its brother (R57 & R58), also replaced the screen resistors (R61 & R62) with 1K ohm because I am running it now with JJ 6V6S instead of 6L6. Did all this with the help of the "Unofficial Fender Hot Rod Deluxe Owner's Guide", since I am new to tube amps and their issues.

So last night, I put the new tubes in: 2x JJ 6V6S, 2x JJ ECC83s (12AX7), 1x JJ ECC832 (12DW7) in V2. Tubes from tubedepot.com. Started to bias - recommended for 6V6 is ~40mV. I'm at 22mV near the lowest part of the pot's range, all good, turning it up a bit, 25mV, turn it up a bit more, not even halfway on the bias pot. The voltage starts running away. It climbed to 60mV in about 5 seconds, with no sign of stopping, before I turned the bias pot back down and it settled back at 22mV. So I'm a little worried, but I let it warm up for 10 minutes or so, then try again. This time I get it to hold steady at 30mV or so, so I turn it around and start playing through the amp.

10 minutes later, I notice that the amp is getting really hot, and I look at the tubes - orange glow on the plates, and I quickly measure bias - 120mV! Nuts! Turned it off. The problem remains - runaway bias above about 25mV.

My plan is to check the coupling caps, and clean the tube sockets - haven't done that yet - but I was wondering if you all have any advice beyond that. Could it be the bias pot itself? How would I go about checking that? Advice appreciated! Thanks!

Jesse
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Old 5th March 2009, 06:40 PM   #2
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p.s. plate voltages are 480 and 481V on V5 & V6.
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Old 5th March 2009, 07:24 PM   #3
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Default Re: Runaway bias on Hot Rod Deville

Quote:
Originally posted by EatMoreArt

My plan is to check the coupling caps, and clean the tube sockets - haven't done that yet - but I was wondering if you all have any advice beyond that. Could it be the bias pot itself? How would I go about checking that? Advice appreciated! Thanks!
Hi Jesse,
Cleaning and perhaps tightening the sockets is a good idea as well as just replacing ths coupling capacitors. But I suspect the new 6V6 tubes are the problem. This is what happens when all we get is shoddy imported Chinese garbage tubes. They are very adept at copying the look, but fall woefully and terribly short on substance and quality. This goes for EVERYTHING they make! Including aftermarket car parts, televisions, home appliances, clothing, and you name it. EVERYTHING!!! Try different new tubes. When you get a pair that work right, face Mecca, bow deeply and consider yourself lucky.

End of rant.
Victor
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Old 5th March 2009, 08:03 PM   #4
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Hi HollowState,

thanks for the reply. But, eh? I have heard nothing but good things about JJ tubes, which I believe are manufactured in the Slovak Republic.

That said, I did order a matched quad, so perhaps I'll try swapping for the other two tubes. Unfortunately I don't have anything else to try - the guy I bought it from had old and mismatched tubes in there that I don't trust.

J
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Old 5th March 2009, 09:22 PM   #5
GordonW is offline GordonW  United States
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Um... on 480V on the plates, NO 6V6 is going to last long at over 25ma.

The plates on the 6V6 are rated at 12 watts... which equates to 25ma at 480V! Even if you subtract the cathode voltage (probably around 15-20V or so), it's still going to be at max rating at 26ma or so!

Running it at over 25ma is simply overloading the tube... and it is quite reasonable to expect the plate to start outgassing, and the tube runs away...

You will need to get the plate voltage down to around 320V or so if you want to run a 6V6 at 40ma. I usually suggest around 350v plate and 35ma idle... seems to be a place where 6V6s run well. Just under 12 watts, once cathode voltage is subtracted from the plate voltage...

Remember- the plate dissipation is the PRODUCT of the plate voltage times the cathode current (roughly)... and that needs to be at or below the rating for the tube... no matter WHAT tube.

Regards,
Gordon.
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Old 5th March 2009, 10:02 PM   #6
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Hi Gordon,

thanks! Sorry, should have been more clear - I am measuring at the bias test point, so I believe what I'm reading is the *combined* plate current of the two power tubes. So my measurement of 25mV would equal ~12.5mA per tube. Right?

J
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Old 6th March 2009, 12:43 AM   #7
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Update: it's not the coupling caps - I unplugged the output tubes and measured the voltage on the grid socket contact, on both sockets. I read about -50VDC. According to Geofex, if the voltage is more positive than -15V, then the caps are suspect.

The next thing I did was swap the pair of 6V6s for another pair of the same brand. This time, I was able to turn the bias pot all the way up, and read 35mV from the bias test point, where it stayed steady. BUT - I then swapped back to the first pair, one by one, and the bias still remained steady around 35mV.

Conclusion: the tube sockets are to blame - either a bad connection between tube pins and socket, or a cold solder joint. I ordered some Deoxit, so I'll try spraying that in there. Thanks for your posts guys! Hopefully I will have no more problems!

Jesse
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