Brownface Deluxe - Power Tube problem

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by IceTre, Mar 5, 2015.

  1. IceTre

    IceTre Supporting Member

    Messages:
    762
    Joined:
    Dec 25, 2011
    Location:
    Denver, Colorado
    I've had a '61 Fender Brownface Deluxe for about 2 years. (And yes, the tone is sweet.) I was trying some NOS power tubes, and after playing for a couple of minutes, the amp got very noisy. I looked in the back, and one of the power tubes was glowing white hot. I turned it off right away. After they cooled down, I replaced them with a different pair of tubes, and the same thing happened to the tube in the same slot (the one closest to the preamp tubes). I then tried a third pair of power tubes-- cheap current production tubes this time-- and it wasn't as bad, but it did get a little noisy, although less so, and the tube in the same slot glowed a little brighter. So I've got a problem but don't know the cause or where to start looking.

    I did take the chassis out before this happened, but only to look at something to compare to an amp build project. I didn't replace any parts or modify anything. It's possible I bumped something and disconnected something taking it in and out, but I don't think so.

    Also, I hadn't played the amp in awhile. Don't know if that's a clue.

    The filter caps appear to be fairly new, like they had been replaced by the previous owner. But that doesn't seem likely anyway--why would a shorted filter cap affect only one of the power tubes? So I think the problem must be elsewhere.

    Any ideas?
    Thanks,
     
    Last edited: Mar 5, 2015
  2. slider313

    slider313 Silver Supporting Member

    Messages:
    7,433
    Joined:
    Jul 28, 2005
    Location:
    NC
    Change both .1 blocking caps, which are connected to pin 5, of your power tubes. You may as well change both of them.


    [​IMG]
     
  3. schmidlin

    schmidlin Member

    Messages:
    5,267
    Joined:
    Dec 23, 2008
    Location:
    Sunny Ohio
    My first guess would be a leaky blocking cap between the PI and a power tube. That would throw off the bias on that tube. Pull the power tubes and check the voltage on pin 5 of each tube. They should be consistent. (and I assume this is fixed bias)

    But if you don't know how to do something like this safely, see a tech.
     
  4. zenas

    zenas Member

    Messages:
    5,970
    Joined:
    Jul 1, 2011
    Yes it's fixed bias. Which means you should check the bias when swapping tubes these didn't get a bias pot so you change a resister (if needed) or install a pot.
    61 might have the yellow Astron coupling caps which go leaky way more often than the later blue molded ones. Common to see a mix of blue and yellow on the early tolex amps.

    Only thing I;d do before swapping the coplin caps is check the solder joints after the bias splits at the 220K resisters. Check those resisters too.
    [​IMG]
     
  5. IceTre

    IceTre Supporting Member

    Messages:
    762
    Joined:
    Dec 25, 2011
    Location:
    Denver, Colorado
    Thanks guys. I'll measure the bias. My amp has the blue coupling caps, see photo below (right side). If it turns out one is leaky, what would you recommend for a replacement? I would think these play an important role in the amp's tone.

    [​IMG]
     
  6. RiftAmps

    RiftAmps Member

    Messages:
    204
    Joined:
    Mar 18, 2014
    Location:
    Brackley, UK
    Orange Drops always work well in that application, I've also had excellent results with Mallory's.
     
  7. neils

    neils Member

    Messages:
    1,766
    Joined:
    May 1, 2011
    Did it sound fine before all these tube swaps? I ask because I was trying some 6V6 tubes in a Vic 20112.

    At some point in this swapping one of the contacts in a power tube socket just got too spread away to make good contact on the tube pin. The noise and squealing was insane. Every tube I tried did it. Both tubes were super hot and when someone saw a photo said they were redplating. Very bright blue/white glow.

    At some point I got in there and drained caps etc and used a small pointy probe and bent all the pins in a bit. Fixed it.

    Neil
     
  8. pdf64

    pdf64 Member

    Messages:
    5,143
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    Had the amp been in cold / damp storage?

    I'm thinking that there isn't much gap between a board eyelet for the 6V6 control grid and a B+ eyelet, and that it wouldn't take much dc leakage to pull the grid up to an unsuitable Vdc.

    As mentioned, always check that the bias is suitable when changing power tubes in a fixed bias amp.

    This will help to avoid subjecting such old transformers to fault current, which may be very beneficial.

    The 6V6s that got white hot may be damaged / unsuitable for further service.
     
  9. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Aside from a leaky blocking cap, pin 5 of the power tube might not be making contact with that socket pin. Cleaning and retensioning would be my first step. Absolutely check/adjust the bias as well.
     
  10. Ronsonic

    Ronsonic Member

    Messages:
    3,313
    Joined:
    May 14, 2006
    Location:
    Sunny Tampa, Florida
    Coupling caps and tension of pin 5 are the usual causes of this sort of problem.

    Coupling cap is easy to check - with the tubes out measure the negative bias voltage at both pin 5s it should be the same on both. If you see something like a couple of tenths off it could be a measurement error, but even a volt difference is a symptom. Always replace both. They're the same age, from the same source and have been subjected to the same use.
     
  11. IceTre

    IceTre Supporting Member

    Messages:
    762
    Joined:
    Dec 25, 2011
    Location:
    Denver, Colorado
    Hey guys, I just got back on this tonight. I've been so busy the past few months that I had to just set this amp aside and use others. But here's what I found tonight:

    With the power tubes out, I measured -30.1dc to -30.2dc on Pin 5 of each power tube. I say -30.1 to -30.2 because there was some oscillation (from the tremolo circuit?). However, both tube sockets measured basically the same. So can I assume the coupling caps are ok?

    I tightened all the pins in each of the two power tube sockets.

    I put in a pair of Tung Sol 6V6's. I measured -29.8dc on pin 5 of both. After a few minutes, the tube closest to the preamp tubes had a glowing on the vertical plate inside-- same as I noticed before. However no noise. I powered off and swapped the tubes. Powered back on, and the glowing problem moved with the tube. So perhaps that tube got damaged when the problem originally happened.

    I tried a pair of Groove Tubes (I'm afraid to try my expensive KCA NOS tubes). The tube in the suspect slot didn't have a glow on the vertical plate, but the top of the tube glowed more than the other. I swapped slots, and the one tube continued to glow at the top, but now the other one in the suspect slot glowed on the plate. I plugged a guitar into each channel and it sounded ok, no noise. But I wonder how long it would have lasted; the glowing of the tube in the suspect slot seemed to be getting brighter. So I turned it off.

    Any suggestions? Perhaps the amp is ok now after tightening the pins, and the tubes just got damaged previously, when the pins were loose? Maybe I just need to buy another pair of inexpensive tubes and verify?
     
    Last edited: May 11, 2015
  12. pdf64

    pdf64 Member

    Messages:
    5,143
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    Don't just stick tubes into a fixed bias amp without checking that the operating conditions are suitable for them.
    That means checking the plate / cathode current, voltage between plate and cathode, calculating the dissipation and making adjustment as necessary.
    If the above process is not followed then occasionally tubes will redplate and become damaged, and tone / performance / operational life will generally be compromised and inconsistent over tube changes.
     
  13. zenas

    zenas Member

    Messages:
    5,970
    Joined:
    Jul 1, 2011
    Yeah "fixed" bias only makes sense when you think the bias voltage is fixed for a set of tubes. A different set of tubes may need a different "fixed" voltage.
    Glowing plates means it needs some ajustment.


    How old are the electrolytic caps?
     
  14. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Aside from retensioning the sockets, and even before, you should clean all the socket contacts too. 60 year old metal oxidizes and won't make a good connection with your tube pin. I'd try that next.
     
  15. Laurent Brondel

    Laurent Brondel Supporting Member

    Messages:
    2,093
    Joined:
    Dec 9, 2011
    Location:
    West Paris, ME
    Check the plates voltage, I have a '61 Deluxe as well and it runs very high (445V with a 35mA bias). I've had NOS Visseaux 6V6's on it for some time, and they seem fine.
    JJ's would take the high voltages too. Not sure about other modern 6V6's. A lower bias means higher voltage too, maybe your tubes can't take it.
    I installed a bias pot on mine, it makes swapping tubes easier.
     
  16. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Even 400V at 35mA bias is insane unless you're trying to destroy your 6V6s or severley shorten the tube life. Where did you get the idea to bias 12 watt tubes at 15 watts when the maximum they should be set for is around 9 watts?
     
  17. Laurent Brondel

    Laurent Brondel Supporting Member

    Messages:
    2,093
    Joined:
    Dec 9, 2011
    Location:
    West Paris, ME
    Is this addressed to me? Voltages are too high at lower bias and are too close or even exceed the filter caps ratings.
     
  18. Blue Strat

    Blue Strat Member

    Messages:
    30,029
    Joined:
    Aug 2, 2002
    Location:
    Sterling, VA (not far from Washington DC)
    Running the power tubes under meltdown condition isn't the cure to protecting the caps. Either changing the rectifier type, using a variac or replacing the caps with higher voltage ratings would be the way to go.

    The plate voltage, if under 450, is less likely to kill the power tubes than running at over 100% maximum power dissipation, at idle.
     
  19. swiveltung

    swiveltung Member

    Messages:
    14,567
    Joined:
    Feb 5, 2011
    Location:
    PNW
    Yes. Please check your bias & plate voltage with the tubes you want to use. You are risking wiping out your Output Transformer which will devalue your amp.. Those amps can run as high as 460 V which is too much for many tubes but JJ's. Figure this out. 35ma is way too high for a Deluxe.
     
  20. IceTre

    IceTre Supporting Member

    Messages:
    762
    Joined:
    Dec 25, 2011
    Location:
    Denver, Colorado
    Here's the voltages I measured:

    Plate on both output tubes: 430Vdc.
    Screen on both: 418Vdc.
    Grid on both: -29 Vdc.
    Cathodes of course are at ground.

    Comments? Any other measurements I should take? Maybe buy a pair of JJ 6V6's given the high plate voltage and try them? Maybe change the rectifier from a GZ34 to something lower, like a 5R4, to get the plate voltage down?

    I'll spray some contact cleaner on the pins per Mike's suggestion.

    Thanks for all the help.
     

Share This Page