Results 1 to 24 of 24

Thread: BF Pro Reverb Shutting Itself Off

  1. #1
    Junior Member
    Join Date
    Nov 2007
    Posts
    20

    BF Pro Reverb Shutting Itself Off

    I have a BF Fender Pro Reverb that has been behaving oddly. It started out by shutting itself off, then it either 1) Wouldn't come back on at all, 2) Would come back on but shut off again when the standby was flipped.

    I can think of a LOT of things that can cause an amp to shut off, but very few that will do it without blowing the fuse, and the fuse does not blow (its the correct value). There is no loose wiring in the AC to PT section, no shorts, etc.

    Installation of a different rectifier tube alleviates this, but the tube in it was a brand new JJ and lasted about a week, my guess is the bad tube is a symptom not the cause. Other symptoms: There is a pop when the standby is turned to play mode, and the tubes flare blue briefly when coming off standby.

    Whats been done: I just refurbished it with new filter caps (power, pre-amp, bias filter), new cathode bypass caps, new plate resistors, 3 prong chord, all new tubes. Biased at a conservative 50% plate dissipation at idle. All JJ tubes. Vibrato ticking fixed per Fender's service bulletin.

    I'd suspect the first stage filtering, but they are new Sprague 80uf caps, which is well within the first stage capacitance that a GZ34 can handle (caps in series, 40uf). The pre-amp caps are 25uf each. I tested all the filter resistors, all are fine, measured the caps on a meter, all are very close to spec. Choke reads 97ohms.

    The screen grid resistors are suspect and I'm going to replace them, but I cant see where they'd be the culprit here.

    Any ideas?

    Thanks guys

  2. #2
    Old Timer
    Join Date
    Jun 2007
    Posts
    4,925
    I hate when that happens!

    When you say "shutting itself off", what do you mean? Is the whole amp going dark (no power) or is just the audio cutting out?

    What are the supply voltages (AC and DC) doing when this happens?

  3. #3
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    It goes dark. No lamp, no heaters, no nothing, its as if the power switch was turned off, but it isn't. Its also not a power down sort of phenomena, where you leave the standby in the play position and turn the power off and it dies off slowly; its pretty much instantaneous when it happens. Acts a lot like a blown fuse, but the fuse does not blow, which is what has me scratching my head. I've seen bad rectifier tubes blow fuses, bad PTs, shorts any number of places, but I've never seen an amp just go dark without blowing the fuse, much less come back to life afterwards as if nothing happened.

    It seems to be the rectifier going bad thats causing that, as another rectifier tube gets it to come back to life, although whats causing the tube to go bad in the first place is a mystery. I suspect its the same thing that causes the "pop" and the momentary blue flare when the standby goes off, but what is causing that, I don't know. The caps and resistors all check ok and the caps are new; I'm not sure how to check for a bad choke but at least the choke is not 'open', its reading 97ohms.

    Of course over capacitance in the first stage can cause a rectifier tube to give up, but its a GZ34 seeing just over 40uf at the first stage, thats well within what it can tolerate. Thats why I was suspecting a bad choke not isolating the first stage from the screen grid filter, which would push that up to around 66uf, but thats only a guess.

    I'm going to replace the screen grid resistors in any case as they are old stressed carbon comps; but if they were bad, I could see the tube not conducting and no sound or bad sound, but not really blowing a rectifier tube.

  4. #4
    Old Timer
    Join Date
    Jun 2007
    Posts
    4,925
    Well, if the whole amp goes dark, it's not the rectifier tube, or the screen grid resistors causing the problem.

    The problem must lie in the AC primary wiring for the symptom that you describe to happen. Switching on or off the standby is probably just causing whatever is loose to make and break contact. Try this if your meter has an audible continuity test setting. Attach the meter leads to the ac plug's hot and neutral prongs, and turn on the power switch. Your meter should start beeping, as the total resistance of the primary wiring should be well under 50 ohms. In fact it will probably be under 10 ohms. Now while it's beeping and driving you crazy, start flexing the AC plug where it attaches to the wire. Also flex the two prongs of the AC plug itself. If the beep does not drop out, move on to where the AC cord passes thru the chassis strain relief. Keep going along the path of the primary wiring, pushing and pulling on every wire and connection you see, and hopefully something will cause the continuity to break and there will be your problem. Try cleaning the AC switch, as sometimes they can become carbonized or oxidized. Check the fuse holder and cap, sometimes they don't quite fit together correctly or the internal spring gets soft.

    Best case, is a wire has come loose, worst case is a problem inside the power transformer.

    Hope this helps.

  5. #5
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
    Join Date
    May 2006
    Location
    Denver, Colorado
    Posts
    3,045
    I had an old Fender in recently that did this and it turned out to be a bad fuse holder.
    When the holder got a little warm, from being a bad connection I presume, it would just open up and let go of the actual fuse, creating an on-off switch.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  6. #6
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,479
    Yep, you are losing the mains voltage. SOme connection or part in the primary circuit is intermittent - bad cord, bad fuse holder, bad fuse, bad switch, bad PT primary, bad solder anywhere along there.

  7. #7
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    Primaries was my first thought as well, but I couldn't find anything there at first. I did the test for continuity and nothing seemed to throw it off. I checked the fuse holder, and the spring had disappeared, so I replaced it, but I'm not really sure this was the culprit.

    If the PT primary is bad, how can I test for it? Is it possible for it to go bad intermittently? I've had PTs go bad before and blow fuses like no tomorrow, that I know how to test for; but I've not seen one be intermittent and not blow fuses, so I'm not sure how to check for something like that.

    Thanks for the replies guys!

  8. #8
    Old Timer
    Join Date
    Jun 2007
    Posts
    4,925
    The wires going inside the transformer end cap are often crimped connected to the actual coil wires of the transformer. These can come loose or the lead wire can break internally, where it enters the end cap.

    You can try pushing and pulling on the leads or tapping the transformer end cap to see if it will cause the connection to break. If the transformer isn't filled with the black tar-like stuff, you can remove the end cap itself and check the connections.

  9. #9
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    Assuming that the standby POP is a separate issue, any ideas? Here's what I've done there: Replaced the screen grids with 5 watt 470ohm wirewounds; replaced the grid stoppers with NOS 1.5k carbon comps, tried putting a 600v
    .047uf cap on either side of the standby (gator clipped in to see if it would work before soldering). Tried a different set of output tubes.

    No joy, it still pops loudly when the standby goes off and has a little aftershock a few seconds later as well.

    Filter caps are new. Anything else I should try?

  10. #10
    Senior Member Twist's Avatar
    Join Date
    Nov 2007
    Location
    Springdale, Arkansas
    Posts
    316
    If it does it often you could pull the chassis out and connect your AC voltmeter to the two primary wires of the P.T. While watching the meter turn the amp on and wiggle the AC cord and tap the fuse holder and power switch. If the amp dies and you still see 120 volts then it's probably your P.T.

  11. #11
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    Did that, havent been able to get it to die since I replaced the rectifier, although I haven't pushed the amp either.

    Right now I'm trying to nail down that pop on standby issue; once thats fixed I'll turn it up and put it through its paces and see if it stays on or not. Just wiggling things etc., at low volume, it seems just fine.

  12. #12
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    Anyone have ideas on the standby pop? THat one has me stumped, did everything I could think of and nothing seems to help it.

  13. #13
    Noodle of Reality Steve Conner's Avatar
    Join Date
    May 2006
    Location
    Glasgow, Scotland
    Posts
    7,251
    Well, did it pop before you replaced all those components? Some amp circuits probably always popped. It's actually quite a difficult design gotcha to make the standby switch work silently.

    I've seen many amps where the tubes flash blue when B+ is applied. I don't think it means there's a problem.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  14. #14
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    It did that before.

    All Fenders have some little pop, but I have quite a few vintage Fenders and this is well outside the 3 std. deviation range for volume; its LOUD. I have another Pro that is less than a year older among other things, same circuit (AA165), no pop; so it shouldn't be a design issue.

  15. #15
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,479
    Switch it on and off with a clip lead, see if that makes a difference. It could be something as simple as pitted contacts on the switch itself.

  16. #16
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    I replaced the switch with a brand new 15 amp Carling. If anything, the Pop is louder than before.

    Its definitely not coming from the power amp. It mostly goes away if the phase inverter is removed. If all the preamp tubes are removed, its dead silent. If all but V4 and the phase inverter are removed, its more of a loud thud than a pop, but still unacceptably loud.

  17. #17
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    Ok so just for giggles, I re-replaced the new filter caps and all filter cap resistors I installed in case something was bad. No change.

    This is driving me nuts. Everything I've been able to research as a possible cause has been fixed/replaced, and the problem remains.

  18. #18
    Noodle of Reality Steve Conner's Avatar
    Join Date
    May 2006
    Location
    Glasgow, Scotland
    Posts
    7,251
    What problem? The amp works, doesn't it? When you consider what the standby switch does, it's a miracle that it doesn't make a loud thud or bang in every amp.

    Have you checked the dropper resistors and bypass capacitors to the preamp stages? If these are in good order and wired correctly, the plate voltages on the preamp should come up slowly, over a period of a second or two after flipping the standby switch. If you had a bypass cap open or a dropper resistor shorted, the plate voltage to that stage would come up instantly, which might explain the pop. If you replaced the dropper resistors with the wrong value, or ones with too low a voltage rating that are arcing internally, there's another possibility.

    If you were feeling really OCD, you could borrow a digital storage scope and compare the turn-on transients in every stage to your other Pro, until you find the difference. ;-)
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  19. #19
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    The pop is loud enough to make me fear for the vintage speakers.

    All bypass caps are new and correct, plate resistors are new and correct, all filter caps and resistors thereof are new and correct.

    Not sure what you mean by "dropper resistor"; are you talking about the pre-amp tube biasing resistors?

  20. #20
    Noodle of Reality Steve Conner's Avatar
    Join Date
    May 2006
    Location
    Glasgow, Scotland
    Posts
    7,251
    In the AA165 schematic on FFG:

    http://www.ampwares.com/ffg/schem/pr...a165_schem.gif

    the dropper resistors are the ones between nodes B, C, and D: the 1K 1 watt and 4.7K 1 watt ones. Changing these resistors for higher values and/or making the 20uF caps bigger might be one way to reduce the popping problem.

    However, did you check that your PI tube has both sides working and reasonably matched? Also, do the volume controls affect the pop?
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  21. #21
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    I see; yes those were all replaced with 2% w watt fireproof; no effect. Those are what I was referring to as filter section resistors. Both 220k, the 1k, and the 4.7k were replaced, no difference at all.

    I havent measured the PI or tried a balanced PI, although I did try several different new tubes there, no difference.

    Also, I used 80uf Spragues for the big caps for 40uf total (actually measured a touch more) and tried two different brands of 25uf/500v caps for the pre-amp and screen grid filters; again, no effect.

    Ultimately, the source of the pop is a big slam of AC on the Power Tube plates at start up; I know its coming from the pre-amp somewhere, but where, I dont know. Most things I can think of that would cause that, other than what I've tried, would result in contant hum etc; i.e. bad coupling caps, bad tone caps. The amp is fairly quiet after about 30 seconds, but that first pop is VERY loud.

  22. #22
    Junior Member
    Join Date
    Nov 2007
    Posts
    20
    I put in a .047uf/600v both across the standby and from the standby to ground. That eliminated the big pop when the standby goes to standby mode, but not when it goes to play mode.

    I've never had a choke go bad, but theoretically, if they do, what symptoms are you likely to see?

  23. #23
    Old Timer Amp Kat's Avatar
    Join Date
    Jul 2006
    Location
    Baton Rouge,LA
    Posts
    1,492
    I doubt it. Try another switch which could be part of your other problem also. Pops are caused by bad connections and a semi cold solder joint anywhere on the HV side could cause it. A loose ground connection from the HV source to ground on that ground side will cause it also.
    KB

  24. #24
    Old Timer
    Join Date
    Jun 2007
    Posts
    4,925
    You said that the amp did this before all of the work was done on it, so I would guess that the problem lies elsewhere.

    Have you tried to isolate the source of the pop? Does removing any tubes have an effect on the pop? If you remove all of the tubes does it still pop?

    I've had chokes go bad by shorting to the case (ground), maybe yours is arcing internally and will eventually fail. Try swapping the choke with another one. Another source could be the output transformer, same idea, maybe an arcing primary winding.

    Whatever the cause, trying to isolate the problem should be your first step.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Reverb noise in Fender super reverb 65 (vintage)
    By albania in forum Maintenance, Troubleshooting & Repair
    Replies: 9
    Last Post: 06-08-2012, 08:14 PM
  2. AA270 Twin Reverb, reverb on both channels
    By JMadson in forum Maintenance, Troubleshooting & Repair
    Replies: 8
    Last Post: 10-21-2007, 02:51 PM
  3. Deluxe reverb, No Reverb!!!
    By deluxe reverb92 in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 07-14-2007, 03:35 PM
  4. BF Deluxe Reverb, reverb tank
    By weeping6 in forum Build Your Amp
    Replies: 6
    Last Post: 02-25-2007, 06:27 PM
  5. Reverb mods for '70's Super Reverb
    By Steve A. in forum Mods & Tweaks
    Replies: 5
    Last Post: 07-22-2006, 12:22 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •