Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Blind Lemon on July 30, 2014, 08:24:10 pm
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Goodness its been a while. Hope everyone has been doing well. I have kind of slowed down a little. (LOT)
Anyway helping a friend with a Silverface Pro, blows fuses intermittently, like when you put it on standby or take it off. But not every time. Does not do it with power tubes removed. Thought it may have been the OPT so I put a new one in, had it on the shelf, didn't help. It still acted the same. Thing needs to warm up a bit before it happens. Noticed the Rect arching so I replace it, didn't help. (JJ GZ34) checked the 2 series Caps before the SB switch, 2 F&T 350v by 100m, so that should be fine.
PT is the only thing I haven't replaced. :BangHead:
Thoughts?
BL
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You're onto the right suspects. Too bad fuses are expensive these days. I guess that to see if something you try/replace works to fix the problem, you have to flip the amp in and out of stby several times?
Partially shorted choke? You could temporarily replace the choke with a 10-50-100 ohm power resistor and see if that solves the problem.
How old are those filter caps?
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The F&Ts on this side of the SB switch look new, old spragues on the other side of the switch. I keep thinking it has to be on this side of the SB switch because it blows fuses when putting it on SB also.
BL
BTW. couldn't get it to break on a light bulb.
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+1 on the choke and filter caps.
Maybe check the bias supply components? Vibrato circuit could have something to do with it? I've not much experience, but like you said it still blows fuses with the standby on. Heater voltages ok? Heater CT making a solid connection to the chassis?
Just throwing things out there.
Hope this helps,
Peace,
Aaron
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Had a similar experience w/super R... tensioned pwr tube sockets,& changed pwr tubes...problem gone.
Footnote---when I put pwr tubes in other build i noticed arcing. Maybe something to look into. Also, wouldn't blow when brought up w/variac.... would occaisionally blow when taken off sb. Should have tested again after I found the short in the pwr tube but didn't want to blow anymore fuses. Maybe crank'er up in the dark n see what happens....or? Best regards...sNp
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Oops... blows before coming out of stdby...just re-read....Confuscius was right, It is better to remain silent and be thought a fool than to open my mouth and remove all doubt... come to think of it I'm not even sure if thats how to spell Confucious...
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betting one of the older caps is getting weak. choke is usually go/no go.
--pete
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Ok, update.
Replaced input caps and resistors. Voltage was near 540vdc with switch open, 470vdc closed. It was too high so I put in a 5U4GB (had GZ34) and a 600 ohm 25 watt resistor before the first caps.
Seems to have worked, I have not been able to duplicate the problem.
Not ready to say it's fixed yet.
BL
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The 5U4 draws 3A heater current, GZ34 draws 1.9A heater current. The PT might not like that.
Brad :icon_biggrin:
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The 5U4 draws 3A heater current, GZ34 draws 1.9A heater current. The PT might not like that.
Brad :icon_biggrin:
The AA1069 had a 5U4GB as standard according to this http://www.thevintagesound.com/ffg/schem/pro_reverb_aa1069_schem.gif (http://www.thevintagesound.com/ffg/schem/pro_reverb_aa1069_schem.gif)
but it probably was one of those things that changed over time
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Anyway helping a friend with a Silverface Pro,
My mistake.
Brad :icon_biggrin:
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Hmm, putting it on standby and taking it off standby?
Fuses blow if the current limit is exceeded
So putting the amp on standby makes no sense since the amp is cut off from the current source
Unless there is backward current from the amp back to the main filter caps on the hot side of the standby switch?
Can't see how that can happen?
Maybe the switch is bouncing and not closing immediately therefore allowing a very quick and large discharge?
Try putting the main caps on the cold side of the standby switch and see what happens
Just my wild arse guesses while enjoying a Friday night beer :icon_biggrin:
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I wrestled with that also and it makes no sense. All I know is almost everything got replaced on the hot side of the standby switch 'crept the PT. I have not been able to duplicate the problem since I did that.
Makes no sense. It's FM.
BL
PS. Margaritas for me tonight.
PSS It still blew fuses after. I put a new switch in it.
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Does it blow the Mains fuse with no tubes?
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Nope
BL
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Which model is this? What size fuse? I see some use 2A SloBlo, others use 2.5A SloBlo.
Are there any other symptoms? Does the amp sound normal?
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Does it blow the mains fuse with just the rectifier bottle in?
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I keep thinking somethings going on with the power tubes and haven't seen where a different good set of tubes was tried or sockets were checked & cleaned . maybe I missed it. Plate
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Man, I left this one hanging.
It ended up being a bad JJ GZ34, I finally noticed the rectifier arc and put another in and had no more problems.
BL
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... Noticed the Rect arching so I replace it, didn't help. (JJ GZ34) ...
... Voltage was ... too high so I put in a 5U4GB (had GZ34) and a 600 ohm 25 watt resistor before the first caps.
... I have not been able to duplicate the problem.
First post mentioned arcing and a rectifier swap. Did you swap one bad GZ34 for another bad GZ34?
A later post mentioned the 600Ω resistor and rectifier swap to 5U4. We can't know if you did both before re-testing, but it's usually best to do one at a time. Otherwise, you're never sure what solved the problem.
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Yep, ended up with 2 that we're bad took out the 600 ohm riz and left in the 5U4. Have had no problems since.
BL
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Sorry to hear about that bit of bad luck! That's really unfortunate...
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had this same problem with a couple of TSL100's i take care of.
scratched my head for a bit. checked all the usual culprets to no avail. i finally looked at the fuses with my glasses on and what do you know ...... not sloblo grrrrrrr. i had grabbed the wrong box of fuses not wearing my glasses lol
getting old is soooo much fun.
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Glad you finally got to the bottom of it. I have a 1974 Pro Reverb someone made into a head with black grill cloth and snakeskin tolex. I guess they cut down the combo cab and made it into a head. It was a total wreck when I got it where someone who didn't know what they were doing had attempted countless mods. I removed all the butchery and rewired blackface specs--sounds great. Anyway, when you work you way through a difficult problem like that, it's good to getum purring again. Platefire