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PT that goes "boing" on cold starts

66.215.25.188
Posted on November 24, 2014 at 22:05:38
FenderLover
 The Fender Showman Reverb needed a power tranny. Original one blew, partially due to the nominal OEM fuse replace by a 20-amp one.

I replace the Schumacher with a Mercury Magnetic eq. All is ok... good power. But, on cold starts, like after the amp has been sitting for a couple of hours, the PT will make a distinct "boing" on start-up. Almost like a spring going off.

I've used MM PT on other smaller amps. Never have this issue.

Any worries? Anyone else have this happen?

Thanks! And an early Happy Turkey Day.

 

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RE: PT that goes "boing" on cold starts, posted on November 25, 2014 at 05:17:31
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 The boing is obviously a vibration. When it is a problem with a transformer it usually comes about from too great of a load on the transformer. It will boing and heat up rapidly. If you don't have the heat rise, it is just annoying, but it might be possible to fix. Most solid state amps in sheet metal cases boing.

The vibration could be inside the transformer like the bobbin vibrating, the lams vibrating, this could be the source of the sound. The only way to tell if the transformer itself is boinging is to unbolt it and hold onto it when turing the amp on. Likely you will just feel quiet vibration instead of hearing a boing. If it does boing in your hand, send it back.

The metal amp chassis is accentuating and amplifying the noise. There could be a cap (or some other part) in the amp that picks up the mechanical vibration from the transformer and beats against the chassis. Rubber washers may help. Stiffening and damping the chassis will help too.

 

RE: PT that goes "boing" on cold starts, posted on November 25, 2014 at 06:05:33
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 Does it do this with the secondary disconnected? If not, something downstream might be causing a current surge.

 

RE: PT that goes "boing" on cold starts, posted on November 25, 2014 at 07:30:28
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Could be the same magnetostriction effect that happens with a H/K Cit. 2.

In addition to the mechanical damping other posters have suggested, turn on surge control seems to be in order. I'm assuming vacuum rectified B+, Therefore, put a NTC inrush current limiting thermistor on the primary side of the power trafo. Select between a CL-90 and a CL-130, depending on the amount of current being passed.


Eli D.

 

RE: PT that goes "boing" on cold starts, posted on November 25, 2014 at 07:36:07
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 "The Fender Showman Reverb needed a power tranny. Original one blew, partially due to the nominal OEM fuse replace by a 20-amp one."

was it ever determined *why* the fuse was "upgraded" in the first place?

Anyway, that boing sound is the tranny vibrating from inrush current filling up the caps. Did you modify the filter section at all? Upgrade caps? That could be part of it...


-SF

 

Thanks!, posted on November 25, 2014 at 08:08:41
FenderLover
 1.) Some tech or the owner along the way must have thought the 20-amp fuse was a "good" fix for an amp which must have constantly blown fuses. I've seen this several times, in older Fender and Marshall amps.

2.) The amp has solid-state diode rectification.

3.) Amp now hums. Volume independent 120Hz. No hum with only 6L6GC's. Hum when PI is inserted (12AT7). Any 12AT7. Checking lead dressing and voltages around the PI.

 

RE: Thanks!, posted on November 25, 2014 at 08:38:12
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 Ok, so sounds like bad filter caps. That would cause 120Hz hum, blown fuses, and a loud transformer. Diode rectification is more prone to inrush current spikes than a tube rectifier because of the lower impedance anyway, so some inrush will always be there, but 120hz hum is a near guarantee of ripple on your power supply, which points to bad filter caps.

 

RE: Thanks!, posted on November 25, 2014 at 08:47:37
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 So, B+ rectification is by SS diodes. Therefore, put the surge limiting thermistor between the diodes and the PSU filter. That allows an O/P tube bias supply, if present, to turn on and gives the filaments a bit of time to start heating.


Eli D.

 

RE: Thanks!, posted on November 25, 2014 at 13:13:29
Ray Moth
Audiophile

Posts: 2784
Location: Jakarta, Indonesia
Joined: November 10, 2003
 The delay caused by a thermistor is only a tiny fraction of the warm-up time of even the fastest heater. The real benefit is to moderate the surge current at turn-on.

 

RE: Thanks!, posted on November 25, 2014 at 15:53:01
drlowmu
Manufacturer

Posts: 9730
Location: East of Kansas City
Joined: January 10, 2005
 Hum...will it hum if you reverse the AC cord in the wall, 180 degrees? Try that, with a cheater if necessary, and give a listen. EZ to do.

Jeff Medwin

 

Fixed! Yay!, posted on November 25, 2014 at 18:27:45
FenderLover
 Ok, fixed. The 22mfd/500VDC filter cap that supplies the phase inverter failed. Interesting, as this is the second F&T 22mfd/500VDC which has failed on me in the last two years. A new one failed on a Deluxe Reverb.

I replaced the last two 22mfd caps with Sprague 20mfd/500VDC ones. All other caps were replaced one-by-one to rule out failures. Starting with the first 80mfd/450VDC cap.

Reversing the power cord didn't work, for this amp.

BTW... anyone try the 20mfd/600VDC Sprague Atom cap, yet? Very $$$ (nearly $20 a pop from AES).


No more hum. Thank goodness!


Thanks, for the replies.



-----

photo newfiltercaps_zps1d7b3a20.jpg

 

RE: Fixed! Yay!, posted on November 25, 2014 at 18:46:17
Sherwood Forest
Audiophile

Posts: 240
Joined: October 7, 2014
 Cool, thanks for the followup. 120hz hum was the dead giveaway, good on ya for being able to tell the difference from 60hz hum. Most people get that wrong.

So, haven't looked at the schematic, but I'm fairly sure that the cap that failed would have had a resistor or two in front of it, and that it's one or two caps from the first filter stage. As such, the amount of current that it could leak was somewhat limited, and it probably didn't contribute all that much to your inrush situation. Unless the tranny "just does that" when it inrushes, if you want to rule out a cap as cause, pay most attention to the ones with the least resistance between them and the diodes; the first filter section really. Couldn't quite tell from your post if you replaced them all or not... Sounds like you got a bad batch of caps there...



 

RE: Fixed! Yay!, posted on November 25, 2014 at 19:37:20
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 FWIW, if the B+ downstream from the choke was nominally 470 volts with a 117 volt primary it would be just under 500 volts at 124 volts on the primary. If the tubes weren't fully warm and drawing their normal current then if the standby switch was flipped too soon that could cause the caps to see over 500 volts. They are rated for 550 surge so that shouldn't do any harm, but note that the (2) 100 uf/350 volt B+ caps are in series so you have a 700 volt effective rating. They did that for a reason.

500 volts for those 22 uf caps seems pretty close to me. I'd be tempted to use a couple 47 uf 350 or 400 volt caps in series there just 'cause I'm a chicken! 23.5 uf at 700 or 800 volts sounds good to me.

Just my thoughts...

 

Sprague Atom 20mfd/600VDC rated caps, posted on November 26, 2014 at 02:56:50
FenderLover
 Ended up replacing all the main filter caps. And the bias supply ones, too. I upgraded the bias supply to two 100mfd/100VDC.

I was thinking of stocking up on those 20mfd/600VDC Sprague ATOMS. For the reasons that Jim noted. Kinda $$$ at $20 each. Big, too. They will fit under a Fender amp "doghouse." Anyone try these yet?

-----

As for 60Hz versus 120Hz, I learned it, this way:

60Hz is lower than an open low E-string. Standard tuning.

120Hz is a little higher than an open A-string.

If you have a guitar, difference is apparent.



 

RE: Sprague Atom 20mfd/600VDC rated caps, posted on November 26, 2014 at 05:12:36
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012
 You are right, those 600 volt rated Atoms are painfully expensive. I use the 450 v and 500 v Atoms mostly. Sprague seems to keep the same qualities up and down the voltage rating of the Atom line. What that tells you is that the 600v caps are simply more expensive to manufacture. That is why cheaper caps fail at +500 volts. Consider them an insurance policy.

I will use a Solen 630v as a first cap to take the beating and heal. I will also either use a vacuum tube rectifier or use a 20% voltage standby switch with silicon diodes. I hate the screaming. I need to silence the lambs.

 

RE: Sprague Atom 20mfd/600VDC rated caps, posted on November 26, 2014 at 12:33:20
Kyle K
Audiophile

Posts: 174
Location: Mililani
Joined: November 6, 2000
 If those 600V Sprague Atoms are made like the old ones used to be, it's simply two 40uF caps in series. There are no equalizing resistors, so I would guess the two caps would be rated at 350 - 400 volts each to be safe. Since the case on the section with the positive terminal would be at approximately half the applied voltage, they put them in a cardboard tube before applying the plastic shrink sleeve for safety.

 

Thanks, for that update!, posted on November 27, 2014 at 09:56:05
FenderLover
 Prolly best to added the two 40mfd caps, in series, yourself. Side-by-side with say, 220k-ohm, 2-watt leveling resistors underneath.

Then, the long length of the 40mfd/600VDC "single" cap by Sprague won't be an issue.

Happy Thanksgiving!!!

 

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