Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jukelemon on August 23, 2025, 07:23:25 pm
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Hi all!
In my experience, it is rare to have a PT go short. Typically something else fails prior to this happening and/or the fuse does it job and removes this possibility.
I got my first amp with a shorted PT primary. Fender PRRI.
Amp came in with a good fuse (it was replaced apparently after the failure/burning smell).
Upon hearing what the person experienced, I removed the rectifier and power tubes and brought the amp up with a variac and was testing the AC side of things. Went to the secondary and nothing. So powered down and tested the primary winding and sure enough it was open.
I don't own a tube tester so I cannot determine right now if it was/is a shorted tube. I tested all power supply caps and none are shorted. None of the secondaries are open.
Nothing on the board is burnt.
So I suspect either a rectifier or 6v6 short.
BUT...in all the other amps that have had these tube shorts occur...the fuse always blows way before a PT failure.
So then I start searching and I am amazed at how many Fender RI PRs have had PT issues.
Do you all think it is a matter of cheap PTs that have very little tolerance to shorts or?
Of course...i am also assuming that the fuse that was in their before this was the right one. It could have had a 5a fuse in there for all I know.
Just curious if anyone has experienced this on these Fender Reissues.
I have a new PT on the way so will know more in a few days.
Thanks!
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Too much current causes a mains fuse to blow. Merely replacing the fuse without fixing the current-draw problem won't fix the problem.
Some typical power supply failure modes
B+ short to ground through a B+ wire contacting the chassis or some other low potential pathway, or a shorted/burnt component like a resistor (which has itself been subject to too much current draw)
Loss of output tube bias causing thermal runaway in output tube(s) resulting in meltdown of electrode cage resulting output tube short resulting in B+ being dumped in an unwanted manner causing overcurrent in PT
Short in rectifier causes high VAC to be dumped onto reservoir filter cap, causing failure of electrolytic dielectric barrier, resulting in cap short whereupon B+ gets dumped to ground, causing overcurrent in PT
PT winding short
Idiot replaces mains fuse with tinfoil when mains fuse keeps blowing
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I've enountered PTs with a thermal fuse build into the winding, in case the transformer gets too hot. They can't be replaced, so basically the transformer bricks itself. (I don't know if that's the case for your PT)
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Does this still have the original IC filter caps? If so, do any of them measure as a short or low resistance? I'd be suspecting either the reservoir or screen.
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They can't be replaced, so basically the transformer bricks itself.
Merlin, that's interesting. Have you ever stripped a PT down to see where the TF was placed? I have run into a lot of TF in computer work and they are easily acceptable.
Are they fused into the winding of the coil and buried in deep? When you run into these, do you send them out to be rewound or just chuck 'em.
Or are you like me and would cut the coil for my copper stash?
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I haven't seen one for a long time, I just binned them, except for one where the fuse was brought out to terminals so I was able to bridge them and it came back to life.
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Does this still have the original IC filter caps? If so, do any of them measure as a short or low resistance? I'd be suspecting either the reservoir or screen.
Yes they are the ICs and no, all filter caps are perfectly fine i.e. no shorts or low resistance which is why I am thinking a tube short.
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Too much current causes a mains fuse to blow. Merely replacing the fuse without fixing the current-draw problem won't fix the problem.
Some typical power supply failure modes
B+ short to ground through a B+ wire contacting the chassis or some other low potential pathway, or a shorted/burnt component like a resistor (which has itself been subject to too much current draw)
Loss of output tube bias causing thermal runaway in output tube(s) resulting in meltdown of electrode cage resulting output tube short resulting in B+ being dumped in an unwanted manner causing overcurrent in PT
Short in rectifier causes high VAC to be dumped onto reservoir filter cap, causing failure of electrolytic dielectric barrier, resulting in cap short whereupon B+ gets dumped to ground, causing overcurrent in PT
PT winding short
Idiot replaces mains fuse with tinfoil when mains fuse keeps blowing
Understood but why would the PT go before a fuse stopped the excessive current draw unless the fuse was not the right one in the first place? In other words, this seems to indicate that a higher amperage rating was used?
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I've encountered one transformer that had not blown fuses but had a dead primary, I was wondering if it could simply be faulty at manufacture, can they simply pop from overheating and have material faults
Can a bad batch of winding wire have thin spots somewhere or problems with the enamel coating etc?
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Idiot replaces mains fuse with tinfoil when mains fuse keeps blowing
Understood but why would the PT go before a fuse stopped the excessive current draw unless the fuse was not the right one in the first place? In other words, this seems to indicate that a higher amperage rating was used?
I highly doubt Fender would let an amp off the production line if it was not properly fused. So if the fuse that was in there wasn’t right-sized, some subsequent appliance user muse have ignored the warnings and used the wrong fuse.
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Idiot replaces mains fuse with tinfoil when mains fuse keeps blowing
Understood but why would the PT go before a fuse stopped the excessive current draw unless the fuse was not the right one in the first place? In other words, this seems to indicate that a higher amperage rating was used?
I highly doubt Fender would let an amp off the production line if it was not properly fused. So if the fuse that was in there wasn’t right-sized, some subsequent appliance user muse have ignored the warnings and used the wrong fuse.
Agree.
The board states a 2008 date code so the amp is certainly not new and had a lot of time to be messed with.
One of the 6v6 GT emblems on the glass is very very burnt and very different than the other matching tube. I'd say that one shorted and red plated significantly if I were a guessing man.
Appreciate the insights!
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if I were a guessing man...
..you'll smoke a new set of PA tubes, maybe take out a transformer.....
suggestion, no PA tubes till everything is fixed n verified, especially a stable bias VDC at the pin-side of sockets. maybe consider 1 ohm monitor R's PA tube cathode to ground.
in my career, 50% of the fuses were saved by the catastrophic condition the fuse was meant to protect against.
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if I were a guessing man...
..you'll smoke a new set of PA tubes, maybe take out a transformer.....
suggestion, no PA tubes till everything is fixed n verified, especially a stable bias VDC at the pin-side of sockets. maybe consider 1 ohm monitor R's PA tube cathode to ground.
in my career, 50% of the fuses were saved by the catastrophic condition the fuse was meant to protect against.
No,,I agree. I am not guessing and then just dumping in a new set of tubes. I say guessing because I am waiting on a new PT and just talking out loud really.
Like I mentioned, one of the 6v6s is really burnt and thus my comment.
Interesting comment about your fuse experience...I would not have expected to hear that. 50% is a lot
Appreciate the comments Shooter!
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I've encountered one transformer that had not blown fuses but had a dead primary, I was wondering if it could simply be faulty at manufacture, can they simply pop from overheating and have material faults
Every time a transformer is powered on the wire will flex slightly with current inrush, and over time this can cause fatigue and a break. This is fairly common in very small transformers (<6VA) because they use very fine wire. But I suppose it might be possible in a larger transformer if there was too much tension or a knick in the wire at manufacture?
This amp uses a half-wave bias rectifier, so a short in the bias supply would lead to net DC and core saturation. So there's another possible fault condition.
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Merlin, have an answer for that question. My last yr in HS I got a job at NWL Transformer and the big boys suffer with the same issue. Almost every Trans that blew and came back for analysis, usually failed due to a speck of contamination in a winding. The contamination chewed its way through the insulation eventually going Boom!
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New PT is in. All secondaries are now what they should be and I have an appropriate Neg VDC bias supply both at the bias pot and pins.
Rectifier was installed and I now have healthy VDC down the line -albeit high given there is no load from 6v6s.
Pretty sure given the charring on one of the 6v6s that a short/meltdown occurred.
Going to put in a new set of 6v6s and bring it back up on the variac.
Power supply caps are ok so unless I am missing something, all signs point to that one charred 6v6.
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New PT is in. All secondaries are now what they should be and I have an appropriate Neg VDC bias supply both at the bias pot and pins.
Rectifier was installed and I now have healthy VDC down the line -albeit high given there is no load from 6v6s.
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Going to put in a new set of 6v6s and bring it back up on the variac.
If you've already powered it up and the voltages at all High Tension nodes are okay without 6V6s in, then there's no need for the variac. Just check you've got -ve bias voltage on the output tube socket grid pins before you pop the 6V6s in.
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New PT is in. All secondaries are now what they should be and I have an appropriate Neg VDC bias supply both at the bias pot and pins.
Rectifier was installed and I now have healthy VDC down the line -albeit high given there is no load from 6v6s.
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Going to put in a new set of 6v6s and bring it back up on the variac.
If you've already powered it up and the voltages at all High Tension nodes are okay without 6V6s in, then there's no need for the variac. Just check you've got -ve bias voltage on the output tube socket grid pins before you pop the 6V6s in.
Yep. Checked -VE at both the bias pot and 6v6 pins
Thanks
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Idiot replaces mains fuse with tinfoil when mains fuse keeps blowing
Understood but why would the PT go before a fuse stopped the excessive current draw unless the fuse was not the right one in the first place? In other words, this seems to indicate that a higher amperage rating was used?
I highly doubt Fender would let an amp off the production line if it was not properly fused. So if the fuse that was in there wasn’t right-sized, some subsequent appliance user muse have ignored the warnings and used the wrong fuse.
Bear in mind that the equipment's mains fuse is there to protect the building and people in it, eg from the equipment failing and setting the place on fire.
Any damage protection it brings to the equipment itself (eg saving the transformers if a valve draws too much current) is a bonus. The mains fuse spec can't discriminate between an amp that's being cranked hard for a few hours, and a valve that decides to slowly cook itself, it has to be specified so it doesn't blow if nothing's wrong.
To protect the equipment, additional fusing is necessary.
On that topic, I think the HT fuse is Marshall's greatest contribution to valve guitar amp design :laugh:
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Out of curiosity, what is the DC resistance of the transformer primary winding?
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Out of curiosity, what is the DC resistance of the transformer primary winding?
There is no resistance :) It is a open winding.
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Out of curiosity, what is the DC resistance of the transformer primary winding?
There is no resistance :) It is a open winding.
Therefore, it may well have been internally fused.
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Out of curiosity, what is the DC resistance of the transformer primary winding?
There is no resistance :) It is a open winding.
Therefore, it may well have been internally fused.
Agree.
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New PT in. new 6v6s in. Amp is back to 100%.
Apart from having to add spade connectors...fairly simple and the Hammond PT matched perfectly.
I am now 100% sure that charred 6v6 was the issue. 1st time seeing a shorted tube take out a PT as opposed to the fuse.
Of course I am also going on the man's word that the fuse that was in there was the original 2a :)
Appreciate all the replies.
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2A fuse seems like overkill to me on those amps. The originals had a 1A sloblow installed, as did the original deluxe reverbs.
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Out of curiosity, what is the DC resistance of the transformer primary winding?
What about the new transformer? Which Hammond did you get?