Hoffman Princeton Reverb Fuse Sizing

[purpletele]

Another amp maintenance / repair question.

I have a Hoffman Princeton that I built 2 years ago, I believe I tested a transformer set before settling on Classictone.  I think I smoked an Allen Transformer.

When I built the amp I ended up using upgraded transformers but I failed to inquire on the revised fuse requirement.

Currently,

The amp works fine at my house with a 1 amp fuse.

The amp went to a show a couple of months ago and blew a fuse shortly after plugging in.  I wasn't there.

I replaced the fuse with a 1 amp replacement to see how long it would hold.  It played really nicely for quite a while until I took it to a jam session and it snapped within seconds of being at a different location.

I have attached the cut sheets of the transformers I used.

Do I have a case of an undersized fuse or should I be looking for a latent error?

Thanks,

Brian V

[purpletele]

Here is a video of the amp in a Mather 1 x 12 Tall boy.

[SILVERGUN]

The PT you listed is for a Champ.
Is that the correct model?


If so, you are exceeding it's current capacity and smoking transformer #2.

[PRR]

The running current is under 1A but the starting current will be higher. If it blows instantly, or 10 seconds after turn-on, it's starting surge.

These really want to be SLO-BLO.

[purpletele]

The PT you listed is for a Champ.
Is that the correct model?


If so, you are exceeding it's current capacity and smoking transformer #2.

I will verify, but I believe that is the transformer set that I used for two Princeton Reverb's as recommended by the Classictone Rep.  The cut sheet references the Bronco and Princeton series, but that doesn't resolve anything.

BV

[purpletele]

The running current is under 1A but the starting current will be higher. If it blows instantly, or 10 seconds after turn-on, it's starting surge.

These really want to be SLO-BLO.

I had a 1A Slo Blo in the amp

[shooter]

was the amp being played when the 2 fuses snapped or just idle?

If idle, I'd look for some slop on the input side, power cord tangs, loose fitting fuse, ....

If it was strumming;

I've been re-biasing 6V6 types using MAX signal conditions, and idle.  mostly the older 2 prong gone 3 prong.  for whatever reason, at 80% to max they jump big for longer than expected.  Most leave closer to 55-58% idle.

[DummyLoad]

that 40-18019 power transformer is in NO way suited for use in a 15-20w amp. 8-10w SE would be pushing it really hard.


you want something similar to the attached.


--pete

[DummyLoad]

also, take of the back panel, play the amp really hard while keeping an eye on the rectifier tube - look for flash-arc - fuse pop. may have a bad rectifier.


bet that trans runs on the warm side. that PT is for a TWEED princeton - not a PR.


https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5f2a.pdf


--pete

[purpletele]

that 40-18019 power transformer is in NO way suited for use in a 15-20w amp. 8-10w SE would be pushing it really hard.


you want something similar to the attached.


--pete

Yikes!  Thank you.

I'll pull the chassis and verify the transformer, and then I'll check watch the rectifier.

BV

[purpletele]

was the amp being played when the 2 fuses snapped or just idle?

The fuse burned while idle

If idle, I'd look for some slop on the input side, power cord tangs, loose fitting fuse, ....

Thank you, I'll check connections.

If it was strumming;

I've been re-biasing 6V6 types using MAX signal conditions, and idle.  mostly the older 2 prong gone 3 prong.  for whatever reason, at 80% to max they jump big for longer than expected.  Most leave closer to 55-58% idle.

[sluckey]

I have a 2A SLO BLO fuse in all of my 6V6 amps. Same for all my 18 watt EL84 amps.

If I had a Princeton Reverb it would have a 2A SLO BLO also.

[purpletele]

also, take of the back panel, play the amp really hard while keeping an eye on the rectifier tube - look for flash-arc - fuse pop. may have a bad rectifier.


bet that trans runs on the warm side. that PT is for a TWEED princeton - not a PR.


https://el34world.com/charts/Schematics/files/Fender/Fender_princeton_5f2a.pdf


--pete

I ordered a new Classictone PT as recommended.  I really like this amp so I don't mind changing out the transformer to get it right.

I installed a 2A Slo Blo and it jumped to life again, it's been at least two hours of playing and sitting idle.  It seems to have a slightly louder hum than some of my others amps, but certainly not disruptively loud at all.  The louder hum has always been there.

In summary, the 2A Slo-Blo fuse is holding, and I will be prepared to change the transformer when it's delivered.

thanks for the heads up

BV
 


I like the 10" speaker with this circuit, but it does sound good with a 12" as well.

 

[mresistor]

Just an FYI - to anyone building a PR -  the Hammond 290AX is suitable and fits and is a laydown transformer to look like original.

[Colas LeGrippa]

Why don t you clip an ammeter on the ac supply cord to know how many amps the amp is drawing ? For ex if it draws 750mA put a 1,5 A in the fuse receptacle. If you don't have an ammeter equipped woth a clip, put your voltmeter in series with the ñine!, black or white lead. Take an old lamp ezxtension which leads are very eady to take apart


Colad

[Colas LeGrippa]

Sorry for typing errors

[purpletele]

Why don t you clip an ammeter on the ac supply cord to know how many amps the amp is drawing ? For ex if it draws 750mA put a 1,5 A in the fuse receptacle. If you don't have an ammeter equipped woth a clip, put your voltmeter in series with the ñine!, black or white lead. Take an old lamp ezxtension which leads are very eady to take apart


Colad

Colas,

Thanks for the note, I have the amp disassembled and ready to receive a new transformer.

BV

[Colas LeGrippa]

Ooooops !

[purpletele]

I pulled the chassis of the Princeton to prepare to replace the transformer and I did a double take on the tranny number.

Reading the number upside down I wrote down 40-18019 which is an incorrect transformer.

Reading the number right side up it is actually 40-18109, which I have posted.

I look at the wiring and I look at the cut sheet and I am not sure how I arrived at the configuration that I have in place.

I have Brn/Black going to the 120 input and I have black white/brown white going to the hot side of the switch.

If I understood the wiring diagram before then I have lost that skill, I don't recall how I interpreted this diagram.

Can I get some clarification on the wiring diagram to understand what I thought was correct? 

[DummyLoad]

looks right in that photo. grey & white & blue are taped individually and left unconnected - do NOT twist the grey blue and white togher or connect them to anything - i usually install a terminal strip to attach unused transformer wire(s) to. 



--pete

[purpletele]

Thank you Pete ^^^^

A couple of days ago I had installed a 2 amp fuse and ran the amp until it finally pooped out, which leaves the following options.  The fuse did not burn during this failure.

1. The power tubes or rectifier were failing and finally gave up the ghost.
2. The transformer is incorrect or wired wrong.

I think I'll go through and document voltages, install new power tubes and see if I was just chasing a fuse sizing issue.

Any advice is welcome!

Thanks you

BV

[purpletele]

Princeton Troubleshooting Update

I've had the amp up and running for an hour with new power tubes and a new V3 tube.

V4 was microphonic so I replaced it with another 12 AY7 and the tube still sounds microphonic but it doesn't pop on the board or with the wires when chop sticking.

1. Is a 12AY7 the best valve for that V4 position? 

2. I found that I am running a 5ar4 as a Rectifier in lieu of a 5uG, I will correct that with a new tube order.  Will that have a detrimental effect on the transformer?

The voltages are consistent with my original voltages in 2018.



I have a spare Classictone 40-18066 if anyone needs one.



 

[purpletele]

Princeton Troubleshooting Update

Played the heck out of the amp.  The transformer was indeed quite warm.

After approximately 3 hours of being powered on the power tubes burned up.

1. Would the 5ar4 Rectifier create that condition that is taxing the power tubes to death?

Thanks

BV

[sluckey]

Would the 5ar4 Rectifier create that condition that is taxing the power tubes to death?

No.

[DummyLoad]

use the violet leads of the high volt secondary - e.g., the 630VCT leads. 

your B+ is too high.

--pete

[purpletele]

use the violet leads of the high volt secondary - e.g., the 630VCT leads. 

your B+ is too high.

--pete

Thanks Pete,

That makes sense.  I'll give that a shot.

BV

[purpletele]

use the violet leads of the high volt secondary - e.g., the 630VCT leads. 

your B+ is too high.

--pete

I made the change and reset the bias, the amp seems to be operating cooler.

The plate voltage lowered to 298 V and I reset the bias at 40mV.

I'll run it tonight and tomorrow at work and see if it is happy.

Thank you, that's fun making corrective tweaks on these amps after a few years.

Brian V

[purpletele]

use the violet leads of the high volt secondary - e.g., the 630VCT leads. 

your B+ is too high.

--pete

Pete,

That seemed to have solved the issue, I've had the amp running for about 4 hours now without issue.

Ordered a used 5u4g Rectifier to get the real feel of that amp, those babies are pricey

Thank you,

BV

[DummyLoad]

use the violet leads of the high volt secondary - e.g., the 630VCT leads. 

your B+ is too high.

--pete

Pete,

That seemed to have solved the issue, I've had the amp running for about 4 hours now without issue.

Ordered a used 5u4g Rectifier to get the real feel of that amp, those babies are pricey

Thank you,

BV

glad to read you got things sorted.   


--pete