Princeton Reverb Power Transformer - 65 Reissue

[CJ]

got a dead skunk layin in the road, this time a Fender Princeton Reverb "65" which is a reissue,

probably has a dead internal fuse which they use on the offshore stuff,

schematic says they run 445 on the plates of the 6V6GT which is not a good idea with today's tubes,

original Princeton runs at about 420 plate,

core was 100 EI, 63 lams at 0.018"

606 EIA code is Schumacher, sad to see them go offshore,

 

[CJ]

outer section is the 6.3 VAC heater wind, done bi-fi, 23 turns,

we might bring that down on the rewind, everybody seems to run heaters at 6.6 VAC these days,

 

[CJ]

next up is the HV secondary, air dry epoxy makes life hard as it does not get soft with the heat gun, we just un-did the first section for 775 turns and guessed that the next section would be the same as this is a full wave center-tapped design,

 

[CJ]

next up is the 5 VAC rect wind, also done bi-fi with 18 turns,

 

[CJ]

finally the shield and the fuse over the primary,

 

[CJ]

and yes the fuse was blown for no apparent reason, no heat signs inside the primary, the wire insulation did come off as we were unwinding, you see this a lot with the combination of cheap wire and strong solvents in the impreg. compounds, i wonder if they isolated the screen foil ends as to not form a complete turn? i should have payed more attention to that,

will get a print up soon for the winding crowd, better to rewind with good wire and insl. then spend 100 bucks for the same thing that might pop again, and we can reduce the plate voltage to 1964 levels during the rewind,

primary wire is always heavy so it does not melt before tripping the circuit breaker at the house,

 

[CJ]

just noticed something interesting, solid state rectifiers feeding the tube rectifier, improves reliability while keeping the sag/compression syndrome that people like,

 

[musipol]

445V on the plates, IMHO, is a bad idea even with NOS 6V6's. Fender must be following the mesa guide for tube longevity.
Don't know why the high voltages..............doesn't improve the sound and the end result is not that much louder.

 

[AnalogPackrat]

Yeah, 445V is bad. When I bought my '77 Deluxe Reverb in 91 or 92 it had no 5U4 in it but still worked. Popped it open when i got home and found 3x 1N4007 in series soldered across each half of the rectifier tube socket. B+ was 485V! Removed that crap and fast.

 

[soapfoot]

An old RCA, Sylvania, Tung Sol etc. 6V6 can handle 450vdc on the plates just fine.

It’s well beyond spec on the data sheet but getting the plate power dissipation in range (60% quiescent is my target) is much more important in my experience

 

[Rusan]

When I bought my '77 Deluxe Reverb in 91 or 92 it had no 5U4 in it but still worked. Popped it open when i got home and found 3x 1N4007 in series soldered across each half of the rectifier tube socket. B+ was 485V! Removed that crap and fast.

I've noticed that the later silverfaces all tend to have high B+. Makes you wonder if maybe CBS didn't hire some ex-Ampeg engineers in the '70s, lol.
AAMOF, I have a 135W UL Twin Reverb on the bench right now, made the week of October 3, '77. The raw B+ indicated on the schematic is 500V. Thankfully, it does say that's with 120V on the primary, not 115, or else with 120V the 6L6s would be emitting X-rays.

 

[AnalogPackrat]

Maybe CBS was fighting in the early volume wars. I think i ended up using a 5V4 in my DR after undoing the hack. I need to open it back up and clean up some of my mods based on new info.

Are you winding a new PT or buying a replacement for that Princeton?

 

[shabtek]

not too OT, I have an ampeg vt 40 or 22 can't remember--the combo. bought it from a garbage picker for $5 25 years ago and it worked straight away but the output tubes were red hot and blue. last time I messed with it all electrolytics replaced and output tubes replaced but same thing. is there an easy way to knock down the B+ or do I need to be less shabby?

 

[soapfoot]

Are we sure the B+ is your problem in that Ampeg?

Does it have enough bias? What are you measuring for plate current?

 

[musipol]

As I recall vt22 was like a v4, so 7027 tubes with 500+ volts on the plates. You also get to deal with those wonderful ampeg pcb’s. I would think about modifying it for adjustable bias.....and at least check the dissapation on the power tubes.

 

[Flatpicker]

The 5AR4 only drops 10V. You can lower the voltage by using a rectifier tube with a higher drop.

According to Eurotubes JJ 6V6's are tested at 500V and will supposedly handle anything below that.

 

[Rusan]

I have an ampeg vt 40 or 22.....is there an easy way to knock down the B+....

What about the old zener trick - a string of 10V 5W zeners from the negative side of the bridge rectifier to ground? If I'm thinking of this the right way, you shouldn't have to even change anything in the bias circuit because C- is taken from the same point as B+. Or, I could be wrong...

....outer section is the 6.3 VAC heater wind, done bi-fi.....next up is the 5 VAC rect wind, also done bi-fi...

CJ, why would they bifilar wind the filament windings? Is it to reduce leakage inductance and therefore heat gain from the higher current windings, or so smaller gauge wire can be used to net a smaller cross sectional area in the winding window, or what?