Princeton- Power Transformer Issues (Solved, includes audio)

[andYz00m]

Hey there,

Using a mojotone kit to build a franken-princeton with no reverb or trem in the circuit, and with a PPIMV.

Built everything and tested voltages with the cheap kit tubes (all new JJ tubes) and the GZ34 started flashing and going nuts after heaters warmed up. My current limiter light bulb started flashing and the pilot light started flashing. Turned the amp off, no fuse blown.

Upon testing the voltages out of the PT with the recti removed, the 5V heater secondary is reading 96VAC on each lead... I also tested with the B+ lead disconnected from pin 8 of the GZ34 and the 5V secondary is still is getting way too much voltage.

6.3V and HV secondaries are reading correctly and bias circuit is showing -31V like it should.

Clearly I should have tested the PT before putting the rectifier tube in there. Is it possible a bad rectifier tube could have damaged the PT? Or is it more likely the PT was bad all along?

Any help welcome.

 

[Rusan]

A shorted rectifier can certainly fry a PT if the fuse doesn't blow. Your dim bulb circuit obviously prevented the fuse from blowing, but with that in mind I would think it should've also limited the current enough to prevent arcing across the transformer windings. Have you checked that you were supplied with the correct value fuse?

Actually, I suppose abnormal current through the transformer for several seconds that's ALMOST high enough to blow the fuse might cause the windings to heat up enough to make their insulation fail. That would especially apply to the HV secondary, with its thinner wire, very long length, and high DCR. Of all the windings, it would experience the highest IR² heating effects, and the point along its length that's 96V away from the C.T. is where the unthinkable has happened.

You didn't mention whether you've already checked across the rectifier tube's cathode and plate pins for a short, but duh, it's obviously toast. You usually see that in DH rectifiers, but I've also seen new IDH rectifiers fail immediately. One was an early Mullard EZ81, definitely high quality stuff, but something wasn't right.

Something you didn't mention is the first filter cap. If it's bad, it could've caused the rectifier to arc and short, which could've effected a nuclear chain reaction that culminated in the 5V winding shorting to the HV secondary. Although, the cap would've probably swelled or exploded. A shorted output transformer is also a longshot, but not impossible.

Check across the all PT leads for resistance readings. The 5V winding is obviously shorted to the HV secondary somewhere along its length, rather than the primary, because being shorted to the primary anywhere would cause a reading of 120V on the 5V winding. Plus, they're typically physically separated anyway.

But, whether the rectifier or the PT caused it, is purely speculative as far as I can deduce. Maybe somebody here whose powers of deduction exceed mine (that shouldn't be too difficult!) can offer additional insight.

I just hope Mojo makes it good for ya.

 

[andYz00m]

A shorted rectifier can certainly fry a PT if the fuse doesn't blow. Your dim bulb circuit obviously prevented the fuse from blowing, but with that in mind I would think it should've also limited the current enough to prevent arcing across the transformer windings. Have you checked that Mojo supplied you with the correct value fuse? If it's too high, well, there ya go.

1A 250V sloblo, proper fuse.

You didn't mention whether you've already checked across the rectifier tube's cathode and plate pins with a VMM for dead shorts, but duh, it's obviously toast. You usually see that more in directly heated rectifiers than indirectly heated, but I've nonetheless seen new IDH rectifiers fail immediately. One was an early Mullard EZ81, definitely high quality stuff, but nonetheless something wasn't right.

Tube is not shorted. Plates are not shorted together and cathodes to plates not shorted either.

Something you didn't mention is the first filter cap. If it's bad, it could've caused the rectifier to arc and short, which could've effected a nuclear chain reaction that culminated in the 5V winding shorting to the primary or HV secondary. Although, the cap would've probably swelled or exploded.

Its a mojo (4) 20uf can cap, looks to be fine (no bulging or other visible issues). I dont have an ESR meter to test it.

Check across the all PT leads for resistance readings, and maybe that'll give you some insight as to what happened.

5V secondary shows 0R :/ shes dead jim. HV shows 250Ohm.

Seems like maybe a bad GZ34 blew my PT.

 

[andYz00m]

Solved.

Pulled the power tubes and tested the PT and recti, somehow the tube survived unlimited current draw from the power tubes. PT and GZ34 are fine, supplying B+ properly with no power tubes installed.

The PPIMV circuit needs a cap between the viper and the grid resistors supplying the bias current. When the MV knob is set to 0 the wiper is grounded essentially sucking the bias current from the grid and allowing the powertubes to runaway.

 

[Rusan]

Whew, glad it wasn't the power trafo! But, I'm a bit confused as to how there was 96V on the open, unloaded 5V winding, but now it's gone. Of course, I'm pretty easy to confuse though! Was it 96V from the 5V winding to chassis ground where the HV secondary C.T. is connected?

As for the 5V winding's DCR, it'll typically test close to zero ohms because it's very short in length and wound with thick wire. It's my fault that I didn't clarify; I meant to check DCR from one end of the HV winding to each end of the 5V winding, then repeat the same with the HV C.T., and finally the other end of the HV winding. You then have a "map" of where the short is located.

Anyway, moving on to the PPIMV, that's a new one on me! I presume it stands for "Post Phase Inverter Master Volume?" Are you placing it there so you can achieve a more authentic cranked-up tone at low volume by including all the small signal stages, versus placing it between the mixer tube and PI?

I don't see how it would work as drawn, or am I missing something? Would you not instead:

1. Remove the grid leak/bias feed resistors altogether, and tie one end of each section of a dual 250K VR to the .1uF coupling caps.

2. Tie the wiper of each section to its corresponding 6V6 grid stopper resistor.

3. Connect the other end of both VR sections directly to C- bias, where the power rail decoupling cap will act as AC ground for the signal.

In other words, simply replace the grid leak resistors with a dual gang pot; basically a grid leak resistor with a sliding tap.

Unless you're driving power tubes into AB2 operation, there's so little control grid current flowing that no matter where the wiper is pointed, the C- voltage at pin 5 should remain pretty constant.

One caveat I do see with doing it as I described, is that when the MV is turned down you'll be shunting the loop NFB to ground as well, which is definitely gonna change the tone and especially the feel. Of course, when the amp is cranked to the point of clipping the finals, there's no more forward gain remaining to drive the NFB loop, so if it's the full-cranked sound you're looking for that'll maybe even get it that much closer anyway.... but, it'll also definitely loosen up the feel, especially without the effects of the speakers being driven hard.

I of course don't know exactly what tone you're after, but since you have no verb or vibrato I presume you're omitting the 3.3M padding resistor (and 10pF bleed cap), which will significantly increase the signal amplitude to the mixer tube and PI. A thought here: cathodynes don't clip in the same manner as LTPs, due to their inherit rectification effects when overdriven as a result of their particularly unique geometry of half cathode follower/half common cathode stage. The PI might draw grid current if pushed too hard, and it'll be the worst, farty sounding distortion you've ever heard. That'll of course limit how hard you can push everything, whereas putting the MV between the mixer tube and PI will let you drive the front end to the moon. Just a thought.

If the PI does clip, I suppose you could solve the problem by DC coupling it to the mixer tube with a very large value grid stopper resistor, instead of moving the MV.

 

[andYz00m]

Whew, glad it wasn't the power trafo! But, I'm a bit confused as to how there was 96V on the open, unloaded 5V winding, but now it's gone. Of course, I'm pretty easy to confuse though! Was it 96V from the 5V winding to chassis ground where the HV secondary C.T. is connected?

Yeah I'm a dingus and was reading one leg of the 5V to ground which had the HV CT on it. Voltage between the two 5V secondaries was correct, especially once the power tubes were pulled.

I don't see how it would work as drawn, or am I missing something? Would you not instead:

1. Remove the grid leak/bias feed resistors altogether, and tie one end of each section of a dual 250K VR to the .1uF coupling caps.

2. Tie the wiper of each section to its corresponding 6V6 grid stopper resistor.

3. Connect the other end of both VR sections directly to C- bias, where the power rail decoupling cap will act as AC ground for the signal.

In other words, simply replace the grid leak resistors with a dual gang pot; basically a grid leak resistor with a sliding tap.

Could totally go this way! The way I did it keeps the original grid/bias resistors and C- in place all the time and just attenuates the signal from the PI using a dual gang pot for each of the PI signals between coupling cap and bias resistor. I also needed to put a 100K resistor between the wipers as when I added the caps before the wipers to keep the C- from shorting when the MV pot was grounded (at zero), the caps were then bridged and shorted together, halving the value... Anyway, got it sorted. I attached the circuit used below.

I wanted a MV as I want to use this for a surf band and have some chimey cleanish stuff but also be able to use it in the studio and crank the preamp stage up but pull the output volume back a tiny bit. It has a 25W greenback in it... its got lots of facial hair.

I of course don't know exactly what tone you're after, but since you have no verb or vibrato I presume you're omitting the 3.3M padding resistor (and 10pF bleed cap), which will significantly increase the signal amplitude to the mixer tube and PI. A thought here: cathodynes don't clip in the same manner as LTPs, due to their inherit rectification effects when overdriven as a result of their particularly unique geometry of half cathode follower/half common cathode stage. The PI might draw grid current if pushed too hard, and it'll be the worst, farty sounding distortion you've ever heard. That'll of course limit how hard you can push everything, whereas putting the MV between the mixer tube and PI will let you drive the front end to the moon. Just a thought.

If the PI does clip, I suppose you could solve the problem by DC coupling it to the mixer tube with a very large value grid stopper resistor, instead of moving the MV.

I left in the verb 3.3M and bright cap in there right before a 470K grid stop for the third triode. (has the NFB). I hadnt thought to put the MV between gain stage and PI. Might have to try that next time.

This thing sounds great. I'll post some audio after I take it to the studio tomorrow. Thanks for the help!

 

[Rusan]

I wanted a MV as I want to use this for a surf band and have some chimey cleanish stuff but also be able to use it in the studio and crank the preamp stage up but pull the output volume back a tiny bit.

I left in the verb 3.3M and bright cap in there right before a 470K grid stop for the third triode. (has the NFB).

Gotcha. Back many years ago when I did this stuff every day, I remember a couple of local players wanting a modern, saturated high-gain tone from their old Fenders, AAMOF in the '90s there were hacked and butchered vintage Fenders and Marshalls everywhere. When I first got the internet, I'd constantly see on forums where people were discussing how to modify their vintage amps to get a hotrod overdrive tone, and unscrupulous "tube gurus" were selling the DIY brain surgery kits to do it, you only needed to supply the drills, jackhammer, sheet metal nibbler and explosives.

I automatically assumed you were going for more of an overdriven tone (which is perfectly fine to do with a kit amp patient!), hence my lengthy discourse; sorry! I see now, you're going in the completely opposite direction.

Since getting back into this a bit about a year ago following a nearly 15 year hiatus, I can see that EVERYTHING has changed, and the '80s - '90s fad of modding actual vintage amps is much more highly frowned upon these days, especially if it involves drilling or punching the chassis.

BTW, I told those guys who'd asked me about modding their blackfaces, "Just get an overdrive pedal," and the response both times was something like, "But I want an all-tube overdrive, from inside the amp, like "Mr. Guru" does with his mods to make it sound more like a Marshall." To which my response was, "Buy a Marshall."

Can't wait to hear those sound clips!

 

[andYz00m]

Here is some recorded audio!

American Jazzmaster > Fender 6G15 (All controls at 3.5)> Franken Princeton

Stager SR-2 Ribbon > 1073 > Apollo

I think this amp has a shitload of low end, still need to dial in the tone stack to account for the 12" speaker. I would pair it with a different mic if I had more time.

Sounds pretty good though for a quick demo! I didnt add any samples of it with the preamp gain pegged and the master pulled down but it has serious hair. Sounds like a hot rod deluxe kinda. Probably wont use it that way, I have tons of other amps that do high gain.

Also the schematic I posted was actually modified slightly. The resistors meant to help the caps from combining when MV was set to zero is individually on each leg of the dual gang instead of bridged across the wipers. Amended version attached.

Lemme know what you think!

 

[Rusan]

Oh yayusss! I couldn't stop smiling while listening to the clip!

First of all, that's some great playing you did there! Dick Dale would've given two thumbs up to the first section, for sure. I was diggin' the jazz blues, sprinkled with some flavors of maybe a little Stevie Ray Vaughan and Motown at the same time. And, you did a superb job on that '50s rock 'n' roll, Mr. Berry...

The tone was downright bodacious! That little guy has some majorly SERIOUS blackface mojo, a sound I could be very happy with, long term. You did a fantastic job; a very clean looking build with tone for days. Very nice, pat yourself on the back!

I was just thnking how that back 35 or 40 years ago, everybody in my area wanted a Twin or Super but would "settle" for a Deluxe if they had to but still wanted the big 6L6 amps. Nobody wanted a Princeton, and Champs were regarded as being somewhat lower than raw sewage, even for practice purposes. Myself, I've always thought Princetons and Deluxes were the epitome and embodiment of blackface chime and sparkle, while the Twin and Super actually ran a close second place in those departments. And, a cranked Champ is an institution all its own! If it's chicken pickin' on a Tele then I'll take a Twin, but for playing nearly anything else on a Strat, Jaguar, Jazzmaster, Mustang, guitars with P90s and even PAFs, or whatever, I've always had a preference for the 6V6 amps.

Admittedly though, surf guitar does pretty awesome on a Super Reverb...

Thanks for sharing the great tones with us, and congrats on a superb build! BTW, did you also build the 6G15, or is it an actual factory Fender?

 

[andYz00m]

Oh yayusss! I couldn't stop smiling while listening to the clip!

First of all, that's some great playing you did there! Dick Dale would've given two thumbs up to the first section, for sure. I was diggin' the jazz blues, sprinkled with some flavors of maybe a little Stevie Ray Vaughan and Motown at the same time. And, you did a superb job on that '50s rock 'n' roll, Mr. Berry...

Ha! Thanks man!! I was trying to give a range of tones for the demo >.<

I was just thnking how that back 35 or 40 years ago, everybody in my area wanted a Twin or Super but would "settle" for a Deluxe if they had to but still wanted the big 6L6 amps. Nobody wanted a Princeton, and Champs were regarded as being somewhat lower than raw sewage, even for practice purposes. Myself, I've always thought Princetons and Deluxes were the epitome and embodiment of blackface chime and sparkle, while the Twin and Super actually ran a close second place in those departments. And, a cranked Champ is an institution all its own! If it's chicken pickin' on a Tele then I'll take a Twin, but for playing nearly anything else on a Strat, Jaguar, Jazzmaster, Mustang, guitars with P90s and even PAFs, or whatever, I've always had a preference for the 6V6 amps.

The amp that gets used in the studio the most is my old silverface champ (with treble and bass controls). I just finished an EP for this surf project and I used exclusively that amp... This princeton sounds very similar but a bit louder and has a bit more range and options.

did you also build the 6G15, or is it an actual factory Fender?

The tank is a 90s fender reissue (PCB version) that I repaired and added a surfy bear pan to. It had some ground issues and some bad filter caps. Got it for VERY cheap.

Thanks again for the feedback!

 

[AnalogPackrat]

Sounds good. Classic Fender tones all the way.

 

[Rusan]

The amp that gets used in the studio the most is my old silverface champ (with treble and bass controls). I just finished an EP for this surf project and I used exclusively that amp...

About 5 years ago, a coworker asked if I'd look at a very old guitar amp that his wife's brother found in a barn (yes, literally a BARN). She played guitar, and wanted it restored to nice condition if it could be brought back to life. When he brought it to work, it was a '75 Champ. Wow.

The grille cloth was flawless and the Tolex had only a few very minor tears (that glued down almost invisibly), but the hardware was pretty rusted. The speaker was shot, but it had the original RCA tubes in very strong condition. I gave it a detailed cleaning, replaced the handle, corner guards and chassis straps, did a full time/age electronic restoration and fixed issues like the heater string, and installed new tubes so the originals could be stashed back.

I've heard a lot of Champs, but for some reason that one sounded extra amazing. It was NOT easy to return it!

The tank is a 90s fender reissue (PCB version) that I repaired and added a surfy bear pan to. It had some ground issues and some bad filter caps. Got it for VERY cheap.

I love those kinda finds! I think I mentioned somewhere else, that I have an original TS-808 from '79/'80 that a friend gave to me when it died. I opened it up and remelted one solder joint, and it's been with me for over 30 years now.

The biggest one I ever ran across was a Carvin MTS3200 2-12" combo in a pawn shop, back around 2003. I plugged into it, and it wouldn't make a sound. I told the owner, who said he always tested everything before he took it, but was really busy when this guy had came in and didn't. He was pretty upset, and said it would cost more to fix it than he could make off of it.

I told him I had a little Squier combo and a Roland Blues Cube 1-12", if he was interested in them. He said he'd take 'em, because it not only solved his problem but he could move small, cheap solid state amps a lot faster than big, expensive tube amps. So, we traded, and were both happy.

I don't remember the exact price, but I'd gotten the Squier used for far under $100, and somebody had GAVE me the Roland. The Carvin was in near mint condition with $450 on it, IIRC, and cost something like $800 new in the late '80s. I gave it a ride home, and 15 minutes, one resistor and one 12AX7 later it was working perfectly. Still does.

 

[soapfoot]

In 2008 I bought a non-working Univox Superfuzz on Craigslist for $50, and the guy delivered it to my house for that price.

a squirt of contact cleaner in the switch and it’s been sounding great ever since