Help troubleshooting fender princeton

[seavote]

http://www.prowessamplifiers.com/sch...a964_schem.pdf

http://deeringamps.com/workbench/princeton_layout.html

hi ,im a novice i know a bit but im confused about where to go to get my brothers blackface princeton up and running again.about 18 -20 years ago he had an overdrive switch put in and the rectifier tube bypassed with a bridge diode. the tech installed a monster transformer for the B+ voltage and added another transformer. from what i could make out the transformer secondaries put out 34V. he used this to supply the 4 tube heaters (in series??) and to make a -34V regulated for bias supply to the power tubes. no, i dont know why he thought it was a good idea,maybe he was experimenting and threw in whatever transformers he had on hand. after about a month the amp burst into flames and was ressurected about a month ago for me to repair. we decided to go back to stock. i removed the 2 transformers, got whatever components i needed to put the -34V bias supply back to stock,replaced all electrolytics,replaced two burned out resistors,replaced the power tubes and had the 12ax7,7025 and GZ34 tested.all tested good.i checked for continuity where ever i could through out the entire circuit.
i had a transformer from a dynaco sca35. it had the correct hi voltage secondary and a 6.3V heater supply but no 5V supply for the rectifier tube heater. i got around this by using a bridge rectifier(rebuilt,not the one the "mod" tech did) when i plugged the amp in i got the correct AC Voltages up until the exit from the transformer. after that low MVs on the B+ and a high voltage( i think about 40Vs but dont recall)on the heater supply(with no tubes).
so i went and bought a hammond 290ax. a direct replacement transformer. took out the bridge rectifier, installed the rectifier tube. powered the amp up and now im getting only voltages in the mv range from all 3 legs of the secondary.
on the layout (see link above)diagram the transformer has a green and yellow ground wire that the hammond transformer does not. the red and yellow ground wire from the transformer is grounded on one of the transformer screws coming through the amp chasis.
another note: i replaced the filter caps canister with 4 individual caps, twisted there negative legs together and grounded them at the same screw.

im not sure where to go from here. hopefully im missing something very obvious(again im a novice)
id find it hard to believe the new transformer is at fault. is there a way to test it with no load without burning it out??
please any suggestions are appreciated.
thanks and sorry for rambling,i just wanted to give you forum members as much info as possible. i'll shut up now and hopefully one of you can take it from here.

[bnwitt]

Start off by posting a voltage chart on the amp. With it turned on and volume on zero.

[seavote]

cant open your file of the voltage chart. im signed in but the forum tells me im not when i try to view your file.
the schematic i linked to does have some voltages at different nodes of the circuit. if you need to know the voltages in the amp im working on i cant get them till mon but they will all be in the mv s. there is next to 0V coming from all 3 legs of the secondary. with and without the rectifier tube attached. i am getting between 115-120V AC from the on off switch and the fuse, going into the transformer. its looking like its a bad transformer, i just did n't want to belieave my brand new tranny was bad.

[bnwitt]

So you've got 120 volts to the primary of the PT and with the HT secondary center tap grounded you've got no voltage present on the secondary legs between each other or from each one to the center tap?

[seavote]

correct. No Voltage to speak of Low mv readings.

[bnwitt]

and that's with your meter set to AC right?

[seavote]

ac yeah.
problem solved. while i did "check" the fuse(more than once), i never "tested" the fuse. it looked fine but was not. DOH!!! so i learned a valuble lesson. B+, heater, voltage to regulator all read fine. i did not have a guitar handy (my work place, i do this during lunch break) but tomorrow i'll give the amp a run through. i'll check some other voltages as per the schematic. at full volumn there is some hum . but not more than most old fenders. will grounding the heater supply make an improvement. if so.. where do i ground from and to in the circiut? thanks

[bnwitt]

If you do not have a center tap on the heater secondary (no green/yellow or similar coloring wire)but only the two green wires, this can cause hum and you can make an artificial center tap with two 100 ohm 1/2 watt resistors to reduce the hum. This is typically done at the pilot lamp in a fender amp. See attached sketch. I usually use heat shrink tubing on the resistor leads that connect to the lamp socket so they don't short out the winding. You can ground the twisted leads on the downside of the resistors to the chassis with a soldered ring terminal and a bolt through the terminal ring hole to the chassis. See the attached picture.

[seavote]

thanks bn, i'll give this a try. i think i have 2 100 ohm 1/2 watt on hand. i've not posted much here. i've been doing some projects at the prodigy pro DIY/or/Lab forum, but being a guitar player i'll naturally end up building some amps. since i have a good amount at my disposal its not a priority however. my brother also has a black face fender pro reverb that has been dormant since he found it in someones trash about 14-15 years ago. it just sounds like its underwater so i'm thinking. replace the electrolytics and replace the power tubes. i'll leave any preamp tubes that test good. the princeton had a mullard 12ax7 and the pro reverb has a telefunken 12ax7 . i'd hate to replace it unless its necessary. again thanks for your help. this is another great forum/people helping other people

[seavote]

whoa!!! this amp sounds great!! (dead quiet with the grounded heater supply,thanks bn) new EH 6v6 power tubes all the rest are old stock. the amp was using a diode bridge but i got rid of it and reinstalled the rectifier tube .12ax7 is a mullard. playing through a carvin 12 speaker i had gathering dust.not sure which part of the circuit makes it sound so great but building an amp with this amp as a basis(add reverb and overdrive channel,i can lose the tremolo)is now on my DIY project lists. to bad for me my brother is coming for dinner next saturday and i'll be relinquishing the amp back to him.

[bnwitt]

Congratulations on the repair Seavote. Those old Princeton's are great sounding amps indeed. The Black Face Princeton Reverb (BFPR) AA1164 is also a great amp and it does have reverb. A good overdrive pedal in front of it gives you the overdrive you seek. I like the fulltone OCR for that purpose. If you're interested in building one of those, I did a layout of a Hoffman style BFPR that one of the other hoffman forum members designed (Hot Blue Plates). It sounds great and is very quiet on idle as well. Here is a link:

http://www.el34world.com/Forum/index.php?topic=4770.0


Barry

[seavote]

thanks barry. while a pedal is fine there really is nothing like pushing a vacuum tube to saturationwhat i had in mind was an overdrive channel(additional preamp stage) with a foot padel to switch channels. do you know where i can find info on something like this. i'd like to know how to do it and also understand what im doing. my guess at how this works??? the foot switch kicks in the additional gain stage and this signal goes through the preamp stages of the "clean channel" and the aditional gain causes them to saturate. do i understand this correctly??

[bnwitt]

Seavote, what you don't seem to understand is that a pedal will push your first input triode to saturation by boosting the output signal of your guitar pickups. You don't need an extra gain stage to do this.

It is sad that today everyone thinks that an amp should do everything tonally and many have forgotten that the original fantastic tones on the great songs of rock and roll involved simple amplifier circuits with external pedals. Trying to get everything out of ones amp usually leads to dissapointment and over complicated circuits that experience multiple failures. But, you do what you think you need to do.

[seavote]

"a pedal will push your first input triode to saturation by boosting the output signal of your guitar pickups. You don't need an extra gain stage to do this."

true. when i think pedal , i think distortion pedal. i havent heard many im crazy about. perhaps just a gain boost pedal would work best for me.
That said, i'll tell you my current rig consist of an Ampeg Gemini G15 and a Chandler tube driver. thats it! the tube driver (ive had it for about 20 years) spent most of its existence in a closet and in my attic. after i got it home from ther music store i could never get it to sound good with any number of amps ive had. about two years ago i hooked these two up together and was blown away!

"It is sad that today everyone thinks that an amp should do everything tonally"

it certainly would be great if one could.options are always good to have. the closest ive seen is the johnsen millenium. i bought one when they first came out. it takes some work and experimenting but some great sounds can come from this beast. im more of a amp distortion and reverb guy so many of the amps effects go unused. but its nice to be able to have a stable of different tones available at the touch of a footswitch. truth be told . when playing in my Top40/wedding gig i used a decent amount of the presets. when jamming with friends or playing more fun"for me" projects i tend to use only about 3-5 presets a night.
also the johnsen has a particular tonal character ( as all amps do) no matter how much tinkering you do it still sounds like a johnson and even though it gets close it will never sound like a fender, a marshall, etc.

put your bfpr files away for a rainy day. thanks!

[bnwitt]

You know my gigging career was back in the 70's and back then I played solid state amps exclusively. I had the same experience with pedals not sounding good with my amps as you mentioned. I bought all of the pedals the famous players were using but they didn't sound right. When you think about it, a tube screamer can't make a transistor scream well.

Then I built my first vacuum tube amp and starting putting my old pedals in front of it and Wow! What a difference. When you remember that the greatest guitar tone of classic rock and roll came from simple tube amps with different external effects pedals, it makes sense to go down that road in one's search for tonal nirvana. I found that the perfect cover band setup was two amplifiers on an A/B switch with two separate pedal boards all miked to the PA. Usually a Vox AC/30 and a Marshall Plexi but it varied. As a service tech, I have learned that the more bells and whistles one puts into an amplifier, the more potential there is that the amp will have a failure. Two different sounding amps with lots of effects pedals is a very versatile rig indeed. Have fun on your tone quest.