Fender Pro reverb 1966... Real dumb question but I got to ask

[Smitty02]

Hi guys A friend asked me a question the other day and I didnt know the answer he has a 1966 fender pro reverb and it is blowing fuses he sent me some pics As we live in different states but often talk shop and gear, He has had this amp for a long time a years ago he tried to plug a solid state tube style bridge rectitfer in it and it didnt last long and had blown fuses ever since.. He said he had a friend work on it years ago they replaced some of the filter caps with a hodge podge of funky caps I told him he should test the 5v taps as he went out and got a new rectifier tube a 5ar4 or such think but is afraid to put it in, I seen the pics and the caps look horid and one is leaking badly I told him not to try it yet and bring it to me when he gets a chance but I told him to check the power transformer without any tubes in the amp, and explained how.... but he stumped me, he said he has got good B+ at the red wires to rectifier 344v on both reds,,, but he only had ONE yellow wire showing 5v ac to the rectifier the other yellow wire shows near nothing... says it reads in milli volts and it is the yellow wire that runs to the filter caps... the heaters filaments are fine also SO my stupid question is.... is this normal for one of the yellow taps to read 5v and the other one to read nearly nothing ???? I could not answer this question cause I dont know LOL I have not worked on many tube rectified fenders I though both the yellow wires were or should read 5v ac Am wrong ??? did he damage the tranny with trying to use a solid state plug in style rectifier?? He said it is not blowing fuses with no tubes in it at all,, but i told him to wait till I get back to him before he tries to plug in a rectifier tube and risk damaging that... and also replace the filter caps too ??? So what the verdict , are both yellow wires sapose to read 5 volts AC ??? or is this right to only have one yellow reading 5V I think it is the original 022798 ? off the top of my head.. If it is bad... What would be a good replacement tranny that would fix this and help hold the value of his amp Thanks Guys

[Bruce / Mission Amps]

First of all... you can not read the yellow rectifier filament wires with respect to ground for AC voltage... they are floating above ground. You measure across the two to get 5vac.

Secondly B+ is DC voltage expression and you would not read B+ on the two red wires.
You read high voltage AC on the red wires. Measure at the rectifier tube socket, with respect to chassis ground if a standard Full Wave rectifier.

Thirdly, about 80%-90% of blown fuse complaints are simply bad power tubes related and maybe a few bad rectifier tubes.
Not blowing the fuse with no tubes installed is your first major clue.

There are lots of high quality trannys out there using the 798 reference.
From what you have described, the PT being the issue is not on my list of uh ohs.

[Jazz P Bass]

Can't say that I quite follow what you are trying to fix,
You seem hung up on the 5 Vac rectifier cathode winding.
If you have B+ Vdc, then the 5 Vac winding is working.
When the rectifier gets up to speed, the B+ voltage dc 'rides' on the 5 Vac voltage.
I have attached a schematic, maybe it will make more sense.

[Smitty02]

Wow thank you He said he got 344v each on he red wires to ground, yes I realize the reds are not rectified yet and are ac, but being a babe in the woods compared to most on this site such as your self that are REAL electronic engineers.. I dont know all the terms and stuff, but do understand some stuff and i am always looking to learn. I will email him now and tell him.. he has to check at the ends of both of the 5v wires..and it is not considered B+ when it leave the tube rectifier..an his tranny is most likely ok Im sure he will be pleased to hear this and we both learned something today.. though I beg the question ... why did he get a reading of 5v ac on one wire to ground and not the other ? Im sure its got to do with the floating reference err I think rather LOL or forget it,, Im sure you probably dont want to wait time explaining it. I can read up on it, I really need to get a good tube amp book... I was just looking for a quick answer cause i couldnt find it right off and we are emailing at the moment Thank you very much Bruce for your reply...

[Smitty02]

Thanks Jazz I am not sure what circuit he has in it I was under the impression it was a AA165 . Also I never did look into how a tube rectifier actually works, I mainly deal with bridge rec's and bad joints and sockets and stuff, though I have rebuilt and fixed bunches of amps with the help of you and other kindred spirits... I dont know all the lingo and still dont know much, other than how to make stuff work.... I cant design a amp but i can fix em LOL and I am getting better reading schematics these days Take care Jazz Peace ..... and thanks You"ve helped me out in the past an Im thankful... I read alot of the stuff on this site and others and am always looking to learn... cause this broken down body cant work no more. My head still works, so a old dawg can learn new tricks...and feel useful again. wish I would have done went to college and used my head for a living and not my body..

[drewl]

It's possible the plug in ss rectifier is putting out a little more voltage than the rectifier tube and the flaky filter caps can't handle it.
Also might have a higher current capability, where the rectifier tube might sag a bit and not over stress the filter caps.

I've been running a Weber Copper cap in my '67 pro Reverb for years as it helps the power tranny run a little cooler.

[Smitty02]

I got the amp this weekend and had a chance to go over it real good. This amp was worked on before years ago I guess and I fix'd up the nasty work and who ever was in it left some parts out and was not real good at soldering, filter cap board was in terrible shape also. I just ordered a full set of caps for it replaced some toasty drifted resistors dug out my old rca foot switch and stubb'd in some temporary filter caps and fired it up with a bulb limiter... All looked ok, after some measurements I went to full power and amp is fine. I will get the new caps and probably replace a few more resistors and Im sure it will be doing its thing.... Nice amp I guess the story is its been sitting for years now un-working due to the shoddy repair attempt once I get the parts it will be a very nice 1967 BF Pro reverb again... It was missing the .001 cap on the phase inverter that was causing the amp to have very weak out put. I really didnt notice till I went through it and tested and was fixing some way out of spec resistors and found the missing cap.. Voltages are pretty good too. Im sure my friend will be very pleased when he gets it back cause its been a door stop for years.... It is the AA165 curcuit BTW I have to call him and let him know it will be fine now and see if he wants to do any mods while I am putting it back together cause someone drilled a single hole in the back for something ?? but I can use it for something Im sure ... Thanks for the input guys

[Jazz P Bass]

The .001 cap was missing?
The signal chain was then broken.
How the heck did it work at all?
Some weird coupling action going on there!

[Enzo]

Crosstalk, normally masked by the amp operation.

[Cygnus X1]

Do the safety updates while you have it...in case they haven't been done.
Death cap removal, three prong plug, proper hot wire routing etc.

[Smitty02]

Yes ... Already done Cygnus I dont usually get going on a amp until I convert it to a grounded plug fuse the hot and connect the neutural to transformer right make the ground longer than the others and use star washers to transformers mount bolt on chassis.... Many high resistors on this rig a 820 a touch over 1K... I will be replacing many of the 1k's I do have on issue that is got me at the moment V5 pin one voltage is reading a little over 400v and pin 6 is around 280v ?? like its backwards... talked with my friend and he told me that last recalled it had a master volume mod that wasn't done right or never finished ? which might explain why cap was missing.. anyways I am waiting new filters caps and did the mid mod for the one hole he had drilled in the chassis. Waiting for parts before I go any further, but may use the ground switch for the NFB mod my friend likes what he read on that and wants to give it a try....Anybody try that on a Pro ?? If so i would like your opinion,, weather its worth it or not, or better to use a pot rather than a switch to dial it in better ? Ive never done this mod before, sounds like it could be interesting...He would like to keep the amp as close to stock without switching out part value's, Make it easy to convert back to stock and no drilling in chassis. Though I will put some 1 ohm 1% resistors in it for biasing, also requested... other than the V5 voltages and drifted resistors amp looks like its going to be a gem again... He will be coming to town again this weekend and I look to have it done and ready cause he is bringing the cabinet with him so we can hear how it sounds proper with the original CTS speakers...

[soundmasterg]

Yes ... Already done Cygnus I dont usually get going on a amp until I convert it to a grounded plug fuse the hot and connect the neutural to transformer right make the ground longer than the others and use star washers to transformers mount bolt on chassis.... Many high resistors on this rig a 820 a touch over 1K... I will be replacing many of the 1k's I do have on issue that is got me at the moment V5 pin one voltage is reading a little over 400v and pin 6 is around 280v ?? like its backwards... talked with my friend and he told me that last recalled it had a master volume mod that wasn't done right or never finished ? which might explain why cap was missing.. anyways I am waiting new filters caps and did the mid mod for the one hole he had drilled in the chassis. Waiting for parts before I go any further, but may use the ground switch for the NFB mod my friend likes what he read on that and wants to give it a try....Anybody try that on a Pro ?? If so i would like your opinion,, weather its worth it or not, or better to use a pot rather than a switch to dial it in better ? Ive never done this mod before, sounds like it could be interesting...He would like to keep the amp as close to stock without switching out part value's, Make it easy to convert back to stock and no drilling in chassis. Though I will put some 1 ohm 1% resistors in it for biasing, also requested... other than the V5 voltages and drifted resistors amp looks like its going to be a gem again... He will be coming to town again this weekend and I look to have it done and ready cause he is bringing the cabinet with him so we can hear how it sounds proper with the original CTS speakers...

I did the NFB disconnect mod with my brother's '67 Pro Reverb and I recommend it. Putting it on the former ground switch allows for minimal mods to the amp and you get a cool, new feature. On my brother's I made some other changes too...it was a transiton model between blackface and silverface....it had a blackface panel, an AA165 circuit on the box, but had some parts of the later circuit. It was also sporting some of the blue molded mallory caps and some of the later blue silverface caps, and thankfully very few of the brown turd caps. It was already non-original in some ways, so he was ok with tasteful mods. I did the mod to put reverb and trem on both channels, which puts the channels in phase so you can use them at the same time if you want, and then modded channel 1 with .022 caps in the tone stack instead of the .1 and .047 or .033. I also used the blue Sozo caps to replace the brown ones. The speakers were missing so we went with two Weber 12F150's, but one is a 12F150A, which is slightly brighter....My brother is very happy with it and it sounds and works well.

Greg

[Smitty02]

Cool... I got the parts today and am just starting to install . Thanks for the insight Greg I am looking forward to finishing it up and hearing this amp with the mods through real speakers as i just use a old monitor speaker for testing and the horn dont help in that LOL.. I may be getting a Bassman thats been hacked up for a next project and he wants to do it up.... Really looking forward to that..