'65 Pro reverb transformer change question

[modusmongo]

HMMMMM. I opened the cap pan lid. I see the 5 caps. BUT, the resistors are in reverse order from what you describe. See photo. I removed them and tested them. They have the right resistance values. But the 1k was between the first and second blue cap, then the 4.7k between the second and third. I'll look for 3W replacements, but please let me know what you think about this reversed arrangement of the resistors (and the fact that they test ok at the proper values on my multimeter).

Thanks again :-)

[MWJB]

Well, if they are the wrong way round, your phase inverter will run at lower than normal voltage and your preamp tubes will run at higher than normal voltage...I doubt anything will blow, but it won't sound right. Where does the brown wire from the 4.7K go? Can you give us a shot of the whole cap pan? Those resistors look like 1W carbon film, I'd use 3W rated metal oxides.

Do you get dc voltage at the + end of the 2 rightmost caps?

[modusmongo]

Okay. I replaced the resistors in the correct order. Voltage is still relatively the same at all previously tested pins. The positive ends of the two rightmost caps in the cap pan read 436vdc. The brown wire goes to the center of the photo below (left side of the circuit board). Cap pan photo also provided. Question: the left most cap is inverted, that is it is negative end below and positive end above. All the other caps are positive end below.

[riz]

modusmongo, where in France are you? I'm in Paris for a couple of weeks... PM me if you're nearby! If you're still having trouble maybe I could help!

[modusmongo]

Hey Riz, What brings you to the land of baguettes and frogs legs? Are you on vacation? I live about 4 hours to the south-east of Paris, so we won't likely hook up, but if you make it down to Dijon, let me know and we'll have a (semi) cold one! Peter

[riz]

A French girl, what else?! Actually there's plenty else, I love this place! I do miss being near my shop and tinkering, however, too bad you are a ways away. There may be a road-trip to Lyon, I'll let you know if we can stop along the way! Good luck with the amp, it will rock when you get it sorted out. Just take your time with the things you don't understand yet. Old Fenders are a great place to start learning this stuff--my first major "victory" was bringing back a Super Reverb that wouldn't make any sound, then an old Supro, and then, and then...!! You maybe should increase the wattage on those power resistors, as was recommended, I usually use 2-watt flame-proof. They are easy to get and cheap at my local electronics store. Good luck!

[modusmongo]

Good luck with the frenchie ;-)

This amp was my work horse in the '90s after I found it in a pawn shop in RI for $400 in a 4 x 10 cab. I put it in a 2 x 12 which I stole from a '68 twin (JBL 120's?), and the combination was immediately spectacular. The sound is so sweet and warm, and unlike the twin, I could play it at bar gigs set on 4 or 5, so it had some grit when I wanted it.

I just finished shielding, rewiring and modding my '73 strat (parallel/series pull pot), and I want that max Fender sound!

Thanks for the advice about the power resistors. I'm going to get the amp working again, then I'll open another thread to get as much advice as I can on what to replace and what to leave inside.

I'll come see you if you're in the area. Enjoy your stay!

Peter

[MWJB]

Modusmongo, the 80uf cap at the far left is the right way around. The first 2 filter caps are in series, thus must be arranged with the "-" of the first cap connected to the "+" of the second cap. The cap second from left is getting the B+ voltage on its "+", the left hand cap has its "-" to ground, which is correct.

So you have B+ voltage at all blue filter cap "+", this means that either you have a break in the B+ supply between the farthest right cap & the preamp tubes that it supplies, we know this because the farthest right cap shows no voltage drop from its neighbour, therefore no current is being drawn across the 4.7K resistor. Alternatively ALL the 12A#7 preamp tube plates fed by the righthand cap could have open resistors, or open cathode resistors...but this would seem very unlikely indeed.

[riz]

Yeah a twin is just too freakin' loud! Are you getting proper voltage from your stepdown trannie? I recently got a 1:1 isolation trans for bench work, and it puts out a bit higher voltage than it gets on the primary, so I run it with a variac. Just a few volts higher, but if it gets 117v it puts out like 124... I think a replacement PT eventually would be a good idea if you want to gig it over here. I've toyed with the idea of moving here myself and have pondered what I'd do in your situation... Get it working first, and then look into swapping parts as you mentioned. Those film resistors are good and hold spec well, but some folks will advise carbon comps in some places for "tone". Here's an idea: if you have a program like Protools or Audacity (which is a free download), use it as a signal generator. Make a little thingie like the "listening amplifier" in this link: Amp Tools
I'm assuming you don't have a scope, so maybe this will help you trace the signal from the input and see where you're losing it. I've never tried this myself, as I have a scope. But it might work for you. I have found a scope/signal injector to be the best tool ever for sussing out these kinds of problems, and without some kind of aid you are just shooting in the dark. I use Audacity myself at this point, it works fine for basic troubleshooting, but I'll get a "real" signal generator someday. MAKE SURE TO DRIVE THE AMP INTO A PROPERLY SIZED LOAD RESISTOR when using the signal injector, you don't want to create more problems. I get 25w 8-ohm resistors cheap at my local store and series/parallel them however I need to make the proper resistance with enough power handling. It's good to at least double the power handling capability of the load resistor to the output of your amp. In your case, 100w should do for testing purposes.
You posted some voltages earlier, I need to go back and have a look at them. At a quick glance I noticed a lot of zeros in the preamp... Maybe some of the mods have gone wrong. When did it stop working, what happened?

[Jazz P Bass]

Quote:"
Voltage report: DC voltages pins 3, 8, 1, & 6 preamp tubes 1 - 6 (assumeing 1 is closest to the output tubes)

tube 1 - 69, 69, 324, 214
tube 2 - 0, 0, 422, 367
tube 3 - 0, 0, 0, 0
tube 4 - 7, 7, 422, 422
tube 5 - 0, 0, 0, 0
tube 6 - 0, 0, 0, 0"
No wonder the amp does not work.
These preamp voltages are not looking good.
First : post the voltages as Pin #1 & 3/ Pin #6 & 8.
Tubes 3, 5 & 6 have no plate voltage.
You must check out why.

[modusmongo]

At a quick glance I noticed a lot of zeros in the preamp... Maybe some of the mods have gone wrong. When did it stop working, what happened?

hi again. There are only two mods that I know of on this amp. One in the input and another for the reverb dwell. They are both very simple.

The amp started to lose volume during gigs and I stopped using it because I had another setup I was interested in trying. For 15 years it has sat unused, and now there is no sound at all. But it's funny, once in a while, it will put out a VERY feeble sound (like 1/100th of a watt I imagine), then an hour later, I'll fire it up to do more tests, and there's nothing.

I'm sure that it's something very simple, but as you say, I'm "shooting in the dark". The listening amp is a brilliant idea! Since I have no output at all, couldn't I just plug the amp into a cabinet for a load? If I understand the idea, I could start at the input and follow the signal until it breaks. Cool!

Thanks ;-)

[MWJB]

You know what is wrong, your preamps are drawing no current/have no voltage. Do a continuity test from the farthest right cap in the pan (preamp cap) to the junction of the 100K plate resistors at pins 1 & 6 of the preamp tubes. If there is no continuity, the wire feeding these points is broken. What do you get?

[modusmongo]

Thanks MWJB. I'll do this tomorrow after my boy goes to his mom's house ;-)

Thank you everyone for your help!

[modusmongo]

FIXED!! Thank you SO much. I followed the yellow wire back to the circuit board, and it was detached. First one! I'm very happy. The amp makes a bit of noise, but I have the soldering iron right next to it, and it is open. I'll be posting another thread concerning the replacement of old components and asking advice on "tone" decisions. I also need to learn how to bias it. Anyone want to suggest a link?

MWJB, you are my hero!

[MWJB]

Add a 1ohm 2W resistor between each 6L6 socket pin 8 and it's ground connection (use the chassis ground that is already there, don't try & resolder them unless you are a total massochist) make sure the 1ohms are matched - "zero" your meter by putting the leads together, if it reads 0.3ohms with the leads together (no resistance) use resistors that read 1.3ohms. Read your plate current at pin 8 of each power tube in millivolts, this converts to to milliamps thanks to Ohm's law. Set to 30-35mA/mV.

Or, buy some bias probes like the TAD biasmaster, this will save you having to drop the chassis every time you install/bias up new tubes.

"MWJB, you are my hero!" I only worked out what was wrong because you told me, you found & fixed the break...we'll go 50/50 on the heroics if you like ;-)