'65 Pro reverb transformer change question

[modusmongo]

Hi folks. This is my first post. I have a '65 Pro reverb that is sitting in my garage in France. It had power issues in the states and I brought it here hoping to fix it (I stopped gigging with it because it would suddenly loose volume). I'd like to swap out the transformer for a multi voltage one (so that I can select the voltage for whererever I happen to be), or at least, put a 220V transformer in it so that I can play it here in France. I don't want to use a step-down transformer.

I am good with a multimeter and soldering iron, and I have the schematic. What transformer do I need, where can I buy it, and what should I look out for when making the swap?

Thanks!

[modusmongo]

OK, no bites ;-)

I do not know why the amp was losing power (volume would drop 10+db in the middle of a gig), and I would like any advice on what to look for. The amp had all resistors, caps and diodes replaced about 15 years ago (it was pretty much completely rebuilt) and probably has not had more than 200 hours of use since then. I am now in france, so I have two problems: 1) I'm using an external 300watt stepdown transformer to power it. is 300w enough? 2) It makes no sound now, although the tubes do seem to warm up.

As I said before, I am good with a soldering iron, and multimeter. I can also follow directions. Where should I start??

Thanks for any help.

[txstrat]

300w is enough for the amp. It needs roughly no more than 150w (va).
Have you ever worked in an open amp running under high voltage? If not, you might leave the work to an experienced technician. Those voltages CAN EASILY KILL YOU.
If you still want to check it on your own, start from the outside. Pull the power tubes and test the voltage readings in the tubes sockets. Pin 3 has to have around 440vdc. Pin 4 slightly less. If you don't know which pin is what: the little nose is in between pins 1 and 8 (for an octal power tube socket). Pin 1 is left of the nose. All other numbers follow counterclock wise.
Check the voltage between pins 2 and 7 (heaters). Report back, when you have the readings.

For the dropping in volume I wouldn't think it's a transformer fault in the first place.
First you should try different (known good) preamp tubes. If you don't have new tubes you might interchange them with each other and see what happens.
Also the filter caps could be too old (people say they last about 15 years and that's what they are).
For more possibilities look here: Low Power or Loss of Volume

[modusmongo]

Hi txstrat. It took me the winter to get this project going. I checked the voltage at pins 3 & 4: 449vdc and 447 respectively. The heaters have NO voltage although the preamp tubes are glowing slightly. What next? ;-)

[MWJB]

Heaters are AC, they will show half the voltage if read from pin 2 or pin 7 to ground. Your preamps wouldn't be glowing without the heaters.

Does it make sound?

[modusmongo]

Heaters are AC, they will show half the voltage if read from pin 2 or pin 7 to ground. Your preamps wouldn't be glowing without the heaters.

Does it make sound?

Hi. Thanks for chiming in! 2.9 vac on each side of the heaters (2 & 7). Interesting, since I plugged it in today (first time since last fall) the voltage at pins 3 & 4 has dropped to 420vdc from over 450. I swapped the 12ax7s and it does make some sound! very little, but sound. I don't have any 12aT7's. Can I put a 12ax7 in there?

What should I look at next?

[MWJB]

Check dc voltage at pin 5 of the 6L6s.

Check plate current in mA.

Check AC voltage at pins 4 & 6 of the rectifier. Check dc at pin 8.

You can put a 12AX7 in any preamp slot in a BF/SF Fender...whether it's what sounds best in there is another matter.

[modusmongo]

-45 vdc at pin 5 of the output tubes (they're marked "5881" is that the same thing as a 6L6?). 330 vac at both pins 4 & 6 of the rectifier tube; 427 vdc at pin 8.

How do I measure the plate current?

Thanks fopr your help!

[MWJB]

"How do I measure the plate current?" Either with bias probes, or with 1ohm 2W resistors inserted between pin 8 of each tube socket & ground (measure dc millivolts at pin 8). When you get some probes or 1ohm resistors, set the plate current to 30-35mA per tube.

Your bias is on the hot side (too little voltage, much current). Until you measure the plate current, turn the bias pot to give maximum negative voltage at pin 5...if this makes the amp sound thin & harsh, reduce the negative voltage slightly. I'd like to see -50v there at least (voltages at pins 3 & 4 will rise slightly).

Nowadays there is no specific difference between a 5881 and a 6L6. Whatever difference there ever was does not really apply to your amp & the conditions that the tubes need to live in.

[modusmongo]

Okay, I got the voltage up to -50vdc at pin 5. The voltage also went up a bit on pins 3 & 4 - around 435vdc. The amp is still very quiet. Even on 10, I can bearly hear it. I hear my fingers on the strings better than I hear the amp. As for the plate current, I need to read up on that. Can you suggest a good place to start learning? Thanks.

[MWJB]

OK, plate current does not seem to be your primary problem. For now just establish a way to measure it & set it as described.

Is the sound low in both channels? Do the Pro speakers sound OK with another amp, what does the pro sound like through another cab? Attach an AC RMS voltmeter accross the speaker terminals & play, reading will jump around, determine an average & report back. If you use the Normal channel, you just need to fit V1 & V6, work through all your 12A#7 tubes & see if the situation remains the same.

Record dc voltages for 12A#7 tubes (V1 to V6) pins 3, 8, 1 & 6.

Amp unplugged from the wall, measure DC resistance from each power tube pin 3 to the red wire that meets the circuit board (comes through the chassis) at bottom left corner.

Provide a hoto of yuor PT wiring (though you have heaters, necative bias & B+ voltages, all in reasonable proportions, so at a cursory glance all looks OK here).

[modusmongo]

I don't remember ever having sound in channel 2. Something about a mod, but that's 20 years ago. The amp no longer has its cabinet. I've tried two different speaker cabs with the same result. Right now, it doesn't make any noise at all.

I've swapped out every tube except the rectifier with no change. The only thing I can report is that the 12ax7 next to the output 5881 makes noise when I jiggle it, the rest are silent; and that when I accidentally touched two pins at the same time, the amp made a good old fashioned "plugging in the guitar" sound - LOUD! The volume pot was making some noise in the speaker, but now I'm hearing nothing.

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

Here are some pics from left to right from the rear. Please tell me which red wire to make the resistance tests with pin 3 of the 5881's

[MWJB]

I thought you said you had a new power transformer?

What's the pot coming off the reverb channel?

I'd guess that the 4.7K 1W resistor in the cap pan, between the PI & preamp caps is open, replace this and the 1K 1W with 3W metal oxides.

V1 is nearest the Normal channel inputs, V6 is next to the power tubes, you follow the signal.

[modusmongo]

I thought you said you had a new power transformer?

Nope, I started this thread because I was thinking that I should replace the transformer because of power problems AND the fact that I live in France where the voltage is different. Thanks to txstrat, I am going to continue using the power inverter that I have, and find out what the real problem is with this amp.

What's the pot coming off the reverb channel?

That is a "dwell" mod for the reverb. Quite nice.

I'd guess that the 4.7K 1W resistor in the cap pan, between the PI & preamp caps is open, replace this and the 1K 1W with 3W metal oxides.

LOL. You're talking to a musician, not a technician ;-) What do these two items look like (a google image search has left my ignorance untarnished)? I see several doohickies on the "circuit board" to the left (from the back) which say ".047". Two are blue, and one is black, they're big cylyndrical jobbers.

[MWJB]

Unplug from the wall, remove the chassis. Turn the chassis over so that you can see the transformers, PT to the left, choke to it's right, then the OT halfway down the chassis, next you have a square tin held down by 4 screws. Before you remove the screws, flick the standby into "play" mode to drain your caps. When you remove the cap pan lid you will see 5 big caps. At the bottom of the far right cap there is a yel/violet/red resistor bridging the cap to it's neighbour on the left, then from that cap to it's neighbour is a brown/blk/red resistor. These are the ones you want to change. If the caps themselves haven't been changed in the last 20 yrs, then these could benefit from being replaced too.