Ampeg R12-R Reverberocket power transformer replacement

[beatnik]

Hi everybody

I have just bought a vintage Ampeg R12-R in need of repair. The first thing that need to be sorted out is the power transformer - h.t. secondary winding is open

Drop-in replacements are available in the U.S. from fliptops.net and Mercury as well, but I couldn't find anything here in the UK or EU and importing from the U.S. would be quite expensive

I have been looking at the Hammond catalog and although they don't list a replacement for the Ampeg R12-R there is one that suits a Tweed Deluxe that seems pretty close in ratings and it fits the chassis without drilling additional holes

https://www.hammfg.com/files/parts/pdf/290BEX.pdf

I am not sure wether or not this would be suitable for the Ampeg R12-R

The tube rectifier is a 5Y3 so 3A are OK

The other tubes are 3x 6SL7 at 300mA each, 1x 6SN7 at 600mA and 2x 7591 at 800mA each. This makes a total of 3.1A which is slightly above the transformer rating

Changing the 7591 output tubes to 6V6 would drop the required filament current to 2.4A but honestly I would love to keep the 7591 tubes if possible

The secondary h.t. i know nothing about, since it's open I cannot measure it, and I couldn't find any specifications online. I have tried emailing Mercury and see if they are kind enough to share that information, but I also thought it might be worth to ask for help here as well, hopefully there are some Ampeg experts around

[Leo_Gnardo]

That Hammond looks just about right for your Ampeg. If I'm not mistook, Mouser has a presence in the UK and they have a selection of Hammond iron in their catalog, that should save you a lot of shipping & customs cost. We're not much Mercury fans on MEF. Prices are sky high, plus you'll have to ship & pay duty - on the inflated price. OW!

[beatnik]

Hammond are easy to get here in the UK and they usually are my choice but in this case I was thinking the 6.3V rating might be ok for 6V6s but not so ok for the 7591s and despite the common trend seems to favour the 6V6 in this amp, I was really looking forward to trying out the 7591 version.

[Enzo]

Carefully note the wires locations. You plan to replace the transformer, so remove the old one. Now you are at the what is there to lose stage. Take off the end bells and see if PERHAPS the reason it is open is repairable. The wires that it is wound with have to join the insulated wires that exit the thing. That joint may have failed on your HV wires. Also, since the winding wire is small gauge, it may have broken, and one place that can happen is right at the terminal for the insulated wire. If it snapped there, you might be able to scrape off the enamel and solder a patch wire up to that terminal.

Seems worth a few moment effort.

[beatnik]

Thanks for the tip. I am definitely gonna try that.

However I must say the power resistors look like they got pounded quite a bit, and also the output tube sockets are a bit loose, electrolytic caps have never been replaced and now more than 50 years old. The guy who sold me the amp has given me the invoice / report from previous service and they reported a burnt area on the pcb and I can see they cleaned it next to the power resistors.

I am really afraid something must have taken the poor transformer down, but it will be worth trying to see if the wire is broken at the terminals

I have located a company in the UK that can rewind the original transformer at a decent cost. It's just slightly expensive than what mouser charges for the Hammond, but won't deprive the amp of the 7591 tubes

[beatnik]

Got the transformer back from the doctor. I have spent around 80GBP for the rewind, well cheaper than buying a new transformer

I have been thinking of adding a fuse on the ht secondary in order to protect the power transformer from similar failures

I am not entirely sure if a fuse between ht centre tap and ground would be safe enough for a tube rectifier circuit

If I am not mistaken I remember diode rectifier need two fuses, one each for each ht winding leg, is that the same for a valve rectifier ?

Since I don't know the exact rating of the power transformer, how can I figure out the correct fuse current rating ?

[beatnik]

As i dig deeper I am discovering new things about this amp

The 5Y3 tube that was in the amp when I received had one shorted section. This is probably what caused the power transformer to fail

My amp has 5Y3 tube rectification and 7591 power tubes. There are labels inside the cabinet with tube configuration and the schematic says it's version R-12R-A. This is basically a transition model between R-12R (6V6 and 5Y3 rectifier) and R-12R-B (7591 and diode rectifier)

I have noticed the can multi section capacitor is actually the one Ampeg used for the diode rectifier version. this means the first cap section after the rectifier tube is 60uF

From my understanding this would be 3x times the value recommended for a 5Y3 rectifier, which is 20uF. I suppose 60uF is not optimal and might have shortened tube life, hence the power transformer fault

It seems Ampeg got in the new batch of capacitors for the diode rectifier but still had leftover stock of 5Y3 rectifiers, and the R-12R-A was born.

My idea would be keeping the 5Y3 rectifier, so I am probably gonna replace the filter capacitors with 20uF, like the previous 5Y3 versions of this amp

[Enzo]

From my understanding this would be 3x times the value recommended for a 5Y3 rectifier, which is 20uF. I suppose 60uF is not optimal and might have shortened tube life, hence the power transformer fault

Over the recommendation for sure, however, the amp worked for what 40 years? I don't think we can say this for that re the cap size.

[g1]

You could add protection diodes to the rectifier tube. This will prevent your new transformer from damage if you get another shorted rectifier tube.

[beatnik]

yes the diodes would save the power transformer in case the rectifier is shorted. they probably also increase tube life ?

but me thinking that even with the diodes there are other possible failure modes that can damage the power transformer, by too much current draw. and the fuse would cover a lot of possible scenarios, at least as far as the pt is concerned.

i am still wondering wether a single fuse from ht ct to ground (like early Marshall JTM 45) would cover most faults or two separate fuses on each end of the ht winding would be safer

the next tricky part will be getting the right fuse rating

[beatnik]

the repaired power transformer has arrived and i have been doing some more servicing on the amp.

the power tubes cathode resistor looks a bit worn out. i am thinking it would be better to substitute it but I cannot find a 140 ohm resistor of at least 10W rating, closest values are 130 and 150 ohm, I am wondering if these could work without upsetting the bias of the power tubes ?

[Justin Thomas]

That's a 10% tolerance on that 140R, anyway... What's it measure? If it's bad, then I'd err on a little cooler, but either should work.

Justin

[Enzo]

Where did you look? My first stop is always Mouser, and I found these right away.

http://www.mouser.com/ProductDetail/...i1XBTa6fmkU%3d

Given the choice you offered, I would also pick the 150 ohm as it cools the tubes a little bit.

And don't think of it as upsetting the bias. Bias is just a more or less thing anyway. Consider that cathode bias has no adjustment, and anything from hot tubes to cool ones will get plugged in there, so right from the factory that resistor was just approximate.

[Jazz P Bass]

https://www.fliptops.net/catalog/p-1...power-resistor

[beatnik]

thanks for your replies

i think i will get a 150 ohm resistor with through hole leads, that's easier to fit on the circuit board than the chassis mounted one

stay tuned