Ampeg R12-R Reverberocket power transformer replacement

[Enzo]

The flip tops one has wire leads.

[beatnik]

the flip tops one would be perfect but unfortunately shipping to the UK is too expensive for just one resistor

i am giving some attention to the power supply capacitors

the 6V6 version schematic shows an additional 20uF on node D. This cap is not present in the 7591 versions. I have attached the schematics for reference

Is there a reason behind this change or Ampeg just wanted to save some money on capacitors ?

I am planning to replace the capacitors with the following values

node A : 20uF
node B : 40uF
node C : 10uF
node E : 20uF
node F : 20uF

Do you think it would be better adding the additional capacitor on node D or it's not really necessary ?

[g1]

You must also consider the change to the resistor after that node. It's not just the cap that was changed.

[beatnik]

you are right, on the 6V6 version that resistor is 10K , on the 7591 version with tube rectifier it's 47K instead

i would expect more voltage drop with the 47K resistor, but I don't understand why the missing capacitor

[Tom Phillips]

...the power tubes cathode resistor looks a bit worn out...

My comment is that the resistor doesn't look "worn out." We don't always go by the look. I suggest that you measure the resistance as previously suggested and replace only if it is out of tolerance. The sandbox housing is not the active part of the resistor so it doesn't matter if it looks bad. (Yours does not look burnt up) Inside is a wire wound resistor which will continue to give good service if it tests OK and the solder connections are good. The 10W resistor actually looks to be in better shape than just about every other part in the photo you posted.

[Enzo]

It might help if you put your location as UK in your profile. It impacts the advice we deliver.

[g1]

you are right, on the 6V6 version that resistor is 10K , on the 7591 version with tube rectifier it's 47K instead

i would expect more voltage drop with the 47K resistor, but I don't understand why the missing capacitor

The caps at point A and B are also smaller in the version with the cap at point D. That could be a factor.
The circuits fed by point E are also different. Everything is inter-related. The capacitor is just part of several changes, you can't just separate it from everything else.

[beatnik]

OK I see. I thought it's recommended to have a filter capacitor on every power supply node.

This is just my second amp restoration project. I am learning a lot from your advice

So would you confirm the cap choice list on my previous post is fine ?

Basically the only change compared to the original schematic is the first cap after the rectifier is 20uF instead of 60uF, but standing to the 5Y3 datasheet this is the recommended maximum capacitance value

[g1]

I think your choices are fine, but you may notice the difference going all the way down from 60 to 20uf at the first node.
The data sheet I have lists 32uf as the max. capacitance for 5Y3. I've seen many examples using 40uf with no major issues.
The 60 they used was probably ok with the tubes of years gone by, but I wouldn't trust a modern production tube into that.
I think you could go at least to a common 33uf for your first node, but your choice of 20uf (or common 22) should be fine.

[beatnik]

Thank you.

I have found a JJ multisection cap that would do the job for a decent price and it's 20+40+20+20uF

[Justin Thomas]

The more the CE can caps prices go up, the more JJs I buy... the only thing I don't like about the JJs is that the diameter is so much larger than the old Mallorys. But, still not worth 3x the price - I don't generally get to work on stuff that needs that level of restoration!

Justin

Edit: then again, they fit nicely inside the chassis, too.

[olddawg]

The more the CE can caps prices go up, the more JJs I buy... the only thing I don't like about the JJs is that the diameter is so much larger than the old Mallorys. But, still not worth 3x the price - I don't generally get to work on stuff that needs that level of restoration!

Justin

Edit: then again, they fit nicely inside the chassis, too.

Or you can hollow out the old cap can and stuff it with modern capacitors that are much smaller than ancient ones. I've done that a few times.

[Justin Thomas]

Well, I could, but I'm not sure I trust my handiwork that much! Have you seen my amp builds? :P
But, that's probably a worthwhile skill for all of us to practice at...

Justin

[olddawg]

Well, I could, but I'm not sure I trust my handiwork that much! Have you seen my amp builds? :P
But, that's probably a worthwhile skill for all of us to practice at...

Justin

It isn't that tough. I usually use a Dremel tool with a grinding wheel. But it can be done easily with a pair of dikes and a file. Just make sure you save the bottom plate and take your time. I just make the appropriate connections to the appropriate components, wrap it with electrical tape (and sometimes a cardboard insulator), and put it back together with 5 minute epoxy. Then let it set with a clamp or rubber band or something over night. I'm not impressed with modern can caps that are available.

[Ted]

Farnell is my first choice for components in the UK. They are a serious component supplier and don't charge premium prices just because something is going in an old amp.
The have a minimum order price of £20.

Sometimes specialist stuff for Ampegs has to come from Fliptops, but for most parts there is are alternatives if you use a little imagination. For multi-section can caps I use Hotrox who supply both F&T and JJ parts.

I am not particularly hung-up on using original form-factor electolytics. Modern made-by-the-million high temperature caps are reliable and cheap

Wirewound resistors go on forever. They are designed to get hot. They can look a little tatty after 50 years. But if it works, it works.