View Full Version : 500c Power Supply Capacitance Increase - How Much?


MoreCowbell
03-19-2013, 05:50 PM
I'm refurbishing a newly acquired 500c to include rebuilding the power supply. I have spent the past couple of days on AK and cannot find much info about the power supply capacitance like I do with almost every other tube amp. I did see one person suggest increasing the three 40uf sections of the filter cap to 47uf but that's it.

When I purchase new caps for the power supply filter, what values should they be? Stock? Increased to xxuf?

Thank you,


-Paul

larryderouin
03-19-2013, 06:33 PM
Leave them as stock as possible. Due to changes in 1970-71 for capacitor values and variances built in, going from a 40 to 47 is allowable. There is a Doubler circuit that involves the isolated cap and 1 section of another cap you can bump them up to 220uf with no problems.

If you don't want to mess with stuffing them, or having 47/450v caps bunched up all over the place, Hayseed Hamfest custom builds Can caps for the 800-C/500-C. I've rebuilt 2 in the last 2 months and am on my 3rd rebuild. The can caps drop right in and solder right up. Roughly $99.00 shipped for all 3. Note that these can caps are temp rated for 105*C vs 55 or 65*C that the JJ or other cans have. See the Buglegirl 800-C thread for pics on what they look like.

Larry

SoundOfSound
03-19-2013, 06:48 PM
:lurk:

MoreCowbell
03-19-2013, 10:50 PM
Larry,
I am familiar with your valuable contributions on this site and appreciate your help, but I am confused by your answer. You state that I should leave the power supply caps as stock as possible (40uf) but then go on to reference them at 47uf.

I will likely restuff the cans, so what values should the power supply caps be?

Just out of curiosity, what happened in 1970-1971 regarding capacitor values?

Thank you,


-Paul

larryderouin
03-20-2013, 12:19 AM
Your 40uf caps have a variance of probably at least +20%/-20% which means that you can go UP to 49.4uf (technically) and DOWN to 30.6UF at minimum. GOING DOWN IS BAD as the new cap will put you out of spec for insufficient filtering.

In the early 70's the industry redid the way they number values on caps over 1uf. Instead of multiples of 5 (ie:5-10-15-20-25-.....40-45-50 ad infinatum) they redid so it more correctly resembles the SUB 1uf values (ie: 22-27-33-39-47-56-68-100-220-330-ad infinatum). Less confusion. So the manufacturers make 22uf 27uf 33uf 47uf 56uf 68uf 100uf and on and on. The new values are tighter in spec usually no more than 20%. The old caps had variances of up to +90%/50% with +50%/-20% being a very common value. Can caps were held to a tighter 20/20.

So that's why i said go up to a 47uf. It's within the 20% of the original and doesn't hurt the unit. 47uf is actually the preferred replacement value. It's easy to get, whereas a 39uf is almost impossible (you'd have a better chance of finding a 20-year old virgin in California) to find in the voltage and size you need.

The 450 and 500V caps can be 450V. 500V caps are rare and the voltages inthe amp don't go that high even under initial surge. The 450 caps have a surge voltage of 500V anyway so they can handle it. I put them in my Sansui and Fisher units with a B+ of 430 and have had no problems. If you restuff, Nichicon makes a pencil type cap that'll fit nicely.


So you'd need 220uf/250V (2), 47uf /450V (6), and 22uf 350V (1) Going from 50 to 47 is less than 1% so that's fine. Doubt you'll find 56uf in 250V or 350V. That's why I said 47uf 450V (6)



Larry

pdm4606
03-20-2013, 12:49 AM
I recently re capped my amp.
The parts list says 4700mf +150%, -000.
I went slightly over as the power supply has a humungous trans. and
I feel it would handle the load.
My pick were 4 10,000mf caps.

Paul

bhamham
03-20-2013, 02:46 AM
There's different schools of opinion on capacitance size in vintage tube gear. The conventional, conservative approach is to keep them close to stock values - nothing wrong with that. Another approach is to increase them 2 to 3 times their original values - noting that capacitance following a tube rectifier should be equal or a bit less than allowed according to its tube data.

Some believe had Fisher and other manufacturers access to the 'modern' capacitors of today - smaller and more heat tolerant - they would have increased the values in their designs, but were limited to physical constraints and a cost imperative.

On several Fisher 400 rebuilds I've doubled the size of power supply capacitors. I think it offers smoother filtration and more stable operation. Many believe it deepens the bass too.

I wouldn't drop the rating from 500v to 450, there's no shortage of caps out there rated 500v. You'll likely be running close to 450v to the 7591 plates and the added 50v margin would be good insurance.

Just offering a different opinion to what is so commonly stated here.

larryderouin
03-20-2013, 05:01 AM
Bruce. That section of the cap (C87D) is working the screens for the 7591's which run @ 375(schematic and 375-390 real world. The Anode runs 430 which is off the Doubler caps. I haven't had a problem with 450V caps in 800-C or 500-Cs I've built with either individual caps or Hayseed Hamfest cans'. His can's are built with a 450VWV/500VSV (Working voltage/Surge Voltage) rating and I get the same basic caps he uses. Nichicon. With that section working @ no more than 390, theres 60V headroom. If it was running 410 or higher due to Wall voltages being higher yeah I'd put a 500V cap in it. Sprague ataoms are a royal pain in the arse to fit in these things. Like taking a Full stick of Dynamite and cramming it into a pack of fags......Cigarettes for us COLONIALS! But even with 125 at the wall I get at most 390V on the screens.

the plates are run off the Doubler caps (200uf/250V x2) for a 500V rating. And they are running 430-440V depending on wall voltage. So 60-70V headroom is sufficient.

I've seen designs where a Filter cap and other caps in circuits were only 1-2 volts higher than the working voltage with absolutely NO headroom for surges. And they ran fine. Once I found it out, I put in somewhat higher rated caps for 50V headroom. But the receiver had already run 25 years like that with no problems. Scary.


Larry

bhamham
03-20-2013, 05:16 AM
Point taken Larry.

Just me, but I'd rather use a 500v rated cap as wall voltages are higher and surges do happen. That's one area where the Fisher engineers were right to rate that cap 500v IMO. You don't have to go to Sprague atoms - there's plenty of options. I'm not so fussy with PS caps - even get 500v ones from Just Radios and never had any problems.

Chappy
03-20-2013, 05:59 AM
Before you go doubling the capacitance in the power supply you may want to read Mr Gillespie's article "Techniques to Maximizing Power Tube Life".

http://www.tronola.com/html/maximize_tube_life.html

Not an issue if you are using new tubes and don't mind wearing them out fast, but if you have irreplaceable vintage power tubes that extra capacitance can be very damaging during power on and power off cycles.

Chappy

bhamham
03-20-2013, 06:44 AM
Before you go doubling the capacitance in the power supply you may want to read Mr Gillespie's article "Techniques to Maximizing Power Tube Life".

http://www.tronola.com/html/maximize_tube_life.html

Not an issue if you are using new tubes and don't mind wearing them out fast, but if you have irreplaceable vintage power tubes that extra capacitance can be very damaging during power on and power off cycles.

Chappy


Here's another of Dave's articles advocating 'tripling' the capacitance for improved performance for the downstream after the first cap of the B+ supply in an Eico HF-85. Might be 'apples to oranges' comparing a preamp to an amp but power supplies are power supplies.

http://www.diyaudio.com/forums/tubes-valves/96203-hf-85-rebuild-advice-4.html#post2843269

sony6060
03-20-2013, 09:23 AM
I recapped my Fisher 400. I kept close to the original can capacitance except the voltage doubler. I used 2x 680uF in the doubler to replace the 100uF & 200uf that produces about 20 volts of ripple. As a bonus to better filtering I also installed a 1.5HY @ 225ma choke and a 10uF poly type cap (LC section) after the 680uF series caps. I believe the Fisher design group would have much enjoyed to have modern capacitors available back then. I view the unbalanced stock doubler as a compromise even with the 200uF cap feeding 1/2 wave into the one can cap to offset some of that imbalance.

audiodon
03-20-2013, 10:26 AM
I've always stayed stock, not for any philosophical reasons or technical know-better, but because I didn't know any better and using the stock values presented no problems. Just refreshing the power supply caps does wonders to these units.

I did increase the voltage doubler caps in two recent X-101-C upgrades and did notice some increased bass, which would lead me to believe that the power supply has somewhat more reserve.

With sony6060's recent power supply thread, I've started examining best practice on this issue again.
:lurk:

Chappy
03-20-2013, 03:04 PM
Here's another of Dave's articles advocating 'tripling' the capacitance for improved performance for the downstream after the first cap of the B+ supply in an Eico HF-85. Might be 'apples to oranges' comparing a preamp to an amp but power supplies are power supplies.

http://www.diyaudio.com/forums/tubes-valves/96203-hf-85-rebuild-advice-4.html#post2843269

It is apples to oranges. I had to read his article several times to fully absorb it all. It really is a good read and he backs up his argument with years worth of testing and analysis. I`ll be putting a scope on my 500C at sometime in the future to determine if it has these harmful pulses during startup and shutdown.

Chappy

MoreCowbell
03-20-2013, 11:19 PM
OK. I'm going to restuff the power supply cap can with 47/47/47uf @ 500/450/350WV. I usually get my power supply caps from JustRadios.com but they don't list a 47uf @ 500WV. AES does so I'll use them this time.

Larry et al,
Thank you for your help and for explaining the capacitor changes that occurred in the 70's.


-Paul