Questions on modern radial 'lytics and PS resistors

[JWK]

I've got a SF Bassman that's been hacked and unfortunately has had a hole drilled through the face plate to add a handy-dandy master volume. This hacker (whoever that person was) was a real genius. Two 100 ohm balancing resistors were added to the heater winding, but the center tap was left attached to the chassis. A HA HA HA!!! Hysterical. Then there's me. Not much of a genius either, since I have to ask these questions.

Anyway, I want to try to replace the caps with modern radials. I don't know much about electrolytics and I've done a search but haven't come up with a decent understanding of what I need. I plan on typical values, for 47uF on the plates rather than the 35uF of typical vintage value. The rest will be 22uF. So I can stack two 100uF with balancing resistors to get 50uF, or use an array of four 47uF caps also with balancing resistors. Space will be an issue, but I can build a new board for the caps and resistors and forget about the dog house cover for that matter. However, from my measurements I have found that I could squeeze in stacked 250v caps under the cover to suit my purpose. Soooo....

1. 250v or 350v rated caps? Any good reason to go with the 350v caps over the 250v?

2. Ripple Current? I don't quite understand it, but I gather that the higher the better? Does it really matter when you're using two or four of these together (if I have this right, the ripple current capability increases when you combine the caps together)?

3. Temperature - just go with the 105C since the chassis will be operating upside down? I would guess that 85C caps would probably be fine in a head with the tubes on top facing up and lots of ventilation, yes? Info I need for future projects I have in mind.

4. What do I need for the balancing and dropping resistors? I see most people use metal oxide, but what power and voltage rating? I've done quite a search on this and have seen numerous sources that say things like, "...and watch your working voltage rating" but never say what that really is. Mouser has 2 and 3 watt metal oxide with a working voltage of 350 and 5 watt with a working voltage of 500. Fender used 1 watt resistors back in the day, but that doesn't mean it was a good idea, just inexpensive. But then, I don't know what the working voltage rating was on those resistors back then.

Thanks to anyone who can help me learn about these things.

[Enzo]

I have to ask, why on earth do you want to stick radials in there when appropriate value axials are available readily and at reasonable prices? The amp is physically laid out for axials. Axials are also readily available in 500v, while the radial I see in general supply catalogs usually top out at 450v.

But since electrically it doesn't matter, I will move on...

1. 250v or 350v rated caps? Any good reason to go with the 350v caps over the 250v?

What voltage will they serve? A 500v supply won't want 250v caps, 350v gives plenty of headroom.

2. Ripple Current? I don't quite understand it, but I gather that the higher the better? Does it really matter when you're using two or four of these together (if I have this right, the ripple current capability increases when you combine the caps together)?

it is of course important in power supply design, but in amp repair and rebuilding it is not something I have ever found the need to be concerned over.

3. Temperature - just go with the 105C since the chassis will be operating upside down? I would guess that 85C caps would probably be fine in a head with the tubes on top facing up and lots of ventilation, yes? Info I need for future projects I have in mind.

In general, in tube amps I prefer the 105C caps for heat reasons, but the plain old 85 degree caps they built a zillion Fender amps with have lasted decades too, even on amps with tubes hanging below. The cost difference is not much, so why not? If you are stocking up, 105 degree caps probably won't make any reliability difference at all in little transistor things like pedals.

4. What do I need for the balancing and dropping resistors? I see most people use metal oxide, but what power and voltage rating? I've done quite a search on this and have seen numerous sources that say things like, "...and watch your working voltage rating" but never say what that really is. Mouser has 2 and 3 watt metal oxide with a working voltage of 350 and 5 watt with a working voltage of 500. Fender used 1 watt resistors back in the day, but that doesn't mean it was a good idea, just inexpensive. But then, I don't know what the working voltage rating was on those resistors back then.

If you have voltage ratings on the resistors, they are parallel the caps, so just think like they were caps. 350v in a series pair means 700v, so...

Wattage? Just apply Ohm's Law. You have two equalizing resistors in a cap stack, so each sees half the voltage, so in a 500v supply, each has 250v across it. Seems to me I see 220k a lot, but let us use 250k so I can calculate in my head. 250v across 250k ohms means 1ma of current, unless I am thinking funny. But whatever you are using, figure out the dissipation then use resistors rated for two or three times that.

[pdf64]

Your plan seems bizarre to me!
There's no reason given why not to use the cap format that the amp was designed for.
Whatever, if you're intending to get rid of the dog house cover then adequate provision must be made to protect the user from high voltage terminals.

[JWK]

Thank you Enzo and pdf64. I tried replying twice and lost all my text twice because I can't figure out how to save my post after my log in has timed out. This time I saved it on leafpad to copy/paste. I'm not much of a computer geek.

So it's been many years since I've hung out here. Since your post I did another search to see what people are using for recaps and stumbled on some posts referencing Antique Electronics Supply. It's probably been 10 years since I've had a good look at their stock. I see now you can get caps for an AB165 for about $15 if you buy the "generic" and about $33 if you go F&T. So yeah, it would be pretty silly to spend the time and effort to make radials work in there. I don't think I even knew about F&T caps 10 years ago.

The information is still valuable to me. I actually had to look up Ohm's law to make sure I was doing it right. It's incredible how much I've forgotten. I'm going through all my KOC books and looking stuff up again. It's coming back fairly quickly. I didn't have a lot of hands on experience, so I suppose it's easier to forget stuff if you don't use it.

I'm on the right track now. After I get the Bassman up and running, I'll have to decide which of the four or five projects that have sitting in my basement for the past 10+ years to attack. Due to circumstances, I actually have some time on my hands.

Thanks for the jolt.

[Chuck H]

+1 to all above and JM2C...

Axials are getting a little less common with time. Fewer options and I've had some performance problems with one brand that use to be considered top shelf. There are a lot of very good radial caps available and a lot more options. Radial caps seem to be a better value for the better quality lines. Almost certainly because of the laws if supply and demand. For these three reasons I design with radials now. I also retrofit them into axial mount service occasionally but only because I have them on hand. WRT repairs and replacements just use axials for axial mount designs until good ones can't be had for a reasonable price anymore but I think it's a good idea to use radials for new designs or teardown mods and such.

[Silvertone Jockey]

Sometimes you just have to use whats available when it comes to filter caps. Re-capped a RF linear amp for a friend today. Went to the local electronics store, the kind with real parts, not cell phones and the latest i(couldn't care less) devices.

Anyway the shop has always been well stocked with the usual garden variety HV electrolytic's in your favorite flavors and packaging, but not so much today. I usually ask the counter guy for a few (fill in the blank) caps and he returns with a few top shelf stuff candidates to choose from post haste, saving me having to dig through all the lower shelf specimens to find what I need that reside on my side of the counter.

Not today, after waiting for him to return with the requested 100uf 450VDC parts and waiting.........and more waiting, we took matters into our own hands and started searching the less noble stock as well. Nothing but an odd this or a single radial that when axial is what we need. Ended up buying smaller value caps and had to wire twice as many up to get close to the as-built capacitance value the circuit called for. Went back to the counter and the guy was still digging in what used to be boxes of plenty, when we told him to forget it and ring us up instead.

Luckily modern caps are significantly smaller than the ones made 30 years ago, so twice as many fit in the same space with room to spare

[Enzo]

For a parts store to stock 450v caps these days is about the same as expecting them to stock parts for VCRs and cassette decks.

[Chuck H]

For a parts store to stock 450v caps these days is about the same as expecting them to stock parts for VCRs and cassette decks.

I was living in the silicon valley in (roughly) 1998 when Quement Electronics was in a continuing process of downsizing. I watched as things like 450V caps, previously available off the shelf, were replaced by multi conductor ribbon cables and power supply parts were replaced by power supply modules. By 1999 the store, which use to occupy about as much footprint as a grocery store, was reduced to an office sized room with catalogs on shelves and one clerk at a desk. They had rented out all other floor space to other businesses. Analogous to this was the Mouser catalog growing from about as thick as a typical magazine to the size of a phonebook. The four local Radio Shack stores quickly reduced to two and those changed from their old MO of catering to electronic DIY to something more like electronic appliance stores. And those last two were on shaky legs with mega stores like Fry's and GoodGuys moving in. I don't think any of them survived there. We still have one here on Whidbey though. It's across the street from the Country Store. A local "Feed and Fuel" type place where you can gas up your old pickup and buy chicken scratch in one stop.

Maybe that's why I moved here. It's changing just slow enough that I might keep up.

I haven't expected to see 450V caps on a shelf for fifteen years.

[Enzo]

Been close to 30 tears ago now, but one day I was in the local parts store, and I was purchasing some 600v mylars and a few other tube amp related things. I was rebuilding juke box amps at the time, though most of my work was then in the arcade industry. The counter guy asked, "What are you using THESE things for?" I told him I was working on some tube amps from juke boxes. He then asked me if I knew anyone looking for a job working on tube amps. He told me Steve was leaving as the repair guy at a local music store. I knew Steve, so I dropped in on him at work to see what was up. He was leaving the area, and their advertising had not come up with any good candidates. A brief chat, and he took me across the store to meet the manager, who offered me the job on the spot. This was a Friday, and he said, "Start Monday."

SO even with my years in live music, and a lifetime of tube amp and other electronics, had it not been for that parts counter guy wondering what I wanted those obsolete high voltage parts for, I might never have gotten into pro audio service as a profession.

[Silvertone Jockey]

For a parts store to stock 450v caps these days is about the same as expecting them to stock parts for VCRs and cassette decks.

This store actually has quite a few of them on both sides of the counter, just sold out of the size I was looking for. There are more than enough HAM's and CBer's in the area to justify stocking them

[Justin Thomas]

Sometimes I go to a Radio Shack in town (we have TWO!?!?). The clerk asks if they can help me find anything. I ask specifics, and half the time it's "maybe I can get it from our online store?" I decline because prices are a bit high. The other half the time it's the "what planet did YOU just step off of?" And the store in town that SHOULD carry "components," VA Electronic Components, said, we stopped carrying parts ten years ago. I can still get shrink wrap, zip ties, and any possible computer connector, though... so I do what I can to support local biz, but...

A few years ago I called around town asking every vaguely electronic store in town if they could order me an ocilloscope. "What's that? Never heard of it. And what's it for?"

Justin

[patlaw]

Parts are a major reason that I try to focus on repairing a few specialty items. Unless I want to go broke paying freight, it's going to take a week or more to get the parts I need. It's possible to stock a few regular parts, like standard value caps. I ordered parts from Mouser, Antique Radio, Jameco, and eBay on March 3. None has arrived yet. Hopefully SOMETHING will arrive today so that I can get a couple of units off my bench. The USPS website showed that they should have arrived by Thursday.

[nashvillebill]

For a part-time hack like me, I order from Mouser usually. Great selection, good prices, and usually delivery is 3 days or sometimes even only 2, shipping regular UPS. (They're in Dallas I think, so their recent bad weather may have impacted shipping) One important thing for me is that I'm pretty confident I'm getting genuine parts (not counterfeits that are sometimes on eBay) and also that they are reasonably recent. Who wants e-caps that have been sitting on somebody's shelf for 10 years?

Jameco has decent prices too, but nowhere near the selection of Mouser. AES has some of the specialty stuff that Mouser or Jameco doesn't carry but they can be a little pricier.

[patlaw]

Bill, I totally agree. Unfortunately, I went through my parts bins a couple of days ago and found a pack of 10 electrolytic caps from one of the suppliers mentioned above that have a 1993 date code. It's something that never occurred to me to check when I bought them a year ago. When the catalog says "Value", be careful.

[Chuck H]

Sometimes I go to a Radio Shack in town (we have TWO!?!?). The clerk asks if they can help me find anything. I ask specifics, and half the time it's "maybe I can get it from our online store?" I decline because prices are a bit high. The other half the time it's the "what planet did YOU just step off of?" And the store in town that SHOULD carry "components," VA Electronic Components, said, we stopped carrying parts ten years ago. I can still get shrink wrap, zip ties, and any possible computer connector, though... so I do what I can to support local biz, but...

A few years ago I called around town asking every vaguely electronic store in town if they could order me an ocilloscope. "What's that? Never heard of it. And what's it for?"

Justin

The Rat Shack here still has a few drawers devoted to components. I sort of know the inventory so I go in whenever I need something they may have. They have a tendency to NOT re order such parts once they run out. And I know why. Whenever I ask about some specific part I can't find, the kid behind the counter typically goes "deer in the headlights" on me. I don't expect they ever even open those drawers when placing stock orders.

There was an enormous electronics salvage warehouse in the industrial/business park area of Santa Clara, Ca. called Halted. I wonder if they're still there. I'll definitely look in on it if I get back there some time.

EDIT: Ha! Halted is still there using the name HSC. Looks like the same building I remember too. Good news for old timers in the area. There was a Fry's across the street from them last I was there. Wouldn't it be novel if the Fry's went belly up instead

EDIT2: Fry's is still there too. Funny, Fry's address is indicated as in Sunnyvale. What a difference "across the street" makes. No matter. Now anyone in the area knows where to go. I know we have a few members in that area.

P.S. I think I've been misrepresented by the arrangement of your sig Don't trouble about it though. I actually thought it was sort of funny.