Electrolytic capacitor question.

[Mike_J]

I built a 5f6a bassman about twenty years ago and have made many other amps since that time. I don’t play them much. In the case of the 5f6a amp I am somewhat concerned about the possible condition of the electrolytic caps due to lack of playing time in addition to the age of the caps.
I don’t want to replace the caps unless it is absolutely necessary. I read somewhere that caps can be reformed if all tubes are removed, a solid state plug is put in the rectifier tube socket and a 100K – 3 watt resistor is installed in series from the standby switch to the first filter caps. Clip a voltmeter across the 100K resistor and turn the amp on. Supposedly this limits the current flowing into the caps and reforms them eventually. Apparently when the voltmeter reading drops to between 20 to 30 VDC.
Has anyone tried this method? I got it from an old geofex article I printed off about the same time I built the amp. Does anyone see any reason I shouldn't try the amp before doing any steps to reform the caps?
 

[vampwizzard]

Ive found a couple of articles describing that method so it seems legit. Do you have an ESR meter handy? or a meter that can measure capacitance? Id probably rather just replace them for 20 bucks in parts than roll the dice.

[pdf64]

I remember reading that the modern ecap production process is different to that used a few decades ago. Such that they don't need forming and don't deform, at least to anything like the same degree. But when they're knackered, that's it, reforming won't help.
It might have been RG Keen that wrote it 

[Mike_J]

Ive found a couple of articles describing that method so it seems legit. Do you have an ESR meter handy? or a meter that can measure capacitance? Id probably rather just replace them for 20 bucks in parts than roll the dice.

Thank you for your response. I have a rather inexpensive multimeter that only measures capacitance to the nf range which is nowhere near the 20uf or greater rating I would need for these electrolytic caps. I have no idea how I would measure ESR. I will probably fire the amp up tomorrow and see how it sounds and if there is an unreasonable level of hum. It was quiet the last time I used it so I will replace the electrolytics if it hums now. It was my first amp build and I never was satisfied with the tone that came out of it. I bought a 59 bassman LTD maybe 10 years ago which sounded good in the guitar store to do a comparison with this amp. They pretty much sound identical as they should using the same value components. Still don't know what I expected out of this amp. Have built some amps I really like the tone of and others that are just okay. I have never accepted just okay well. I know there are some original 59 bassman amps that had great tone like you get right before your amp starts smoking because your tubes are biased way too hot. Haven't figured out how to accomplish that safely yet as far as this amp is concerned. Going to try again.

[vampwizzard]

I built one for a guy out in nashville.. outlaw country guy and he loved it. Something about them really cranked does the trick. Cranked bassman and a tele is 70s springsteen all day.

Which transformers did you go with? Have you played other amps that you really like?

edit: Theres also a ton of ways to modify it per Rob.. https://robrobinette.com/5F6A_Modifications.htm

[Mike_J]

I remember reading that the modern ecap production process is different to that used a few decades ago. Such that they don't need forming and don't deform, at least to anything like the same degree. But when they're knackered, that's it, reforming won't help.

Thank you for responding.  Is knackered a technical term that means something pretty bad?

[2deaf]

It might have been RG Keen that wrote it

RG Keen comes on other forums as a false prophet that flat makes sh stuff up thinking that everybody there is too ignorant to know.  He has taken several serious spankings from me because I frown on that sort of thing.  He has displayed an astounding lack of knowledge about electrolytic capacitors, so you might want to take what he says with a grain of salt.   

[Mike_J]

I built one for a guy out in nashville.. outlaw country guy and he loved it. Something about them really cranked does the trick. Cranked bassman and a tele is 70s springsteen all day.

Which transformers did you go with? Have you played other amps that you really like?

I am pretty sure you are on to something here. A friend of mine came over and played the amp through his pedal board and it sounded great but at a lot higher volume than I would play it at. The PT is a Fender part for a tweed bassman. The voltages are per the schematic. The output transformer is the latest version Magnetic Components put out for the tweed bassman. Mark said it is interleaved like the originals. Don't know about the iron versus the originals. If I remember correctly he even used a zinc coated bell cover as per the originals because he couldn't think of anything else he could do to make it any more original. The amp I made that I like the best is based on the Dumble Steel String Singer. What I like about it is it has that Texas tone you hear in the introduction to Leave my Little Girl Alone. I also made a Dumble HRM that does a better than Marshall Marshall sound using EL34s. Those are the amps I like best. Like the tweed deluxe I made as well. It has a three way switch that allows for different V! preamp tube biasing. Good for single coils and humbuckers. 

[vampwizzard]

I dont think the materials used in the transformers matter as much as the values/ratings (former transformer designer here).. so if theyre the same theyre the same.

Listening to In Step, they compressed the hell out of the bassmans. If you can, give Springsteens first live album a listen. Single coils, cranked bassmans. The sound is very similar but you'll probably need a little help to get yours to tighten up. Probably something like Robs "Voicing a lead channel" mod. If you want to go all out, you can do the dumble manzamp conversion to el34's.. Frenchie started a build on that this month of memory serves. https://www.youtube.com/watch?v=cc7l5an99xE

[PRR]

Thank you for responding.  Is knackered a technical term that means something pretty bad?

"A knacker ..or knacker man is a job title used for the centuries-old trade of persons responsible in a certain district for the removal and clearing of animal carcasses (dead, dying, injured) from private farms or public highways"
"'Knackered' meaning tired, exhausted or broken in British and Irish slang is commonly used in Australia, Ireland, Newfoundland, New Zealand, and the United Kingdom."
https://en.wikipedia.org/wiki/Knacker

[glass54]

Correct pdf64 and PRR.
In Aussie, after a physically hard day, one gets "knackered" But after a a good evening's rest complete with a beer  and a good meal, they are fit and ready to do it again the next morning.
However, when it comes to Electro Caps, once knackered, they are history ie File 13 (Bin, Waste Basket or Trash can  )
(Occasionally I get customers who want to keep them ie old electros as their originals  )
Kind regards
Mirek

[pdf64]

It might have been RG Keen that wrote it

RG Keen comes on other forums as a false prophet that flat makes sh stuff up thinking that everybody there is too ignorant to know.  He has taken several serious spankings from me because I frown on that sort of thing.  He has displayed an astounding lack of knowledge about electrolytic capacitors, so you might want to take what he says with a grain of salt.   

My experience is that he's a pretty reliable source, such that I had to double take your post to be sure you weren't being ironic(?). I'd be grateful if you could point me to somewhere he's made stuff up, so I can reassess.
The only 'problematic' threads I can recall RG Keen being embroiled in involved MEF users Mooreamps or soundguruman

[Mike_J]

Opened the doghouse for the electrolytics to see what was inside. Two Mallory TC69s (100uF @ 350VDC) and two Mallory TC75s (20uF @ 450VDC). All said made in the USA. Was curious because it was probably the late 90s when I put them in the amp. Probably bought them from some electronics store in Houston where I was living at the time.


Looked on ebay to see if anyone is still selling these caps. Somebody is trying to sell the TC75 at $50 each. The TC69s are only going for $12.50 each. Needless to say I scrambled to the capacitor bin to see if I had more of these but sadly only bought extras of the 8uF @ 450VDC. Doubt that I paid much more than $1 each for these caps. Absolute insanity that someone is asking $50 for a capacitor that has been sitting on a shelf for over 20 years.

[SILVERGUN]

https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Capacitors&ORDER_ID=!ORDERID!


It's a safe bet to switch to the FT brand. You'll see a 22uf/500V and a 100uf/350 on that page.

[Mike_J]

I have used FTs in most of my other builds without complications or complaints. Thank you for your comment.

[sluckey]

Yes, dump the Spragues and switch to F&Ts. Much smaller and cost effective. F&Ts are also excellent quality.

[Mike_J]

Remember now why I did the e caps using the blackfaced amp method instead of the four 20uF method called for in the schematic. Purchased some Sprague atoms 20uF @ 600VDC to do the job. Unfortunately they were huge. No way to get those under the doghouse. Running two 100uF in series with 220K - 1W resistors across them at the first filter stage provided 700VDC of protection instead of 450VDC which the smaller resistors would have provided. This is how Fender did it on the AB763 amps which used GZ34 rectifier tubes. Additionally, 50uF at the first stage versus the 40uF per the schematic provides a little more filtering. Anybody think that difference is enough to impact tone materially in this amp? The 40 watt tweed versus the super reverb sound quite a bit different. I know the tone stack arrangement has a lot to do with it but the filtering is different as well.

[Mike_J]

Yes, dump the Spragues and switch to F&Ts. Much smaller and cost effective. F&Ts are also excellent quality.

The caps in the amp say Mallory on them. Were they the same as Sprague during that era? Not that it matters because they need to be replaced. Would you use 20uF 500VDC in parallel or the 100uF 350VDC in series for the first filter stage. Is 500 volts adequate for the first filter stage?

[Mike_J]

I think the question has been answered. Doug's schematic and layout for the 5f6a shows five 22uF 500VDC caps for the amp. I will order five of the caps. Leave the 8uF in the preamp section and see how it works. Can replace it with the 22 and see what the difference is. Thanks to all who replied.

[sluckey]

The caps in the amp say Mallory on them. Were they the same as Sprague during that era?

No. I mispoke. Sorry.

[2deaf]

I remember reading that the modern ecap production process is different to that used a few decades ago. Such that they don't need forming and don't deform, at least to anything like the same degree.

I don't know and that's why I didn't comment, but I do have an opinion about it.

They're calling the things "aluminum electrolytic capacitors", so they have a dielectric composed of aluminum oxide and some sort of electrolyte.  There are liquid and solid electrolyte constructions for aluminum ecaps, but we seem to use the liquid ones in our amps.  There is only so much aluminum oxide that can be deposited in vitro and the rest has to be formed by electrolytic oxidation when voltage is applied to the completed capacitor.  The production process employed to get to the point of initial voltage application doesn't seem relevant to me.

The problem with large time periods with no voltage application is that the aluminum oxide layer on the anode slowly reacts with the electrolyte.  This causes a decrease in the withstand voltage of the dielectric, a decrease in capacitance, and an increase in leakage current.  The dielectric can be re-formed by electrolytic oxidation when voltage is applied to the aged capacitor.  To me, these seem to be inescapable features of this type of capacitor and the production process again seems irrelevant.

Like I said, I don't know, but the way modern production ecap makers still recommend re-forming after long term storage reinforces my opinion.

[shooter]

fwiw;
I've re-formed half dozen 20yr old caps, IIRC used 150k @ caps working VDC, cooked them over night, AFTER I verified they weren't getting hot, a few did heat up, so I strapped up a power cord, wired to caps and plugged into 120vac from 50'.  blew a 20A breaker on one, blew cap on another 


 These are a couple 10,000uF and a couple 7,000uf used in an audio build

[Mike_J]

fwiw;
I've re-formed half dozen 20yr old caps, IIRC used 150k @ caps working VDC, cooked them over night, AFTER I verified they weren't getting hot, a few did heat up, so I strapped up a power cord, wired to caps and plugged into 120vac from 50'.  blew a 20A breaker on one, blew cap on another 


 These are a couple 10,000uF and a couple 7,000uf used in an audio build

Seems like reforming them would be best at their working VDC values. Don't think I would want to do that in any of the amps I own as it is my understanding that an electrolytic cap causes quite a mess when it explodes. I have an extra 100 watt Plexi PT lying around somewhere. Might be good to make a board that takes B+ from the PT run both wires through 1K diodes and then a 150K resistor and then through 3w dropping resistors corresponding to the amp the caps are being used in. Not advisable for just a few caps but if you are like me who makes amps and just throws them in an unused room and doesn't do anything with them then eventually something needs to be done if the amps are going to be used again. Thanks for the 150K suggestion.

[bmccowan]

I do not doubt the science on electrolytic capacitors. But there also seems to be somewhat of a mystery too. Back when they were $40 items I bought at least 20 Hammond organ amp chasses of various model numbers. The cap cans in those old amps all seem to be good. Doubting those findings, I have replaced some with F&Ts - no practical difference. Now we all know that most organs sit around for years in Grandmas house. I do start those amps up on a Variac, but within an hour I have them up to full voltage. That's not enough to re-form them, but I do it just to see if something is wrong.
I also have repaired dozens of old Gibson amps. Most had cardboard shell firecracker filter caps inside the chassis - typically right above a hot tube. They are always bad. So I think there is much more at play here than just the de-forming due to age or disuse.

[mresistor]

A lot of the late 70's early 80's filter caps Fender used, in the aluminum cases, read just fine, and sound just fine, look just fine..
Also if Allesandro and many here use F&T lytics, that's good enough for me.  I use them all the time.