Filter Cap replacement in Fender amp

Ray Anderson · Post by **Ray Anderson** » 13 Aug 2013 3:42 am

I recently ordered the Caps for this amp , there were 5 of them. 2 of one value and 3 of another. The originals were Astron minimites and were qiute large in physical structure with what appeared to be a wax type coating on a paper/ cardboard cylinder. The Caps I ordered were F&T of the same values or as close as I could get them and when they arrived I was surprised at the size of them, they were about half the bulk in size as the older ones. Will this be acceptable or not for use as replacements or do I need to search further? This amp is a Bandmaster and circa '66/67. Thanks in advance

Eric Philippsen · Post by **Eric Philippsen** » 13 Aug 2013 4:52 am

There will be others who will chime in here and many are very knowledgeable about this subject. That being said, there will be those who will say that "the larger the cap the better."

It has been my experience that the brand and age of replacement caps are most important. (It goes without saying that proper installation is critical). Some brands just don't cut it, period.

I have used F&T caps and have enjoyed the results.

Ken Fox · Post by **Ken Fox** » 13 Aug 2013 5:23 am

Newer caps are much smaller and far more efficient. If you want the larger size get the Sprague Atom. It has been said on the Fender Forum that when opened up the Sprague atoms had a smaller cap inside!!

F&T are awesome caps for sure.

Ken Fox · Post by **Ken Fox** » 13 Aug 2013 5:28 am

If you order 5 then you only got the filters under the cap can. You should go ahead and replace the bias cap on the bias board and the electrotytic bypass caps (25uf at 25V) on the main board. As the cathodes only have a few volts at the most on them the 25 volt caps are fine. I use 50 volt there as they are physically closer in size to the old 25uf caps.

Tim Marcus · Post by **Tim Marcus** » 13 Aug 2013 7:01 am

F+T caps are the way to go! They have totally changed the way I design amps because of their small size. We can do things now that would never have been possible in the 60's

when you install them you are going to notice a HUGE difference in the feel of the amp!

Chris Boyd · Post by **Chris Boyd** » 13 Aug 2013 7:12 am

Will changing electrolytic caps change anything tonally in the amp? I realize they convert ac-dc and need changing due to leakage/age...Thanks...

Ken Fox · Post by **Ken Fox** » 13 Aug 2013 7:24 am

Perhaps with the cathode bypass caps there could be an audible difference? However they are for filtering the ripple out of the DC supply as well as preventing inter-stage coupling in the preamp section(as well as filtering).

I have been using the F&T caps for a a few years and really think they are top of the heap.

Chris Boyd · Post by **Chris Boyd** » 13 Aug 2013 7:31 am

Thanks Ken...

Tim Marcus · Post by **Tim Marcus** » 13 Aug 2013 10:18 am

Chris Boyd wrote:Will changing electrolytic caps change anything tonally in the amp? I realize they convert ac-dc and need changing due to leakage/age...Thanks...

Yes

In a tube amp everything affects the tone - that's the beauty of them: simplicity

Chris Boyd · Post by **Chris Boyd** » 13 Aug 2013 1:04 pm

Thanks Tim..

Ken Fox · Post by **Ken Fox** » 13 Aug 2013 5:55 pm

I have found that the majority of the older cathode bypass caps drift from their 25uf rating to as high as 150uf. That allows more bass to pass thru the audio chain. A fresh set of cathode bypass caps can definitely tighten up an old Fender amp's sound. As the filtering caps are primarily for filtering the supply they have little effect on tone, when in good condition. As they develop a higher ESR they will drag down the B+ voltages in the amp and have a definite effect on amp performance and tone. If the cathode caps go open they will produce a thin tone in the affected gain stage. If a filtering cap goes open in the B+ it will result in rippled DC and hum will be present in the audio output. A high ESR with a bias cap will drag down the voltage their as well, not a good thing.

Rich Hlaves · Post by **Rich Hlaves** » 14 Aug 2013 2:38 pm

To go just a little further on what Ken and Tim said.

Changing those cathode bypass caps given their age should restore a good deal of "Spank" to your Bandmaster. That feel of having dynamic control over your playing right from your fingertips. While doing those I'd meter all the bypass resistors and change anything that is out of spec. I typically just change them out if I'm in there. Grid resistors fail over time as well and cause hiss, pops and crackle. Not a bad idea just to swap 'em and be done with it specially if you are going to gig this amp.

I'm an F & T convert as well. I started using them in amps when I started to see bad Sprague Atoms ordered brand new and some with very short service life. I've never gotten a defective F & T.

I still use Sprague for bypass & bias however. My issues were all with high voltage electrolytics.

Tim Marcus · Post by **Tim Marcus** » 14 Aug 2013 2:42 pm

I have seen plenty of defective 25/25 sprague caps as well. I have been using them for years but every once in a while you get one that fails within the first 100 hours.

Ray Anderson · Post by **Ray Anderson** » 14 Aug 2013 3:58 pm

Thanks a heap guys, I'm going to order the rest of my list, now that I know that physical size is not a hurdle. I plan to replace some resistors as well on the tube sockets 470s I think is the values. Should I go with carbon or metal film ( I think that was the 2 choices). As always plenty of helpful knowledge from a lot of unselfish folks.

Sam Rothwell · Post by **Sam Rothwell** » 14 Aug 2013 5:49 pm

I Echo the good sentiments about F&T capacitors. I've also been repairing amps for very many years. I much prefer F&T over anything else. They, F&T also make a 22 uf X 25 V X2 (2 caps in one can, axial lead) that will work on the preamp board of most fenders. The older blackface amps used the two caps in one can quite a bit and these are a direct replacement. Warning due here too though. There are lethal voltages inside all tube amps. Be Careful!!!!

Tony Prior · Post by **Tony Prior** » 15 Aug 2013 1:46 am

Ken Fox wrote:Newer caps are much smaller and far more efficient. If you want the larger size get the Sprague Atom. It has been said on the Fender Forum that when opened up the Sprague atoms had a smaller cap inside!!

F&T are awesome caps for sure.

very true...

If I can find it , I have a photo around here somewhere of a cap that I pulled from a Fender amp, it was a Sprague, large blue..I cut it in half, inside was a capacitor element about 1/4 the size of the blue can !

AND

I would not recommend changing caps if you have never done it before and at minimum do not have a volt meter that can read +500 VDC . After the change you must check and confirm the HI DC voltage which can be as much as +465 in a Fender amp...commonly in the +435 range... you can't just change caps and go to a gig...after the change you need to also check and possibly reset the power tube BIAS voltage. Oh yeah, the bias cap should be changed at the same time as the big filters...

Changing caps is only part of the required maintenance for an older Fender amp...there are many other things that go along with this as part of the "tune-up". Quality techs like Ken and Tim will look at many other things while changing caps..they will confirm proper circuit functions in many area's...

It's never about the size of the cap, "the bigger the better" it's always about the capacitance value and the rated DC voltage rating.

Tony Prior · Post by **Tony Prior** » 15 Aug 2013 1:51 am

oh by the way, Caps do not convert AC to DC, the rectifier does that..tube or solid state....the Caps filter the raw DC from the rectifier....

Very HI DC voltages live inside these amps...treat them with total respect and caution...

Rich Hlaves · Post by **Rich Hlaves** » 15 Aug 2013 7:40 am

The mention of changing the 470s on the tube sockets is a good thing. I forgot about those. Metal Oxide is great in that application. It wouldn't hurt to change the 1K5 resistors on the power tube sockets as well.

You might want to swap out and upgrade the dropping resistors on the cap board too. 1 watt metal oxide and I think the values are 4K7 and 1K.

Tim Marcus · Post by **Tim Marcus** » 15 Aug 2013 8:17 am

and here is where the argument begins...

I do not like metal oxide anywhere in a tube amp, except maybe the 470 1W in the bias circuit. They do not react the same as carbon comp under heat and stress - thats part of the magic!

I will use a carbon metal film in a few spots that are prone to noise, but keep it original. Chances are values have drifted a little but that shouldn't matter. As long as they are within 10% the amp will still sound good.

If it were me doing this work, I would keep the amp as original as possible but replace old electrolytic capacitors. I might put in new interstage coupling caps to get some of the snap back, but keep those old Carbon Comp resistors in there! Unless they are behaving bad, they will sound better than any modern equivalent