BFSR Power Supply Capacitors

[Big Mike Simpson]

The original 5 Mallory caps in the 66 BFSR power supply were:
3 20mfd/500v
2 70mfd/350v



In one of my amps (65BFSR) that I recapped I used:
3 20mfd/500v Sprague Atom
2 100mfd/350v Sprague Atom

It was a couple of years ago and I don't remember the reasoning behind using the 100mfd/450v in place of the 70mfd/350v other than that was what Antique Electronics had in stock.

I had a well known amp repair shop recap another BFSR and they used:
2 20mfd/500v Sprague Atom
1 100mfd/500v Sprague Atom (in place of one of the 20mfd/500v)
2 220mfd/350v Ruby Cap (in place of the 70mfd/350v)


What's better or does it matter?
Is it a good choice to use the 100mfd/350v in place of the 70mfd/350v?

[Dacious]

Using bigger caps will provide more filtering and therefore quieter operation. It will also - to some degree - facilitate bass response. DC Power coming off even a full wave rectifier is like a series of sawtooth spurts. The caps act like little batteries, charging off the spurts then releasing power downstream in a smoother flow.

Bigger caps provide more smooth power so bass will be enhanced somewhat. The flipside is it takes more out of the transformer and rectifier to charge them and there is a limited charging time per AC cycle - most tube rectifier manufacturers recommend a certain range of capacitance vs voltage and current draw depending on various factors.

Big Marshalls often use a lot more filtering than big Fenders, and use SS rectifiers rather than tubes for efficiency. Bigger caps are dearer and physically larger so real estate is an issue.

100mfd vs 70 is not a difference that you'd notice much, but I'd query the use of 220s a little. It would be hard to fit them in the doghouse, apart from all else. If the amp works and sounds OK they probably do no harm.

They may have used them because they had them.

[zook]

Originally Posted by Big Mike Simpson

The original 5 Mallory caps in the 66 BFSR power supply were:
3 20mfd/500v
2 70mfd/350v



In one of my amps (65BFSR) that I recapped I used:
3 20mfd/500v Sprague Atom
2 100mfd/450v Sprague Atom

It was a couple of years ago and I don't remember the reasoning behind using the 100mfd/450v in place of the 70mfd/350v other than that was what Antique Electronics had in stock.

I had a well known amp repair shop recap another BFSR and they used:
2 20mfd/500v Sprague Atom
1 100mfd/450v Sprague Atom (in place of one of the 20mfd/500v)
2 220mfd/350v Ruby Cap (in place of the 70mfd/350v)


What's better or does it matter?
Is it a good choice to use the 100mfd/450v in place of the 70mfd/350v?

The Two 70/350 caps are wired in series with leveling resistors in parallel with each. That means the total capacitance is 1/2 the individual capacitance. Two 70's in series= 30uf total. Two 100s in series-50uf just a little more.

I usually replace the Fender caps with 30/500s and 100/350's. I think if you check you'll see that the 100uf caps you used are 350 volts not 450volts.

The Sprague atoms are used because they are similar in size to the originals.

Winnie

[Big Mike Simpson]

Yes you are correct... the voltage rating on the 100's I used previously was 350v (I corrected my first post) and also the middle 100 cap that the repair shop used was a 100/500v.

Also... What is the difference between the 5U4G and the 5AR4 rectifier tubes? The AB763 diagram shows to use a GZ34 (cross referenced to 5AR4) and the later AB768 diagram shows to use a 5U4G.

I have always tried to put in the correct tube that the schematic calls for but this 66 BFSR that I bought had a 5u4G in it. Besides the obvious difference in bottle shapes the 5U4G has only 4 pins where the 5AR4 has 5 pins.

Is the 5AR4 and the 5U4G interchangeable in a 66 BFSR or is the 5U4G the wrong tube for the circuit?

Here is some info I found on the difference in Rectifier tubes.
http://www.guitaramplifierblueprinti...ectifiers.html
Rectifier substations / cross reference:
GZ34=5AR4 GZ32=5V4 GZ31=5U4 GZ30=5Z4


Also thanks for the advice. I will go ahead and use the 3 20mfd/500v Sprague Atom and 2 100mfd/350v Sprague Atom in the 66 like I did on the 65 before.

[Tremo]

I always use the 100 uf at 350 volt caps in the seriesed positions since those are readily available, and the increase from 35 uf total to 50 uf total isn't so much as to make the amp sound too stiff. It doers help with the filtering and low end. However, for the rest of the caps, I stay with the 20 uf at 500 volt units.

You have to be very careful not to use too big a cap on a tube rectifier, because the repetitive peak charge currents will be extra high and will destroy the tube. I once saw the 220 uf caps used on a SR, installed by a semi-famous tech who should damn well know better, and it wiped out a perfectly good Mullard 5AR4. Do not hang more than 50 uf on a tube rectifier.

[DrewB]

Originally Posted by Tremo

Do not hang more than 50 uf on a tube rectifier.

Are 5Y3s and the other small bottles okay with this much capacitance, too?

[Dacious]

Are 5Y3s and the other small bottles okay with this much capacitance, too?

That depends - something like a Tweed Deluxe with ~350 Vdc/70ma would be OK. As you get nearer the tubes' design limits for peak inverse voltage of about 700 volts (say +ve 350 to -ve 350Vac) and current (80ma), the specc'd capacitance comes down. If you have a cap/choke setup rather than all-capacitor filter network it's also different. Tweed Deluxes used 3 x 16mfd but they were 20% tolerance components. My homebrew's been running fine with 3 x 22mfds on the original rectifier for several years.

Again, as noted - a Fender with tube recto will be different to a SS Marshall. Ampeg VTs used large-value cap arrays off a high voltage - but at fairly small currents.

[Big Mike Simpson]

It sounds like I should replace this set of caps (20mfd/500v, 100mfd/450v, 220mfd/350v) in the other 66 with the 20/400 and 100/350's too...

I am a bit disapointed... I sent an amp to a well known shop... My only instruction was "make it work correctly and make it reliable"... after very little playing it blew a fuse and I have just let it set since I had many other amps. I did not want to pay to ship it back. The repair (caps, tubes etc) and freight was not inexpensive. Now I open it up to see what was used for comparison and from what you guys are saying the 2 220/350's are too large of a value for a rect tube.

If this sounds confusing it is because I have more than one amp. One is a 66 BFSR in a std 4x10 SR cab (serviced by a shop) and I am servicing the second 66 BFSR that I bought as a head and put in a Vibrolux 2x10 cab and I am servicing it to get it up to gigworthy.

[Tremo]

Mike, absolutely, get those 220s outta there. They will cause the rectifier to short. Then you will blow fuses. Replace the 220/350s with 100/350s, and use 20/500 everywhere else.

Sorry Paul, I know you don't like it when I say this stuff, but the guy who put those 220s in that amp is a feckin idiot.

[Big Mike Simpson]

Thanks Tremo, I try to replace any component with the same value replacement when possible. I am learning to work on my own amps for the same reason I learned how to do guitar repairs...

[zook]

Originally Posted by Big Mike Simpson

Yes you are correct... the voltage rating on the 100's I used previously was 350v (I corrected my first post) and also the middle 100 cap that the repair shop used was a 100/500v.

Also... What is the difference between the 5U4G and the 5AR4 rectifier tubes? The AB763 diagram shows to use a GZ34 (cross referenced to 5AR4) and the later AB768 diagram shows to use a 5U4G.

I have always tried to put in the correct tube that the schematic calls for but this 66 BFSR that I bought had a 5u4G in it. Besides the obvious difference in bottle shapes the 5U4G has only 4 pins where the 5AR4 has 5 pins.

Is the 5AR4 and the 5U4G interchangeable in a 66 BFSR or is the 5U4G the wrong tube for the circuit?

Here is some info I found on the difference in Rectifier tubes.
http://www.guitaramplifierblueprinti...ectifiers.html
Rectifier substations / cross reference:
GZ34=5AR4 GZ32=5V4 GZ31=5U4 GZ30=5Z4


Also thanks for the advice. I will go ahead and use the 3 20mfd/500v Sprague Atom and 2 100mfd/350v Sprague Atom in the 66 like I did on the 65 before.

The 5U4 requires 3 Amps. The others require less, 5Y3 uses 2 Amps. 5AR4 1.7 (or 1.8) Unless the amp was designed to use a 5U4 it won't have the amperage the tube requires.

Winnie

[Big Mike Simpson]

Thanks for all of your help on this... here are come pics of the my amps.

Can anyone tell what these two white wires were added for?




Here is a pic of the 220 caps before I replace them with 100/350's

[Big Mike Simpson]

also...
Here are a couple of pics of the original condition 66 BFSR that I am putting in the Vibrolux cab.

Original Power supply caps


What are these white square resistors on the power tube sockets? I don't remember seeing these in my other BFSR's... are these the screen grid resistors?


untouched... aint it purty?


66 BFSR Projects

[Tremo]

Originally Posted by Big Mike Simpson

Thanks for all of your help on this... here are come pics of the my amps.

Can anyone tell what these two white wires were added for?




Here is a pic of the 220 caps before I replace them with 100/350's

Get that 100 uf cap outta there as well.

[Tremo]

Originally Posted by Big Mike Simpson

Thanks for all of your help on this... here are come pics of the my amps.

Can anyone tell what these two white wires were added for?




Here is a pic of the 220 caps before I replace them with 100/350's

Why did that idjit replace all those resistors and all those old good blue tubular caps with orange drops? Sheesh, you can bet he replaced parts that didn't need replacing. Just to run up the bill? Did he give you all the old parts he took out?

[Dacious]

Looks like he added screen grid resistors which is not a terrible idea, although sandboxes is a bit OTT. He has used Sprague Atom bypass caps on the preamps which is good.

The white cables look like he's bleeding some speaker output back into the preamp section - positive feedback(????????????).

IMO, Mike, find someone close to you who can read a schemo and get them to check what has been done to this amp and return it to original at which time it will probably sound as good as it's ever going to. I'd also be asking for your old parts back - if he hasn't already installed the blue signal caps in someone else's amp (like his own).

I've done some stuff like this to my own amps - I'd never do this to someone else's valuable BF Fender even if they paid me.

[Glen W]

Those white wires look to me like shielded wire for the vibrato channel's 12AX7 grids.

[Big Mike Simpson]

The 66 that has had all of the work done to it had been worked on before I bought it and I don't remember if the blue caps were still there when I sent it to the repair shop. I don't remember if the white wires were there before I sent it or not. It looks like they are attached between v2 and input2 (the low gain input which I almost never use on my amps), I don't think I tried input2 when the amp was working. Of the parts returned to me there were no blue caps... there were some power supply caps and some small caps and resistors. I have not done any work on this amp myself untill now... it was all "professionally" done. I will replace all 5 power supply caps, rectifier tube and fuse and see if it will power up and play.

The second amp here I bought as an original amp that was cut down to a head sized cabinet... ya they cut up an original 66 BFSR to make a head cab out of it... go figure... It appears to be untouched inside and all caps are still original, what a pity. It is the one with the square white resistors on the power tubes. I bought it for a spare and since I have 3 other 4x10 Supers I decided to put this one in a Newell 2x10 Vibrolux Reverb clone cabinet. I set it up and played it about a half hour and it sounded great. The next time I turned it on I let it warm up for a minute and when I put it on standby it made a funny noise... like pfewww... sort of a low pop and a going away whistle. When I turned it back on it howled a little and I shut it down. I thought I would replace the power supply caps since they are 40 years old... and see if that helped.

[Big Mike Simpson]

Well ...
Thanks to everyone for their help.

On the 66 BFSR that bought as a head and put into the 2x10 cab... this amp was untouched inside.
I replaced the power supply capacitors and the rectifier tube and cleaned all the pots with radio shack "tuner cleaner" as reccomended in one of my amp books. It fired up and sounded good except that the vibrato had a noticeable static tick. After reading around the net I decided to try a different tube in the V5 and the problem was solved. I think I am going to order some Weber speakers for this amp as I have high hopes of this being my #1 gig amp as long as it stays reliable.

Then there's the other 66 BFSR...
This is the one that had 220/220/100/20/20 caps in the power supply and had blown a fuse. I bought this amp on ebay a couple of years ago and it had been "serviced" by some amp shop (their sticker was on the chassis) for the previous owner. After I bought it I had it checked out and instructed a well known repair shop to do whatever it needed to make it reliable and sound good in stock form without any mods. (See story posted earlier)

Well... I replaced the 220's with 100/350's and replaced the middle 100 with a 20/500 so it once again had 100/100/20/20/20 Sprague atom caps, I removed the 5U4 and installed a "known good" 5AR4 and replaced the fuse. The amp fires up and does not blow the fuse but the volume is VERY low and it sounds distorted as if there was an overdrive pedal but there is not. I have switched all of the tubes for other "known good" tubes and even plugged in the 5U4 to see if the circuit had been modified to use this tube... still very low volume with distortion... This is on both channels and in all 4 inputs so I "assume" that it is not the preamp section. The speakers are not the problem, I tested them with the other BFSR and they sound fine.

V1 through V6 are all NOS JAN Phillips and the power tubes are Sovtek 5881WXT's, the bias reading on my "Bias Rite" is currently (pardon the pun) set at 34ma... and the rectifier is a Sovtek 5AR4... I am relatively certain that the problem is not any of the tubes and I am sure it is not the speakers.

Still... very low volume and distortion....

Any suggestions?

[Tremo]

You need test equipment to go further with the diagnosis. Sig gen and o-scope.

At least you have it powering up reliably without blowing fuses, so that much is now OK.