One question on fliter cap values on fender AA764 champ build.

[diagrammatiks]

just keep in mind that there is a max value for the reservoir after a tube rectifier.

other then that anything goes.

fender is routinely used fairly small caps in the power supply. The twin reverbs used 40/20uf and the marshall leads used 100/50uf

[Intergalactic Tourister]

Man is that the piggy back with reverb . If so I had one of those in 1966. Great amp If this is the amp I am thinking of I don't recall mine humming at all, only if I got near it with the guitar.

It has two channels... reverb and tremolo on channel 2. It is the one with 2 6L6 tubes and yes the head fits inside the speaker cabinet.

Interesting yours didn't hum back in the day. I'm betting anyway its other components too... like I said ceramic discs... and maybe the increase in mains voltage as well. All that and more will gladly contribute to noise/hum.


***Edit: I meant the 1484

[lowell]

You need not "rip out" your can cap to try 20uf. Put a 40uf cap in SERIES with the can cap and voila: 20uf. Be sure to add parallel blender resistors so each cap shares the same voltage. IMO if I were you I'd mod the amp to solid state rectification as a tube rectifier is pointless in a Champ. Then I'd us a 100uf reservoir cap, 100ohm, 47uf cap. I would pull your B+ from the 47uf. If you want to introduce some sag increase the Rscreen and rebias.

[catnine]

You need not "rip out" your can cap to try 20uf. Put a 40uf cap in SERIES with the can cap and voila: 20uf. Be sure to add parallel blender resistors so each cap shares the same voltage. IMO if I were you I'd mod the amp to solid state rectification as a tube rectifier is pointless in a Champ. Then I'd us a 100uf reservoir cap, 100ohm, 47uf cap. I would pull your B+ from the 47uf. If you want to introduce some sag increase the Rscreen and rebias.

I see what you are saying lowell , I think , you are talking about a SS rect and then adding a 100uf cap with a 100ohm dropper then the 47uf cap , are to talking about adding the 100uf cap to the existing three caps that are already in there. This is not a cap can but three individual caps . Part of the reason I did not go with a SS rect is that the B+ is already 373 VDC . Seems I would have to add a lot more resistance in the rail to get that to where the 5Y3 put the B+.

[Intergalactic Tourister]

Put a 40uf cap in SERIES with the can cap and voila

Either way it is still a pain in the backside.
If connecting a cap in series... you still have to unsolder one end of the cap... attach some clips... and hope you don't electrocute yourself in the process and or have a clip come unloose and fry other components.

And in several of my cases removing a filter cap means removing one or more components to get to it... not to mention cold solder joints tend to happen more in these situations and then you have to remove solder and resolder... like I said wait for someone to respond who has done it. If you don't want to wait that long just remove the whole cap and put the smaller cap in and hope you won't want to take it back out.

I see what you are saying lowell , I think , you are talking about a SS rect and then adding a 100uf cap with a 100ohm dropper then the 47uf cap , are to talking about adding the 100uf cap to the existing three caps that are already in there.

I don't see any reason to switch from tube rectifier to SS rectifier. Your initial curiosity was about using a smaller cap- not gutting out the whole power supply.

[lowell]

"Either way it is still a pain in the backside.
If connecting a cap in series... you still have to unsolder one end of the cap... attach some clips... and hope you don't electrocute yourself in the process and or have a clip come unloose and fry other components."

No pain, lift the ONE wire to the positive side of the existing filter cap. Solder the negative of the new cap THERE, then the one wire to the positive of the new cap. No clips, possible shorts etc.... However since it's not a can cap, replacement IS indeed just as easy.

"I don't see any reason to switch from tube rectifier to SS rectifier. Your initial curiosity was about using a smaller cap- not gutting out the whole power supply. "

There was talk of hum issues in single ended PA's. This mod is a great fix for that and has the benefit of saving $ on a rectifier tube, as well as avoiding the likelihood of the tube rectifier going bad at some point as they often do. No "gutting" necessary. Diodes can easily be placed on the pins of the rectifier socket. Also easily undone at a future point.

[Intergalactic Tourister]

No pain, lift the ONE wire to the positive side of the existing filter cap. Solder the negative of the new cap THERE, then the one wire to the positive of the new cap. No clips, possible shorts etc.... However since it's not a can cap, replacement IS indeed just as easy.

You're definitely right, and I agree, but...

I can replace it with a 22 uf , it's a lot of work because they are seperate caps and there is not a lot of room to work -catnine

And...

Changing that cap is quite a job , this is a allen amps chassis -catnine

I'm taking his word for it... that he'd much rather find out on this forum before actually doing the cap job.

There was talk of hum issues in single ended PA's.

"The amp is quiet , sounds good with no issues from low levels to full up ." -catnine, first post

No "gutting" necessary. Diodes can easily be placed on the pins of the rectifier socket. Also easily undone at a future point... [adding] 100uf reservoir cap, 100ohm, 47uf cap. I would pull your B+ from the 47uf... introduce some sag increase the Rscreen and rebias.

Fair enough. Perhaps a partial gutting of the power supply. But the overriding factor is a practical one. His layout and chassis doesn't allow easy access for experimentation. He just wanted to know what a smaller cap would do in his build before opening his amp up.

[catnine]

The main issue to me with a ss rect is the B+ voltage increase that will be quite a bit above the 5Y3 . The amps B+ is already high enough . I already had to use a 680 ohm bias resister to get the 6V6 to idle at no more than 14 watts. I do realize a SF champ does not need a tube rect because it does not sag from voltage drop at the same time if the ss rect renders another 20 to 30 VDC it's going to raise the entire amp that much then I am going to have to find a way to lower the B+ with some value of a rail resister and just waste voltage and then re bias which in effect will change the 6V6 clipping. Yes if the PT was a lower voltage unit then I would be fine using a ss rect as the newer like the champ 600 is designed to operate.

[lowell]

Yes you have a good point there with the B+ voltage. And if hum is not an issue then why mod it? If you like sag though you could mod to ss rectifier and use a 1k-5k Rscreen. Then due to the lower screen voltage you'll need to bias hotter which could in turn bring you closer to where you want to be bias-wise and clipping-wise. Just a suggestion obviously it's your amp. I've however done this with great results.

[catnine]

I found this "Ripple current and valve rectifiers:

When the reservoir capacitor charges up with each input cycle, it draws a large current pulse from the rectifier and power transformer. This is known as the ripple current, and the larger we make the capacitor the larger it will be.
Calculating the ripple current is fairly lengthy, but sufficed to say it is usually about five times the average current drawn. In this case we can expect it to be nearly 1A! Luckily this does not mean we need a transformer rated at 1A (which would be as expensive as it would be huge) as the current pulses are of short duration, which the transformer should be able to handle. The current rating on silicon diodes already takes ripple current into account, so 0.5A diodes would still be safe to use. However, this is why you should not use a ridiculously large value capacitor, as the increased ripple current would put a great strain on the rectifier and power transformer. (Although the diagrams shown here depict a full wave rectifier, it could also be a bridge rectifier, it makes no difference.)

from this site The Valve Wizard..

That is if you really want to drive yourself nuts. If you go through all of the info on that site you are told that diodes and more likely to fail than rect tubes.

I must say I am a bit confused by the voltage if the ripple , I understand what ripple is yet how many times will the reservoir cap be recharged during current pulses and if the reservoir cap is larger and charged since it is a storage unit would it then not make sense that it would not be fully drained each cycle? and 5 times the current is draw in each cycle so if my champ draw 40mA and my PT is 100 mA then I need a 200mA PT . Is this nuts or what?

[Enzo]

Nobody said the cap will be fully drained each half cycle. The charge in the cap may only drop a little each half cycle, but the charging current to restore it can still be large.

[Intergalactic Tourister]

I must say I am a bit confused by the voltage if the ripple , I understand what ripple is yet how many times will the reservoir cap be recharged during current pulses and if the reservoir cap is larger and charged since it is a storage unit would it then not make sense that it would not be fully drained each cycle?

The cap is recharged every pulse- give or take, of course. But if the cap is fully discharged you want it to be able to handle that situation. If the cap is fully charged, somehow, then you'll still be fine.

It is the 60hz (US) pulses. So the B+ going into the reservoir cap is not full on 100%. It is like an incandescent light bulb... try turning it on-and-off as fast as you can flip the light switch. You'll notice that the bulb will never get to its fullest brightness or wattage. This is due to you making it pulsate at the switch. Same concept. The mA draw will not get to its fullest.

and 5 times the current is draw in each cycle so if my champ draw 40mA and my PT is 100 mA then I need a 200mA PT . Is this nuts or what?

If 40mA is the draw... times 5 = 200mA. But it pulsates... and the other filters caps are also supporting the larger one with respect to mA. So I don't believe your calculation takes all factors into account. In other words you have more than one big filter cap in your amp.

Further it depends if the mA is average or peak. I don't have the specs of your amp, but I believe that if 40mA were peak then you'd want about 30% higher rated PT.

[catnine]

Nobody said the cap will be fully drained each half cycle. The charge in the cap may only drop a little each half cycle, but the charging current to restore it can still be large.

Ok , I don't know what to do , just leave it alone or change the 1st cap to a 22UF that I do have . There are all sorts of opinions and ideas out there from diodes to protect the rect tube and fuses for everything and on and on . All I know is I looked at my 73 SF champ and re-placed the original cap cna with a 40/20/20 that was stock and old and you can tell if it was changed . I know I had an 80 champ that blew the fuse when I turned it on and was told by a fellow who never saw the amp that I needed a new 5Y3 and a fuse , this worked . And not to take a 67 super reverb out of a heated club after three hours of playing out into Chicagos Jan -20 winter wind because the tubes will blow whcih they did .

I mean really if the PT blows it is almost less effort to buy a new one or live without the amp than go insane . If the 5Y3 shorts maybe it will not hurt anything , There is always risk, I could walk out the door right now and be run down by a loaded garbage truck being distracted by an un- leashed pitbull or a wasp landing on my back or some young female walking down the street .

So what do i do here ? What I don't want is hum . what I don;t want is some sort of help me please bottom notes screaming for help and tone stack calculations . Just a reasonable version of the 73 champ I once had and that is what I did build.

I didn't even know what a cap can was until I re capped the 73 champ , rather a 77 mm bass amp then a 71 . And the music master bass amps did not have a cap can . I thought that silver thing hanging near the tubes was something other than a tube because of the tube chart .

At the time I have the 73 SF champ I was told to take out the 470 ohm cathode resister and repalce it witha 1k 5 watt , I did that and it sounded fien after I remove the horrid torres gain mod and installed a weber sig 8s alnico , it had tone to die for and that is what I was trying to re-create.

[catnine]

The cap is recharged every pulse- give or take, of course. But if the cap is fully discharged you want it to be able to handle that situation. If the cap is fully charged, somehow, then you'll still be fine.

It is the 60hz (US) pulses. So the B+ going into the reservoir cap is not full on 100%. It is like an incandescent light bulb... try turning it on-and-off as fast as you can flip the light switch. You'll notice that the bulb will never get to its fullest brightness or wattage. This is due to you making it pulsate at the switch. Same concept. The mA draw will not get to its fullest.



If 40mA is the draw... times 5 = 200mA. But it pulsates... and the other filters caps are also supporting the larger one with respect to mA. So I don't believe your calculation takes all factors into account. In other words you have more than one big filter cap in your amp.

Further it depends if the mA is average or peak. I don't have the specs of your amp, but I believe that if 40mA were peak then you'd want about 30% higher rated PT.

The PT is rated at 100 mA on the HT and a old champ is 70mA plus the PT from weber which is what I got has a 4 amp 6.3 and a 3 amp 5 volt so it's more thana stock champ and I went for weber because of the cost and i have used the same PT for three other builds with never an issue but they were 6G2 princeton builds without the trem . Whne the amp is on and warmed up it draws about 40 mA .

I am just going by the 6V6 max cap for a 5Y3 , don't know how to measure ripple current.

[Intergalactic Tourister]

It's more simple than that.

Don't put a reservoir cap that is too big for your rectifier tube. Check the rectifier tube data/spec sheet.

Using a 47uf cap instead of 40uf will be fine. I did exactly that in a hifi amp... and noticed no difference - even the lowend.

Putting in the 22uf cap will most likely be fine as well if that was the original plan.

It's not really worth expending the all the effort here.
My mains are 117v. A mile away my friend has 125v mains. Having a 47uf cap my be better at one of our mains than the other with respect to lowend.

And vice versa... a 22uf cap may be better on the other persons mains voltage with respect to lowend.

In general, it is all ok. But the external variables are never ending.