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First filter stage question in amp's PSU

71.93.223.76
Posted on August 17, 2014 at 13:20:12
FenderLover
 Why does Fender use a 220mfd/100VDC + 47mfd/500VDC, pair of caps in series, in the first filter stage of their Deluxe Reverb Reissue? C31 and C32. The 220/100 cap has a 100k-ohm, 1/2-watt resistor across it. And the 47/500 cap has a 470k-ohm, 1-watt resistor across it.

I guess the question is, will the 100VDC rated cap be ok in this portion of the PSU --- using this setup? And will a 220mfd/500VDC rated cap work just as well, as a substitute, for C31?

Or should I replace both cap C31 and C32 with 80mfd/450VDC caps, with 220k-ohm, 1-watt leveling resistor across each cap?


Thanks!

 

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RE: First filter stage question in amp's PSU, posted on August 17, 2014 at 16:14:34
BofService
Audiophile

Posts: 741
Location: Atlanta
Joined: February 28, 2003
 I'm having trouble figuring that out too. Maybe it is a mistake. Note that ALL the other Deluxe Reverb schematics use 450v caps.

 

It's not a mistake., posted on August 17, 2014 at 16:41:30
FenderLover
 I've redone several of these DRRI's. I'm helping a dude with re-capping his amp. And those are the caps in the first stage.

The '68 Custom Deluxe Reverb (new 2014 models) also have the very same setup, in their first stage.

The 220/100 cap is a puzzler. As it only drops the overall mfd in the first stage to around 38.7 mfd. Only about 9 mfd below the second 47/500 cap.

Odd

 

FWIW..., posted on August 17, 2014 at 17:29:57
Steve O
Audiophile

Posts: 12359
Location: SE MI
Joined: September 6, 2001
 ...the cap and resistor values and positions and cap voltage ratings tend towards equal charge on each cap when in service. The combo strikes me as a clever means of obtaining a 600V rating using available/surplus parts. I could be wrong too, esp since the design was copied for the reissue.

 

Thanks! Can a 220mfd/500VDC cap..., posted on August 17, 2014 at 17:58:14
FenderLover
 be substituted for the stock 220mfd/100VDC cap, with no issues?


Vous remercier, pour votre réponse!

 

RE: First filter stage question in amp's PSU, posted on August 17, 2014 at 18:30:32
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 I would guess that the voltage rating wasn't quite what they needed, so they went to their parts inventory, grabbed that 220uF cap, setup the divider and called it good.

 

Can't see why not..., posted on August 17, 2014 at 18:52:59
Steve O
Audiophile

Posts: 12359
Location: SE MI
Joined: September 6, 2001
 ...at least technically. Since everything seems to affect guitar amp tone, there may be aesthetic reasons to stay with original values and ratings. You be the judge.

 

This amp is a DR reissue, posted on August 17, 2014 at 20:26:16
FenderLover
 With cheap PCB's. Not a "real" DR (whatever that is... LOL). So I'll go with the two 80mfd/500VDC caps. Thanks, for the answers.
.
.
My "real" 1968 DR:

-----

photo front2_zps85db53be.jpg


photo circuit_zpseccad054.jpg

 

RE: It's not a mistake., posted on August 18, 2014 at 07:10:44
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 The schematic shows about +400V at that point. If that's correct, there's no reason the caps would need a 600V rating. You and I might build it that way, but I've seen factories do worse without adversely affecting product reliability. I'm betting they've had trouble with the 5AR4s. There have been some really poor quality copies (imports) of the original design over the last ten years. Fender probably found out the hard way that reducing the input capacitance was necessary to keep the more marginal examples alive. That's just a guess, of course, but I can't see any other reason for doing this.

 

RE: It's not a mistake., posted on August 18, 2014 at 07:44:40
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 TK, when the amp is in standby the only load is the 570k ohms.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: It's not a mistake., posted on August 18, 2014 at 11:12:08
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 "when the amp is in standby the only load is the 570k ohms."

Yes, I see that now. So, maybe the voltage increase was more than the 500V cap alone could handle. It would be interesting to measure this on a live amp. In any event, I think FenderLover's idea to use two 80uF caps is reasonable. I'd probably use a pair at 330V, rather than 450V, just to be sure they stay formed.

 

That all sounds good to me. (nt), posted on August 18, 2014 at 12:02:21
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 .
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: It's not a mistake., posted on August 18, 2014 at 16:17:18
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 From the C-D manual and various other sources, it is fairly common (especially in military gear) to run electrolytics at less than the rated voltage to extend their lifetime. Usually this is done at between 50% and 80% of the maximum rated working voltage.

 

RE: First filter stage question in amp's PSU, posted on August 18, 2014 at 18:40:56
Gingertube
Audiophile

Posts: 545
Location: South Oz
Joined: October 8, 2004
 For capacitors in series.
The same charge current flows through both caps.
The voltage the cap charges up to is INVERSELY proportional to the capacitance.
Therefore the 47uF cap will charge to 220/47 = 4.7 times the voltage that the 220uF will charge to.
This is re-inforced by the 100K/470K voltage share resistors.

So a 100V rating for the 220uF is fine.

If that 220uF had 100V across it then 47uF would have 470 Volts across it.

Hope this makes sense to you.

Cheers,
Ian

 

Understood, but..., posted on August 18, 2014 at 20:25:16
FenderLover
 would it be ok to replace a 220mfd/100VDC rated cap with a 220mfd/500VDC one? Keeping the 47/500 intact. With same leveling resistors?


Much thanks!

 

RE: Consider this, posted on August 19, 2014 at 04:28:28
Russ57
Audiophile

Posts: 3754
Location: South Florida
Joined: November 16, 2001
 The two caps in series, 220uF and 47uF, add up to 38.73uF. What I'd think you would want is a cap within the rating of the rectifier tube and rated above the no load voltage on the transformer. You could measure what the voltage is and add a safety factor.

In other words, why pay for a 220uF/500VDC cap when a single 40uF cap might do better. Given you would be replacing two caps with one perhaps a film cap would fit. Likely it would perform better and last much longer.

 

RE: Consider this, posted on August 19, 2014 at 06:15:57
FenderLover
 Thanks! I've redone 5 or 6 of these DRRI main filter caps. The last two I used two 80mfd/450VDC F&T (for C31 and C32) and replaced R65 & R66 with 220k-ohm, 1-watt resistors. The first few DRRI's got Sprague ATOM 100mfd/350VDC caps, with 220k-ohm resistors.

I asked about the 220mfd/500VDC cap replacing the 220mfd/100VDC because someone on the Fender Amp Board is redoing his amp's caps and wanted to use this higher voltage rated cap. I'm not sure what effect the voltage rating has on the GZ34, which sits next to it.

Thanks!

 

RE: Consider this, posted on August 19, 2014 at 09:33:26
Russ57
Audiophile

Posts: 3754
Location: South Florida
Joined: November 16, 2001
 Using a higher voltage cap (than needed) results in longer life. No problems at all other than increased size and cost.

I like to avoid series caps when I can. Not long ago it was hard to find high voltage caps. Today, because of variable speed drives, good high voltage caps are easy to come by.

 

RE: Consider this, posted on August 19, 2014 at 19:12:36
FenderLover
 Thanks! There are few oddball things with these reissued amps. If you get a chance, look at the new '68 Custom Deluxe Reverb schematic. More head scratching circuitry mods.

 

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