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power supply filter cap replacement question

N

NationwideBass

Dec 20, 2006
198
0
Northern Virginia
www.myspace.com
#1
i have a 69 Ampeg BT-25 that has never had the filter caps replaced, and they're looking bad. I'm trying to find the replacement caps right now. The service manual specifies them as 2500mf/90V, but on the caps themselves it seems to read 2500uF/90 (or 80 on one). My question is, would this work as a replacement across the board?

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And if not, what should I be looking for? Thanks!
 
#4
Most people don't know how to access the extended character set from their keyboards including me. ;( I've forgotten! MF or MFD is an acceptable substitute. All Electronics has some small 6800MFD at 80V snap in caps for $1.49 each. I bought a bunch of them for some projects I have coming up. They are for PCB mounting but you should be able to work around that.

Paul
 
#5
NationwideBass said:
Yup. I know. Thats part of my confusion. The manual lists it as mf and the cap read uF.
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Old farts like me will tell they mean the same thing. There's no way an amp that old will use anything with more capacitance than can be expressed in microfarads for the filters. On old skits, the two terms are interchangable.

And it's not actually a 'u', its the greek symbol pronounced "mew"
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I knew that, hence I said a "u" symbol, not actually 'u.'
 
#7
BassmanPaul said:
Most people don't know how to access the extended character set from their keyboards including me. ;( I've forgotten! MF or MFD is an acceptable substitute. All Electronics has some small 6800MFD at 80V snap in caps for $1.49 each. I bought a bunch of them for some projects I have coming up. They are for PCB mounting but you should be able to work around that.

Paul
Click to expand...

I didn't see the 6800MFD caps, but they have this one:

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Is there any reason you suggested a 6800 MFD when the caps I'm replacing are 2500?
 
#8
NationwideBass said:
I didn't see the 6800MFD caps, but they have this one:

[Invalid or Expired Link Removed]

Is there any reason you suggested a 6800 MFD when the caps I'm replacing are 2500?
Click to expand...
The higher the capacitance the better the filtering. Cap technology has come a long way in 40 years, so you can get a lot more from the same physical size, just be sure the voltage rating is no less than that of the original. If the original was 90v don't go lower.
 
#10
billfitzmaurice said:
The higher the capacitance the better the filtering. Cap technology has come a long way in 40 years, so you can get a lot more from the same physical size, just be sure the voltage rating is no less than that of the original. If the original was 90v don't go lower.
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... only as long as the rectifier can take the added current draw. With tube and old solid state rectifiers, adding too much filter capacitance can be a death sentence.
 
#11
Psycho Bass Guy said:
... only as long as the rectifier can take the added current draw. With tube and old solid state rectifiers, adding too much filter capacitance can be a death sentence.
Click to expand...
True, though in '68 the cap was the weakest link of the chain and going up to as much as 10,000 uF is seldom problematic. OTOH, hi-fi poogers who decide that increasing their capacity from 4700uF to 30,000 uF and in so doing toast their PS aren't at all uncommon.
 
#12
NationwideBass said:
I didn't see the 6800MFD caps, but they have this one:

[Invalid or Expired Link Removed]

Is there any reason you suggested a 6800 MFD when the caps I'm replacing are 2500?
Click to expand...

A) The more capacitance a filter has the better it performs. I needed 22000MFD at 80V. I found this value for $200$+. I prefer to parallel 3 -r 4 of the 6800 mfd units at $4.50 to get the same value. Sorry, They must have sold out ot the larger units. I checked their site but didn't see anything suitable. Try a google on electolytic 80V and see what comes up.

Paul
 
#13
Most modern Electrolytic caps have a +/- 20% tolerance range anyway so you can go up a step to 3300μF or 4700μF without issue. 6800μF would also work.

One thing not mentioned is the operating temperature of the caps. Ideally you want at least a 105°C rated cap in order to prevent premature failure due to overheating. A lot of regular caps are only rated at 85°C. Bass amps are not always the most friendly thermal environments.

Hope this is of use.
 
#14
SB100 said:
One thing not mentioned is the operating temperature of the caps. Ideally you want at least a 105°C rated cap in order to prevent premature failure due to overheating. A lot of regular caps are only rated at 85°C. Bass amps are not always the most friendly thermal environments.
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Temperature is more of a concern with tube than SS, but this points out why adding a fan to a tube chassis is a good idea. Not to cool the tubes, but to cool the rest of the components that can be cooked by the heat from the tubes.
 
#19
the answer is that the "mu" was not on any standard typewriters.

OK, so I just totally lost 2/3 of the audience there, with that "typewriter" thing.......

Anyhow, "m", would potentially 'stand for" the English phonetic spelling of the correct "mu" Greek letter (which I am too lazy to look up the keycode for), and "u" kinda looks like it.

In any case, the original poster's 2700uF ! at 100V would be fine electronically for the original 2500 uF at 90V. If they fit well mechanically, that's even better, but I'll bet they are somewhat smaller than the originals.

The folks suggesting significantly larger values need to consider that older units may have weaker rectifiers, and won't like the higher peak re-charge currents of larger-than-spec capacitors.

Not only that, but the newer capacitors probably already have lower "ESR", or internal resistance, so the currents are already potentially larger.

So....... close-to-original value capacitors are generally the best idea overall. That won't result in any worse performance than the unit has had for all these years.
 
#20
billfitzmaurice said:
Temperature is more of a concern with tube than SS, but this points out why adding a fan to a tube chassis is a good idea. Not to cool the tubes, but to cool the rest of the components that can be cooked by the heat from the tubes.
Click to expand...

Valves can and do work reliably at far higher operating temperatures than solid state. A lot of transitors are rated for maximum case temperatures of less than 70~80°C. Go much over this an the tranny is likely to cook as it's Junction temperature will be achieved and one that happens it's all over for the tranny. Valves on the other hand will keep on rising in temperature untl the glass bottle fails at several hundred degrees C. I measured the temp on the outside envelope of an EL34 at over 170°C after a hard night of giggig a few years ago. I won't get in to just how hot valves can run, but will simply say that the BBC used to heat buildings with th heat dissipated from their large valve RF amps in the 50s through to quite recently.

Solid state gear is far more prone to thermal issues, in the professional audio world there hasn't been a power amp made since the early to mid 80s that didn't use fan cooling and frankly if you've got a less than 15 yr old solid state bass amplifier over 200Wrms and it's not fan cooled, then it should be; as most bass amps I've seen in recent years are woefully short of heatsinking on the output stages and run dangerously close to thermally destructive temperatures in many cases.
 

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