Bassman 100 Filter cap help

[kniqht152]

Hi everyone!

First off a happy holidays and a happy end of the world to you all . Now to the issue at hand, a few months ago I did a recap job on my Fender Bassman 100 mostly because it still had the stock caps in it and I was very worried that one day they might just explode on me. Unfortunately I think the older stock filter caps sounded much better then the Sprauge Atom replacements I put in. Since then I sorta left it alone and figured that well at least I won't have to worry about them dieing anytime soon. It wasn't until recently that I realized that the older caps had much worse tolerance levels and may in fact be a different value then the ones I put in to replace them. I bought a Cap-meter and measured the values of the stock caps that I took out.

These are the measured stock values:

125uf
125uf
23uf
23uf
23uf
----------
319uf total

These are the values of the new ones I put in:

100uf
100uf
20uf
20uf
20uf
----------
260uf total


That makes a net loss of about 60uf in total capacitance that I can figure. Now can someone please explain to me which caps I need to replace and with what value in order to regain the missing 60uf of capacitance? Preferably where to purchase said caps as well? I know how to read a schematic but I'm not sure of what caps are in parallel and series in the bassman schematic. That and I'd like one of you experts to double check my ideas anyway.

bassman schematic

Thanks.

[kniqht152]

I may have solved my own problem.

Would replacing the new 20uf's with 40uf's do the trick? They look like they're all in parallel therefore they should just add up linearly right??

Can someone validate/enable me here? Thanks!

[Jazz P Bass]

That is correct.
Parallel capacitors add the value.
The voltage rating is the lowest value if they are different.

[52 Bill]

Your earlier calculations were wrong. The 2-100uF caps are in series so the capacitance seen by the circuit is actually half that value. So 62.5uF versus 50uF.

Before you start changing caps, tell us what the tonal difference in the amp is and why you feel that the caps are the cause. A difference of 3uF in the 20uF caps probably isn't enough to make a big sound difference.

[Bruce / Mission Amps]

Your earlier calculations were wrong. The 2-100uF caps are in series so the capacitance seen by the circuit is actually half that value. So 62.5uF versus 50uF.

Before you start changing caps, tell us what the tonal difference in the amp is and why you feel that the caps are the cause. A difference of 3uF in the 20uF caps probably isn't enough to make a big sound difference.

Besides all that... old electrolytic capacitors more frequently test out and read a much higher capacitance then new!!
Why, a number of reasons but mostly because they are starting to go bad, the capacitor tester usually doesn't put 450vdc-500vdc on them and they caps are not under any current load.
That was a novice assumption on your part.
I'm with 52 Bill..... something else is going on... like the bias voltage and or plate voltage has changed because the new filter caps are working right and you hear a cooler bias setting or higher plate voltages, etc.

[kniqht152]

Thanks for the reply guys,

That was a novice assumption on your part.

I am a novice.... So that makes sense

Before you start changing caps, tell us what the tonal difference in the amp is and why you feel that the caps are the cause

Please keep in mind that this is not a a stock bassman, atleast the pre-amp portion anyway. I've had it for a good many years and have modded it (with help) into a sort of dookie-mod plexi circuit and use it for guitar.

Lets see before swapping out the stock filter caps with new ones the amp had sort of a crunchy mid-high end but a very smooth and sort of controlled sounding low end. Not too much and not too little.
Since swapping out the caps it seems to me like the low end characteristics have changed. They seem slightly less smooth and more apparent. Like there is less compression?? Just a different low end sound to it now, you could mess with the low control as much as you like but it just can't hit that smooth sound.

The filter caps were the only things that were changed, same guitar, same tubes, same everything. Didn't mess with the bias at all and I noticed the change right away after the swap. The amp itself sounds fine it's just missing that low-end smoothness it had.

Now I'm tempted to put the old ones back in, but I'm worried about them going, are there any brand new caps on the market that sound like old failing caps?

[kniqht152]

Excuse me please while I did up a dead thread for a minute here...

So I decided to "test" my memory of how the old caps sounded and put them back in the amp. Afer all my whole debate here was hinged on the fact that the older caps sounded better then the new.

After about 3 hours of playing it was more then clear that the old caps sound like ****. The bottom end of the sonic spectrum was wayyyy to lose and the whole amp lost any articulation that it had, simply put it sounded like mud.

Obviously I put the new caps back in and became much happier. This just goes to show you that sonic memory is not 20/20, people get used to a sound and then anything different almost always sounds worse.

[daz]

Excuse me please while I did up a dead thread for a minute here...

So I decided to "test" my memory of how the old caps sounded and put them back in the amp. Afer all my whole debate here was hinged on the fact that the older caps sounded better then the new.

After about 3 hours of playing it was more then clear that the old caps sound like ****. The bottom end of the sonic spectrum was wayyyy to lose and the whole amp lost any articulation that it had, simply put it sounded like mud.

Obviously I put the new caps back in and became much happier. This just goes to show you that sonic memory is not 20/20, people get used to a sound and then anything different almost always sounds worse.

Thats why when i make changes i always play it like that for a while then once i have a good idea what it sounds like i go back to the old part/design/whatever and compare. VERY often my ears will play tricks on me so i A/B them till theres zero doubt in my mind. Just recently i began trying mine with much higher V1 voltages and after just maybe a week so far i have had it like this and back to the lower voltage it initially was about 3-5 times each ! I keep going back and fourth till i am sure. Ears are unbelievably tricky ! It's unreal how different things can sound form even one minute to the next leat alone one hour or day to the next. But at some point with enough listening they can no longer trick you and you begin to hear it for what it really is and it stays that way. That IMO is absolutely by far the hardest thing about designing a amp or part of a circuit to get the best from it. If not for that phenomenon you could accomplish a years worth of design advancement in about a week !