Problem with power supply filter capacitors in late-50s Gibson GA18 (GA16)

[slidincharlie (Carlo P)]

Good day and merry Christmas everybody :-) .
I have an old Gibson vintage amp (that I DO love) that is labelled GA-18.
Actually its circuit matches perfectly (except for the filter section) a schematic labelled GA-16T (attached) instead of the one labelled GA-18T (also attached). So let's stick to the GA-16T schematic.

The amp has got a 20+20+20+20 uF @450V multisection cap in the power supply section, with two of the positive lugs soldered together (see attached photo_1). This latter joint goes to the 470R resistor and to the OT center tap.
First question: is that a parallel connection? What are the final capacitance values of that can cap? 20+20+40 uF? Or is that a series connection yielding 20+20+10 uF?

Now the main problem and second question:
The GA-16T schematic calls for 20, 10 and 10 uF filter caps. Why that increased filter configuration in my amp? I believe that the Rubycon can cap (see attached photo_2) is a replacement unit from previous maintenance work - maybe someone decided to change the original filter configuration? Does it make any sense?
For completeness of information, a few years ago I replaced the original tiny output tranny with a 15W Hammond 1750E tranny meant as a replacement for Fender 5E3 Deluxe amps.
The amp sounds gorgeously, maybe cleaner than expected for an old 15W tube amp. I am thinking of a cap job just to prevent possible failure of the old electrolytic caps in there (the bypass caps are old silver-coloured Spague units), although the can cap date stamp might suggest a 2008 (?) manufacturing.
I am wondering what filter caps should I install.

Thanks in advance,
Carlo

[Chuck H]

It sounds to me that your current filter configuration is 40uf-20uf-20uf. The higher capacitance over the schematic value is only of benefit. I don't think you would hear a difference with the lower values. When that design was first made the values in the schematic made more sense from a practical manufacturing perspective. Caps like that were comparably expensive. Later technologies made higher values more affordable.

I haven't needed a multi cap can in a long time so I don't know who might make them. The last one I bought was JJ. If you're interested in individual units I like Nichicon PW series. I gave up on axial caps for power supply filtering some years because of their typically lower performance and higher price.

[Chuck H]

It looks like CE is making a lot of configurations for multi section can caps. I haven't used them. Here's a link to their 20/20/20/20 @475V

https://www.tubesandmore.com/product...f-electrolytic

Your can looks skinnier but it's hard to know since caps are smaller every year. Perhaps the CE can is just shorter.?. A little research and measuring is in order for fitting.

[Tom Phillips]

...The amp has got a 20+20+20+20 uF @450V multisection cap in the power supply section, with two of the positive lugs soldered together (see attached photo_1). This latter joint goes to the 470R resistor and to the OT center tap.
First question: is that a parallel connection? What are the final capacitance values of that can cap? 20+20+40 uF? Or is that a series connection yielding 20+20+10 uF?...

Yes. It's a parallel connection giving you 20+20+40 uF. Or, stating the values in the order they appear in the power supply circuit, think of it as 40+20+20 uF.
It is common to find such "upgrades" when caps are replaced. The 40 uF first stage cap puts more stress on the 5Y3 rectifier tube during initial turn on than a 20uF would in that position. 20uF is the max recommended value per the old 5Y3 data sheets. However, in your amp circuit, the larger capacitance value is not a serious problem. Such changes are common in guitar amps and obviously your amp is not damaging rectifiers. The effect of changing back to 20uF would be easy to test by disconnection one of the two terminals currently wired in parallel. The trade offs are sag vs hum level. If it was my amp, I would go with the value that gave the tone & response & hum level that you like best. I personally would not be concerned about using either capacitance value. The manufacturer originally used the 10uF values due to cost control. In the "olden days" the higher capacitance values that we commonly used today were relatively more expensive.

[nosaj]

Hayseed hamfest will make what you need.
nosaj

[slidincharlie (Carlo P)]

Thank you guys.
My main doubt was about caps' values.I don't care about a new multisection can cap since it would cost me a lot including shipping. First of all I'll check for tone and hum as per Tom's advice, then I'll shop for new caps.
Thanks again.