SF vibrolux reverb - filter cap size limit

[g-man]

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[eleventeen]

20, 22 uf is fine. Nothing to even think about.

While you're at it, I would strongly urge you to replace the 100K ohm plate resistors with Doug's 3 watt metal film resistors or equivalent. I did this while doing caps on my Deluxe, Princeton, and Pro, all of which were silverface or older and it made quite a difference in the amount of hiss coming ALL 3 of 3 amps. A very noticeable difference, much quieter.

On two of those amps...the hiss got quieter after I left the completed amps on for maybe half an hour, and from then on, they were quiet. They will show improvement when you first turn them on (after changing these Rs) but on 2 amps, it almost seemed like the resistors had to burn in...and then the amp was even quieter.

[g-man]

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[eleventeen]

Yes, 6 total resistors, if you have the chassis in front of you with the tubes closer to your belly, near V1-2-3-4,  they are arranged in pairs in that upside down "V" config.

Just one of those things, for me. If the amp is open and not intended to remain stock down to the solder joint, I do this as a matter of course on old black/silver/face Fenders. I can almost guarantee it will make the amp noticeably quieter on hiss. For $2 in parts, I just just throw 'em in there and don't think about it much.

[eleventeen]

When you speak about changing the bias filter caps....carefully compare the schematics from an AB763 blackface Vibrolux Rev compared to an AA270 VibRev which is likely what you have. One is a "bias balance" control (in the AA270) and the older style is...well, the older style.

I have no opinion as to which config works better; my preference would be for the older style, but my point is, you should compare and ponder the schematics before changing parts values. Changing a 16ufd filter cap to a 20 or 22 ufd is *not* changing parts values, in my book. Fooling around with bias circuit thinking it is similar to an AB763 bias is not a good idea.

You could go a tad bigger on the filter caps, but you should not do so for the first filter cap the rectifier sees. Note that in a VibRev the two 16 ufds are in parallel forming a 32 ufd cap. Some folks like to mount filter caps closer to the preamp tubes instead of under the filter-cap can. There are plusses and minuses to either way.

[g-man]

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[eleventeen]

Most folks in your situation would opt for completely changing the amp over to AB763 circuitry, and there's not much to it. Remove the little caps on the 6L6's, change the PI plate resistors to the 100K/82K  and change the bias circuit. Change the 100K resistors in the AA270 to 220Ks as in the AB763. This is not meant as a complete list....and I'd be surprised if the PI changes would make any audible difference. The most painful thing is to change the bias circuit over, requires a new pot of 10K instead of 100K. Still not any kind of big deal.

I have not looked at the 2 schematics with religious intensity, but the bias voltage is also fed over to the trem circuit----which needs good-sized negative volts, probably to shut off the oscillator. Just something to keep in mind---you could probably feed the osc grid anything from -30 to -50 volts and it would shut off the osc. OTOH, you may have to futz with it.

Obviously, the decision to do all this depends upon whether this is YOUR amp (to keep) or one you wish sell as stock or a customer's amp.

[g-man]

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[g-man]

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[HotBluePlates]

... AA270 has a pair of 50 uF. I'm not sure why they have the 2 x 50uF caps in the AA270, but I have seen some re-caps where these were replaced with a single 100uF. ...

I'm in favor of the AB763 bias pot to easily adjust bias when changing tubes. ...

The earliest Vibrolux Reverb I found with a quick look is the AA964. It has the single 25uF 50v bias filter after the diode.

Your AA270 has a 50uF -> resistor -> 50uF. Does your B+ supply give cleaner d.c. with 1 filter cap, or with several separated by resistors (and maybe a choke)?

The way your amp has it now will give a cleaner and more stable bias voltage. I'd keep that aspect, even if you move the output of the 2nd 50uF cap to the end of the 10k bias pot to create and adjustable bias supply.

So for new filter caps I'm planning to go with 1 x 40uF and 3 x 22 uF to replace the original 32 uF (2 x 16 uF in parallel) and 3 x 16 uF. Does that seem reasonable to you guys?

Will the 40uF fit under the cover? It seems like 1 cap in place of 2, but is likely longer/wider and may not fit under the original filter cap cover.

I'd just get all 20-22uF caps (whatever value the brand you choose offers) to replace the originals. If you choose modern electrolytic types instead of Sprague Atoms, the new caps will likely be much smaller than the originals due to manufacturing improvements.

I have heard good things about the Miec 600V electrolytics. Anyone else used them?

Never tried them, but I've bought other stuff from Just Radios, and turned out fine. Many companies make high quality electrolytics these days, so you will have a hard time going wrong.

[g-man]

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[HotBluePlates]

IF you are certain you made no wiring or soldering mistakes, you could just flip on the power. You might want to do that with no output tubes until after you check pin 5 and verify negative voltage is present.

If you aren't sure you made no errors, or if you used questionable caps, you might want to fire up first with a lightbulb limiter. That will limit current to the amp and prevent big sparks in the event of a mis-wire or short or failing cap.

Either way, leave the output tubes out of the amp until you're sure B+ is right and bias voltage exists.

[g-man]

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