Ampeg SB 12 information please?

[Neil H]

I'm ordering those replacement caps for the three big paper wrapped ones, and the blue ones, as suggested. So sometime over the next month or so those will get swapped out.

I'm also simply ordering that pair of old style 7868s.

I'm curious as to what difference if any, in tone or timbre might occur from replacement of 60 year old capacitors?

And if I wanted to brighten the tone of the amp a bit, what would be the change one could make?

[martin manning]

I'm curious as to what difference if any, in tone or timbre might occur from replacement of 60 year old capacitors?

Expect any 120 Hz hum to be reduced, and a tightening and strengthening of the bass.

[Stevem]

While you’re in there replacing the filters if you want to experiment with overall tone and clarity then do this.
Get a few .2 uf and .02 uf caps @ 600 volts like these .

Wire them in parallel and then wire them across the filters .

See what you like best, bypassing all 3 filters or whatever works for your ears / needs.

[Neil H]

I love the helpful information given here!

Thanks ...

Neil

[Neil H]

Going through the caps again.

So the big one, 30uF 600V seems easy enough ... like the Miec one ... it's an axial, does that work?

And the 20uF 450V electrolytic ... axial or radial, either?

Next ... the 2x 20uF 450V ... Miec has a can type 20uF + 20uF, for $14.99 ... that would be the correct one?

And the two blue, 50uF 50V ... which ones are they?

[martin manning]

Do you have the schematic for your amp identified? It would help to point to that for comment.

[Neil H]

Yes, it's posted in earlier posts of this thread. The early SB-12, with 7868 tubes. Prior to the later builds, with 6L6 tubes.

And there's a picture of the undersides showing the entire layout of parts.

There were comments that the paper covered caps and the blue ones should be replaced, but probably the other caps are fine. I'm not sure exactly what the blue caps ... are ... other than their listed value in the schematic, which I gave above. 50uF 50V. But what type of cap are they? I've never seen blue caps before.

[didit]

Hello --

Recommend basing evaluation/analysis against this schematic. Trace your amp out against it starting at the rectifier and then power amp and then the preamp. Then help us help you by annotating your pictures with the respective components. Unfortunately this schematic hasn't uniquely enumerated each of the components -- e.g., C1, C2, ...
---

Ampeg_sb12_schematic_10_65.pdf

Not having compared the Joe Piazza schematic with this so cannot comment on his renderings but some limited past experiences puts more faith in those that are "Ampeg" tagged.

To answer your questions: the smaller bright blue 50uF/50V are doubtless are for the bias circuit; and yes, either a dual 20uF/450V and 20uF/450V, or maybe simpler just three 20uF/450V (or 500V) would cover renovation of the power filtering. The choice should be ease of doing the actual rework. I see the metal straps holding the original caps are riveted. If this was on the bench here I'd put in three 20uF/500V axials, using some zip ties to fasten and anchor. Might recommend a radial for the 30uF/600V dependent on where leads need to connect -- it's a bit obscured in shadows etc. but shorter leads are generally recommended.

..

[Neil H]

Just got a pair of 7868s from TubeWorld ... I suppose one should put them in simply to test operation?

[didit]

Sure. A minor bit of advice would be to learn enough about setting the bias that you're confident in swapping a fresh set in. Check the status of the installed set. Prepare to start "cold" on the new set and then adjust to taste, within acceptable dissipation limits. You'll need some tools.

For some background on the topic there's good reading material here ::

https://www.aikenamps.com/index.php/white-papers

If this seems more than you'd want to tangle with then recommend getting experienced help.

..

[Neil H]

Thanks for the link. The papers are both very informative and often a hoot to read. Like ...



* Be sure to turn the power off to the amplifier before discharging the electrolytic filter capacitors, or you will get an unpleasant surprise.
* Be sure to remove any capacitor-discharging jumper wires before re-powering up the amp, or you will get another unpleasant surprise.

And the ever-sage advice:

* Never work on amps around small children - you will be embarrassed when they pick up the language you use when you touch live wires.

Setting the bias does seem a bit ... opaque ... at the moment.

[Neil H]

The Ampeg logo plate and bulbs arrived from Fliptops.net a couple days ago. I got them installed tonight.