Gibson Gibsonette recap - couple Qs.

[f13ndus]

I'm going through my 1957 Gibsonette yard-sale find and tuning it up, as a way to teach myself more about tube circuits. I'm about to recap it (it's not a museum piece, so I'm probably going to replace all of them, but keep the originals around (of course)) and I've got a couple of questions. I've attached a highlighted schematic.

First, about the filter capacitors - they've been replaced in the past with some big (100uf 450v) chinese electrolytics that can't be good for the rectifier - and right now, there are only two in the amp. The schematic shows four filter caps, I believe (highlighted in yellow) and the two it's got are installed are in place of the 10uf right by the output transformer, and in place of the last 10uf that's for the preamp power (I think). I'm probably going to replace these with the "correct" value caps, or close to it. Is is dangerous/a terrible idea not to have the four caps on the schematic in the amp? The amp has been working with just the two big caps in it, until I found the schematic I didn't know anything was amiss. What if I used a new JJ multicap - I believe they have a 4-section 40-20-20-20, would that be appropriate? Or does anyone have any other suggestions for what to use?

Secondly, the coupling caps (highlighted in blue) are currently the sprague "bumble bee" bakelite/striped type. Can I test to see if they've drifted in value (I only have a DMM)? If I replace them, what would be good to use - I know this is a matter of personal opinion as far as sound - but I'm not sure if these are non-polar or not? Should I use something like Orange Drops, or get some nice (read: expensive) paper-in-oil types? Should I bother trying different values?

And lastly, the caps highlighted in green. Embarassing question: what's the right term for their role in the circuit? (Cathode bypass? I really need to get an old RCA manual...) I think these are electrolytic caps, should I just use any old axial type with appropriate values? How do they affect the amp's tone?

I will of course be checking resistor values and voltages while in there, and replacing as necessary.

As usual, I'm asking a ton of questions... I appreciate any help/advice you guys can give me. I've had a lot of help from here with bringing other amps back to life, and I'm always reminded of how much more I have to learn!

[stokes]

Sounds like sombody hacked up the power supply with those 100uf caps.I would go with the values you see on the schematic.If you want to stiffen the filtering some,you could go with 40uf's in place of the 20uf's.If you are concerned about using a 40uf as the first cap the 5y3 sees (I wouldnt worry about it,tho) use a 20uf and for the second use the 40uf.Use the 10uf listed for the pre-amp stage and I would go with a 20uf for the screen supply.Make sure you have the dropping resistors in between the caps,I suspect some were removed when the 2-100uf caps were installed.The caps you have highlighted in green are cathode bypass caps,they affect the gain of the stage and the frequency response and should be changed with as close to the original value as you can find.I wouldnt bother with the paper/oil caps for the coupling caps.I've experimented with them in a couple of amps,actually 11 amps,and only found them to be an improvement in 2 of them.Go with the orange drops or the yellow Mallory axials.I find the orange drops to be rather harsh in most circuits,but you may like them.I would just replace those "bumble bees",dont even bother testing them,they are quite prone to leaking when they get old.

[f13ndus]

Sounds good to me. I'll get some caps ordered this weekend... those coupling caps are .02uf, not .2 uf, right?

Also, If the dropping resistors in the power supply aren't there, what wattage rating should the replacements be?

[stokes]

The first one (doesnt show a value,but I would use 5 or 10k there) should be 10watt the 10k/5w and 22k/2w. The coupling caps are .02uf.

[Enzo]

You very likely have resistors way of value. Remember back then most amps were made with 20% tolerance parts anyway, so a 10k resistor could have measured 12k when it was new and still be in spec.

Your caps? DOn;t worry about caps drifting oiff value, what they drift off from is low leakage. Those .02 caps - replace them. There are only two of them. Small coupling caps tend to get leaky. This is a very old amp.

Then the electrolytics - -you have four filter caps and a couple cathode bypass caps. Lytics tend to dry out over time, and lose their function. At this point, replace them. You could measure their leakage or ESR or whatever, but their value won;t tell you much. I often encounter old dried up lytics that measure higher in capacitance than their printed value. But they don't work in the circuit.

[f13ndus]

Yep, I'm just going to replace all the caps. There are only 8, after all... no reason not to.

the power section resistors, the first unlabelled one in the schematic was there when I got the amp, but crumbling apart; it was 200ohm and I've replaced it with a 220ohm 10-watt which should last. The others, I'll have to go through the amp when I get home from work and see if they're there...

as for all the others, I'll be checking them with the multimeter. Don't really want to replace any I don't have to, but carbon comps definitely drift...

thanks for the help, everyone, I'll be back with an update when I get the new parts in...

[TD_Madden]

Think I measured that unmarked one at 500 ohms in my Gibsonette....need to check again, I guess!

[f13ndus]

Does yours have the same tube lineup? I know Gibson seemed to change these every week I did use a 450ohm resistor there for a bit, but the amp just wasn't as lively...

With a 5Y3 in, I measured 314V on the plates, with the new 220ohm in place... I think that's right, I'm going by memory (got it written on the amp chassis with sharpie so that I've always got it handy and I can remove it with rubbing alcohol!)

I think I might still have the old busted resistor at home, actually... I'll look for it to re-measure.

On a completely unrelated note... well, vaguely related... I just found this on eBay:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=270342470588

It's the Gibsonette before it was named as such... otherwise known as the Kalamazoo Electric Amp (KEA)... and I'm very tempted to fire sale a few of my other pieces of gear and pick it up, especially since it's only 1 state away from me... apart from the field coil speaker and octal preamp tube, it's probably almost identical to the one I've already got... chassis layout definitely looks the same (and now I know why the power transformer in my amp is labelled "GIBSON KEA", and why my noval preamp socket it mounted in an octal-size cutout w/ an adapter...) I've got some GAS!!!

[TD_Madden]

I'm sure it's the same...white-covered cab, 5Y3, 2x6V6, 12AX7

I put a WarehouseSpeakers "vintage 10" into it and it sounds pretty good for what it is.

[f13ndus]

I've got one of the Fender/Eminence blue frame alnico 10" speakers from the reissue '59 Bassman in mine, I love the way it sounds!

My cabinet had seen far better days (probably why I got it for $25) so I got it down to bare wood, stained and sealed it, replaced most of the hardware with brass bits (kept the original handle) and put on some grill cloth from an old organ... it's not original finish, but it wasn't when I got it (can you say "spraypaint"? ), and looks pretty sharp now!

I wish it was gig-worthy without having to be miked, but hey, I can start to build a bigger amp once I learn more about how to work on this one...

[TD_Madden]

"Does yours have the same tube lineup? I know Gibson seemed to change these every week I did use a 450ohm resistor there for a bit, but the amp just wasn't as lively...
With a 5Y3 in, I measured 314V on the plates, with the new 220ohm in place... I think that's right, I'm going by memory (got it written on the amp chassis with sharpie so that I've always got it handy and I can remove it with rubbing alcohol!)......"




Interesting, the schem indicates 320v on the plates, and of course that is based on 110v (or so) wall-voltage. I definitely need to recheck my voltages!