Gibson GA-15rvt suggestions

[Cliff Schecht]

I actually like the EQ on those things. They are by no means distortion monsters but they do a nice clean. I have a thread on a very similar amp and really didn't have to do too much to get a nice sound. The couple things that really bring up the sound are increasing the small coupling capacitors to at least 0.01uF (I used .022uF IIRC). You can play around with bypassing the first stage cathode with a cap too, but I don't remember needing to do this.

The other big change is what you mentioned, changing the speaker. The amp I repaired had a CBS Fender speaker in it. I can't really make any recommendations though, I'm not very knowledgeable on speakers.

The first time the owner of the amp heard the repaired amp, they used it on a country album that some dudes were recording in the studio that day. I even got to sit in and do some cheesy alt country soloing .

[Phil_S]

So any thoughts about the dual filter caps? I ran across some at the Torres Engineering site who was selling the 20uf caps, as well as 30uf that they were pitching as "better"...I thought this was a little dubious (his site's pretty heavy on the sales pitches ) and figured I should stick with what's in the amp, at least to start with. (Actually got my parts from thetubestore.com, where I've gotten a few things fro in the past.)

Is there anything to be gained by raising that value above 20 uf (what would now be 40uf in this amp, I guess?)

The filter caps on this amp were multi-section 20u can caps mounted sideways in a ring riveted to the chassis (see pic). Today's technology allows the manufacture of caps that are physically smaller and to much tighter tolerance. This amp has solid state rectification, so it won't be a problem increasing the filter cap values, though I think this is a waste beyond the first two caps (plates and screens of the power tubes). Going up to 30u or 40u will stiffen the power supply and take the flab out of the bass. I don't think it will address your ice pick tone issue, though.

I think what Cliff says is good advice, and it is quite possible the amp he had was a "good one". Apparently, Gibson had a habit of building the amp du jour with whatever parts happened to be in stock. There may also have been undocumented design changes, like the input coupling cap that appears in built amps but isn't on the schematic. What I'm getting at is, all amps of the same model probably aren't equal, and I believe what you report is more typical, and what Cliff reports is the exception.

I have pictures of the "kinda long" amp from the owner. If you want more, send me a PM with your email.

[willowhaus]

Re: the filter caps - I didn't think they would do much for the bright tone, otherwise I would expect the amp to sound better already. The dual caps are already there, put in by a previous owner. I was just trying to understand why he did it, and also wondering if a change that large might be detrimental to the rest of the circuit.

I also have a question about the resistors coming from the input jacks. The "long" mod talks about a minimal difference changing these around - am I correct in assuming that he's basically altering the freq. response of each input, so that #1 sounds a bit different from #2, etc, or is there something else going on there?

[Phil_S]

Maybe those were the filter caps he was able to get. I'd leave them alone. I don't think it makes much difference one way or the other.

On the input jack resistors, as I understand it, these are typically meant to filter out interference and perhaps modestly attenuate the signal. You can build an amp without them. I've been told anything from about 10K to 100K works OK and the standard you on a pair of input jacks is 68K. 2x 68K is effectively 34K because they are parallel. I think yours has 3x 100K, essentially the same thing. I don't think there's anything to be gained here. I'd move on.

I think you're in the right place with the caps and the notch filters.

[willowhaus]

well...did the cap mods, and the amp sounded good - for a few minutes. Except, there was this occasional dropout, like a little click & then the amp comes back. Suddenly, no sound...and smoke starts drifting up.

I kill it in a hurry, and opened it up again. Not seeing anything burnt...so I put tubes in & fire it up again, out of the case...and it appears to be the transformer mounted inside, next to the filter caps. Looking a little closer, I can see some evidence of past tinkering - odd bits of newer wire that were added & then clipped off here & there, for instance. [img:15:15]http://cacophony.aspinock.com/Smileys/d ... cepalm.gif[/img]

It appears to only have 2 black wires, connecting to the filter caps - as a matter of fact, it looks like both wires are connected to the + sides of one pair of the caps, which just seems wrong to me...might that explain why the thing starts cooking (literally) when powered up?

What is that transformer's function?

[Cliff Schecht]

Sounds like a choke. Are the tubes you are using verified good? Does the power stage turn on and bias up correctly?

[Phil_S]

The choke goes between the first and second 20u filter caps. The first cap feeds the plates and the second cap feeds the screens. Then, there is a 1K dropping resistor between the 2nd and 3rd filter caps. The choke is not on the schematic. Consult the GA20-RVT schematic to see where the choke goes.

Smoke? Once you let it out, it doesn't go back in. Let's hope it was some wire insulation. If you think it was the choke, you'll need to meter it to see if it is open or fused (0 Ohms or infinite Ohms). Disconnect it from the circuit to check it.

If you need to replace the choke, try to get one that is physically close in size, and preferably one that fits the existing mounting holes. Chokes are cheap. Because it is after the power tube plates, not much current is passing through it. I'd guess anything between 3H and 20H is acceptable. The "generic" Fender chock is probably fine. You might get by without it. You could try substituting a 5W wirewound resistor of something between 500 Ohms and 1K Ohms.

[willowhaus]

I didn't see smoke when I had it open, but there was a sizzling sound - the wrap around the xformer core (paper?) was bubbling at the edges. I'm assuming this was due to excessive heat, and it would likely have smoked if I'd left it on. I really didn't want to test that theory, though.

[Structo]

This is why you should power up an amp that you are not familiar with or have made changes to the power supply with a light bulb current limiter.

It can keep the smoke in if there is a short or other wiring fault.

Pretty cheap insurance.

If you start with a 25w light bulb it limits the current to less than 250ma to the amp.

What happens is, you connect the limiter, turn on the power to the amp.
If there is a filter cap before the standby switch then this will cause the light bulb to briefly flash, then go down to a low glow.
Flip the standby switch to play and the light bulb will flash again then go down to a low glow.
If the light stays brightly lit after either switch is closed, then you have a short or other wiring fault and you should immediately turn the power off.

It's a good idea to put a power switch on the light bulb current limiter to be able to kill power quickly.

If the 25w bulb test passes muster then power down and screw in a 60w light bulb and repeat the test.
If it passes that, then install the tubes and try it again.
The bulb will glow a little brighter because of the tube load drawing more current.

Good luck.

REMEMBER: Anytime there are power tubes in an amp you must have a speaker load connected or you can fry the output transformer and tubes.

[josewails]

well...did the cap mods, and the amp sounded good - for a few minutes. Except, there was this occasional dropout, like a little click & then the amp comes back. Suddenly, no sound...and smoke starts drifting up.

I kill it in a hurry, and opened it up again. Not seeing anything burnt...so I put tubes in & fire it up again, out of the case...and it appears to be the transformer mounted inside, next to the filter caps. Looking a little closer, I can see some evidence of past tinkering - odd bits of newer wire that were added & then clipped off here & there, for instance. [img:15:15]http://cacophony.aspinock.com/Smileys/d ... cepalm.gif[/img]

It appears to only have 2 black wires, connecting to the filter caps - as a matter of fact, it looks like both wires are connected to the + sides of one pair of the caps, which just seems wrong to me...might that explain why the thing starts cooking (literally) when powered up?

What is that transformer's function?

I believe this is an "interstage coupling transformer" not a choke. Used as the p.i. instead of r/c network. -Joe-

[Phil_S]

The one mounted inside is a choke (only two wires?). The interstage transformer is mounted on the outside, basically opposite the eyelet board.

If you had one go bad, pray it isn't the i/s tranny, as that is made of unobtanium. Well, you can pop for one at Mercury Magnetics for around $90, IIRC. At choke would be about $10 and any generic one will do.

[Cliff Schecht]

You could probably find an acceptable replacement for that I/S transformer on ebay. They're very very common and not used anywhere appreciable anymore. If you can find the specs, I will look through my collection of transformers and see if I can't find you a usable replacement there..

[Phil_S]

This is what I know about the interstage transformer. It is the same part in the GA20-RVT and the GA15-RVT. Mine blew and I was lucky enough to find the original part on eBay. This is the record from what I could measure.

I tried a 1:3 or 10K:90K generic from Hammond, and it was not right.

The part number is T-1001-D. The EIA manufacturer's code is 757 or Grand Transformers.

The laminations measure approx. 1 5/8" wide, 1 1/4" high, and 5/16" thick (measuring across the lams on the bottom).

The bell dimensions are approx 1 1/4" across, 3/4" high, 1 1/8" wide (going across from right to left when looking at the end bells).

Primary DCR (red to blue) is 724 ohms. Secondary DCR is 3436 ohms (green to green/white), 1834 ohms (green/white to yellow CT), 1603 ohms (green to yellow CT).

Finally, the turns ratio: 5.43vac input to the primary gives 12.92vac across the outer legs of the secondary. Let's call this 1:2.4 turns.

I don't have an LC meter, so I can't tell you the H on each side, sorry about that.

But the OP is talking about a transformer with two black wires going to the filter caps. This is without doubt in my mind the choke.

[willowhaus]

Well, I finally got around to ordering a replacement choke. One (hopefully not too dumb) question: is there a directional flow to these, i.e. does it matter which one of the 2 wires is connected to one point or the other?

[Phil_S]

It'll run in what ever direction you put it in there.