Gibson GA15-RVT Explorer – more gain less treble – question
Question:
Hi – long time reader first time poster… I have a mid 60’s GA15-RVT that I have recently been working on. I have converted it to a three prong, replaced the electrolytics, and put in a new (used pro jr) output transformer, and replaced the original speaker with a Eminence 102 alnico (35watt I believe). Also, after lots of reading on AGA and elsewhere on the net I did the mods outlined by John King and others here: http://groups.google.com/group/alt.guitar.amps/browse_thread/thread/f… The results are pretty good. The amp is very quiet hiss wise and sounds good enough that I have used it on several smaller gigs (generally dimed on the volume). Sorry for the long windedness, I’m getting there 
My dislikes about though are that it’s still is too bright – I usually run the bass on 8-10 and the treble on about 3. I would also like for it to go into OD just a little more. All out it just breaks up slightly. The gain increases if I up the treble – but like I said it is allready too trebly. I am using this with single coils BTW. Another thing I have noticed is that this is partly the speaker I believe. When played through my 2×12 with a Weber Blue and a G12H-30 the bass is a lot better. While I know a 10" is not going to get the same results I think it should get closer. So, after all that my questions are – is there a way to get it to break up a bit sooner, like additional tweaks to the circuit? How about adding a sag resistor? Would changing the value of the volume pot allow me to tame the brightness, yet drive the preamp more by turning it to a higher value? And finally any suggestions on a speaker? I was thinking maybe a Weber 10F150-O. (Warm, big bottom, early breakup, very crunchy, loud – is it’s description). Thanks – I have already learned a TON from the knowledge shared here on AGA.
Response:
- Hide quoted text — Show quoted text – > Hi – long time reader first time poster… > I have a mid 60’s GA15-RVT that I have recently been working on. I have > converted it to a three prong, replaced the electrolytics, and put in a > new (used pro jr) output transformer, and replaced the original speaker > with a Eminence 102 alnico (35watt I believe). > Also, after lots of reading on AGA and elsewhere on the net I did the > mods outlined by John King and others here: > http://groups.google.com/group/alt.guitar.amps/browse_thread/thread/f… > The results are pretty good. The amp is very quiet hiss wise and sounds > good enough that I have used it on several smaller gigs (generally > dimed on the volume). > Sorry for the long windedness, I’m getting there > My dislikes about though are that it’s still is too bright – I usually > run the bass on 8-10 and the treble on about 3. I would also like for > it to go into OD just a little more. All out it just breaks up > slightly. The gain increases if I up the treble – but like I said it is > allready too trebly. > I am using this with single coils BTW. Another thing I have noticed is > that this is partly the speaker I believe. When played through my 2×12 > with a Weber Blue and a G12H-30 the bass is a lot better. While I know > a 10" is not going to get the same results I think it should get > closer. > So, after all that my questions are – is there a way to get it to break > up a bit sooner, like additional tweaks to the circuit? How about > adding a sag resistor? Would changing the value of the volume pot allow > me to tame the brightness, yet drive the preamp more by turning it to a > higher value? And finally any suggestions on a speaker? I was thinking > maybe a Weber 10F150-O. (Warm, big bottom, early breakup, very crunchy, > loud – is it’s description). > Thanks – I have already learned a TON from the knowledge shared here on > AGA.
I may be able to help you on a couple of points: My main guitar for dialing in the tone on my Explorer is my favorite Gibson 335 copy. Of course I have played other, single coil, guitars through it, and I find it a bit "edgy" with them, just as you do. I think all you need to do is play around with the values in the tone stack to dial in the frequency peaks where you want them, instead of where they worked on my 335 copy. That might take care of the tone, but I’m not sure where/how to get more drive from the circuit. If you’re going to try another speaker, I’d suggest that being the first change you make, and work from there (or go back to the Eminence if you discover you liked it better). A "sag resistor" shouldn’t really make the amp "break up" (saturate) earlier, but will instead change the character of the dynamics some. Perhaps it might give a bit of "tweed" feel to the dynamics of the amp; which lacks such feel due to its use of diodes for rectification instead of a tube. Probably the easiest way of getting more drive from the amp would be to put a clean boost pedal in front of it. Effectively adding a gain stage to OD the preamp section (albeit solid state in most cases). From the looks of your header, you’re somewhere near me? Contact me directly if you want. My email isn’t munged. Regards, John King
Response:
I wonder, what is the value of the bypass cap on V1a and or has it drifted? I’ve got the Explorer’s big brother, the Minuteman GA20-RVT, a 2 channel amp with the second channel nearly identical to the Explorer. I noticed that the bypass cap was on the schematic but missing from the amp. Putting one in really improved the bass response. If you’ve got one in there, you might consider tweaking the value. IIRC, the schematic calls for 20mf, but you might try something a bit higher (33?) or lower (10?) just to see. Also, when I did the cap job, I used a JJ 2-section can cap rated at 32mf + 32mf in place of the 20+20. With a SS rectifier, I figured nothing bad would happen, and it did have an favorable impact on bass, making it more punchy. I admit that I don’t understand why, so please don’t ask me. I also think my amp is a little on the brittle side. I’ve been meaning to put it on the bench for quite some time now for the JK mods, but never seem to get around to it. It also needs some maintenance for a popcorn noise. At first I wasn’t sure what to believe about speaker advice, but I want to suggest that you experiment some. Speakers generally make a bigger difference than the output tranny on a small amp like this. Welcome to the world of Gibson amps. Regards, Phil
Response:
Hello mojowrkn I have a GA35-RVT. After installing a Weber C10N and increasing the cathode bias capacitors I got it to sound just like a Fender Deluxe Reverb. BORING. I put it back to stock, except for the Weber spearker. Yes, it is still a bit bright unless I roll the treble way down, but it sounds quite unique this way, kind of like an exaggerated blackface fender amp — very scooped mids. There are at least two sounds out of this amp that I really like, bright and brighter, and none of my other amps can duplicate these exactly. I have thought about putting a power amp section "tone cut" potentiometer and treble bleed capacitor in through the external speaker jack, a la Vox AC30, but so far have decided it isn’t worth the effort. Have fun experimenting.
Response:
What tubes do you have in there?
Response:
> What tubes do you have in there?
My GA35-RVT uses a couple of 6EU7, a 12AU7, OA2 (gas tube regulator), two 7591 power tubes. I think the unusual tone stacks and anemic cathode bias capacitors are responsibile for the brittle tone of some of the white panel amps. In my case the stock CTS speaker was inefficient as hell, the cone was stiff, and the sound was way too bright as well — probably had gone to hell with age. Chris Taylor Boston
Response:
- Hide quoted text — Show quoted text ->What tubes do you have in there? > My GA35-RVT uses a couple of 6EU7, a 12AU7, OA2 (gas tube regulator), > two 7591 power tubes. I think the unusual tone stacks and anemic > cathode bias capacitors are responsibile for the brittle tone of some > of the white panel amps. In my case the stock CTS speaker was > inefficient as hell, the cone was stiff, and the sound was way too > bright as well — probably had gone to hell with age. > Chris Taylor > Boston
The GA-5, 15RVT and 20RVT out of the "white panel" series are really very different animals from the 35,45, and 55RVT with the circuit you describe. I’ve had a couple of the 45 and 55, but not the 35 like you have. I’m not a real fan of that circuit. The whole "Williamson" phase/power section and the OA2 voltage regulator is just a bad design for the tube amp tone most folks are interested in (IMO). The smaller amps from that series are much more simple in design, and I like them because they’re easy to tinker with. The only way I’d buy any more of the larger white panel series, is if I got them dirt cheap in hopes of turning a profit on them. In fact, I’ve still got one GA-45RVT Saturn (2x 6L6 and 2-10") I’ve tinkered with it endlessly to get it sounding right; all to no good end. I’ve threatened before to strip the board and use the chassis, cab, iron and maybe the board to make a totally different amp from it. I still hope to get around to doing that with it; because it seems hopeless as-is. John
Response:
Hello John. I wouldn’t mind at all having a lower power white panel like the 15RVT or 20RVT after all the good things I have heard about these on AGA. The GA35-RVT is pretty much a one-trick pony that doesn’t have the flexibility and nice break-up that people want these days in small amps, but I do like the sound. And yes, I sank way too much time and money into mine relative to the sonic results, but I had fun restoring it and will use it at least occasionally for recording. Doesn’t the 45 have a weird mid control circuit? Maybe that is part of the problem.. Chris Taylor Boston MA
Response:
Hi guys – thanks for the quick responses. To answer a few questions, the tubes are all used older American stuff, I can’t remember which is which. I have also tried a set of JJ EL-84’s that where matched. The filter caps are the original value of 20 mf (a multi cap can from Vibroworld). On the V1a bypass cap, if I have identified it correctly, this is the 2 MFD 25V described in John’s Mod #3. I originally replaced it with a Sprague electrolytic of the same value, and then per John’s mods I added another 25uF, 25V Sprague in parallel. Would it be safe to go further by upping that value a little more? It did increase the bass I think. Please correct me if I am talking about the wrong cap.
Response:
> Hi guys – thanks for the quick responses. > To answer a few questions, the tubes are all used older American stuff, > I can’t remember which is which. I have also tried a set of JJ EL-84’s > that where matched. The filter caps are the original value of 20 mf (a > multi cap can from Vibroworld). > On the V1a bypass cap, if I have identified it correctly, this is the 2 > MFD 25V described in John’s Mod #3. I originally replaced it with a > Sprague electrolytic of the same value, and then per John’s mods I > added another 25uF, 25V Sprague in parallel. Would it be safe to go > further by upping that value a little more? It did increase the bass I > think. Please correct me if I am talking about the wrong cap.
That’s the one. I think 25 is about as high as I would go. Maybe you can look at the tonestack to take some of the edge off.
Response:
Any suggestions? I notice that the treble pot is a 2 meg, anything there that could be changed? Caps? Thanks again guys. Here is the schematic: http://www.freeinfosociety.com/electronics/schematics/audio/gibsonga1…
Response:
> Any suggestions? I notice that the treble pot is a 2 meg, anything > there that could be changed? Caps? Thanks again guys. > Here is the schematic: > http://www.freeinfosociety.com/electronics/schematics/audio/gibsonga1…
There are 3 or 4 caps between V1a and the volume pot that you might fiddle with. Have you tried using Duncan’s tone stack calculator? http://www.duncanamps.com/tsc/
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