From: John King (kingsx@swbell.net) Subject: Gibson GA-15RVT Explorer Caps and Mods Update... (kinda long) View: Complete Thread (5 articles) Newsgroups: alt.guitar.amps Date: 2003-06-21 15:18:02 PST In a thread some weeks ago, several folks were discussing the shortcomings of the mid 60's "White Panel" Explorer. They're a bit tinny and thin sounding as original equipped. I have one that had already undergone a speaker transplant, from the puny original 10" CTS, to a 12" Eminence / MojoTone:http://www.angelfire.com/blues/rockinjohn/amps/ga_15rvt/ga_15rvt_thmbs.html
The bigger, better speaker was an improvement, but it really just accentuated the weaknesses of the amp itself.
First things First:
The first order of business was a new 3 wire cord, death cap removal, and fresh filter caps. All went well there and the amp was near dead quiet. The EL84 power tubes are low hours, and the amp is cathode biased, so I just gave that a quick check and proceeded to the pre amp mods...Miles O'Neal had suggested some starting places for bringing the tone out and increasing the gain. As it turns out, his recommendations were very close to what I ended up keeping after several trials. I did add a few of my own along the way too though ;=].
Here's a *stock* schematic for any who want to follow along:
http://www1.korksoft.com/~schem/gibsonamps/ga15rvt.pdf Here's a list of the mods, and their progressive results:
Mod #1
Bypass R1 with a permanent jumper soldered to its leads (input #1 resistor), add parallel 15k resistor to R2 (input #2 resistor), and leave R3 (input #3 resistor) at its original 64K.This did open up the tone just a bit. but the difference between inputs is just barely noticeable. Input noise was increased very slightly.
Mod #2
I discovered that all inputs ran through a .0047 Orange Drop capacitor that isn't on the schematic, on their way to V1A grid. I reasoned that this was a possible bottleneck, and decided to try a .0047 tantalum cap in parallel to see what happened. This *really* opened the tone up, and increased punch and volume considerably, but the tantalum was noisy as hell! I replaced both with a .015 OD, and that was better yet, and reduced the noise back to acceptable levels.Mod #3
Despite the schematic's lack of a cathode bypass cap for V1A cathode resistor (R5), I discovered that it did indeed have a 2 MFD 25V cap on the board. I parallel jumpered it with 25uF, 25V Sprague axailytic, and did the same to the 2 MFD cathode resistor (R16) bypass cap (C8) on the second gain stage (V2A).This increased the gain a bit, and brought the lows and mids almost up to par. The amp was sounding better with each step now, and it had plenty of gain for my classic rock and blues styles, but the tone wasn't quite there yet...
Curiously, this mod seemed to have brought noise levels down! (Don't ask me why...)
Mod #4
The amp was singing beautifully by now, with plenty of mid-range feedback but there just want enough high harmonic content to get proper pinched notes or pick attack.Miles had suggested changing C1 from .0047 to .2uF. I chose a smaller increment to start with, and temporarily scabbed a .01 OD onto the existing OD. It was good. The harmonics were there in any pickup position on my test guitars, the lows were more than sufficient. I decided to try it with the .01 OD only. It wasn't as good, so I added the .0047 back in parallel with the .01 and decided to keep it that way. This mod really did a lot to open the tone up all across the spectrum. However, a small bit of noise was induced with this one.
Summation:
Changing the un-documented .0047 to .015 (Mod #2) was possibly the biggest tone/ volume difference, but being at the very front end, it is prone to noise, so keep the leads short and use a low noise cap.Swapping C1, (Mod #4) made a hugh tonal difference too, and was certainly worthwhile.
Playing with the input resistors, and increasing the cathode bypass caps for V1A, and V2A, (Mods #1 and #3 respectively) were less drastic in their immediate effect, but I believe that both added to the sum total of an overall *very* much improved amp.
The end results are tonally a hit with me! This amp now absolutely nails the Santana "Black Magic Woman" lead tone, and the Clapton "Woman Tone". There are just enough harmonics to do a bit of ZZ-Top. The tone controls are effective, and controlling the right frequencies. Clean tone and headroom is also improved considerably. Of course, at volume settings above 7, there is a bit more noise than the nearly dead quiet I had after the filter cap job, but it is an appropriate noise level compared to other amps of this gain level. Overall, I'm very, VERY happy with the results.
In the near future, there will probably be a simple web page detailing this adventure.
John King
http://www.angelfire.com/blues/rockinjohn/rockinamps.html