|
Dirty-South Blues Harp forum: wail on! >
Amp repair advise
Amp repair advise
Page:
1
rbeetsme
1007 posts
Dec 26, 2012
1:06 PM
|
This summer I picked up a late 50's vintage Gibson GA20, looks original. Bought it online from a longtime guitar guy who swore it was in great shape. Arrived looking fine, but pots are a little scratchy and the guitar inputs cut in and out, sound bad. Will work on with the mic input but still cuts in and out. I didn't pay much for this so I was willing to get it looked at, but my local amp tech (well known and very talented) always takes about a year to get to anything. The amp came with the tubes removed, so I installed them, old tubes, not well marked. Any chance a tube change would do any good? Any ideas a non tech person might try? BTW: I also have an earlier 50's version of this amp that works fine, killer amp, tubes are different so I can't just swap them.
Last Edited by on Dec 26, 2012 1:07 PM
|
LSC
349 posts
Dec 26, 2012
1:28 PM
|
Start with getting yourself a can of spray contact cleaner. Should be available from most any electronic supply store. Be sure it's proper contact cleaner and not WD40 or some such. WD40 is great for freeing nuts and bolts and the like but it is oily. Contact cleaner evaporates very quickly and doesn't leave any residue.
You should be able to remove the chassis fairly easily to access the pots. Be sure to unplug it first. Spray a bit of cleaner, don't drown them, into the pots while working the knob from lock to lock several times. That should clean out any dirt or moisture and eliminate the scratchiness. Spray a bit of cleaner into the inputs as well. Stick a plug into it and just make sure the clip is making contact. Reassemble. Plug it in and test. This usually cures these sort of problems. If not, probably best to take it along to a tech for a service. A good idea with vintage amps anyway.
If your local guy is that slow find someone else but be sure he's experienced with vintage amps. If there really is no one local you'll have to ship it to someone like Skip Simmons but that should be a last resort. Gets pricey shipping amps around. Having said all that if you've had it since the summer it doesn't sound like your desperate to get it back anyway. You can always ask the local guy if he can move it up the line. Some techs will do an expedited service for an additional fee.
----------
LSC
|
rbeetsme
1008 posts
Dec 26, 2012
2:02 PM
|
Thanks for the suggestions. Well acquainted with Skip. I often recommend him too, was looking for user tips.
|
5F6H
1464 posts
Dec 27, 2012
4:40 AM
|
The pots may be noisy due to being dirty, or because the coupling caps are leaky and you have dc voltage on them...JD & LSC's tips will help with the first case scenario, but if cleaning doesn't solve the issue get a tech to check over the coupling caps.
Cutting out - Check pots & jacks are tight to the chassis. Does this cutting out happen randomly, or when you are playing particular things, like under a big signal? Do you have the correct designation tubes installed (e.g. no higher gain subs). You say it is late 50's, this should have cathode biased preamp tubes, with no capacitor in series from the input jacks to the preamp tube pins, is this so?
-----
www.myspace.com/markburness
http://www.facebook.com/markburness
|
rbeetsme
1010 posts
Dec 27, 2012
4:35 AM
|
5F6H, I'll pull it out and check all of the above. May post a pic. I have found 4 different schematics for this amp, all with different tube layouts. Will make an effort to figure it out. It looks to me like the 3 preamp tubes are all 12AX7s, none are marked.
|
rbeetsme
1011 posts
Dec 28, 2012
8:28 AM
|
Ok, I pull the back off the amp, pretty simple, very clean. tube line-up left to right is 5Y3, 6V6GT, 6V6GT and 3 - 12AX7 pre amp tubes. The only schematic I can find shows the pre-amp line-up as: 6SL7, 6SJ7, 6SJ7. I check the 6 series pre-amp tubes, all look larger and have a large guide pin like a power tube. It doesn't look like the pre-amp sockets were ever changed. Now what?
|
5F6H
1466 posts
Dec 28, 2012
10:29 AM
|
Follow the heater wires from pins 2 &/or 7 of the 6V6s (should be a wire at each 6V6 pin 2 and one other from each pin 7, but not all amps follow the schems so one "leg" might be grounded). Which "12A#7" pin no's do they run to? Count clockwise from the keyway. Gibson were fond of using other 9 pin types like 6EU7 and 5879, first thing to do is to check that the "12A#7" you have really are just that. You could theoretically have similar looking, but differently functioning tubes in the wrong sockets.
----------
www.myspace.com/markburness
http://www.facebook.com/markburness
|
Post a Message
|
