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Gibson AC30

1/22/2006 2:51 AM
Chris/CMW amps
Gibson AC30
Did like the GA20 and GA40 and a buddy was impressed by the rare and famous GA79RVT (stereoamp, also mentioned in the GT amp book).  
they're a pita to service, never be the same as one of the online/etc. schematics, and lots of parts mounted under the board.  
About VOX copying Gibson:  
What I did hear or read: the TB-tonestack was copied from a Gibson GA77 SCHEMATIC. On the schematic there was an error (wiring mid-R and bass-pot) and VOX did copy that with "pretty chimey results".  
Hope this helps,  
1/22/2006 12:10 PM
Thorny old tweed gibson amps
The tweed era (gibson's version of tweed - the lighter color tweed) amps mostly sound fantastic. I have repaired and owned several of them. However - everything you all say is true about the amps:  
- they never match the schematics  
- they are a mess inside compaired to a fender  
- the components are mounted on the top and bottom of the board - a recap means disassembling everything to do it properly.  
- I have had some with odd design problems from the factory that if you do repairs by the hour can be a nightmare to fix and then pay for by a customer.  
- Another amp repair friend that knows a lot more than me got one in that was not correct from the factory in the 60s - he fixed it and it turned out to be a decent amp. I wonder if it was just a design problem/revision or sloppy work and quality control?  
- I get a feeling these problems are a lot more common with the gibsons than the fenders and other amps.  
Would I trade a tweed deluxe for a tweed gibby? No way. But the gibsons did have advanced features earlier than fender even if they were not always implemented very well or consistently.  
The maestro amps of the same age were quite similar just had cheaper speakers. Same with some of the Epiphone models as well. I care less for the mid 60s era gibson amps (after the tweed era).
1/22/2006 12:22 AM
Steve A.
So how about Epiphone amps?
    A friend has a pair of Epiphone EA-50's which are supposed to be the same as the Gibson GA-5 Skylark, which uses 1-6EU7, 1-6C4, 2-6AQ5 and 1-6X4 tubes:  
    I'm not very impressed by these amps at all, and figured that new tubes might help them. Or do you think that it is a lost cause? :(  
Steve Ahola
1/22/2006 6:12 AM
Rick Adams
Steve, the EA-50 has a bridged-T circuit in place of a tone stack. That drops a huge amount of gain out of the preamp circuit and gives you a very poor man's Blackface emulation. Personally, I really like the EA-50 (I used to own one until I got offered a price I couldn't refuse) but it's really best suited for low volume rockabilly IMO. You can take out (or simply bypass) the bridged-T filter though, and you'll get a heck of a lot more gain. You'll still have the undersize OT, though, which is the common denominator in most old Gibson/Epi amps. They're also such a rat's nest that they aren't much fun to work on.  
I think a pair of these would be a lot of fun, especially with decent speakers.
1/22/2006 3:30 PM
Steve A.

Rick said:  
Steve, the EA-50 has a bridged-T circuit in place of a tone stack. That drops a huge amount of gain out of the preamp circuit and gives you a very poor man's Blackface emulation.  
    So are there different versions of the EA-50? I don't know what a bridged-T circuit is but I don't think that the schematic I posted has one...  
    Assuming that the schematic is the same as my friend's amps (a big "if") it looks like the 2nd stage uses a split plate load (with two 47k resistors) to cut the signal voltage in half, and that is followed with a whopping 250pF coupling cap!  
    So the 6EU7 uses a 9 pin socket which could be rewired for a 12AX7- right? The 6C4 is a 7 pin miniature tube, but it is listed as a power tube (up to 5.5 watts in RF applications). I guess that a 9 pin socket would fit and it could be wired up as a cathodyne or LTP PI... (So why are they using the 6C4???)  
    As for the 6AQ5's, is that a decent power tube or should the sockets be replaced with 9-pin ones and rewired for EL84's?  
    Just wondering...  
Steve Ahola
1/23/2006 6:18 AM
Rick Adams
Steve, my EA-50 followed the schematic that is in the Jack Darr book on p. 114. I'm at the wrong computer to upload it right now but you probably have it anyway as I believe the whole book has been scanned and is on the net. It's certainly out of print in real form nowdays. This version of the schematic is nice in that the physical "T" is shown on the schematic so you can see why it's called a Bridged-T. I seem to recall that the Joe Piazza schematic is the same electronically but not laid out the same as the one in the Darr book.  
The bridged-T in this one is just three components: two 220k resistors and a 500pf cap. In the EA-50 it was actually an orange Sprague thing shaped like a rectangular wafer with three legs. It's placed between the first and second triodes just before the volume control. I think you can just put a jumper across the whole thing to bypass it.  
The 6AQ5's are very good sounding tubes, being just a low voltage version (250 max) of the 6V6. They just put out less volume than the 6V6.
1/29/2006 12:19 AM
Steve A.
These are sounding REALLY NICE...
    One of them had been modded extensively and had a problem of cutting out intermittently. The other one (purchased on ebay) had a really loud hum, although the seller insisted it was working perfectly before he shipped it. :D  
    Of course the heavily modded one turned out to be a real nightmare. The wise ass amp tech who did the mods thought it was really cool to just touch the leads together before soldering them, without even giving them a full turn. This is a point-to-point amp with just the tube sockets and a few terminal boards to secure all of the parts. There are plenty of joints in mid air with nothing holding the parts together except a small dab of solder. And many of the added parts were not properly inserted into the tube socket pins but just held in place and tack-soldered.  
    I cleaned up the soldering and noticed a few wires shorting out, which is where all of the gain had gone. I wired up the heater leads to the power tube cathodes to keep the amp nice and quiet, thinking it was going to be a mellow jazz amp. When I plugged it in after doing the work it ripped my head off with its gain and distortion, a lot like a tweed champ but much smoother. THIS IS A GREAT AMP! The mods include a pot with a rotary switch to replace the on/off switch; this pot is kinda like a tone control and kinda like a gain control! The tiny coupling caps have been replaced with .022uF's and 0.047uF's. I had a lot of fun playing that amp... kinda like my Kustom Tube 12 practice amp, only the Epi is all tube.  
    So I take a look at the other amp and it is immaculate. I am sure that no parts have been replaced since it left the factory and I think that the metal back cover has only been removed a few time for inspection (half of the screws were brown and half of them were silver- would the factory have done that?) The noise is coming from all 3 power supply caps being bad, so I replace them with three 20uF/500vdc Atoms. I try it out briefly before moving on to the rest of the repairs (like replacing the 2 wire cord and maybe wiring the heaters to the output tube cathodes). Darn... this amp sounds even better than the other one! Not as much gain and distortion, but still enough to have a lot of fun. I try replacing the 6EU7 preamp tube and 6AQ5 power tubes, which fills out the bottom end a lot: this is a really nice amp!  
    Here are some chassis pix of the "virgin" amp:  
    And here is the schematic:  
[right click to save it to disk]  
Steve Ahola

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