Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: El Rollo on February 28, 2016, 03:02:46 pm
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Hi,
I recently bought a Vox AC4TV to play Harmonica through because it was the only tube amp I could find in my price range. I own a Epiphone Acoustic Studio 15C SS amp that was just too clean and no way to get the sound I wanted even with a tube overdrive pedal. Back to the Vox the previous owner swapped out the 12AX7 with a 12AU7 I guess for more headroom before breakup since the AU7 is 60%of the gain of the AX7. Well the amp is much better than the SS amp in tone and close to getting the sound I wanted. I saw on another board of changing the values of some of the caps in the audio chain. So I picked up some caps to change C5, C4 and C6. C5 I changed to 22nf from the 4.7nf to get more bass response. C6 in the schematic calls for a 10nf but there was no C6 installed on mine so I cleared the holes on the board and installed a 47nf cap. C4 is a 220pf and it voices the amp towards the bright side so I replaced it with a 470pf Silver Mica that I had on hand ( I have been repairing Ham Radio gear for a few years now). After putting the circuit board back and reassembly Man what a change I am now getting much better Tone control and am getting close to that Chicago Blues sound I a looking for. I guess my next move is making a larger cabinet for a 2X10 setup I think that will put me where I want to be with this amp. Now I want to know if I should change the AU7 back to the AX7 to see if that puts it more toward the sound I am looking for. Also if I want to add a Reverb circuit where would I inject in into the signal path before or after the Preamp stage. I am new to Guitar amps or really any amp running in class A but put a RF amp in front of be running class AB2 with 3KV on the plates I am quite fine. So what do you think guy's?
Thanks for any ideas,
From the Island of Hawaii
El Rollo.
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I think the 12ax7 will be an improvement; it will give you less headroom which, I understand, is harp players want. If it gives you too much early breakup, then try a 12ay7. Further tweaks would be putting in 22uf cathode bypass caps if it doesn't already have them. Sounds like you're on the right track though!