Ampeg "Guitaramp", 1956?

[vibrating string]

Well, I received the Weber 12A125A and hooked it up on the bench last night and I like it. It sounds a little crisper than the old Jensen. There is a little more bottom too, I think. I will know more when it is in the cabinet again....maybe tonight.

I went around the apron of the old Jensen P12Q with a layer of contact cement to shore up a couple of little tears that I made accidentally last week and reinforced the rest of the apron with a couple of layers. I am sure this is a sacrilege to the purists. But the speaker sounds excellent and clear.

Larry

[jjmt]

I started using liquid electrical tape for reinforcing the surround.

I didn't even know that existed. Sounds super-useful.

[vibrating string]

Is the cone type on the old P12Q similar to the 12A125A? I started using liquid electrical tape for reinforcing the surround. It dries better than the contact cement and remains pliable.

The cone appears similar, with slightly wider corrugation spacing. The old Jensen cone does not have the sheen of the new Weber. Both speakers sound excellent, outside the case.

I ran into a peculiar issue though. When everything is put back into place in the case, the sound CRACKS quite badly on loud passages. This happens with both speakers. While the cracking sound is happening, I can abruptly stop it by tapping on the preamp section of the circuit. I suspected a bad connection somewhere. When I removed the electronics and put everything on the workbench, it sounds clean. I suspect a contact issue that only shows up when the screws tighten the electronics to the case. I am looking into this.

Larry

[vibrating string]

Followup: I cannot reproduce the sound cracking issue with the upper (preamp) section on the work bench with either the original Jensen or new Weber speaker. So I fashioned a rig to hang the upper section in the same orientation as when installed. Could not reproduce the crackling sound. I re-installed the upper section and tightened the hold down nuts in steps until they were tight. At no time did the crackling sound return. The amp is all back together and I suspect that one day I will hear that sound again. I listened to the crackling audio file sent in by JJMT recently. My crackling sound is a bit more severe and only at louder passages.

I have already replaced all electrolytic and foil caps except for the Astron .1MFD caps along with several resistors. SO I have been playing the amp it sounds clean. Wait and see time.

Larry

[vibrating string]

It has been over a week and I have NOT been able to reproduce the crackling sound on the old Ampeg. The amp sounds clean, but I never pinpointed the culprit. It is all back in the case with the new Weber speaker. It sounds great and the tremolo effect is so sweet.

One change I did make was to add a 500k resistor to the high end of the 1M volume pot to limit the maximum volume where I never go and give me a slightly less sensitive volume knob. I don't know if this is the best way to attack that issue, but I figured that this is what voltage dividers do well.

Larry

[jjmt]

One change I did make was to add a 500k resistor to the high end of the 1M volume pot to limit the maximum volume where I never go and give me a slightly less sensitive volume knob.

Hi. I know it was me that pointed you at the volume pot, and what you've done will certainly have the desired effect. But having thought about it a bit, I would ask if your amp has a negative feedback loop? Negative feedback reduces an amp's gain (volume), and if it isn't working (open/off-spec resistor) you would certainly see your amp go to full volume prematurely. Since there is no schematic, nothing to do but trace the lines from the speaker side of the OT.

[vibrating string]

I did work out a good bit of the GuitarAmp circuit in sections. I see no negative feedback loop on the preamp or on the 6V6 power amp sections. There is no sign of the tube outputs feeding back to the grid inputs, which is how I imagine the negative feedback would appear. I have not removed the hardware again from the case to verify that, but I think my schematics would picked that up.

At this point, the amp sounds great and I am happy. It is not as quiet as my Gemini VI, but the hum is fairly minor and I love playing on it.

Thanks for the feedback! (No pun intended.)

Larry