Ampeg J-12 Jet too quiet

[Axtman]

I recently pickup up a 1968 Ampeg J-12 Jet combo guitar amplifier. The amp has great tone but is too quiet. The volume is not so soft that something is broken. Since amp uses a pair of 7951 output tubes pumping out 20 watts (?) I would assume it would be on par loudnesswise with a Fender Deluxe Reverb.

Any ideas on how to get more volume without seriously hacking it up. Thanks.

[Leo_Gnardo]

I recently pickup up a 1968 Ampeg J-12 Jet combo guitar amplifier. The amp has great tone but is too quiet. The volume is not so soft that something is broken. Since amp uses a pair of 7951 output tubes pumping out 20 watts (?) I would assume it would be on par loudnesswise with a Fender Deluxe Reverb.

Any ideas on how to get more volume without seriously hacking it up. Thanks.

How about we look at the real power, not what Ampeg or someone else claims. I have a similar amp, converted to 6V6 back when 7591 were rare & expensive if you could find them. Has those funky triple triode pre tubes too. Measured a heroic nine watts at clip. Not loud enough to hurt anyone's ears at full roar, BUT ahhhh the tone! Mic that up for recording, twist the volume control to 8, 9, 10, it sounds like a big stack of whatever brand you choose to imagine, just perfect. My Jet has a Recoton Jensen C12Q speaker, good enough for the job, and cheap. You might try different speakers with yours, anything more efficient than what it has will bring up the volume.

To truly boost the volume, you would have to beef up the power supply with a higher voltage power transformer. For a kool recording amp like this, I'd avoid that & let it be what it is.

[Axtman]

Leo,
I am not trying to turn a Ford Pinto into a Ford Mustang. I am just surprised how gutless it is with a pair of 7591 output tubes. My theory is that Ampeg quieted the amp down in order to preserve the clean headroom.

I am currently comparing the amp's schematic to Fender schematics. I might change out the cathode resistors, plate resistors and coupling resistor. R7 at 5.6M seems ridiculously high. I am puzzled how the tremolo circuit works off that resistor.

[eschertron]

How do you know that the output tubes have any life left in 'em? Got a new pair to try?

edit:
I agree adding cathode resistors will give some gain.
just for grins, lift the NFB resistor.

[Justin Thomas]

I've had a 66 J12-T with a similar sort of issue for years. I just leave it on the back burner, but I know exactly what you're talking about. It was significantly quieter than my Champ was. I've owned several small Ampegs over the years & the Jet is definitely not working right.

Justin

[g1]

I think Leo was trying to point at the supply voltages. I haven't looked at schematics, but if it's running at significantly lower voltages, (with reference to the Deluxe mentioned) you are not going to make a big impact without changing the iron.

(edit: I see Justin comparing his to a Champ. That is a different story. )

[Justin Thomas]

One other question: speaker?

Justin

[mozz]

Have you tested the resistors? Especially R7. Check the cathode cap C7. If you plug into the bright channel, you have 47k going right to ground. Add a cap across R5.

[eschertron]

If Leo is singing its praises, why are we in a hurry to modify it? (I know, I suggested mods myself :slap: )
If part of it is broken let's fix it first?

[Dave H]

It looks like it doesn't have much gain.
Does it start to distort when you turn its vol pot right up?

[Axtman]

I put a sine wave signal into the input, cranked the controls and measured the voltage across a dummy load at the speaker. I got 4.7 volts across and 8 ohm load. P=Vsquared/R so (4.7x4.7)/8=2.76 watts of raw animalistic power!

[Jazz P Bass]

First off:
What is the ac voltage of the input signal?
(I use 100mv)
What frequency is at the input?
(I use 1K)
What ac voltage is actually reaching the output tube grids?

[TelRay]

Hi!
i see this post faded away a year ago, I too have a J12 and always wanted "more power" (maybe that's why i got a fender twin... hahahahah)
i am guessing that if you are getting less than 3W there's something fundamentally wrong with the amp and I am guessing if you ever found out what it was.

[Axtman]

I got back to the amp and checked ALL the resistors (out of circuit). Only 2 were out of spec and replaced. I also put two good 7591 tubes in it and installed a new speaker. The amp is a bit louder but really should about as loud as a Fender Deluxe.

I am toying with the idea of replacing some of the resistors in the preamp section to make it more of a Fender topology. Or maybe I will just leave it stock design.

[Chuck H]

What you have to do is set up a test. First tack a capacitor to the ungrounded end of C4. Just one end of the cap. You'll be testing at the unconnected end. This cap can be any standard coupling cap type value of, say, 10n to 100n. Now put a 100mV input signal to the amps input. Trem off and tone at 5. Monitor the output signal as you increase the volume knob. Can you get the amp to clip? If yes then leave the amp adjusted so it's just starting to show flatted points on the wave form (clipping) and use your scope to check signal at unterminated lead of that capacitor you tacked in. Do you see any clipping? If not then there's really nothing to be done (EDIT: Other than repair and maintenance). If you DO see clipping there then the signal from the preamp is clipping BEFORE it gets to the power tubes. In this case it may be possible to increase output a bit by modifying the preamp to deliver enough clean signal that the power tubes clip before the preamp.