12at7 in Vox AC15C1

[formulaq]

Hello everyone. After doing a lot of research on amps, i have decided that the ac15c1 is the best one within my price range. However, I prefer the clarity of amps like the fender twin. After reading up a bit on tubes and their effect on amps, I've thought about replacing the v1 position tube in the ac15 with a 12at7. (Possibly a mullard cv4024, havent decided yet but something under $75 preferably). Has anyone done this? Is this a step in the right direction? I'm looking for a clearer, better clean tone. Also, the v1 slot on the vox should be the one closest to the power tubes, correct? Thanks so much.

[Stevem]

No, the first two gain stages would be in the preamp tube the farthest away the output tubes, the preamp tube closest to the output tube is the phase inverter tube which we also cut back and call the PI tube for short
.
Note that with the way new tubes are today that unless you get the current preamp tubes in your amp tested than a new 12at7 that is up to spec may have as much or more gain than a 12ax7 making a apples to apples test untrue!
I would recommend to you to try a 12au7 in the V1 spot as they are rich sounding and its gain should still let you have enough drive to get full clean volume out of the output stage!

[formulaq]

^oh wow i wouldnt have thought of that.. Thanks so much.

[rp]

Note that with the way new tubes are today that unless you get the current preamp tubes in your amp tested than a new 12at7 that is up to spec may have as much or more gain than a 12ax7 making a apples to apples test untrue

Good Lord, glad I got a good supple of old tubes, might be 2 lifetimes before I use up my 12AT7s, 3 before I use up my 12AU7s.

Just to add to the OP, I never liked the tone swapping 12AT7/12AU7s for 12AX7s. IMO best way to experiment w/ V1 gain is to stick within the family: 12AY7 -> 5751 -> 12AX7. Different 12AX7s have different gain themselves, Amperex vs Mullard vs Telefunken vs USA, and from tube to tube, though I doubt you are looking for such a subtle shift if you are considering a 12AT7. And my experience here is with vintage tubes, no idea about new 12AY7s etc. Pulls off ebay are pretty cheap and usually fine IME.

[Stevem]

A 12ay7 with its AF of 40 would be a better pick than the 12au7 I posted about with a AF of 19.5.
You might also want to go to the guitar amp blue printing site and read both section of his preamp tube primer before you send even dollar number one!

[Teleguy61]

Good advice here, but the basic thing is tube rolling in a Vox is not going to turn it into a Fender Twin Reverb.
They are very different beasties.

[rp]

Good advice here, but the basic thing is tube rolling in a Vox is not going to turn it into a Fender Twin Reverb. They are very different beasties.

That's the long and short of it.

One other thing, plugging into the low gain input isn't much different from a lower gain tube, and nothing cleans up your tone like rolling back your guitar volume, and it's free. Hell, took me 20 years to realize the volume knob was actually useful for something other than a mute for a break.

[halaanes]

I contacted Eurotubes for advice, and according to them installing a JJ-5751 Standard or Gold Pin in V1 will increase headroom by 20-30 percent. That't what I'm doing with my AC15C1 Cream. The tubes are on their way.

[M Fowler]

I think I'm going to get some JJ ECC832/12DW7 tubes to play around with watching which triode does what in the circuit this tube could be a good way to change the character of the gain circuit.

Isn't there another tube that is opposite of the 12DW7 100%/20% to 20%/100%?

12AY7 or 5751 work very well to lower the gain some.

Mark

[Teleblaster]

I think I'm going to get some JJ ECC832/12DW7 tubes to play around with watching which triode does what in the circuit this tube could be a good way to change the character of the gain circuit.

Isn't there another tube that is opposite of the 12DW7 100%/20% to 20%/100%?

12AY7 or 5751 work very well to lower the gain some.

Mark

Hi Mark,

Did you make any lee-way on this? I have some 12DW7s coming and would like to try the same thing. I'm an electronics laymen who can just barely pull off a mod on my Champ 600 and certainly not a builder by any means.

This is V2a and V2b we're talking about? How can you tell which pins would work with the cathode follower and which would work with the gain stage?

With the JJ ECC832 and all other 12dw7s, it goes like this

Section 1, Pins 6,7,8 : 100 gain
Section 2, Pins 1,2,3 : ~20 gain

With the JJECC823, it is the opposite with the low and high gain sections switched.

A user at metroamp.com was nice enough to make and share some AC15C1 schematics seen here:
http://forum.metroamp.com/viewtopic.php?t=39805

Thanks. I'm looking forward to hearing about your progress.