Replacing V3 with 12AU7 to cut volume in PV C30/DB?

[Steve A.]

With the new tube covers on the Classic 30 and Delta Blues there is not a lot of places to mount a master volume control. So I figured why not try replacing V3 with a 12AU7 or 12AY7? V3 has the pre-driver and the cathodyne driver stages. Here is a chart I've saved which give ballpark figures for the amount of gain from the various 12A_7 tubes:

MU FOR 12A_7 PREAMP TUBES AND DAIRY COWS

12AX7 [100]
5751 [70]
12AT7 [60]
12AZ7 [60]
12AY7/6072 [44]
12AV7/5965 [37]
12AU7/6189 [20]

So is there any downside to doing this? Will my El84's get flabby from lack of strenous exercise?

I figure I can always swap the 12AX7 back in if I need the full power of the amp.

This just seems to be too easy to be right...

Steve Ahola

[Rob Mercure]

Steve,

Just a picky bit of technogargle - which I'm sure you actually know - "gain" isn't the same as the amplification factor and in most BF Fenders, which I'm familiar with right off my head, the stage gain is only around 30-36 while the 12AX7/7025 sections have a mu of 100.

Not being off the top of my head (fleas, lice) familiar with the C-3-Oh (a starwars prototype?) would a 12DW7 pin out to let you use the 12AU7 section where you want it?

Rob

[Steve A.]

Steve,

Just a picky bit of technogargle - which I'm sure you actually know - "gain" isn't the same as the amplification factor and in most BF Fenders, which I'm familiar with right off my head, the stage gain is only around 30-36 while the 12AX7/7025 sections have a mu of 100.

Not being off the top of my head (fleas, lice) familiar with the C-3-Oh (a starwars prototype?) would a 12DW7 pin out to let you use the 12AU7 section where you want it?

Rob

I look at those numbers only as a rough guideline and know that a 12AX7 has a lot more gain than a 12AU7. But I'm not sure how that 100:20 ratio translates into actual gain or power- whether you look at the square root of the ratio or the cheroot of the cigar...

I also know that 12AT7's draw a lot more current than 12AX7's not to mention a big difference in impedances. I'm not sure if that is true with other members of the 12A_7 family.

Thanks

Steve Ahola

[Enzo]

REmember that cuting power in half only reduces volume 3db.

You could always make one of those volume control in a box things and stick it in the effects loop. Make the cords long enough so you can chuck it into the bottom. Now you have a master volume that works on both channels and at any time you can dial it back up for a gig or whatever.

I don't know how much more current a 12AT7 might draw, I do know it can HANDLE larger currents. The external circuit components are in charge of the current. But in any case, ONE 12AT7 is not going to tax the amp, and impedance-smimpedance, plug one in there and see how it sounds if you like, won't hurt it. I think it will affect headroom in the PI more than volume. You know a favorite trick is a 5751 in V3 for that very purpose.

You could also make a very short patch cord with a couple resistors in one of the shells as a voltage divider. A pad. It would be like a permanently set master volume. Stuff your pad-cord into the FX jacks and wail quietly.

You want to get real fancy? Make a short patch cord with a big fat #70 plug on one end. Mount a tiny trim pot inside it and drill a hole in the shell to access it, and make a mini-master that way. Set it with a tiny screwdriver.

[Steve A.]

You could always make one of those volume control in a box things and stick it in the effects loop. Make the cords long enough so you can chuck it into the bottom. Now you have a master volume that works on both channels and at any time you can dial it back up for a gig or whatever.

I just got one of those volume pedals and was thinking that I could plug it into the effects loop if I wanted to. What resistance pot would you recommend.

I don't know how much more current a 12AT7 might draw, I do know it can HANDLE larger currents. The external circuit components are in charge of the current. But in any case, ONE 12AT7 is not going to tax the amp, and impedance-smimpedance, plug one in there and see how it sounds if you like, won't hurt it. I think it will affect headroom in the PI more than volume. You know a favorite trick is a 5751 in V3 for that very purpose..

I've never liked the sound of any of the 12AT7's I've tried. My comments on the impedance had to do with replacing the 12AT7 in a blackface PI with a 12AX7- supposedly the impedances would be off and you would really want to replace some of the resistors in the PI.

I just remembered that one of the 12A_7 replacements was not a good choice for the modern PCB amps with the tiny 1/8W screen and cathode resistors- someone here reported those resistors burning out on an amp- for some reason I think that was a 12AU7...

You could also make a very short patch cord with a couple resistors in one of the shells as a voltage divider. A pad. It would be like a permanently set master volume. Stuff your pad-cord into the FX jacks and wail quietly.

You want to get real fancy? Make a short patch cord with a big fat #70 plug on one end. Mount a tiny trim pot inside it and drill a hole in the shell to access it, and make a mini-master that way. Set it with a tiny screwdriver.

One disadvantage to using the FX loop like this is that it does engage the SS circuitry of the loop (which would otherwise be bypassed). But I say "all tube? all schmube!"

Steve Ahola

P.S. I just plugged in a quartet of NOS GE 6BQ5's from the mid-60's and it smelled like something was burning so I unplugged them and put the stock JJ's back in. I remember there being threads about certain pins being used in the old 6BQ5 tubes that aren't usually being used in the EL84's manufactured today...

Any thoughts on this? Thanks!

[Enzo]

Look at an old data sheet for the tube type, then look at the board layout drawing. Newer ones don't include the traces, but I believe the older C30 layouts include traces. The boards for the DB are the same except the extended part under the controls for the trem. See if any of the "unused" pins are connected by trace somewhere unwanted.

SO try the loop volume and see how it sounds. It may introduce that one transistor emitter follower stage, but can you hear the difference?

I'm not a fan of the 12AT7 either, but I think the 5751 sounds good there. If you have one, try it.

[tubeswell]

I found that I quite like the 1957 GE12AT7WA (6201) in V1 of the C30 if you change the power supply resistors for the B+ and B++ to between 4k7 and 6k8 (instead of 10k). Really nice cleans tho' not as syruppy as a 5751. Not bad in V3 either.

[Rob Mercure]

As far as members of this "family" of tubes with the same basing the 12At7 really do stand out when you use them - as compared with the AU,AZ,AY,BZ, etc. Perhaps one of the reasons is the the AT was never designed as an audio tube but instead as an RF mixer - but Fender, as pragmatic as ever, found that they had the desired gain and current/power handling capabilities coupled with the right price. If you start comparing 12AT7s in your collections you'll notice that there aren't any "long plates" while all the other 12A_7s have both short and long designs over the years. Another thing is, for all practical purposes, all 12AT7s sound pretty much a like while the variations within the 12AX7, for example, cause some folks to pay more for an example than I've paid for some cars (Telefunken E88CC going for a $1K on e-beast recently!). Not the most musical of tubes that's for sure.

Rob