Fender Reverb Mod Questions

[wizard333]

I'm in the process of moding some Vintage Fenders, basically putting a high gain channel where chnl 1 used to be.

I borrowed the 3rd gain stage from the verb channel, used 2 stages there and changed the 3.3M resistor to 470k. Reverb still works but has more usable range on the knob, channel stays very clean until 8 or so then gets a nice bluesy grit. Bandmasters only used 2 stages after all. So far so good.

Channel 1 using 3 gain stages is at a very nice high gain JCM800 flavored tone, messing around with values but pretty damn happy with it. I found 3 stages can give meltdown gain, I'm actually going to back it down a tad; why do people use 4 or 5 stages? hmmm.

Anyway, what I was wondering is, it would be nice to have another triode available. The Fender reverb is using a coupled 12AT7 as the driver and half a 12AX7 for the recovery.......so

1) What should I expect if I de-couple the verb send triodes and use only one triode for send and one for return, will it
a) Work with the existing verb xformer?
b) Be too wimpy? Should I switch to a 12AX7 or is the current of the 12AT7 needed here instead of the voltage gain of the 12Ax7?

2) Should I instead scrap the existing circuit and just use a circuit that uses a single 12Ax7 for send/return?
a) again, will that work with the existing verb xformer?

Thanks!

[octal]

Not sure I'm too enthusiastic about the idea of modding vintage Fenders for cascaded gain stages, but I do have some info for you.

The 12AT7 reverb driver circuit operates very close to the tube's limit of plate dissipation with the two triode sections in parallel. If you use one section for something else, the single remaining triode will almost definitely be over its max plate dissipation. The impedance ratio of the reverb transformer would also be wrong by a factor of two- i.e. I _think_ it's a 22K:8ohm tranny. You would need a 44K:8 ohm tranny to get a good match.

12ax7s make lousy reverb drivers for many reasons (high plate resistance, lower max plate dissipation, etc.)

my .02.

Nathan

[wizard333]

You could always change resistors to change the bias though right?

There are a number of single stage send/return designs for 12ax7s; the ones on Marshall800s or Boogie DCs leap to mind. I guess that would require a different verb xformer though.

I didnt know the acceptable plate impedance for a 12AT7 at ~180v (44k?); any idea on the plate impedance range for a 12ax7?

[TheElectricMoron]

On the reverb front - there are several one tube reverbs out there. The Fender 75 used half of a 12AT7 for the driver, and the Kalamazoo Reverb12 also used a single 12AX7 for drive/recover. But I can tell you for sure that the K'zoo worked WAY better with a 12AU7 in that slot. There is also a thing called a D'lite Reverb that has a single tube 'verb and the schemo is somewhere at the Hoffman amps site.

Charlie

[TheTinMan]

I'll have to admit I cringe a bit thinking about all of those mods to a vintage Fender. However, here's an idea for the reverb - use a 12DW7 one-tube reverb. Use Section 2 (pins 1, 2, & 3) for the reverb driver. It's very similar to a 12AT7 triode. Use Section 1 (pins 6, 7 and 8) for the recovery since it's just like a 12AX7. Then have fun with the "extra" triode you just freed up. BTW it is the current, not gain, that you care about for the reverb driver.

Tubenit here and on the Hoffman forum has successfully used one-12AX7 reverbs on multiple circuits.

Still, I hope you're doing this on donor chassis and not working vintage amps.

Cheers,

Chip

[TheElectricMoron]

Like I mentioned above with the 12AU7 in the K'zoo, I use 12AU7's in every reverb I have. Fenders love 'em. And they're dirt cheap and available by the bagfull. It you're gonna wire up a single socket for the 'verb and plan to stuff a 12DW7 in it, try a 12AU7 too.
Like the Man said, current is the driver and a 12AU7 will move some current.

Anyway, have fun with it.

Charlie

[wizard333]

Its a vintage amp for sure. However, everything I do on vintage amps is easily and totally reversible; I dont drill holes in the chassis. I use the extra input holes,a single off/on/on switch instead of off/on and standby, and the extra speaker jack hole to mount pots if I need. I run a parallel line off the single speaker wire with a female 1/4 to get an extension speaker jack if I need the hole for a pot.

The only thing you don't want to do to a vintage amp is drill holes, everything else is reversable. When I bought this particular one it had a blown OT, so I put in a Mercury with 4/8 taps. In any case, you can always change the OT and put the vintage one away if you ever want to go back to stock; probably safer since you wont blow the vintage one and a good new one can sound as good or better and have multiple secondary taps.

For 95% of the people that use these amps, the "normal" channel just sits there and cooks a 12AX7 for no reason. What I'm doing is leaving the classic Fender clean channel alone but adding a very usable gain channel. Its not a copy of another high gain amp circuit (try that in a Fender and see what happens....sounds like crap mostly) or anything like that, its a ground up channel design that makes a vintage Fender sound like a modded JCM800. And its all totally reversible. Heresy? Shoot me then. Most people prefer Fender clean and Marshall gain, my self included, and no amp company does a good job of combining those in one box; usually one or the other, often both, is compromised and you get a sort of Rivera effect with so-so clean and gain. I have yet to hear a stock Marshall with a good clean channel, or a Fender with a good gain channel.

As far as the driver/recovery, I've used 12AU7s in lots of fenders as the coupled-triode driver; never tried only half as driver/half as recovery. I agree its great in a Fender; reduces some of the over abundance of gain in the verb drive and makes the range of the knob more useful.

Its always an easy option to pull the vibrato circuit and use that 12AX7; however I like to see how far I can go and keep the effects and clean channel intact, i.e. not lose verb or vibrato. I'm probably being a little silly there since hardly anyone seems to use the built in vibrato.

[max_lwedge]

Here is the typical single tube reverb used by a lot of D style amp builders. Most real deal Dumbles are dry. You might think this circuit wouldn’t deliver enough reverb, but it actually has an excellent useable range. Here is a long thread on it:

The Amp Garage :: Log in


A normal Fender reverb transformer is typically used. The impedance may not be perfect, but it is cheap and works fine. The single tube reverb mixing resistor, R1, does not incur the signal loss that the BF Fender reverb does, so it works well without the extra recovery stage. A lot of people think an amp with this reverb sounds better because there is much less effect on the dry signal than with the BF reverb. If you want wetter reverb, R1 mixer can be as large as 270K. Use as small as possible. If the reverb is turned down, the circuit is close to transparent.

I prefer a 12AT7, others 12AX7. IMHO, you don't need a 12AU7 or 12DW7 for the extra current. It's been said that there is not enough signal to drive them, I haven't tried it. The AT works well and makes life simpler whenever you are sourcing tubes. With the 12AT7, I use either 3k3 or 4k7 for R4 and R6, the cathode bias. You can make R3 a trimmer for a dwell control if you want, it depends on the tank and your taste. The R10 resistor is there mainly to keep full B+ off of the plate; it is usually 10k to 47k. Some people bypass it, some put a small power cap on it, and some people leave it alone, including me. The reverb pot says 2M, but I always used 3.3M because I already have them. I typically use a 100k plate resistor instead of the listed – I shoot for 200v to 210v on the plate.

If you stuff this in a non-reverb 2 channel BF or SF Fender, it would go between the blocking cap for the PI and the mixing resistors for the 2 channels. Most models have blocking caps coming off the plates before the mixers, but some odd ones, like the 6G16, don’t. In those odd cases, you’ll need a cap to keep DC off the reverb pot.