1973 Twin Reverb...Reverb Circiut. Running Out Of Ideas

[Chuck H]

As mentioned, an au7 makes an excellent reverb driver, BUT the classic Fender circuit isn't designed to use one and the result is lower drive to the tank. Because of this I've seen some posters mistake the circuit for gain dependent, which isn't the case. The reverb driver circuit is primarily for watts, not voltage gain. Since the au7 is actually better at making watts than the at7 it's the more ideal reverb tube, but the circuit should be designed for that specific tube. The lynch pin for using a au7 in Fender circuits is the reverb drive transformer. This part being incorrectly valued for the au7 results in the low gain you experienced. So stick with the at7 tube. The ax7 you have in there is fine, but not ideal and it's working harder than it should.

[Leo_Gnardo]

Something to consider, ever since Fender started building-in their reverb, it's been missing the "dwell" control that was on the separate reverb box. "Dwell" was a volume control on the reverb drive. You could turn dwell all the way up and slap the tank hard for full surf reverb, or dial down for a spectrum of gentler reverb effects. Seems the full surf slap was the level chosen for built-ins, but not everyone needs or wants that. We've gotten used to it as part of the Fender sound. One could add a dwell control; I've offered it but no takers. OTOH dialing down the drive a bit by removing the cathode bypass cap has turned out to be a reasonable and reversible choice for those seeking a mellower reverb. Swapping in a 12AU7 another choice, and you don't even have to go inside the amp.

Yes a 12AX7 takes a beating in the reverb drive. One thing working in your favor Keith is that 2K7 in the cathode, tends to limit the current thru the tube. Still would be a good idea as Chuck says, get a 12AT7 in there. Also, there's a cap parallel to that cathode resistor, right? That will help keep the gain up to the max.

Good to see you got your Twin sorted out Keith. Up the hill there in Kamloops you'll be rockin' the Canadian Rockies proper-like. I can hear ya, all the way down here.

[Enzo]

An old trick on fender amps with the Normal and Vibrato channels, is to unplug the reverb return cable from the back, and plug it into the usually unused Normal input up front. Now that channel's controls act on the recovery.

[Drewline]

I'd forgotten about the Ice Cube. The 470K resistor trick I mentioned previously was what we did to get more gain way back when. These days I use it as a quick & dirty reverb circuit test. I've never had a problem or seen a problem from using a 12AU7 as a reverb driver in a BF/SF amp. I look at it as another viable option for the reverb. Enzo's post about plugging the reverb return into the normal channel is also another option that works for some folks.

[Chuck H]

I've never had a problem or seen a problem from using a 12AU7 as a reverb driver in a BF/SF amp.

In case this is a response to what I said, I didn't mean to imply that it was a problem, just that it reduces drive to the reverb tank.

Not long ago I did a 1 tube reverb. As in one triode as the driver and one triode for recovery. I used a 12dw7. The au7 triode and a standard Fender reverb transformer were used as the driver circuit and the ax7 triode was used for recovery. I'd estimate about 3/4 the amount of reverb drive the standard Fender circuit has. But that's with a less than ideal transformer impedance and keeping the driver triode to the max spec of 300V. Results were very satisfactory so I didn't fuss about it.

[66 Kicks]

The 12AT7 has somewhere around twice the transconductance as the 12AU7 at the same operating point. This means that it will take somewhere around twice the input signal to the grid of the 12AU7 to get the same current swing as a 12AT7. So when you put a 12AU7 in a Twin reverb driver, you're going to be driving the tank around half as hard. Also, if you put a 12AU7 in a stock Twin driver with a 2.2K cathode resistor, it will draw around twice the current as a 12AT7.

The 12AU7 can be an analogous driver to a 12AT7 by increasing the 2.2K cathode resistor to 5K (or so) and doubling (or so) the magnitude of the input signal.

[66 Kicks]

66 Kicks thanks very much for pointing this out. You are right mine is not an AA270. The schematic you sent is the correct one. Is there a layout available for this same circuit? I'd love to have a copy.

I don't have one and a quick search came up empty.

[Chuck H]

The 12AT7 has somewhere around twice the transconductance as the 12AU7 at the same operating point. This means that it will take somewhere around twice the input signal to the grid of the 12AU7 to get the same current swing as a 12AT7. So when you put a 12AU7 in a Twin reverb driver, you're going to be driving the tank around half as hard. Also, if you put a 12AU7 in a stock Twin driver with a 2.2K cathode resistor, it will draw around twice the current as a 12AT7.

The 12AU7 can be an analogous driver to a 12AT7 by increasing the 2.2K cathode resistor to 5K (or so) and doubling (or so) the magnitude of the input signal.

Good additions! Transformer impedance not withstanding. Since there isn't much option to increase grid signal with the typical Fender design it's a reasonable consideration to increase the coupling cap size above Fenders 500pf value. The lower frequencies add a lot of signal. And since we're basically just shaking the springs on this end of the circuit, and not trying to EQ things much it works just fine as long as you don't get carried away. I used a 1500pf.


I biased for proper idle current by using my scope to see that the driver was centered when the grid signal rose enough to start compressing the wave form. This circuit IS basically a power tube after all.

[66 Kicks]

Not long ago I did a 1 tube reverb. As in one triode as the driver and one triode for recovery. I used a 12dw7. The au7 triode and a standard Fender reverb transformer were used as the driver circuit ...

What did you bias the au7 at?

I did the same thing except I put a 6.8K resistor in series with the transformer and ran it at 360V. It put 204 ma_rms into 4.1 ohms at 500Hz biased at -9.5V.

[Chuck H]

What did you bias the au7 at?

I did the same thing except I put a 6.8K resistor in series with the transformer and ran it at 360V. It put 204 ma_rms into 4.1 ohms at 500Hz biased at -9.5V.

I can't remember well enough to promise accuracy. It's a customers amp. IIRC it was right at 10mA with 305Vp. I never measured the actual output because the circuit worked and sounded good. Well, except for the hassle of quieting tank microphonics in a 2x12 combo cabinet and a strange oscillation that was only stopped when I grounded the speaker baskets. So my troubleshooting on these issues ran me right up to delivery time.

[g1]

There's a layout for the AA270 in here (pg.2):