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

[keithb7]

Cathode cap installed. Only had a 2.7k resistor. Tried it. Much better. Not quite as much reverb as it should have yet, but we're 60% there I'd say.
I'll pick up the 2.2K tomorrow.

I'll wrap this thread up by saying thank you again everyone for your assistance, and being patient with a rookie.

[loudthud]

I remember seeing a discussion some years ago about some Fender amps having the wrong value on some schematics in the reverb drive cathode circuit. The value on the schematic tended to burn up a 1/2W part because the current was too high. A 1W part might be a more reliable option.

[Chuck H]

I remember seeing a discussion some years ago about some Fender amps having the wrong value on some schematics in the reverb drive cathode circuit. The value on the schematic tended to burn up a 1/2W part because the current was too high. A 1W part might be a more reliable option.

And some examples also left Fender with 1W CC reverb driver cathode and PI bias resistors.?. I guess there may have been some differences depending on who was supervising during which working shift too

[Enzo]

And the lack of the parallel cathode bypass cap would not cause the resistor to burn up, it would only cause a reduced gain in the stage.

[pdf64]

There are various mods to tame the reverb level in BF / SF Fenders, removing the V3 driver cathode bypass is one of them.
That also acts to help the reverb channel to overdrive more pleasantly, which may be useful if the master volume is turned down to facilitate that.

[Drewline]

In my own amps I like to use a 12AU7 rather than a 12AT7 as the reverb driver since I don't use much reverb. A long ago I learned a trick which I've since found to be very helpful in troubleshooting reverb circuits in these amps: Solder a 470K resistor between 2 RCA plugs & plug it in place of the reverb tank. Now the reverb knob is a gain boost. A quick way to check the reverb circuit without pulling the chassis when you have no reverb.

[Leo_Gnardo]

In my own amps I like to use a 12AU7 rather than a 12AT7 as the reverb driver since I don't use much reverb. A long ago I learned a trick which I've since found to be very helpful in troubleshooting reverb circuits in these amps: Solder a 470K resistor between 2 RCA plugs & plug it in place of the reverb tank. Now the reverb knob is a gain boost. A quick way to check the reverb circuit without pulling the chassis when you have no reverb.

I always wondered whether a 12AU7 might be more at home than 12AT7 driving reverb. Any circuit changes suggested?

I find anywhere from 270R to 2k2 used as cathode R's for Fender reverb drives, bypassed with 22 or 25 uF. To let the drive tube run cooler, I favor increasing the lower value R's to 2k2. And if the reverb's getting smacked too hard, I remove the cathode bypass cap to drop the gain a notch.

Your bridging reverb drive & return with 470K is the basis of the old "Ice Cube" boost mid 70's. I'm sure they got it from somewhere.

[66 Kicks]

This Twin is not an AA270. The AA270 doesn't have a master volume nor does it have that little 220K/.01uf feedback network in the PI. The reverbs are slightly different, but the same idea. Attached (hopefully) is a schematic that matches this Twin.
Fender_twinreverb_sf_100_mv_noboost.pdf

[Jazz P Bass]

In my own amps I like to use a 12AU7 rather than a 12AT7 as the reverb driver since I don't use much reverb. A long ago I learned a trick which I've since found to be very helpful in troubleshooting reverb circuits in these amps: Solder a 470K resistor between 2 RCA plugs & plug it in place of the reverb tank. Now the reverb knob is a gain boost. A quick way to check the reverb circuit without pulling the chassis when you have no reverb.

Wasn't there a gimmick back in the late 70's that used the reverb for a gain boost?

And that's what it was. A 470K resistor.

The Cube something or other.

[66 Kicks]

I don't see how you guys figure the 1/2W cathode resistor might burn up. After doubling the milliamp numbers on the Average Plate Characteristics graph, the most wattage across any cathode resistor appears to be 1/10W. Even if the plates are connected directly to the 450V node with no transformer, the maximum appears to be 1/7W. The Fender setup in this Twin with a 2200 ohm cathode should only have 1/18W across that resistor (5ma and 11V).

[Leo_Gnardo]

Wasn't there a gimmick back in the late 70's that used the reverb for a gain boost?

And that's what it was. A 470K resistor.

The Cube something or other.

Ice Cube. I'm not sure the value of their resistor, possibly 220K but same idea. Late 70's I'd drill a hole back of a Fender (horrors!), mount a resistor & pot so the user could dial in an OD level.

Not to be confused with JBL's Ice Cube lightweight power amp, an early class D fizzle. I have one here, broken, as they do, if anyone wants to dissect it.

[Jazz P Bass]

JHD Ice Cube.

[keithb7]

This Twin is not an AA270. The AA270 doesn't have a master volume nor does it have that little 220K/.01uf feedback network in the PI. The reverbs are slightly different, but the same idea. Attached (hopefully) is a schematic that matches this Twin.
[ATTACH]37644[/ATTACH]

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.

[Chuck H]

Keith,

How's the reverb level coming along? Did you install a bypass cap and fuss with the final bias resistor value? A few posts ago it sounded like you were on top of the root problem in this thread. I'm just interested if that's been solved for and we've moved on to a more ethereal discussion on the matter.

[keithb7]

Chuck, I had the 12AU7 in the reverb driver when I mentioned that I felt like I was 60% of the way there. I put in a 12AX7 and its all there now. The circuit calls for 2.2K resistor. It had the old original carbon comp resistor in there. I had a new 2.7K at 3%. It's pretty close and it sounds great. I think I'll leave it unless you recommend otherwise. I am happy. It's fixed. Thanks.