hoffman AB763 with reverb dwell and tone control (torevibestyle)

[LaszloS]

hello brothers, I am in the process of building a deluxe reverb AB763 I would like to add a dwell and tone control to the reverb, like the fender standalone style I have a torevibe and I really like the range of sounds the reverb can produce but I am tired of carrying it around with me all the time and i would like to have this included in the amp.

I have searched the forum extensively and found a few topics on dwell but none on how to implement the tone.  I have looked at the schematics for the ab763 and the torevibe and was thinking I could cut and paste but I am new to this game and I am not sure if doing that would mean other values would have to change.

so I was wondering if there is anyone who can cut it to me plain how this can be implemented.
thankyou kindly,LZ

[tommytornado]

Email me @ tomcat.arnold @ gmail . com  (no spaces of course) and i'll email you an allen old flame schematic.  It has a 3 knob control setup and fender style 2 tube  reverb circuit.

[Searing]

there's a section in Gerald Weber 2nd book about doing this to a Twin with the first channel.  I did it, bit changed it back, thought the reverb sounded funny.  Coulda' been in my head.

[LaszloS]

^^emailed

^would that be Tube guitar Amplifier Essentials?

[sluckey]

If you just want to add Dwell and Tone to the existing AB763 circuit, look at the first pic.

Hoffman has a circuit to add verb to a Plexi style amp. It could be easily adapted to the DR preamp. More info in the Library of Information (link at bottom of this page). A simple tone control could be added to bottom of Mix pot (see 6G15 schematic). Look at second pic for Hoffman's schematic.

Or, you could just build the entire 6G15 circuit into the DR.

[Searing]

His 2nd book is called "Tube Amp Talk for the Guitarist and Tech".

[LaszloS]

Or, you could just build the entire 6G15 circuit into the DR.

I thought about that but the 6g15 circuit has the 6k6 tube in it though that would be hard to fit in there. Not really sure what it does either.  everything else about the circuit is very similar.

thanks for the schems.

[Fresh_Start]

I put a Dwell control on a Princeton Reverb build and can be useful.  However, as you reduce the signal going into to reverb driver, you have to boost the reverb recovery that much more.  You might get an increase in noise as you turn the Dwell down. 

Just based on the two Blackface builds I've done, my personal preference would be to have a bass cut instead of a treble cut for the reverb circuit.  You could get there by having two cathode bypass caps on the reverb driver stage - keep 2uf in there all the time and have a small (5K?) pot to dial a 25uf pot in parallel.  Just an off-the-cuff thought.  However, that all depends on taste, speakers, the rest of the amp, etc.

Just 2 cents worth,

Chip

[PRR]

> the 6k6 tube .... Not really sure what it does

The reverb tank needs a "power" amp to drive it.

Not a lot of power. Part of a Watt. A 6V6 (4+ Watts output) is more than is needed. A 12AX7 (0.1 Watts output) is less than some users could want.

In the stand-alone, at the time, 6K6 was an obsolete radio power tube, good for several Watts, but after the 6V6 came out the 6F6 had become so darn cheap that Leo used it as a part-Watt power amp. Supply of 6K6 eventually became uncertain (for mass production) and modern clones tend to use 6V6. EL84 would also work dandy.

As you say, in a "full chassis" the big hot tubes are a problem. And since Fender made good profit on the stand-alone reverb, they did not want the built-in reverb to be as MUCH verb as the standalone. And by this time the 12AT7 was fading from TV set uses, again cheap at the time. Using both sections gets you near 0.25 Watts. Less bulk than 6F6 or 6V6, less heat than 6F6 or EL84.

Some people don't need the BIG reverb rolling in from Catalina, just a touch. And there's a real difference in power between "BIG" and "touch". A hard-worked 12AX7 seems to be enough for many.

[LaszloS]

wow! so if i added an el84 right before the reverb tank it would get that explosive reverb. rollin in from catalina is a great sound.

and an el84 that's the same output impedance for the reverb ot?

[HotBluePlates]

wow! so if i added an el84 right before the reverb tank it would get that explosive reverb.

Yes, and it has been done by Fender for on-board reverb in the VibroKing, among others.

But an EL84 is $$ when you can still buy NOS 6K6's for cheap. I bought a bunch on ebay one time in a lot of maybe 20-30. In the end, after shipping, I think I still paid only a little over $2 per tube. All were NOS, but a good number were 6K6G (coke-bottle shape) which are taller than the 6K6GT (same basic size as a 6V6). The 6K6G is too tall to easily fit in a standalone reverb, but you might have plenty of room in a normal amp chassis.

I eventually sold the reissue reverb unit I had, but still use the 6K6's from time to time to lower the distorted volume of my tweed Deluxe copy a little. The power supply voltage in that amp really amounts to "tube abuse" but so far they've held up fine and I'm not looking to make a pair last the next 10 years or more.