ValveWizard's No-Transformer reverb driver

[Lectroid]

Hello all,

Has anyone ever tried out Blencowe's transformer-less reverb driver for a high-impedance tank? Read about it here:

http://www.valvewizard.co.uk/reverbdriver.html

I have a source for 4F... tanks (1425-ohm input impedance) from old Hammond organs.  My next project is a single-channel Deluxe Reverb AB763 with reverb but no tremolo.  His discussion and reasoning make sense to my limited understanding, and I think it could be adapted to the AB763 eyelet board pretty easily. 

Has anyone ever tried it and do you have any wisdom/warnings to impart?   

Thanks!

[T Wilcox]

Havent tried the Merlin version but I built a Benson clone that does not use a transformer that sounds excellent using a 12DW7. Schem attached

Todd

[Lectroid]

Thanks, Todd

Never heard of a Benson amp before, but it's an interesting design. Uses only 1 low-mu driver tube for the reverb where VW uses 2 in parallel.  And you don't see too many paraphrase PI.

[thetragichero]

i've used this one tube reverb for a hammond organ build using a 12dw7 (which i got from the organ). the au7 half for the driver and ax7 half for the recovery

[Williamblake]

I tried the cascode driver but liked an EF80 as driver better. The cascode driver is intended for medium impedance but it didnt even drive those tanks better than an EF80. Also EF80 is a cheaper tube.

[sluckey]

Here are two proven transformerless reverb circuits that sound very good. I've owned both of these amps. Still have the Magnatone. The Magnatone circuit seems a bit unusual at first glance but makes more sense as you study it.

     http://sluckeyamps.com/magnatone/Magnatone_M10A.pdf
     https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_g15_gemini_ii.pdf

[Lectroid]

Thanks everyone.  I was hoping for a discussion of Blencowe's design but I got a whole lot more to think about/look over for my target AB763 DR.

@thetragichero: I thought about using the Hammond native circuit (the iron is from an AO-43).  Did you find the reverb strong (deep?) enough with just one 12AU7 section? 

@Williamblake:  Do you have a schematic for the EF80 circuit? Sounds interesting.

@sluckey:  The Magnatone design is real similar to Blencowe's except it doesn't have a series resistor after the coupling cap, which he recommends. 

Is using two 12AU7s in parallel what give us more current output?  Or is it to get the output impedance into the right ballpark for the [4F...]  tank?

Also, why is the V6-pin 8 cathode connected to the cathode resistor of V7?  I can see it's a voltage divider but I don't get the intended function.

Thanks again!

[sluckey]

@sluckey:  The Magnatone design is real similar to Blencowe's except it doesn't have a series resistor after the coupling cap, which he recommends. 

Is using two 12AU7s in parallel what give us more current output?  Or is it to get the output impedance into the right ballpark for the [4F...]  tank?

yes and yes

Also, why is the V6-pin 8 cathode connected to the cathode resistor of V7?  I can see it's a voltage divider but I don't get the intended function.

That's the dry signal path. The wet (reverb) signal is applied to the grid of V7B and the dry signal is applied to the cathode of V7B. The two signals are magically mixed together inside the tube. No mixing resistors needed. Kinda neat, huh? Might be more obvious if you look at page 4 of that pdf.

[Lectroid]

Is using two 12AU7s in parallel what give us more current output?  Or is it to get the output impedance into the right ballpark for the [4F...]  tank?

yes and yes

So to match the driver's signal to the 'high' impedance of the 4F- tank (1425 ohms), the signal from the 12AU7(s) tank driver should have (relatively) higher current and lower voltage, is that right? The closer the match, the greater the power transfer? 

That's the dry signal path... Might be more obvious if you look at page 4 of that pdf.

Ohhh...you mean page 4?  Where it has "Dry signal path" highlighted in blue?      Damn--I've got to learn to read one of these days...

sluckey, thanks again for all your help.

[Williamblake]

This is the EF80 driver i am very happy with. It also has an EL95 driver but only one is used dependig on the tank.

[thetragichero]

@thetragichero: I thought about using the Hammond native circuit (the iron is from an AO-43).  Did you find the reverb strong (deep?) enough with just one 12AU7 section? 

yes it sounded great. i didn't use the stock reverb circuit because i used the 14H choke between the plate and screen power supply nodes

[Lectroid]

i used the 14H choke between the plate and screen power supply nodes

How did that work for you?  I've put that into my schematic but wasn't sure if it would make a difference.

[sluckey]

I used that little choke in my Plexi6V6 amp with no issues.

Dude is using that choke in his Plexi6V6 that uses EL34s.    https://el34world.com/Forum/index.php?topic=28945.0

[Lectroid]

Good confirmation, thanks!   

[thetragichero]

it's great because it saved me ~20 bucks
also it works and provides a lot of filtering, which is especially nice if you're concerned about first filter cap size and the 5u4 (I'm not, but also add the big Milwaukee 64 ohm resistor found in the chassis before the reservoir capacitor)

[Lectroid]

One more question:

How close to the PT did you mount that choke?  Do you have a pic?  Just curious.