New DR project sanity check?

[Lectroid]

Hello all,

Here's my current amp project which is based on an AB7563 Deluxe Reverb. It's a single-channel amp based on Hoffman's one-channel AB763 design.  I'm re-using the iron from a Hammond AO-43.  My hope is that the final build will be somewhat stronger than a DR, maybe able to support 6L6s, making it closer to a Pro Reverb.  But the basic idea is to make a AB763 DR clone without the tremolo.  See the schematic below.

Specific questions:

1. I copied the reverb driver from Merlin Blencowe's site, to drive a Hammond high-impedance reverb tank directly without a transformer.  His circuit is attached below.  Elsewhere on the forum I found a discussion of how I need to alter the reverb input voltage divider to adjust for the input level to the driver:

    https://el34world.com/Forum/index.php?topic=24270.msg260641#msg260641

I think I'll end up in the 6-10VAC range with my input signal, but I left the resistor labeled as "TBD" until I can test the actual circuit.

That said, my design (ha-ha!) is very much just a cut and paste job based on limited knowledge so I'd like to hear opinions on whether it will work the way I've stitched it together or whether there's a better way.

2. The Hoffman circuit has a 1M grid leak resistor on the reverb driver, where the VW design shows only a 170K grid leak. Why does the VW design have a smaller value for that grid leak?  Is it solely to act as part of the voltage divider, or is there something else I'm not understanding here?

3. In this configuration, once I've determined the value of the "TBD" resister, could I still replace it with a pot as a Dwell control (as detailed in other posts)?

Any other thoughts and critiques gratefully accepted.


Rich

[Williamblake]

"I copied the reverb driver from Merlin Blencowe's site, to drive a Hammond high-impedance reverb tank directly without a tranformer."

Be prepared to change a lot of it. What tank do you have exactliest? Maybe get more different tanks cheaply.

[sluckey]

I copied the reverb driver from Merlin Blencowe's site, to drive a Hammond high-impedance reverb tank directly without a transformer.

Why? Your life would be so much simpler if you just used the Fender circuit.

[Lectroid]

Thanks for the quick replies,

@willliamblake
The tank is a 4FB3A1B (also on the schematic).  Another user on the thread I referenced made it work okay and he didn't mention having had to make radical changes to do it.  What changes do you think I'll have to make to use Blencowe's circuit? 

@sluckey
Yes, I could build the Fender circuit after buying a new tank and reverb transformer.  But I get Hammond tanks and amps cheap from a Hammond repair tech.

And Blencowe makes a case that the Fender circuit is inefficient, for driving a 12AT7 voltage amplifier hard  into a transformer to match impedances with the 8-ohm tank.  His solution (as I understand it) uses a high-current, high-z driver stage to begin with, and achieves near-perfect impedance matching.  (So he says!).  His solution seems cheaper, certainly fewer parts. 

That said, I don't have anything near your experience.  I don't want to be a know-it-all, or claim that Blencowe is The Last Word.  I want to understand what's going on rather than just copy known circuits.  If my understanding is wrong or my original questions miss the point, please correct me.


Thanks again!
 

[PRR]

> 170K grid leak. Why ...?  Is it solely to act as part of the voltage divider.....

Looks like that to me. Dunno why you don't mention the 1Meg.

Where does Merlin say "more efficient"? And why does it matter (a 1/4 Watt amp embedded in a 40W amp)?

[Williamblake]

I tried the 12au7 driver and tweaked it and didn't like it. But with E-type or lower impedance tanks. I also didn't like Merlins cascode driver i must admit. I finally used EF80.

[Lectroid]

I tried the 12au7 driver and tweaked it and didn't like it. But with E-type or lower impedance tanks. I also didn't like Merlins cascode driver i must admit. I finally used EF80.

Thanks.  Can you expand a little on how you tweaked it and why it didn't work for you?  And how did the EF80 work out?

I have a 4F tank, exactly what he shows, that I got nearly free.  I want to use it if possible. 

[Lectroid]

> 170K grid leak. Why ...?  Is it solely to act as part of the voltage divider.....

Looks like that to me.

Thanks for answering my question.  Good to have know I saw it correctly.

>Dunno why you don't mention the 1Meg.

It's in the other thread I linked.  It discusses how to select that resistor for the right voltage to drive the 12AU7's grid signal.  I think you were in on that discussion at one point.  So I left that resistor as "TBD" on my schematic. 

>Where does Merlin say "more efficient"? And why does it matter (a 1/4 Watt amp embedded in a 40W amp)?

On his "Spring Reverb" page he says:  "Low impedance reverb tanks seem to be persisting in valve amp design, which makes little sense since valves are not well suited to driving low impedance loads. By taking advantage of high-impedance tanks (usually intended for op-amp drivers) we can build much simpler valve drivers."

Okay, you're right, it's just a tiny corner of the larger amp.  I admit that directly quoting his "much simpler" would have been more accurate than "more efficient."  However, it does sound more efficient when, by more closely matching impedances, you can transfer more power to the tank.  Correct me if I'm wrong about that, but that's what I got from his discussion.  Beyond the minutiae of a particular amp, I'm trying to learn the smartest ways to look at larger design issues.

Once I get that "TBD" resistor nailed down, would I be able to swap it out for a pot to create a Dwell control?

Or I may chuck the whole idea and just buy  a 4A tank and a transformer and build the Fender circuit. 

Can I assume that everything else on the schematic looks OK to you?

[octal]

I've built Merlin's circuit, it works well enough, but I think it's actually far less efficient than transformer driven reverb in terms of power consumption vs drive signal imparted to the tank coil- a resistor loaded triode amp is less efficient than a transformer loaded triode amp. I think the reason for this is that in a resistor loaded amp, the ac signal goes through the plate load resistor to what is essentially an AC ground (the B+rail) and also through the coupling cap, out into the load. All the AC signal that's dissipated in the plate resistor is wasted.


Valves are perfectly well suited to driving low impedance loads- through a transformer! See, for example, the output stage of a tube amp!
I also think the benefits of constant current drive for a tank are overstated. It's easy to get the the same results with filtering and EQ. Having said that, there are good sounding cap coupled reverb circuits out there, have fun and knock yourself out!

[Lectroid]

Hey octal,

 You gave me a lot of real information to digest and you answered a lot of questions I had about the constant current driver, questions I'm not fully able to even articulate much less answer.  I just learned a lot.  Thanks very much.   

Based on your thought, and given that this is only my third amp, I'm going to go with the classic Fender circuit and leave my 4F tanks on the shelf for now.

One more question:  Do you have specs on the Fender reverb transformer primary and secondary windings?  I'd like to know how the signal is being manipulated.

[sluckey]

This is Hammond's drop-in replacement reverb transformer...

     https://www.hammfg.com/files/parts/pdf/1750A.pdf

[Lectroid]

Just spent fifteen minutes looking for that exact page...     Many thanks.

My copy of the DR schematic shows 8.7V at the cathodes of the 12AT7 with a 2.2K cathode resistor.  Ohm's Law gives 4mA current going through both cathodes, or 2mA to each section's cathode.  Am I looking at that  right? 

If the combined plate resistance of 25000 ohms is also split between the two sections, that seems to imply a voltage drop on each plate of (12500 ohms X .002A), or 25 volts drop on each plate. 

Actually the schematic shows a 5V voltage drop through the primary of the transformer; Fender shows Node B = 415, with 410V at the plates.  Obviously I'm wrong somewhere. 

[pdf64]

25k is the primary impedance.
That would equate to 50k load per anode.
Primary resistance is noted at 1065ohms. That’s where the idle VDC comes from.

[Lectroid]

@pdf,

Many thanks for explaining that, it was exactly where I was unclear.  Now I can draw a proper load line. I am understanding design theory more and more but still have a lot of gray areas.  How to look at parallel resistances is obviously one of them.

Thanks again,

Rich

EDIT:  Somehow ended up posting two versions of this.  Please see below.

[Lectroid]

25k is the primary impedance.
That would equate to 50k load per anode.
Primary resistance is noted at 1065ohms. That’s where the idle VDC comes from.

@pdf,

Thanks for straightening me out.  I can build an amp but still have a lot of gray areas when it comes to design issues.  How to look at parallel resistances is obviously one of them.

I want to draw a proper load line. So each plate has a load resistance of 50K.  With a cathode resistance of 2.2K for both tubes, would each section's cathode have an effective resistance of 4.4K?  Am I looking at that right?

Assuming DCR stands for "direct current resistance," what's its significance here?  I thought the idle VDC would be found at the quiescent point once the load line is plotted.   

[acheld]

I was looking over your schematic, and your choke is listed @ 14H.   The usual for a DR is 4H (125C3A/022707, @ 50mA)  and 4H (125C1A/022699, @90mA).   The 125C1A should work fine for both the Super and the DR, but is a physically bigger (and heavier) part. 

[Lectroid]

@acheld,

I'm re-using all the iron from a Hammond AO-43 amp, and that choke came along with it.  Originally it served a reverb circuit in the organ.Another member said it worked fine for a PS choke.  AFAIK, 14H means it supplies more filtering than the 4H choke without causing much voltage drop.  14H is probably overkill, but it was free and I want all the power I can get in this build.

[pdf64]

…
I want to draw a proper load line. So each plate has a load resistance of 50K.  With a cathode resistance of 2.2K for both tubes, would each section's cathode have an effective resistance of 4.4K?  Am I looking at that right?
…

Yes.
Merlin goes through the SE output stage design process here http://www.valvewizard.co.uk/se.html