Wiring a multi tap OT in a blackface Super Reverb

[mrhudson27]

Hello, all. I posted the following on the Steel Guitar Forum recently & haven't made headway. Just joined in the hopes that I could get some direction here. It concerns my attempt to swap the original OT n a '65 Super Reverb for a Hammond multi-tap OT to allow 4/8/16 Ohm switching. The primary leads are easy enough to figure out- not the secondary! I really only need 4 & 8 ohm options, so I can dispense w/ the rotary selector, but if I can figure out how (if possible) to wire in the selector I have, I will. Thanks!:



I carved out some time & got everything together. I went ahead with Ken's suggestion & bought a rotary impedance selector w/ jack already wired. The P-H1600 selector is designed for the 1600 series Hammond OTs, and since the applicable 1600s & 1700s are multi-tap 4/8/16, I figured I could just look at the diagram for wiring a 1600 & make adjustments as needed to suit the 1760L that I'm using.

I can't.

Looks like the 1600 series all have 5 secondary wires, each rated 0, 4 or 8 Ohms, and combinations therein result in 4/8/16 switching. The 1700 series have 4 secondaries: COM/4/8/16. Here'ss the 1650N, which appears to be the closest equivalent to the 1760L:

https://www.hammfg.com/files/parts/pdf/1650N.pdf

And the 1760L I'm installing:

https://www.hammfg.com/files/parts/pdf/1760L.pdf

And hookup/data on the rotary switch:

https://www.tubesandmore.com/sites/default/files/associated_files/p-h1600_wiring.pdf

Based on this info- anyone see a way to wire the 1760L to this switch for 4/8/16 ohm options? And a bonus question: the selector actually allows for 4 positions. Can position four be configured as a "nothing happens" setting as opposed to "magic smoke" setting in the event that it accidentally gets flipped all the way over?

That's a lot of info. I thank you all in advance for reading/doing my homework.

[sluckey]

I'd do it like this...

[mrhudson27]

Well hot damn. This forum paid dividends already! Thanks Sluckey- I'll give that a go this weekend. Bonus question: can I use a multimeter to ensure I've wired properly? I (being new to the tech side) can test impedance from a driver, but don't know how to verify the other end is set to like a particular load.

[sluckey]

You can use your DMM to check the switch operation ***BEFORE*** you connect the OT wires. But once you connect the OT everything will read near zero ohms. Very hard to tell what's what. But this is such a simple circuit you can just check everything with your eyes.

There are ways to check that the proper impedance tap is connected to the speaker jack, but it involves more than just a DMM and might be a bit too involved for this thread.

IMO, that little $1.95 switch is too light duty to be used as an impedance selector. I highly recommend this $12.95 switch. And as a bonus, it's a no-brainer to wire it up.

     https://www.tubesandmore.com/products/switch-rotary-1-pole-3-position-0

[PRR]

....IMO, that little $1.95 switch is too light duty to be used as an impedance selector....

+1.

I was surprised CE was selling it as fit for duty. Those things are often rated 50mA (0.050 Amps). 50 Watts at 4 Ohms is 3.5 Amps. While the 50mA toy switch "may" stand more when not playing LOUD, 70X more seems optimistic. The other switch says, and looks-like, 6 Amps, which is 144 Watts @ 4Ω, and I'd use it cold-switched at 200W.

[pdf64]

Might the 50mA be a switching current rating, with the contact non switching (steady state) current rating being much higher?
Realistically, the switch won’t be getting actuated whilst the amp is cranking out power. Or at least, it shouldn’t be.

[glass54]

+1
The $1.95 switch is best used for signal level switching (Preamps).
If you want a robust/reliable speaker switch definitely go with the $12.95 unit.
You won't be sorry 
Regards
Mirek

[PRR]

Might the 50mA be a switching current rating, with the contact non switching (steady state) current rating being much higher?...

Hmmm, maybe I didn't say it clearly? 70X more seems optimistic.

I'll put 4 dogs on my 2-dog porch. But not 140 dogs (even 101 Dalmatians).

[mrhudson27]

Thanks all! Gonna order the $12.95 selector. Incidentally, the ”$1.95” option was $11.95 from the same supplier! No sleep lost.

Sluckey- you say the wiring will be a no brainer, but I don’t think you know who your dealing with yet. Imma keep coming at you w/ 101 level questions like “how do I wire the existing input jack to the new switch (which I plan on putting in the “Aux” jack hole of the chassis”?

Srsly, tho. Thank you all a ton. I’ve been trying to put this together for over a month so I can run into one (two if I feel like it) JBL K130s (8ohms each), each in their own enclosed cab. Right now I’m limited to running both cabs (4ohms) from the stock OT which refers see 2.

I’ll continue to do my best w/ self study of tube amp fundamentals, but til I get my sea legs I truly appreciate the hand holding.

[sluckey]

...

[mrhudson27]

Thanks again Sluckey!

[dude]

You could also just skip the switch and use two output jacks but not shorting jacks, so you’d not want to turn the amp on with out a spk connected. Use the 4 and 8 ohm taps only tape the 16

[pdf64]

You’ll also need to decide what to do about the NFB.
As there’s not a 2ohm secondary output available, something will change.
I suggest to use the 4ohm connection for the NFB sample, and to change the 820ohm resistor to 1k2, or the 47ohm resistor to 33ohms.

[sluckey]

Fender used 820/47 NFB divider only on 8Ω speaker loads such as Deluxe Reverb or Vibroverb. He used 820/100Ω for 2Ω and 4Ω speaker loads such as Super Reverb and Twin Reverb. So, I would just connect the NFB wire to the 4Ω tap and not mess with the voltage divider resistors.

[pdf64]

Oops, yes 
So yeah, using the 4ohm connection will increase the voltage that the NFB is sampling by 1.414. Hence to avoid altering that aspect of SR operation, increase the 820 to 1k2, or alternatively, decrease the 100ohm to 68ohm.

[sluckey]

There is no correct amount of NFB (within reason). The amount of NFB is very subjective. Leo approved of 820/100Ω divider for 2Ω and 4Ω speaker loads. I would leave it alone. I suspect if mrhudson27 was interested in keeping the SR operation intact, he would not be pursuing this OT change to begin with.

[pdf64]

That’s a reasonable supposition of course, but it’s also feasible that aspect of the SR (ie the lowest degree of NFB of the big amps) might be something that was wanted to be retained.
Whatever, it’s out there for consideration now 

[mrhudson27]

Thanks guys!

What the hell is a negative feedback wire?

I like the idea of connecting it to the 4ohm and forgetting about it. Once isolated, solder NFB wire to the 4 ohm designated tab on the selector switch along w/ the 4 ohm secondary wire, yes?

I'll be in and out of this and numerous other chassis (pl), so I'm good w/ keeping it simple & researching later- I can always change it. Gonna hang on to the OEM 2ohm OT in the event I ever want to change it back for some impractical reason. It's a rusty dinged up dumster save, so it ain't shiny mint.

[pdf64]

… solder NFB wire to the 4 ohm designated tab on the selector switch along w/ the 4 ohm secondary wire, yes?
…

Yes, that should work fine.
Negative feedback helps the amp to keep a more linear response, and avoids the speaker cone flapping around at its bass resonance. The relevant resistors are R64 & R53.

If the OT is wired up with incorrect polarity, ie such that the negative feedback becomes positive feedback, the amp may sound weird, and possibly howl at full power. So leave sufficient  spare wire to pin3 to enable it to be moved to the other pin3.

But provided that the blue primary wire goes to V7pin3, it should be ok.
https://el34world.com/charts/Schematics/files/Fender/Fender_65_super_reverb.pdf

[dude]

Seems mrhudson27 is not ready for fine tweaking the Neg Feedback just yet.., that’s fine. As pf64 says, if you get a howl at start, just switch the plate wires to power tubes, the blue and brown to pin 3. But you’ll
probably be fine.
If you want, you can put the feed back wire to the 4 ohm tap and then try it on the 8 ohm tap. Use whatever sounds best to you, one way will clean up the tone a bit, the other a little more distortion or you’ll notice no difference, either way will “not” hurt the amp.