Modify output ohm options on DR clone

[Heywood_ja]

I am building a mojotone deluxe reverb kit, but want to make a mod to allow for options on output ohms.

My plan is to sub the 8ohm output transformer (mojo773) with their 768sp tweed deluxe ot that has 4/8/16 taps.

I have 2 2x12 cabinets i have built (8 ohms parallel). That i would use as ext cab ( only 1 at a time of course)

I am thinking of running the 8 ohm out to the internal combo speaker out, and the 4 ohm tap to the ext out.

Any guidance on wiring?

I am thinking of adding a switch so it would power one or the other.

The original ot is 3500 ohms input, and the substitute is rated at 4000 ohms.

(Besides the wires being shorter it has the same footprint)

[Phil_S]

I'm not quite sure I follow what it is that you want to do. Nevertheless, I have an answer that will work.

IMO, 3.5K vs 4K primary probably doesn't matter very much. You may hear a difference and you may or may not prefer one over the other. I wouldn't concern myself with it.

I suggest 3 output jacks, one for each tap. Wire the sleeve with a common ground wire. When working with a single speaker (or speaker cab) use the jack that matches the impedance of your speaker. When using two cabs, use the jacks that are half the impedance of the speaker.

In other words, when using two at the same time, plug a 16 ohm load into the 8 ohm jack and an 8 ohm load into the 4 ohm jack. This works because the speakers are now wired in parallel, making them worth half of what they are, if you allow me to say it that way.

If you want to use a pair of 16 ohm cabs, then you will need to have an intermediate thingy [1] that allows you plug in both in parallel and then connect to the 8 ohm output jack.

I hope this is clear.

[1] Thingy: a technical term.

[NickC]

............... when using two at the same time, plug a 16 ohm load into the 8 ohm jack and an 8 ohm load into the 4 ohm jack. This works because the speakers are now wired in parallel, making them worth half of what they are ................

A 16 Ω speaker in parallel with an 8 Ω speaker yields a total impedance of 5.333 Ω. I'm guessing that would be okay connected to the 4 Ω OT tap. But I don't understand using both the 8 Ω and the 4 Ω OT taps simultaneously??? I've never considered doing that (using two OT taps at the same time), so it's an alien concept to me. Is that okay/safe on the OT? TIA

[morcey2]

............... when using two at the same time, plug a 16 ohm load into the 8 ohm jack and an 8 ohm load into the 4 ohm jack. This works because the speakers are now wired in parallel, making them worth half of what they are ................

A 16 Ω speaker in parallel with an 8 Ω speaker yields a total impedance of 5.333 Ω. I'm guessing that would be okay connected to the 4 Ω OT tap. But I don't understand using both the 8 Ω and the 4 Ω OT taps simultaneously??? I've never considered doing that (using two OT taps at the same time), so it's an alien concept to me. Is that okay/safe on the OT? TIA

If the speakers were just wired together in parallel, the 5.333Ω calculation would be correct. In this case, they're connected to different secondary windings on the OT. If you look at it from the perspective of the primary, it makes more sense.

Assume a 4k primary with 4,8, and 16Ω secondaries. With a 16Ω speaker connected to the 16Ω secondary, the primary is 4k. Same with 8 and 4Ω connected to their corresponding output. Now, move the 16Ω speaker to the 8Ω output and the primary sees 8k instead of 4k impedance. Same with an 8Ω speaker in the 4Ω output.

When you take the radical step of putting the 16Ω speaker in the 8Ω tap and the 8Ω speaker in the 4Ω tap, each of them is reflecting 8kΩ to the primary. 8kΩ || 8kΩ = 4kΩ. Exactly what the primary is supposed to be.

Matt

[Darkbluemurder]

If the speakers were just wired together in parallel, the 5.333Ω calculation would be correct. In this case, they're connected to different secondary windings on the OT. If you look at it from the perspective of the primary, it makes more sense.

Assume a 4k primary with 4,8, and 16Ω secondaries. With a 16Ω speaker connected to the 16Ω secondary, the primary is 4k. Same with 8 and 4Ω connected to their corresponding output. Now, move the 16Ω speaker to the 8Ω output and the primary sees 8k instead of 4k impedance. Same with an 8Ω speaker in the 4Ω output.

When you take the radical step of putting the 16Ω speaker in the 8Ω tap and the 8Ω speaker in the 4Ω tap, each of them is reflecting 8kΩ to the primary. 8kΩ || 8kΩ = 4kΩ. Exactly what the primary is supposed to be.

Excellent post, thanks a lot!

Cheers Stephan

[NickC]

............... when using two at the same time, plug a 16 ohm load into the 8 ohm jack and an 8 ohm load into the 4 ohm jack. This works because the speakers are now wired in parallel, making them worth half of what they are ................

A 16 Ω speaker in parallel with an 8 Ω speaker yields a total impedance of 5.333 Ω. I'm guessing that would be okay connected to the 4 Ω OT tap. But I don't understand using both the 8 Ω and the 4 Ω OT taps simultaneously??? I've never considered doing that (using two OT taps at the same time), so it's an alien concept to me. Is that okay/safe on the OT? TIA

If the speakers were just wired together in parallel, the 5.333Ω calculation would be correct. In this case, they're connected to different secondary windings on the OT. If you look at it from the perspective of the primary, it makes more sense.

Assume a 4k primary with 4,8, and 16Ω secondaries. With a 16Ω speaker connected to the 16Ω secondary, the primary is 4k. Same with 8 and 4Ω connected to their corresponding output. Now, move the 16Ω speaker to the 8Ω output and the primary sees 8k instead of 4k impedance. Same with an 8Ω speaker in the 4Ω output.

When you take the radical step of putting the 16Ω speaker in the 8Ω tap and the 8Ω speaker in the 4Ω tap, each of them is reflecting 8kΩ to the primary. 8kΩ || 8kΩ = 4kΩ. Exactly what the primary is supposed to be.

Matt

Thanks for that excellent explanation. I see it now. Before, I'd never use but one OT tap at a time, and make whatever connections were necessary to present the exact correct load to the corresponding secondary OT winding. This new way you explain opens up new possibilities and greater understanding. Thanks!

[morcey2]

Thanks for that excellent explanation. I see it now. Before, I'd never use but one OT tap at a time, and make whatever connections were necessary to present the exact correct load to the corresponding secondary OT winding. This new way you explain opens up new possibilities and greater understanding. Thanks!

Glad I could help. I have a Hi-Octane that I regularly use with a 16-ohm Relic-30 (short-lived Guitar-Center special from celestion) AND an 8-ohm, 2x12 cab loaded w/ greenbacks. It's a great mix. It does open up lots of options, especially when you can't buy the perfect speaker for each amp because of fundage limitations.

Matt

[pdf64]

A DR OT should have a 6k6 primary; even that seems rather low for 6V6
Use of an OT with a 4k primary under the normal DR operating conditions may result in the plate dissipation rating of the 6V6 being exceeded.
Why is a 4k primary OT being used?

Regarding the use of the OT taps, the advice from Aiken is:-

Q: Attempting to simultaneously operate a 4-ohm and a 8-ohm speaker from the 4-ohm and 8-ohm secondary taps of an output transformer...not a good idea---but, why?
A: If you run a 4 ohm speaker on the 4 ohm tap, it will reflect back, say, 8K to the primary. Now, if you also connect an 8 ohm speaker to the 8 ohm tap, it will also reflect back as 8K to the primary, so the tubes now are seeing a 4K load instead of the 8K they require. That's why it is a bad idea. Now, if you connected an 8 ohm speaker to the 4 ohm tap, and a 16 ohm speaker to the 8 ohm tap, the reflected impedance would be correct.

Pete

[morcey2]

A DR OT should have a 6k6 primary; even that seems rather low for 6V6
Use of an OT with a 4k primary under the normal DR operating conditions may result in the plate dissipation rating of the 6V6 being exceeded.
Why is a 4k primary OT being used?

The diagram on the mojo website shows a 7kΩ primary, but incorrectly shows the impedance between the CT and each of the legs as 3.5kΩ. That's where the 3.5k value came from. The Tweed deluxe version shows the type of thing, 8kΩ with 4k on each side instead of 2k.

It should just show 8kΩ CT or 7kΩ CT instead, but it's their doc.

Matt

[Heywood_ja]

Thanks everyone-

What I wanted to wire was the 8 ohm to the "Speaker" jack

(This is the Internal Jensen C12Q Speaker 35 Watts Nominal)

And Wire the 4 ohm out put to the "EXT. SPKR." jack.

For my 2x12 cab with 2- 8 ohms wired in parallel

( 1 or the other, not both)

I would run the 16 ohm to a terminal strip.

I was never intending to have both output used at the same time, and that is why I was considering a switch to only have one active.

But now you bring it up, I have the 2- 2x12 cabinets, one Dean Markley Open back with DM speakers (custom Eminence)
The second cab is a closed back I built with 2 WGS ET-65s.

The DM cabinet has two jacks, so I run a line from the amp to the 1st cabinet (4ohm) , and then a line from that cabinet to the second (also 4 ohm) does that give me an 8 ohm load ( Z1+Z2) or does it give me a 2 ohm load (Z1xZ2)/(Z1+Z2)

[NickC]

.......

The DM cabinet has two jacks, so I run a line from the amp to the 1st cabinet (4ohm) , and then a line from that cabinet to the second (also 4 ohm) does that give me an 8 ohm load ( Z1+Z2) or does it give me a 2 ohm load (Z1xZ2)/(Z1+Z2)

Two 4 Ω cabs in parallel is 2 Ω load. The cab speaker jacks are almost certainly wired in parallel.

[Heywood_ja]

Here are the schematics for the OTs
"Stock"
http://www.mojotone.com/transformers/Fe ... OJO773.pdf

"Mod"

http://www.mojotone.com/transformers/Fe ... O768sp.pdf

This is the standard AB763 circuit

http://mojotone.com/Amp%20Kit%20Schemat ... 6&gc=clear

http://mojotone.com/Amp%20Kit%20Schemat ... 6&gc=clear

[Heywood_ja]

Two 4 Ω cabs in parallel is 2 Ω load. The cab speaker jacks are almost certainly wired in parallel.

That's what I thought, not sure how it could be wired for series.

But my playing isn't really worthy of being louder anyway!

I can just use my band master for the second cabinet, and run the other output form my dual output guitar.

[Heywood_ja]

I was thinking of using the hole for the ground switch to put the output selector. Recommended switch type?

Would I need to change the jacks from their current 3 prong open circuit to a different kind?