Hammond 125B multiple outputs

[Greg_L]

I'm building a project with a Hammond 125B OT.
125B.pdf

I want...if possible...4, 8, and 16 ohm speaker outs with a 22.5k primary impedance for a single 12AU7 output tube in push/pull.

The 4 is pretty easy...use wires 5 and 6.

The 8 and 16 options though share a wire....wire #4.

What do you think is the best way to have independent 8 and 16 ohm jacks that have to share that wire?

Should I put them on a DPDT switch? Or use switching jacks?

[loudthud]

Because the power level is so low, a 3 pole 3 position switch such as

Link: https://www.tubesandmore.com/product...les-3-position

should work. You only need two poles. If you can find a 4 pole 3 position version, use two poles in parallel.

[tedmich]

Whats the design?

I got 125B for building a Robinette Slo Micro Naked and decided against using that OT because the bass response drops so much.

I am going to try a 12BH7 output tube and an Antek MP-15W75 OT which will be at 15k into 8/16 ohms

[mozz]

Whats the design?

I got 125B for building a Robinette Slo Micro Naked and decided against using that OT because the bass response drops so much.

I am going to try a 12BH7 output tube and an Antek MP-15W75 OT which will be at 15k into 8/16 ohms

Push pull triodes with a torridal output trans, might not sound like a guitar amp. No low watt designs with a dual pentode?
I don't have a answer to the OP question, just nit picking.

[Greg_L]

Whats the design?

I got 125B for building a Robinette Slo Micro Naked and decided against using that OT because the bass response drops so much.

I am going to try a 12BH7 output tube and an Antek MP-15W75 OT which will be at 15k into 8/16 ohms

I'm building the micro 800.

I already have several real full size 2203s and 2204s so I thought this would be a fun little project.

[Greg_L]

Because the power level is so low, a 3 pole 3 position switch such as

Link: https://www.tubesandmore.com/product...les-3-position

should work. You only need two poles. If you can find a 4 pole 3 position version, use two poles in parallel.

I'd like a jack for each though, if possible. Instead of a switch. But I'll do whatever it takes to get 8 and 16 ohm taps.

My other idea was to use switching jacks. If I could run the 8/16 ohm in-common wire to the tip of one jack and have it shunt to the other jack via switch when not in use. Then I could use the remaining independent 8 and 16 ohm wires to the sleeves of each jack respectively. Does that make sense? Maybe I need to draw it.

[J M Fahey]

+1 on what Mozz says.

NO Guitar amp at any power uses power triodes, way too polite weak sissy no balls.
You might as well build an SS amp.

If anything , build a *single* pentode amp low power, say using 6AQ5, ECL82, etc.
Not a Push Pull, of course, but the real thing: bite, growl, sustain, what the doctor ordered.

TONS of single pentode amps around since forever, maybe there is a reason for that.

But .... twin triodes? .... sounds like an armchair_designer/you tube thing to me.

But ... but ... didn´t Marshall and Bugera add a twin triode 1W amp to their catalogs?

Yes, most probably because they know they can $$$ell a few, no market niche is ignored.
That doesn't make them sound better

[Pedro Vecino]

For a similar project with a 12AU7 in pushpull I used that same transformer and a 3-position 4-circuit Alpha rotary switch using double circuits for safety. I leave you a small diagram below.
Those types of amps, if well designed and with musical criteria, can sound surprisingly good.
This is a small sample from a professional guitarist who uses it a lot in his studio.
(If you do not have a Facebook account, you need to click on the x at the top right to access it and turn up the volume in the video window when it appears).

https://www.facebook.com/10000082109...6218044415536/

[Greg_L]

For a similar project with a 12AU7 in pushpull I used that same transformer and a 3-position 4-circuit Alpha rotary switch using double circuits for safety. I leave you a small diagram below.
Those types of amps, if well designed and with musical criteria, can sound surprisingly good.
This is a small sample from a professional guitarist who uses it a lot in his studio.
(If you do not have a Facebook account, you need to click on the x at the top right to access it and turn up the volume in the video window when it appears).

https://www.facebook.com/10000082109...6218044415536/

Wow this is cool, thank you.

[tedmich]

I've recently become aware of another 12AU7/12BH7 (22.5k) OT here
https://primarywindings.com/product/...t-transformer/
They say its good to 85Hz and it is inexpensive, at less than half the 125B

they have a variety of OTs/PTs with big Union Jacks on them so theyre made...?

[Greg Robinson]

I've used the Hammond 125H with the secondaries mismatched for 2:4/4:8/8:16Ω for a primary impedance of 20k with good results.

[Tom Phillips]

I've recently become aware of another 12AU7/12BH7 (22.5k) OT here
https://primarywindings.com/product/...t-transformer/
They say its good to 85Hz and it is inexpensive, at less than half the 125B...

The sellers text just says "Typical bandwidth is 85Hz – 6kHz." That statement leaves a lot of unknowns. To properly compare the performance to the Hammond, you would need see the actual response curves and know the measurement conditions. Historically, manufacturers would specify bandwidth at the -3dB points. However, I have recently seen specs that used -10dB points. I'm also curious why the upper limit of the transformer you found is only 6kHz. Although it could be argued that higher frequencies are not needed for a guitar amp, the relatively low HF spec seems to me like an indication of a lower quality transformer.

[Greg_L]

If I want independent 4,8, and 16 ohm jacks, I need wires 2+4 for 8 ohm, and wires 1+4 for 16 ohm, can I just run #4 to a jack and put a switch on the #4 wire between two jacks?

[tedmich]

The sellers text just says "Typical bandwidth is 85Hz – 6kHz." That statement leaves a lot of unknowns. To properly compare the performance to the Hammond, you would need see the actual response curves and know the measurement conditions. Historically, manufacturers would specify bandwidth at the -3dB points. However, I have recently seen specs that used -10dB points. I'm also curious why the upper limit of the transformer you found is only 6kHz. Although it could be argued that higher frequencies are not needed for a guitar amp, the relatively low HF spec seems to me like an indication of a lower quality transformer.

on the opposite end of the spectrum, this Lundahl OT has impeccable specs! ; )
https://www.don-audio.com/Lundahl-LL...ut-transformer

this one (from these guys https://www.musicalpowersupplies.com/) is more reasonably priced https://www.ebay.com/itm/16695621999...Bk9SR7aW4ovLZA

I've asked for frequency specs.

that offer a few types

[Tom Phillips]

If I want independent 4,8, and 16 ohm jacks, I need wires 2+4 for 8 ohm, and wires 1+4 for 16 ohm, can I just run #4 to a jack and put a switch on the #4 wire between two jacks?

Per your diagram, you have shorted the secondary pins #1 to #2. That's critical problem.
It looks like your thinking is to use the switch shown to connect the 16 Ohm jack but the circuit leaves the 8 ohm jack connected all the time.

I'd still recommend that you re-consider using a multi section rotary switch to select your output impedance.
What do you think?