Output Trans question

[pda041376]

Ok, I'm an idiot and ordered a single ended OPT for a firefly build. Just noticed the oversight this morning so new order is already in for the correct tranny. My question is: I have a couple of AO-43 chassis sitting around, can I just pull one of those and drop it into the firefly build?

I know the specs for the AO is 7.2K primary
running an 8 ohm secondary and the firefly build was calling for a hammond 124A with 2&4 hooked up which (looking at the tranny spec) would be 22.5k at 8 ohm.

Is this thought just a disaster waiting to happen or can the 43 donate its OPT for the cause with minimal issue?

[sluckey]

What do you think?

[pda041376]

I think maybe with resistors to up the resistance to the primary side it may be possible. But I'm no expert, so *shrug*.

[pda041376]

Or would that even work? Does the resistance have to be between the sides of the tranny iternally?

[shooter]

if you take 7.2k * 3 you get real close to your 22.5k
so to make the math folk happy you need to multiply 8 3 times

[pda041376]

So that will load everything correctly (or near enough) and avoid total chaos?

[sluckey]

Use a 24Ω speaker load... 3 x 8Ω in series, or a 16Ω and 8Ω in series.

[pda041376]

Awesome, many thanks.

[AmberB]

Has anyone tried building a Firefly type amp with a 6SN7 as the power tube instead of a 12AU7?
I wonder what the output load would need to be for that?

[DummyLoad]

get the right part: hammond 125c - works as ~22K to 2 or 4 or 8 or 15Ω

--pete

https://www.hawkusa.com/manufacturers/hammond-mfg/power-products/audio-transformers/125c?gclid=CjwKCAjwkdL6BRAREiwA-kiczKhgbwgftc5KViRaZpSsnC3h6l4yQ-uaNZPrWCdcyKurNHrSHMEKmhoCAxAQAvD_BwE

[pda041376]

Hm... I thought A was the right part, per the build sheet in the pcb version. Guess A through E all have same ratings just different wattage (based on the info sheet)? I'm good with 3 (A) watt instead of 8 (C), I just wanted to slap this together while I waited on a couple of things for an AO-39 conversion.

[HotBluePlates]

... 6SN7 as the power tube instead of a 12AU7?
I wonder what the output load would need to be for that?

I haven't built a Firefly.  However, 6SN7 has a similar internal plate resistance as well as a similar plate dissipation rating (yes, the 6SN7 can dissipate more if you choose).

If you keep the same supply voltage as a 12AU7-Firefly, the 6SN7 should drop-in with no change to the load presented by the output transformer primary.

It's important to realize that an "ideal load" of an output stage yields maximum clean output power; you can use a non-ideal load and have less output power.  Also that the "ideal load" is found by looking at supply voltage, possible plate voltage swing, and the desired output power (with an eye towards the tube's plate current limitations).

For a triode, there is a relationship between the tube's internal plate resistance & the load impedance (load is 2x bigger).  Still, if you get "non-ideal" the amp still works but doesn't push as much power to the speaker.

[AmberB]

Two 6SN7 output tubes and an 11K to 8 ohm push/pull output transformer...   

[HotBluePlates]

Two 6SN7 output tubes and an 11K to 8 ohm push/pull output transformer...   

Making the tiny amp bigger?     Wouldn't a Champ be in the same range with fewer sockets?

[AmberB]

Two 6SN7 output tubes and an 11K to 8 ohm push/pull output transformer...   

Making the tiny amp bigger?     Wouldn't a Champ be in the same range with fewer sockets?

Where's the adventure in that?  :-)