Reflected impedance - 16 ohm speaker on 8 ohm tap

[pjd3]

Hi there, would really be interested in your feedback on this.

In transforming my old Bogner Alchemist into a "marshally plexish" kind of amp, I'm using the PT and OT that came with it if I can.

The amp was originally designed with two 6L6GC and an OT with an 8 and 4 ohm tap. The circuit I will be installing will be using two 6V6''s which I understand want to see more of an 8K primary, all other things being conventional. (compared to two 6L6's which would want to see a 4K primary imp.)

Regarding the reflective nature of transformer impedance, can I expect with any confidence that putting a 16 ohm speaker on the secondary of this OT will create an acceptable impedance situation for 6V6's, OT and speaker? Or are there any other elements I'm not aware of that could create a less than optimum situation? The whole idea was to reuse the chassis and tranny's of the Bogner Alchemist for something useable and classic. I heard the transformers were fairly decent in the Alchmist (and it did sound not bad) so I decided to go for it. And after just spending nearly $900 on the Vibroverb build it just made good sense.

Thanks for your feedback,
Phil D.

[pompeiisneaks]

In looking at the datasheets for 6v6 vs 6l6, yes the 6L6 tend to want around 4k impedance, and the 6v6 seem to want around 8k, depending on the voltage of course, 6v6 like lower voltages than 6L6, 285VDC for 6V6 vs 400 for 6L6, (6v6 says max 315VDC in the datasheet I'm looking at for an old tung Sol)

So you can expect roughly double the impedance of the 4k if you call the 8ohm tap a '16 ohm tap for 6V6' .

You will want to do some voltage dropping for the 6V6's as well so you don't fry them, maybe a mosfet dropper like is somewhere here on the forums.

~Phil

[R.G.]

Yep.

Remember: Transformers don't HAVE impedances. Transformers have RATIOs.

[pjd3]

Thanks Phil,

Yes, that's what i'm going for, that placing a 16 ohm speaker on the 8 ohm tap would reflect through the OT in the 6V6's seeing in the area of 8K impedance. I do for the most part get to some extent the ratio aspect of transformers, that naturally the ratio of windings will constitute an absolute DC ohmic value, but that the impedance is something contingent on AC frequency and the vector additions of inductance/resistance/capacitance in the load and tubes. I think that 1KHz is the standard frequency that marketed speaker impedance is based on.

My concern was that I was missing something, that there may be something other than impedance to consider, like the DC ohms needed to be something or another for desired frequency response. I always assume that I'm going to miss something! But great. If doubling the "impedance" of the speaker over the tap impedance will set things straight for the 6V6's with this particular OT then very good!

The Bogner Alchemist PT has 330-0-330 for its high voltage so, if I put a GZ34 tube rectifier in there that would place about 400VDC on the plates of the 6V6's, maybe a little less. I understand that that is a pretty OK voltage for 6V6's. The Alchemist used diode rectification and I recall seeing 425 volts on the 6L6's years ago when I measured the plates. Grab a 120vac/5vac for the rectifier heater current and that should do it.

Thanks again Phil, sure helps remove mental sticking points so to move on in the process! Some of us "freeze" when we are in the dark!

Phil D.

[Faze]

You could also use 4 6v6's to drive the transformer at its rated impedance.

[pjd3]

True Faze, I see that would indeed work. I was originally attracted to the design due to the 2 6V6's and the lower stage volume I'd be able to get (and the lower cost since I just built a single channel Vibroverb)!.

As long as there is no "other" issues with the 16 ohm on the 8 ohm tap I'm willing to see how that works out.

Thanks,

Phil