Bassman transformer switch

[martin manning]

I'd go for a small resistance value, even if it is only a 2 or 3W, and that will probably be fine. If you don't have something like that, Radio Shack should have something close, and it will be much smaller physically. If you were to use that portion of the secondary between the 4 and 8 ohm taps it would require a 0.7-ohm load to show a 4k impedance at the primary, so a 7-ohm resistor would be a 10x load.

[FUCHSAUDIO]

I meant from hot to ground Martin. It at least assures no no-load condition, even with no speakers attached. Not sure how between taps would have any benefit.

[martin manning]

Hi Andy, shorting or placing a resistor between taps makes a completed circuit and a place for the magnetic field energy to go when no load connected same as a resistor to ground. Rethinking, the R to ground doesn't mess up the PI operating point as I suggested above. My attraction to the between-taps short is that like the simple shorting jack method, it doesn't require a great big resistor, and it is taken completely out of the circuit in use.

[FUCHSAUDIO]

interesting MArtin, I will give it a try !

[doveman]

So back to 5-10 ohms for me then if I use Martin's method?

[martin manning]

That's my recommendation. 4R7 or 10R, 2 or 3W.

[martin manning]

I sent an e-mail to David Allen and asked him about this shorting the taps together technique of no-load protection. He questioned its effectiveness since he knows that Heyboer transformers are constructed with two separate secondary windings which are joined at the common ground lead.

This is not a problem, and I think I'd just short the two taps together with no resistor in between as in my original proposal. The two windings are in-phase but since they have different numbers of turns there will be a voltage difference which will force current to flow through both of them and dissipate the field energy. I modeled this arrangement in SPICE and confirmed that to be the case.

[doveman]

I appreciate your help. I'm assembling it as designed ... to make sure it works. But I think at some point soon I will want to try something to have access to the options. I also ordered a impedance switch that will be here next week from Mouser. It's nice to have options. I think what you suggest sounds like a good option. I talked to David about it and he was not sure just looking at it. But all the guys at Dr Z website were against me trying the VVR on my Route 66. Even Dr Z says he's not sure it works well ... we've talked about it). But it blows me away when I play it.

So you never know. But it's not a real big deal at this point.

But ... I learn something with all these experiments ... and every time my gear gets better.

[martin manning]

... I learn something with all these experiments ...

Me too! With only two taps all you need is a SPDT switch, which you could mount in the second jack's position. Use the switching jack for the safety short and you are good to go.

[doveman]

... I learn something with all these experiments ...

Me too! With only two taps all you need is a SPDT switch, which you could mount in the second jack's position. Use the switching jack for the safety short and you are good to go.

The amp has an open hole unused for tremolo close to the two speaker jacks, as well. I have a few good options.

[SixStringBender]

I know this is a little over a year old, but I have a suggestion. I was looking for specs on the TO35MT and I found my answer, kind of. Anyhow...

You can use a 13A switching jack in your extension jack with both taps wired to it. It then wires to the 12A main jack. The NFB goes to the main.

You can use the 13A as an SPDT switch. All you need is an unwired plug. A plug in the jack will switch the tap. Pull the plug and it switches back to the tap that is closest to the signal lug. No two taps are connected at the same time and there is always a tap being routed to the 12A main jack. It behaves exactly like an SPDT toggle.

With the 13A extension jack you even have the option of using an extension cab. Depending on the output tubes installed you can use a standard cable for a parallel connection or a crossed over cable for a series connection to get the load you need.

For example, I'm using the TO26 in a Blues Junior with 5881's and JJ 6V6S (not at the same time of course). With the 5881's I don't use the dummy plug in the 13A. The 13A sends the yellow tap to the 12A main for use with my 8 ohm speaker. With 6V6S's I put a dummy plug in the 13A to switch the green tap to the main jack with my 8 ohm speaker.

And I still have the option of connecting an 8 ohm cab in parallel with 5881's or 6L6GC's. Or an 8 ohm cab in series with 6V6's.

Elegant?