RS OT Impedance switching

[springhead]

Hi,

I've seen the drawing of the DPDT switch method of putting the two 4 Ohm windings in parallel or series, and then in series with the 6 Ohm winding. Slightly different arrangement to the RS datasheet but gets you 8 or 16 Ohms.

Has anyone tried this and does it work ok with regard to output phase, and hence the negative feedback path? I'm assuming you just take the NFB tap from the switched/live side of the arrangement? Alternative would be to always take it from the top half of the 6 ohm winding.

I'm trying to decide whether to try this arrangement, or some sort of internal plug/socket - maybe hidden under the turret board.

Thanks!

[Roe]

+ 1 on this

[springhead]

I just drew it out and it looks like the toggle switch arrangement keeps the overall phase the same in 8 or 16 ohm mode. Plus the relative phase of the 3 windings is preserved, I thought it looked like there may be a cancellation in there. The difference is which side of the paralleled 4 ohm pair the 6 ohm winding connects to.

Apologies if this is going over ancient ground! I've just got an original RS OPT and am planning a JTM 45 build with it - would be great to have switchable impedance and it does look possible with this scheme.

I'll put a sig-gen/scope on it to check the phasing works correctly before I commit to drilling/wiring.

Final check for anyone who has successfully installed/wired this arrangement?


thanks,

[shakti]

I would be very interested in seeing this. I built a block logo JTM45 myself with C M transformer which has the tagboard exactly like the original. I wanted to have it switchable, but never figured out how. Truth be told, I've gone back to 16 ohms because I prefer the overall sound that way. Since the negative feedback can only be wired to the speaker jack, you end up with NFB dependant on the impedance setting. It doesn't sound right at 8 ohm to my ears. However, with this switchable setup, perhaps it is possible to wire negative feedback permanently to the 16 ohm tap?

[springhead]

I spotted it here:

http://forums.vintageamps.com/viewtopic.php?f=1&t=97540

If the feedback is from the speaker jack then the feedback level in the 8 ohm position with 8 ohm load should be the same as in the 16 ohm position with 16 ohm load. Different cabs/speakers will have different frequency dependent loads and hence have an impact on the feedback. But I'd like to know how this works in practice from someone who has implemented this, hence this post!

[Roe]

27k@16ohms=13.5k@4ohms=19.1k@8ohms

[springhead]

I guess you can either use a rotary switch or 3PDT to also switch the feedback resistor, or use a separate switch. Or maybe take the feedback from the top end of the 6 ohm winding?