Sunn Transformer Taps -- Speaker Impedance

[lochry]

I tried to post this earlier but it did not show up.  If it is redundant, my apologies.

I have a question about the speaker jacks in my '67 Spectrum II.

I did a cap job and added a 3-prong cord and the amp sounds great.  My No. 1.

When I had it on my bench, I noted that one of the speaker jacks is a double shorting jack.  Pretty common in British amps, but haven't seen one in a US amp before.  I mentioned this to a pal and we have a disagreement about the jack's purpose.

My Assumptions:

(1)  The speaker jacks (labeled 4 and 8-ohm) are independent in that each taps the transformer at its stated load.

(2)  When the 8 ohm jack is used, the 4-ohm tap is shorted out. 

(3)  When two speakers are attached, the shorting jack is used to default both jacks to the 4 ohms tap so the amp is looking for two 8-ohm cabinets in parallel.

My pal says NO.  He says the shorting jack is a no-load protection device for the transformer (I know Fender started using this trick in the early piggyback amps in case someone fired up the amp with no speaker load).  He says that when two speakers are used, the amp is still looking for a 8-ohm load AND a 4-ohm load.

I'm no expert, but my rudimentary knowledge of transformers tells me that is like saying you should shift into 1st gear AND 3rd gear before releasing the clutch.  Also, the suggested design makes no commercial sense -- Sunn would have to set up its cabinets for alternative loads (I can see switchable 4/16 load, but a 4/8?  Don't think so).

So . . . am I right, wrong, half-right? 

Thanks for clearing this up.

[JoeArthur]

I tried to post this earlier but it did not show up.  If it is redundant, my apologies.

I have a question about the speaker jacks in my '67 Spectrum II.

I did a cap job and added a 3-prong cord and the amp sounds great.  My No. 1.

When I had it on my bench, I noted that one of the speaker jacks is a double shorting jack.  Pretty common in British amps, but haven't seen one in a US amp before.  I mentioned this to a pal and we have a disagreement about the jack's purpose.

My Assumptions:

(1)  The speaker jacks (labeled 4 and 8-ohm) are independent in that each taps the transformer at its stated load.

(2)  When the 8 ohm jack is used, the 4-ohm tap is shorted out. 

(3)  When two speakers are attached, the shorting jack is used to default both jacks to the 4 ohms tap so the amp is looking for two 8-ohm cabinets in parallel.

My pal says NO.  He says the shorting jack is a no-load protection device for the transformer (I know Fender started using this trick in the early piggyback amps in case someone fired up the amp with no speaker load).  He says that when two speakers are used, the amp is still looking for a 8-ohm load AND a 4-ohm load.

I'm no expert, but my rudimentary knowledge of transformers tells me that is like saying you should shift into 1st gear AND 3rd gear before releasing the clutch.  Also, the suggested design makes no commercial sense -- Sunn would have to set up its cabinets for alternative loads (I can see switchable 4/16 load, but a 4/8?  Don't think so).

So . . . am I right, wrong, half-right? 

Thanks for clearing this up.

Half right maybe.

Your point #1 - wrong.

Your point #2 - wrong, the 4 ohm tap is left open, not shorted out.

Your point #3 - correct.

Notice that #1 and #3 are contradictory.

Unfortunately, for a number of surprised but now in the market for a new output transformer Sunn owners, Sunn did not use a shorting jack like Fender.

And do you really think it makes sense for an amp to be looking for two different value loads?

[lochry]

Thanks for the schooling.      Now if I can figure out how to rebuild my transmission.

[po-mo preschool]

Wait a minute, Joe, I think lochry's point #1 was that if you use the 4-ohm output alone with a single cab, the transformer will be expecting a 4-ohm load, and ditto for the 8-ohm output. (My technical vocabulary and understanding are not anywhere near either of yours, sorry.) That actually isn't contradictory with #3. I really hope this is the case, in fact, because this is my understanding of these Sunns based on other threads on this site, and it is my intention to use my Sorado with a single 4-ohm cab.

[JoeArthur]

Wait a minute, Joe, I think lochry's point #1 was that if you use the 4-ohm output alone with a single cab, the transformer will be expecting a 4-ohm load, and ditto for the 8-ohm output. (My technical vocabulary and understanding are not anywhere near either of yours, sorry.) That actually isn't contradictory with #3. I really hope this is the case, in fact, because this is my understanding of these Sunns based on other threads on this site, and it is my intention to use my Sorado with a single 4-ohm cab.

Yes you are correct if #1 means only one jack used with a single cab.

I did interpret that both jacks were being used - because of the paragraph after point #3.

Thanks for catching that.

[xsolarusx]

Sorry to dig up an old post, but this seems to deal my question.

I have two 612S cabinets.  Connecting them in parallel, they would be running at 3ohms.  Running them parallel from my head(71 Solarus, with the 4ohm/8ohm taps), would I run a cable from each cabinet to each jack, and it would run at 4ohms?

I figure I should consult those in the know before I go blowing my gear up.

Thanks!

[loudthud]

Check my reply in the thread below. I hope it answers your questions.

http://sunn.ampage.org/sdp/index.php/topic,3833.0.html