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Post subject: Extension Cab with a ’79 UL Twin
Posted: Wed Mar 03, 2010 1:17 am
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I have a '79 135 watt UL Twin running on 1 pair of output tubes (TAD ToneBones with EL84s) which means that the spkr jack is looking for an 8Ω load instead of the standard 4Ω. I'm using a Z-Matcher between the spkr jack and spkrs (2x8Ω wired parallel) and its set to 8Ω on the amp side and 4Ω on the spkr side.

I read in another forum that the UL Twins have an 8Ω tap on the output tranny that is engaged when ext jack is used and that the spkr and ext jacks are wired in series.

“If that is a 135 watt Twin then there is a way to get at the 8 ohm tap. Those amps had the UL transformer and had a 4 ohm tap for the main and an 8 ohm tap when using an extension speaker. Thru the jack switching contacts, the external 4 ohm speaker would be placed in series with the internal 4 ohm load and switched to the 8 ohm tap.”

“If this is a '78 (135W/Ultra-Linear) Twin, the speaker jacks are wired in series. The Ext. jack has a switch in it which changes the amp to 8 ohms. Yes really... that's why it says 4 ohms below each speaker jack, not 'Total Load 4 ohms' below both of them, like the older models do. Which makes things very simple, because you can just plug your 4-ohm cab into the Ext. jack and everything will be fine.”


So now I’ve got a few questions concerning adding an 8Ω cab to the equation.


1. Running on 1 pair of output tubes means that both the spkr jack and ext jack are each looking for 8Ω, correct?

2. If both jacks are used at the same time, the amp will be looking for 16Ω total load, correct? (the 8Ω tap on the tranny is looking for 16Ω because 1 pair of output tubes are pulled)

3. If I plug the stock spkrs of 4Ω (2x8Ω wired parallel) into the spkr jack and an 8Ω cab (2x16Ω wired parallel) into the ext jack, the amp will see a 12Ω load because the jacks are in series when both are used?

4. If all the above is correct, then the impedance mismatch will be 4Ω. Will this mismatch be "within tolerance" to the output section of the amp? Any chance of burning anything up?

The Z-Matcher does not have a 12Ω setting.


Many Thanks!

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Posted: Wed Mar 03, 2010 11:30 pm
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Very interesting. If I have this straight, two 8ohm loads will do the trick, right? Could you use the Z matcher on the stock 4ohm load to reflect an 8ohm load to the amp? Then using both speaker jacks, one Z matched and the other 8ohm cab, you would present a 16ohm load. Or am I missing something here? As I understand it, the UL OT is less critical on the secondary side, in the sense that the load can be higher to some degree. On the primary side, it's the tubes that adapt to the demand and of course the negative circuit will react differently. I can't see any problems if you can achieve that 16ohm load. If you're going to run the 12ohm load, the 4ohm speakers will be getting more power, so there will be that imbalance. Can't say I'd be comfortable with the lower ohm mismatch with what little knowledge I have about the UL transformers, more research is inorder for me. Art

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Posted: Thu Mar 04, 2010 12:29 am
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aclempoppi wrote:
If I have this straight, two 8ohm loads will do the trick, right?


Art,

You are right. And there's an error in my question #1. With 2 tubes pulled, the 4Ω tap will be looking for 8Ω and the 8Ω tap will be looking for 16Ω.

I suppose I could use the Z-Matcher between the stock spkrs and amp to make the spkr jack see 8Ω there and plug the 8Ω cab into the ext jack. Both jacks in series would then be 16Ω and match the 16Ω that the 8Ω tap is looking for and there would be equal power distribution to all 4 spkrs.

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Post subject:
Posted: Thu Mar 04, 2010 1:16 am
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Yeah, seems like that scenario will be fine. Talk about errors, I was referring to the negative feedback circuit, missed that before I hit "submit." BTW How do you like the EL84's in your amp? Art

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Posted: Thu Mar 04, 2010 2:26 am
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They sound real sweet, lots of clean chime when not overdriven. Not much headroom using only 2 though, total output being about 22 watts. If I need louder clean I add 2 6L6GCs to them. I don't plan on using the extension cab and 4 output tubes.

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