Extension Cab with a ’79 UL Twin

[5er Driver]

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!

[chuckocaster]

[steve25]

That amp runs near 500 volts across 4 6l6 output tube plates, so I can not see how 2 EL84s will hold up for more than 1 hour in that amp!
You had better cut that amps fuse back to a 2 amp FAST BLOW type!!

[5er Driver]

Chuck - Those would be ideal.

Steve - I've been using these for about 2 years with no issues yet.

http://www.tubeampdoctor.com/product...roducts_id=669

[steve25]

Do you pray for them(EL84s) alot at night, or is your A/C line voltage very low? LOL

[phantomman]

Examining the owner's manual that came with my '78 UL Twin Reverb, no mention is made of the impedance requirements when using the extension speaker jack. However, a careful study of the provided schematic confirms that the OT does indeed have two secondary taps -- 4 and 8 ohms -- which are accessible via the self-switching jacks on the rear panel. In fact the schematic illustrates every possible speaker configuration that Fender intended for this chassis since it was also used as the platform for the Dual Showman Reverb (4 ohms), Quad Reverb (4 ohms), Super Six Reverb (5 ohms), and Vibrasonic Reverb (8 ohms). Assuming that those TAD adapters and/or the EL84s do not self-immolate (as Steve25) opined, you should be alright.

HTH

[5er Driver]

Thanks phantomman. I figured I'd set it up like this: 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.
Thanks again.

[phantomman]

I would periodically push four 8-ohm JBL D120Fs with my UL TR and while the output power was equally distributed among the speakers, they didn't produce identical perceived volume since one pair were mounted in the amp ("infinite" baffle) and the other two were installed in a Bandmaster cab (sealed "bass reflex" design). But it sounded great nonetheless and worked well, especially for outdoor venues.

HTH