Dual Showman Low Volume

Danny Naccarato · Post by **Danny Naccarato** » 18 Mar 2003 6:24 am

I am by no means an electonic whiz, but my understanding, in layman's terms, is this. My amp is rated for 8 ohm speakers. It has 2 speaker jacks, 1 main and 1 external. If I add an external speaker, it needs to be 8 ohms. When plugged into the 2nd jack, the xformer will switch to 4 ohms, which is what you have with 2 8 ohm speakers. So, 2 plugs will trip the xformer to 4 ohms. If the amp sees another plug, whether or not it is connected to anything on the other end, in the case of my amp, 1980 Vibrosonic 135 watt, Ultra Linear etc, then it wants a 4 ohm load. I have a 1501-4 BW in mine, connected to the main Speaker jack, with a plug in the External Jack connected on the other end to nothing....a simple Geo L right angle plug. So, the impedance is matched correctly. It thinks there is 2 of "something" connected and has switched to 4 ohm. If I decide to put the EV 15-8 back into it, I will simply remove the plug from the Ext Speaker jack. But, if I want to go with 2 speakers, they need to be 8 ohms each.
I know I didn't technically explain that right, but in my simple way of understanding it, I hope that helps.
Danny

Ken Fox · Post by **Ken Fox** » 18 Mar 2003 6:16 pm

Actually, you are right on there Danny! When I mentioned how it worked above, I was talking about the jack on the Twin reverb, 135 watt model. I forgot you had the Vibrosonic! The aux jack on a Twin amp (remember these amps are not like other Fenders!) will wire an external 4 ohm speaker in series with the internal 4 ohm speaker for an 8 ohm. On a Twin of this era only, you could use an 8 ohm internal speaker (instead of a 4 ohm load) by using a shorted jack in the external jack position. On yours, it is already at the 8 ohm tap with the internal speaker plugged into the main. You are correct, plugging in an open plug into the aux speaker jack will cause it to switch to the 4 ohm transformer tap.
This same 135 watt amp was also used for the Quad Reverb, Dual Showman and Super Six Reverb. None of these amps had the switching speaker jacks.

Rick McDuffie · Post by **Rick McDuffie** » 18 Mar 2003 6:23 pm

So... Ken... I'm getting confused. On my '68 Dual Showman, if I plug one 4 ohm cabinet into the main speaker output (and leave the other jack empty), am I going to be running at maximum efficiency... or not?

Bill Hatcher · Post by **Bill Hatcher** » 26 Mar 2003 1:57 pm

If you have a 68 Fender amp then you possibly have one of the very early amps that absolutely sound horrible due to changes made by the design geeks when Leo was out of the picture. These amps need to be swapped back to earlier "Black face" specs. Otherwise they suffer from lower output and poor tone.

First place to redo is the bias circuit. Just disconnect the resistors as they are now and put back in two matched 220K resistors and wire the bias adjustment and circuit as per the Dual Showman AB763 schematic.

The old bias circuit allows the bias to be adjusted up and down on BOTH tubes. The 68 circuit does not do this. It balances the bias. Most of the amps I have worked on that have the 68 bias circuit will not allow the correct current draw throught the power tubes.

If this were my amp and a tech had already swapped out the transformers I would be leary. I have never seen a Fender amp that had BOTH transformers burned out at the same time.

Have your tech put the amp on the bench under a dummy load and check the RMS output of the amp. You should easily see 80 watts and if you look on a scope there will not be much of a square wave at all!! Twins and Showmans are loud and clean.

I just took a 68 Fender Bassman that I got for $25 and totally put it back to ab763 Bassman specs. Incredible difference in the sound. Just wonderful!!

Ken Fox · Post by **Ken Fox** » 26 Mar 2003 4:40 pm

There is a schematic for the AC568 (manufactured on or after 5th day of 1968) Showeman over at Schematic Heaven. This the the so called "dreaded" schematic. I just recently learned about this critter on a Twin Reverb a fellow forumite has. There was an attempt to use cathode resistors and bypass caps on the output power tubes on this design (ala the old Tweed design). The bias is not a true bias adjust either. It has fixed bias on one set of power tubes and adjustable bias on the other pair of tubes (to allow power tube matching). With a bit of patience and good tech skills this amp schematic can be converted back to Blackface specs. You are correct that a 4 ohm load is optimum for your amp.

J Fletcher · Post by **J Fletcher** » 27 Mar 2003 5:55 am

My 2 cents. Ken, regarding the external speaker jack on a 60's or early 70's Super Reverb, if an 8 ohm speaker is connected to it, and the four 10 inch speakers are still connected to the normal speaker jack, then the total load is 1.6 ohms. This will reflect an impedance of 3K2 back to the plates of the 6L6's. This doesn't seem to be a dangerous operating condition to me.
On a Pro Reverb of similar vintage, using an 8 ohm external speaker will result in a total speaker load of 2.7 ohms, and a reflected impedance to the 6l6 plates of 2K7 ohms, which is farther away from the designed reflected impedance of 4K, than the Super Rev situation. Just wondered why you think the external speaker jack on the Super Rev isn't a good idea.....Jerry

Bill Hatcher · Post by **Bill Hatcher** » 27 Mar 2003 8:35 am

Gerald Weber has a book out on amplifier repair stuff. He says that Fender output transformers are very tolerant of impedence mismatches--as long as specs are not off by more than 100% then things are fine!

Ken Fox · Post by **Ken Fox** » 27 Mar 2003 5:24 pm

Jerry, I can see your point on an 8-ohm extension speaker on a Super. However for me it is a matter of believing that long-term damage can be done by reflected voltages to the primary windings and tube plates. Even Gerald Weber's book (Desktop Reference) states that the aux. speaker jack on a Super was a bad idea. He further stated that it would be best to rewire the speakers to a series parallel arrangement for a 4-ohm load and then using an extension speaker. I suppose if you are not pushing the amp to the limits every night, then this damage would be negligible. <FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Ken Fox on 27 March 2003 at 05:25 PM.]</p></FONT>

J Fletcher · Post by **J Fletcher** » 28 Mar 2003 4:08 pm

Ken, I just wonder why it's more of a problem using an extension speaker with a Super Reverb, than with other Fender amps. The way I see it, the extension speaker has less effect on the reflected impedance in a Super Rev, than in any other Fender tube amp of 60's or early 70's vintage. So I would think it would be safer to use an extension speaker, 8 ohm, in a Super Rev than in , say a Twin Rev, Pro Rev, Deluxe, Princeton, Bassman, Vibrolux, Bandmaster, Showman, etc.
Just wondering why you singled out the Super Reverb as being problematic?....Jerry

Gino Iorfida · Post by **Gino Iorfida** » 28 Mar 2003 4:55 pm

Super rever problematic due to the fact it's OT is designed for a 2ohm load, going lower than that is too cloe to a short circuit. Granted a short is safer than an open condition, still not ideal, due the massive amount of current it would pe pumping

Ken Fox · Post by **Ken Fox** » 28 Mar 2003 6:51 pm

What Gino said. As you appoach "zero" ohms things change expotentially. If a dead short blows a fuse, then as you approach a short a lot of power is being developed, likely more than the winding can handle. If it works for you, go for it! I am just expressing my opion based on my limited knowledge of electronics and things I have read. I am not trying to spread an mis imformation.

Long and laborous debates on this issue have surfaced on the Fender Forum over the last 4 years. The general consensus is that an impedance match up or dowm 100 percent is OK. Others on the forum do not agree and have gone to great lengths to show the math/engineering involved to calculate signal reflections to the primary windings and plates. Most of that discussion was put on by "Old-Teleman". The man is a brilliant engineer. He spends most of his time over on the amp forum at the Weber site now. It is intersting to read his replies, the man is a bundle of Fender amp knowledge! I generally tend to look more at the theoretical as opposed to what seems to work. I guess that is just personal thing!