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Post subject: Reverting UL amps back to BF circuitry
Posted: Wed Jul 04, 2012 9:19 am
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Anyone do this keeping the UL OPT? By removing screen tap off OPT and wiring it to PS tap? I've only done a couple of these amps, by going to original spec'ed or NOS BF OPT's, as the UL ones were blown.


Thanks! And Happy 4th. :D


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Post subject: Re: Reverting UL amps back to BF circuitry
Posted: Wed Jul 04, 2012 10:17 am
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I've done precisely two UL-to-BF conversions. Both of them were royal PITFA's and took untold hours to make them play correctly. They were such a fussy job to backdate, I never even bothered to do my own ultra-linear '78 Twin Reverb. I refuse to accept such commission work any more.

Arjay

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Post subject: Re: Reverting UL amps back to BF circuitry
Posted: Wed Jul 04, 2012 9:10 pm
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Never attempted that job. But I wouldn't consider it without the OPT swapp. Just too skittish of a set-up, like Arjay said! The entire component deletion/change and the lead dress, seems hardly worth the effort. Besides, ultralinear isn't necessarily a bad design, for folks who can dig it. JMO Art

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Post subject: Re: Reverting UL amps back to BF circuitry
Posted: Thu Jul 05, 2012 7:58 am
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One of the tube amp design books I have been studying has one chapter on UL amps (Guess that's all the coverage the author felt they deserved. :lol: ). From reading the design details it appears that the amps can be designed for either maximum power or minimum distortion depending on where the screen is tied into the output transformer. It also appears that the amp is more tolerant of changes in speaker impedance (both phase and magnitude) as it changes through the frequency range.

Did Fender have a particular design goal in mind when they built and marketed these amps or were UL amps just "the latest thing" (although it appears they had been around for some time already) that every manufacturer had to do to stay competative?

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Post subject: Re: Reverting UL amps back to BF circuitry
Posted: Thu Jul 05, 2012 11:04 am
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bluesky636 wrote:
Did Fender have a particular design goal in mind when they built and marketed these amps or were UL amps just "the latest thing" (although it appears they had been around for some time already) that every manufacturer had to do to stay competative?


I think multiple engineering teams designed this one, Bill. The committee in charge of the output stage had maximum clean volume as their primary parameter while the guys running the pre-amp show decided the amp was too clean and gave it a master-volume control to saturate the initial gain stages. When that didn't yield sufficient "dirt" the project manager foisted that miserable push-pull boost switch on us. Thus, "a jack of all trades and a master of none" was the result.

:roll:

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Post subject: Re: Reverting UL amps back to BF circuitry
Posted: Thu Jul 05, 2012 11:32 am
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Sounds typical. :lol:

So I am guessing that they were not very successful in the marketplace because of those compromises. I guess I can see a place for ultralinear amps in the hi-fi world, such as some Dynaco power amps, but don't really see the value in a guitar amp. Did Fender do any UL bass amps? That would seem to be a logical choice unless they are cost prohibitive.

Here's an easy description for those that may be just tuning in:

http://ampbooks.com/home/amp-technology/ultralinear/

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Post subject: Re: Reverting UL amps back to BF circuitry
Posted: Thu Jul 05, 2012 6:25 pm
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bluesky636 wrote:
Sounds typical. :lol:

So I am guessing that they were not very successful in the marketplace because of those compromises.

Did Fender do any UL bass amps? That would seem to be a logical choice unless they are cost prohibitive.


Actually, the sales figures were quite impressive for the U/L Twin Reverbs -- which is why you see so many listed on Ebay now. Aging folks (like me) are loath to pack them to a gig because of the weight and they can't seem to give them away due to the popularity of smaller, lighter combos.

The Bassman 135 was a close cousin of the U/L Twin Reverb, utilizing all of the power supply and most of the phase inverter and output stages. However, it was less of a commercial success due to the proliferation of Acoustic 370's, Ampeg SVT's, and various Sunn platforms.

HTH

Arjay

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Post subject: Re: Reverting UL amps back to BF circuitry
Posted: Thu Jul 05, 2012 8:07 pm
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Retroverbial wrote:
bluesky636 wrote:
Sounds typical. :lol:

So I am guessing that they were not very successful in the marketplace because of those compromises.

Did Fender do any UL bass amps? That would seem to be a logical choice unless they are cost prohibitive.


Actually, the sales figures were quite impressive for the U/L Twin Reverbs -- which is why you see so many listed on Ebay now. Aging folks (like me) are loath to pack them to a gig because of the weight and they can't seem to give them away due to the popularity of smaller, lighter combos.

The Bassman 135 was a close cousin of the U/L Twin Reverb, utilizing all of the power supply and most of the phase inverter and output stages. However, it was less of a commercial success due to the proliferation of Acoustic 370's, Ampeg SVT's, and various Sunn platforms.

HTH

Arjay


That 135 definitely appears to be a beast. Except for the tone stacks, there does not seem to be much of a difference between the two channels.

http://ampwares.com/schematics/bassman_135.pdf

Power supply looks to be an interesting design.

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Post subject: Re: Reverting UL amps back to BF circuitry
Posted: Fri Jul 06, 2012 6:24 am
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Bill, that is a great article that you provided. Much thanks!!! Art

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Post subject: Re: Reverting UL amps back to BF circuitry
Posted: Fri Jul 06, 2012 2:14 pm
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aclempoppi wrote:
Bill, that is a great article that you provided. Much thanks!!! Art


It's basically an extract from the book by Richard Kuehnel that I have been reading. He has a book on preamp design and another on power amp design. Plus he wrote the book I have analyzing the 5F6A Bassman. Lots of math in all three, but it brings back memories. :lol:

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