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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Thu Aug 13, 2015 8:27 am
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The smaller cap will reduce bass response.

The silverface amps generally have more bass than the BF versions. This is mostly due to the larger OTs that the SF amps sport.

The AB763 Super Reverb SF has much more bass in the Vibrato channel than the Normal input.

I trim bass response at the first cathode cap. Going from a 22uF to a 2.2uf Poly cuts the bass in half and improves tone with the higher speed capacitor.


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Thu Aug 13, 2015 9:26 am
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TimsAudio wrote:
The silverface amps generally have more bass than the BF versions. This is mostly due to the larger OTs that the SF amps sport.


Please indicate which schematic(s) for a blackface amp that made the leap to silverface cosmetics where Fender specified BY PART NUMBER a different output tranny for the later version.

Arjay

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Fri Aug 14, 2015 4:52 am
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Ya this is NOT the UL model, not even a master volume so..............
Just trying to get her back as close as possible to the famed 1965 Super reverb as possible

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Fri Aug 14, 2015 8:43 am
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Retroverbial wrote:
TimsAudio wrote:
The silverface amps generally have more bass than the BF versions. This is mostly due to the larger OTs that the SF amps sport.


Please indicate which schematic(s) for a blackface amp that made the leap to silverface cosmetics where Fender specified BY PART NUMBER a different output tranny for the later version.

Arjay


Trick question there Arjay...
The early silverfaces used the old BF OT. The story I got it that the later CBS OT was the same part number, but used different laminations. The permeability was greater and produced better transfer characteristics for more gain and slightly more power. It also produced more subsonic bass response that gobbled the goodness out of the midrange. It would have sounded real good in the showroom with all that bass, but it was also the main reason the SF amps were panned by the pros of those days. As Leo described it,"Yea we got some bad iron".
If you compare the response curves of the BF and SF versions, they have similar schematics, but very different response curves. I can only attribute the differences to the "bad iron"


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Fri Aug 14, 2015 9:19 am
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Here is the basic outline of the mods I'm doing to my 74 Super reverb

http://blueguitar.org/new/schem/fender/ ... bf_mod.pdf

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Fri Aug 14, 2015 10:25 am
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The blue guitar mods don't address the differences in the OT that I described. It kinda throws a wrench into the SF to BF mod theory.
Mostly, bass response is a matter of taste and your application. If you are playing solo in your living room its great, all that bass sounds like you have a bass player locked in with your low notes.
If you are in a band situation and actually have a bass player, your subsonic response will argue with his notes acoustically and create nasty harmonic interactions.
So a SF to BF mod needs to have a baseline response curve for the BF and then match the SF to the same curve.
This requires a BF to get the original curve and a setup to measure it and the SF.
You need a flat response sine wave signal generator, a true RMS AC voltmeter and dummy loads to measure the response from 20Hz to 20KHz.
I have graphed over 250 amps and the tone balance differences between SF and BF amps is clear and unmistakable.
You can make circuit compensations for the differences in the OT, but to effectively match a SF to BF specs means you need to be able to measure both.
In general, I replace the V1 & V2 cathode caps to 2.2 Poly to reduce the bass response. Cutting the value of the PI coupling cap seems to reduce bass and mid response.


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Fri Aug 14, 2015 11:24 am
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Well................................
I've "blackfaced" an awful lot of Silverface circuits and can tell you the difference is absolutely stunning on most !
as far as OT's or PT's carrying the same # but different specs well............................. :roll:

Lets just say I'm betting this Super reverb is gonna be extra sweet when i'm done 8)
And hey if its a placebo effect then I'll be blissful and ignorant !

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Fri Aug 14, 2015 11:56 am
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TimsAudio wrote:
The early silverfaces used the old BF OT. The story I got it that the later CBS OT was the same part number, but used different laminations. The permeability was greater and produced better transfer characteristics for more gain and slightly more power. It also produced more subsonic bass response that gobbled the goodness out of the midrange. It would have sounded real good in the showroom with all that bass, but it was also the main reason the SF amps were panned by the pros of those days. As Leo described it,"Yea we got some bad iron".
If you compare the response curves of the BF and SF versions, they have similar schematics, but very different response curves. I can only attribute the differences to the "bad iron"


I'd like to see some substantiation for the source of that "story".

And while it's certainly possible that FMI received a batch of "bad iron", Leo Fender's affiliation with the corporation had for the most part ceased by then and he'd moved on. I'm betting he wouldn't have cared one way or the other and likely not made such a statement.

I stand ready to be proved wrong though.

Arjay

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Fri Aug 14, 2015 4:59 pm
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Well, so am I. I'll try to track that story down.
I do remember that the reason that I explored the issue in the first place was because of the discrepancies in response curves I discovered between SF & BF amps. That much I will stand by


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Fri Aug 14, 2015 6:37 pm
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That's good enough for me, Tim.

But anecdotes such as these (if substantiated) provide a wonderful insight into Leo the man and the early day-to-day FEIC operations at Fullerton. As such, I have a keen interest in both. If you're able to track down an article, video, or interview, please post links -- I love reading this stuff.

Arjay

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Sat Aug 15, 2015 5:54 am
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All done !
Yes I went from the bass setting at 4 to 6 , mids 6 treble 5 and now she sings !
Blackface punch !
PS all the suppressor caps are GONE ! and not going back ! If I get oscillation problems I'll rewire the whole chassis with cloth wire like i did the 69 bassman a few years ago that had that ridiculously thin wire.

Pretty much waiting on weber now :mrgreen:

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Sat Aug 15, 2015 12:41 pm
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Great news! I don't think stability will be an issue. Use good 6L6GC tubes.

Interested to see what you think about those re-coned CTS speakers.

:D


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Sat Aug 15, 2015 7:22 pm
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Me too Beemer !!!!
Me Too !

the GE 6L6GC's showed up and I cant get em within 6ma of each other and the readings follow the tubes , not the sockets so....................
Right now I have a strong engl branded pair in there

I have three RCA that are too far apart too so................... :roll:

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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Sun Aug 16, 2015 1:23 am
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Chris,

You can mitch different brands, no problem. GE and an RCA which match maybe kinda fun to hear.

I've used output tube 7mA apart. No problems. Some ppl actually like the tone of mis-matched tubes.


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Post subject: Re: Road worn 1974 Super Reverb restoration
Posted: Sun Aug 16, 2015 5:28 am
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I was trying to just use the GE tubes but the spread on the current draw was enough to leave one too cold or too hot
34ma one one leaves the other at 29ma

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