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Post subject: "Snapshot" analysis of Vintage Fender Amp
Posted: Fri Feb 28, 2014 12:53 pm
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I just purchased a ca. 1977 Super Reverb which has a killer overdrive tone in the Preamp Section. The Reverb Channel sounds terrific even with the Master turned down. My question is: How can I preserve this sound even when I would have to repair the amp and replace bad components? How can I take a momentary "snapshot" of the amp's circuit status? By "status" I don't mean any controls settings or so, but the status of the active and passive components and how they contribute to the sound.
Should I measure all the component values? Or do I better measure all voltages and take some sort of frequency response curve?
I am sure that other vintage amp owner's have done this in one way or another. Anyone who would like to share some advice?
Thanks
Tom


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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Fri Feb 28, 2014 2:34 pm
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Well, you can check both the voltages and the values of Caps & Resistors, that'll give you the 'snapshot' you're looking for. But, that's no guarantee you'd be able to reproduce these exactly.

There is wide variation between Resistors, and to a lesser extent, Capacitors. You might have to buy/test 10 (or more) different Resistors to find those which match your current pieces exactly.

And, even if you do that, the sound you want is also the interaction of all these various components. Some of these may now be unavailable, or manufactured in a different way than the originals (even though values may be the same), or are burned-in or tired, yet still functional.

A lot of what people think or atribute as that 'Vintage Sound' are mis-interpreting a tired, 30-45 y.o. Amp as that 'Vintage Sound', but are disappointed when they refurbish the amp because it doesn't sound the same. Yet, in many cases, the refurbished amp comes much closer to the original sound than the amp did before the rehab.

And you also have to account for wear. A refurbished Amp is likely to improve in Tone w/ play over many months or even years. Just as the Amp did before the work when it had many, many hours of use and hundreds of service cycles.

The best you can really do is follow the schematic precisely, or as precisely as possible. Many tout OD and Film resistors as more accurate and longer-lasting, but they also 'color' the Tone differently than the OEM Carbon Comps do. Sticking w/ CCs gives you a better chance of retaining the orig. sound.

Also, research replacement parts such as the Electrolytics. The OEM parts are no longer available, but there are several replacements from different Mfgrs. I think the gen'l concensus is that the Sozo brand electrolytics come the closest to the OEM Astrons... I'd say 85-90%, but still not exact.

cheers!

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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Fri Feb 28, 2014 5:45 pm
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I recently repaired a '65 SR and was worried that some of the components would take away from the "vintage" sound, but it turns out the last person that "repaired" this was less concerned about the tone than I am. It sounded good when I got it (it was motorboating on the vibrato channel) but I found a few suspicious parts and replaced them with what are considered to be better tone-capable pieces.

For example it had an IC 80/350 (#1 in the can) and a blown #2 original cap (causing the motorboating) so I replaced them both with TAD 80/350 caps. I also replaced the 25/50 bias cap with a TAD and the 470 ohm resistor (the resistor was almost 600 ohms) as well as the grid resistor on the last 5881 which was also way out of spec, and the pre-amp 100K resistors.

When I tested it it sounded just ok to me, but I was also testing through a 15" speaker at the time. When I took it back to the owner he put it in the original 4x10 cabinet and played through it for a while. I was impressed with the tone and he said it was "alive" again. It really did sound like it had a personality, and after several hours of playing he says it sounds amazing again. Apparently, like speakers, capacitors have a "break-in" period before they sound like they're going to until it's time to change them again.

So the point is, the components you choose CAN affect the tone, but as Lightning said, unless you have a perfect memory and bought it new you can't know what it sounded like originally and it probably won't sound like it did before you refreshed it. It should sound a lot better if you use good components.

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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Fri Feb 28, 2014 5:51 pm
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jimolson1963 wrote:
When I tested it it sounded just ok to me, but I was also testing through a 15" speaker at the time. When I took it back to the owner he put it in the original 4x10 cabinet and played through it for a while. I was impressed with the tone and he said it was "alive" again. It really did sound like it had a personality, and after several hours of playing he says it sounds amazing again. Apparently, like speakers, capacitors have a "break-in" period before they sound like they're going to until it's time to change them again.


The secondary tap on the SR's output tranny is spec'd for 2Ω......what was the impedance of your single 15-inch speaker?

Arjay

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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Fri Feb 28, 2014 10:45 pm
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Thanks for your input guys. On my special SR, the Vibrato channel provides a lot more gain than the Normal channel. If I turn the Master down to 2 or so and play the Vibrato channel, the amp almost sounds like a Boogie Mk 1 in overdrive. When I then plug into the Normal channel (Master still at 2) the amp sounds very clean, but not a bit weaker. Is this normal for a 45w Silverface SR considering the Master works for both channels? I was always under the impression that the Normal and Vibrato Channel sound quite similar on Silverfaces or do at least provide similar gain. Could the pull-boost circuit in the Vibrato Channel (which I never engage) could have to do with this? From what I know, the pull-boost has no influence if disengaged. The amp has never been modded and both channels have identical input tubes from current production (TAD).
Cheers


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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Sat Mar 01, 2014 12:38 am
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frodobaggins, Lord of the Rings, fan eh? :D

You need to replace the electrolytic caps to have a reliable amp and check the bias, at the minimum. I doubt that doing this will grossly effect the reverb section, one way or the other.

If you don't change worn and tired parts, you may end up with an amp that makes no tones. Reverb or not. A Super Reverb amp should be properly brought up to spec. Then, you should get a good 25 years of service.


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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Sat Mar 01, 2014 12:56 am
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BMW2002Ti wrote:
If you don't change worn and tired parts, you may end up with an amp that makes no tones.


+1

That's the bottom line.

Arjay

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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Sat Mar 01, 2014 11:47 am
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It's an 8 ohm cabinet. It doesn't say that on the schem, his 4x10 cabinet is only 2 ohms?

Retroverbial wrote:
The secondary tap on the SR's output tranny is spec'd for 2Ω......what was the impedance of your single 15-inch speaker?

Arjay

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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Sat Mar 01, 2014 11:56 am
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That's correct -- 8Ω drivers, wired in parallel.

This is not marked on the AB763 schematic but drawings for all subsequent circuit revisions beginning with AB568 clearly indicate the individual speaker impedance.

Arjay

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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Sat Mar 01, 2014 1:28 pm
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Ah, so it does. I will remember that!

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Post subject: Re: "Snapshot" analysis of Vintage Fender Amp
Posted: Sat Mar 01, 2014 2:39 pm
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The SR shares its speaker impedance and wiring configuration with the 5F6-A Bassman. Not so with the 4-speaker Quad Reverb and Bassman 10 though -- they're wired in parallel but use 16Ω drivers for a total system load of four ohms.

Arjay

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