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Post subject: 1973 Super Reverb Basket Case
Posted: Mon Dec 29, 2014 2:56 pm
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The story of my silverface.
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This old amp has been kicking around my father's place for 30+ years. He bought it used and abused from another working musician but didn't use it a whole lot after. Dad's in his 70s now, doesn't play guitar much anymore and when I saw the cabinet, missing a speaker and no "head" in it, sitting on the veranda I asked him if he had the rest of it and wanted to part with it. He gave it to me on the condition that I take it away and never bring it back. :D

I found the head and reverb tank elsewhere in the house packed it all up and brought it home.

I started with the head. It was in better shape than I could have hoped for, some surface rust from sitting in the damp, the only mod was a 3 prong power cord that looks to have been done correctly.
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The power tubes were mismatched by about 10 years, but when I look at the schematic I don't think it is a huge deal. Regardless, a pair of matched power tubes are now on order.

I probed a few key spots for ground faults etc.. and the amp was at the very least safe.

After basic visual inspection I did a smoke test, using a 2x12 4 ohm cabinet as the amp load. It didn't explode, catch fire or otherwise set off the fire alarms in the house. I probed a couple of spots then said to hell with it, plugged in my strat and tried it out.

The Pots were very dirty, which led to a noise violation with the wife. I did a quick "spin clean" and tried again.

The normal channel worked fine. Why they made this amp 2 channels is beyond me, but there it is...

The reverb channel was giving me noise and no reverb. The RCA cords were suspect and the one connection at the tank was corroded. I replaced the cables and cleaned the reverb tank connections (and inspected the guts) and I had reverb. I will order "vintage" cables at some point, but the heavy modern gold ones I'm using will do just fine until a purist smacks me upside the head.

I replaced 2 very weak triodes (though I had to use a 12ay7 instead of a 12AT7 on the reverb preamp, just for now) and have the rusty reverb tank working again, though I have a lot more TLC to do on the tank. I am lucky, the critical parts are spotless, though the enclosure looks like it sat in a snowbank for a year. I am starting to think someones basement flooded.
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All new triodes are on order. (And I have a good, quiet reverb channel even as it sits now, even with the under powered 12ay7.)

The Vibrato/Temolo isn't working. I have no pedal and so did the rca cord hack and still no go. The pot was busted so I replaced it with a pot I had on hand.

I swapped one of the old 12AT7 tubes for the 12ay7 I had put in the rev/Vib amp section and still no go.

After studying the schematic for a minute or twenty I figured I had might as well order a replacement LDR. I double checked and I see no light coming from the one on the board. If that's not the problem I'll sit down with my oscilloscope and trace it out.

So that's where the head is, functioning, sounds sweet through a 2x12, but it's slightly crippled. I have ALL new tubes on order including the rectifier.

Today I tackled the cabinet. Someone replaced the Fender Grill Cloth with doubled up nylon screen door material. (Ugh) Oh well - it's an easy, cheap, low priority fix that will let me "make it mine."
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The sad news, I did a mechanical inspection of the three AlNiCo speakers left in the cabinet, one is seized with a torn cone courtesy of my left thumb, one was missing it's dust cover and is 'scratchy' and one is fine. I set aside the two bad speakers, they will eventually go to be hopefully repaired. I know who had the 4th speaker. I doubt I will convince him to part with it. :roll:
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I tested the remaining speaker with a hybrid head that handles 8 ohms (An Orange Micro Terror.)
The tone was what you would expect from a vintage style AlNiCo speaker, though the sensitivity was pretty low (much quieter than the modern voiced cab that I usually run it through.)


At this point I will call that speaker "good". I ordered an Eminence Legend 1028k AlNiCo speaker to replace the missing one and to allow the Max 4 ohm head to be mated back to the cabinet once its refurb is finished. I will send the bad speakers out to be repaired, but the costs are adding up, and with Christmas just over they will have to wait in a box for now.

This amp is being repaired for my own use in my basement and for home recording. If I were to gig with it I'd replace the capacitors and maybe even do a complete unsolder/resolder job on it. As it is, it's a salvage job with an awesome amp for a reward.

Any thoughts, comments or advice?


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Mon Dec 29, 2014 3:05 pm
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I just came up with a theory as to why this thing is so rusty.
Transporting an amp in Frigid Canadian winter, cooling it to -20 then bringing it into a hot stinking sweaty humid bar = instant condensation where you can't see it. I never saw an amp cover for it, so this makes some sense.


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Mon Dec 29, 2014 6:05 pm
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Someone was very kind and sent me a PM regarding something I wrote.

I wrote the head was 4 ohm max - it is of course a 2 ohm head - this is dictated by the power tubes and the output transformer. I was so wound up in what I was doing I lost clarity in my post, and I appreciate the manner in which my mistake was corrected. I came up with this 4 ohm number as it is commonly cited by laypeople and techs as the maximum impedance you should run this head into.

When you run a super at something other than 2 ohm load, you run the amp out of spec and it causes the power valves to run differently. You can't judge the sound of the amp when running at 4 ohms if it is to be run at 2 ohms when you are done. It was relatively common for players to disconnect one or two speakers to intentionally make the silverfaces a little less bright/clean and reduce the output.

If I were to ditch the cabinet and permanently run it with 2x 8 ohm speakers, I would swap the transformer - which is the correct fix. Another option might be swapping the power tubes and/or rebiassing them cooler. The best option would be 2x 4 ohm speakers.

For now, I feel secure running it into a nominal 4 ohm load, which is fairly common practice, but once I get the 2 bad speakers repaired, will absolutely run it at the full 4 speaker 2 ohm load to reduce the risk of damage to the output tubes.

James


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Mon Dec 29, 2014 7:45 pm
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" Though I had to use a 12ay7 instead of a 12AT7 on the reverb preamp, just for now. "

Don't run that 12AY7 for long, you'll burn it out. Use a 12AU7, if you have one --- while waiting to get a 12AT7 replacement. The reverb section of this amp runs the 12AT7 much hotter than the 12AY7 is designed to handle.

Much of the rust looks like surface oxidation. Light polish and elbow grease could make the amp look much better. I like Simichrome. Full restoration of this amp can be a worthy project. It's tone is like no other Fender amp (or other make amp, for that matter).

By the arrangement of the bias supply filter caps & the type of wires used, I'm guess the amp is 1970-80 era. Do you have any photos of the tranny's stamped numbers? And the serial number on the back of the chassis? One speaker looks to have an CTS (137) number and "732" = 1977, 32nd week.

Check the wiring layout of your amp against the AA270 schematic:

http://ampwares.com/schematics/super_reverb_aa270.pdf


Close-up shots of the irons, speakers (cleaned), and circuitry will help ID the year made.


-----

Some photos of my SFSR:

Note: My bias circuit has been re-wired to AB763 (bias pot), from OEM balance pot.



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Last edited by BMW2002Tii on Mon Dec 29, 2014 7:52 pm, edited 1 time in total.

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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Mon Dec 29, 2014 7:49 pm
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Oh yeah... welcome to the SR club! :mrgreen: :D :mrgreen:

Mine front and back:


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Mon Dec 29, 2014 9:19 pm
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Great post! Sounds like you have a nice re-furb going there. Welcome to the Super Reverb club!

I have done a couple re-furbs on a couple Bantam Bass Amps here on the forum and a small one on my BF Super Reverb. Lots of enjoyment returning an amp to full factory specs.

Here is a pic of my BFSR:
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These are very tuneful amps and I love mine which with the exception of one speaker that I replaced with a period correct matching one, it is all original and has the CTS Ceramics.

Make sure you keep the updates coming as you progress, this is helpful for a lot of folks here.

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Last edited by T2Stratman on Thu Jan 01, 2015 10:08 am, edited 1 time in total.

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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Wed Dec 31, 2014 12:44 pm
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Tubes came in, she'd be as fresh as a daisy now but I looked at the wrong document and ordered a 5AR4 instead of a 5U4, so the old rectifier is going to have to do some overtime until I pick up the right tube. There's no use stressing the old girl out more than I already am.

Now, those paper filter caps concern me. They are still fine, but for how long? It breaks my heart to replace functional original components but man... those things are both old and notorious.

Keeping in mind this is going to be a basement queen when I'm done, should I go ahead and do a full Capacitor job (at least the electrolytics) or wait for them to fail and deal with the fall out?

I think I just answered my own dumb question.


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Wed Dec 31, 2014 6:54 pm
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5AR4 will work fine. In fact, it draws one less amp than the 5U4GB, so the power tranny may run a tad cooler. Plus, has slow start-up feature. Check the plate voltage of the 6L6GC's. May need to re-bias the amp. Something that you should do whenever to replace a rectifier. Same type or different.

I'd do a complete electrolytic cap change-out.

Good luck! Tell us how it sounds. :D


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Wed Jan 07, 2015 7:08 pm
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jruddy...how goes the progress on the SFSR refurbishing?

T2

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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Sat Jan 10, 2015 7:24 pm
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I re-capped one of these (actually it was a '74 Twin but what's a difference in models between friends?) and repaired the trem circuit a few months ago. It's a good sounding amp! The one I worked on had the end of a filter cap blown out and it made a huge mess in the can.

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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Fri Jan 16, 2015 1:03 pm
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Been out of town for a couple of different family emergencies, such is life as you get older eh?

Today I got back at it, replaced the LDR and I now have the Temolo/Vibrato working, though there is a slight sharp tick tick tick. I've read about this tick before, I think I can use a capacitor to eliminate it, and will google it as soon as I'm done typing this.
(The spider I ordered was supposed to be Fender, but I suspect it's not. These are the pros and cons of hitchhiking, er.. shopping on-line.)

Thanks for the heads up regarding the rectifier, I think that delayed start up might be "easier" on those old caps until I get new ones in - will definitely plug it in and do a re-bias. It's a premium tube and it cost a pretty penny, so it must be better, right? ;)

The missing speaker is long gone, passed on from my nephew to a buddy of his to someone else. I could spit nails at this kind of thing... But the Eminence 1028K I bought to replace it is a real beauty and in my ever so humble opinion a great match for the amp. It's AlNiCo, has a similar frequency response to the Jensen, and looks and sounds the part. Weber's and Jensens are not as easily available in Canada and I can get Eminence at a good discount within 2 days of ordering them.

Finding a reputable speaker recone service in Canada for the 2 Vintage AlNiCo speakers is proving to be a bit tricky. Asking around has gotten me everything from dumb looks to recommendations for companies that recone big stadium PA equipment. I want a guitar amp guy who does vintage. I would rather not ship them back and forth over the border to the USA, don't want them to disappear in the chaos that is customs...

Right now when testing the amp I am running 1 original AlNiCo, 1 Eminence 1028K and then on the external jack 2x12 cabinet with a couple of 8 ohm eminence speakers (Texas Heat / Wizard). This at least gets me in the 2 ohm region for cranking it up to see if anything pops, though that 2x12 cab adds a lot of bass to the sound, which sounds good, but not right. :D

I'm thinking on holding off on the recone, packing away the speakers and throwing in a couple of vintage voiced ceramics. Some guys swear by this sort of combo and with one original speaker gone dodo on me, all hopes of keeping it original are out the window anyways. Not 100% sure.

The brass brush has been getting a workout and the head is looking less like something I found in a dumpster. No pics until I'm done folks. ;)

I spent a couple of hours in -30 weather looking through boxes for the missing footswitch and chassis straps. I will continue the search when the weather warms up though I'm not optimistic. There are replacements available. That "screen door" grill cloth has been on it since the late 1970s. I didn't even know they made plastic screen door material back then. It's on the list to go, I have my eye on some aged silverface style tolex.

Christmas bills are in, I probably won't drop a bundle on caps until February, so I'll be scrubbing rust, checking wires and solder, getting rid of the tick tick tick for the next little while. Oh and dropping in that alternate rectifier and re-biasing the amp.

Cheers.
Jim


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Fri Jan 16, 2015 5:57 pm
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Jim...sounds like you're a fan of plan your work, work your plan! Not being in a hurry is certainly a key to getting it right the first time.

Ceramic speakers in a Super Reverb can sound great, my '65 has the CTS Ceramics in it and my 72 had the CTS alnicos and both of these amps sounded like a Super Reverb should. It would be a tuff choice between the two.

Keep testing and poking through the wiring to get the amp back to specs. Enjoy the process and if you need anything just give us a shout out.

Sorry to hear you had family emergency issues.

T2

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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Fri Jan 16, 2015 6:19 pm
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Well I'll be...

I sat down to bias the amp and before I started I took a second look at the bias section of the schematic. (AA1069)

In this incarnation the "bias" pot simply lets you match the tubes' grid voltages to each other. There is no overall "bias" pot like in the AB763 form of this amp. Is this one of the reasons people "blackface" these amps? Being able to set how hot or cold the power tubes run seems like a mod I will want to add to the list of things to do.

Since it's not really "adjustable" I'm not going to run it with the higher voltage rectifier. The old one is putting out a near perfect B+ anyways.

LESSON:
I should have checked the bias on both power tubes as soon as I swapped the tubes, but I had made a bad assumption - I figured I was only worried about hot & cold, not one tube versus the other. When I put matching JJs in, the grid voltages were forced to be unmatched by this pot. (D'oh) I thought it had a lot of even harmonics with the new tubes, but I had chalked it up to the new triodes.

NOW that I have it adjusted correctly the amp is as clean as can be at low volume. I haven't had a chance to crank it up yet but I hope it will have that nice mix of even and odd harmonics that I expect from the design.

Then I ran into a problem with the vibrato that I had fixed earlier and nearly lost my mind, but it turned out to be a mechanical failure with the socket.

Next up, a cap job.


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Fri Jan 16, 2015 7:57 pm
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It's a HUM or BALANCE pot. Check the idle bias current on both tubes. If they are in the 60-70% ballpark --- adjust the pot until you get the least hum.

Now, recheck the idle bias current. You should be closer to matched readings. The better the original match of the 6L6GC's, the lower the hum --- after adjustment. This works pretty darn well with matched tubes. But, the amp may become noisy as the tubes age and go out-of-matched specs.

To reduce or increase the idle bias current, you need to play with the dropping resistor (before or after diode) or C(minus)-to-ground resistor (on pot).


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Post subject: Re: 1973 Super Reverb Basket Case
Posted: Sat Jan 17, 2015 11:31 am
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BMW2002Ti wrote:
It's a HUM or BALANCE pot. Check the idle bias current on both tubes. If they are in the 60-70% ballpark --- adjust the pot until you get the least hum.

hmm... I'm going to have to look at how push pull pairs work again, not sure how it effects hum and inquiring minds need to know. :?:

She's running cold, just under 50% - 448V / 32.5mV on each tube. [**Correction 438V / 32.5mV**]

The 15k C+ to ground resistor measures out at 14.45k, the resistor before the diode measures just over 1k and then there is a 3.3k in line between the diode/cap and the pot that isn't on the AA1069 schematic that I've been working off - but I've been looking at the wrong schematic, I have an AA270 amp not an AA1069 amp. :oops:

There is a mess of other changes at this end, I'm going to have to try and trace it out - *I think* someone might have fused the 6.3V line but I haven't looked into it yet.

I swapped out the rectifier tube for the higher voltage 5AR4 which gave me 466V / 50mV (over 75%)
This is way too hot and there was a lot of buzz - I didn't bother trying to reduce the buzz, just yanked the 5AR4 and put the old rectifier back in.

I'm wondering if, once the cap job is done, I need to sit down and do the "mythical" break in for those power tubes for a couple of hours and check again? I haven't put much through them but I wouldn't think it would have made that much of a difference.

I'm thinking of adding another pot to replace either the 3.3k or 15k pots in order to allow manual biasing, or should I just do the blackface bias mod?

hmmm....


Last edited by jruddy on Sun Jan 18, 2015 1:17 pm, edited 1 time in total.

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