Ok, so my tablesaw is my "bench" right now. But here is my new project:
Here's the full album. I'm new to flickr, so let me know if there are problems with this link:
https://flic.kr/s/aHBqjAKr8a
Ok, so my tablesaw is my "bench" right now. But here is my new project:
Here's the full album. I'm new to flickr, so let me know if there are problems with this link:
https://flic.kr/s/aHBqjAKr8a
Some notes:
Before I started working on this one, I played it through 2x10s @ 4 Ohms & it sounds strong & punchy.
It's hard to see in the photos, but the chassis around the power tubes is lightly "toasted". That makes me think it was either running the wrong impedance or the bias was/is off.
As seen in the photots, I have already replaced the 2 prong power cord w/a 3 pronger, switched out the bright switch caps, & bypassed the polarity switch. YouTuber Uncle Doug has videos detailing the conversion of the polarity switch to a negative feedback loop, but that's a little out of my depth
While it's possible that the idle bias is/was incorrectly set or the amp was regularly pushing a mismatched load, that ever-so-slight discoloration around the power-tube sockets isn't necessarily indicative of a developing issue. Inspect the sockets for any evidence of thermal damage then measure the screen-grid resistors for excessive value drift -- if everything appears copacetic, proceed with the bench check.
"When injustice becomes law then rebellion becomes duty."
Link to the flickr album worked perfectly. It looks mostly "unmolested", as Uncle Doug would say. Which is good, since you won't have to correct someone else's mistakes. From the date codes on the transformers and doghouse caps, it is indeed a 1969. On mine, in addition to changing out all of the electrolytics, I also changed the doghouse resistors and replaced everything on the bias board, just for good measure.Originally Posted by Cogs
Good luck with the project!
Edit: What Uncle Doug does with the polarity switch is to make the negative feedback loop switchable between on and off. Fender put negative feedback in the circuit for a reason.
"When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top
Gotcha. I hate being "that guy", but how do I measure the screen grid resistors? Are those the ones underneath each power tube, bisecting the socket?
The screen grid resistors are on the side of the power tubes sockets. The grid stopper's are closer to the tube base. I just noticed from looking at your photos, that your amp is the same circuit revision as mine. AA-768. You can tell by the big ceramic 150ohm 7 watt resistors from cathodes to ground on the power tubes. This is known as the hybrid fixed/cathode bias circuit.
"When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top
@cogs I don't know your level of experience working on vintage tube amps, but there is a very good series of videos on Psionic Audio's YT channel. Go there and search for: "Time Machine '68 Super Reverb Part 1" He has about 6 videos servicing the amp, and goes into great detail. I watched all the videos again today for the 2nd time.
"When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top
Thanks. I have very little experience
One thing I do differently than he does is change out the filter caps in the doghouse one by one, so I don't get the wires mixed up. He fully removed the eyelet board and cleaned it, but I haven't found that necessary. And on your '69, there shouldn't be a need to create new bus wires under the doghouse eyelet board like he did, because Fender most likely didn't use the long capacitor leads for bus wires like the amp he was working on. (at least on my '69, the caps came right out)
Observe the polarity of the 2 reservoir caps (the 2 biggest ones), as they are wired in series on that amp. I usually just put a piece of masking tape on the chassis and write down the polarity of all the doghouse caps before removing any of them.
Also make sure you get the polarity correct on the bias cap. (positive to ground) I replaced the bias resistor and diode also, using metal film or metal oxide resistors, same thing with the doghouse resistors, even if the old carbon composites measure OK. You can buy electrolytic cap kits from various sources, just select your amp and circuit (AA768), but I don't think they come with any resistors. Let me know if you want to find the website I buy the cap kits from.
"When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top
All the tube sockets appear to be original & in great shape. The power tubes are German made.All the little buggers have some dampening wraps on them, Mesa Engineering
I like Uncle Doug's stuff, but DANG that Psionic dude is a college of knowledge. I have a new thing to do now, lol
Ya, Lyle at Psionic Audio designed and built his own boutique amps. I don't think he's currently selling them, as he's a one-man shop. I joined his YT channel a couple years ago when he had a couple thousand subscribers, now he's approaching 40K. Every few weeks he does a live Q & A on the channel, usually Sat or Sun afternoon. And on his current videos, he has improved his camera equipment and microphone setup.
"When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top
Doing the cab in Sonic Blue Tolex. I'm pretty happy w/these corners:
Real nice job, Rob.
Got your holes plotted out for the tilt-back legs?
"When injustice becomes law then rebellion becomes duty."
Really nice, damn corners always give me fits!
I don't, but that will be an easy addition. Right now I cant bring myself to spend $38 for a pair. I'm waiting on a deal
I hate this part. I've done some without any problems whatsoever, & other times they've turned out so bad I have to scrap the piece & start over again. This time around I matched the vinyl w/some enamel spray paint & hit the corners just in case the seams had a gap. So far so good
Time to fit the chassis.
IT DOES NOT FIT
It is somewhere between a skosh & a smidgen too long & has dug into the tolex on the end w/the pilot light. This is a critical point, because I can tear & stretch the tolex & then its game over.
Time for the bottle jack!
You need a new tape measure! I usually drill the chassis mounting holes before tolex.
If it's just a little, trim the tolex on either side back to just clear the faceplate, (the chassis doesn't need to).
Last edited by Bill Moore; 07-09-2023 at 06:31 AM.
I did drill the holes before I covered the cab. I just didn't check the fitment
Never try and force the issue......just get a bigger hammer.
Looks swell, Robert!
"When injustice becomes law then rebellion becomes duty."
Looks like you made it fit! Looks great!
Beautiful work and I love the custom look!
Born in Fullerton, California in 1952
Thanks for all the compliments. Completing this project was very gratifying. The silver panel & blue lettering certainly limits the number of colors that might look appropriate.
But now I have to pull the chassis back out & find out why the reverb is so weak
Did you clean all of the jacks and plugs for the reverb-tank cables?
"When injustice becomes law then rebellion becomes duty."
Now that you mention it, I cleaned the jacks on the tank & the chassis, but I did not clean the plugs because they are brand new. It just didn't occur to me. Best to break out the isopropyl alcohol & check that off the list
A friend recommended that I install a 7025 in the reverb slot instead of the 12AT7, but I'd rather figure it out & make it work properly
A 7025 would provide more gain but the substitution may not remedy the root cause.
"When injustice becomes law then rebellion becomes duty."
Exactly my thinking. Time for a little YouTube research
I just looked at a couple of my amps. One has about 1/8 inch gap between the end of the faceplate and the side of the cabinet on the pilot lamp side, and it's flush up against the cabinet on the other side. On another amp the gap on one side is a bit smaller than 1/8 inch on one side and flush on the other. So even the Fender factory didn't always get the top holes drilled perfectly.
How did you resolve your issue?
"When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top
I managed to get the chassis into the cab almost all the way, probably 2 inches shy of the front before it got wedged in. At that point it was stuck, & I didn't want to risk forcing it one way or another & tearing up the tolex. So I put the amp on its side, & put a bottle jack & a block of wood inside the cab just under the chassis, & began to extend the jack slowly. As soon as it was snug against the sides, I turned the cab upright, being careful to support the chassis as I resumed extending the jack. After a couple more pumps of the jack, the cabinet sides bowed out just enough to let the chassis move. I quickly installed the straps, screws, & kep nuts, & centered the chassis inside the cab as best I could. When the chassis was finally tight against the top of the cabinet, I turned it back on its side & released the jack.
It will be a bear to service, but that's all on me. I will remove it from the cab myself if & when I have to send it to a technician.
There's a guy on the Bay that builds beautiful cabs, Refugio I think. I ordered a 5E3 cab from him, and he sent an e-mail wanting to know if I planned on using Tolex or Tweed! I never used Tweed, but I guess it's thicker?
If you could peel back the thin part of the tolex where it is binding against the chassis, you could use a small block plane and maybe a chisel to remove a bit of wood from the cabinet sides where it's too tight. Probably 1/16" on each side would do it. Once the tolex is glued back down it wouldn't be noticeable.
"When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top
Tolex and Nubtex are about 30% thicker than linen tweed. As well, contemporary Tolex and Nubtex are about 15% thicker than vintage material. It's not a factor for new Fender equipment though because no one mills true 3/4" lumber anymore -- planks used for enclosures actually measure 11/16". The thicker Tolex and Nubtex ensure that metal corner protectors still fit snugly when installed.
"When injustice becomes law then rebellion becomes duty."
That's probably true. I know that Nubtex (the "rough" stuff) is thicker than standard or "Bronco" vinyl. Heck, I'm learning that there are three grades of Bronco that are different thicknesses. Really adds meaning to the term "wiggle room", lol
I'm not confident that I could do that without making a hash of it. Sad to say at this point it would be easier for me to just build another cabinet.
The good news is that my OCD drove me to create.four cabinets. I will follow your advice before I lay down any adhesive on the next one, & I will fit the chassis first
It's not something I ever thought about until seeing your photo this morning, but it looks like Fender made the cabinet opening (including tolex thickness) about 5/32" wider than the chassis width. In the case of the Bandmaster Reverb in my picture, the chassis isn't centered in the opening (its flush on the other side), but once the screws are removed, the chassis can be centered and easily removed from the cabinet.
"When You're Riding Down the Highway at Night, And You're Feeling that Wild Turkey's Bite" ZZ Top
Yup, as Lyle from Psionic Audio says, "Leo designed these amps to be serviced". That's one of those details that you never see until you do, & then you can't unsee it! I just wish I had been paying more attention
The RCA jacks on the amp were pretty shiny, but I hit them with some rubbing alcohol & a q-tip anyway. The jacks on the tank were a different story, being heavily oxidized.
It seems to be improved but some of that might be my imagination. My '72 has FAT reverb at about 2.5, & this one is the same only around 3.5 or 4. Might just be some variable that escapes me
Posting Permissions
Reply With Quote
