Can anyone help with why the capacitor values are different from factory specs? Should I change them to factory specs? Got this amp in a trade it looks to have upgraded capacitors and was rebuild at some point. Not sure why there's a Mexican capacitor in the one position. It sounds great but want it put back to factory specs.. I.E. I want it to sound like 1968! Any help and advice would be greatly appreciated. Also the normal channel does not work.
The tube chart may say AB763, but if it's a 1968 it's possible it is AC568 circuit revision. Are there 150 ohm 7 watt resistors connected to the cathodes of the power tubes? They will most likely be light colored rectangular resistors. See the example in the bottom left of the picture below.
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Correct. The AC568 was an emasculated sad-assed mess, capable of only about 65 watts of output.Originally Posted by Michael Smith
The tube chart may say AB763, but if it's a 1968 it's possible it is AC568 circuit revision. Are there 150 ohm 7 watt resistors connected to the cathodes of the power tubes? They will most likely be light colored rectangular resistors. See the example in the bottom left of the picture below.
As to the non-operating normal channel, I'd start the troubleshooting by checking the V1 tube. An easy method is to substitute a known-good 7025 or 12AX7 into that socket and note any change(s).
"When injustice becomes law then rebellion becomes duty."
It's definalty a AB763 circuit, By the serial number it's a very early 68' Silverface, Drip edge, black line twin reverb which still used the infamous AB763 Blackface circuit till May of 68' How can I upload a picture? The amp has definatly been rebuilt, with Sprague Atom caps theres 2 Caps that dont belong though, 1 is in the main Cap circuit its a 450v 10uf made in mexico and the other is a cheap 160v 100uf cap. Normal channel works fine with a known good tube! Thanks for the suggestion!!
Last edited by Outlaw17; 02-19-2022 at 10:14 AM.
Upload your photo's to an image sharing site (I use Imgur). Then click on the photo and copy the BB link, and paste it into your chat reply (Ctrl & V), See example below. As far as I know you have to copy and paste the photos one at a time, but you can put multiple photos in a chat reply.
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I have removed the handles lol and the cheap seat vinyl it was covered with to find the original tolex its aged nicely.
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From what I can tell, they have upped the value of the bias capacitor (the upright (radial) cap), which is not uncommon. The original schematic specified 25uF/50V. Later Fender schematics upped the value to 50uF/70V. I don't know the value of the smaller filter capacitor I have circled. It should be a 20uF/525V.
What is the value of the first dropping resistor? The white rectangular one circled? It should be 1Kohms, 1 Watt (although higher wattage is OK). Also I can't see the values of the balancing resistors (they are hidden under the 2 largest filter caps in the doghouse). They should be 220K Ohm, 1 Watt.
Your circuit is not the infamous AC568, as your power tube cathodes are grounded directly to the chassis.
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Whatdid that?
Looks like they trashed the original grill cloth as well.
If you have a pair of 16-inch tilt-back legs marked "PAT. PEND.", now's the time to use them.
How about the knobs......'snowman' types?
"When injustice becomes law then rebellion becomes duty."
So the cap you circled values are 10uf 450v, The resistor is a 2w 1k . All in all it sounds great but needs some tlc. And yep pretty sure the grill cloth is old seat covering lol. I got it from Mexico on a trade for a $300 Guitar!!
Last edited by Outlaw17; 02-19-2022 at 11:54 AM.
If it were my amp, I would replace the bias cap with a 100uF/100V and the 10uF/450V filter cap with a 20/525 (or 500V if that's what is available). The 1kohm 2 Watt resistor is fine although it looks odd.
Do you have the ability to check/set the bias for the power tubes?
How about a photo and info on the speakers?
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8ohm speakers.
I think the Tube Works 12 G is an Eminence 100 Watt speaker. There should be a label with some codes glued to the sides of the magnets. Looks like the speaker baffle has been replaced, as I think the originals were painted black.
Now that I can see a glimpse of one of the balancing resistors under the reservoir cap, it is a 220K ohm. Looks like the original carbon comp resistor. I normally change those out for Metal Oxide's, but if they measure OK that could probably wait until the next cap job.
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I currently don't have to tools to bias but will be getting them very soon. I'm completely comfortable working on live circuits would mostly like to correct and wrong components. The baffle was defiantly replace with cheap pine plywood. Ill be restoring this to original also..
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I couldn't find much info on the speakers. I plane to replace these as well.
You can set the bias with just a simple multi meter capable of measuring resistance (ohms) and volts. Using bias probes is easier. Check out "Uncle Doug's" YT channel and search for "biasing a fender amp" or similar title. Other's have similar videos. I use an Excel spreadsheet to input the resistance from Pin 3 of the power tubes to the center tap and the plate voltage, to derive the plate current (miliamps) and plate dissipation (watts).
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67-9531 is Eminence, 1995, 31st week. They may be perfectly fine for that amp.
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Yep I was watching Uncle Doug yesterday! I may give it a try tomorrow. I would imagine it needs new tubes as well There were 5 different 12AX7s and 1 12AT7. In that mess was a OS GE JAN 12AX7WA tube!!
Honestly they sound pretty good!
You should have 12AT7's for the reverb driver and phase inverter. I think those are V3 and V6. From one of your photos, the power tubes look to be somewhat new Sovteks. They may have a date code on them.
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The original baffle board was ½" MDF with a ¼" pine surround and logo shelf, similar in construction to this......
The correct speaker for an authentic 1968 restoration is an Oxford 12T6 or JBL's D120F. Twin Reverb's with factory-installed Lansing's featured a JBL logo at the lower right corner of the baffle board which required a small rectangular logo shelf similar to this......
"When injustice becomes law then rebellion becomes duty."
Thanks for all the great info! Looking forward to restoring this amp to its former glory with the JBL's if i can find them for a reasonable price... Would love to see that chrome dome threw the grill cloth!!!
Yep i ordered a set of 12at7 for that, Not sure what dumbass was in this amp but geez louize!!
anyone know where i can find the drip rail? I'm missing one piece and all the corners..
The original baffle board was ½" MDF with a ¼" pine surround and logo shelf, similar in construction to this......
The correct speaker for an authentic 1968 restoration is an Oxford 12T6 or JBL's D120F. Twin Reverb's with factory-installed Lansing's featured a JBL logo at the lower right corner of the baffle board which required a small rectangular logo shelf similar to this......
Was thinking of using reclaimed long leaf pine that i have from New Orleans. The house i got it from was 109yrs old. It's aged nicely an di think it would be a great tone wood.
The JBL speaker factory upgrade in 1968 was quite pricey. $638.50 vs 499.50 for the "standard" speakers.
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I sent a message to a guy I know here who restores old amps. If there is a current source, he will know it.
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There was a vendor on Ebay who sold the drip-edge extrusions. No corner pieces though -- you'd have to salvage some from an original (presumably derelict) cabinet like I did. The stuff is a bitch to work with and you'll need to reduce the standard height and width dimensions of the baffle board to accommodate the moldings.
This shot of a '68 Showman valance panel shows the correct grill cloth and the precise logo placement (relative to the top and left side)......
"When injustice becomes law then rebellion becomes duty."
My contact does not have a source for the drip edge material. He confirmed the corners are of the same material hand cut and bent.
Amplified Parts does sell period correct grill cloth, metal protective corners, tolex, handles, etc.
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Awesome! Thanks so much
Awe well.. I'll see what i can come up with, I'm in no hurry to restore the cabinet. But the amp i will correct the wrong parts very soon
There is another alternative.
Periodically I've seen people parting out relatively new silverface Custom Twin Reverbs on Ebay, including complete cabinet assemblies. Those enclosures will have the drip-edge molding, semi-correct grill cloth, authentic logo, tilt-back legs, etc. You might even find a new control panel too. Might be worth some thought......
"When injustice becomes law then rebellion becomes duty."
Good luck. In the last year I've done a 68 Bassman, 69 Super Reverb, 77 Deluxe Reverb and am waiting on a few parts for a 79 Vibro Champ. Also planning to build an external speaker cabinet or two.
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Thanks everyone I'll post updates as i go with the restoration.
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"When injustice becomes law then rebellion becomes duty."
I just sold a pair of Orange Frame JBL's] from a 79.
I have another question, What's the correct speaker placement for a 1968 twin? Side by side or one higher than the other??
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