Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: rafe on September 08, 2019, 04:45:24 pm

Title: Guess what followed Me home AB568??
Post by: rafe on September 08, 2019, 04:45:24 pm

Some things just can't be passed up . I picked up a 68 super reverb. pretty unmolested . Has the 2 prong  5u4gb all brown turds and white Mallory's . The doghouse has all the original orangish yellow caps (one shot it load) The amp worked but was low volume on the reverb channel ;the other channel had volume. I kind of hate to do more than get the doghouse taken care of at this point Oh and the cord of course. Original speakers although it looks like one may have been replaced in 74 with the same speaker with a different label round blue instead of the square blue.I was playing with the idea of keeping the receptacle for power ;but think I'll bypass all that stuff and leave it in place . I checked the fuse and it was a 6amp instead of 2 amp …..masking a problem?? So it has been a while …...black to fuse, white to switch and green to ground?? I will look around to see what I can find too. The death cap is the only blue molded . It really looks nice in there. Photo files are too big ...i'll post later

 

Title: Re: Guess what followed Me home AB568??
Post by: rafe on September 09, 2019, 05:53:43 pm

Wired up the three prong keeping the socket hot  and ordered caps and 2amp fuses from Doug. Love those alnico's …..I went over the tolex  wasn't too bad now it's good 1/4 inch hole in grill will almost disappear can't wait to hear it :)

Title: Re: Guess what followed Me home AB568??
Post by: shooter on September 09, 2019, 06:06:00 pm

worked on a '65 version awhile back, I ask owner about the plug, he kept it 2 prong, He also signed my disclaimer  :icon_biggrin:
He liked it best with cool bias ~ 55% idle

Title: Re: Guess what followed Me home AB568??
Post by: rafe on September 11, 2019, 07:44:37 pm

Got the caps in tonight , still low volume on the reverb channel, No reverb no tremolo . It is quiet otherwise no static ,hum or noise in general . I will go over the tube sockets (clean) as well as the pots and inputs. I'll clean the reverb connections too. See what happens after that. Any suggestions or leads are appreciated;  one of the 25/25 white Mallory's has a burn like spot on the casing (not on the ends) goes to V5 pin 8 thought it might be that but it's probably just flux spatter

Title: Re: Guess what followed Me home AB568??
Post by: sluckey on September 11, 2019, 08:46:04 pm

Sounds like multiple issues. Divide and conquer. Reverb and tremolo issues are seldom related. No reverb and low volume may both be due to a bad V4 or circuit component.

Tip... Tremolo on this amp requires a ground on the tip of the tremolo jack. That ground is usually provided by the footswitch but can be provided with a shorting plug or a gator clip test lead.

Title: Re: Guess what followed Me home AB568??
Post by: rafe on September 12, 2019, 05:32:46 am

Thanks S, I'll start checking and cleaning after work, what amazes Me is that those 68 caps in the doghouse were pretty much doing their job; even though one had spilled it's guts. From the initial looking over everything inside the chassis; I really don't think it has ever been touched or modified. Really a nice find R.

Title: Re: Guess what followed Me home AB568??
Post by: rafe on September 12, 2019, 10:36:09 am

"WE" are still not worthy Steve ;)  :worthy1: After I read your post this morning and before I went to work (short Day) I pulled V4 it had a little white halo around the top . I came home about an hour ago put in a good 7025 and it is pretty magnificent; I don't know why the AB568 has an aura of distain about it; it seems very darned fantastic to me. I will still do the pots,and other aforementioned maintenance procedures before I button the chassis back up.   Good reverb & Tremolo Too ……...

Title: Re: Guess what followed Me home AB568??
Post by: Willabe on September 12, 2019, 11:22:25 am

Quote from: rafe on September 12, 2019, 10:36:09 am

..... I pulled V4 it had a little white halo around the top .

That means the tubes vacuum went bad, sprayed silver getter did it's job, absorbed the atmospheres gasses and turned white.

Quote from: rafe on September 12, 2019, 10:36:09 am

....I don't know why the AB568 has an aura of distain about it; it seems very darned fantastic to me.

CBS ran those amps colder, too cold, than the Fender black face amps.

Here's what CBS changed;

1. CBS put in 2000pF/.002 caps on the 6L6GC grids to kill off any parasitic problems caused by bad lead dress.

(These make a large difference in the amps tone, for the worse. They bleed off a lot of signal/harmonics/sparkle/chime to ground. This is why you can't just bias the power tubes hotter to warm up the amp like a black face. If their in your amp, try snipping the ground end and see if it stays stable. If not stable then you have to play around with the lead dress. Fender/CBS put them in just as a precaution. CBS/Fender was selling sooooo many amps and they didn't want to take the time with trouble shooting them or taking the time to do the lead dress correctly. Not all those amps are unstable without them. Some say only ~50% are unstable without those grid bleeder caps.)

2. They changed/lowered the PI (12AT7) plate R values from 82K/100K down to 47K.

(You can try changing those and listen if you like it better. They will add a little more drive/fuller/fatter tone.)

3. Lowered the 6L6GC grid return R's from 220K down to 100K, in part because of the bias balance.

(You can try changing those to 220K IF you also change to adjustable bias. Or if you stay with balance bias you could try 180K, maybe even 200K, lower value because of the resistance in the bias balance circuit. They will add a little more drive/fuller/fatter tone.)

4. They used a combination of grid and K bias.

(Snip out the 150R's and cap on the 6L6GC's K and then run a wire from the K's to ground. Much mo' better, put a 1ohm R between the 6L6 K and ground to measure bias current, should be done anyway. Then you have to re-bias. Some say the combination bias effects the amps tone for the worse. I suspect that cap, run from K to K, more than the K R's.  :dontknow: )   

5. They used bias balance instead of adjustable bias, some/many prefer adjustable bias.

(This can be changed to adjustable bias pretty easily, if you want/prefer. With matched tubes you don't really need bias balance. With adjustable bias you can set the 6L6's to run hotter or colder. Plus, different tube sets, even the same brand, will all run hotter or colder with the same -dcv (bias voltage) setting.)

You won't know if you like it any better then it sounds now if you don't try these things.

If you do try them, do them 1 at a time and listen for changes. (Except for 2 and 3, I'd do those at the same time.)

The parasitic caps on the power tube grids will most probably make the most noticeable difference, #2&3 next most difference, then #4.       

If it were my amp, I'd do all the changes I listed without hesitation.

It fattens up/warms up the amp and is more lively, gives it a little more drive. CBS ran them colder than Fender did and CBS sales plummeted when they made these changes because of it.

Title: Re: Guess what followed Me home AB568??
Post by: rafe on September 13, 2019, 09:17:50 pm

Thanks Willabe, I printed that out; I will likely do some if not all of that.

Title: Re: Guess what followed Me home AB568??
Post by: Willabe on September 13, 2019, 09:52:09 pm

Please let us know what you try and how you like it, if you like it.  :icon_biggrin:

Title: Re: Guess what followed Me home AB568??
Post by: 2deaf on September 13, 2019, 11:31:17 pm

Quote from: rafe on September 08, 2019, 04:45:24 pm

Photo files are too big ...i'll post later

I hope you haven't given up on that.

Title: Re: Guess what followed Me home AB568??
Post by: rafe on September 14, 2019, 10:03:15 am

I hope you haven't given up on that.




I have been trying....New Computer is (was?) fighting Me. The caps photo is not too good , But the date code is 68 as is everything else on it; that I've seen so far

Title: Re: Guess what followed Me home AB568??
Post by: rafe on September 14, 2019, 10:07:07 am

The guts of the 68 super

Title: Re: Guess what followed Me home AB568??
Post by: rafe on September 14, 2019, 10:27:31 am

Not done yet! No reassembly until then .

Title: Re: Guess what followed Me home AB568??
Post by: rafe on October 18, 2019, 06:48:33 pm

Here is the update the super;I have not gone any further with it (purchased bypass caps etc. ) but they are working fine for now (not leaking voltage) I have put the chassis back in and buttoned it up and moved it to the living room. I have been playing it a lot daily and here is my reasoning. If I started tearing into it before I knew what it sounded and responded like, I really would not know the difference  Kind of like comparing Apples to no apples So that is the new plan, I did print out the plan Willabe and when I get back in there I'll post the results :)

Title: Re: Guess what followed Me home AB568??
Post by: Willabe on October 18, 2019, 09:38:50 pm

Quote from: rafe on October 18, 2019, 06:48:33 pm

I have been playing it a lot daily and here is my reasoning. If I started tearing into it before I knew what it sounded and responded like, I really would not know the difference  Kind of like comparing Apples to no apples.

Good plan.  :icon_biggrin:

Title: Re: Guess what followed Me home AB568??
Post by: Platefire on October 18, 2019, 10:29:33 pm

The lead dress don't look too bad. I've got a 74 Pro Reverb that's a lot worst. Nice original Super Reverb there--congratulations!

I always like to hear the story of the hunt of how and where you picked it up but---that's up to you. I don't really hunt anymore. Seems tweaking my old builds to straighten out my rookie mistakes is enough to keep me occupied anymore:>) Platefire

Title: Re: Guess what followed Me home AB568??
Post by: rafe on October 19, 2019, 09:00:25 am

Thanks Guys, Platefire I wasn't really hunting on the last three. BUT I did open the curtain and peek.....Facebook market place ; a 64 GA 19-RVT falcon within 5 miles of my house. It had a hum and no reverb but not beat up ; The guy knew what He had and what they were going for on E-bay but I guess He wasn't willing to pay His tech to go over His amp; and He has others ... Got it at less than half of what I can get for it. ....
I cleaned up the tube sockets and all the inputs and outputs including the reverb pan and it's awesome . I had watched some good videos on this amp and decided i'd like one if the opportunity arose; it did. It had been recapped at some point and a 3 prong was installed . Pretty crappy job ( Sloppy looking ) and one wire on the 3 wire was literally hanging by a thread. I really like this falcon and will address the caps and remove and change some tone robbing components later this winter. That amp was in my living room up until this week ....NOW 
The Super, Same Curtain, but a longer trip 8 miles. It was not online  long and and I was on it like white on rice. The 68 drip edge and a 66 mustang for less than A badly beat up later silver face would run you . Again the Guy knew what it was worth and didn't care. I told Him I want it and I'd give Him what He was asking for it, but that He could get a lot more for them. He played out for years and can't play anymore and they just sat in the Man cave room for the last 20 year with little use.....The strings on the mustang are supposedly on it for 20 years and I am afraid to change them because they sound and play so well.    and NOW for something completely different
 This other amp was a bit of a bust; I traveled 40 miles to get it.https://el34world.com/Forum/index.php?topic=24898.msg269090#msg269090 It does have 2 nice 10" alnico speakers in it so all is not lost .......I might put a GA 5t to them.....I have tube amps; is an understatement! :laugh:

Title: Re: Guess what followed Me home AB568??
Post by: Platefire on October 19, 2019, 10:07:26 pm

Thanks for filling me in on your amp source and actions. I use to be on the hunt with all that was in me but never could turn up any real good classic amps locally. I did fine a Gibson GA40 LP amp, a Gibson Atlas Bass head, late 90's Mesa Boogie Rocket 440, two Silvertone 1482's and 1974 Fender Pro Reverb. Everything else were hi fi/reel to reel/Pa amp conversions. Most outstanding thing I guess is I once had an Allen Accomplice kit complete with cab given to me. It's one sweet amp which is basically a single channel Blackface Deluxe with extras(less tremolo).

I did take a look an your un-Identified amp. It looked more like a old Gibson than anything else to me.Happy amping! Platefire
 

Title: Re: Guess what followed Me home AB568??
Post by: Sonny ReVerb on October 25, 2019, 10:22:48 am

Here's what I did to my (similar) Vibrolux Reverb AB568 drip edge (https://www.tdpri.com/threads/silverface-vibrolux-reverb-rehab.487100/). I changed it to adjustable bias but kept the balance pot (https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Balanced_Bias_With_Single_Balance_Pot). Kind of the best of both worlds.

Title: Re: Guess what followed Me home AB568??
Post by: rafe on November 03, 2019, 01:17:20 pm

Here is a photo, I am definitely enjoying this amp. I am planning on getting in there and addressing the power tubes (cutting out the micro caps) and such but I'm in no hurry at this point . I appreciate tall the information on it. this amp has been on at least an hour a day since I buttoned it up and is inspirational to say the least! I have an old 335 that I seldom play ….and It shines when in the super so I have been playing it more. Strat is good too.

Title: Re: Guess what followed Me home AB568??
Post by: rafe on December 01, 2021, 04:24:58 pm

Did a little more on this amp had an issue, replaced the bypass caps and that solved it, I also took the time to clean out the dust around the reverb tank and speakers I noticed two of the speakers had lost their dustcaps so I pulled the speakers and reattached them (they were in the grillcloth . 3 speakers are the 4th week of 69 one is a 74 replacement same part # but has a shiney metal larger dustcap .... :dontknow:   clipped out the .002 caps on the 6l6's no problem there. I also cleaned up the control panel and it looks good other than those  corrosion lines that appear over time there aren't too many. I am going to post a question about my meter over in the tools section ....It is baffling me!