1966 Super Reverb Maintenence

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1966 Super Reverb Maintenence

Postby vez on Thu Jun 22, 2017 6:17 pm

This amp has needed some work for quite a while. About five years ago I changed the filter caps, including the one at the bias pot and added a three-prong line but the entire amp needs a going over. I have a technical background but, I’m new to this and I'm learning a lot. I made a tool to discharge the caps and I've boned up on safety procedures.

So far I have found and fixed:
The bias on the grids of the power tubes was at +62v so; I brought it down to factory spec +52v. I also found the plate of preamp tube V2 was about 100v low. The missing voltage reappeared after I replace the tube. I was disheartened as it was a Telefunken 12ax7. Strange because it still tests super strong. I found an open ground in the tremolo oscillator, a bad volume pot and a broken connection on the resistor that grounds where the positive feedback connects to the coupling cap to the grid of the phase inverter.
I sprayed the bad volume pot with this stuff called CRC. That was a mistake, to my dismay it seized the pot's shaft. Apparently CRC is pure solvent w/o lube. So, in the meantime I installed an Alpha replacement pot, ordered some Detoxit, and disassembled the seized pot, which was 1966 Stackpole and soaked it in Liquid Wrench overnight. That loosened up the shaft so I cleaned it up and gave it a light coat of Detoxit. (I noticed someone on Ebay selling '66 250k Stakepole pot for $250!).

There are some components on this amp that are not original. I bought this amp around 1972 and the only work done was when I replaced the filter caps. This amp came in a homemade head cab with no reverb tank (but the seller gave me a Fender black face reverb unit). The Power transformer is dated 1969. The "MIDDLE" pot is not original. And the pilot lamp has a couple 100 ohm resistors to ground. The coupling capacitors between the phase inv and the power tubes are 0.15uf rather than 0.1uf and are labeled "Duranite". They do not leak but I was thinking of replacing them with a better brand like Sprague.
I have found no leaking coupling caps. I will replace all the 1 watt resistors and the two screen resistors. All voltages on the plates, cathodes and grids are in the ballpark except
The cathode voltage on the reverb driver reads +9.4vdc and should be +8.4 and the cathode for the tremolo driver reads 13.3vdc and it should be 17vdc.
The amp sounds fantastic but no reverb tank

What’s your opinion, should I replace these "Duranite" caps?
What is the voltage rating for coupling caps in this position?
Should you replace all the cathode bypass caps?
Should I replace any resistors whose values are more than +/-10% ?
Should I do something about those cathode voltages that are off?
Thanks for listening and replying!
-Rick
Images are HERE http://goo.gl/photos/vpkv5SEdHXh795iF6
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Re: 1966 Super Reverb Maintenence

Postby jaywalker on Thu Jun 22, 2017 10:32 pm

If you like the sound I say leave the duranites, (original were .1) I would replace the tan elytics 25/25's and resistors outside of +/- 20%. otherwise play it! my 2 cents
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Re: 1966 Super Reverb Maintenence

Postby JJman on Thu Jun 22, 2017 11:51 pm

Those cathode voltages don't look to bad but I would change those original cathode caps. Keep for posterity.
-If it says "Vintage" on it, it isn't.
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Re: 1966 Super Reverb Maintenence

Postby stratele52 on Fri Jun 23, 2017 9:05 am

1- Replace Duranite caps with 0.1 mfd Malory Orange drop capacitors.

2- Replace bypass caps ; these are electrolytics like filter caps and they could be dry or out ouf value.
Use Sprague Atom 25 MFD 25 volts ( or more volts ) filter caps

3- Replace all 100 K tubes plate resistors ; they make noise with years

4- Voltages are good

5- Keep all old parts

Dont put cheap parts / caps if you want keep the good tone an quality of you amp
Filter caps ; I don't know what you put, I only use Sprague or F& T , never Asian made caps like IC ( Illinois)

Sprague Atom; https://www.tubesandmore.com/products/c ... ectrolytic

Malory Orange Drop ; https://www.tubesandmore.com/products/c ... ypropylene
F& T caps ; https://www.tubesandmore.com/products/c ... nd%3DF%26T
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Re: 1966 Super Reverb Maintenence

Postby vez on Fri Jun 23, 2017 1:18 pm

stratele52 wrote:1- Replace Duranite caps with 0.1 mfd Malory Orange drop capacitors.

2- Replace bypass caps ; these are electrolytics like filter caps and they could be dry or out ouf value.
Use Sprague Atom 25 MFD 25 volts ( or more volts ) filter caps

3- Replace all 100 K tubes plate resistors ; they make noise with years

4- Voltages are good

5- Keep all old parts

Dont put cheap parts / caps if you want keep the good tone an quality of you amp
Filter caps ; I don't know what you put, I only use Sprague or F& T , never Asian made caps like IC ( Illinois)

Sprague Atom; https://www.tubesandmore.com/products/c ... ectrolytic

Malory Orange Drop ; https://www.tubesandmore.com/products/c ... ypropylene
F& T caps ; https://www.tubesandmore.com/products/c ... nd%3DF%26T

Thank you everyone for your replies.
Stratele52, if I replace those Duranite caps, what voltage rating should I use? The Duranites are 600v but I'm sure the originals were not 600v were they?
BTW my filter caps are all Atoms.
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Re: 1966 Super Reverb Maintenence

Postby stratele52 on Fri Jun 23, 2017 2:00 pm

Originals one are rated 400 volts.
More volts are ok too, just more expensive.
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Re: 1966 Super Reverb Maintenence

Postby jaywalker on Fri Jun 23, 2017 5:29 pm

Go with 600v like the orange drops, mallorys or even Sozo blues in place of the duranites. For the tan electrolytics TAD (Tube Amp Doctor) has dual 25uF/25v caps then you need one single 25/25 - Tubes and More has them. Some like to recommend using 25uF/50v Sprague caps to replace the 25/25 electrolytics. another 2 cents. You might try replacing the elytics first then play it a bit then try it out with the duranites replaced. See which way you like it better.

p.s. the two resistors going to ground from the pilot light bracket are a good thing...leave as-is.
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Re: 1966 Super Reverb Maintenence

Postby stratele52 on Fri Jun 23, 2017 8:27 pm

Tube amp doctor is good for people living in european country.

Where vez live ?

Which circuit this 1966 Super Reverb ? AB568,AA1069, AA270 ?
These circuits use two 100 ohms resistor ( on pilot light ) for heater artificial ground.
It is not only a good idea it is a must if you want a quiet amp because the Power Transformer don't have heater ground on winding.

A isolating cardboard is missing under the pilot light to prevent short circuit if lamp housing turn.
I see that one time too on Fender amp

Be sure the amp use the right fuse if you don't want to blow the PT
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Re: 1966 Super Reverb Maintenence

Postby pdf64 on Fri Jun 23, 2017 8:48 pm

vez wrote:... I also found the plate of preamp tube V2 was about 100v low. The missing voltage reappeared after I replace the tube. I was disheartened as it was a Telefunken 12ax7. Strange because it still tests super strong...

It looks to be a high gain tube; if it sounds good then leave it in there, 0.5mA extra plate current is no big deal.
If it has an audio problem, it may still work fine as the trem oscillator V5.
My band:-http://www.youtube.com/user/RedwingBand
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Re: 1966 Super Reverb Maintenence

Postby vez on Sat Jun 24, 2017 3:56 am

stratele52 wrote:Tube amp doctor is good for people living in european country.

Where vez live ?

Which circuit this 1966 Super Reverb ? AB568,AA1069, AA270 ?
These circuits use two 100 ohms resistor ( on pilot light ) for heater artificial ground.
It is not only a good idea it is a must if you want a quiet amp because the Power Transformer don't have heater ground on winding.

A isolating cardboard is missing under the pilot light to prevent short circuit if lamp housing turn.
I see that one time too on Fender amp

Be sure the amp use the right fuse if you don't want to blow the PT

I live in Northern Virginia.
The amp came in a homemade head cabinet so there is no tube chart sticker. The serial number stamped on the chassis reads, A17941. Every component I've inspected on the amp has a 1966 date code except the power transformer which reads 1969. I assumed the circuit was AB763.
I guess in 1969 some catastrophic event blew the the power transformer. That's probably when the Duranite caps were installed along with a new PT. I guess the tech installed the blue molded Duranites in keeping with the color scheme of the Mallory caps :D But he must have known what he was doing if he installed those 100 ohm resistors.
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Re: 1966 Super Reverb Maintenence

Postby stratele52 on Sat Jun 24, 2017 8:47 am

AB763 ; could be right.
On another photo, I see the isolating cardboard under the pilot light to prevent short circuit if lamp housing turn.

I can't see why put these two big caps, they never fail and have nothing to do with a blow PT.

Did you check the fuse ?
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Re: 1966 Super Reverb Maintenence

Postby slider313 on Sat Jun 24, 2017 1:55 pm

If the amp is from 1966, then it can only be an AB763. Looking at the pictures confirms this. I would replace every electrolytic cap in the amp. Those dual 25uf@25v cathode caps are well past their shelf life and could be leaking. That may be the reason your cathode voltages are off spec, or not. I would also leave the large Duranites, as long as they're not leaking dcv.
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Re: 1966 Super Reverb Maintenence

Postby vez on Sat Jun 24, 2017 5:31 pm

jaywalker wrote:TAD (Tube Amp Doctor) has dual 25uF/25v caps then you need one single 25/25 - Tubes and More has themselves.

Thanks Jay, I like this idea. I have lots of Orange Drops. I bought box lot of them on EBay.
Below are photos of coupling caps Fender used as the signal exits the 2nd preamp stage on the Normal and Vibrato channels respectively. I am surprised because I didnt think ceramic and the brown "turd" caps were not known for tonal quality. I'm tempted to replace them. But maybe it's those crappy components in vintage amps that impart tonal character.
What is the reason for that 10pf cap parallel with the 3.3M resistor on the " dry line" going to the grid of V4 preamp? It seems like this is a drastic high pass filter?
Another question: Is there a test I can perform on the coupling caps I'm installing to make sure they are not installed backwards. I saw a video illustrating the black line on the caps is not to be trusted as the tail end. But he's used a scope which I don't have. Thank you, Rick

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Re: 1966 Super Reverb Maintenence

Postby jaywalker on Sun Jun 25, 2017 2:43 pm

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Re: 1966 Super Reverb Maintenence

Postby vez on Sun Jun 25, 2017 3:08 pm


Thanks Jaywalker. That's very helpful.
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