Help Restoring '71 Fender Pro Reverb and '69 Twin Reverb

[blackba]

I am working on getting a '71 Pro Reverb and '69 Twin Reverb going for a friend. He just purchased them, so I have no idea yet if they function. The Pro Reverb is the priority for me to get going. Plan to use a variac on both amps the first time I power them up to bring them up to voltage slowly.

So starting with the Pro Reverb, here are my observations. Has August 6, 1971 stamped right on the chassis. Its been blackfaced and recapped. Everything looks to me to be in order. Here are my questions:

1. Should I replace the 470ohm 1watt resistors on the power tube sockets? I often see these replaced. If replacing what type and wattage of resistor should I use?
2. I noticed they used 22uF 50V electrolytic caps, I assume this is fine since its close to the stock 25uF value. Any reason to replace these? I have some 25uF 25V and 25uF 50V caps on hand
3. Its missing a 25uF 25V cap according to the schematic and layout, should I add one so its per the schematic?
4. I often seen the power switch replaced, is the stock one fine?
5. Here are the transformer numbers (best I can read). Are they original?
Output - 022848 606-1-09
Choke - 022699 606125
Power - J022798 606-1-100
Reverb - 022921 606119

Here are some pics:

 

[blackba]

Now for the twin reverb. This one is an export model. The twin is also missing a 25uF cap and has the silverface 5uF cap in the vibrato circuit. Here are my questions for that amp:
1. They used a couple of 16uF 50V caps, are they fine or should I change them to the stock 25uF value
2. Are the transformer original? Here are the codes:
Output – 022889 606930
Choke - 022699 606-9-32
Power – F022764 606930
Reverb – 022921 606933
Here are some pictures:

 

[PatF]

Cool amps. Probably not much to fix. Looks like most of the electrolytics are pretty new. I really like the black F&T's.

Take the tubes out for the initial variac startup. Build one of those light bulb limiter boxes. There's a good article about cap reforming by Larry at the MetroAmp forum.

The 470 ohm screen resistors I would probably change to 2 watt metal oxide. Use the blackface schematic value. Might be 1 watt. A think lot of people put the 5 watt bricks in there. I recently read some very trustworthy folks at the Hoffman Amp forum advise that it was better to burn up screen resistors than output transformers.

The transformers are good quality. The voltages in the later models like these is sometimes a little too high. If they were mine, I would set up to use the variac full time. And re-voice the preamps.

The 25uf cathode resistor bypass caps are optional. 22u/50v is fine. It gives that stage more gain over the entire freq range. Sounds like a good idea but sometimes causes low end mud. A 2.7K/.68uf on the first stage might sound really good. The lower value .68 allows high frequencies past but blocks the lows.

I take out the polarity switch, death cap, and ext. AC jack and just wire the AC cord regular 3 prong style. Sometimes an IEC in the ext. AC hole. I like the DPST power switches better, but they can be a pain to fit.

Worst case scenario. The shitty fiber circuit boards in these sometimes absorb enough moisture to become conductive. Then it's time for a turret board transplant.

Go the Hoffman Amps Library of Information. All the original Fender schematics are there. Transformer code charts too.

 

[glpg80]

+1 to an IEC connector, proper 3 prong plug, and attaching the ground wire to the ground lug next to the PT in the twin reverb picture. Clearly mark with a sharpie the reference to earth ground using the correct chassis ground symbol.

For the pro-reverb get rid of that chassis-soldered ground. That's a bullshit shortcut for someone who had too short of a wire to go to the wire-nut next to it. Drill a hole and screw in the proper lead-off nut or remove the wire and install the proper length attached to the PT ground reference. Do the same for the pro-reverb in regards to an IEC connector, 3 prong plug, and ground reference to that same lug as the PT.

Before powering on, check to make sure fuses are rated properly.

I'd replace both power and standby switches while I was in there as any arcing over the years will degrade contacts - do it now while you're in there. Not fan of chun-ho IC caps. Stick with Nichicon or F&T for power side. Both are excellent and what personally I use. Don't bother with audio rated capacitors - it's snake oil. You want low ESR.

Anything electrolytic I would be suspect to in the preamp side - especially those high value 25uF capacitors since the voltage on them is nowhere near 25 or 50V on the cathodes. Fuck reforming capacitors - just replace the bastards. This is 2015.

Power it on at first with no tubes plugged in and check plate voltages against the schematics that Pat referenced for you to download. If you don't have a signal generator and an oscilloscope to check AC functionality, just check DC voltages at the plates of each tube to make sure you're getting juice. Do a smell check and make sure nothing is getting hot with no tubes plugged in.

Check bias.

One hand rule for everything as always.

I prefer a high B+ for fenders. It's a matter of preference for either of these amplifiers. The only time I would be concerned is whether there existed tube rectification.

Finally check for dirty or corroded potentiometers.

Anything else you choose to replace or change would be based on a need-to-do or want-to-do factor.

 

[blackba]

Thanks for the help guys.

Both amps are missing an electrolytic cap in the vibrato circuit, its the 25uF across the 2700ohm resistor. I was going to put this cap in both amps, any reason why someone would have removed it?

 

[glpg80]

Strictly a tonal preference - nothing more/nothing less.

You're welcome

 

[blackba]

Update:

Got the amps done and got to compare these to a '69 Bandmaster reverb.

First the '71 pro Reverb. Sounded alot like the bandmaster reverb, but was a little sweeter. Normal channel was more aggressive than the vibrato channel. Vibrato channel was a touch thicker and warmer than my bandmaster reverb. Overall very impressed.

Second was the '69 Twin reverb. Its been a long time since I have played a twin, but I loved this one. Normal channel was fuller and sweeter than the PR and BMR. Vibrato channel had so much authority compared to the PR and BMR and was the thickest sounding. The Twin took pedals the best of the 3 amps. Would love to have this one and turn it into a Twin Reverb head (ala dual showman reverb).