Got a Barn Fresh Silverface Pro AA165 in to Restore(Filter cap job done)

[Platefire]

A musician friend bought this today for $100.00 that has been sitting up in a shed for a long time. I took the chassis out while ago and it looks pretty much stock and don't look like anyone jacked with the circuit on first glance. Got spider webs in the circuit. Original filter caps in the doghouse. The rectifier tube is missing, mis-matched power tubes and all the preamps look like are all old RCA's. It has two fender 12" speakers but someone has been jacking with that because most of the speaker nuts are missing and one of the speaker wires has been cut and reconnected by just twisting the wires back together.

He wants me to get it going. So after I look it over I'll give him a report. New filter caps, power tubes and rectifier for sure. Some of the pres may be OK. After I've cleaned it up a bit would like to test the PT unloaded to make sure it's OK. Platefire

EDIT: PT is good. Did a Light Bulb limited test and got 322.4 VAC on PT secondary reds to rectifier and rectifier producing 436.7 VDC. So I took it off the limiter and got 357.2 VAC on the reds to rectifier and
491.3 VDC from the rectifier. So to me this looks about right for loaded voltages on fender schematic to drop to 340VAC to rectifier and 440VDC to OT/6L6 Plates.

[TubeGeek]

Sounds like a fun repair/restoration!

[Platefire]

Working on an old fender is like eating cake and ice cream in comparison to the Ampeg SVT CL.  

The serial number is A 11 899. According to what I seeing on the net, this amp was made in approximately 1968 if the data below is correct. The circuit is AA165 in the Blackface period but has a Silverface SN??
I guess transition period from Blackface to Silverface. 

Pro Reverb AA165 (blackface)
 A00100 to A02700 - 1965
 A02700 to A09200 - 1966
 A07000 to A10200 - 1967

Pro Reverb AA1265, AB668, AA1069, AA270 (silverface)
 A10000 to A10500 - 1967
 A10500 to A12000 - 1968
 A12000 to A13300 - 1969
 A13400 to A14500 - 1970
 A14500 to A15000 - 1971
 A15000 to A15600 - 1972
 A15600 to A17200 - 1973
 A17200 to A19700 - 1974
 A19700 to A20000 - 1975
 A20000 to A21500 - 1976

Quad Reverb CFA7104 (silverface)

[TubeGeek]

 

I have one of those(svt cl) in for repair this week too! Oh fun.

[Platefire]

OK after a good checkout last night, I think circuit-wise is all original complete with 2 conductor power cord and ground switch. From looking at the circuit and comparing it to the AA165 Schematic/Layout I did notice some differences like 150/7 watt resistors to ground on the power tubes cathode plus a cap between the cathodes and also on the power tube 1500 grid stoppers had a ceramic cap to ground. These were not on the schematic. So apparently this is an amended AA165 circuit. Anyway it don't look like the circuit has been altered or worked on since original assembly best I can tell. The amp is pretty much original stock except the top back plate is missing.

So if I'm thinking right if the customer just wants to re-sell for a profit, it would be best left in original condition---right! but of course if he want a player for himself all the normal upgrades-3 conductor power cord, new filter caps/power resistors, new tubes---not sure how far to go replacing board parts? Platefire

[sluckey]

This is probably your schematic...

http://www.el34world.com/charts/Schematics/files/fender/proreverb_ab668_schem.pdf

[Willabe]

I did notice some differences like 150/7 watt resistors to ground on the power tubes cathode plus a cap between the cathodes and also on the power tube 1500 grid stoppers had a ceramic cap to ground.

You probable already know this but;

That's the dreaded CBS layout/lead dress fix. It's not needed on all the amps but they put it in to make sure the ones that would act up didn't. Most times you can just snip that stuff out and it will be fine. If it starts to act up you gotta play around with the lead dress until it behaves.


                  Brad     

[Platefire]

Willabe

I do know that Fender started to go downhill circuit-wise when CBS took over. Most of the old fenders I've worked on has been pre CBS. I did notice a lot of things in the lead dress I thought to myself--I wouldn't do that! First thing I noticed was the two input jack 68K grid resistors were wired directly to the jacks and the ends twisted together floating in mid air the the grid wire to V1 soldered to it. I first thought someone has jacked around with this but after looking at the layout, that is original. Also I've notice no seperation in lead dress in a lot of cases. I've been going through seperating wires as much as possible to give them a little room to breathe. The board is like the lay of the land here in North Louisiana---hilly Platefire

[TubeGeek]

I agree with Brad, if this were to come to my shop I would advise or educate the customer on what's inside the amp and then ask the owner how he'd like to proceed. 

I'd recommend removing all the added cbs parts and the amp will open up and sound even better in my opinion.

It's up to the owner in the end though.

[Willabe]

First thing I noticed was the two input jack 68K grid resistors were wired directly to the jacks and the ends twisted together floating in mid air the the grid wire to V1 soldered to it.

They never did get that right from the beginning. Those are grid stop R's and for them to work as intended they have to be mounted on the tube sockets grid pin with as little lead as possible from the R's body to the grid pin.

OTOH, Fender and all the others at the time never dreamed anyone would turn up their amp past 4 or 5. So it really didn't matter much.

I did notice a lot of things in the lead dress I thought to myself--I wouldn't do that!

Yep, at that point, CBS, it was all about getting them out the door as fast as they could. Plus they had switched the wire they where using, to me not as easy to work with. So They came up with a universal quick fix for poor lead dress. Problem was it sucked tone an sustain from the amp.

Not all the amps got the treatment but most did. Sales fell through the floor because of it but by the time they stopped with the "fix" it was too late they had got a bad name from it.    

There's whole books on all this and on the net too, nothing new that I'm saying here. Bottom line is, to me anyway, take the CBS "fix" parts out and deal with the lead dress properly, like they should have from the start and you'll have a great amp.

You already know what to do, this ain't your 1'st rodeo.


                    Brad        

[Platefire]

Thanks very much for TubeGeek and Willabe. Appreciate you sharing your knowledge and experiance with me. It's always a different thought process when your working on somebody elses amp.

Also Thanks Sluckey, you nailed it with that AB668 Schematic.

One question I do have before talking with the customer is, would the amp be worth more for re-sell
in its original as is configuration or modified to Blackface circuit including replacement of all parts normally replaced on an amp this old??? Platefire

[HotBluePlates]

First thing I noticed was the two input jack 68K grid resistors were wired directly to the jacks and the ends twisted together floating in mid air the the grid wire to V1 soldered to it. I first thought someone has jacked around with this but after looking at the layout, that is original.

And that's exactly how Fender did it in the pre-CBS and CBS blackface amps, too.

You might look at it as technically incorrect, but those resistors are performing two functions and the way Fender did it generally doesn't cause problems.

The serial number is A 11 899. According to what I seeing on the net, this amp was made in approximately 1968 ...

Maybe the serial number chart is correct, maybe not. I can tell you this: no vintage dealer (or apparently their serious buyers) date a Fender by the chassis serial number. A single chassis or transformer or pot or speaker could sit under a bench for years before finally getting used. Or, it might get used right away.

So the method they use is to gather all the available information to date an amp. But your single fastest way is to look on the tube chart. There's a 2-letter code stamped on the chart with a black rubber stamp. What does yours say?

That will give month and year of manufacture. The pots, speakers and transformers will have their EIA codes, with week and year of manufacture. All four sources are usually compared, along with cosmetic details (faceplate, "drip-edge" trim on silverface or lack thereof, grill cloth color/detail, Fender logo detail) and types of components/layout/wire used and the probably of original or replaced components.

All those sources of date information would be taken together to estimate the amp's date of manufacture, as any one by itself could be an outlier for the reason I mentioned above.

[Platefire]

HBP

My tube chart has no stamp on it with letter code. Looked close, it's just not there. Worked hard on the cab tonight. Rat poop, broken glass and dirt in the bottom of cab. Cleaned it out, tried to take the reverb bag out but one screw is so rusted up, I couldn't budge it. Did slide the tank out one end, it looks OK. It appeared someone had previously removed the speakers and just cut the wires to get them out and at some point reinstalled them and just tied the wire back together and put electric tape over them. They didn't bother putting most of the speaker nuts back in plus the speakers were loose. I spiced some new wires in, soldered them back and put shrink rap over the splices. Put new nuts/washers back on the speaker bolts. Tried out the speakers with another amp head and was hearing a rattle. Apparently one or both speakers has a problem. By the time you figure new speakers, new tubes and replacing old caps/resistors, correcting CBS bo bo's, it will take quiet a bit to bring this amp back to a gigging amp. Platefire

[TubeGeek]

These type of amps can indeed turn into a job but at least it's a fender and not an svt!

Sounds to me like it could be around a $300 to $500 bill for the owner in the end.  If it were mine I would not be concerned about vintage value, it's not the gem people look for so modifying it to pre CBS specs wouldn't hurt in my opinion.  Give all the options to the owner and he can make the call.  These are the kind if jobs I enjoy in my shop :)

[Platefire]

Well the amp is out the door. I tested the tubes last night on my tube tester and one 12AX7, and two 12AT7's tested weak, one the original 6L6's was broke and the GZ34 missing. I found good replacements in my tube inventory. I fired it up this morning on a light bulb limiter and it came up fine.
Hooked up and tested the reverb and it worked fine. Hooked up the foot switch and it made a little hum
when I hooked up the reverb plug. Tested it and the tremolo worked fine but the reverb wouldn't switch off. Sprayed some contact cleaner in the switch and tested it, the reverb started swithing off and on and also the hum was gone. Checked all the voltages and they seemed pretty much on target. If it wasn't for the ratteling speaker, the amp would sound perfect.
It is so quiet that at 3 on the volume, you can't hear nothing or no sound except when you play your guitar. I couldn't believe it was so quiet on 44 year old filter caps. I do a written report on ever amp I repair that I provide the customer that includes what I found on inspection, what I did to the amp, observations/recomendations for the amp and also schematic/layout.  So I called the customer, he came over immediatly, tried it out, provided him a amp repair report, he was happy and paid me. Platefire

[Willabe]

So I called the customer, he came over immediatly, tried it out, provided him a amp repair report, he was happy and paid me.

Very nice!


                      Brad       

[six el six]

You didn't replace the electrolytics?????!!!!!!!!! Huh?!

Always replace old electrolytics.

[Platefire]

Well the guy really don't want the amp because he has back problems and it's too heavy for him to manage. He was trying to sell the amp to me. I don't need another amp--I need to get rid of some I already got. I got the amp up and running and that's the task he gave me. It's decision time for him: #1-Does he want to sell it as an all original vintage amp--then my advice to him was leave it alone and sell it as is #2-If he wants to upgrade it to a player/giging amp for himself he needs to be prepared to make an investment----new speakers--or re-cone existing, 3 conductor cord, filter caps, new tubes and closer look at board parts for tolerances/issues. $$$$. I even told him about it being a Leo Fender/CBS transition amp where many folks mod it back to Blackface specs. It's all in my Amp Repair Report I provided him. To him the amp was an investment opprotunity. He said he might hold on to it because the value will only increase. It's all original with the exception of the upper back plate missing. I use to play in a band with this guy and we worked on 3 different CD's/abums together--so I know him well. A great song writter, singer and guitar player. He now uses something like a POD into a PA for his amplifier. Platefire

[plexi50]

Hey Plate? Barn Fresh is cool! He needs to get at least a cap and speaker upgrade to make it right.

[Platefire]

I have to admit, it was tempting to buy that amp. It had the vintage used road worn look with the dingy speaker grill, all original, price was right and all. I guess I need to change my Forum phrase "How many amps do you need, just one more" to "Gimmi one good amp that's easy to tote" Platefire 

[six_eight]

Working on an old fender is like eating cake and ice cream in comparison to the Ampeg SVT CL.  

The serial number is A 11 899. According to what I seeing on the net, this amp was made in approximately 1968 if the data below is correct. The circuit is AA165 in the Blackface period but has a Silverface SN??
I guess transition period from Blackface to Silverface. 

Pro Reverb AA165 (blackface)
 A00100 to A02700 - 1965
 A02700 to A09200 - 1966
 A07000 to A10200 - 1967

Pro Reverb AA1265, AB668, AA1069, AA270 (silverface)
 A10000 to A10500 - 1967
 A10500 to A12000 - 1968
 A12000 to A13300 - 1969
 A13400 to A14500 - 1970
 A14500 to A15000 - 1971
 A15000 to A15600 - 1972
 A15600 to A17200 - 1973
 A17200 to A19700 - 1974
 A19700 to A20000 - 1975
 A20000 to A21500 - 1976

Quad Reverb CFA7104 (silverface)

Does it have silver metal trim around the edges where the grill cloth/speaker baffle meet the cabinet?

[eleventeen]

I just sold an old blackface Pro Reverb 2 days ago! This was a real old one, s/n A00138 1965 something. (Pro Reverb number 138??) I bot this in the late 70's, and it had been modded to Twin specs. So, it had a SS recto, a Twin P/T and O/T, it retained the smaller sized choke from the Pro which had to be swung at an angle to make room for the extra 6L6. The GZ34 had been yanked and the socket used for one of the add'l 6L6 tubes, and, another 6L6 socket was installed in a punched out chassis hole. This was a fairly standard mod that was done through the 70s by a co called Starz Guitars up here in the SF Bay Area. Very simple, just add two 1500 ohm resistors and parallel the tubes, change fuse to 3 amps from 2 amps, done. By jove, if it had 2 6L6, it had to have 4! They did this to lots of Fender amps from the era.

Over the years, I installed a 3-way switch in the power position, one side lifted two cathodes off ground, so the switch was low-power (40w)-off-high-power (80w) and a 3 wire cord. I never did any further mods to it, and just recently did a light rebuild---just the 100K plate resistors and replaced the electrolytics (and stage-dropping resistors under the can) including the bias filter cap. It was really making bizarre oceanic rodent-like noises to the point where it was almost unusable. After the rebuild-lite it got very quiet again. Nice. It was also in a much-later vintage Twin cabinet---which I switched out at some point many years ago. So it was in effect an AB763 Twin Reverb lacking the middle control.

It was a great amp. I gigged with it for about 20 years and beat the hell out of it. It never failed me other than gradually acquiring the weird noises, but I no longer have use (nor room) for such a loud back-breaker. Was glad to sell it, frankly.

[Colas LeGrippa]

an amp from la grange ? ready to play some zz top then........

[jojokeo]

HBP, Sluckey, or someone please enlighten me on the use of the negative bias voltage in this circuit please?

[HotBluePlates]

HBP, Sluckey, or someone please enlighten me on the use of the negative bias voltage in this circuit please?

Say what? Dunno what you mean.

[sluckey]

It's a bias 'balance' circuit. Actually, this one has some cathode bias as well.

The bottom tube gets some non-adjustable (Fender knows best) negative bias to the grid and the top tube gets an adjustable negative bias. The idea is to adjust the top tube to match the bottom tube. Since the cathodes are sitting on 150Ω resistors, adjustment is simple. Connect one voltmeter lead to one cathode and connect the other voltmeter lead to the other cathode. Adjust for zero volts. That's all there is to it. Zero volts difference only occurs when each tube is drawing exactly the same current. The tubes will be perfectly balanced.

May not be biased as hot or cold as 'you' may want though. That's why people today don't like the balance circuit.

[HotBluePlates]

There's also a fairly cool bypass to the cathode resistors.

5uF from cathode to cathode is electrically the same as each resistor having a 10uF bypass cap of its own. 10uF against 150Ω gives a -3dB point of ~105Hz.

[jojokeo]

It's a bias 'balance' circuit. Actually, this one has some cathode bias as well.

May not be biased as hot or cold as 'you' may want though. That's why people today don't like the balance circuit.

That's something you don't see everyday for sure. I'm guessing that for hotter/cooler bias you just change the cathode resistors then?

Lastly, what about the fixed/non-adjustable neg bias that goes into V5 like that?

[HotBluePlates]

I'm guessing that for hotter/cooler bias you just change the cathode resistors then?

No, you'd leave those alone and the resistors around the bias pot (10k's and the 18(?)k).

Lastly, what about the fixed/non-adjustable neg bias that goes into V5 like that?

It's to assure a quick start-up of the trem oscillator.

When the trem footswitch is in the "off" position (hot and ground not connected), a large negative voltage is applied to the oscillator grid to shut it off.

When you switch the trem footswitch to the "on" position, the 2.2M resistor (and 1M resistors) are quickly moved from a big negative voltage to 0v. The transient that results from that is enough of a "shock" to start the trem oscillator immediately.

Other Fender amps have variations of this idea for jump-starting the trem.

[Platefire]

So if you Blackfaced the AB668 circuit and removed the 150 Ohm resistors and substituted the 1 Ohm resistors, you could at least easily check how hot or cold you were?

When I was working on this pro I saw the oscillator flashing and thought something was shorting out at first and realized that was normal---duh!!!--hadn't noticed that before. I've always been told that the bias trem was better sounding but this pro trem is excellent. Platefire

[Platefire]

Got this amp back to replace filter caps. Looking at the Sprague 20 uf it goes from 500 V to 600V with a big difference in price. The existing paper caps are 525V. Top voltage coming off rectifier is 450V loaded and it goes down from there. Do I need to go with the 500V or 600V?

[PRR]

450V is about the upper limit on aluminum electrolytic. 475V if the dope were clean and you were lucky; these often had a 525V "surge" (few second) rating. I see a lot of 500V caps today. Maybe chemical purity is easier now. Maybe they just round-up.

There are no 600V electrolytics. Sprauge used to put two 350V electros in one tube and call it 600V.

450V running voltage; I'd use two 300V caps series, and the good modern "snap caps" not some old Sprague retro-fit. But I'm not a restorer.

BTW:

> The existing paper caps are

I'm sure these are aluminum electrolytic in paper sleeves.

"Paper caps" used paper as insulator between the foils (fortified with wax or oil) and were values like 0.05uFd, not whole uFds.

[Platefire]

PRR, what about this one?

http://www.newoldsounds.com/index.php?main_page=product_info&products_id=10391

[DummyLoad]

HBP, Sluckey, or someone please enlighten me on the use of the negative bias voltage in this circuit please?

looks like mainly to balance output tubes.

--pete

[Platefire]

What??   

[sluckey]

PRR, what about this one?

I have 2 of those TVA1966s in my Sunn. They are really 2 caps in one tube.

http://home.comcast.net/~seluckey/amps/sunn/sunn.htm

I think they are too big to fit under the Fender doghouse.

I would pull the output tubes and measure the unloaded B+ at the rectifier. You may be able to use a 500v cap.

[Platefire]

OK thanks, Sluckey---I'll try to keep the bear out of the doghouse! Took a look at your Sunn. That's one cool amp! I recall there being a lot around in the 70's and played over a couple setting in with other bands back then.

On the caps, I may be wrong but I think most folks are just putting the 500V in the old fenders. Platefire

[Platefire]

I'm installing a new 3 conductor power cord. On the old ground switch, is it customary to just disconnect it and abandon it in place? Can't think of any other use, already got a standby. Platefire

[g-man]

Deleted

[Platefire]

Installing a 3 conductor cord on this one. Was planning on bypassing the ground switch but wondering also about the accessory plug---abandon in place or install a 3 conductor plug? What do you do on the vintage fender amps with this situation???? Would appreciate some advice from you experianced hands that's done this plenty of times--best fix practice! Thanks Platefire

[sluckey]

I would just replace the cord, leaving the AC receptacle and ground switch both wired. I would rewire the fuse and power switch such that they are both on the AC hot line. IOW,

Power cord black to AC receptacle to ground switch to tip of fuse. Tip Side of fuse to power switch. Other side of power switch to black wire on PT primary.

Power cord white to AC receptacle to ground switch to other black wire on PT primary.

Power cord green to ring lug connected to chassis. Maybe use a screw on the AC receptacle.

[DummyLoad]

lately i've been using F&T in fender restorations.

http://www.tubesandmore.com/products/capacitors?filters=Brand%3DF%26T

the miec line looks interesting too.

--pete

[eleventeen]

As I recall, most every chassis mount 3-wire w/gnd AC receptacle I have encountered required a reasonably larger mounting hole than the 2-wire ones Fender used. Hence, to make this change you'd have to do (what I consider to be) a somewhat irritating piece of metalwork----complicated by the interference the belly of the power transformer creates. Sure, if you have a die-grinder it would be a snap. Going manual, using a file, is not much fun.

Perhaps you can get around this by mounting such a 3-wire receptacle with the mounting plate on the outside of the chassis. Sometimes the plastic header is smaller on the backside of the mounting ring. Regardless, almost certainly, the mounting holes are wider-spaced. Another approach is to change to an IEC-type three wire, detachable power cord and mount the receptacle in the hole formerly occupied by the AC socket, and abandon the hole where the attached power cord goes thru the chassis. Of course, you've already done the work to replace the existing AC cord with a proper 3-wire one. Sigh. Nothin's easy.

I would leave the 2-wire receptacle as is. Some things end up being more pain than gain. In a world where a simple 6-outlet strip is a $3-$5 thing at Home Depot, I don't think there's any massive advantage to having the rear panel outlet being a grounded one. And if your pal is planning on selling the amp, I wouldn't even consider it. My .02.

And, I would abandon the ground switch, disconnect it, and leave it in place.

[Platefire]

Thanks, Yep I will just leave the two conductor plug as is and use the sluckey method.
sluckey---I had to draw a sketch of your described wire up to wrap my brain around it. I agree with your discription of hook up except for one thing on the second line ---you say(ground switch to tip of fuse. Tip of fuse to power switch.)

I think that should say(ground swith to tip of fuse. Base of fuse to power switch) The way you described that part, you would be bypassing the fuse I think! Double check me but that's the way I'm seeing it?? Thanks a bunch, Platefire

[sluckey]

I think that should say(ground swith to tip of fuse. Base of fuse to power switch) The way you described that part, you would be bypassing the fuse I think! Double check me but that's the way I'm seeing it??

You're correct. I should start proofreading! I'll go back and correct the original statement.

[Platefire]

OK, thanks!  Just wanted to make sure. I like your plan better than abandoning everything in place. Appreciate the help!  Plate

[Platefire]

One more thing on the power cord. Is there any "no no" rule about grounding the green on the power
transformer lug--it's the closest thing with a bolt! Closest accessory plug and tube socket screws are original self tapping(no nut). Trying to leave the vintage chassis "as is" as much as possible. Guess I could remove a self tapping screw out of the accessory plug(closest point) and install a small screw, nut and ground lug---but the PT is the closest substanual bolt/nut to grab on to. For some reason I have avoided grounding power cord on the PT in the past. Don't know if it's because of something I read or heard regarding this in the past but need to clairify now. Thanks, Platefire

[DummyLoad]

rule about grounding the green on the power transformer lug

no. it's a mechanical connection to chassis so use it.  :-)

--pete

[Platefire]

Thanks, Man I will:>)

[jojokeo]

Installing a 3 conductor cord on this one. Was planning on bypassing the ground switch but wondering also about the accessory plug---abandon in place or install a 3 conductor plug? What do you do on the vintage fender amps with this situation???? Would appreciate some advice from you experianced hands that's done this plenty of times--best fix practice! Thanks Platefire

This example was made to an original '65 BFDR, is a gem, and is non-invasive allowing amp to be put back into original condition if ever wanted (but this won't happen anytime soon because of how effective it is and the improvement in tone & versatility it has created). I remove the whole grounding mess and replace with a push-pull pot for Presence control. This can be operated normally on "1" which creates "normal" NFB response, dialed up yielding Presence which these amps don't have and is a GREAT addition, and finally can be pulled out to remove all feedback and opening up the amp for more gain and harmonic complexity and earlier break-up. The best part is it was done to my bandmate's amp and I get to hear the results often in playing conditions.