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1964 Fender Showman: what maintenance?

kingsxman

Gold Supporting Member
Messages
6,868
I just picked up a 1964 Fender showman the other day. Very stock. Looks like it might even have all original tubes. Anyway, a few questions: is this a AB763 circuit or AA763? I'm trying to make a list of what I need to do standard mainteance:

* I know I should replace the power caps.
* I need to add a 3 prong cord also.

Anything else recommended? Amp sounds great but I did hear a whining noise last time I fired it up. It hadnt been played for years.

I'm gonna slow the trem down by adding a .022 ufd cap in place of one of the .01 ufds in the trem circuit.

I'm looking for a good place to buy all the caps. Do you hear a difference in spraque vs other brands? Could I buy a AB763 recap kit and that woudl handle the caps?
http://members.goldengate.net/jgermann/Showman/
 

jzilla

Member
Messages
1,498
yeah, it's the AB763. i have one from 65. if you love fender clean, you're gonna love this amp. might want to change the vibrato jack to a 1/4", too. btw, make sure to check out the normal channel. significantly better sounding than the vib channel imo.
 

WaltC

Member
Messages
2,129
You can get the caps at the same places I recommended for the strain relief's for the Reverb power cord (and get another power cord from MPJA too <G>).

I'd replace all the electrolytics, including the cathode bypass ones on the eyelet board and the filter on the bias supply too. Get a higher voltage one than the stock filter on the bias, such as a 47uF/150V or so. It'll last longer and provide better service in the long run.

Careful on the power supply filter caps (under the dog house), the first two are 70uF/350V run in series w/ two 100K (I use 220K 2 or 3 watt myself) "balancing" resistors between them. The first is wired up "backwards" from the others on that board in that it (farthest left) is neg down and pos up (in your picture) and the second (and the other three also) is pos down and neg up.

Ditch the "death cap and leave the ground switch un-wired when you replace the power cord. Be careful and enjoy it, that's one cool setup you've got now w/ the showman and reverb unit too. <G>
 

WaltC

Member
Messages
2,129
Oh... and BTW. F&T makes a two-in-one cathode bypass cap that's a "drop in replacement" for the stock ones and Antique Electronics has them as well. You can replace them with two small spragues 25/25's or 25/50's (my favorites) too.
 

gldtp99

Member
Messages
3,857
Replace filter caps in doghouse (replacing any drifted/damaged power rail resistors while you're in there)--- replace Bias Cap---install grounded three conductor power cord -- rewire PT primary side to modern specs: Hi AC (usually black) to Switch, fuse, PT----other AC (usually white) to PT--- Ground (usually green) to lug bolted to PT stud (add solder lug--- use toothed washer underneath for best ground connection)----remove "Death Cap".
If you're talking about replacing all the caps on the board then IMO this would be a mistake---- don't replace any of the Blue Molded coupling caps unless they're bad (leaking voltage or physically damaged)---the Blue Molded caps are great sounding and removing them would be a poor idea if they are in good shape and it would de-value the amp.
Some would replace the bypass caps on the board (big cardboard caps)---i would, but some like these amps with the orig bypass caps--- they usually drift high and, if so, would contribute to a duller/bassier tone----some prefer it this way--- it sounds more "Vintage" to them----either way there's no danger to long term amp operation if they aren't replaced.
Replace the flakey re-wire job on the Standby Switch/Screen Grid supply---- check Screen Grid resistors for cracking/drifting (i'd just put in new 470 ohm/3-to-5 watt Screen Grid resistors)--- these resistors are subject to a lot of heat being mounted on the 6L6GC sockets, and they're old----might as well replace the 1.5k Grid Stopper resistors (1/2 watt ok) since they've been baked by the output tubes for yrs, too--
Clean tube sockets- re-tension if needed-- clean jacks--- clean pots if needed (i like Caig MCL---- it cleans and lubes).
Check the old resistors on the board, paying special to Plate Load Resistors---- the old Carbon Comp plate resistors are often a source of noise----replace if badly drifted or noisy.
Get a good quad of matched 6L6GC output tubes---- get the best you can afford----i really like USA preamp tubes in these amps: RCA, GE, Sylvania (you can save some $$ by using a new production 12ax7 to run the trem if wanted)---- a good USA 12ax7 in V1, V2--- a good USA 12at7 in V4 (PI).
And before it's plugged in to bias/test-- CHECK the FUSE---- make sure it's the right one--- a 2-1/2 amp Slo-Blo.
When it's up and running into an 8 ohm 2x12, 2x15, 4x12, etc cab that will easily handle 85 watts---- enjoy the Big Fender tone !!!! These are good amps !!!...........gldtp99
 

kingsxman

Gold Supporting Member
Messages
6,868
yeah, it's the AB763. i have one from 65. if you love fender clean, you're gonna love this amp. might want to change the vibrato jack to a 1/4", too. btw, make sure to check out the normal channel. significantly better sounding than the vib channel imo.
THats what I heard yesterday when I fired it up...
 

kingsxman

Gold Supporting Member
Messages
6,868
Replace filter caps in doghouse (replacing any drifted/damaged power rail resistors while you're in there)--- replace Bias Cap---install grounded three conductor power cord -- rewire PT primary side to modern specs: Hi AC (usually black) to Switch, fuse, PT----other AC (usually white) to PT--- Ground (usually green) to lug bolted to PT stud (add solder lug--- use toothed washer underneath for best ground connection)----remove "Death Cap".
If you're talking about replacing all the caps on the board then IMO this would be a mistake---- don't replace any of the Blue Molded coupling caps unless they're bad (leaking voltage or physically damaged)---the Blue Molded caps are great sounding and removing them would be a poor idea if they are in good shape and it would de-value the amp.
Some would replace the bypass caps on the board (big cardboard caps)---i would, but some like these amps with the orig bypass caps--- they usually drift high and, if so, would contribute to a duller/bassier tone----some prefer it this way--- it sounds more "Vintage" to them----either way there's no danger to long term amp operation if they aren't replaced.
Replace the flakey re-wire job on the Standby Switch/Screen Grid supply---- check Screen Grid resistors for cracking/drifting (i'd just put in new 470 ohm/3-to-5 watt Screen Grid resistors)--- these resistors are subject to a lot of heat being mounted on the 6L6GC sockets, and they're old----might as well replace the 1.5k Grid Stopper resistors (1/2 watt ok) since they've been baked by the output tubes for yrs, too--
Clean tube sockets- re-tension if needed-- clean jacks--- clean pots if needed (i like Caig MCL---- it cleans and lubes).
Check the old resistors on the board, paying special to Plate Load Resistors---- the old Carbon Comp plate resistors are often a source of noise----replace if badly drifted or noisy.
Get a good quad of matched 6L6GC output tubes---- get the best you can afford----i really like USA preamp tubes in these amps: RCA, GE, Sylvania (you can save some $$ by using a new production 12ax7 to run the trem if wanted)---- a good USA 12ax7 in V1, V2--- a good USA 12at7 in V4 (PI).
And before it's plugged in to bias/test-- CHECK the FUSE---- make sure it's the right one--- a 2-1/2 amp Slo-Blo.
When it's up and running into an 8 ohm 2x12, 2x15, 4x12, etc cab that will easily handle 85 watts---- enjoy the Big Fender tone !!!! These are good amps !!!...........gldtp99
Thanks. I'll have more specific questions in a bit..but I'm not going to replace all the caps. Only what really needs it.

Alot of these items I havent done before...so its a learning process for me. I've replaced caps but havent done the transformer wireing, etc. I know the dangers of working in an amp..and I know an option is to take it to a tech..but this is something I can defintiely handle with a bit of guidance from the board.
 

kingsxman

Gold Supporting Member
Messages
6,868
Oh... and BTW. F&T makes a two-in-one cathode bypass cap that's a "drop in replacement" for the stock ones and Antique Electronics has them as well. You can replace them with two small spragues 25/25's or 25/50's (my favorites) too.
Thanks Walt. I'll make an order from one place. Is Antique Electronics your favorite spot?
 

WaltC

Member
Messages
2,129
actually Doug Hoffman (hoffmanamps.com) is my favorite, but he doesn't always have everything I need (nor do they all). For what you're looking at, Antique Elec. is probably best this time, but shopping around can often get you just the exact things you want and can save you some money on occasion as well.
 

kingsxman

Gold Supporting Member
Messages
6,868
Replace filter caps in doghouse (replacing any drifted/damaged power rail resistors while you're in there)--- replace Bias Cap---install grounded three conductor power cord -- rewire PT primary side to modern specs: Hi AC (usually black) to Switch, fuse, PT----other AC (usually white) to PT--- Ground (usually green) to lug bolted to PT stud (add solder lug--- use toothed washer underneath for best ground connection)----remove "Death Cap".
If you're talking about replacing all the caps on the board then IMO this would be a mistake---- don't replace any of the Blue Molded coupling caps unless they're bad (leaking voltage or physically damaged)---the Blue Molded caps are great sounding and removing them would be a poor idea if they are in good shape and it would de-value the amp.
Some would replace the bypass caps on the board (big cardboard caps)---i would, but some like these amps with the orig bypass caps--- they usually drift high and, if so, would contribute to a duller/bassier tone----some prefer it this way--- it sounds more "Vintage" to them----either way there's no danger to long term amp operation if they aren't replaced.
Replace the flakey re-wire job on the Standby Switch/Screen Grid supply---- check Screen Grid resistors for cracking/drifting (i'd just put in new 470 ohm/3-to-5 watt Screen Grid resistors)--- these resistors are subject to a lot of heat being mounted on the 6L6GC sockets, and they're old----might as well replace the 1.5k Grid Stopper resistors (1/2 watt ok) since they've been baked by the output tubes for yrs, too--
Clean tube sockets- re-tension if needed-- clean jacks--- clean pots if needed (i like Caig MCL---- it cleans and lubes).
Check the old resistors on the board, paying special to Plate Load Resistors---- the old Carbon Comp plate resistors are often a source of noise----replace if badly drifted or noisy.
Get a good quad of matched 6L6GC output tubes---- get the best you can afford----i really like USA preamp tubes in these amps: RCA, GE, Sylvania (you can save some $$ by using a new production 12ax7 to run the trem if wanted)---- a good USA 12ax7 in V1, V2--- a good USA 12at7 in V4 (PI).
And before it's plugged in to bias/test-- CHECK the FUSE---- make sure it's the right one--- a 2-1/2 amp Slo-Blo.
When it's up and running into an 8 ohm 2x12, 2x15, 4x12, etc cab that will easily handle 85 watts---- enjoy the Big Fender tone !!!! These are good amps !!!...........gldtp99
Gldtop. quick questions: Where are the plate load resistors?

Is this the wireing you'd use for the 3 wire power cord?
http://www.theguitarfiles.com/guitarfile644.html
 

kingsxman

Gold Supporting Member
Messages
6,868
Oh... and BTW. F&T makes a two-in-one cathode bypass cap that's a "drop in replacement" for the stock ones and Antique Electronics has them as well. You can replace them with two small spragues 25/25's or 25/50's (my favorites) too.
Hey Walt, if memory serves me right...these are the caps with the 2 leads on one side. Do these have an affect on tone?
 
Last edited:

WaltC

Member
Messages
2,129
yup, cathode bypass caps do affect the tone of the stage they are present in but not directly and the "quality" isn't as critical, and the F&Ts do a good job. You can always replace each of the "double" cathode bypass caps with two smaller sized individual caps w/ the same uF and voltage rating of the originals (or as close as you can get, I use small Sprague or Illinois 25uF/50V individuals my self but I've used the F&Ts with good luck too).
 

gldtp99

Member
Messages
3,857
Gldtop. quick questions: Where are the plate load resistors?

Is this the wireing you'd use for the 3 wire power cord?
http://www.theguitarfiles.com/guitarfile644.html

That's a detailed description of how to install a grounded, 3-conductor power cord----- i like to wire the power switch before the fuse but either way works just fine.
The plate load resistors are connected to the plates of the preamp tubes (pins 1 and 6 of 12a_7)...............gldtp99
 

kingsxman

Gold Supporting Member
Messages
6,868
yup, cathode bypass caps do affect the tone of the stage they are present in but not directly and the "quality" isn't as critical, and the F&Ts do a good job. You can always replace each of the "double" cathode bypass caps with two smaller sized individual caps w/ the same uF and voltage rating of the originals (or as close as you can get, I use small Sprague or Illinois 25uF/50V individuals my self but I've used the F&Ts with good luck too).
Thanks. BIg diff in price. $10 per "split" capacitor or much less 2 use 2 smaller caps. But...it'll keep it more "original" I suppose with a F&T or other split caps.
 
Last edited:

kingsxman

Gold Supporting Member
Messages
6,868
So..dumb question. I want to see how the bias is on this with the old power tubes. They're "probably" ready to be replaced..but how would I go about biasing it? I have a weber bias plug that I would be using. Whats the current draw/plate voltages supposed to be on these?

FYI, there is one 470 ohm resistor thats out of spec and one 1.5k grid thats bad.
 

WaltC

Member
Messages
2,129
replace the resistors first, then put the bias rite (?) on it w/ the old tubes (does it have the B+ voltage option?) and get a reading (at idle, volume on zero, power and standby on) and let's see what you get. If it doesn't (the bias rite) have the B+ option then measure the DC volts on the plate (pin 3) (CAREFULLY!!!! 400+ DC volts here) and let us know what the ma and DC volt reading on each tube are and we'll go from there.

You gonna' change out the electrolytics first? I only ask 'cause if you do you should carefully start up the amp (I use a VARIAC or a light bulb load limiter myself) w/o tubes first and then w/ tubes to make sure that you have the electrolytics in correctly. Remember the the first two under the dog house are wired in series w/ load balancing resistors and connected differently (usually) from the other three in there (by that I mean that the hot wire on the first of the two series electrolytics and the ground connection on the second of the first two is often (always?) on the opposite side of the eyelet board than the hot and ground wires connecting the remaining three caps. Just pay close attention to the orientation of the + and - poles of *all* the caps and make sure your replacements go in the same way as the originals. More than one person has been bitten by this kind of mistake, trust me <G>.
 

kingsxman

Gold Supporting Member
Messages
6,868
replace the resistors first, then put the bias rite (?) on it w/ the old tubes (does it have the B+ voltage option?) and get a reading (at idle, volume on zero, power and standby on) and let's see what you get. If it doesn't (the bias rite) have the B+ option then measure the DC volts on the plate (pin 3) (CAREFULLY!!!! 400+ DC volts here) and let us know what the ma and DC volt reading on each tube are and we'll go from there.

You gonna' change out the electrolytics first? I only ask 'cause if you do you should carefully start up the amp (I use a VARIAC or a light bulb load limiter myself) w/o tubes first and then w/ tubes to make sure that you have the electrolytics in correctly. Remember the the first two under the dog house are wired in series w/ load balancing resistors and connected differently (usually) from the other three in there (by that I mean that the hot wire on the first of the two series electrolytics and the ground connection on the second of the first two is often (always?) on the opposite side of the eyelet board than the hot and ground wires connecting the remaining three caps. Just pay close attention to the orientation of the + and - poles of *all* the caps and make sure your replacements go in the same way as the originals. More than one person has been bitten by this kind of mistake, trust me <G>.
Resistors and caps first before biasing. I dont have the full blown bias rite..just the Weber BR-AH (found here towards the bottom of the page):
https://taweber.powweb.com/biasrite/br_page.htm
I plug it into my fluke and get my readings that way.
 

kingsxman

Gold Supporting Member
Messages
6,868
One more thing: I'm trying to order all my parts from one place...but one seems to have the right caps and one seems to have the right resistors. Antique Electronics has the caps...but they only go to 2 watts for the 470 ohm grid resistors. Is 2 watts ok for these? I'm guessing no.
 

WaltC

Member
Messages
2,129
2 watts is plenty (low current flow, relatively high DC voltage). The originals were 1 watt'ers.
 

kingsxman

Gold Supporting Member
Messages
6,868
2 watts is plenty (low current flow, relatively high DC voltage). The originals were 1 watt'ers.
Thanks. i made my parts order today. I'll let you know the voltage readings after they come in and I can get started. Probably at least a week is my guess. Ended up ordering from Antique Electronic. Cheaper and had caps closer to what I needed. I opted not to get the dual caps though and will just use a couple of 25/50v spragues to replace each of the dual caps.
 




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