aaronlowther1993

TDPRI Member
Joined
Sep 4, 2016
Posts
68
Age
29
Location
Claremore, OK
I recently purchased three 60's Bandmasters from a local vintage guitar shop. Two AB763, one AA763. I've been in the process of doing some of @robrob 's wonderful mods to each of them along with a Quad Reverb I bought. All are done except one.

I've opened up the last Bandmaster (the AA763) and I'm seeing some things that are concerning to me. I played it before buying it and the amp functioned. However, upon opening it up things are a bit strange. It's obviously been serviced by someone with even less know how than myself. Two power transformer taps are "grounded" by being shoddily soldered to the chassis; Same with doghosue ground. When I pulled the fuse someone had put a 20 amp fuse in. I've swapped to the correct fuse, pulled the death cap and put on a grounded cable. I was about to move on to the next mod when I noticed that when someone re-ran the heater wires they accidently connected Pin 2 to Pin 1 on V6. Pin 1 is tired to ground. I checked the schematic and layout and this looked wrong so I remedied it and decided to check the heater voltage. Between Pin 2 and Pin 7 I get about what you'd expect: 6.6 volts AC. A little high, but, nothing crazy. But between either side of the heaters and ground I get 179 volts when I would expect to get 3.3.
Looking at the caps in the dog house, they absolutely need to be replaced, but none of them are leaking or turning to fluff. But, I'm not sure if there's something else really wrong here. I'd love to get some help here, as while I'm far more confident than I used to be with this, this is getting into an area in which I don't feel super comfortable.

Looking at the power transformer, and checking it's part numbers, this is a Blackface Twin power transformer, not a Bandmaster one! It's an 022756 instead of an 022814. The choke and OT are correct.

Obviously changinging out the PT is in order, right? What would you do from here?
 

Attachments

  • IMG_2937.JPG
    IMG_2937.JPG
    229.6 KB · Views: 63
  • IMG_2938.JPG
    IMG_2938.JPG
    189.1 KB · Views: 61
  • IMG_2939.JPG
    IMG_2939.JPG
    189.7 KB · Views: 74
  • IMG_2940.JPG
    IMG_2940.JPG
    294.9 KB · Views: 74
  • IMG_2941.JPG
    IMG_2941.JPG
    178.9 KB · Views: 73
  • IMG_2942.JPG
    IMG_2942.JPG
    288.1 KB · Views: 73
  • IMG_2943.JPG
    IMG_2943.JPG
    276.1 KB · Views: 70
  • IMG_2944.JPG
    IMG_2944.JPG
    221.8 KB · Views: 70

Jon Snell

Tele-Afflicted
Joined
Aug 31, 2015
Posts
1,308
Location
Jurassic Coast, Dorset. Great Britian.
It looks to me that the mains transformer has been replaced with possibly one 'they' had in stock and I agree, not installed correctly.
To comply with most countries regulations the safety ground must have its own dedicated fixing and the conductor crimped to an eye that is bolted securly to the chassis. Not on a transformer fixing bolt!
Apart from that the transformer is similar. The HT will be about 10volts down but nothing anyone would notice. There is suficient current available for all this amplifier needs to perform as it should.
The secondary ground connections are acceptable if; the soldering was hot enough to make a good mechanical joint. If not, use a solder tag with a nut and bolt and shake proof washer.
Fit the transformer correctly, replace all of the old and tired electrolytics. Replace the 100k anode load resistors around the 12AX7 valves and hey presto, a nice reliable amplifier.
I am pleased you had the forethought to test it first. Some don't and wonder why, after replacing loads of random things and "modding" the amp, they finish up with the same fault that was on the amplifier before they started and they get understandably, confused about a cure.
 

Timbresmith1

Friend of Leo's
Joined
Oct 1, 2010
Posts
3,380
Location
Central TX
I’m thinking the power transformer may be okay to leave in. More heater current available and plate voltage may be similar..,
 

aaronlowther1993

TDPRI Member
Joined
Sep 4, 2016
Posts
68
Age
29
Location
Claremore, OK
I’m thinking the power transformer may be okay to leave in. More heater current available and plate voltage may be similar..,
So, would you have a suggestion as to why I’m getting 179 volts between a single side of the heater and ground? Just so I know which repair (new ground connections, new electrolytic caps, new anode lode resistors) actually fixed it.
 

aaronlowther1993

TDPRI Member
Joined
Sep 4, 2016
Posts
68
Age
29
Location
Claremore, OK
It looks to me that the mains transformer has been replaced with possibly one 'they' had in stock and I agree, not installed correctly.
To comply with most countries regulations the safety ground must have its own dedicated fixing and the conductor crimped to an eye that is bolted securly to the chassis. Not on a transformer fixing bolt!
Apart from that the transformer is similar. The HT will be about 10volts down but nothing anyone would notice. There is suficient current available for all this amplifier needs to perform as it should.
The secondary ground connections are acceptable if; the soldering was hot enough to make a good mechanical joint. If not, use a solder tag with a nut and bolt and shake proof washer.
Fit the transformer correctly, replace all of the old and tired electrolytics. Replace the 100k anode load resistors around the 12AX7 valves and hey presto, a nice reliable amplifier.
I am pleased you had the forethought to test it first. Some don't and wonder why, after replacing loads of random things and "modding" the amp, they finish up with the same fault that was on the amplifier before they started and they get understandably, confused about a cure.
So, would you have a suggestion as to why I’m getting 179 volts between a single side of the heater and ground? Just so I know which repair (new ground connections, new electrolytic caps, new anode lode resistors) actually fixed it.
 

Timbresmith1

Friend of Leo's
Joined
Oct 1, 2010
Posts
3,380
Location
Central TX
So, would you have a suggestion as to why I’m getting 179 volts between a single side of the heater and ground? Just so I know which repair (new ground connections, new electrolytic caps, new anode lode resistors) actually fixed it.
I do not know. I’d be getting those toiletpaper tube caps out of there asap. I’m a better parts replacer than theorist when it comes to guitar amps. Troubleshooting an amp is easier if there are fewer variables.
 

aaronlowther1993

TDPRI Member
Joined
Sep 4, 2016
Posts
68
Age
29
Location
Claremore, OK
That board looks stock. Other than making the necessary evil changes, don't do it.
I understand why people say this, I do. But for me, I never buy something like this without the intention of tinkering with it. I leave it up to you guys to preserve history. I’m going to be dragging this thing on the road, so it’s going to get abused anyway.
 

zook

Friend of Leo's
Joined
Aug 6, 2003
Posts
2,927
Location
Cochise, AZ
Two things come to mind:

1. Are you measuring AC current on the heaters?

2. Disconnect the heater wires from the pilot light and see if there is a short to ground on the pilot assembly. I recently found one that did this with a new assembly.

BTW the filter caps show bulges.
 

Lowerleftcoast

Poster Extraordinaire
Joined
Dec 30, 2019
Posts
5,540
Location
california
I noticed the AA763 does not include *Balance Resistors* on the in series wired 70uF reservoir caps. Betterer practice would install them. Below is the AB763 showing the balance resistors in the red circle. Since you are replacing the filter caps, it is a reasonable upgrade. Each resistor's value can be anywhere from 220k to 470k. One Watt is fine.

FYI:
The Fender amplifiers, near this vintage, that had an artificial heater center tap, would generally install two 100R resistors from the pilot lamp terminals to ground along with the bias ground point. Of course the artificial CT can be installed anywhere along the heater wiring and a humdinger pot can replace the 100R resistors if you choose. Below I show the absence of a hardwired heater CT with a green X.

If this were my amp, I would leave the later model PT in place. OMMV.

InkedAB763_Bndmstr_Schm1_LI.jpg
 

Lowerleftcoast

Poster Extraordinaire
Joined
Dec 30, 2019
Posts
5,540
Location
california
What would you do from here?
From what I see in the pics, this will never be a museum piece. You have described a players amp... "dragging this thing on the road".

I would replace all of the electrolytic caps.
Depending on how I want the amp to respond (touch sensitivity vs tighter bass), I would *consider* changing the values of the filter caps. That said, chances are I would stay with essentially the stock values but modern equivalents (modern 80uF replacing the original 70uF and modern 16uF or 22uF replacing the original 20uF).
I would up the value of the cathode cap on V3 pin3. 100uF to 250uF would provide lower frequencies which might allow the (vibrato) oscillation of the tube to start quicker. There would be no change in sound.
On some amps the cathode voltage on V3 pin8 can rise above 25v so I would use a higher voltage cap there.

As Jon Snell wrote, the plate resistors. Basically, after 60 years, all of the CC resistors are suspect. Besides the plate resistors... the dropping resistors, 470R screen resistors and input 1M and 68k resistors are prone to failure. I would check all of those carefully. I am not advocating changing out all of the resistors.
Many times I replace the plate resistors with new (not new old stock) 1/2W CC resistors. Otherwise I use metal film.

The SS rectifier diodes from those years are prone to failure. Diode technology has improved. The pictures look like the diodes may have been replaced already. The ones I usually see, look like silver metal bodies. In any case if they are old you might consider replacements.

I like seeing the domino coupling cap on the board. The schematic shows it to be 500pF. You might like a larger value there. It should give more *girth* with a larger capacitance. Try clipping in another 500pF cap in parallel with that cap. If you like what you hear, solder in the parallel cap. Iow, keep the domino for looks but increase the capacitance.

I would consider changing the ceramic disc capacitors carrying the signal for silver mica. The 250pF tone stack treble cap and the treble bleed caps. (Leave the vibrato ceramic disc caps in place.)

I would add a snubber cap (47pF to 100pF) on the plates of the phase inverter. This will help prevent ultra high frequency oscillation. It will not change high frequencies that can be heard. It can be installed on the board at the terminals where the PI plate resistors are. An example can be found on the 5F6A Bassman schematic and layout.

Check all the switches, jacks, pots, and sockets and replace as necessary.

If you decide to solder grounds wires to the chassis, make sure the connections are good.

Anyway this and the bleed resistors described in post #14 should make for a reliable players amp.
 

aaronlowther1993

TDPRI Member
Joined
Sep 4, 2016
Posts
68
Age
29
Location
Claremore, OK
Two things come to mind:

1. Are you measuring AC current on the heaters?

2. Disconnect the heater wires from the pilot light and see if there is a short to ground on the pilot assembly. I recently found one that did this with a new assembly.

BTW the filter caps show bulges.
Yes I am measuring AC.
 

schmee

Telefied
Silver Supporter
Joined
Jun 2, 2003
Posts
20,231
Location
northwest
I recently purchased three 60's Bandmasters from a local vintage guitar shop. Two AB763, one AA763. I've been in the process of doing some of @robrob 's wonderful mods to each of them along with a Quad Reverb I bought. All are done except one.

I've opened up the last Bandmaster (the AA763) and I'm seeing some things that are concerning to me. I played it before buying it and the amp functioned. However, upon opening it up things are a bit strange. It's obviously been serviced by someone with even less know how than myself. Two power transformer taps are "grounded" by being shoddily soldered to the chassis; Same with doghosue ground. When I pulled the fuse someone had put a 20 amp fuse in. I've swapped to the correct fuse, pulled the death cap and put on a grounded cable. I was about to move on to the next mod when I noticed that when someone re-ran the heater wires they accidently connected Pin 2 to Pin 1 on V6. Pin 1 is tired to ground. I checked the schematic and layout and this looked wrong so I remedied it and decided to check the heater voltage. Between Pin 2 and Pin 7 I get about what you'd expect: 6.6 volts AC. A little high, but, nothing crazy. But between either side of the heaters and ground I get 179 volts when I would expect to get 3.3.
Looking at the caps in the dog house, they absolutely need to be replaced, but none of them are leaking or turning to fluff. But, I'm not sure if there's something else really wrong here. I'd love to get some help here, as while I'm far more confident than I used to be with this, this is getting into an area in which I don't feel super comfortable.

Looking at the power transformer, and checking it's part numbers, this is a Blackface Twin power transformer, not a Bandmaster one! It's an 022756 instead of an 022814. The choke and OT are correct.

Obviously changinging out the PT is in order, right? What would you do from here?
Well, if you are gonna have 3 BM's, then having one with a bit more headroom via the big PT isn't a bad thing. The chassis has been carved, so if you put in a normal PT there will be a gap etc...
I would replace all the paper elytics and those 470 ohm resistors on the power tube sockets Using 2 watt metal.
 

Wally

Telefied
Ad Free Member
Joined
Mar 17, 2003
Posts
41,757
Location
Lubbock, TX
That TR Power transformer does not have a center tap for the heater winding, so heed Jon Snell’s advice on establishing a virtual center tap.
Kudos for being cautious. Someone who should not have been in any amp was in that one. +1 on replacing all electrolytics.
 




Top