Fender Bandmaster (AB763) issues

[Bernardduur]

Hey all!

I got an old Fender AB763 in that belongs to a blues player I worked for before. He brought in his main amp now as he always complains that the amp sounds bad. He once bought it a while ago from an anonimous seller.

Inside the amp looks like a normal bandmaster although most of the caps are replaced with cheap cheap caps. Values are not of a Bandmaster but from a Vibroverb. The tremolo (Vibrato ahum) was disconnected and turned into a mid control. The PSU caps are all on the urge of blowing as they are rating far under the voltage of the amp

My main issue is the output transformer. I've seen some old(er) Fender amps but I never saw an output transformer that looked like this. It looks more like my old ELK amp OT, it falls apart a bit and bears the numbers: OT - 5A next to the normal p1 / p2 / B, 4 ohm and 0 (post some pics later).

Is this the normal OT for a Fender Bandmaster? Or is it replaced with something else?

Thanks!

[Garydean]

The original OT should be a 125A6A. They are not very beefy which is why you may think it's been replaced. Although, because of it's size many have been replaced by now. Some people replace it with a Bassman OT with little modifications. This mod improves the sound and allows more of the bottom end thru. Sounds like a cap job is more in order for your amp.

hope this helps

Gary

[Bernardduur]

Thanks!

I'll post some pics later; wanna know if it is a better or worse OT they put in there

Cap job done; sure sounds a lot better now.............



I noticed one thing though

With the stock tubes (that were put in there a short time before by the owner, a set of Sovtek 12AX7WB's) the heater voltage on the lamp is 6.33V AC, on the preamp tubes it is 4.56V AC..........

??????

When I replace the preamp tubes all points give the same voltage (6.33V AC); still it is weird! Bad preamp tube?

[MWJB]

"When I replace the preamp tubes all points give the same voltage (6.33V AC); still it is weird! Bad preamp tube?" Possibly, pull all the preamp tubes & replace one by one measuring AC as you go, see if the low voltage follows a tube or a socket.

6.33VAC sound rather low for a mid sixties Fender with the original PT? What kind of plate current are the power tubes drawing?

[Bernardduur]

"When I replace the preamp tubes all points give the same voltage (6.33V AC); still it is weird! Bad preamp tube?" Possibly, pull all the preamp tubes & replace one by one measuring AC as you go, see if the low voltage follows a tube or a socket.

6.33VAC sound rather low for a mid sixties Fender with the original PT? What kind of plate current are the power tubes drawing?

I tried all the old tubes in various sockets and all give the same result. 1 preamp tube (sovtek) gives a drop to 5.2V AC. When I use my "old" leftover tubes it stays at 6.38V AC

I run 2 Ruby 6L6GC's in the power section. I run em now at 44 mA with a voltage over the plates of 417V DC. The bias trimpot is at max with this setup. The voltage selector sits at 230V AC

[MWJB]

"I run 2 Ruby 6L6GC's in the power section. I run em now at 44 mA with a voltage over the plates of 417V DC. The bias trimpot is at max with this setup. The voltage selector sits at 230V AC" Drop the current back to 35-40mA and see what heater voltage you have (you may need to increase the value of the resistor grounded from the bias trim pot).

Are the filter caps new (less than 20yrs old)?

[Bernardduur]

With a lower current the heater voltage stays the same.

Filter caps are brand new.

(is a lower current better in tone / feeling / etc? I always take 70% of the 30W the tubes can give)

[MWJB]

The tubes should sound fine from 30mA up, few modern production 6L6 are really 30W tubes, assume 25W in the absence of hard figures from the manufacturer.

The low heater voltage is a little odd, it would have been 10-14% lower when the amp was built, which doesn't seem right. It makes me suspect something is loading the B+?

Power down & unplug from the wall, connect an ammeter accross the standby & power up, what does the ammeter read?

[Bernardduur]

The meter measures an infinite resistance

From V+ to ground I measure a fast increasing resistance until the resistance is infinite (>20 Mohm)

I set the AC selector switch to 230V;

[MWJB]

We're meant to be measuring dc milliamps, not ohms.

[Bernardduur]

We're meant to be measuring dc milliamps, not ohms.

Ah, now I get the question; it measures 93 mA

[MWJB]

OK, that looks to be fine.

[Bernardduur]

Ah well, she sounds pretty nice now

Picture of the OT

Is this the original Fender Bandmaster OT or something else?

[MWJB]

OT & choke are non original, what's the PT like?

[Bernardduur]

Thanks!

The PT is still original.