68 Fender Bandmaster Reverb help needed

[Stratz]

Hi all, I'm pretty much a novice when it comes to amplifiers.
I do bias my Marshalls and have done small mods but I cant troubleshoot and I dont know much about tube amp design.
I have a 1968 Fender Bandmaster Reverb in nice shape that is acting up. For a while it would drop back to lower power/volume when hit with a power chord and now it seems to be stuck at 1/2 power or so (lower volume and pre-amp tube distortion).

From what I've read the cathode bypass caps may be a good place to start. It has 2 newer caps in the power supply (75uf-450v, is this value too high?), the other three are original Mallory 20uf/500v

The cathode bypass caps are not what I was expecting to see. I though they would all be 25uf/25v axial but they are 25-25-25v. Can these still be found as I think I need to replace them???

One of the other issues is some leakage from one of the chocolate caps. I cant tell which one is leaking so is it advisable to just change them all? There is one .01/600v that looks distorted (see picture). Are these polarized caps?

I'd really like to repair this amp myself this time so any help is certainly welcome. I also need to change the power cord cord to a 3 prong so if anybody has instructions on this I'd appreciate it.
I've attached some pics. If anybody can help with these issues, sourcing parts or notices anything out of the norm please feel free to speak up.
Thanks
Rob.

[The Dude]

In no particular order:

1) The cathode bypass caps are simply 2 caps in one with a shared ground lead. You can replace them with 2 separate caps.
2) Before you go all crazy swapping parts, have you tried swapping any tubes with known good ones?
3) Definitely replace the power cord and remove the death cap so the amp is safe. (picture below)



Also see this:

http://www.tdpri.com/forum/attachmen...ong_layout-jpg

[Stratz]

Sorry about that. Yes, I did replace the tubes with known working tubes. Same problem.
The only tube I didn't replace is the GZ34 rectifier because all I have on hand is the 5U4GB and I dont know if they're compatible.

Is the death cap the 50uf 50v in this pic? Theres also another smaller silver can cap behind it but I cant read the value. Does that stay???
Thanks very nuch for the quick reply!

[Justin Thomas]

Also, F&T makes a dual-cap like the old Mallorys. It can occasionally be difficult to get the leads of newer caps in the eyelets, as they are thicker and you have 2 ground leads now. If you're not afraid of using "cheap" Nichicon/Panasonic/etc. radial lead caps, you probably won't have that problem. The F&T are a couple bucks a pop, as opposed to cents.

https://www.tubesandmore.com/products/C-EM25-25-25

Justin

Ed i t: No, that's not the "death cap." That will be somewhere near where the power cord enters the chassis, probably mounted on the Polarity switch if it has one.

[The Dude]

No, that cap is the bias filter. The death cap will be tied to the ground switch in the rear of the amp and the other side connected to the chassis.

[Stratz]

Thanks Dude and Justin.
I think I'd rather go with doubling up the 25uf 25v caps. It's a lot less money.
Do you think the 50 year old cathode coupling caps could be causing my low output/distortion problem???

Thanks for the updated 3 prong layout. I'm going to use it. First I need to figure out the colors on my new 3 wire cable.
I have Brown, Blue and yellow with green stripe.

[The Dude]

Power cord
BROWN = HOT
BLUE = NEUTRAL
GREEN/YELLOW STRIPE = GROUND

On the caps: Hard to say if that's your problem. I normally just check them. If you don't have a way to do that, it won't hurt to replace them given their age.

[52 Bill]

Can I ask if you have done any testing of the amp or of the components that you are planning on replacing? I wouldn't even replace the power cord until you have fixed the volume problem with the amp.

From your questions, it seems to me that you really don't have much experience in these things and are trying to fix this by replacing a bunch of parts whether they are bad or not. Please think about this and try and take the time to take voltage readings and other measurements to find the source of the problem first. Then replace things that you have read that should be replaced because they are the wrong color or the wrong age.

Have you tried to mechanically press or vibrate the fiberboard to get the volume to change? One of the first things that I look at is the 100 ohm resistor that grounds the phase inverter circuit. It is the resistor that runs parallel to the length of the board. Often because of the physical orientation, one end of that resistor will come loose and it will cause a loss of volume.

[Stratz]

Hi Bill. As mentioned in my first post I am a novice but not completely without some experience with biasing and modding a couple of my Marshalls. I haven't taken any live voltage readings but I've been measuring components along the way.
The only problem is that I dont know what to expect since the components are soldered to other components.
The 100 ohm resistor you mentioned reads 96 ohms.
I tested all three of the original 25uf-25uf-25v cathode bypass caps with an Extech LCR meter and they all read ~36-45uf.
The single 25uf 25v cap reads ~25uf
Problem is I dont know if the readings are good since they're installed along with other components.
I tested the screen/grid resistors and all are in spec. I certainly dont want to replace any parts that I dont have to.

Thats about as far as I've gone. I'm trying to learn how to do this myself because while I dont mind paying a good tech for their services I do not like waiting for weeks or months to get my amps repaired and I own quite a few old tube amps.

I have excellent soldering skills and I know about the electrocution hazards and how to drain the caps down so I figure that's the most important part and possibly, with help from the guys here I may just be able to get this old Fender working properly again.
Thanks
Rob.

[52 Bill]

The 100 ohm resistor you mentioned reads 96 ohms.

The value wasn't my point, but the fact that that specific resistor will break loose at one end and stop making contact.

Testing parts with a meter is fine, but it will not show how those parts react when a high voltage is applied. If you don't feel safe in taking live voltage readings, then perhaps you can get a set of test leads with clip ends and only power up the amp after the leads have been clipped into place.

Do you feel comfortable in using the chopstick method with the amp powered up?

[Stratz]

The value wasn't my point, but the fact that that specific resistor will break loose at one end and stop making contact.

Testing parts with a meter is fine, but it will not show how those parts react when a high voltage is applied. If you don't feel safe in taking live voltage readings, then perhaps you can get a set of test leads with clip ends and only power up the amp after the leads have been clipped into place.

Do you feel comfortable in using the chopstick method with the amp powered up?

Sorry about that bill. I'll check and make sure it's connected or even re-flow the solder joints.

I dont have any problem poking around with chop stick or taking live voltage readings, I just dont know where to take them. This is what I would like to learn. I've been servicing my tube amps for nearly 40 years. I just dont have any formal training or a very good understanding of circuits. Perhaps it's too late to learn. Well see.

[The Dude]

Those caps seem fine. "1/2 power...distortion" makes me think, for whatever reason, one of your output tubes is not working or not being driven (a PI problem). That said, I'm just guessing a bit. I would start by checking voltages on your output tube sockets. If those look good, I'd check plate voltage on all of your preamp tubes. That will at least get us started and may get us going in some direction. There are any number of things that could cause your symptoms.

Edit: It's never too late to learn. I still learn something every day.

[Stratz]

It's never too late to learn. I still learn something every day.

Like it or not

I'll need to find a chart or something that gives me the pins and voltages. The Bandmaster Reverb isnt nearly as common as other Fender 40w amps so it's hard to find a lot of information. I believe the closest design is the Super Reverb.

[The Dude]

http://www.thevintagesound.com/ffg/s...1069_schem.gif

Schematic with voltages. Remember, all voltages approximate. You are looking for something that's way off. Don't worry about a few volts difference here or there. As far as pin numbers, you can just google (for instance) "6L6 pinout" and find diagrams.

[Stratz]

Thanks Dude. The power tube sockets are numbered so I shouldn't have too much trouble. It's the driver tubes I have a bit of trouble with. 1/2 the tube and such.