Fender Bassman 1968 silverface

[soundmasterg]

I can't add much to what Mark pointed out. There are a couple things however.

The amp has been modified from stock. There are some wires with another wire twisted around them in a circle. This was done to stop an oscillation of some sort, probably due to the poor CBS circuit and layout choices. If the amp doesn't oscillate then I'd leave it alone, but there are better ways to fix the problem than what whoever did with that wire loop.

The heater wires should be pulled up as high as you can get them and still have them connected and in the chassis. The grid wires should be pushed down against the chassis. This will keep them away from each other as far as possible with the circuit layout.

That 80uf cap should be replaced with a new one....anything close in value is fine. Check to make sure the bias is set correctly. If it is too hot (too much current) the amp can hum more than normal. The amp may just have a bias balance pot instead of a true adjust pot. If so, adjust for least hum and live with it.

Checking grounds throughout the amp is a good idea. Anything reading over .3 ohms should be resoldered.

Aside from that.....can't think of anything offhand at the moment.

Greg

[thebassman]

Hi Greg
is the twisted wire arrangement the wires shown in 06-141-8.jpg. What other ways are there to sort out the oscillation.

For the heater and grid wires; is there some kind of shielding that can be done to keep them seperate.

thanks

martin

[soundmasterg]

Hi Martin,

The twisted wire thing I was talking about is indeed in that photo. The problem with the CBS era Bassman's was that they mounted some of the parts to the board incorrectly, and the grid wires in that section of the circuit end up being quite a bit longer than they would need to be if the parts are mounted differently. If the parts are switched around, then the plate wires end up being long, but that is ok, and the grid wires end up being short, which is much better. Long grid wires act as antennas and are susceptible to what is around them in the circuit layout. If the amp was changed to the correct layout, then those twisted wires could be removed and the amp would work in that area of the circuit without oscillating, unless something else is wrong. I forget off-hand which parts are the culprits, but it is mentioned in the Dan Torres book I believe. I'd have to look again when I get some free time.

For heater and grid wires, the layout rules are this:

Keep them seperate from each other, keep heater wires twisted around each other, if the wires have to cross, have them cross at right angles to each other. Keep grid wires as short as possible.

In Fenders the heater wires are installed last and should be position up in the air, while the grid wires should come off the board and lay flat against the chassis. You can take a wooden chopstick and poke around while the amp is running and hooked up to a speaker, and you will hear differences in hum level depending on where wires are moved, and if you get the chopstick in the right areas like towards the input, it will increase the hum level just by having the chopstick there. If you do this, be very careful however. The high voltage in the can kill. Only one hand in the amp at a time, and the other in your pocket or behind your back is a safe rule to follow. There isn't really any shielding that can be done on those wires except in the case of the grid wires, you can run shielded cable. It isn't necessary however. If you decide to use it, the shield should only be grounded at the input end and the other end should be cut off and insulated with shrink wrap. Fenders almost never used shielded cable.

Greg

[thebassman]

Hi Greg
Thankyou for the information.
So are the heater wires the grey twisted wires running to all the tube plates?
What colour are the grid wires?

[soundmasterg]

The heater wires are usually green, and all the rest of the wires in most Fenders are yellow, but you can't really tell sometimes since the colors fade over the years. You need to study up a bit on the tube pinouts and see which pins have which function for the various tube types. This is the only way you're really going to know what wire is going where. Once you have this knowledge, you can trace a wire from one point to another and know what and where they are going. The tube plates are pin 3 on the power tubes in your amp, and are not the same as the heater wires.

As an example, for a 12AX7 preamp tube, pins 4, 5, and 9 are for the heaters. Usually, it is wired such that pins 4 and 5 are tied together, so you have a wire going to pins 4/5 on one tube, and pin 9 on the same tube, and then another wire going from each of those to the same point on another preamp tube, or to different pins on the power tubes. The heaters are wired in series in Fenders so you always see a string of wires going the length of the tube sockets. The sockets are keyed and you read the pin numbers clockwise from the key when looking at the circuit side of the sockets. Larger octal sockets for the power tubes have 8 pins, though not all of them are connected. Some octal sockets are used for a tube rectifier and only have 4 or 5 of the pins connected. The preamp tubes have a different socket with 9 pins, though again, not all of them may be connected.

It sounds like you need to get some knowledge about amps in general, so I'd suggest to read some books. There are some free on this website:

http://www.pmillett.com/technical_books_online.htm

The Jack Darr Handbook is a good place to start and I think it is still on there. If not, you can purchase it, along with many others at Antique Electronics Supply. (www.tubesandmore.com)

Some good ones to get started with are the Gerald Weber books (three of them), The Dave Funk Tube Amp Workbook, and as you learn more, the whole "The Ultimate Tone" (TUT) series of books by Kevin O' Connor. Another very good one to get is the RCA tube manual, though I think you can download it from Millet's site above. It will have all the tube data and pinouts.

Greg

[BlueAngel]

You should just replace all of the electrolytic caps in the amp. They have a finite lifespan (usually 10-15 years)

Just reading through this thread again, and although I don't have much to add to the great advice here already, I would mention this. The lifespan of electrolytic caps is FAR longer than 10-15 years - I've seen this written many times but it's a serious exaggeration of the decay rate (with the possible exception of an amp that has sat genuinely unused for almost all that time) and will cause people to panic unnecessarily about getting a cap job done. Of course caps do fail at (or before) this sort of age sometimes, but these are isolated, one-off failures in the same way as other components can do - I consider these 'manufacturing flaws that take time to show up' rather than 'age-related failures'.

The typical lifespan for electrolytics in a guitar amp is at least double that, 25-30 years or so in my experience. It's only when you get above about thirty years that the failure rate really increases noticeably, and even then only for older caps - the technology has improved over the years and caps made in the 50s, 60s and 70s would have had a shorter life than those made in the 80s and 90s. I would not do a routine cap job on any amp made in the last 20 years, unless there was definite evidence it was needed - ie more than one cap failing. How many people have cap jobs done on their amps at only 10-15 years old? And how many amps actually fail as a result of the fact that they haven't been? Next to none on both counts - so few that you can basically ignore it.

My general approach is:

Under 10 years: replace only the failed cap.
10-20 years: replace only the failed cap unless it makes economic sense to do them all at once (eg amps where dismantling/re-assembly is a major part of the cost) or if more than one has failed (which tends to indicate lack of use/bad storage, or low-quality caps in the first place).
20-30 years: replace all caps if any one has failed.
30-40 years: replace all caps if any other substantial work needs doing, even if no caps have failed.
40+ years: replace all caps, even if nothing else needs doing.

Even then I come across a lot of amps of 40 years old or more with their original caps and no signs of problems. (Which doesn't mean I don't change them though.)

It also depends to some extent on the type of caps - the radial cans used in (eg) Marshalls seem to hold up better than the axial tubes used in (eg) Fenders. Modern axials seem to be more consistent with the radials.

[sharkboy]

I mostly agree with you, but it is about the first thing I suspect if an amp is having problems and nobody changed anything in it. I'd have to think about it, but I think I've had more caps go bad than tubes from normal use.

And there's nothing like cap yak inside an amp.

[BlueAngel]

I mostly agree with you, but it is about the first thing I suspect if an amp is having problems and nobody changed anything in it. I'd have to think about it, but I think I've had more caps go bad than tubes from normal use.

With older amps I might agree, the old tubes were great and the old caps have deteriorated, but anything made in the last twenty years will have ****** new-production tubes in (unless the owner fitted good ones) and the failure rate on these is dreadful. I've had a few cases of failed caps in modern amps too, but definitely 'manufacturing fault' rather than 'age-related'. The worst offenders seem to be Fenders from the red-knob and more recent series - they're just bad caps, not old.

And there's nothing like cap yak inside an amp.

I know... worst of all is if you decide to (even carefully, using a Variac) power up an old amp that hasn't been turned on in years to assess it before changing anything, and a cap blows up in your face. That's happened to me a couple of times... not nice.

The best way to get rid of it is actually to put the whole thing in the shower and hose the stuff out of it with hot water - it's water soluble. Then obviously let the amp dry out properly before you do anything else!

[thebassman]

Thanks for all your advice guys.
I have the last few parts now and i am planning on getting it finished this weekend.
I will give it a test after this and let you know the outcome.
Cheers
martin

Just another quick question.
I have had to remove all the old tolex as the wooden cabinet was falling apart and ripped the tolex.
I have taken the plastic vent grills off of the end faces and they have aged quite badly, the plastic has cracked around the fixing screw positions.
Does anybody know whether i can get replacements? I have tried Tube amp doctor and Antique electronics, but no joy.

[BlueAngel]

Just another quick question.
I have had to remove all the old tolex as the wooden cabinet was falling apart and ripped the tolex.
I have taken the plastic vent grills off of the end faces and they have aged quite badly, the plastic has cracked around the fixing screw positions.
Does anybody know whether i can get replacements? I have tried Tube amp doctor and Antique electronics, but no joy.

You're probably out of luck unfortunately. As far as I know those grilles are unique to late-70s/very-early-80s Fender heads.

If the cabinet is in that bad a state, you may be better trying to find a complete replacement one in better condition - even if it means buying a whole dead amp, possibly one that's been hacked up. The rest of the parts - handle, corners, feet, knobs and all internal components - are quite valuable as spares to anyone restoring an old Fender, so you should be able to sell them, possibly for a profit on the whole deal.

[thebassman]

Hi
i have finally got all of my pars changed.
I ended up fitting a full set of pots, volume, and tone.
The weird thing is channel 2 sounds pretty much silent (no background noise or interference) but channel 1 is still very noisy (permaneant background noise and made worse when the tone pots are moved, they still sound scratchy). How could this be?
Does the transformer run both channels of the amp seperatley? or could it be just the wire routing or some other component?

Please help as i am gradually running out of ideas and patience.

[thebassman]

One more thing;
i am thinking of having the transformers re-wound.
Has anyone else had this done?
Is there any transformer scematics available?

[soundmasterg]

I wouldn't get the transformers rewound or consider replacing them either. They aren't the problem.

Scratchy pots means a DC voltage is on the pots, most likely caused by a coupling cap leaking DC. This type of thing can be found with some specific testing, but you need to be very careful if you decide to do it yourself. If you think you want to try, then I can post how to do it. If you don't want to try yourself and ahve a tech do it, then suggest to a tech that a coupling cap is leaking DC on the pots on Channel 1 and he/she should know what to check for. It could also be an oscillation of some type, and this can be found by testing also, but requires specific tools such as an oscilliscope for instance.

Greg

[BlueAngel]

I have a Silverface Bassman 70 here with the same problem - not just scratchy when turning the pots, but constant pop-and-crackle background noise too, only on the Bass channel. There is some slight measurable DC on the tone stack, but not much. I've tried changing the coupling caps, of course. It's not the plate resistors either, which are a common cause of noise problems on old Fender amps. I suspect it's the increasingly common and very nasty Eyelet Board Conductivity Problem.

This occurs where the fiber eyelet board - which is waxed to keep out moisture, but often not very well - begins to degrade and develop very slight conductive paths through the material which like all organic substances is carbon-based. You won't be able to measure the resistance - it's extremely high - but under several hundred volts of the B+ supply it will leak tiny currents between some of the eyelets and cause all sorts of problems, including noise but also sometimes just odd tone and occasionally instability.

Sometimes it's caused by the insulating layer under the eyelet board itself (which touches the eyelets underneath) and if so you can fix it by inserting a new layer of insulating material - plain cardboard works well - between the two. It's not that with this one though. At this point there's no solution other than replacing the board - I've even tried reheating it (with a hairdryer!) and adding more wax, but actually that made it worse, which pretty much confirms it's the problem.

[thebassman]

Mine does sound the same as the normal (channel 2) side is virtually silent, all the pots are silent when moved, but the Bass instrument (channel 1) side is very noisy when static and all the pots are scratchy when moved.
Which is the coupling cap, what is it's value? the only reason i ask is that i am pretty sure i have changed every single cap in the last month.
The only item i could not re-source was the little silver can, just by the rectifier diode group. i think on the schematic it is stated as 0.5A diode.