'67 Bassman Resurrection

['66Cuda]

I am trying to resurrect this 1967 Fender Blackface Bassman for a friend. I have spent weeks researching as much as I can on this amplifier. I have searched dozens of photos of other Blackfaces and can't find one that looks like this one in respect to all of the chocolate drop caps. I have seen plenty with the blue Astron caps with a couple of the red/brown chocolate drop caps dropped in. But I have never seen one with this many chocolate drop caps. Plus, it appears that some of the caps have been changed, while the large filter caps are all original and mostly bad. So what do I have here? Are these bypass caps original? Did Fender switch part bins in the middle of this amp? Why only a couple of the Astrons? Has this thing been modded somehow? My experience level would not allow me to spot it just by looking at it. One other thing....no it has not been in a house fire! Yes...that black stuff appears to be from this thing getting so hot! Anyone care to offer their years of experience to someone trying to learn?

[eschertron]

I'm no expert, but it looks like tool marks on some component leads, and some non-original soldering. So I'd say you are right that it's been fixed and/or modified. You'd do well to draw up the schematic, noting possible differences against a diagram corresponding to the model stamp on the inside of the cabinet. See how many component values you can read (meter) in-circuit, and if you are replacing the PSU caps, other components may become available to read while the power supply caps are out.

What's wrong with it? Does it not power up? Make noise? Not make noise?

Do you have test instruments? What's your previous level of involvement with electronics? High-voltage circuits? A little background on both you and the amp will go a long way towards getting targeted help with the repairs.

Oh, and welcome to the place

[Justin Thomas]

I would not put too much credit towards the cabinet stamp - Fender is infamous for using wrong or old circuit stickers. They make a good "at-a-glance." But don't be surprised if component values aren't EXACTLY right like on the schem. Given your tranny codes, I'd say it's a late 66 or early 67. I've got a BF 67 with loads of Blue Mallorys. As far as a schem goes, look for an AB165 as the first go-round. Whatever circuit is is or ends up being, if you stick with a 50W Bassman head circuit, it'll be worth it.

Justin

[TigerAmps]

Restore ...

I would replace the following:
1) Silver bias cap (50uF/60V) near the power transformer - use a 50uf/100V
2) Under the cover on the tube side of the chassis are the power supply caps; I would replace them.
3) The two black caps on the input side of the amp; I would replace those as well
these are axial multi-section caps (2 caps in one with a common minus/negative) - use four 25uF/25V
4) The dull brown "chocolate caps" I would replace with new "orange drop" type caps.
5) I would check all the resistors with the meter; the 100K plate resistors drift and get noisy with age.

A number of thing will make it run "hot" - bias; base power supply caps; etc.

I would start there starting with #3, #1, then the rest.

Take pictures and make a diagram as to the orientation of components (+/-) and where wires go.
It's not difficult, just be carefully and don't rush.

You can get these components from newark.com, mouser.com or any number of other places.

['66Cuda]

Thanks. I am a hobby fixer with some experience in tube radios and have built my own 5E3 from a kit. I am approaching this from the angle of an AB165. I am waiting on the 22mf caps to arrive and then will recap the top side, plus the bias and multi-level 25/25s on the bottom. I did find that one of the 6L6 tubes tested no good. I will retention the sockets and power up slowly after the top side recap. Amp has sat for over 10 years, not working, so I have no idea what could be wrong. I will bring it up on a Variac and current limiter and see what happens. The digitized schematics are really hard to read, once printed off. I hope to find a good one. There seems to be some fairly broad component drift with some resistors testing 40% or more out tolerance. I will wait to see what it sounds like before I start replacing all of the resistors. I ordered Mallory 150's to replace the chocolate drops, but I have Orange drops on hand. I had read where they were fairly sterile in some Fender amps, so decide to go with the Mallory caps that are recommended so often. Personal preference I guess. My main concern that lead me to post was that I wasn't able to find any other '67 era Bassman images with so many of the drops in it. It made me question whether I actually had an AB165. This thing definitely went through a rough spot somewhere. None of the components visibly appear to be toast, so maybe there is hope for it. I will post after I get it recapped. Thanks!

[Justin Thomas]

The negative feedback loop and the OT wiring will probably be the giveaway. Looks like your negative feedback resistor is 47k? Also, you have a small ceramic cap on the plate load resistor of V1 & V3. Also, I can't tell from your first pic, but there are 2 220k resistors between (electrically, not physically) the 6L6 plates and the grids of same tubes? Also - if your green wire of your OT goes to the sleeve of the speaker Jack & black goes to tip... No other BF Bassman has these features, and the 220k local feedback resistors are unique to this circuit.

It's an AB165. Don't get caught on what specific types of caps are in it or why. Fender by this point was buying caps by the bazillions, and looking for the cheapest prices possible. Someone completely recapped my 62 Concert with modern "junk" and gave it four Celestions. It STILL sounds like a Fender, and a damn good one, at that. I like your choice of Mallorys for this amp, and they're half the size of ODs, too. You'll have an awesome amp by the time you're done.

Justin

[g1]

4) The dull brown "chocolate caps" I would replace with new "orange drop" type caps.

Curious as to why you would replace without checking if they are leaky. Are they known to have leakage issues more than other types?

[The Dude]

I'm tempted, but

[eschertron]

Curious as to why you would replace without checking if they are leaky. Are they known to have leakage issues more than other types?

I'm with G1 on this. Until it has been determined that the unit needs to be wholesale rebuilt, I'd look for problems and fix them. Having said that, if I expected a total rebuild, for myself I'd likely pull the eyelet board out and replace it with a new to-spec board. That would only be if there seemed to be gremlins that couldn't be located. Maybe if the board is conductive, or otherwise damaged. From the pictures, I don't think that's the case, though.

For the knowledge gained by reconditioning and troubleshooting, the technician has to confront each problem and find the fix. Anything else cheapens the experience

['66Cuda]

Thanks for all of the input.

g1 the chocolate drop caps are "known" to be among Fender's worst caps. Assuredly, I will test everyone of them out of circuit, just to see what is going on. I will still probably replace them, as it is a widely recommended upgrade and is fairly inexpensive to do. Of course which cap to use is like asking the age old "Ford or Chevy?" I am not looking to wholesale rebuild, just replace known problems right off the bat, before I ever even try to power up, so I don't damage any of the new components or power tubes.

Tiger Amp that is exactly my plan, only I am going to try and power up before I start replacing the coupling caps. I have checked several of the resistors...more than 10 or so that are way out of spec. My thought was to listen to the amp first and see if we liked the sound. I know many people (Buddy Guy I think) clone amps based on drifted values and not stock values. So this is something I might be able to learn from, but is a ways down the road.

['66Cuda]

For the knowledge gained by reconditioning and troubleshooting, the technician has to confront each problem and find the fix. Anything else cheapens the experience

It certainly does!

[Justin Thomas]

I think all these amps need is a bias adjust IN ADDITION TO the balance. Here's another reason not to mess around with the innards any more than necessary - they're very picky about lead dress. Don't ask how I know. And yes, they make awesome bass amps, when coupled with efficient speakers designed for all-purpose use.
This is all my humble opinion. If you can't tell, I really love these amps.

Justin

[Richard]

I'm tempted, but

Your signature says it all.

[Tom Phillips]

Thanks for all of the input.

g1 the chocolate drop caps are "known" to be among Fender's worst caps. Assuredly, I will test everyone of them out of circuit, just to see what is going on. I will still probably replace them, as it is a widely recommended upgrade and is fairly inexpensive to do. Of course which cap to use is like asking the age old "Ford or Chevy?" ...

People do say that the brown caps are no good but I have never heard the proof. My theory is that they look like shit AND they showed up in the Fender amps when the popular opinion was that the overall sound performance was taking a turn for the worse. However, I don't think it was those specific caps that were the cause of the problem. Just my opinion and I acknowledge the fact that the debate about the sound of various types of capacitors will never end.

In the case of the brown drop caps it seems a bit like like looking at two cars. One has old badly oxidized brown paint and the other is clean shinny orange paint. Right off, everyone seems to think that the performance of the brown car is inferior even before they drive it.

[g1]

People do say that the brown caps are no good but I have never heard the proof.

If it's a sound issue, then that's fine. As you say, the debate will never end, so no need to get into that here.
But if it's a leakage issue, I'd like to hear about it.
In my experience with Fender amps from all decades, no particular type of cap (non-electrolytic) has stuck out as being more problematic than others.