1960 Bassman Weird Oscillation

[Elzorro]

I've been lurking for a while. And reading everything. But I have a problem I can't find an answer for.

I have a 1960 Bassman that sounds wonderful except for one issue.

With the channels jumpered.
Normal volume past 5 and bright volume anywhere above 4.
There is a strange, almost echo type oscillation. If you strike and quickly mute a note you can hear it go Chunk....... chunk ..chunk or something like that.

If you turn the Normal volume up more it gets more severe and as you turn both knobs up more it will become self sustaining as a low thump tap type of thing like 5 or 6 beats per second.

It does not happen when you turn the bright volume all the way up as long as the normal volume is below 5.

It can be heard with volume as low as - bright volume 3 - normal volume 5

I have swapped all preamp tubes.

Like I said the amp sounds wonderful as long as I don't turn the normal volume up past 5.

Does not happen with out channels jumpered.

I have searched, but I don't even know what to call this problem so I don't know what to search for.

I would love to crank both channels wide open.

Thanks for any advice I can get,

joe

[Bob M.]

Hi Elzorro,

I would go over everything in the 1st stage of the amp with a fine toothed comb. Check your grounds, especially the volume control grounds. I would pay special attention to the 1st stage cathode bypass cap and if it's not brand new consider replacing it. Informed techs sometimes think that 250uf cap is too much in that position. As an experiment you might temporarily try a 25uf/25v cap or even a 10uf cap and see if that helps your particular complaint. Really, there's no problem getting beaucoup bass from this amp. Otherwise, I guess you might forgo bridging the channels. Also, check the filter grounding as Leo's filter ground scheme for the 58-60 Bassman is less than desirable. It could be motorboating of the preamp filter cap (you do have newer/replaced filter caps, right?) These are really great amps!

Bob M.

[Elzorro]

Yes the filter caps are new.
The board is unmodified except for the bias section.
The cathode bypass cap is original. I don't have a problem with replacing it.
I will also check the volume control grounds as well.

Some one else replaced the filters so I will double check the filter grounds too.

I don't think I can live without the channels jumpered though ;~)

[Jazz P Bass]

Fender Bassman 5F6-A

I just got finished repairing a 1953 Pro Amp.
What I found is the jumpering thing doedn't work as you would think.
Not like a 1961 and above Bassman anyway.
When you are in one input and dime it, the other input if dimed actually sucks the volume down.

[Elzorro]

I just got finished repairing a 1953 Pro Amp.
What I found is the jumpering thing doedn't work as you would think.
Not like a 1961 and above Bassman anyway.
When you are in one input and dime it, the other input if dimed actually sucks the volume down.

But jumpering a bassman (and JTM) is an age old tried and true thing.

If I mix the channels and turn the normal volume down just enough to where I don't hear the echo, the amp sounds great.
Turning them both all the way up doesn't seem to suck any volume.

Also the amp doesn't seem to go into oscillation without a signal to get it started.
Letting the note decay naturally doesn't seem to do it. Only a quick mute starts the echo which starts the oscillation at 10 or above on the normal volume.



The preamp bypass tests good but with a little high ESR

[Bob M.]

Elzorro,

It's 2010, that V1 cathode bypass cap is 50 years old. Time to replace it. If the wiring is all original, the filter cap ground leads are soldered to the filter cap housing cover. If the screws loosen that hold this cover to the chassis, you loose filter ground and all sorts of silly things can happen. A good idea is to run a ground wire to where the power transformer center tap is grounded and solder it there. These housing covers loosen up all the time, especially on 50 year old amps. It's one of the 1st things I check on tweed designs with a filter housing cover. While you're at it, replace all electrolytics that are original. The amp will sound even better after you do this. Remember, electrolytics are essentially like tires on a car.

Bob M.

[Elzorro]

Elzorro,

It's 2010, that V1 cathode bypass cap is 50 years old. Time to replace it. If the wiring is all original, the filter cap ground leads are soldered to the filter cap housing cover. If the screws loosen that hold this cover to the chassis, you loose filter ground and all sorts of silly things can happen. A good idea is to run a ground wire to where the power transformer center tap is grounded and solder it there. These housing covers loosen up all the time, especially on 50 year old amps. It's one of the 1st things I check on tweed designs with a filter housing cover. While you're at it, replace all electrolytics that are original. The amp will sound even better after you do this. Remember, electrolytics are essentially like tires on a car.

Bob M.

I somehow missed this post.

Thanks,

I will make a secure ground for the filters. It could be the preamp filter getting behind a bit. AND I will get those two last electrolytics replaced before I start doing anything else. It is 2010 after all!

It does seem to come and go a bit in intensity.

[Gtr_tech]

Replace the front end cathode bypass caps and the bias filters as well. Probably time for new plate load resistors in the front end too. Wouldn't be a bad idea to replace the resistors in the pwr supply as well as the output tube screen resistors. All those old carbon comps are tired by now.

[Jazz P Bass]

5E3

The 1953 Pro had leaky coupling caps at the first stage.
0.1uf/ 600V.
I measured 34mv on the PI grid when the instrument volume pot was at max.
150mv when the microphone pot was maxed.
When the caps where replaced I read 4mv on both.

[Gtr_tech]

Yep....that too. Forgot about those old Astron paper/oil caps....they don't age well.

[Elzorro]

Well,

Normal volume had not so good ground. Fixed.
Replaced first stage cathode bypass and the 8uf preamp filter. All electrolytics now new.

Amp sounds great!

Still has the original issue though.

Amp Sounds Great!

I think I'll call it personality and leave it alone from here on out.

The first cathode bypass really tightened it up.
Nice clean bass and the highs are so clean it's downright scary.

Now I can't wait to get my p10r's back from the re-coner.