Mysteries of the Bench: case#1 - Phased and confused

[Randall]

"
In this series of amps, when I run across ultrasonic oscillation, I apply the patented parasite smasher: 470 pF 1KV disc cap across the plates of the drive tube"

By "across the plates", do you mean from plate to plate, pin 1 to pin 6?

[nosaj]

I know the blues jr will oscillate. Put it on a scope, with the back on and the back off, makes a difference. I think it was 50khz oscillation. The cats went running out of the garage when i turned it on.

You think we could market them as a Pest repellant?

nosaj

[Mick Bailey]

The Blues Jr can suffer badly from oscillation - I've had a few that will quickly red-plate when you probe the PI or (especially) the EL84 screens. I've tried the trick of a 100pf cap across R30 and it's worked every time.

[Leo_Gnardo]

"
In this series of amps, when I run across ultrasonic oscillation, I apply the patented parasite smasher: 470 pF 1KV disc cap across the plates of the drive tube"

By "across the plates", do you mean from plate to plate, pin 1 to pin 6?

Yes, that's it. Notice many Fender amps feature a 47 pF cap in this position, a vague attempt to suppress ultrasonic racket. I just go a bit overboard with the concept. Whatever it takes to beat the bug. Those with a lot of patience can try one cap after another, increasing by increments. I confess, I ran out of patience. 470 pF and whomp! it's gone.

[drewl]

For some reason if you pull on your meter leads (I use mini-grabbers) real tightly you can measure without causing oscillation.
For some reason it's always the right side plate/transformer wire that oscillates when measuring bias.

Also moving the ribbon cables away from the board helps and stops hum as mentioned on the Bill M mods site.

[Gnobuddy]

For some reason if you pull on your meter leads (I use mini-grabbers) real tightly you can measure without causing oscillation.
For some reason it's always the right side plate/transformer wire that oscillates when measuring bias.

Probably because that plate/anode has the right phasing to cause positive feedback via stray radiation (from the meter lead). The same kind of thing happens with long-tailed-pair phase splitters too, probing one anode tends to trigger the oscillation, the other one is stable.

I have no explanation for the "pull on the meter leads" trick, presumably it had to do with the all-important issue of layout...pulling the meter lead straight wire reduced coupling back to the input.

Also moving the ribbon cables away from the board helps and stops hum as mentioned on the Bill M mods site.

I've only worked on one Blues Junior, which I owned at the time. It was more stable with the ribbon cables moved as close as possible to the metal chassis. A couple of millimeters of cable movement could stop or start oscillation.

I have also worked on two Super Champ XDs and a Princeton Reverb. Every one of them would oscillate, and you could trigger the oscillation in several different ways - not just by probing output anodes with meter leads, but also, for example, by moving a distortion pedal too close to the amp. The SCXDs were particularly bad when it came to instability.

All of this points to the same thing - Fender Corp. sells a whole lot of marginally stable amps. I could maybe understand that in 1952, since Leonidas clearly didn't understand electronics in much depth. But in much later designs like the Blues Junior or Super Champ XD? Shameful.

I don't envy FMICs current situation, though. Not only are there fewer electric guitar sounds in contemporary popular music, but also most of the kids that represent the next generation of potential buyers have no interest in them.

-Gnobuddy

[Gnobuddy]

The late BillM had on his webpage, the cure for the oscillation

I'm sorry to hear that he has passed on. I respected the fact that his mods always involved real engineering improvements designed by a person with real engineering skills. He never sold snake-oil as so many "amp gurus" do.

-Gnobuddy

[Enzo]

When I read "pull on the meter leads" my first thought is grabbing the leads to pull adds your body capacitance. Same capacitance that causes hum when your fingers get close to a grid wire.

[Gnobuddy]

When I read "pull on the meter leads" my first thought is grabbing the leads to pull adds your body capacitance. Same capacitance that causes hum when your fingers get close to a grid wire.

Interesting thought! Drewl mentioned using "mini-grabbers", so I assume his hands aren't actually in contact with the leads. Just guessing, though.

Body capacitance and/or AC voltage picked up by the body from mains wiring is a funny thing. I remember at one time (1980's?) seeing various electronic gizmos that could be turned on or of by touching with a fingertip. I saw DIY circuits to implement the same thing, typically by touching the gate of a JFET, turning it on enough to trigger a bistable multivibrator (flip-flop).

Turning things on and off with a touch was "nifty" back then. Now (almost) everyone has a fondle-slab, obsesses over it, and spends most of their waking moments poking at it with a fingertip or two.

A local community college has a central three-storey concrete spiral staircase running from the entrance lobby all the way to the top. I keep seeing students climbing or descending this, blindly, by feel, while poking at their phones. Nobody has fallen down the thing and split their skull open yet, but I am worried about it being only a matter of time before it happens.

-Gnobuddy

[Enzo]

Well, pulling on it means SOMETHING was in contact, unless there is a mechanical lead puller. I imagine hands wrapped around something, whether the wires or the meter itself.

[1ampman]

I have a Regular Blues Junior that howls when the volume control is turned past 8. It's Ok up to 6. It seems to be around 100 hz. This happens with nothing pugged in.

[Leo_Gnardo]

I have a Regular Blues Junior that howls when the volume control is turned past 8. It's Ok up to 6. It seems to be around 100 hz. This happens with nothing pugged in.

2 things mostly, either a bad preamp tube, or a filter cap gone bad in the power supply.

[Helmholtz]

As there seems to oscillation involved, it could also be a bad ground contact. E.g. at the output jack, filter cap ....