Bell & Howell Filmosound 179

[Structo]

I've seen some cool handles that guys have gotten at the hardware store.
Like kitchen cabinet handles or the like.
There are hundreds of designs out there, you just have to pick one.

[M Fowler]

Yeah? Well, now you're just bragging. Wink

No I am not bragging I am just saying it is possible to get the amp quiet.

My chassis is 12x8x2 Hammond with cage.

Mark

[xtian]

Do I see the mains (green/yellow) ground sharing it's ground point with the circuit ground? IMO, that is a sure fire way to introduce 60 Hz or other hash from the house wiring...Please fix this, even if you do nothing else.

Did this. Thanks, Phil. See photo.

It is also unsafe because now your circuit is directly connected to the mains, bypassing the fuse and the PT. You must give the mains ground its own private ground point.

What?!? I've moved the mains ground to it's own lug, but I don't see how this made any difference to "bypassing the fuse and the PT." Please explain.

[xtian]

To lower the gain on the input tube, lower the value of the plate load resistor and/or increase the value of the cathode resistor, and/or lower the value of the grid leak resistor.

Phil, I still want to reduce gain in input stage and maybe in PI, but I didn't understand your suggestion. The input stage's cathode is grounded, so that leaves the voltage at the plate as the only element that can be adjusted, right? The resistor there, connecting the plate to B+2, is 220K, and I read about 90vdc at the plate. Doesn't it make sense to increase the value of that 220K resistor in order to lower voltage and thus gain?

[Phil_S]

It is also unsafe because now your circuit is directly connected to the mains, bypassing the fuse and the PT. You must give the mains ground its own private ground point.

What?!? I've moved the mains ground to it's own lug, but I don't see how this made any difference to "bypassing the fuse and the PT." Please explain.

The mains ground wire is a redundant neutral. It performs the same function as the white mains neutral. It is a "return", so to speak, for the hot wire. There is all manner of mayhem and mishap possible in an electrical device, most of which can't be predicted -- we call these "accidents." They often take the form of a short. One possibility is there will be a backfeed. With the mains ground directly attached to the circuit ground, it is possible the ground will become live and then the fuse is last in line, not first.

The PT has an isolation function, too. In addition to changing the mains AC to 6.3V or 250V or whatever, the primary and secondary windings are actually never connected to each other. When you connect the mains ground to the circuit ground, you are, in essence, connecting the primary and the secondary windings. There is, in fact, a direct path created between the two windings when you do this. However remote the possibility of an accident, you've defeated the isolation of the primary and secondary windings.

This might all seem like a stretch. I have been assured at one time or another, by more than one licensed master electrician, that the practice of providing the mains ground of an appliance (read "tube amp") with it's own bolt is in the NEC ("code") for a very good reason and this should not be ignored. I understand the European CE requirements are similar and that they are even more particular about how things are done.

Not being someone who is all that familiar with the NEC, I have always taken on faith that it is not something to be fooled with. Think about it. It makes perfect sense to me.

[Phil_S]

To lower the gain on the input tube, lower the value of the plate load resistor and/or increase the value of the cathode resistor, and/or lower the value of the grid leak resistor.

Phil, I still want to reduce gain in input stage and maybe in PI, but I didn't understand your suggestion. The input stage's cathode is grounded, so that leaves the voltage at the plate as the only element that can be adjusted, right? The resistor there, connecting the plate to B+2, is 220K, and I read about 90vdc at the plate. Doesn't it make sense to increase the value of that 220K resistor in order to lower voltage and thus gain?

I'm a little out of my element here. I am an amateur, not an EE, so I'll do my best, but don't take it as the absolute truth.

As I recall, the input has a resistor to ground. This qualifies as the grid leak resistor. The 5C3 schematic shows a pair of 5Meg to ground. That's the grid leak.

Since you have no Rk installed, the only things you can do are to fiddle with Rg and Ra. I think I'd experiment with the grid leak. I'm not sure I'd want to raise Ra -- the next logical increments are 270K and 330K. I am guessing 250K isn't a meaningful change, but I don't know. I'm going to say, less than 90V is going to sound like poo. You should do something to raise plate voltage and give the input stage more headroom.

The good news is, you've got a high value Rg. Just tack in another in parallel and see what it does!

My inclination, though, would be to simply dress it more like a typical amp, 1M on the grid, 100K on the plate, and something between 820 and 3K on the cathode.

Good luck with this. As I said, I'm feeling a bit out of my depth here.

[xtian]

Thanks, Phil.

Maybe there is a better place to address this. The issue is that the amp is too hot overall--too much gain. If I dime the Vol and Tone, the amp begins to squeal (runaway oscillation, right?).

So I'm hoping a useful side-effect of reducing the gain somewhere will also lower the overall noise at idle.

Adjusting the NFB did not help this issue.

Anyone have a suggestion?

[M Fowler]

Reduce the voltage to the preamp tubes by increasing dropping string resistance will help otherwise decrease the gain of the tube by reducing the bypass resistor or remove any bypass cap on V2.

5m and 270k

160v V1
75V V2

[martin manning]

When you connect the mains ground to the circuit ground, you are, in essence, connecting the primary and the secondary windings. There is, in fact, a direct path created between the two windings when you do this. However remote the possibility of an accident, you've defeated the isolation of the primary and secondary windings.

Phil, the mains ground is typically connected to the circuit grounds via the chassis even if it has its own mechanical (and electrical) connection. The isolation is maintained because the neutral and the mains ground are independent paths to earth ground. The cardinal rule about the mains ground being made with an independent connection is intended to minimize the posibility that that connection will ever be compromised. It's true that the primary and secondary circuits will be connected at the mains panel, but this is the means by which the chassis will be held at ground potential in the event that an above-ground potential becomes connected to the chassis. It is possible to lift or even completely isolate the circuit grounds from the chassis, but that is not common in guitar amps.

[xtian]

decrease the gain of the tube by reducing the bypass resistor

Which is the bypass resistor? Parallel to the cathode bypass cap? If I reduce this value, I'm making a LARGER voltage differential between cathode and anode, increasing gain, right?

[M Fowler]

The 2k7 on V2 if reduced to 1k7 will reduce gain. If the bypass cap 25uf/25v is removed the gain will be even less.

Could start with removal of the bypass cap to see how that helps.

[Phil_S]

Phil, the mains ground is typically connected to the circuit grounds via the chassis even if it has its own mechanical (and electrical) connection. The isolation is maintained because the neutral and the mains ground are independent paths to earth ground. The cardinal rule about the mains ground being made with an independent connection is intended to minimize the posibility that that connection will ever be compromised. It's true that the primary and secondary circuits will be connected at the mains panel, but this is the means by which the chassis will be held at ground potential in the event that an above-ground potential becomes connected to the chassis. It is possible to lift or even completely isolate the circuit grounds from the chassis, but that is not common in guitar amps.

Hmm....OK, sorry for posting mis information. Would I be wrong to think putting circuit and mains grounds on the same bolt is a way to introduce hash from the mains?

[martin manning]

Would I be wrong to think putting circuit and mains grounds on the same bolt is a way to introduce hash from the mains?

Well, given that elevating the circuit ground is a way to reduce such noise, connecting it directly to the mains ground would leave the lowest possible resistance between them, but I don't think that the fraction of an ohm resulting from putting some distance between them on the chassis would make a huge difference. Kind of a moot point, the mains ground is supposed to have it's own dedicated attachment anyway.

[xtian]

Glamour shots.

[Cliff Schecht]

Damn that 6V6 is shoved right up against the PT. That can be a source of hummmm.