Filmosound - Guitar - Ground

[atmars]

First, let me say I have some experience modding amps, but my technical knowledge is limited, so forgive anything I say that seems ridiculous.

I have a Bell and Howell Filmosound amp that I am using for guitar. It is transformerless. I have acquired a isolation transformer for the sake of safety, but I have read that I should ground (attach chassis to third wire of mains plug) the chassis for an additional measure of safety. I gave this idea a try, but the result is a lot of hum. I removed the ground wire from the chassis and all is good again. Is there a way to ground the chassis that does not result in a world of hum?

Here's is a link to the schematic:

http://s294.photobucket.com/albums/m...mosoundL-2.jpg

Thanks!

 

[HollowState]

For some reason your photobucket link won't work right for me. It starts to go there, then switches to the main homepage. Is this a public access page?

Victor

 

[atmars]

See if this works any better

http://s294.photobucket.com/albums/mm109/atmars/?action=view&current=filmosoundL-2.jpg

 

[Merlinb]

That amp has a very annoying ground scheme! It does have an input signal transformer though, so I suspect if you remove the grounding cap from the primary side of that signal transformer the hum may be cured. (I mean the cap connected to the input jack, on the top far-left of the schem, I think it's labelled .0056 20410)

 

[atmars]

Thanks -

i'll give it a try and report back!

 

[atmars]

Test results

Did some testing. I was unsure of my ear, so I installed a temporary switch on the mains/chassis ground and on the .0056 cap.

Results:

Chassis ground and .0056 = crazy big hum.

No chassis ground and .0056 in circuit (original configuration) = least amount of hum.



Chassis ground in and .0056 out of circuit=

No chassis ground and .0056 out of circuit=


Same degree of hum.




The hum of both these settings was about the same intensity but of a different tone color - one was "brighter" than the other. The intensity was a bit greater than the original configuration, not even close to chassis ground and .0056.

Questions:

Is it more important to have a chassis ground and a bit more hum, or forget about the chassis ground, or is there a different way to address the hum issue?

Thanks

 

[tubenoob80]

I know this post is years old but if you look the filmosound 179 models are grounded out on the cathode (pin 8) of the output tubes to lift the heaters off of the chassis ground. There is a special note in the schematic about this that prefaces the entire 179 section. Idk if this helps but I figured I would give it a try. I am pretty new to all of this and have a Filmosound 179 that I am trying to convert to a guitar amp and am having quite a bit of difficulty as this is my first try at building a tube amp. My wires on my transformer are extremely difficult to identify so if you can help me with any ideas about what they are or should be as far as color I would really appreciate it!

 

[leadbelly]

I know this post is years old but if you look the filmosound 179 models are grounded out on the cathode (pin 8) of the output tubes to lift the heaters off of the chassis ground.

Doesn't sound correct, the grounding of the cathodes is more likely because it is fixed bias.

If you are converting a working amp into a guitar amp why do you need to know what the wire colours are for? Just make sure you keep that part wired as is, or analyze the working circuit to understand the colour code.

 

[tubenoob80]

Well it was working, but the filter caps needed to be replaced and when my clumsy fingers started poking around things were broken. Real estate is really cramped and long story short, the amp was gutted. There was just so much crap crammed in there it seemed easier to just gut the thing and start over. I wish I had been careful and left the main structure of the power section intact but I didn't so here I am trying to figure out what goes where. It looks as if there are two red wires, two yellow, a green, maybe a white (yellowed badly), a white with green dots, and two sets of secondaries... One set of green with yellow dots and one set of brown.

 

[Johnne]

Filmosound 385 Power Transformer info.

It looks as if there are: 😛
two red wires - high tension for rectifier tube socket pins 6 and 4
two yellow - rectifier tube socket pins 2 and 8 for tube heater circuit which is a typical 5 volt system
two green - 6.3vac for heater circuit which is pretty stout as there are a ton of tubes to heat up!!
the brown ones are probably the 110volt wires for hot/neutral maybe the other ones are for 240vac?😱

a green, maybe a white (yellowed badly), a white with green dots, and two sets of secondaries... One set of green with yellow dots and one set of brown.

enclosed is a picture of a filmosound 385 power section...good luck...also I have one and will delve into this soon - more to follow!...johnne in seattle