Bell & Howell Filmosound 179

[xtian]

Just bought: http://www.ebay.com/itm/350524941716

Want to strip and rebuild as 5e3 or similar, single channel, tweed-like.

This company does same: http://www.atmarsamps.com/products/index.html

I notice they've kept only a pair of 6v6 and the 5y3, then added two noval sockets for a 12ax7 and 12at7. But they don't show any gut shots or schematics. In action: http://www.youtube.com/watch?v=8kRNa-qG8BE

There's a long thread at the Gear Page <http://www.thegearpage.net/board/showth ... p?t=156790> but not many helpful details. (Yes, I read the whole thing!)

What circuit do you think would work well in this chassis?

[Alexo]

I vote for an octal AC15 - 6J7/6SL7/6V6's!

[xtian]

I vote for an octal AC15 - 6J7/6SL7/6V6's!

I like the idea of all octal. Couldn't find the schem here: http://drtube.com/guitamp.htm#Vox

[xtian]

Fender 5c3 looks promising..

[Alexo]

That's because it doesn't exist. There are some old amps with 6J7 inputs you could copy for V1, or at least some with a 6SJ7 which is very very similar (see a 5C1 champ, i think) and you would do fine just copying the long tailed pi from the AC15 and applying the 12AX7 values to the 6SL7. Then you could cop the tweed deluxe output and power section if you were so inclined....

[Cliff Schecht]

Fender 5c3 looks promising..

I love mine, it can get a into some very high gain territory with the right guitar/speaker combo. I plug into the bright channel and roll back for cleans like how one would play an Express.

Gotta be extra careful about layout and grounding with those 6SL7's though, they pick up any hum that you allow them to. I went with all DC heaters and did lots of tweaking before I was happy.

[xtian]

Gotta be extra careful about layout and grounding with those 6SL7's though.

I am worried about hum, and the chassis is very small and cramped. And 6SC7 seem to be rare and expensive. Maybe the 5e3 is a better choice with its 12a?7 front end. I know the PI is different. What do you think?

[Alexo]

If you're using the 6SL7 as the phase inverter, its hum issues are a little less critical and you'll find it easier to get away with than sticking one in V1.

[xtian]

The project begins! I stripped the chassis (yuck--60-year-old dust). See photos. I think I'll build a 5c3 Deluxe with 6SL7 as preamp and PI. I know the 6SL7 is a different pinout from the 5c3's 6sc7. Weber's Deluxe schematic has a good circuit for the 6SL7.

I'll have only one input jack on my amp. How can I best make use of the two triodes on the input side? Seems like the Deluxe is just set up for a Lo and Hi input (the 75K resistor being the difference). But maybe I can run the two triodes in series, switchable?

I'm used to seeing 1M ref to ground on the input jacks. But the Weber and 5c3 schematics have 5M. Why?

[xtian]

Anyone care to answer?

I'll have only one input jack on my amp. How can I best make use of the two triodes on the input side? Seems like the Deluxe is just set up for a Lo and Hi input (the 75K resistor being the difference). But maybe I can run the two triodes in series, switchable?

I'm used to seeing 1M ref to ground on the input jacks. But the Weber and 5c3 schematics have 5M. Why?

[Cliff Schecht]

First part: You'll find the amp has more than enough gain as is. Another gainstage would probably be too much. Maybe parallel the triodes to drop the noise floor a bit.

For the second part, the inputs are grid leaked bias. It's an old way of biasing triodes that uses the small grid current across a large resistance to develop bias. These stages tend to squeeze the most gain out of a single stage as one can get but don't have any negative feedback/cathode degeneration (i.e. a cathode resistor) which means that the performance of that stage is highly dependent on the tube you put in the socket. I love my 5C3 to death, as well as all of my grid leak biased amps really, but you gotta roll a lot of tubes to find one that's good. Luckily the 6SL7 is incredibly common and very cheap. For 50's-60's NOS examples I usually pay $2-3 a pop.

[xtian]

Ah! Really great advice. Thanks Cliff. See below:

First part: You'll find the amp has more than enough gain as is. Another gainstage would probably be too much. Maybe parallel the triodes to drop the noise floor a bit.

I like this. Example schem? Keep the 75K input resistor? Use a 0.1u cap for each triode, or one common one? Two input resistors or one common one?

you gotta roll a lot of tubes to find one that's good. Luckily the 6SL7 is incredibly common and very cheap. For 50's-60's NOS examples I usually pay $2-3 a pop.

Want to sell me some tubes? Or recommend where to buy?

[xtian]

Welp. Got it wired up. B+ voltage looks OK with 5Y3 installed. Now I have to wait until 6SL7 tubes arrive to find out if it works!

Man, this was a driving-with-eyes-closed build. I used only components already in my (very small) inventory, indulged in only brief contemplation to solve the layout, and went for it. A real Hail Mary amp. It will be a major PITA if I have to change some of the buried components.

[Structo]

Man they used to cram a lot into those old chassis!

Can you imagine building that original amp?

Is that some kind of funky switch mechanism in the original?
With that bar thingy that goes across to the other jack?

[img:640:478]https://lh6.googleusercontent.com/-ggUa ... ound02.jpg[/img]

Sort of like the Hammond M3 amp I bought on ebay, I thought I could simply convert it but I had to gut it like you did.

Hopefully your transformers are OK.

If I were you since you are not sure it works yet, power it up using a light bulb current limiter before you add tubes.
Then add the rectifier, etc.

Remember in the ad he said that one of the tubes was arcing, so it may have left a carbon track on the socket as well.

What kind of cabinet are you going to put it in?

[xtian]

Can you imagine building that original amp?

No! Must have been on a long assembly line, with simple jobs per person.

Is that some kind of funky switch mechanism in the original? With that bar thingy that goes across to the other jack?

Yes. When a jack is inserted into the input, it switched on the "exiter lamp" and photo cathode tube. I don't grok the details, but I gather this is a way to decode the audio track etched on the film.

What kind of cabinet are you going to put it in?

Don't know. Maybe none. Trying to keep it cheap--I've got less than $100 invested. Let's see if it works!