Did I get in over my head too soon?!?

[janikphoto]

I know enough about stereos (ohms, wattage, etc) to do high-end auto installs, but I don't think I've got a clue about my new venture. I wanted to take some some old parts and put them together to make a nice little tube guitar amp for the house. This is what I got:

http://cgi.ebay.com/ws/eBayISAPI.dll...3122&rd=1&rd=1

Along with a nice old 10" speaker from a console system built in 1953, I plan to build a custom cabinet and start playing. However, it looks like a tube is missing and I have no clue on how to find one to replace it on my own, and forget about me repairing it. I can solder just fine, but I wouldn't know what was broken...

Should I take this to a local shop to have it looked over, or should I just track down the missing tube and cross my fingers?!? I can track down a new power cord easily (I assume) and hard-wire the speaker in (it has an odd looking plug right now), but I'm not ready to learn all about tube repair yet...

[anatech]

Hi janikphoto,
Step one. Get a schematic and look it over. It's possible the socket is for a plug to another piece of equipment (like a projector). There would be a lamp supply and pickup cell connection. You may need to modify this for guitar use.

Don't wail on that poor speaker you have. It will come apart. An instrument speaker is another thing again.

Hey Joe! (burnedfingers), have you done this before? Tubelab would be another good member to look in here. Many others too many for me to think of right now!

-Chris

[SY]

Looks like an auxiliary plug. Four pins from what I can see in the photo.

[wa4htz]

Sy, your eyes and monitor are better than mine 'cause it just looks like a black round spot to me. The 5879 he lists is a 9-pin minature low noise pentode used as a microphone or other low level preamp tube. I don't see any minature tubes on the chassis but it might be inside the black housing which I'm guessing holds a photo-multiplier tube also. I can't for the life of me imagine what they would do with 3 25L6s. Two are probably in the output in push-pull but what are they doing with the third one?

If my memory serves me correctly (it rarely does anymore), Filmosound was a name Bell & Howell put on their early 16mm sound movie projectors. I think I remember seeing that in elementary school but that was a loooooooooooooooong time ago!

Also, and this is the most important part, with 3 25-volt and 2 6-volt heater tubes, this probably does not have a power transformer and therefore has one side of the AC line connected to the chassis. These are very dangerous in that if you plug it in the wrong way, you have the hot side of the AC line connected to the chassis. YOU CAN BE KILLED! If you figure out how to use this for guitar, you MUST get an isolation transformer to run it from.

Hope this helps.

Ken

[janikphoto]

thanks. three pins. And yeah, it must be the input for the projector. That was dumb of me not to think that... It has a mic input that the guitar will plug into, so I won't have to modify the input.

The old speaker won't be abused. It is a lot like other old speakers I've seen in old guitar amps, so I was hoping to give it a try and see how it does. If it doesn't perform well, I'll just get an eminence or other current speaker designed for guitar use.

I'm reading through everything on the noob stickie and hopefully I will be able to do more tube stuff as a hobby. Thanks for the help so far...

[dnsey]

Also, and this is the most important part, with 3 25-volt and 2 6-volt heater tubes, this probably does not have a power transformer and therefore has one side of the AC line connected to the chassis.

IIRC, the transformer for most early B&H projectors was external, in a big Rexine covered box! I suspect that the socket in question is for connection of HT and LT supplies via the rest of the projector.

[janikphoto]

Originally posted by dnsey

IIRC, the transformer for most early B&H projectors was external, in a big Rexine covered box! I suspect that the socket in question is for connection of HT and LT supplies via the rest of the projector.

hmmmm... interesting. I saw the transformer right next to a plug on the side and thought that was the only one for power. I figured this female plug right next to the transformer on the side of the amp had three prongs and was kind of oval shaped, like where a power cord would plug in, so the transformer must be for sonverting ac to dc. Since I don't understand tubes well enough yet, I suppose I could be confusing it with something else...

Now, this plug on the side may not lead directly to the wall outlet and I may have to get an outboard transformer as suggested aabove, but it is definitely where the power comes in. It is even marked 117 volts/50-60 cycles/0.9 amps next to this side plug. This round top plug must be for the audio input from the projector, since the only other input area is a mic plug on the side labelled 'mic'. I guess you could narrate during the movie?!?

Well, I was going to just find a plug and hook this thing up to the wall, but that sounds bad if this thing is sans ac/dc transformer. I'll shelve this project until I can find out a bit more. Anyone know what info I need to track down a schematic? The only things I know for sure is that it came from a supposedly working bell and howell filmsound projector. I found another (not exact same model, but same knobs, mic input, fuse location, and power plug on it. See the fuse and power plug area on this one:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=270046971712

Of course that one has the huge transformer like you say. Mine is labelled 10 watts on the bottom, and i assumed the small trans on mine was due to the low wattage.

[wa4htz]

I'm doing a good bit of guessing here, but I believe the small transformer behind what appear to be 2 of the 25L6-GTs is the output transformer. The 3 pin socket just behind the transformer is probably where the speaker plugged in. These types of connectors were pretty common on old radios and amplifiers intended to be used as a part of a piece of equipment and not as standalone.

Since there is a connector on the side labeled "117 VAC 50/60 CY.", I'd bet lots of money that is the power input and was probably connected to an umbilical from the projector. If the connector is oval like you say and has 3 pins with the center one offset from the outside two, this was a very common power connector (HP used them of almost all of their test equipment before the current IEC connector became popular) and the power cord to fit these is now very hard to find. I just looked at the power connector on the other amp on ebay and all 3 pins are in a row; I think a mate for this connector will be very hard to find.

It would have been quite unusual for them to use 25-volt heater tubes in an amp with a power transformer. If you look at the tube line-up listed for the other amp you showed on ebay, they are all 6-volt heater tubes except the 5Y3 rectifier. The perforated covers on the chassis also point to it needing lots of ventilation and could be over power resistors used for dropping heater voltage.

One other thing, the tube he lists as a "65L7-GT" is probably a 6SL7-GT. I think the best advice has already been said; try to find a schematic of the amp or, if possible, the whole projector. Good luck.

Ken

[dnsey]

Since there is a connector on the side labeled "117 VAC 50/60 CY.", I'd bet lots of money that is the power input and was probably connected to an umbilical from the projector.

Agreed - I didn't see the text. however, the projector transformer may well still have been used to isolate the amp from the mains.
This guy might be able to help with more information or a schematic.

[dnsey]

Here you go - sure to be here somewhere (or something very similar)!
http://www.film-tech.com/warehouse/index.php?category=2;rt=1162474762770]http://www.film-tech.com/cgi-bin/externallink.cgi?http://www.film-tech.com/warehouse/i...=1162474762770[/URL] EDIT: It's an odd link, which doesn't seem to work eternally!
Go to the second uRL, and scroll to "16mm projectors" (near the end of the page. I should think yours would be in the "B&H amplifiers model 120..." manual.