Project questions from someone who's learning...

[noredr2]

Greetings...new to the forum here. Stumbled across it and it seems like an active and helpful community. I'm stepping beyond the small mods I've done on amps and pedals, looking to learn a bit more.

Last week, I acquired a Setchell-Carlson Model 13B. Old head, schematic is attached. It's not working right now because the choke melted down. Tubes warm up, though, so it seems the PT is working just fine. In comparing to the schematic, it's apparent some components have been replaced in the past and some original parts are different than what the schematic shows (mainly different cap values). As it stands right now, given the age of most everything in there, I'm leaning towards replacing all resistors, caps, choke, and OT, but I'm open to suggestions.

My questions for starting out are:
--The 6F5 and 6Y7 tubes are ones I've never heard of before. Anybody have experience with these tube types who can say anything about sound quality/provide comparisons to more common tubes?
--One of the few recommendations I got in another forum was that a resistor (5-10watts, value depending on what's needed for B+ voltage) could be substituted for the choke...any thoughts on this? Otherwise, any recommendations on a suitable replacement choke? No way to really tell values on this one since it's fried and not labeled.

I know this is a decent undertaking, but that's why I'm starting here with a fairly simple circuit. I do have a small amount of experience working in tube amps. Been away from it for awhile, but through this project I'm hoping to learn a bit more about how they're designed and how to do some simple repairs. Sorry if I ask some stupid questions in it all...that's how I learn. Thanks in advance for any input.

[Phil_S]

To me, this looks like a fun project to learn with. Be patient, and I think people will participate.

Here is where you can look up tube data:
http://tubedata.itchurch.org/index.html

Those two tubes seem like oddballs to me and there isn't much info to be found.

For a choke, I think I might buy an inexpensive Hammond or Weber. I don't think the rating in Henries is critial, 4H to 10H is probably fine and voltage rating in this amp couldn't be that high. The generic Fender replacement choke will work and isn't costly. Otherwise, try a 10W resistor in the 50-250 ohm range.

C3 and C4 are electrolytic caps. If original, these are too old to be in use and need replacement. Any value between 8uf and 20uf should be fine.

If replacement parts are +/-50%, at this point, I don't think that's critical. Maybe later it will be an issue.

Can you post a few clear pictures? Gut shots, please.

[Structo]

Are you planning on amplifying a guitar with this amp?

If so you will want to change the input so it has a 1 Meg resistor from the tip of the jack to ground, and add in a 22K or so for a grid stopper.

What is strange is the output impedance listed as 500 ohms.

Also I believe C1 should go away as it appears to be a death cap.

Install a modern three prong cord with a ground and attach the green ground wire bolted to the chassis near where it enters.
C3 appears to be a 3 segment cap can.
I would bump the value up to 20-30uF for the first and 20uF for the remaining two.
Or if you have room you can install separate caps there as well.
It would be cheaper to do that than a can.

A replacement choke is about $15 if you are in the states.

[tubeswell]

My questions for starting out are:
--The 6F5 and 6Y7 tubes are ones I've never heard of before. Anybody have experience with these tube types who can say anything about sound quality/provide comparisons to more common tubes?

Can't offer any opinion about their sound. The 6Y7 is an octal dual triode by the look of things

http://www.radiomuseum.org/tubes/tube_6y7.html

The 6F5 is a single triode in an octal bottle with the anode connected on top

http://www.radiomuseum.org/tubes/tube_6f5.html

2CW

[Cliff Schecht]

The 6F5 is somewhat common for older audio applications, the most prominent example being the Gibson EH-150 series amplifiers from WAAY back in the day. The 6Y7 is a more interesting choice but not surprising being that it's a 6.3V octal dual triode. I assume this tube preceeded the 6Sx7 series.

[noredr2]

Thanks for the input so far…great advice. I’ll have to find some time in the coming week to look in to the information you all posted about those tubes. Thanks on that.

I am looking to use this with a guitar, and I was expecting to have to put some sort of resistor on the front end. Structo, what do you mean by a grid stopper?

Replacing the OT to something with a bit friendlier numbers is one of the things I was planning on doing…I too was thrown by the 500 ohms. The amp came with some speakers, but they’ve seen better days. Plus, I already have a 16 ohm 60 Watt cab around.

I forgot to mention that some previous owner installed a fuse but it doesn’t look like their grounding scheme was super great, so 3-prong will definitely happen.

Gut shots are posted here:
http://s811.photobucket.com/albums/zz36 ... son%2013B/

Hope that link works. Doesn’t seem to be a cap can, just a few electrolytics of similar values all placed in. The wiring isn’t pretty, and that’s another reason why I’m thinking complete strip-down and rebuild. And if that’s the case, I would consider adding a master volume or some other tone controls. Still a bit leary on those oddball tubes, but hopefully I’ll find out more in the coming weeks. Thoughts?

[Structo]

From R Aiken:

A grid stopper is a resistor connected in series with the grid of a tube, usually right at the pin of the tube. It is used to prevent parasitic oscillations and reduce the chance of radio station interference by forming a lowpass filter in conjunction with the input capacitance of the tube.

Lots of good tech info here.

http://www.aikenamps.com/TechInfo_2.htm

[Phil_S]

Nice looking amp. Let's see what it needs. In the end, it's up to you.

I'd agree, the first order of business is a 3 prong plug. Ground the line-neutral on a stud near where it enters the chassis and make the wire longer than the white and black. The reason you do that is, when the line cord gets yanked, the ground is the last to go. Make sure you fuse the hot wire. Clip the death cap, C1. For $3 at Target you can buy a 3-wire extension cord with molded plug. Clip the end you don't need.

It's nice that you already have two 1/4" input jacks. Like someone else said, I'd convert these to standard guitar input jacks. Look at any Fender or Marshall schematic.

All those cardboard covered electrolytic caps need to be replaced. They are simply too old to be relied upon.

You're idea to replace the output transformer is also something I'd do. Here's where you get an real nice 8K to 4-8-16 ohm for a good price.
http://www.musicalpowersupplies.com/3.html The OT15PP.

You can probably sub a 6SC7 for the 6Y7. You may need to tweak R2 and R5/R7. 1200 ohms for R2 might be fine. I think I'd change R4 and R7 to 100K if I were using a 6SC7.

Instead of the 6F5, I might look for a 6AQ7. I'd change R7 to 100K and R3 to about 1K.

You should be able to find 6SC7 and 6AQ7, even if you have to resort to eBay. You might get lucky here for 6F5 and 6Y7 or the other tubes I mentioned. http://www.oldradioparts.com/pg1.htm I've purchased here before and I think he's OK, if a little eccentric. You can probably find a cheap choke there, too.

Take it slow. There is a lot to do inside a tight working space.

Good luck.

[Cliff Schecht]

All of the paper and wax caps need to be replaced too, they fail easily with heat and age. Each resistor needs to be Ohmed out to check for open circuits or way out of spec values. With amp builds like this, I tend to completely tear the guts out, salvage what I can and start from scratch. Simply redoing an amp like this is cumbersome and inevitably ends up with a sloppy lead dress.

[M Fowler]

I can look through my stash of radio tubes and see if have those tubes.

You could just go with 6SL7 or 6SQ7 or similiar for better tone I think.

Mark

[noredr2]

Awesome stuff here...thanks so much. I'm in a holding pattern right now with repairs. Gotta sell some gear before I can buy parts. Once those arrive, I'll have updates and questions. Any recommendations on sources for caps and resistors?

[Structo]

http://www.tubesandmore.com/