Purchased two weird amps

[antiunderscores]

Hey guys,
I recently bought two little projects for the fun of it one is in alright condition another is pretty poor. Neither has arrived yet but im trying to get the ideas ball rolling. My plan is to map both the schematics out replace any old caps or resistors and basically try and get them working. And from there change them to try and get a decent sound out of them. Just a quick run down...

The first is in pretty poor shape the wirings cramped and very messy and it appears to have one preamp tube, a rectifier tube but then two power tubes. Anyone ever heard of a layout like this? Should i add another preamp stage? can you just chuck on in their? heres a link:
http://cgi.ebay.com.au/ws/eBayISAPI....m=180297797026

The second is in better condition but to my untrained eye it also seems to have a pretty weird layout. From the blurry pictures it seems to have 2 12ax7 type preamp tubes, 2 EL34 or similar and a single 6 something 6-G something power tube. heres a link:
http://cgi.ebay.com.au/ws/eBayISAPI....m=220297462696

Also is there a way to tell how many ohm output an amp is? Incase you cant tell im pretty new to this whole thing but from my experience working on stuff is best way to learn.

[Chuck H]

The first is in pretty poor shape the wirings cramped and very messy and it appears to have one preamp tube, a rectifier tube but then two power tubes. Anyone ever heard of a layout like this? Should i add another preamp stage? can you just chuck on in their?

That amp appears to be a seperated power amp chassis for a console unit of some sort. It's also old, rusty and uses some funky tubes. You probably could have saved $34.00 AU + shipping on that one But you could probably save those transformers with a scotch bright pad and a coat of spray paint. You may also get some other parts from it. Trannies are often compatible with different tube types. As far as adding a preamp tube, sure. The heater winding capacity is the thing to worry about when adding tubes. One preamp tube probably won't exceed the MFGs overrate on that winding. But "I" wouldn't bother with that on this amp as it is.

The second is in better condition but to my untrained eye it also seems to have a pretty weird layout. From the blurry pictures it seems to have 2 12ax7 type preamp tubes, 2 EL34 or similar and a single 6 something 6-G something power tube.

It looks to me like 2 -12**7 type preamp tubes a pair of EL84s and a big old "coke bottle" type 5u4 rectifier. Now thats an amp you can sink some work into. The layout does look a little funky but unless your going for really high gain you'll probably be OK if you layout YOUR work carefully.

Also is there a way to tell how many ohm output an amp is?

Yes. You'll need an audio range signal source (signal generator, keyboard, etc.) and a VOM or DMM. It's a long description. I'll go over it if you have those tools.

Incase you cant tell im pretty new to this whole thing but from my experience working on stuff is best way to learn.

Yes. But a couple of good books would help. Also, there are higher than average voltages and stored charges in tube amps. Messing around without knowing the safety rules could actually kill you. Do a web search on tube amp safety and read several articles. Aiken has some really good safety and beginner to intermediate info. Just use search words - Aiken tube amp - and you'll find him.

Welcome to the craft.

Chuck

[martin manning]

Re: Can you determine the correct speaker load for an amp?

Yes. You'll need an audio range signal source (signal generator, keyboard, etc.) and a VOM or DMM. It's a long description. I'll go over it if you have those tools.

Chuck

Can you elaborate on this Chuck, or point to a description I can read?

MPM

[Chuck H]

OK...This has been covered here about a dozen times. You could probably find it with a search here or in the Archives. But here goes:

First you need to determine the turns ratio. To do this you put a signal on the OT secondary and measure the voltage at the secondary and the outer legs of the primary, exclude the primary center tap. So If you hook up a signal generator at 400 to 1000 Hz to the secondary and get a read of 1 volt (for ease of math, actual voltage may vary) and the primary measures 28.8 volts, you have a turns ratio of 28.8 to 1. You divide the output by the input to find this turns ratio. Then square the denominator of the turns ratio. In this case it is 28.8. 28.8 X 28.8 is 829.44. Now multiply that number by the target secondary impedance. lets use 8 ohms for the example. 828.44 X 8 = 6635. So if you ran that OT into an 8 ohm load it would reflect a primary impedance of 6635 ohms.

Some intuition is needed at this point because if you did your math with a 4 ohm impedance in mind you would have come up with 3318. Which would only be appropriate for a four tube amp. Also consider the size of the OT. If it's the size of a tuna can you would have to assume that this OT is for a pair of 6V6 tubes at 8 ohms. Only because obviously it wouldn't be suitable for a 4 X EL34 amp at 4 ohms and a 6600 ohm primary would be common for a pair of 6V6 tubes. If the OT is large it may be suitable for a Quad of EL34 or 6L6 tubes. You have to use your noodle.

HTH

Chuck

[Steve Conner]

When it comes to transformer size vs. power, I like to reckon somewhere between 25-50 watts per pound weight. Big transformers built with modern core steels can get over 100W/lb, but a lower figure is more realistic for tube-era stuff, and especially OTs.

[martin manning]

Thanks, Chuck. In point of detail it seems like you're describing how to determine the load and application for an unknown output transformer. The premise here is we have an amp with unknown output impedance, and that's where the OP is coming from, I think.

The answer is almost the same, except you would start from the impedance of the output stage based on its configuration and tube type, then divide that by (Vin/Vout)^2 as measured across the output transformer with a signal applied to the secondary. That part I understand... the OT is matching power for two different impedances (Pin = Pout), and since P * Z = V^2, Zin/Zout = (Vin/Vout)^2.

MPM

[Chuck H]

In point of detail it seems like you're describing how to determine the load and application for an unknown output transformer. The premise here is we have an amp with unknown output impedance, and that's where the OP is coming from, I think.

The answer is almost the same, except you would start from the impedance of the output stage based on its configuration and tube type, then divide that by (Vin/Vout)^2 as measured across the output transformer with a signal applied to the secondary. That part I understand... the OT is matching power for two different impedances (Pin = Pout), and since P * Z = V^2, Zin/Zout = (Vin/Vout)^2.

The TEST is the same. If you run the above described test, use an 8 ohm target impedance and come up with 6600 ohms primary, but the amp in question runs four big bottles (eg; 4 x EL34's) then you would have to assume the OT is intended for a 4 ohm load and a primary of 3300. There... You have just determined the unknown secondary impedance.

Chuck

[Marc]

The first one might make a good parts donor for a champs tyle amp, unless you're thinking of using the original tubes...

The second one looks like it would be good for a spitfire clone or an 18W watt type. The power tubes look to be EL84's to me, but my eyes aren't what they used to be.

As to the speaker lead try an 8 ohm lead and see how it sounds. Unless you just have to know the optimal load for the the exsisting OT and 8 ohm should be OK.

Marc

[antiunderscores]

That amp appears to be a seperated power amp chassis for a console unit of some sort. It's also old, rusty and uses some funky tubes. You probably could have saved $34.00 AU + shipping on that one But you could probably save those transformers with a scotch bright pad and a coat of spray paint. You may also get some other parts from it. Trannies are often compatible with different tube types. As far as adding a preamp tube, sure. The heater winding capacity is the thing to worry about when adding tubes. One preamp tube probably won't exceed the MFGs overrate on that winding. But "I" wouldn't bother with that on this amp as it is.



It looks to me like 2 -12**7 type preamp tubes a pair of EL84s and a big old "coke bottle" type 5u4 rectifier. Now thats an amp you can sink some work into. The layout does look a little funky but unless your going for really high gain you'll probably be OK if you layout YOUR work carefully.





Yes. You'll need an audio range signal source (signal generator, keyboard, etc.) and a VOM or DMM. It's a long description. I'll go over it if you have those tools.



Yes. But a couple of good books would help. Also, there are higher than average voltages and stored charges in tube amps. Messing around without knowing the safety rules could actually kill you. Do a web search on tube amp safety and read several articles. Aiken has some really good safety and beginner to intermediate info. Just use search words - Aiken tube amp - and you'll find him.

Welcome to the craft.

Chuck

wow, thats lots of replies. I've read a fair bit on tube amps and i know all the safety precautions, etc and im getting to understand schematics but im not 100% on all the lingo you guys use.

In regards to the rusty amp, i can get most the tubes in there or at least similar ones without too much effort but are you saying its basically the power section of an amp without the preamp? I've heard people just cranking a booster pedal as a preamp, is that a bad idea tone-wise or is it just an easier alternative to wiring in a preamp that likely to make everything explode? I'm still interested in getting it up and running so perhaps i'll map the schematics and post them up here so you guys can have a look at it and tell me if its got anything worth working with, then i can ignore your advice and have a go anyway .

Thats good to hear the other amp is in a more workable state but unfortunately im yet to see the wiring underneath so it could be a nasty surprise. Im not aiming for high gain, just something that can get a nice balance between clean and bluesy distortion.

I'll keep you posted once they come arrive, thanks again for your friendliness and helpfulness thus far.

[antiunderscores]

ok, i've mapped out the schematic of the first rusty amp but its a mess (the amp and the drawing) All the components of the amp seem to be in alright condition once your considered all the dates imply they were made in 52. Its a lot better than the pictures looked, it only has a tiny bit of surface rust on the frame and the protective sides of the transformers. Anyway i need a hand sorting out the schematics. I think for the time being i'll have the shape of the tube pins up the top and then map them out from there. Any volunteers to get it into proper format?

The second amp is in really good shape. Its already got a bass and treble control, all the lines are wrapped to be neat, well set out, point to point wiring, there are two wires that are broken. One is practically touching where it joins but another i have no idea where it goes, so i'll try and map the schematics and hopefully you'll be able to tell me where it should go. Its got a spare IO socket in the back and the pair of EL34 both have 3 points that arent used, does that mean its basically running half the tube?

So is there anyone out there willing to help me fix up the schematic so its understandable?

[Chuck H]

and the pair of EL34 both have 3 points that arent used, does that mean its basically running half the tube?

No. Not all the pins need be used to operate the entire tube. By the way, are those el34's in that amp or el84's?

So is there anyone out there willing to help me fix up the schematic so its understandable?

Draw it up as best you can and post it here. If something doesn't look right, you'll know pretty quick

Chuck

[antiunderscores]

they're EL84, im now reading up on the difference. I havent had a chance to go over the schematics and neaten them up, i spent most the day setting up a tweeter in my amp, cone break up to the extreme!

[antiunderscores]

First schematic written!

ok, i've tidied up the drawing of the schematics of the rusty amp so you guys should be able to read it and help me out. At the moment im wondering if it is just a power amp with no pre, where the loose wires connect to and anything else you guys find interesting. cheers.

hope that makes sense
sorry but the file size is 371 kb, if thats too big just winge and i'll re reso it.

[6267]

The rusty one looks like it indeed had all the elements of an amplifier, and is well suited to a low-wattage SE output design, probably something like a champ. You need to see if there is continuity between the green and black wires of the OPT (it is an output) and between the blue and red wires. You should probably also disconnect it entirely and make sure there is no continuity between either of these pairs. The 6sj7 is a good preamp tube if it is not microphonic. The 42 power tube is interesting, it sounds like it is somewhere between a 6k6 and a 6v6, directly heated. The sockets used by these tubes were popular in the time leading up to WWII. This is an OLD amp, or at least and OLD design. You have a few options, in terms of making this something usable. Tubes that fit those sockets are either not in production or very expensive. A 2A3 would fit the socket for the 80 rectifier, but the transformer impedance is probably wrong. The 6sj7 uses an octal socket, so you could use 6sj7, 6sl7, 6sn7, 6sc7, etc. here for preamp duties. I'd replace the weird 4 & 5/6-pin sockets with octals from AES (tubesandmore.com) and build a champ 5F1 type amp. Use a 6SL7 in the one current octal socket (it will be wired up like the 12ax7 in the 5f1 schematic). You could leave the type 80 rectifier and use it the same as the 5y3 (5y3 is an octal type 80). Then, you will need only to replace one of the other sockets with an octal for the 6V6 or 6L6 power tube. You will have one socket you could optionally replace with an octal left, either for reverb driver, tremolo, or another power tube in parallel. Essentially, toss most of the circuit and sockets, and rewire it into a 5f1 or so. You could probably even add reverb using a 6EM7 or 6DN7 as the driver and recovery, or do a simple tremolo with another 6sl7. Good luck, I bet that it has a great sounding OPT for guitar/musical instruments though!

[antiunderscores]

The rusty one looks like it indeed had all the elements of an amplifier, and is well suited to a low-wattage SE output design, probably something like a champ. You need to see if there is continuity between the green and black wires of the OPT (it is an output) and between the blue and red wires. You should probably also disconnect it entirely and make sure there is no continuity between either of these pairs. The 6sj7 is a good preamp tube if it is not microphonic. The 42 power tube is interesting, it sounds like it is somewhere between a 6k6 and a 6v6, directly heated. The sockets used by these tubes were popular in the time leading up to WWII. This is an OLD amp, or at least and OLD design. You have a few options, in terms of making this something usable. Tubes that fit those sockets are either not in production or very expensive. A 2A3 would fit the socket for the 80 rectifier, but the transformer impedance is probably wrong. The 6sj7 uses an octal socket, so you could use 6sj7, 6sl7, 6sn7, 6sc7, etc. here for preamp duties. I'd replace the weird 4 & 5/6-pin sockets with octals from AES (tubesandmore.com) and build a champ 5F1 type amp. Use a 6SL7 in the one current octal socket (it will be wired up like the 12ax7 in the 5f1 schematic). You could leave the type 80 rectifier and use it the same as the 5y3 (5y3 is an octal type 80). Then, you will need only to replace one of the other sockets with an octal for the 6V6 or 6L6 power tube. You will have one socket you could optionally replace with an octal left, either for reverb driver, tremolo, or another power tube in parallel. Essentially, toss most of the circuit and sockets, and rewire it into a 5f1 or so. You could probably even add reverb using a 6EM7 or 6DN7 as the driver and recovery, or do a simple tremolo with another 6sl7. Good luck, I bet that it has a great sounding OPT for guitar/musical instruments though!

Yeh i found some dates, 1952 on the rust ball, fingers crossed the irons are war surplus. It's convenient you mention the Champ as i've been nutting my brain over the 5C1 champ and the AA764 Bronco, so hopefully i should be able to pickup on the schem pretty fast. I just ordered some caps for it a day or two ago, but i'll see which ones i can recycle for the champ design. Was there any flaws in the champ amp i should change to improve anything?

Thanks for the reply, it's really helpful and gives me some hope for this poor little thing. i happen to have a metal chassis-like structure from an old VCR would this be a suitable base for this build? The current ones just so crowded i wouldnt be able to really change anything, plus i can imagine it having insane AC hum.

Hopefully once my assessment from school start to ease i might have a chance to make some progress, i'll keep you guys updated and probably impale you wish more requests for advice. Thanks again for everyones help thus far.