First Build Transformer Problems

[brokennarsil]

Hello everyone, I've got a few dumb questions I just can't seem to get my head around and need some help please.

I'm trying to build my first amp using the parts I have from a gutted old PA amp. I've got the tubes, transformers, and chassis that I want to rebuild into something like a fender 5c5 or some such. The tubes are the same as in the schematic, but the transformers I know nothing about.

My first problem is that I have no way to figure out what ratings my trannies have. I know the old amp was a 30w head and that's about it. If anyone could tell me or point me to a link on how to test these things for voltages, etc. that would be great, because all I have now is a hunk of iron with numbered pins

That being said, if I figure that out, and if my thought process is correct, that still wouldn't mean that I could just wire up the trans using the supplied schematics, right? The caps, resistors and all are designed for those particular trannies, right?

THEN, if that all gets sorted, since I wouldn't want to buy the needed transformers for the application, how could I figure what values of resistors, caps etc I would need to go with the transformers I have?

Sorry about this ton of questions, but I feel like I am getting so close to understanding this, but am just missing one little thing before it clicks. Thanks everyone.

[JHow]

Hmm. You raise a lot of questions. To address the first, you can estimate what the "rating" of the old tranny is (by which I assume you mean how much current the secondaries can take) by looking at what tubes were in the original equipment and their current requirements (find datasheets online). You can then design your new circuit with that as a maximum - so as long as you don't exceed the orginal design, you are probably okay. You need to take into account the current needs of all secondaries: heaters, rectifier, plates, etc.

Sorting out voltages, primaries and secondaries would take a whole 'nother thread.

If you have no electronic knowledge, you will need to do a fair amount of reading before you start. If you have some high school electronics (Ohms Law) but are new to tubes, maybe have a look here:

The Valve Wizard

Start with the section on designing power supplies. He explains about how current needs form the basis of power supply design.

[brokennarsil]

Thank you JHow. It seems I still have a lot of legwork to do to make sure my theory is sound.

I do have very limited electronic knowledge at this point, but the site you linked seems to have good (however steep) info.

And I suppose sorting out the voltages, primaries, and secondaries that you mentioned is what I was looking for in relation to the pins on the transformers themselves. For example, what is pin # whatever for and so on. I heard somewhere of doing resistance checks across the pins to determine something or other, but I'm still not sure what that was all about.

But I have the data sheets for my tubes, and will try to calculate the current requirements for them all and use that as a maximum for my figures.

I'll post back later when I've done more research and see if what I've figured out is correct.

[brokennarsil]

Ha, so I was looking around at some old threads of my own and came across the info I needed! GuitarRock04 kindly figured out the transformer pinouts a while back so I should have what I need now in relation to my pin numbers.

Apparantly the PT high voltage secondary gives 412-0-412. I'm assuming these are the voltages given, with the zero being the ct to ground? If that's the case then, (and I know this is a very dumb question) what is the 412v usable for? In other words, where does it go?

Sorry again for the uneducated questions.

[JHow]

The same site can help with that question, too.

The Valve Wizard

Have a look at the "two-phase full-wave rectifier" section. If valve wizard is giving you too much information, have a look at any basic electronics book explanation of a power supply. The principles used in tube amp power supplies are exactly the same as any other device, with the exception of the use of tube rectifiers. If you live near a university, take a trip to their engineering library and root around. You can still find old texts that deal with exactly these types of problems and give examples using tubes. Just ignore any chapters you find about "waves in ether", but the fundamentals are all there. Also in the library you might see old tube books on radios that can help you get a handle on this stuff. Old radios have power supplies and an audio amp at the end of the circuit.

[brokennarsil]

So the 412 voltages go to either plate of the rectifier?

I read the entire power supply section and that cleared up a lot for me. It's just tough because I'm trying to learn from scratch and having to deal with all the formulae that is presented in those articles. I'll try to look around for some books though, as you've suggested.

Finally though, with all of this being said I have yet another question

I read somewhere that a given amp circut could be used with a transformer thats power exceeds the requirement of that circut, but never one lower, as to prevent stress on the transformer? If this is true could I then use my 412v PT in a circut for a fender pro 5c5? I'm not sure the rating of the transformer in that amp, but I think it's lower than the one I have.

[JHow]

Yes on the rectifier. Yes on the power question, for example, you could safely use a 100mA rated transformer for a circuit that only draws 50mA. As to your specific transformer, if you can't find any published specs, which admittedly may be difficult, it would help to know what tubes it used to power.

[brokennarsil]

The tube compliment was 2x6l6g 2x6sc7 1x6sl7 and 1x5u4.

Other than that, I just know the pinout for the PT and OT. The PA amp that it originally came from was a bogen ch30 which has proven to be the most difficult-to-find-info-on amp in the world apparantly.

Thanks a lot for all of your help, though.

[brokennarsil]

So my noob nature shines through again, and I didn't want to start another thread to ask these simple questions.

In this schematic of the 5c5, I am having trouble understanding where the OT is in the layout page. I'm assuming it's the 'holes' towards the middle/bottom with leads going to the speaker jacks and power tubes?

Also, are there really four instrument-in jacks on this thing? If so, why and how can I reduce it to just one?

[JHow]

Well shoot, you just about have the same tube complement you you are looking for. Transformer should be fine.

Transformer in the layout is mounted on the top of the chassis. The drawing shows wires from transformer coming out of a hole on the left side of the drawing. The layout is draw looking in from the bottom of the chassis.

There are four inputs. You could eliminate some. Someone else with more historical knowledge will have to answer why Fender did that. I guess they thought you might hook guitar and mic for yourself and others on a single amp?

[brokennarsil]

Great! So if I'm not mistaken, all I have left to do now is to purchace the listed resistors, caps, etc and wire it up according to the schematics.

I just hope I have it all figured before I go out and buy all this stuff, but hey it's a learning experience. It almost seems too easy now; makes me think I'm missing something vital.

[JHow]

Pretty much. But you will want to do some more planning before you start soldering.

I would suggest you update the power supply from a safety standpoint. I think most people here would use a DPDT switch, use a modern three-wire power plug, move the fuse to the hot line in front of the switch, and ditch the cap going from the neutral line to the chassis (sometimes called the "death" cap). These deviations from orginality aren't going to affect the audio part of the circuit and provide some margin of safety if things go wrong.

I forgot about it earlier, but a good introductory read can be found at AX84.com - The Cooperative Tube Guitar Amp Project

Look for the document called the "P1 Theory" document. It gives a basic discussion of the basic amp parts, including power supplies, with examples.

You will also want to give a thought to your ground scheme. Ground schemes are important to getting a quiet (meaning no hum or buzz) amp. If you search on "hum problem" or "buzz problem" here and read the threads you will find that many many noise problems result from grounding issues. Good planning up front can help you there. The time spent drawing out your own layout and thinking about avoiding ground loops, keeping noise out of the preamp, will payoff in debugging time later on.

I have not seen a historical 5C5, but the orginal probably included grounds soldered to chassis or a brass ground "plane" - you can see the ground symbols on the old layout, which could mean that the ground was simply soldered to the sheet metal chassis (requires a big iron). I'm not sure if you are attempting an exact replica, or are just trying to replicate the circuit (for some of these amps you can get a replica brass sheet), but that is not the way most people do it now.

For sure, you will want to run heater lines as twisted pairs, not grounded to chassis as suggested in the layout.

Do some searches on "star grounding" and "buss grounding". The valve wizard link has a discussion of grounding, as does the one below.

New Page 1

Look at his discussion of star grounding under the Tube Tech Pages section of the site.

[brokennarsil]

Thanks very much for all of the info, I will check out the links you supplied!

I knew there would be a snag though, and sure enough upon looking at what I had again, I noticed the OT isn't the original. It's some sort of replacement that has no numbered pins, and no markings as to the model number... So, I guess I'll have to bite the bullet and buy a replacement OT with the specs that the amp calls for.

As far as the hum problems, I was was planning to twist my wires to help avoid that, and I was going to use the supplied tabs on the sheet metal of the chassis for a central ground of sorts.

And I'm not really trying to build an exact replica of the amp. As my first build, something that even functions will be a wonderful sucess to me! I'm probably going to have to redesign the layout of the board though, since my chassis layout doesn't provide the kind of space the original board calls for.

But mucho thanks again for all the help, and I'll post updates as the build progresses.

[brokennarsil]

Sorry, last question for a while.

Will this output trans work for my application?

Mojo Transformers Tweed Deluxe Output Transformer

[JHow]

Have fun. I look forward to the pictures. If you are on a budget, I have had success with the following transformers:

P-P OUTPUT XFMRS - A Thermionic Webplace

The guy there is "Matt", he is very helpful. His OT40PP is designed for two 6L6 and has taps for 4,8,16 ohm ouputs, which gives you some choices as to speaker. You can just shrinkwrap the unused leads and tuck them out of the way, or get three output jacks and then you can use external cabinets, or just run a plug from your combo speaker.

I should add, I have no connection with MP and am not suggesting their product is better than anything else out there, but I am saying I have been happy with their products and service so far - and I think the OT40PP is a reasonable value at $29. I have used their 18W PP OT, and numerous single-ended OTs 8, 10 , 20Watts.