Newb OT question

[volker_77]

I bought an old Masco MA-17 PA amp, and the guy who sold it to me said the output transformer started smoking, then the amp went to crap. I plan on completely redoing this amp, but I have run into a problem.

The photofact for the amp says the original OT had a 7200 ohm primary impendence, 2 6l6's, and put out about 17 watts. Though, I can't seem to find a replacement OT that exactly fits that specification. How much can the OT's impendence vary from this, if at all? Reccomendations?

[bbartilson]

the original OT had a 7200 ohm primary impendence, 2 6l6's, and put out about 17 watts. Though, I can't seem to find a replacement OT that exactly fits that specification. How much can the OT's impendence vary from this, if at all? Recomendations?

We aren't really talking laboratory equipment, so you certainly have wiggle room. In your shoes, you've got to define the goal first. You could:

1. Attempt to restore the thing to original or better fidelity than new.
2. Find the cheapest replacement that would reasonably work.
3. Somewhere in between those two.

As far as the specs are concerned, the transformer doesn't really have a primary 'impedance'. The load on the output tubes is reflected impedance, and is directly in relation to the speaker load on the secondary, and the turns ratio of the OT. So the number of secondaries, and their appropriateness for the load(s) you would like to drive with the amp are at least as important as the intended primary impedance.

Having said that, 10% leeway either way is acceptable to get the thing working again. So that means 6.5k to around 8K ohms reflected impedance. I would tend to slant toward the lower end of that range, but that's just me. There are choices readily available.

A couple of caveats - not having seen the schematic, does the stock transformer have screen taps? Is it cathode or fixed bias? Judging from the rated output I would guess no on the screen taps and yes to cathode bias, but can't say for sure without seeing.

If we can get some notion of the circuit topology, folks here ought to be able to narrow it down to a very few good recommendations. Don't forget to measure the physical size of the thing, and how it mounts. The new part has to fit in the old hole, regardless of how good an electrical match it is.

Best,
-Bill

[volker_77]

Thanks for the reply

I guess my goal is to turn it into a guitar amp. Originally being a PA, I have read that such a goal isn't too hard. I would like to remain as faithful the original design as possible though. That said any help from anyone more learned than me is welcomed. I started a sort of half-assed blog for this little project of mind a while back. http://blikethetubes.blogspot.com/ If you go there I have the schematics and such; if you feel so inclined.

[sportster4eva]

That's a good basis for a guitar amp. For that particular application, a Hammond 1620 or a 1750h might be a good OT to replace it with. You can get the 1620 for around $40 and the 1750h for around $35.
One thing you might want to think about, The preamp tubes in the one you have are the "loktal" type, which are no longer made, so you may want to either scour ebay and see if you can acquire a bunch as spares, or convert to octal base tubes. The 7f7 is equivalent to a 6sl7, as you see in the schematic that amp could have used one or the other, and the first tube, the 7c7 can be changed to a something like a 6sj7/6sj7gt. Here's a great site for looking up tubes, and there's a button at the bottom that lets you compare tubes with similar characteristics. http://www.nj7p.org/Tube.php
You could also use a 6sl7 in the first hole too for an added gain stage, or so that you can use all current production tubes, but the pentode that's there has plenty of gain already. I have this exact same amp in my collection. I love the old retro look.. a real sleeper!

[bbartilson]

I guess my goal is to turn it into a guitar amp. Originally being a PA, I have read that such a goal isn't too hard.

Given that you have a good schematic from which to work, I would say that your goal is indeed reasonable.

The fact that the chassis is quite open on the top, combined with the fact that the current OT transformer is a 'stand up' variety leaves you with a lot of options, and not much metalworking to install a different transformer.

So now we're back to the goal, the budget, and your skill level when it comes to electronics. So we need the answers to:

1. What's your budget for this replacement transformer? As little as possible, or are you willing to put some money into a replacement? Remember that I don't know whether you're 14 or 87 years old. Neither one matters, but if you're younger, you may not have much of a budget at all. If $40 is a lot, I probably have a used part which you could obtain at very little expense.

2. What sort of 'voice' (if any) do you want the thing to have? You're in a good position, in that you don't HAVE to do all of the work at once. But if you have an ultimate plan, that can help you make the initial choices. Fender, Marshall, and Vox are what most of the basic DIY circuits center around. The OT is a big component of that sound, and you're wide open at this point. This is a good thing.

3. What's your general level of skill? Be honest. There's no shame in not knowing much, because you're in the right place to learn. I don't have any trouble helping a newbie, but there are basic tools and skills you need. Doesn't matter whether your involvement was formal classroom training, some other scenario, or none at all.

But, there's no sense in researching and buying an output transformer if you haven't got a soldering iron or a meter, and at least a few other hand tools. Besides, there's plenty of reading material online and off that can help you to fill in the gaps and ask questions when you don't understand something.

Think about those, answer, and we'll get you going on your project.

Regards,
-Bill

[volker_77]

Thanks for the replies

First off I would like to extend a heartly "Thank You" to those who have replied. It's really helping me out. Secondly, since bbartilson is giving me itemized steps I will just answer his questions to the best of my knowledge and encourage anyone else 2 cents.

I can deal with $40 dollars. I know you can find em on Ebay for cheap, but I think I'm leaning towards hammond. There readily accesible and I have heard good things.

The voice of the amp doesn't really matter to me that much, I'm more concerned with ease of the transformation, because this is the first time I've ever actually attempting to do this, which brings me to your last question. (But if given a preferance, I'd like a Fender black face sound.)

I'm extra virgin at this amp building thing. Though I have had some electronic theory in high school and college with some breadboarding and such. I got a sodering iron, which I know how to use fairly well. I've also changed pick ups. So I'm somewhat comfortable of getting the parts out and following the schematic, and I have read a lot on AX-84.com, and other homebrew amp sites giving me an understanding of the terminology and what is involved, but I as far as knowing how to alter circuits and exchanging this for that, and how many watts etc. I get a bit lost.

Hopefully this can be of some use.

[bbartilson]

Excellent answer, and just what was needed for me to point you further.

Out the the Hammond line, I would lean toward the 1750H. It's a good functional match for the old part, and is a reasonable replacement for a Fender amp output.

Only drawback is that it only has an 8 ohm output. Don't mistake that for a problem. I just tend to prefer OT's with multiple taps. My preference is not yours, and unless you have a stack of different cabinets you intend to use with the amp, it's not really a consideration. I'm just trying to be complete in the info I throw your way so you make an informed choice.

You mentioned black face Fender. When I think of that sort of circuit I think of this amp/kit maker named Allen, as he's big into that scene. He has a variety of transformers that would work in your application as well. They are about the same money as the Hammond, but in some cases give you an extra output tap.

His stuff is at:

http://allenamps.com/parts.php#transformers

Also, the cat from Mission Amps (I believe his name is Bruce?) posts here quite often, and has an 8K primary transformer that might well work. Being based toward the 5E3 Fender circuit, it's more a tweed oriented part than black face, but certainly worth considering.

I'm not new to electronics by any means, but like yourself, I'm very new around here. There may well be other cats with appropriate parts to sell or replacements to suggest. If that's you, please jump in and make me aware of yourself. Overall this seems like a very cool community. I'm glad Enzo told me about it.

-Bill