Output transformer ??

[distorto]

Hello,

I'm trying to learn how to test unknown OT's to marry them to speakers. In most cases I know which type of power tubes the amp used. I just don't know the impedance load the Trannie is looking for. I am using the calculations mentioned in post #7 of this HC thread (the poster got it from somewhere else)

Output transformers - Harmony Central Musician Community Forums

My question is this: How much leeway do I have for matching speaker loads to transformers. For example, assuming I have an amp that uses 2 6V6's (8000k) and using a filament transformer to feed 6.3 volts ac to the OT's secondaries I find that one transformer is 48.8:1 so I do the rest of the math and I learn that the transformer wants see 3.359312012899758 ohms Am I safe with 4 ohms? (forgive me if this is a stupid or obvious question) I would imagine it would be fine but????

Or let's say another tranny works out to be 27.2 :1 which, after math , I learn wants to see 10.813148788927336 ohms. Is 8 OK?

Learning this and how much leeway I have will open up a whole new world for me in the conversion realm.

Thanks a bunch

Chris

[RWood]

I think you are right on in your thinking.

I just went through the same exercise with a replacement OT for a Princeton, and the way I looked at it was "is this better suited to a 4 ohm or an 8?", because that is what we have available to use. (Actually Weber also offers a 3.2 ohm speaker, at least they did). In my case it was obviously closer to a 4 ohm, as it is in your first example.

Remember that a speaker's impedance is frequency dependent, so it is really a moving target. We are just trying to get the most appropriate match, or the "lowest mismatch".

RWood

[distorto]

Thanks RWood!

Yeah, I have a couple of chassis I want to make into guitar amps and a bunch of parts I've salvaged. The one I'm tackling first is and OLD philco radio amp that used #42 power tubes with a #80 Rectifier. As far as I can tell it seems to be well suited for 6V6's or EL84's I was thinking of making an 18 watt lite. The OT that came with the Philco seems a little wimpy size wise. This is the one that works out to want to see 3.4 ohms with 2 EL84's.

I also have a more robust tranny that was part of a salvaged organ amp that wants to see 10 ohms. I guess I'm just trying to decide if I should use the more substantial xformer with a slightly greater mismatch or physically smaller one that is a little closer.

Thanks again for your reply!

Chris

[RWood]

Okay, I have a better picture now of where you are coming from.

Personally, I would go with the bigger OT and use an 8 ohm speaker. But there is nothing saying you can't try both - swapping out OTs is pretty easy.

It also depends, to a degree, on what speakers you have lying around that you might want to use in this amp.

The 18 watt lite IIb is a great circuit - you'll love playing through it. I made one from an Imperial radio (mid '50s) and then added an EF86 channel, switchable coupling cap values, etc.....a great test bed. Also a very rewarding project. Have fun.

[distorto]

Hey thanks again for the input!

An 18 watt has been on my want list for some time. I don't have any EL84 amps. I have a 5f1 champ, a 5e3 Deluxe, a 6G2 Princeton and a Sovtek Mig 50. So I'm trying to convert 2 chassis I recently scored into a couple of classics that fill in any sonic blanks that those leave (The second chassis is nearly a 5C8 Twin already)

I'm leaning towards going with the more robust xformer in the 18 watt I'd have to come up with a speaker/cab though. I have a 2 x 10 60's vox cab here that is 4 ohms (can be wired to 16) So that gets me back to leaning towards the 4(ish) ohm xformer.

I do have early 60's C12,PS Jensens that came out of an organ. It's weird because on measures 13.3 ohms and the other 11.6??? I think I get 6.9 with them in parallel which seems close enough to 8. So I'd just need a cab.

Anyhow, sorry to blather on and thanks again for you input!

Chris

[distorto]

RWood,

Just wanted to pop in to say that the 18 watt IIb is finished and you are right - it is an amazingly fun circuit to play through. I'm actually quite blown away by this thing!

I ended up going with the 4 ohm(ish) original transformer and I'm glad I did because it sounds fantastic through my 60's Vox 2 x 10 cab.

One last question: I used one 25 watt 25 ohm power resistor on one leg of the mains to a primary lead on the PT. This took my B+ from too high to right where it should be and lik eI said it sounds fantastic. Is there any thing wrong with this approach to getting the working voltage in the amp lower? Most of the info I find searching deals with zeners. This (power resistor) seemed to be an easy and straight forward way for me.

Thanks

Chris

[RWood]

Chris,

Congrats on your build; I'm glad that you like the way it sounds. Now I want to build another!

Vox cab?? You know your PI has an unused grid that is just waiting to see an EF86 channel come before it!

The only two considerations I can think of about that 25 watt power resistor are sag and heat.

Did you go with a tube recto or solid state? If SS, then the sag induced by that resistor might be just right to sound like a tube. And if you already have a tube rectifier, see what you think when the volume is on 11 and you are hitting power chords. Are you okay with the recovery time?

And the heat - just make sure the resistor is in a well ventilated spot, no wires going across it, not near anything that might melt.

Can you post some pics of your build?

Rwood

[distorto]

Yeah,

I'm pretty happy with the sound of this thing. I went with a 5Y3GT rectifier. I don't notice much sag but I haven't dimed it much. It sounds great half way up and under. I'm pretty impressed with the way this fills out so fast. At 9:00 it gets full and starts to compress a bit. Kind of reminds me of an AC15.

[RWood]

Looks great! And that power resistor is sufficiently away from everything.

What kind of cabinet, if any, are you planning to put it in?

[distorto]

Thanks,

The layout of the knobs switches is a bit spread out because i used existing holes except for the inputs, which I drilled myself. The pilot light assy. and bakelite knobs are from the original Philco radio.

I'm going to do some sort of Marshall-esque head cab. I'm thinking I'll have to leave some extra space for air flow around the Power tranny. Two of Its edges are flush with the chassis...so t'll be longish for an 18 watt.

Chris

[distorto]

Here is the finished head. It sounds awesome. Thanks for all your help!!

[RWood]

Looks great! Nice tolex job. Have you cranked it up yet?

[distorto]

Hey thanks. It was my first time working with tolex. I haven't really gotten the hang of getting tidy corners yet.

I have cranked it and it's a great lead tone. I tend to use it more at or below 12:00 where it is very full sounding and dynamic.