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Old 13th May 2008, 11:26 PM   #1
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Smile Wind my own output transformer for 100w bass amp

Hello this is my first posting on this forum, ever!
I thinking of building a 100w bass guitar amp. Really haven't nailed down
a design yet but I'm thinking the most typical would be four 6l6's
in a push pull configuration.
I know there are tons of kits out there. And there are off the shelf transformers available.
That doesn't stop me from wanting to "do it myself" !!!

Now I know there is a thread on this forum that is exactly what I need. But there is so much info on this forum that I can't seem to find it.

So I thought I would start with the most complicated part (at least to me) first.

The output transformer. Now from reading here and other places I've figured out that two 6l6's in push pull are needing a 4k primary. Now does that mean that four in push pull need 2k????
And is that impedance on either side of the center tap? Or is it the entire winding?
I would like 4 and 8 ohm taps on the secondary also.
I think I will leave it there because I don't have a clue. I've been reading about interleaving, bifilar windings and I pretty confused as you can tell. So feel free to post your designs or point me to other threads and links.
Thanks in advance.
Dennis
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Old 14th May 2008, 12:02 AM   #2
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Its OK if the OPT is slightly higher impedance than the tubes,
slight reduction in power out in exchange for greater control.

Maximum power is transferred when "impedances match",
but that equation doesn't tell about steady state losses.
And loudspeaker impedance varies with frequency, xmax,
and cone breakup...

Remember that DC resistance of your plates will not be the
same as impedance (slope of the plate curve at that point),
though both are measured in Ohms.

What would be bad is OPT lower impedance than the tubes.
Just don't go lower and you will be fine. Throw some extra
turns on your primaries for insurance.
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Old 14th May 2008, 12:21 AM   #3
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Fender Bassman 100W complete project with opt details:
http://www.audiyofan.org/wiki/index...._de_sortie_DIY

I doubt you will find anything better.
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Old 14th May 2008, 12:43 AM   #4
cerrem is offline cerrem  United States
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For BASS amp 100W would be minimum.... Since bass needs more power due to lower frequencies....
I prefer using the JJ KT88 valves or 6550A valves.....
You can push the Plate B+ to about 700V safely and then make sure the screens are about 350V , no greater.....this is IDLE VOLTAGES....
Figure you will get roughly 100W per pair at those voltages with a 5K plate load in Class AB1..... So 4 tubes would be about 2.5K plate load, which to answer your question is refered to as Plate-to-Plate when talking push-pull...
Impedance matching for maximum power transfer only works in transmission lines and RF applications when using for example S-parameters 50 ohm or 75 ohm line matching , or when you are dealing with 600 ohm balanced lines ect ..... For pentode tubes you want the biggest plate load that will stay in the "SATURATION REGION"...this will give you the most power output for the lowest distortion, also refered to as the "optimum plate load".....
The saturation region of a pentode is the section of the curves that go almost flat approaching constant current, of course they don't go perfectly flat but almost, that represents the plate resistance of the pentode tube and is quite high... The region to the left that is almost vertical is refered to as the DIODE region and has a very low plate resistance...you don't want your load-line to intersect this region for your application...For starters you need the correct curves....by making sure you have ones for 300V screen since your 350V screen voltage at idle will droop down under full load... Also assume for now your 700V plate voltage supply will droop to 600V , which is not that far off what will really happen... So with 300V screen voltage selected curves, draw your load-line from 600V up to where you intersect the 0-BIAS curve , making sure you stay to the RIGHT of the knee of the curve, since this will make sure you stay in SATURATION region...
It's judgement call when the plate resistance drops by more than 25% as you move to the LEFT of the 0-BIAS curve, don't go that far... Then multiply this Load-Line by 4 and you get roughly 5K plate-to-plate load....
So you need a transformer to handle 200W at 2.5K load for 4 KT88 valves this will produce about 707V of AC signal across the full winding at full power output... I can tell you right now it will be roughly a 3" stack of EI-150 using M6 core material.... You will want to keep the flux density about 13,000 at 20Hz at 700 volts AC for an acceptable bass OPT..... This 3" satck of EI# 150 would be 26.7 cm^2 of effective area......this would equate to roughly 2271 turns on the primary.... You have roughly 280mA of RMS AC current so a safe wire gauge for the primary would be #26 or #27 AWG...... Modest amount of interleaving is OK, keep in mind this is a bass amp not a MacIntosh Bi-filar wound OPT....
You can figure the rest out I am sure....

Best Regards
Chris
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Old 14th May 2008, 05:52 AM   #5
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Default bass output transformer

Thanks everyone for your replies. I actually was considering the bassman circuit.

cerrem
Is that 2271 turns of #27 the entire winding with a center tap? This would be good for the 4 tubes with a plate load of 2.5k?
I calculate a winding radio of about 17:1 if you use 8 ohm speaker.
That would be roughly 147 turn on the secondary. Any Idea of the secondary wire size?
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Old 14th May 2008, 02:01 PM   #6
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I'm still searching the forum for info. Still no detailed winding info to be found for the lowly push pull transformer.
There is a lot of info on winding SE transformers.

I guess what I need is detailed instructions from someone who has actually done this.

100 watt bass guitar push pull output transformer. Using 4 6l6 tubes.
It would be nice to have 4 and 8 ohm outputs.
thanks
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Old 14th May 2008, 05:07 PM   #7
cerrem is offline cerrem  United States
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Quote:
Originally posted by audiojerk
I'm still searching the forum for info. Still no detailed winding info to be found for the lowly push pull transformer.
There is a lot of info on winding SE transformers.

I guess what I need is detailed instructions from someone who has actually done this.

100 watt bass guitar push pull output transformer. Using 4 6l6 tubes.
It would be nice to have 4 and 8 ohm outputs.
thanks
If you want to wind one...you need a winding machine, even if it is small, with a counter.... Then you need to cut the insulating sheets by hand for the proper widths, these need to be applied for each layer... Solder the lead-out wires..Then stack the steel laminations, VACUUM impregnate with varnish to now make the insulations electricaly sound, then bake... the biggest problem nowadays with making transformers is nowing what materials to use and what and where to get them.....
First get a transformer book...Like by " Ruben Lee" then you will have the basic equations that you can design with...

Chris
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Old 14th May 2008, 05:56 PM   #8
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Hi Chris Thanks for the reply, again.

This is what I found in my junk box. It's an old 10 hy 250ma. choke.
I think it might make a good donor core. Here are some dimensions.

2 x 3 1/4 x 4 inches. The laminations appear to be aprox .014 thick.

The thing weights about 7 pounds with the copper still on it.

Let me know what you think about it. What kind of watts is it good for?

Sorry about the delayed responses. They still have me on moderation.
How long is this going to go on for?
thanks
Dennis
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Old 14th May 2008, 06:45 PM   #9
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Chris do you have plans for a winding machine you describe? I think I saw one or two on this forum but I can't find them.
How elaborate does this have to be?
I have rewound a few power transformers with nothing more than a hand cranked apparatus.

Oh the winding window on that core is about 2" x .75"

Chris does this core have an official EI number?
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Old 14th May 2008, 08:25 PM   #10
cerrem is offline cerrem  United States
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Sounds like a 1 + 3/8 lamaination...
(3.437" x 4.125")

Window: ( 2" x .687" )


Chris
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