You guys have helped me in the past to understand the various taps on a line transformer. I have a transformer like the one shown below.
In a push-pull amp, I understand to use the 8k tap for top tube and common terminal for bottom tube. B+ goes in between on the 2k tap.
Is it possible to use the 4k and 1k (or 2k 500ohm) taps for screens?
In a push-pull amp, I understand to use the 8k tap for top tube and common terminal for bottom tube. B+ goes in between on the 2k tap.
Is it possible to use the 4k and 1k (or 2k 500ohm) taps for screens?
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It will work, but you will have a missmatch. The feedback won't be the same for both tubes.
Al lof the 70V transformers I tried had lousy frequency response. They seem to be designed for PA operation with limited BW. A measure of primary inductance and leakage inductance was consistent with that.
I suggest doing a frequency sweep before spending a lot of time building anything with them.
Al lof the 70V transformers I tried had lousy frequency response. They seem to be designed for PA operation with limited BW. A measure of primary inductance and leakage inductance was consistent with that.
I suggest doing a frequency sweep before spending a lot of time building anything with them.
Look at the "Voltage or turns ratio to output" column.
If 500 is 8X then 1000 is 11.313X and 4000 is 22.627X
If you use the 2K connections the 22x tap is 6.62 turns from the Center Tap but the 11x is only 4.68 turns from ct.
Not really balanced.
You could try it and let us know what you think of the sound.
However:
The 8000R side windings at 70V is designed for about 9mA. (0.625W)
Are you using this to build a 1.25W push-pull amp?
If 500 is 8X then 1000 is 11.313X and 4000 is 22.627X
If you use the 2K connections the 22x tap is 6.62 turns from the Center Tap but the 11x is only 4.68 turns from ct.
Not really balanced.
You could try it and let us know what you think of the sound.
However:
The 8000R side windings at 70V is designed for about 9mA. (0.625W)
Are you using this to build a 1.25W push-pull amp?
Imagine the chart shows the primary 'turns' for the column marked voltage or turns ratio.
For the 2kPP setup, the 4kohm tap is 6 turns away from the CT tap (ie. 2kohm tap), and the 1kohm tap is 5 turns away from the CT. Normally in a UL setup, the UL taps are the same number of turns away from the CT - if the turns to each UL tap aren't the same then the screens are being driven to different voltage levels - so not a perfect match.
Same situation for the 1kPP setup.
For the 2kPP setup, the 4kohm tap is 6 turns away from the CT tap (ie. 2kohm tap), and the 1kohm tap is 5 turns away from the CT. Normally in a UL setup, the UL taps are the same number of turns away from the CT - if the turns to each UL tap aren't the same then the screens are being driven to different voltage levels - so not a perfect match.
Same situation for the 1kPP setup.
Thanks. I'll give that a shot and see what I get.
I'm just messing around and seeing if it'd work. The transformers I'm working with are a little dull in the high-end but bass is great. I'm working with a lot of old parts here and I don't even know if my tubes are all that great either.
I'm using a concertina PI and just for fun I placed the concertina's output caps across the 70v transformer's outer windings just to hear it. It sounded surprisingly good but I left the B+ on the CT. Funny thing was, ALL noise was eliminated that way. When I removed B+ all the noise came back. I can't figure that one out.
I have a negative feedback question too. Doesn't global-nfb put the cathode's DC across the opt's secondary and the speaker? Or is the resistor in series with the nfb minimize any effects of a small DC current on the secondary?
The dc bias on a cathode is only a volt or two, and that bias voltage is divided to a large level to the lower cathode resistor used for the common feedback function. And then that dc voltage is divided again by the series feedback resistor and the speaker resistance. So effectively there is almost zero influence on the speaker.
I think the chart in the first post may need clarification. When using the 8K and Common with the 2K as center this would yield 8K plate to plate. In the case of a AB PP amp, 8K/4 for a 2K loadline. In class A pp then 8K/2 for 4K load line.
There is no symmetrical configuration for UL, see 8K plate to plate example. Using number of turns specified on he Speco 7010 transformer spec sheet.
There is no symmetrical configuration for UL, see 8K plate to plate example. Using number of turns specified on he Speco 7010 transformer spec sheet.
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This transformer is typical of the transformers fitted inside small PA type speakers, (factory use etc). If the 70 volt line from the amplifier is set across the 8k and common, it will produce 0.625W into 4 or 8R. If set across 500 and common it will produce 10W to the speaker because the lowest impedance matches the highest power output. The volume is set on the amplifier to produce 70V p-p maximum.
I tried the SE opt in PP. Not good. The 70v one seems to work well but I'd guess it's limit is around ~15k. It would probably work quite well for a guitar amp.
I'm designing an amp for my car and needed something quick for bench testing it. I have a thread on that. Maybe you guys can help with some of those questions too.
http://www.diyaudio.com/forums/car-audio/243986-diy-tube-car-audio-amplifiers-2.html
I'm designing an amp for my car and needed something quick for bench testing it. I have a thread on that. Maybe you guys can help with some of those questions too.
http://www.diyaudio.com/forums/car-audio/243986-diy-tube-car-audio-amplifiers-2.html
Any of these suboptimal OT work best with very low output impedance finals. Things like 6080's are the way to go. With low output impedances they can drive the high transformer capacitances and get response out past 20K.
Application of Plate to Grid feedback can lower the output impedance of output tubes to acceptable levels allowing for the output tube to brute force the capacitances out of the picture. Global feedback around the OT will have a similar result but is correcting for the poor frequency response of the OT after the fact rather than correcting for it before the transformer. Local feedback is a better approach than gNFB aroung the tranformer.
Shoog
Application of Plate to Grid feedback can lower the output impedance of output tubes to acceptable levels allowing for the output tube to brute force the capacitances out of the picture. Global feedback around the OT will have a similar result but is correcting for the poor frequency response of the OT after the fact rather than correcting for it before the transformer. Local feedback is a better approach than gNFB aroung the tranformer.
Shoog
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