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A Loftin-White style SE with EL34
A Loftin-White style SE with EL34
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Old 9th March 2019, 08:44 PM   #21
Sheldon is offline Sheldon  United States
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Join Date: Dec 2003
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A bit pedantic, perhaps. These are direct coupled amplifiers, but not really "Loftin/White".

The original here: Loftin White: September 2007

It was based on a pentode input tube.

There's a simple schematic of a triode version, halfway down the page.

Note the connection from the top of the output cathode resistor, to the top of the Unbypassed input cathode resistor. That's the crucial difference, and it cancels power supply noise. I've built a 6SN7/801 version of this. It's quiet as a grave and uses only film caps throughout because it doesn't require a lot of PS filtering. Took me a while to get my head wrapped around the concept.

Back in their day, caps were large and expensive - relative to their capacitance. Note that the original version uses only a half-wave rectifier.

Sheldon

Last edited by Sheldon; 9th March 2019 at 08:47 PM.
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Old 9th March 2019, 11:55 PM   #22
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Sheldon View Post
There's a simple schematic of a triode version, halfway down the page.
The one on the right embodies the LW principles. The green line represents the output AC current loop.

Last edited by Sheldon; 10th March 2019 at 12:02 AM.
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Old 10th March 2019, 11:29 PM   #23
BJosephs is offline BJosephs  United States
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So I noticed a hum pretty quickly and just had time to troubleshoot it today. At first I thought it was heater realated (Iíd forgotten to provide a ground reference) but it turns out to be power supply noise. Thereís about 10mV of ripple at the speaker terminals. To mitigate it Iíll add a noise cancelling cap from b+ to cathode but Iíll need to wait for a parts shipment to do it. Beyond that I tweaked the values a bit so I have 90% dissipation with 375v across the power tubes. Output power seems to be about 7W peak. I also tried bypassing the e88cc cathodes but I didnít hear a difference so then I added 6db of cathode feedback. The sound cleaned up quite a bit but I think I like the more ďliveĒ character of leaving it out. Iíll probably build a switch for it.
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Old 14th March 2019, 10:50 PM   #24
BJosephs is offline BJosephs  United States
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Next update: I swapped the 100uF cathode bypass caps for 40uF film and added 4uF caps on each channel from B+ to the cathode for noise cancelling. The cancellation is quite effective though I can still hear the hum if I get very close to the driver, overall I consider it successfull. I used a film for the large value because the tolerances are tighter. I probably should have gone with a 50 and a 4.7uF the better match the mu of 10.5 but started getting spooked over the size/price.

I think the prototype is done and I now need to turn my attention to a proper enclosure.
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Old 15th March 2019, 04:55 PM   #25
VictoriaGuy is offline VictoriaGuy  Canada
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A Loftin-White style SE with EL34
Quote:
Originally Posted by BJosephs View Post
added 4uF caps on each channel from B+ to the cathode for noise cancelling. The cancellation is quite effective though I can still hear the hum if I get very close to the driver, overall I consider it successfull.
Just a guess on my part, but I think you'll find the hum will decrease once you re-build the amp on the final chassis and shorten up the wiring.

Interesting circuit; thanks for posting!
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Old 18th March 2019, 12:23 AM   #26
BJosephs is offline BJosephs  United States
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Originally Posted by VictoriaGuy View Post
Just a guess on my part, but I think you'll find the hum will decrease once you re-build the amp on the final chassis and shorten up the wiring.

Interesting circuit; thanks for posting!
That would be nice. I hadnít considered external sources.
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