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Old 31st January 2004, 07:07 PM   #1
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Default direct coupling

please , tell me your opinions about direct coupled tube amplifiers[ no capacitors in signal path] i have see one circuit , and i have many questinos about this project. thanks
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Old 31st January 2004, 11:50 PM   #2
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The best known direct coupled SE amp is the Loftin-White.

What is it you're looking for?

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Old 1st February 2004, 03:26 AM   #3
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Default direct coupled

Hi 2a3. Other opinions may be different, but I sense better and more bass yet the midrange seems more open. The highs seem clearer, cymbals seem to have more shimmer. The overall inner detail seems better too. More space around the individuals, that sort of thing.

The down side would be to keep the plate current constant with varying AC voltage changes. I don't think it is a huge problem though. I take it it is SE?

If it's not too difficult to modify, I think it is worth the mod, for the experience if nothing else.
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Old 1st February 2004, 05:16 AM   #4
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Other opinions may be different,
Yes, mine is...

This is a MAJOR compromise,or shall I say challenge, it can work fine but it has it's caveats..
Definetely NOT beginners territory...

With modern boutique caps getting better and better as we speak, I'd opt for that coupling cap nothing else being defined here.

If not, you'd better know electronics and you won't be asking in the first place.

Just my opinion and with all due respect to anyone else's.

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Old 1st February 2004, 10:03 AM   #5
Vortex is offline Vortex  Hungary
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yes or no?

I'm working on a parallel single ended 4 x 2A3 based class A and I've also seen this no capacitor project ...

check this out

Loftin White Circuits

My question... if there's any need of NOT using capacitors at all...

I think the Black Gate capacitors and some other brands too make such a quality today, that such a conventional design with capacitors could easily beat Loftin White designs.

I don't know.

"A pair of mono 8 watt Class A parallel single-ended no feedback amplifiers with all Audio Note silver-wired circuit and power supply, Audio Note silver foil signal capacitors and tantalum resistors, Black Gate caps everywhere, all Audio Note silver wired 0.2mm R36 C-core output transformers. They utilize 4 NOS 2A3, 2 NOS 5692 and 2 NOS 5U4G tubes."

(Audio Note Kageki) ..

I'm confused about this no-capacitor design.

Any ideas still ? I'd like to know more about that all.

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Old 1st February 2004, 02:30 PM   #6
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Loftin White circuits should reach high levels of audio but at a cost. If anything happens i.e. the tubes sockets are dirty then max B+ would be at the grid of the output tube. This would be biasing the tube to draw high amounts of current, thus damaging the tube. Loftin white is not recommended due to tube aging of driver could be earlier than output tube, mechanical connection needs to take place for electrical connection. Alot of elements for things to go wrong. Try monkey design which takens the B+ for the driver tube from the cathode biasing of the output tube. Has a balancing thing to prevent tube damage.
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Old 1st February 2004, 02:56 PM   #7
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In the Mullard 3-3, an older SE design, the screen of the EF86 driver is fed from the cathode of the EL84 output tube. Mullard argued that direct coupling reduces phase shift in an amp, and thus much greater amounts of negative feedback can be applied without causing instability.
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Old 1st February 2004, 05:28 PM   #8
SY is offline SY  United States
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If you mean "direct-coupled all the way to the speaker," lotsa luck. Intellectually, the idea is appealing. Practicality is another issue. Rule one is that tubes fail. Rule two is that failed output devices can destroy expensive speakers very quickly- speakers tend not to like the sudden imposition of a couple of hundred volts from your low-source-Z high-current supply.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 1st February 2004, 05:40 PM   #9
dhaen is offline dhaen  Europe
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If you are hoping to remove all caps from the signal path, I'm afraid you'll fail. In simple designs (with an output transformer) you'll still have the power supply caps in series; in more complex designs, where you add another voltage rail, you'll add caps.
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Old 2nd February 2004, 01:57 PM   #10
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Default PP design

Hi 2a3. I agree with Frank, it is an undertaking to say the least. My test PP requires adjustment every few days or so, about 1 ma. off. Output tubes change plus the added problem of the driver sections changing balance and transfering that unbalance to the output tubes.

If I add a compensation circuit to the DC amp, then I would have to build another DC coupled amp and compare, which takes alot of time, which I am not willing to do right now.

A little tip I heard at another chat site that seems to improve the sound is running the output leads of the OPT thru the chassis, connecting the speaker wires to them, and using the output jacks as merely clamps. This can rid one of speaker wire connectors, a solder connection, the output jacks from handling current, and another solder connection to the jacks.

Does look a little ugly though, from the back.
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