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Old 31st October 2005, 01:59 AM   #1
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Default TX coupled, pentode linestage

Hey-Hey!!!,
I have a buffer stage, faux-pentode cathode follower lnestage. It is quiet with a simple, single stage L-C filter in the PS. The output TX is an Intact Audio unit. Nickel core with a gap to live with some DC.

Adjust idle current by varying the faux-g2 voltage.

It takes ~15 mins to warm up, and flesh out the sound. I haven't got a good reason why. Certainly the OPTx isn't getting all that warm, so it may be the power supply stabilizing.

Here is the hand drawn schematic in .pdf B+ of ~250 works quite well as delivered by a salvaged SCA35 power tx. AC on the 5687 heater.
cheers,
Douglas
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File Type: pdf buffer_linestage.pdf (55.7 KB, 183 views)
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Old 31st October 2005, 10:06 AM   #2
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Hi Douglas,

Would you please explain the "faux pentode" descriptor to a triode-using know nothing. What advantages would this have over a basic 5687 CF using a constant current sink ?

How much leeway do you have for adjusting your cathode bias? It would seem to be very dependant on your transformer.
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Old 31st October 2005, 10:25 AM   #3
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Hey-hey!!!,
On the pentode plate curves, after the diode-slope portion, they are flat( compared to triodes anyway).

Examine also the cascoded triode plate curves. After the voltage below Ec2, the curves are quite pentode-like. With a MOSFET in place of the upper triode, what do you think the plate curves will look like?

Try drawing a vertical line through the upper gate voltage on the lower triode's plate curves. Turn the g1 lines horizontal where they intersect this line. Operate in the horizontal portion.

On adjusting bias, there is a fair bit of leeway. AC flux in the core is quite small, and the L does not fall off until curent passes 20 mA for this particular wind and gap.
cheers,
Douglas
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Old 31st October 2005, 02:00 PM   #4
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here's a different link
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Old 31st October 2005, 08:01 PM   #5
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So, if you want it, go to either Intact Audio and register and log in, or go here:

http://audioroundtable.com/GroupBuild/Projects/

The Audioroundtable Group Build Projects page and download it. That forum does not require any registration.

If you are intersested in my PP amp design with a similar hybrid cascode PP front end, look at Merlin V2. You will need an OPTx with two taps...
Cheers,
Douglas
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Old 1st November 2005, 10:50 PM   #6
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Hi Douglas,

Nice design...does it sound good? In which subforum at intactaudio are have you posted this design?

I've got a AVC from intactaudio to try out...gotta try it before Christmas..

Cheers,
Bas
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Old 1st November 2005, 11:08 PM   #7
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Quote:
Originally posted by Bas Horneman
Hi Douglas,

Nice design...does it sound good? In which subforum at intactaudio are have you posted this design?

I've got a AVC from intactaudio to try out...gotta try it before Christmas..

Cheers,
Bas
Hey Bas,
It is in the transformers section. It has another home on AudioRoundtable's Projects folder.

It sounds brilliant. I had to crawl back into my previous design, and find where I had made mistakes. Turns out I must have been soldering in the dark from one of them I found.

Principle complaint is its warm up time. it is always a bit on the thin side for the first 10 mins or so.

It is made with some seriously inexpensive parts though. RCA jacks salvaged from a Kenwood L-07c, power Iron from a SCA35, volume control is the famous Radio-Shack $2.99 stereo 100k by Alps. Chassis is an old Thyratron PS chassis.

I think it is time to roll the two designs together and build with a DHT amplifier. A 10/801A I think would be best. I have a nice filament choke that is just the ticket for quiet DC on the filaments.
cheers,
Douglas
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Old 1st November 2005, 11:12 PM   #8
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Quote:
I think it is time to roll the two designs together and build with a DHT amplifier. A 10/801A I think would be best. I have a nice filament choke that is just the ticket for quiet DC on the filaments.
I'd be very interested in that design...since I got some nice 801A's in a tubes for transformers trade recently.although I'll probably default to some plate choke and parafeed the 801A's.

Regards,
Bas.
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Old 2nd November 2005, 04:10 PM   #9
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How does this design compare to Guinevere?

Or was she not a design of yours?Click the image to open in full size.
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Old 2nd November 2005, 04:30 PM   #10
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I've put up your schematic on my site for easy reference...If you mind..let me know and I will remove it asap.Click the image to open in full size.
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