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TX coupled, pentode linestage

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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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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.
 
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
 
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
 
Bas Horneman said:
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
 
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.
 
Hey Bas,
Guinevere is one of mine too. It was an exercise in how simple and versatile an inexpensive linestage could be done. It has a single stage LC filter, tube rectification, and costs less than $200. The PT was a custome Heyboer unit, and can be rigged for up to 400-0-400 for use with a VR tube shunt regulator, and it is way long on curent headroom( it oculd run a PP 2A3 amp comfortably ).

The circuit can be done for any desired pair of triodes, with care to idle current and MOSFET heat dissipation. It can be done with all sorts of interesting permutations and combinations, up to and including an all-tube active load circuit.
cheers,
Douglas
 
They are both quite good, and in slightly different ways. Both are nearly vanishing, and acting just as a box with a volume knob. One of course has gain. The all-tube version is slightly better, but is ridiculously more complex.

Some comments from those who have heard the TX circuit: revealing and neutral. Adds nothing, and takes nothing away. I sometimes miss the bloom and euphonics of a metal based 6SN7WGT, but even that can be tiresome if you know it is there.

I wish I could give a set of comparable linestages to measure it against, but I am strictly DIY...:)
cheers,
Douglas
 
Doug, :cheers:Could you provide the details for the 1:1 TX used in your buffered-linestage ? Brand, specs ect... Thank-you

hey thor61,
It is an Intact Audio piece by Dave Slagle. It is a bi-filar wind on a 3/4" stack of EI75 80% Ni lams and is gapped for ~15 mA. IIRC it has something like 15 Hy of inductance at that current level. if there's more you need don't hesitate to contact Dave.
cheers,
Douglas
 
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