The "Muscovite" 6S3P Tube Phonostage

[kevinkr]

All parts for final power supply have been ordered, it is essentially the supply shown in this thread, but I have decided to build one for each channel, but sharing raw filtered DC and filtered screen supply. I will probably also use 6BD5GT as pass tubes and DC filament power on the cascode error amplifiers in the supply.

I'll post some details on the supply build as I proceed

[kevinkr]

Here are some minor variations on the theme previously expressed. Essentially the changes are two fold (and not implemented in my design), the first eliminates the dependence on grid current for biasing the upper half of the cascode, and the second (previously shown) removes the battery in the choke based mu-follower from the signal path.

As always there are trade-offs, the quality of the power supply becomes even more critical with the fixed bias version of the cascode in exchange for hopefully more consistent behavior over the life of the tubes, and in the case of the mu-follower the quality of the coupling capacitor may impact sound quality. Both add additional complexity, although relatively minimal cost. Just an alternative to experiment with if so inclined.

[kevinkr]

All resistors in this design in both the supply and pre-amp circuitry are metal film.

Pre-Amp:
I've older Holco H4 in most locations in the audio path, but the cascode plate load resistor is a 2W Roederstein 10K 1%, and series resistor in the EQ is a Caddock MK-132. Nothing terribly exotic or expensive here.

The EQ caps are selected, closely matched REL RT polystyrene and foil type with a base tolerance of 5% - quite cheap actually.

The most expensive parts are the coupling caps which are TRT Dynamicaps, but the Russian FT-3 should work well for the 0.22uF. (Leave plenty of room)

The 6S3P seems quite a bit more forgiving wrt to VHF oscillations than the 5842 or D3A or any number of other high transconductance tubes, no signs of any oscillation using my 100MHz Tek scope. Probably not a good idea to omit the grid stopper on the input however, both for stability and potential RFI.

Power Supply:

I'm stacking Panasonic 450V caps in the current supply to achieve 50uF @ 900V. Each of these caps has a resistor across it to assure that voltage divides equally across each cap.

The output cap is film.. There is no reason why all the HV supply caps could not be film if cost is not a concern.

Power transformer: Edcor XPWR-228-120
Choke: Edcor CXC100-5H-200mA
Filament: Antek AN-0509

Note that in theory at least an EI transformer for the filament supply should provide more isolation to EMI on the power line - in practice in a half dozen or so designs where I have used these toroids there has been no problem.

[GTHICM]

Having heard your previous designs and this new design, I have to say that this latest "Muscovite" is substantially better. And this was with the temporary power supply.

What chokes did you source for the phono preamp? I think that the Magnequest EXO-01 were the only ones I found that had the DCR you specified and oddly enough, some makers do not list DCR in their specifications.

[kevinkr]

I am the proud owner of several pairs of MQ EXO-01 which get recycled from time to time into new projects. This is one of those pairs and they will get to stay in their new home a long time.

[kevinkr]

Note that if anyone is intrepid enough to build one of these things I will be happy to oblige by posting the BOMs here..

Once I build the permanent supply I will post pix and any enhancements that may cross my mind.

Mine is getting quite a lot of use..

[Carlp]

I'm now looking at this thanks to your suggestion on another thread, and may give it a go over the summer, so a BOM would certainly make life easier. PS is a moving target, I know, but will you include that? Thanks.

[kevinkr]

Hi Carl,
I'll post BOMs over the next few days, the supply shown is essentially the supply I am building, I am planning to experiment with 6BD5GT in place of the 6V6 pass tubes, other than that no significant differences.

[kevinkr]

Here is the first cut of the BOM. I will add the power supplies sometime over the next few hours to days, and update here. Just download the latest version when available - will not change the existing BOM. Note that I have listed recommended parts, do not feel bound by my recommendations, although you may end up with something that sounds a bit different as a result.

Edit: Updated bom with high voltage power supply added 2012-06-09

Edit2: Updated bom with filament supply added 2012-06-10

[Carlp]

Excellent. Thank you Kevinkr. It's even in spreadsheet form! I hope to get to this over the summer, which for me begins in a few weeks as the kids get out of school. I'll probably start off with a parts bin breadboard version and modify from there, though I'll no doubt use the Edcor x-formers from the start. I saw no listing for the power xformer but put in an inquiry about it. Others are quite reasonably priced, even for a die-hard miser like me.
Carl