Magnefest 801-A parafeed preamplifier

Yes you read that right the first time. Magnefest not Magnequest. It’s a fest of Magnequest iron.

This is my first DHT based amplifier. I should have done this earlier. The 801a is really a special tube I think.

RCA 801A

RCA 801A

It is a fest of Magnequest transformers but most of them are below the chassisplate. The only one visible is the Abbie.

Magnequest Abbie

100H/25mA

Magnequest Abbie

The Abbie is not part of the standard line but a one-off product. When Mike put them up for sale on his forum all those years ago I jumped at the opportunity. I just love stuff that has not been built on plastic bobbins. Part experience (10 dollar jobbies I bought a decade ago from Triodeelectronics), part emotion…whatever.

And he built them for his own pleasure and/or projects, gotta be good then right?. Here is what Mike had to say:

Here’s what these chokes are… they are designed to carry 25 madc of dc current and deliver approx 100H and they have a DCR of approx 315 ohms. They can be used both as a power supply filter choke and\or as a plate choke.
Let’s call’em the Abbie chokes… these are layer wound with all kraft paper insulation. They are wax impregnated. They have color coded teflon lead wires. All M6 laminations. And they are designed to be fitted with bell end caps (no channel frame is large enough to fit this trans).
They weigh approx 2 and 3/4 pounds each. They are 2 and 5/8th’s inches tall. And about three or so inches deep (from outside bell end to outside bell end). And they are about 2 and 3/16ths inches wide.
This is a premium layer wound choke… with superlative performance. The calculated temp rise is approx 5 degrees C. It can be used comfortably as a power supply input filter choke. It can accomadate in excess of 600 vrms at 100 hz. It will run coool and quiet.
It can also be used well as a plate choke. This guy will run typically at approx 10kg or less of total flux density when used as a plate choke.
Normally the iron that we use in our finished products will not be offered as catalog items or as standalone products.
That said… being the boss does have it’s advantages. So when I wanted to keep stacking these guys… I thought… no problem… I’ll build some extras and offer them up on the MQ forum. It’s probably a once and done deal though…
I’ll fit these with all brass hardware and your choice of steel bell ends or brass bell ends for $275 a pair plus shipping.

O yes…they also had brass endbells. I just love those as well..

But ofcourse there are other transformers/chokes in the mix. AE-Europe power transformer, a Lundahl LL1673 choke used in the common mode configuration followed by another Lundahl in standard series mode and a Sound Carrier choke ( the C3). I like the Lundahls and the Sound Carriers because they have several layers of insulation between the layers of wire. Which I believe lowers the capacitance of the chokes. I tried to get away with 2 psu chokes but ended up with three since I only wanted to use “oilers”

For attenuation I have used K&K Audio shunt attenuators as I like the concept:

Shunt attenuator concept

Shunt attenuator concept


And it looks pretty. I used a Shinkoh 10k/1 Watt resistor for the series duty. Since I only had to buy 2 I splashed out decided to put in my order to Acoustic Dimension.

The K&K Audio attenuators are made with Seiden switches and Vishay smd resistors. Seiden is from Japan. I also used Seiden for my input switching.

Seiden switch

The type I used for input switching

This story is turning out to be a tale of stuff from all over the world. Lundahl from Sweden, Sound Carrier from Croatia, bought from someone in Slovenia. Seiden from Japan. Padouk probably from Africa cut and made into a chassis by someone in the Netherlands. Rod Coleman pcb’s and the parts I ordered from England. The Magnequest, RCA 801a from the USA. The Attenuator from the USA as well. Shinkoh from Japan. Don’t remember where I got the Lundahl chokes from. Continental Capacitors from the USA I guess via a Canadian parts store. Obbligato from Hong-Kong made in Singapore with German winding machines.

Shinkoh non-magnetic resistors

Shinkoh non-magnetic resistors


KandKAudio Shunt Attenuators
sm.resistor.front_

sm.resistor.back_

Please bear with me. As I’m jumping from one thing to another. When I started this post I was thinking some pictures and the schematic..bla bla bla. But as I wrote this I wanted to tell the story behind the story (not THAT interesting…but nevertheless…no editor and this is my own blog). For instance the padouk chassis was made by the guy who builds these things for AE-Europe. AE-Europe usually orders oak. But the builder sent this one as well..because I guess he had some extra padouk lying around. So AE put this one on sale. Which I found funny because it is many times more beautiful than the oak versions to me. So I got this chassis I like more for less than it would to get the oak. :)

The power supply for the linestage:

Magnefest 801-A powersupply

Magnefest 801-A powersupply

Main psu part 1

Main psu part 1


The power supply for Rod Coleman filament heaters (just one drawn):
Raw power supply for Coleman regs - schematic

Raw power supply for Coleman regs

Raw dc supply for the Coleman reg

Raw dc supply for the Coleman reg

And finally the schematic (still playing around with the parafeed cap)(input switching not drawn):

 Magnefest 801-A parafeed  preamplifier

Magnefest 801-A parafeed preamplifier

Insides showing a copious amount of Magnequest:

Magnefest insides

Magnefest insides

And an older picture before the power transformer was taken to a seperate chassis because the magnetic fields affected the Abbies. Now the amp is so quiet I can’t hear if it is on or off:
Picture of linestage 801-A

Picture in operation (in the corner you can see the power transformer has been taken away…with quick makeshift woden plate for the oiler cap):

801-A linestage in operation

801-A linestage in operation

Various:
The 801-A’s filaments are starved. I read that Thomas Mayer put them on 6V in his linestages. I thought it would be funny to put it on 6,3V just like most newer idht’s.
It became a three chassis amplifier. Planned to put it all in one…but reality got in the way.
I purposely made it old school. I.e. without solidstate rectification or CCS. (Except for the Rod Coleman filament heaters)