Tubelab SE Parafeed Monoblocks (5 channel)

[xcortes]

I am in the process of designing/building a set of five 300B amplifiers for a 5.1 channel system. I already have the 97dB/W efficient speakers in place. I came across the Tubelab SE boards and I fell in love with them for the simplicity that they would represent in building: PCBs and no need for drilling five chassis sounds like heaven. Moreover the comments around the amps are very positive so I'm leaning towards them.

I already have all the iron (Magnequest) as follows:

Power transformers: PGP 8.1 (120V primary, secondaries 350-0-350V 75 mA, 6.3V 2A, 2X 2.5V 1.25A)

Power supply chokes (instead of R6): RGC-6 (10H/75ma/180ohm DCR)

Plate chokes: EXO-03 (60 mA, 30H, 344 ohms DCR)

Opts: EXO-50 Permalloy (5K:16,8 or 4 ohms, 20 watts max)

From what I've researched I would need minor modifications to build the amps as monoblocks and parafeed. Initially I would just screw the amps to wooden boards and put them on a high place out of reach. Later I will find how to cover them.

I think that I will be setting the 300Bs to run at one of the WE classic operating points at 400v, 87 v bias, 5k load and 8.3 watts.

I would very much appreciate any comments,

xavier

 

[audiowize]

While you're at it, you might ditch the mosfet follower and use some Magnequest grid chokes in place. You might also look into adding something to balance the hum, and order up some CMC's to bring back some of the dynamics lost with standard DC supplies on DHT filaments. Also R30 should be decreased a bit for 300B use.

On the other hand, they'll probably work pretty well in stock configurations, just be sure not to exceed the current rating of those plate chokes you plan to use.

 

[xcortes]

just be sure not to exceed the current rating of those plate chokes you plan to use

I think that I will be setting the 300Bs to run at one of the WE classic operating points at 400v, 87 v bias, 5k load and 8.3 watts

At that OP the current is 60mA so I'm fine. I guess.

 

[audiowize]

Ah, I didn't notice that, yeah, you should be right under 60ma. Just don't layout your iron like this:

http://www.tubelab.com/images/glow45-1.jpg

 

[kevinkr]

Current rating on that secondary HV rating sounds really low for a 300B amplifier - what about the power factor when using a capacitive input filter, not to mention the current required for the driver circuitry??
A 125mA secondary rating would still be pushing it for a single 300B..

You might just squeak by with SS rectifiers, choke input and no other load on that transformer. Fixed bias would be recommended because the plate voltage will be in the low 300V range due to the choke input and after going through that plate choke. You would be constrained to no more than 60mA of plate current. I run my 300B amp (not at all similar to the tubelab design) at 80mA/400V fixed bias into a 3.2K load and get about 9W @ 5% THD...

This set of transformers might actually work better with the 45.

 

[xcortes]

Current rating on that secondary HV rating sounds really low for a 300B amplifier

Hmm, I don't know. The Tubelab page says: "the power transformer needs to provide about 640 or 650 volts center tapped at 150 mA". But this is for a stereo amp and mines will be monoblocks.

Also, if I bias the 300B for 60mA, I have 15mA for the driver which I understand is set to 12mA.

the plate voltage will be in the low 300V range due to the choke input and after going through that plate choke

I haven't simulated this exact power supply but I remember simulating a similar one, albeit with solid state rectifiers, using this transformers and a 300B circuit and I was getting a B+ of about 430V. The plate choke will reduce 11V so I'm still above 400V. In any case I can go to a lower voltage OP and still get 6 watts.

kevin: I'm trying to learn and understand, not to confront your views

And thanks for the help