Fairchild 660 from scratch

[fazeka]

There's nothing really gained by using tubes in the B+ regulator and the same regulator circuit could be done using mosfets. It would save money and heat, or would it be blasphemy?

Not blasphemy, thanks for your design! Just to confirm, looks like J4 = control voltage, J7 = signal amp and J8 = 6V6 grids?

Also, it seems the STW8NK80Z is not readily available from the major suppliers, at least in the US. Some saying mid 2023 availability!

 

[fazeka]

Thanks DaveP for checking in! I really appreciate it!

Re: the Hammond 1645. I would hope that the schematic on their site is up-to-date (I've posted it a few posts back) but you're right, sometimes what happens in production doesn't manifest itself into the documentation until (much?) later...

That said, do you know how it is wired differently now?

Thanks,
Chris

The Hammond 1645 that I used was old stock, I believe that the secondary is wired differently now, check out before purchasing!

 

[fazeka]

Thinking about Heikki's PS design (still a noob so forgive me)... is there any advantage to tubes in the power supply to allow a softer "ramp up" to B+ voltages (a softer start-up) than the MOSFETs?

 

[beatnik]

one of the advantages it gives tubes and power supply capacitors an easier life, but in fact soft start can be implemented with solid state regulators too

 

[fazeka]

Thank you beatnik for that explanation.

Another question: I noticed DaveP used two OEP X187B transformers for the control amp input. I'm confused as to how he wired them when comparing his schematic to the original (which has only one control amp input transfomer). So it seems that DaveP's schematic ("T7") shows TWO control amp input transformers... OK.

It seems that the OEP X187B primaries are wired in parallel... and the secondaries of these two are wired in series? Yes?

Thanks all for helping me confirm my understanding! Very appreciative!

Chris

 

[Heikki]

Not blasphemy, thanks for your design! Just to confirm, looks like J4 = control voltage, J7 = signal amp and J8 = 6V6 grids?

Also, it seems the STW8NK80Z is not readily available from the major suppliers, at least in the US. Some saying mid 2023 availability!

J4 = unregulated B+ voltage to control amp. J7 = signal amp B+ and regulated voltage to control amp (12AX7). J8 = 6V6 screen grids.

The mosfet doesn't need to be STW8NK80Z. Almost any N-Channel mosfet with high enough voltage rating should work.

Receiving tubes don't need or benefit from soft start. Direct coupled cathode followers can be damaged on start up if the B+ is present before the cathodes are hot. There is a simpler fix for direct coupled cathode followers than soft start circuit.
https://www.valvewizard.co.uk/dccf.html

 

[fazeka]

You guys see any issues in using a Hammond 812A for the control amp input?