The Simple 45 -- Any Advice or Updates Before I Attempt to Build?

[thekak]

Hello--

I'm interested in building this amplifier:
https://positive-feedback.com/Issue1/single.htm

Looks like it has Doc B's signature at the bottom!

I found an article about a build that has generated a question:
https://dagogo.com/the-simple-45

Here is the relevant excerpt:
"Steve’s West Coast experience came into play in regard to the Constant Current Source driver input. He custom-built boards with a much higher AC impedance than the original design for additional hum rejection and a far steadier current. These are both attributes of using CCS instead of merely a resistor and his Gary Pimm-influenced design rally delivers the goods."

What exactly is being changed per the article from Positive-Feedback? Or does the Positive-Feedback article incorporate the changes mentioned here? (Changes that may have been made to the design from the Chinese restaurant napkin as opposed to the Positive Feedback schematic?)

Looks like the MPS4250 has become obsolete. Am looking for a substitution. Am not terribly familiar with solid state parts. Using the "search similar" function on Mouser is not returning a whole lot of results--if I hone in on what appear to me to be crucial operating points, I get 0 results for alternatives.

Thanks for any help you can provide,
K

[Paul Birkeland]

You'd have to post two schematics to really know what they are talking about specifically.

It's also important to point out that this design is old, and those who worked on it might caution you that you can do a bit better by modern standards.  For example, when you turn the amp on, you'll put a B+ level of voltage on the grid of the 45, and this can be pretty hard on the tubes.  PJ worked out the soft start circuitry that we use in our current products that pulls the driver plate to 0V at startup and lets it very slowly rise up to the proper number, and this is a very good idea for these directly coupled circuits. 

The CCS in there is not something we currently use, and PJ might have some commentary on what you can expect with it vs. something like our current C4S.  A PN4250 should work just fine in place of the MPS4250.

C6 is better placed between L2 and T2, then the bottom of T2 can tie back to the top of R8.  This keeps the B+ off the primary of the output transformer and lessens the importance of the quality of the cathode bypass cap.

There's no ground reference for the 6.3V winding shown, and that will make some noise problems if you don't assume that there's a CT that you'll ground as you build it.

[thekak]

Thank you for the response, Paul.

All of your points are well taken.

Protecting the 45s is top of mind. In my first scratch build with direct coupling, I believe I obliterated a couple 45 NOS VT-231. The stakes are a bit higher with this one!

I could always buy a soft start kit from the DIY Audio Store--I really don't want to burden you guys with throwing together brand new solutions to the schematic. It seems easy enough to incorporate.

Per the second article, valve rectification is an option. I'm partial to choke input power supplies anyway.

If you think the CCS is nothing special, I could pursue a "gyrator" for the input tube:
https://wtfamps.com/gyrators/
http://www.bartola.co.uk/valves/for-sale/gyrator-pcb/

The Wauwatosa Tube Factory article mentions DC coupling AND the uses the 5965 as an example at darn near the OP of the Simple 45--these are strong signals in favor of going this route!

As for the iron, I don't really know if Magnequest still exists. I posted on the Magnequest board here and sent an email to an aol.com address I found on a dated looking website.

My friend Larpy (who encouraged me to buy my first Bottlehead kit) showed off his Eros with Sowter SUTs and it sounded really nice. So I took a quick look at their offerings and there appear to be comparable components on offer: OPT model 8983 and choke model 8985 (40 mA - 50 H - 374 DCR).

Again, thanks for the tips, modifications, advice, and the hobby!

K

[Paul Joppa]

Mike (Magnequest) has retired and hardly ever even posts these days.

There are a number of current source designs out there and even kits. We sell a few as upgrades, e.g. for the S.E.X. amps but it's specific to that amp, so there is no general-purpose guide to using it in other circuits. The original current source is good enough to achieve the intended low driver distortion.

If you do use that design, the safest way to avoid arcing on startup would be to take the current source power from the top of the cathode resistor (R8) and not use R6,7.

I'd make R3 an adjustable resistor (100 ohm trimpot) so that you can adjust the 45 grid voltage to set the bias point, since drifting bias is the biggest issue with direct coupling after arcing.