A small friedman-inspired amp using an inexpensive power supply

[AX84CH]

I have been working on an experimental build that uses a 280v SMPS for power and a small 8k output transformer from a Randall RM20 for a pair of 6P3S tubes.

The preamp is Friedman-inspired and is the sound I am always chasing, not entirely unlike the AX84 Hi-Octane of old. 

Most of the parts were already on-hand and this was intended to be a low-dough build.  For the FX loop, I used a trick from Merlin's first preamp book for the send and a basic recovery stage.   

Global negative feedback is used to tame the overall tone and provide a depth circuit for some additional girth.  I didn't bother with a presence control since relaxing the NFB seems to do about the same thing for me on a lean amp like this.

It has turned out really well so far and has prompted me to start considering a version 2

Your comments and suggestions are welcome-

[passaloutre]

Care to share a link for your power supply? Also what is the capacitance multiplier?

[kagliostro]

@ passaloutre

https://www.electronics-notes.com/articles/analogue_circuits/transistor/capacitance-multiplier-circuit.php

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@  AX84CH

Interesting build 👍

where did you get those PCB ?

Franco

[AX84CH]

Care to share a link for your power supply? Also what is the capacitance multiplier?

I think I bought mine from Ebay the first time but available from a lot of places.

https://www.amazon.com/dp/B0D9VCYBLB

[AX84CH]

@ passaloutre

https://www.electronics-notes.com/articles/analogue_circuits/transistor/capacitance-multiplier-circuit.php

Thanks for linking that.  Someone gave me a couple of prebuilt multipliers years ago.  I have read about them only at a high level and have used them in a couple of projects but have no real command of how to design them.

@  AX84CH

Interesting build 👍

where did you get those PCB ?

Franco

The helper PCBs and terminal strips were made at JLCPCB, though I have had OSHPark make them as well.   I really enjoy using the helper PCBs for projects because the let me have better heater layout than I would normally have and convenient point-to-point wiring around the tube socket.

Terminal strips are sort of expensive so I just started getting these little PCBs made and made a little closer to what I want in a terminal strip. 

All of this is a work in progress for hobby builds.

[kagliostro]

Thanks for the replay


Franco

[zircontweezer]

@AX84CH is that Chris Hurley? so much to thank you for if it is regards Joker/Joe.

[AX84CH]

@AX84CH is that Chris Hurley?

This is me. 

[JPK]

Welcome Chris! You got me started in building amps. I built the High octane kit back in 2010. The November shortly after. I'm up to 7 now. Interested in your thread.

[DummyLoad]

https://www.ebay.com/itm/267165428920

Seems similar to the unit you're using. No cage though.

--Pete

[DummyLoad]

Looks like this one:

https://www.amazon.com/dp/B0D9VCYBLB?tag=threadproductcarousel-20&linkCode=osi&th=1&psc=1&language=en_US

--Pete

[DummyLoad]

This is the one I'd zero in on... 30bux. 300V @ 300mA; enough for 20-25W amp without working it too hard.

Still, I prefer linear PS for tube amps. JMO.

https://www.amazon.com/Vacuum-Preamplifier-Switching-Transformer-100-265V/dp/B0DZGR1HPR/ref=pd_sbs_d_sccl_2_1/141-4689532-8312021?pd_rd_w=SY8Ej&content-id=amzn1.sym.aa738fbd-ad05-4d11-aae2-04b598db6305&pf_rd_p=aa738fbd-ad05-4d11-aae2-04b598db6305&pf_rd_r=PXC57AFWBQH84PVYSA2Y&pd_rd_wg=cdqpX&pd_rd_r=cf9d8605-54ab-40e5-9375-719858e5fb22&pd_rd_i=B0DZGR1HPR&psc=1

--Pete