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

---

@  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

[AX84CH]

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

I have ordered one of these to try and should have it shortly.   I'm not sure it will be radically different though an extra 20v is welcome. 

I would probably prefer a linear supply as well, all else being equal.   If these end up being workable for a segment of projects, they are considerably less expensive and lighter than a linear supply.  Of course, everything is a tradeoff.  I'm pretty happy with the amp I built so I would say it has been a general success.

[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.

--Pete

I have ordered one of these to try and should have it shortly.   I'm not sure it will be radically different though an extra 20v is welcome. 

I would probably prefer a linear supply as well, all else being equal.   If these end up being workable for a segment of projects, they are considerably less expensive and lighter than a linear supply.  Of course, everything is a tradeoff.  I'm pretty happy with the amp I built so I would say it has been a general success.

I have one on my bench as I type this doing a burn-in. Have only a resistive HT load on it, 4x 300Ω 25W resistors in series; the stack measures 1190Ω total. With that load the PS delivers 285V, so the calculated current draw is ≈240mA - Meh! typical Asian over-reach spec hype. I just viewed a video another buyer posted just a bit ago and they were seeing similar results. So, probably will return it and leave a 2-3 star review with my eval.

Link to lab results

--Pete

[AX84CH]

I have one on my bench as I type this doing a burn-in. Have only a resistive HT load on it, 4x 300Ω 25W resistors in series; the stack measures 1190Ω total. With that load the PS delivers 285V, so the calculated current draw is ≈240mA - Meh! typical Asian over-reach spec hype. I just viewed a video another buyer posted just a bit ago and they were seeing similar results. So, probably will return it and leave a 2-3 star review with my eval.

Link to lab results

--Pete

Hey Pete-

I notice you don't have a load on the filament supply.   I don't know if it is the case on the one you have but the B+ on the one I have seems to behave better with a load on the filament supply.

I will get around to testing both of these a little more.

Also on the one I have, as I increase the current demand by biasing the amp hotter, the plate voltage is creeping up a bit.  I think it was 285 with ~110mA versus 280 with ~70mA

[AX84CH]

The one I got appears a little different from the one in your picture

https://www.ebay.com/itm/157252283200

[DummyLoad]

Yeah, I saw that same pattern PS on Ebay as well, seems like several accounts to same vendor, USA stock BS, etc., bulk ordered and shipped from China; why they state the 4-7 days ship transit time. 


On the unit I have, adding the 6.3V load did not change things. Test results in link.  3x 10Ω 5W resistors in parallel, for 3.3Ω total load, or about 1.92A current load. Some of the resistors were cooking at ≈130°C. 


--Pete

[johnnyreece]

Yeah, I saw that same pattern PS on Ebay as well, seems like several accounts to same vendor, USA stock BS, etc., bulk ordered and shipped from China; why they state the 4-7 days ship transit time. 


On the unit I have, adding the 6.3V load did not change things. Test results in link.  3x 10Ω 5W resistors in parallel, for 3.3Ω total load, or about 1.92A current load. Some of the resistors were cooking at ≈130°C. 


--Pete

It looks like your jumper is set at 280.  Thanks for sharing the testing results!

[DummyLoad]

Yeah, I saw that same pattern PS on Ebay as well, seems like several accounts to same vendor, USA stock BS, etc., bulk ordered and shipped from China; why they state the 4-7 days ship transit time. 


On the unit I have, adding the 6.3V load did not change things. Test results in link.  3x 10Ω 5W resistors in parallel, for 3.3Ω total load, or about 1.92A current load. Some of the resistors were cooking at ≈130°C. 


--Pete

It looks like your jumper is set at 280.  Thanks for sharing the testing results!

It was! Thanks for pointing that out. I was completely oblivious to the existence of that jumper   Seeing 304VDC now with the 1200Ω load bank.

Link to test results.

--Pete

[AX84CH]

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

This time I have ordered one of those exact units to try.  The other one I got is considerably larger.  I could use the 300V without the doubling in size.

[passaloutre]

Yeah, I saw that same pattern PS on Ebay as well, seems like several accounts to same vendor, USA stock BS, etc., bulk ordered and shipped from China; why they state the 4-7 days ship transit time. 


On the unit I have, adding the 6.3V load did not change things. Test results in link.  3x 10Ω 5W resistors in parallel, for 3.3Ω total load, or about 1.92A current load. Some of the resistors were cooking at ≈130°C. 


--Pete

It looks like your jumper is set at 280.  Thanks for sharing the testing results!

It was! Thanks for pointing that out. I was completely oblivious to the existence of that jumper   Seeing 304VDC now with the 1200Ω load bank.

Link to test results.

--Pete

Living up to your username lol!

How clean is the output? Good enough for preamp work, or would it need some extra filtering?