JCA50H - My fix for eliminating mains hum - use DC heaters

[quickclicknick]

I received a new Jet City JCA50H back in March and try as I might I could not get rid of an annoying level of mains hum and buzz. Note that I live in the UK and so my experience may not be the same for other regions as we have around 240V and 50Hz at the wall socket.

I tried all of the standard DC elevation tweaks taken from the Valve Wizard http://www.valvewizard.co.uk/heater.html but nothing made any real difference on my amp. I will accept that it may be possible to arrive at a quiet amp using AC heaters but for me I decided on a far easier solutionwhich for me has other benefits too.

Here is my strategy for creating a separate DC power supply for all 5 x 12AX7 preamp tubes. It is very easy to implement but I am sure will meet some disapproval from purists as it breaks away from any traditional approach using transformers.

My solution was to buy a very small 5V (adjustable up to ~5.8V) switched mode power supply (Ximeng Electric SMUN SMB-20-5 rated 5V@4A at a cost of a few dollars) and break the heater jump link at JP5 and leave only the 6L6s running of the stock PT. The PSU fits snuggly in the space next to the IEC mains input socket and is attached by two screws through the rear of the chassis.
I also took the oppurtunity to create a DC elevation as the Cathode follower stages exceed the maximum H-K voltage for a 12AX7 by a lot. The actual way I have implemented this is a work in progress but at the moment is just two resistors connected across the HT. The -ve DC supply to the 12AX7 heaters is grounded as far as AC is concerned with a 2.2uf 400V polypropelene cap.
The amp is totally quiet as far as hum and buzz are concerned and there is only the tiniest bit of high frequency ripple getting into the signal and that is lost in hiss. (My next mods will be to fit higher power plate resistors in the preamp to reduce hiss but for now it is quiet enough).

The heater voltage for the preamp is running at 5.8V, which is within +/- 10% of 6.3. I may consider changing the value of the resistors in the PSU to alter the adjustment range to get it a little higher at a later date. The total heater current for the 5 x 12AX7s is around 1.7A. The PSU runs only slightly warm to touch.

A 'feature' of this setup (that is annoying) is that at a cold start the 5 x 12AX7 heater filaments are a very high load and the PSU goes into a cyclic short circuit protection (called hiccuping) with an audible clicking. After about 4-5 seconds of this pulsing enough current and so heat has got into the filaments that things stabilise and altough it isn't ideal I can see no ill effects from this behaviour. Maybe a 0.5-1ohm resistor in series would stop this but it would also lower the heater supply and so for now it is as it is.

This strategy was chosen for another reason. It means that I can now fit EL34s or KT66 or KT77 (maybe even KT88) without any fear that the stock PT will be overloaded as I have removed 1.7A of load from the stock configuration.

I hope that this experience will help others and may inspire others to do similar or better than I have done. I bought the power supply direct from China and it was marketed as an LED strip light PSU. The cost was $5.60 and so I don't think anyone would argue that this isn't a cheap and simple upgrade!

I have some pictures and will post if anyone is interested.

Cheers

Nick

[quickclicknick]

Photo of power supply

Photo of power supply connected to preamp heaters

[Zozobra]

Very nice. I hadn't thought of using an SMPS for the heaters before. There was talk over on slocloneforums of getting some universal tube amp SMPS power supplies made but nothing came of it. It would be cool to have a power supply with a tunable output voltage and the high frequency AC output means much smaller filter caps could be used too.

[quickclicknick]

Thanks! As soon as I ran into the idea of going to DC and that I would need at least a small toroid and a bunch of hefty electrolytic caps the SMPS idea hit me.

The 1st version used a Dell PDA brick. That worked fine but was running at 5.3V and being sealed not easy to adjust.


I then ordered direct from China via AliExpress.

[Tortuga]

This is certainly welcome and relevant to my interests - I've been going a bit buggy with all the loud hum & hiss coming from my 50H, ever since getting it into an environment where I could actually crank it up.

Forgive the newbness, but can I bug ya for some clarification / details? So, DC power supply run 6.3V at 2A would fit the bill properly? Any chance you can point out where JP5 is on your pics, and is it just a jumper (or pair of jumpers) that gets clipped? What's the large, black, capacitor-looking thing near your Crunch pot in the 2nd picture? I'm having a bit of trouble understanding how you implemented the DC Elevation circuit - can you help me with that?

Thanks!

[quickclicknick]

You can bug me for details...sure! But bear in mind that it is a work in progress and so don't do exactly as I have done. Question it and do it better!

JP5 is a blue and black link cable that is on the stock JCA50. It runs north to south looking at the PCB with the controls towards you. Look for the source between R54 and R52. It is marked as JP5 (jumper 5). The destination end is just north of the orange relay and next to R10. It is marked JP5. This is the start of the heater bus for the preamp tubes.
On mine it went through many a wrangle while trying to reduce hum and so all I have left is a photo showing the twisted remains but if you look on another photo elsewhere (or on yours) you will see it.

I disconnected and removed this cable and attached a DC PSU supply to the preamp end. (the longish blue and brown twisted cable from the PSU in my photo).

The black cap is indeed a hack for the grounding part of the DC elevation. It provides an AC ground path and links the new DC heater negative to the nearest ground I could find, which is on the south side of R4. It is a 2.2uf 400V Solen, which is way overkill in terms of cost and quality but all I had to hand.
*NOTE* You do not NEED to use any DC elevation, my DC heater mod works just the same (in terms of being quiet) if you ground the negative as I had it in this picture

I just got to be a bit experimental following good advice from Zozobra. He set me looking into DC elevation properly in the first place and I was horrified to find that V4 pin 3 especially is running beyond the recommended maximum H-K voltage.

With regard to what PSU rating will work you need at least 2A and ideally at 6.3V. My chosen PSU can in theory supply 4A. You will not find many off the shelf PSUs or suitably small bricks that can readily be dialled to 6.3, most are 5V or maybe 6V or 12V.
One thought I had was to split every 12AX7 to run series heaters and buy a 12V PSU instead. This would halve the current required and also probably be a far easier startup load. I hope that someone else does it this way as it's probably a better approach, but also a lot more work and involves ruthless and brutal cutting of traces on every 12AX7 socket.

[quickclicknick]

Here is a summary of the minimum steps I followed to convert JCA50H to DC heaters for the preamp:-

Desolder the AC heater supply link cable to the preamp at JP5 next to R10, insulate the bare cable ends and tie back out of the way.

Solder 6.3V DC supply +ve to JP5 on PCB nearest to R10.

Solder 6.3V DC supply -ve to JP5 on PCB nearest to orange relay.

Solder a link wire between the -ve at JP5 (or on pin 9 of V5 if it is easier) and ground (I used the end of R4 nearest to the control pots) but you could use the main ground connection at the edge of the board.

The DC supply should be 6.3V @ 2A minimum however in my experience, as the startup current is momentarily much higher, a 2A PSU may struggle at a cold start.
I tested with various PDA and laptop bricks before choosing the one in the photos. All of these made the JCA50H completely quiet as far as hum and buzz goes. I settled on the 4A PSU as it is the largest that will fit inside the chassis, has adjustable output voltage and it runs cool.

[Tortuga]

Definitely thinking about doing something like this very soon. Thanks for sharing this! I will be linking this to the main JCA thread as soon as I can.

Question - if the stock xfmr is already putting out 6.3v AC, could I simply rectify and filter that to get the clean DC I need? I realize that wouldn't provide the additional current overhead to run the other tubes as you describe, but if I'm OK (for now) running 6L6s, perhaps that makes for a pretty simple diy project that can be done without having to source a power unit as you did that is small enough to fit into the chassis.

[quickclicknick]

Recitifying the stock preamp supply by adding a bridge rectifier and some big caps after the jumper capble JP5 was my first attempt too. I couldn't get it to work though. There isn't enough spare voltage (the PT is 6.3V-0-6.3V AC) and you lose some in the conversion (I should know how much but I can't remember the math). I think I got it running with about 4.8V, which is way too low.

You may do better than me though as I didn't persevere and was already resigned to building something extra to run fatter output tubes.

[quickclicknick]

I have moved the small SMPS for the preamp heaters from inside the chassis to on top of the chassis. Previously I had it installed inside but it was a bad idea as during long term tests yesterday it became clear that my little PSU was getting too hot. If the chassis had been re-installed inside the head shell it would have surely cooked itself to death.

Here is picture showing the new power supply location (it has a green LED) and also showing my new Harma KT66-Retro output tubes and Mullard NOS ECC81 fitted in the V5 phase inverter position. I can report that fitting the KT66s was like opening the curtains in terms of the improvement in the sound.

[Zozobra]

Did you increase the screen grid resistors for KT66 use? The screens are pretty flimsy!

[quickclicknick]

That is a very good point! I'd done a little research and thought that they were 470 ohms, so the same as 6L6, but revisiting just now says that they should be about 1K.

Is 1K a good value?

[Zozobra]

What is the plate and screen voltage? If you know the OT primary that will help in working out what sort of SGR is sensible although that will require finding a datasheet with the g2 current curves on too. This all said I'd use 1K as a minimum as 3.5W max for Wg2 is pretty skimpy! You wont notice the difference much unless you're ragging on it hard and you may like the extra screen compression anyway so I'd use the biggest value you can get away with tonally. Don't be afraid to go high. I know guys who use 3k3 SGRs in KT88 amps just because fuck buying a quad of KT88s because the screens blew out prematurely!

[Tortuga]

Looks great, Nick - I think you made the right decision moving that thing out of the chassis.

I'm still torn over this solution versus installing raised, twisted heater wiring as shown in the latest postings on the JCA thread. Probably going to go with twisted heaters, as it's less expensive (and I'm a cheap bastard) and I can still do this later if I see fit. Midnightlaundry posted about upgrading the rectifier diodes in the JCA thread as another idea for lowering the noise.

Great to hear about the KT66s - hope you guys get that working reliably. Would love to give that a try at some point.

[quickclicknick]

Thanks for the heads up on the twisties and the diodes, very interesting.

I won't have time to mess with this for a while as I'm travelling but will be swapping the screen resistors to 1.2K as a start and will measure all the voltages to work out exactly what the Wg2 dissipation is. Then I can go even higher if required.