London Power's Power Supply for Preamps (PSU-PRE)

[London Power]

Have you tried the PSU-PRE Power Supply Kit for Preamps? Post your comments or questions here!



The PSU-PRE power supply kit for preamps is low-profile and will fit inside a 1U rack chassis. It uses a pair of DC, well-filtered, standard Hammond 229-series transformers to create the heater and plate supplies. The plate supply has two nodes that can be individually tapped with paired wiring. The heater supply can be wired for either 6V or 12V; 12Vr is preferred for use with London Power preamp kits and for using the ERK Electronic Relay Kit for channel switching. The heater supply also supports the switching circuits.

The regulated heater output can support up to two/four 300mA heaters at 6Vac or up to two/four 150mA heaters at 12Vac. There are three Links on-board that set the heater transformer wiring and regulator output to "6V" or "12V". The regulator keeps the heater voltage hum free.

There are two basic versions:
PSU-PRE-12 can support up to two 12A_7 tubes - enough for a 2-channel preamp
PSU-PRE-24 can support up to four 12A_7 tubes - enough for a 3-channel preamp

Read more about the PSU-PRE at https://londonpower.com/electronics/tube-preamp-power-supply/ .

If you're interested in London Power's tube preamp kits, see this page: https://londonpower.com/preamp-kit-selection/ .

[K O'Connor]

Hi Guys

Note that both versions of the PSU-PRE come with the same number of regulator BJTs, which means that in the 2-tube version these can be stood up on the PCB rather than being bolted to the chassis. For the 4-tube version, the BJTs should be mounted as the photo shows and bolted to the chassis. The kit notes show how to align everything.

PSU-PRE includes mounting hardware for the board and BJTs.

Have fun

[John Bourke]

Attached is my put together but not yet powered version:        I'll power it up tonight and see if it's a power supply or a smoke machine.

I do wish I had read the previous comment about the BJTs before assembling - as you can see, I've got them underneath.  I may take them out and resolder them. 

Went together pretty easily.  The only thing I struggled with at all was figuring out which bits went where, which took me a little bit of sorting.  I hope  I managed to get it all figured out.

I put the terminal blocks on so I can screw the proper wires in once I get it in the chassis.  This way is easier.  As you can see, I had to hack at the 5mm input one a little to fit the 7mm pitch on the board and clear the jumper for lnk2.  the output ones are a good fit for the smaller 4mm pitch blocks (a bit of a tight fit, but worked well enough - might be 3.5mm on the board)

Next step, plug it in and test the outputs.  Then move on to the bass preamp.

[K O'Connor]

Hi John

The pitch of close-spaced wire blocks on London Power PCBs is 0.1", so 2.54mm if you are using metric terminal blocks, although I have only seen metric in even millimetres: 2mm, 4mm, etc.0.1" is usually 0.1"

The mounting direction for the power BJTs is in the kit notes. It's my experience that very few people actually read the notes but do like to lose them a lot. You can leave the BJTs as they are instead of disturbing your connections and reheating the semis unnecessarily.

Do you have a Power Limiting Safety Socket? This is the Mandatory Project in TOT (Tonnes of Tone) (yes, metric tons here but inches on the PCB). The PLSS is essentially incandescent light bulbs in series with the device under test, which limits power provided the correct wattage bulb is used appropriate to the DUT power expectations. Using the PLSS saves $$ in fuses and in other components not yet burned. The most likely error in a new build is a solder bridge.

[John Bourke]

Hi John

The pitch of close-spaced wire blocks on London Power PCBs is 0.1", so 2.54mm if you are using metric terminal blocks, although I have only seen metric in even millimetres: 2mm, 4mm, etc.0.1" is usually 0.1" 

The mounting direction for the power BJTs is in the kit notes. It's my experience that very few people actually read the notes but do like to lose them a lot. You can leave the BJTs as they are instead of disturbing your connections and reheating the semis unnecessarily.

Do you have a Power Limiting Safety Socket? This is the Mandatory Project in TOT (Tonnes of Tone) (yes, metric tons here but inches on the PCB). The PLSS is essentially incandescent  light bulbs in series with the device under test, which limits power provided the correct wattage bulb is used appropriate to the DUT power expectations. Using the PLSS saves $$ in fuses and in other components not yet burned. The most likely error in a new build is a solder bridge.

Ah, yes, that makes perfect sense.  In any case, the metric ones I had seemed to have been made to work.

Makes sense about the load bank...  Are the values for the bulbs something you can share here and I can figure out a way to get them set up?

[K O'Connor]

Hi John

You have it back to front: PLSS is NOT a load; it is on the mains side to restrict power that gets to the DUT.

Use the lowest-wattage bulb you can find for the initial test, say 25W.

Have fun

[John Bourke]

Ok, next question:
I did the initial test as you said above, no smoke, all good.  Gave it full power, still no smoke, but I'm getting 16ish volts from the LV side and 420ish volts from the HV side.  Have I done something wrong?

Voltages I've measured are:
Input: 122.6VAC
LV transformer out: 7.76 VAC
HV transformer out: 159.2 VAC

outputs:
16.16VDC
432VDC

[K O'Connor]

Hi John

No loads and high mains AC.

It looks like the LV PT "out" is one winding, reflecting the lack of loading. The PTs have dual secondaries, both of which must be used. In the case of HV this is taken care of; for LV you decide their orientation (series / parallel) for the heater voltage desired. In this case series for 12Vdc heaters.

The PTs have poor regulation because they are small and open voltage can be 20% for 12VA and 30% for 6VA units measured with 115Vac input. Add the percentage difference of your high mains.

Raw Low-V will be the full DC produced by the bridge output and is easily 16v to 18V. For W3 to be so high you might have any of D3,4,5,6 reversed. If those are as they should be, try a load of one tube heater.

[K O'Connor]

Hi John

One other thought: make sure QP1 is MPSA92 NOT -42. use lots of light and a magnifier to read it. The wrong sex BJT could give high output or no output.

[peteotto]

Yes I have! 

Looking at the date of the build...it was over 10 years ago.  (2010-11-13)  Holy smokes!  Has it been this long?

The PSU works great and is quiet, no complaints. 
If I remember correctly, first it started as a PSU for the guitar pre-amp, but the crappy solder gun crapped out and did some damage to the PCB. 
Then there were about a dozen calls to Kevin. 
In the end it is currently being used in the Bass pre-amp.  Still being used today.

I highly recommend the PSU 

[K O'Connor]

Hi Guys

Thanks for your feedback and kind support

Note that the PSU-PRE peteotto has is the original one released in 2008. The PSU was designed to support up to six 12A_7s. The transformer regulation was such that at a lighter load the heater voltage could be problematic, so towards the end of supply of these boards we were including a heater regulator board. The new (current form) PSU-PREs are available in two power ratings and incorprte the heater regulator.