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PSU testing
Hey folks,

So I've built my first London Power standalone preamp kit and unsurprisingly it doesn't work. This is my first amp project; I'm used to building pedals. But I have to start somewhere. It's the M-Pre (hot rodded 800 version) with the Pre-PSU for two tubes.

So what I did was build it like I build a pedal. Basically followed the instructions, put it together and turned it on. Of course in hindsight I should hve been constantly testing as I went, but as i said, it's my first one.

So the tubes didn't come on and of course there's no sound. So what should I do first? Check the psu, I guess. So what would be the best way to go about that. I have some basic electrical knowledge (mostly safety stuff) and of course a dmm. Any advice would be greatly appreciated.


Hi Steve

Did you build the safety socket for your test bench? It's basically light bulbs that are in series with the thing you wanna test and it works great! I used to blow up a lot of fuses testing things I built - although I've gotten better at building things that don't blow the fuse the first time Big Grin

I got TUT3 a while back and there is a suggestion to build a little bit then test it, then build more. Actually maybe that was in the Standard project from TUT5? Anyway, it makes sense to start with the power supply and test it. Once you know it is working okay you can forget about it when you build the next part and power it up from the supply. You still need the light bulbs as you go through the step by step building and testing. When you get to the part of adding tubes you need to increase the wattage of the bulb cuz of the heaters.

I don't blow fuses when I use the light bulb protector. Once you feel sure that everything is working, you can bypass the light bulbs and just be on fuse protection. It's cool even though maybe it takes a bit longer than just shovelling everything together ans slamming on the juice Smile
Hi Guys

Steve, as Nauta said, it is best to build and test incrementally AND to use the Power Limiting Safety Socket - the Mandatory Project in TOT (Tonnes of Tone).

With that in mind, you build the PSU-PRE Power Supply for Tube Preamp first, which you have the option to build in two steps since it really is two power supplies on one PCB. Suppose you make that incremental choice and build the high-voltage plate section first because it is simplest. Follow the assembly notes, wire the mains for your local voltage, then plug the PSU-PRE through the PLSS fitted with a low-wattage bulb (25-40W).  You will see the bulb brighten then dim if everything is okay. There is no load yet and if you measure the DC voltage at the W2 output block it will be a couple of hundred volts - lower than the voltage with full mains voltage.

If everything is okay then you would stuff the heater supply and test its output voltage at W3.

Both W2 and W3 have the positive end of the supply on pn-1 and the negative end at pin-2. Once LNK8 is added, the heater supply is "programmed" to look like a negative supply where W3-1 is at ground and W3-2 is -12V (or -6V if that is how you've set it).

Remember to install LNK8 to join the DC grounds together.

Remember to add the chassis wire to CHS.

You can make the testing increments even smaller in the case of the PSU-PRE. Were you to do the mains wiring and mount the transformers only, you could verify that mains wiring is correct by measuring the PT secondary AC voltage. You should see >200Vac out from T1 and >12Vac from T2 if LNK4 and LNK5 are in place for a 12V supply. You would measure these AC voltages by carefully probing the middle two holes of where the respective bridge rectifiers are mounted.

You mentioned that everything is assembled, so you can still test what you have WITHOUT TUBES first, using a low-watt lamp in the PLSS. making sure that all the appropriate links are in place for 12V heaters on both the PSU and M-PRE British-Style Tube Preamp Kit, and for the mains voltage for your area, it is best to clip the black meter probe to ground using an alligator test lead. This lets you probe voltages with only one hand and the other on the power switch.

With the power 'off', you can do a resistance check to verify the ground connections are intact. The meter probe clip above should be on the circuit ground NOT the chassis as the ground-link-isolator on PSU-PRE places a small resistance between these points. With the RED meter probe, check PSU W2-2, W3-1, M-PRE VP-2, and the sleeves of the jacks. All these should read zero, or as close to zero as your meter allows. As a reference, wipe the meter probes together to see what the lead resistance is prior to performing any resistance check.

Turn the power 'on' and see if there is high voltage at PSU W2 and then if this voltage is getting to M-PRE VP. Within M-PRE, you can follow the voltage along the string of 10ks (21,19,20) and see that those connections are intact by the presence of voltage.

Similarly, you can check for heater voltage on PSU W3 and follow this through to the tube sockets pins-4 and 5.

If voltage is not present at any of the points, turn the power 'off' and see why.

If everything is okay without tubes, add the tubes and change the lamp to 60W or so and power up. You'll see the lamp glow dim - if it is too bright try a 100W bulb. Voltages will be a bit low, so the -12V will look like say -10V and the +320V will be <300V. Still, you should now see voltage drops across M-PRE 's supply resistors (R19,20,21). You only have to probe one end of these Rs to see that the voltage is dropping as you move from VP to the input stage.

Ideally, if you have a sinewave generator and a scope you can do a signal test through the preamp and verify proper operation of all the controls. Without these pieces of test equipment, you can just go to an audible test.

An audio test should begin with the output level set to zero, or if you are using the plexi wiring, that the monitoring equipment has its input level set to zero to will avoid being surprised by any loud noises, buzz or hum.

Common errors would be solder bridges, reversed diodes, reversed bridges - should cause the PLSS bulb to light bright. It used to be that semiconductor packages had printed lettering on them - silk screened with ink or paint. The modern way is to burn the lettering in with a laser, which leaves very faint grey text on the usually black background, which is very difficult to see without bright light and rotating the part slightly to see the reflective difference. For this reason, it would be easy to reverse the flat BJTs (QN1-4) or the bridges.
Hey guys,
Thanks for the replies and for the minute detail. I have a lot to learn. Awesome. 

I didn't build the plss yet mainly because incandescent light bulbs aren't available here in Belgium anymore. Would the more modern replacements be as effective?
Hi Steve

They have to be incandescent. Everything else has SMPS supplies built in and halogen is inappropriate with zero protective qualities. You can likely find surplus bulbs or even get them from China.

Have fun
Ok. I'll have a search online to see what I can find.

Thanks again
(11-21-2019, 06:58 AM)SGillespie Wrote: Ok. I'll have a search online to see what I can find.

Thanks again

Hi SGillespie,

I found this 60W incandescent lamp here:

as well as this 40W one:
and another one here:

and a 25W one:
also here:

Here is a 100W one:

These are in the UK, but I don't think there should be a problem ordering some.
Hope this helps.
Formyx Amplifiers

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