Buying a bench power supply

[vampyretim]

Originally Posted by mhennessy

I'll hopefully get a chance to check them out over Christmas - if so, you have first refusal

Thanks Mark,

That would be great.

[poppydog]

I have one of those Tenma power supplies from CPC (30v, 3amp), it was given to me as a gift last Christmas. I am currently on the 3rd supply...

The first one was very good until the digits on the display disappeared, that one was returned. The replacement CPC sent was no good due to the cooling fan sounding like a DC10, that one was also returned. I was going to get a refund, however I thought I would give it one more go. The replacement was sent and the fan was still noisy however it wasn't as bad as the previous, although it is still quite noisy. If it gets any worse I will get a refund. CPC were good about it however they did drag their heels a bit when sending the replacements.

I wouldn't recommend one, I also will be in the market for a good second hand replacement after Christmas.

Regards
Poppydog

[MrBungle]

Thanks for the review. Will stay away

I’ve got a very nice HP supply (E3630A) but it doesn’t kick out enough current per channel. Think I’ll look out for a second hand TTi PL330.

[Boom]

If you can find one the Coutant LQT100 or LQT200 are bomb proof. I've had mine for years. It's twin output good for 30VDC each at 1A. The 200 is good for 2A.

[Sinewave]

Originally Posted by poppydog

I have one of those Tenma power supplies from CPC (30v, 3amp), it was given to me as a gift last Christmas. I am currently on the 3rd supply...

The first one was very good until the digits on the display disappeared, that one was returned. The replacement CPC sent was no good due to the cooling fan sounding like a DC10, that one was also returned. I was going to get a refund, however I thought I would give it one more go. The replacement was sent and the fan was still noisy however it wasn't as bad as the previous, although it is still quite noisy. If it gets any worse I will get a refund. CPC were good about it however they did drag their heels a bit when sending the replacements.

I wouldn't recommend one, I also will be in the market for a good second hand replacement after Christmas.

Regards
Poppydog

Odd, I've been using mine for a few years now and haven't had any issue with it at all.

[G0HZU_JMR]

If you can find one, a cheap option is the classic old Weir 561 bench PSU. I have one here and I bought it over 20 years ago for just £15 at a rally and it's still going strong. I had to clean the control pots and meter selection switches a few years ago but besides this it's still working OK today. It says 0-60V 1A on it but I'm not sure if it will actually deliver 1A at 60V. I've only ever used it at a few hundred mA up at 60V. But it is a small and solid little PSU if you don't mind the analogue metering and dated looks. It has an adjustable current limit as well.

[WME_bill]

I remain puzzled by the replies. Vampyretim seems to be seeking a power supply with two 60v outputs. So all the comments about the usual 30v1A or 2A alternatives surely will not help him.
Buying a glossy modern unit is one alternative, but why ignore older second hand units. At least you can get the service manuals for the older units.

I have a later Weir to that mentioned by G0HZU-JMR, the Weir 761 but it only does the 30v2A or +-15V2A. I have one spare here if you want it. I would expect the circuit of his Weir 561 to be very similar indeed. I also have spare a Coutant LQT200 , dual 15V2A with tracking or series or parallel. A very versatile unit.

The standard workhorse is the Farnell LT30/2 is dual 30V 2A. Always sells. There are less common variants up to 50v 0.5A. You can of course put the two outputs in series to get 60V.
I also have a Solartron AS1414, 60V1A and another AS1412 at 40V5A both with decimal voltage setting and good metering. Early very sophisticated units.

These are all pretty standard designs with variable voltage and current limiting and analogue meters. Easy to service.
I don't think I have ever needed to know that my bench power supply is giving a voltage accurate to within a few millivolts.

For audio amplifiers, I would have thought 60v at 1A should be ample to get it working, even if it will not permit you to give a full power 100watt low frequency test.

If you are nervous about a second hand unit, as you can repair an audio amplifier, a little bench power supply will hold no terrors.
Service manuals are always a point. I have manuals for all those I mention.
wme_bill.

[Station X]

Originally Posted by WME_bill

Vampyretim seems to be seeking a power supply with two 60v outputs.

That doesn't seem to fit in with his requirement that he wants it to aid in the repair of valved guitar amplifiers.

I was going to recommend a Farnell E.350. It has AC LT supplies for heaters along with a current limited HT DC 0-350V output.

[dave cox]

For valve gear, I don't think a variable-voltage / current limited supply is so essential, but I will be interested to hear what others think. In any case, it is quite a job to regulate high voltage and make it robust in the face of fault conditions (or risk terminal damage to the DUT). I have a 350V/1A one under my bench and it is of truly of epic proportions and its forced air cooling sounds like a jet engine! ( its not for sale )

For 'modernish' solid state amplifiers you will need split rail, so an option is buy 2 60V/1A. This would be enough to get most things working. Good to follow others experience on which ones are robust, simple and work. I have come across PSU's with undesirable features, for example one that had a variable current limit also had a non-variable foldback current limit that strangled the output at low voltage - WHY DID THEY DO THAT ?

dc

EDIT:
I should point out that 'foldback' PSU had swtched transformer taps so it wasn't a power issue ...

[mhennessy]

Originally Posted by Station X

That doesn't seem to fit in with his requirement that he wants it to aid in the repair of valved guitar amplifiers.

I was going to recommend a Farnell E.350. It has AC LT supplies for heaters along with a current limited HT DC 0-350V output.

It quickly became apparent that there are two distinct "needs" here - a PSU for valve gear, and another PSU for solid-state amps. Still, like multimeters, you can never have too many power supplies

A dual 30V/2A unit will be fine for power amps - it's what I use - but a pair of these will give 2 lots of 60V if needed. Luckily, dual 30V/2A units are ubiquitous. 0-60V supplies are less so. The Weir 460 is the only one I've used, and that was a long time ago. It only does 1A.

It's worth saying that when you go beyond 30V, you generally find SRC-based pre-regulators to keep the dissipation under control. These are generally reliable, but this is another set of circuitry to understand and potentially troubleshoot.

I've no experience of the E350, but it looks like a nice unit

[G0HZU_JMR]

I did a quick google for the Weir 561 and there doesn't seem to be much on the web about it. So here's a quick image of mine below. It doesn't look very sexy or modern but it has been very rugged and reliable over the years.

I have a TTi electronic load here (as seen in the image below) and I connected it up and did a few tests. These tests prove that the Weir 561 can deliver 1A at 60V and the ripple is very low. Much less than 1mV even at 1A output. I also did a few transient checks by configuring the load to switch between 1A and 0.1A at various switching rates and with various slew rates and it appeared to perform very well with no sign of any change in the output due to the switching of the load. So this old PSU seems to be working fine.

I wonder how well some of the cheapo Chinese PSUs would cope with transient loads, especially the ones that use a switching regulator. I've not used this electronic load very much but it does also offer an arbitrary waveform input to simulate analogue loads so it could be used to model loads that vary with speech or music I guess. However, I've never tried this. I bought the electronic load a few weeks ago at a local company liquidation auction and I'm still learning how to use it and what to expect from the various PSUs I have here when I test them with this load.

I also have the Weir 400 30V 1A model that looks very similar and also a Coutant LA400 PSU. I bought them all on the same day at a rally and the seller even included the manuals. These three PSUs have been amongst the most reliable kit I have ever purchased and they all cost £15 or less. I think the Coutant was a tenner including the original manual.

[G0HZU_JMR]

As a comparison, I tried the same transient check on a Thurlby PL330DP dual programmable supply. This is a dual 32V 3A supply and it is nearly new. The results are good but not as good as the old Weir 561 when both are tested at 30V output with a 0.1A to 1A load transition. The Weir 561 shows no change in output voltage during the transition from 0.1A to 1A but the Thurlby PSU shows a brief 20mV spike (away from 30V DC) on each switching point. i.e. it shows a spike as the load changes from 0.1A to 1A and another spike as the load switches back to 0.1A.

The load is set to switch very quickly in this test and the slew rate for the load transition is set to 2A per ms according to the TTI electronic load. So this is a fairly harsh test. I'm not sure how important the transient performance is for a PSU for a hifi amplifier but the Weir 561 is very good here and appears to be in a different class to the Thurlby PL330DP in this respect. But both are 'good enough' I think.

[See_Mos]

Valve amps usually have the power supply on the main chassis so you don't really need a separate PSU but you could always build one from a scrap (?) amplifier.

For transistor work if I need one, I use this - a TTI CPX200 dual 35v @ 10 amps with adjustable current limit. This is suitable for most solid state amplifiers, even high power and it is rare to find one that will not perform at reduced power.

By the way, it's not as dirty as the photo suggests.

[GrimJosef]

Originally Posted by dave cox

... In any case, it is quite a job to regulate high voltage and make it robust in the face of fault conditions (or risk terminal damage to the DUT). I have a 350V/1A one under my bench and it is of truly of epic proportions and its forced air cooling sounds like a jet engine! ( its not for sale ) ...

I would agree entirely with Dave. Fault modes in valve amps quite often involve high-voltage breakdown. A high-voltage psu needs to be able to withstand a sudden short-circuit and to switch itself off or, at the least, reliably limit the current when that happens. If you're trying to detect whatever fault it was that has burnt out a mains transformer's HT secondary then you do need to be able to deliver the full HT voltage (the naughty part might well not break down or go into thermal runaway at 75% of it).

Sadly while there are plenty of low-voltage supplies available there are hardly any high-voltage ones in current production. A DIY build might take time but so might waiting for the right second-hand one to turn up.

Cheers,

GJ

[6AL5W-Martin]

hello Tim,
I think you need 2 power supplys, one for the HiFi, and another for the tubes.

The PSU for HiFi have to deliver a split voltage, +-30V is great.
The PSU for the tubes have to deliver 6,3V AC, a adjustable negative DC of -100 up to 0V, and a adjustable high Voltage up to at minimum +350V.

I use for my tube repairs an old heathkit, IP2717 ( =IP17)

greetings
Martin

[vampyretim]

Thanks for all the advice.

I'm going to wait for Mark to dig his out of the attic.

Thanks again guys and merry Christmas.