LUMLEY: ST70 issues

Hopefully you can help with a problem I have with my Lumley ST70 Reference power amp.

My set up is as follows: Esoteric X01 D 2 CD player, Linn Sondek LP 12, Thorens 124 Mk 2 with Van den Hul Super One cartridge, Magnum Dynalab MD 90t tuner, all feeding into a McIntosh MC2300 preamp, Cambridge Audio 840W power amp and B&W 802D speakers and wiring loom is by Kimber Kable. Mains power is through a PS Audio mains regenerator set for 230 volts. The sound is nicely smooth, detailed and with good bass when its present from source. My room is 8.0m x 3.8m x 2.5m. However, I like the valve sound and some time back acquired a second-hand Lumley power amp.

From the start, I had difficulties and here is the sequence of events spread over a period of time: New set of valves throughout with four Sovtek 6550 valves. The other valves are ECC82 and ECC83, two of each. New valves fitted, I switched on as per instructions. At this time my speakers were Audio Plan Kontrast Mk 3, well regarded and a relatively easy load. Immediately, there was loud hum which I took to be mains. 

In the back of my mind's recesses, I recalled something I read that some amps react to the earth wire in the 3 pin plug. So I experimented with a twin wire mains. Bingo, hum vanished! 

Next followed a house move and a once in a lifetime upgrade as above. Having settled in, I hankered after that valve amp. Still using the twin earth mains but not happy about this I decided to give it a try. I had at least one pleasant hour listening to some nice jazz on the radio when there was a bang, a flash and then silence on the right hand channel! 

I handed over to the valve specialist who concluded that the cause of the problem was a defective ECC82 valve and having replaced blown valves, fuses he then played it for several hours as a test.

After about fifteen minutes, that loud hum was back. Behind the hum, it was clear that there was lovely sound. So close to nirvana, but yet so far... 

Remembering my trick with the twin mains no earth, I tried it again. Fantastic, only the slightest of transformer noise, not much. I was just ten minutes into a nice relaxing listen to classical music on Lyric FM, when ‘PHUT’ and the left hand channel went dead. Fuse only this time, I think. My guesses are as follows: 

Lifting the earth is not a good idea and are there any other consequences to this?

The speaker output taps are 8 ohm; no 4 ohm is available but the 802s have a minimum impedance of 3.2 ohms. But this amp has big transformers and weighs in like the proverbial battleship. The valve guy said it was all point to point wiring and soundly made. 

So I consult your good selves with the hopes that you maybe able to pin point or guide me as to what is wrong. The hum that vanishes with no mains earth connected concerns me. Why is this and is the rest of my equipment then vulnerable?

You have a few problems here. That of hum that disappears when the amplifier's earth is disconnected is caused by a ground loop. It is not a good idea to remove the earth on the amplifier as in our view all valve amps need an earth. You can fit an earth lift resistor though, of 3-10 ohms, between incoming mains earth and the chassis. Make it as low a value as possible.

Alternatively, leave the earth connected and disconnect other items in the system to see which one is acting to cause the hum loop. It is likely to be one or both record decks. Disconnecting an earth here is another possibility, providing there is no exposed metal work.

The bang and phut business is another problem altogether. I find it odd that an ECC82 should cause the original 'bang'. It is a small signal valve, rarely fails and if it does high value anode and cathode resistors limit current, preventing either a bang or a flash.

Bangs and flashes are, most commonly, caused by output tube failure. The anode warps under heat, touches another electrode and there's a flash and bang as the HT shorts out. If the cathode receives the HT then the bypass capacitor, if there is one, may well be damaged or blow. Usually, an HT fuse is fitted to ensure the output transformer primary does not burn out. When the fuse blows the anode may cool down and the valve continue working after fuse replacement – until the next time!

I appreciate that the right channel blew first then the fuse on the left, but all the same the amplifier needs checking to see if there is damage. Cathode resistors and capacitors, and transformer primaries all need passive measurement. Then, with output valves in place, the HT and all voltages should be measured, especially at the cathodes and grids.

You really need a manufacturer's circuit diagram with voltages for this. Only an experienced engineer should do this because the voltages are dangerous. If all is well then the amp. can be run. If there is another flash and bang, an output tube is definitely dodgy. Best to replace both on the failing channel with a new matched pair, or perhaps do this in any case before running the amp.

I suspect you have inherited an amplifier with a problem and it needs expert attention. And it may just be that the anode volts are too high for reliable running (high HT gives more power, less reliability). I see the 6550 is rated at 680V max in fixed bias mode but this is very high and, I suspect, fantasy island! Just 400V are recommended for cathode (self) bias. Your engineer needs to consider this. I have no experience with 6550s because I didn't like them and never used them.

Finally, most loudspeakers nowadays use a 4 Ohm bass unit and are best driven from a 4 Ohm tap. Driving a lower impedance from an 8 Ohm winding is not a good idea, as it raises distortion considerably. Driving a higher impedance like 8 Ohms from a 4 Ohm tap or winding is more tolerable however, as a higher impedance is reflected back into the primary, so although load matching is sub-optimal anode current is reduced, and distortion with it.

To be frank, a more up to date and capable amplifier, one with a 4 Ohm output, would be your best bet. I think you bought a duffer; it may have been cheap but it was no bargain. If you want as close as possible to that elusive valve sound but from solid state then the default (go-to solution) is LFD

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