Hot tubes!

[Markpab66]

He certainly is! Glad I bought a second hand laptop with an optical drive at Christmas now- knew it would come in handy for something!

[John Caswell]

Hi Mark,
Yes it is Class A up to a certain level, and it is push-pull.

John

[Markpab66]

Thanks for the info John. Does that mean that at low levels each pair of output tubes are both handling the full sine signal together, before at higher volumes splitting apart, and one handling the positive part of the signal, and the other tube the negative? I'm only used to push pull transistor amps, where one transistor does the positive half of the wave, then hands over to the other transistor to do the negative at all volumes.

[Phil Y]

My understanding is that it is the output transformer that makes class A push pull possible.
Both valves of a pair are biased in class A (the middle of their curves) but crucially they are driven in anti-phase so when one goes up, the other goes down, like the two ends of a see-saw. The outputs (anodes) are then connected to the opposite ends of the output transformer primary. The primary sees (at any one moment) one end positive and the other negative with resulting current flow. This is coupled to the secondary in the normal way.
Most p-p valve amps traditionally operated in AB1 because you can get more power from a given set of valves that way.

I like the WAD/WD approach, it gives decent output power but with no crossover distortion.
A good comprise.

Phil.


I stand to be corrected on any of the above!

[Markpab66]

I see, of course transistor amps don't have output transformers, the transistors are fed from a transformer, and the chips then do everything themselves. Valve amps really are different animals, not just a case of putting a transistor where a tube was and it would still work fine.

[Markpab66]

I had a humming problem coming from the right channel, could only hear it when there was no music playing through it, but it was bugging me even so. If I unplugged the RCA input from the right channel it went dead quiet. Took the bottom off the amp, checked all the caps' ESR, all were fine. I then thought it must be a ground loop of some kind, so unplugged each component to see if the hum went, but no joy. I then got an extension cable, and plugged the amp into it, using a socket on the other side of the room - hum completely gone! Maybe it's a good idea for tube amps to have their own dedicated power supply, solid state seem to be fine using common sockets with other components.

[Richard]

Hi Mark,

Two things come to mind,

Firstly check your house sockets have earths. Easiest way to check for LNE being present and wired the correct way around is with a plug-in tester such as used/sold by caravan and camping outlets or maybe you know someone who has one you can borrow. It can be re-assuring anyway to check all your house sockets with something like this, https://www.ebay.co.uk/itm/334602905076

If earths are ok it may be your extension lead has provided an "earth lift" of an ohm or two. That might indicate you should have a good look at the way the signal ground has been lifted inside the amp. You should be able to find 10 ohms between signal ground (such as the metal shell of a phono socket) and the mains earth IEC pin. If not (less or none) check to see if signal ground is shorted to mains ground.

[Markpab66]

I believe you've found the problem, Richard. If I put my meter on the IEC earth terminal and measure to an RCA plug, or to the chassis ground on the rear panel for a turntable I get virtually zero resistance - 0.2 ohms.

[bob orbell]

I doubt this is your problem, but, there should be a 10 ohm 2 watt resistor connected from the star earth tag on the power supply board to the earth post. Bob

[Markpab66]

I'll do some investigating later on, will have to carry the amp upstairs again for examination - quite a workout with the weight of the amp! Thanks Bob.