Class A/B whats the difference?

[JimN]

Thanks Ray - yes, I've read that the possible AC heater supply problem can be checked by disconnecting it and temporarily using a 6V battery. If the hum goes, then that's the cause eh? It wouldn't be rocket scince to have an off-the-shelf stabilised DC transformer inside.

Hi, Lionel,

Unfortunately, as I recall finding out during my first year as an electrical engineering apprentice (ever such a long time ago), you can't transform DC!

All you can do is transform the AC supply to something like the required DC voltage, and then rectify it. There's a mathematical formula for working out the required AC voltage to give a particular DC voltage when rectified and smoothed.. All to do with sine-waves, sines, cosines, trigonometry, etc. I'd tell you all about it, but I learned it so long ago that I've now forgotten the detail.

JN

[Didier]

one hum-reduction technique for the perfectionist is to rectify and smooth the 6.3v heater supply. This means that a.c. stops at the power supply and there are no 50Hz alternating currents anywhere in the audio side of the amp.

This in commonly used in high end HiFi valve preamps and amps.

Didier

[RayL]

If you convert AC to DC using a full-wave rectifier then the DC voltage will be just a little less than the AC voltage (due to the losses in the rectifier). If you then add a large smoothing capacitor in parallel on the DC line out of the rectifier, the DC voltage will go up by about 1.5 times (so 4v AC becomes 6v DC, for example).

If better smoothing is required, the usual method is to add a series choke or a series resistor followed by a second parallel capacitor. However, as soon as a load is applied to the resulting DC line, the effect of all of the series components (the rectifier, the choke/resistor and the impedance of the transformer winding) is to act as a potential divider and the line voltage 'sags'', so the 'x 1.5' will not be accurate as soon as a load is applied.

Lionel's suggestion of a stabilised power supply is therefore a good one, because with a stabilised supply you start with a DC voltage that is higher than you need and the regulator fixes the output at a lower voltage.

Ray

[RayL]

Lionel, one more thought on your humming amplifier. Unless the hum was there from new , then it is almost certainly caused by electrolytic capacitors in the power supply drying out. Before you rush around doing star earthing or arranging DC heater supplies, change those big 'C's!

Ray

[Graylion]

OK guys - that's as clear as mud! Only joking! Yes Ray, the hum was there from new but I think it's 100Hz, not mains. Haven't used it for a while (I have 8 amps to choose from!) so I can't remember. How about a 6V lantern battery or 3 - in parallel for the heaters? They cost me nothing as I work as an Engineer on the motorways but what is the current draw for the heaters to a bucketful of valves? The Carlton has 2xEL34 and 3xECC83. I'll let you know when I next dig it out and experiment with the valves & reverb tank. It wouldn't be difficult to wire in a stabilised DC supply - but haven't people thought about that before? I can't have been the first! How do you get a "pure" DC supply of around 6.3V?
Cheers, Lionel

[Amanda]

Hi Lionel,

You could try connecting a 100 preset resistor across the heater supply,
with the centre tap (slider) connected to chassis, always assuming
that one side of the heater supply is not already earthed.

By adjusting the preset, you should be able to reduce the hum, as the
adjustment of the preset will "balance" the heater circuit against ground.

Heater hum reduction can also be achieved by having the wires to the heaters
twisted!

A valve with a leak from heater to cathode can also cause hum.

Regards

Amanda

[Graylion]

Thanks for all the advice people. I'll dig out the Carlton one day (when I've finished buiding the new shower room!)and have a look at its insides! I know about twisting the heater wires as I plan to build an all-valve amp soon from a Ceriatone kit. I hope it's hum-free when I've finished, but I'll know hwo to bl;ame if it isn't - Amanda! I'll try valve swapping first as it's easiest. Cheers, Lionel

[rogera]

Hi Lionel - if you do try 6V lantern batteries then the current draw will be 450mA for each EL34 and 300mA for each ECC83.

That will give you a total of 1.8 Amps.

[Graylion]

Many thanks Roger - quite a draw for batteries. I would try it with at least 2 of the big lantern cells. Maybe I'll find another more obvious cause for the hum though. I'm really just being picky as it's the loudest hummer of all my valve amps so far. OK, it's the highest output at 100 watts, but the hum is annoying when I have a 47-year old amp that's quieter!