Power Supply Design - Current Draw

[Littlewyan]

I've just finished building a new amp which is basically a JMP Master Volume with a 15W Power Section. Its the standard Marshall circuit except it has 2 x 6V6s in the power section and uses different transformers.

I was testing the amp tonight running it flat out with a sine wave being pushed into it and whilst that was happening I measured voltages throughout the amp. I measured the 6V6 Cathodes and got roughly 67mA off each Cathode.

Does this mean the current draw in total for the power section is 134mA? Or can you not measure it that way? I only ask as my PT is only rated for 120mA on the HT and its a non centre tapped secondary for the HT.

The HT sags from 330V down to 300V which isn't too much and the PT becomes warm to touch after about 10minutes.

[pdf64]

What is the PT HT Vac with all secondaries unloaded (eg no tubes in place), and then under full drive conditions?
I seem to remember that the PT's Iac is = Idc with a CT bi-phase rectifier, but Iac is 1.414 Idc with a FWB rectifier (assuming cap input smoothing and no ripple).
If the PT doesn't get too hot to touch then it's probably fine; the classic amps tend to have a fair bit of HT winding sag.

[Stevem]

What are they idling at?
I find that other then in SE that any type 6V6 but for a JJ brand starts to sound too soft and broken up on the bottom end when pushed hard if they are idling at over 22 ma.

[martin manning]

I guess you're measuring voltage across current sense resistors at the cathodes? If so, and with a sine wave input, then you're getting some average or RMS voltage for something that looks like half-wave rectified DC. The power tubes are conducting this current alternately (except for the AB overlap), so I don't think you have anything to worry about. The fact that the PT just gets warm under these conditions is confirmation of that. Is your rectifier a FW or FWB? As Pete says the available Iac is less for a FWB, a factor of 0.62, but you'd still be ok, I think.

[Littlewyan]

The PT HT Vac with secondaries unloaded is 272VAC and with the amp under full load its 243VAC.

They are both idling at 28mA and it sounds ok so far but I haven't tried it without an attenuator in a practice room yet.

I was indeed but I just measured the current by putting my meter between the secondary winding and the bridge rectifier and measured 140mA at full load. On the Tube Town website it says this PT is rated for 120mA at 250V :\. I read earlier about the current for FWB being different to a CT PT. I think mine still goes over a bit. I've emailed Tube Town though, they asked for the current reading which I've just given them so will see what they say.

[martin manning]

If your meter is true RMS, that should be a good reading. At 17% over the spec, it doesn't sound too serious, and how often are you going to play it flat-out?

[Littlewyan]

Every time as the amp is running near enough flat out with the master and pre amp controls on 5/6! I can get a better PT for £35 that can handle 200mA.

[Littlewyan]

Just received a response from Tube Town, they said I'm overloading the transformer and will damage it. They obviously don't think there is any leeway with it. So I'm going to order this other one which will give me plenty of headroom.

Bit annoying really as I was trying to use up parts when building this amp and now I'm stuck with a spare PT!

On the bright side this PT has two taps on the secondary winding, 190V and 275V, so I could put those on a switch to give me a low power option!

[martin manning]

They are being conservative, as I guess they should be. The only thing that will damage the transformer is heat, and the insulation is rated for something like 105 or 130C.

[Littlewyan]

Yeah I guess they can't say it'll be fine as they're making themselves liable if it melts itself. It was warm after only 5minutes though and if you think it could be running flat out for 2 hours then its a bit of a risk. Also its a Toroidal Transformer, not sure if they can't take as much heat as an EI transformer?

[pdf64]

Every time as the amp is running near enough flat out with the master and pre amp controls on 5/6! I can get a better PT for £35 that can handle 200mA.

Yes but even full on heavy rock isn't as demanding as a full power square wave.
eg there will be songs, so pauses between them, and there might even be gaps between some of the notes in the songs
All these will momentary breaks will give the PT chance to cool.
So I suggest to 'suck it and see', eg if the PT gets super hot in use, then look to replace it with a higher spec type.

[Littlewyan]

I've already ordered a new one anyway. Funnily enough the PT I ordered is the one I originally wanted but it wasn't in stock at the time.

Also when measuring the amp's output I did notice that after a short while of running it flat out the wattage would decrease. I suspect this could be the PT getting hot, resistance increasing in the wire and the HT going down. Of course I didn't think to measure the HT at the time as I didn't realise what was happening -_-.

[Phil_S]

Oh, never mind. <deleted>

[Littlewyan]

Installed the new PT today and the amp actually sounds better tonally I think. The idle HT is now 385VDC, however when playing clean that drops to 365VDC and flat out its at 350VDC. PT handles it no problem. And it seems to sag like an early 2204 would (They ran at roughly 380-390VDC).

The amp also has slightly higher output too. 20W Clean and 38W flat out. The OT is only rated for 18W but I'm sure it'll be fine.