Power supply problem

[krzanik]

I've just finished building an amplifier using the schematic found here :http://www.angelfire.com/vt/audio/se6aq5schem.gif. It's a Single ended 6AQ5 amplifer that I used 6V6GT's in rather than 6AQ5s. I checked with Wade, the creater of the schematic, first and he gave me the go-ahead to use the 6V6GTs. When I put in the rectifier (5Y3-GT) and check the voltage it's steady at 300V dc. I built the power supply (I only had one 40uf/450 for the power supply, it called for two 40s and one 20) so I used 1 40uf/450 and two 20 uf/450 for the power supply caps. When I turn it on with the rectifier in place and start taking measurments B+ etc the voltages are all too high. B+ is something like 460V and the B and C voltages are around 450 and 447. I built the rest of the cuircuit exactly as called for. I put in all the tubes clipped my volt meter up to ground and B+ and turned on the power. As soon as the B+ hit 350 (and I knew it wasn't meeting it's design parameters) I turned it off. Can someone tell me why the power supply voltages are so high? Thanks.

[billr]

One of the main reasons for voltages to be way up there is that there is no load on the power supply.

are you actually taking current through the amp?

try it in bits.

I can't get to the image, but i imagine that there is a driver, then a phase splitter, then possibly some more drivers, then the output stage.

when you measured the voltage on the amp, did you clip the negative lead to the psu, then prod around the amp?

If so, is the negative rail on the amp properly connected to the power supply?

are the heaters coming on?

If the answer to the above is yes and yes, then look at the following as a series of options.

1. take all the valves out, see what the power supply creeps up to.
2. put the first stage valve in, do you get any volts on the cathode, if you do, it's conducting, if you don't your return path to the psu is suspect.
3. the first stage is unlikely to present too much of a load on a psu that is designed to drive an amp to watts of output, so the voltage sag with just the first stage connected will not be terribly marked.
4. if the first stage is conducting, repeat the exercise until you get to the output.

Just a thought for you

look forward to hearing what happens.

best wishes

bill

[Miles Prower]

Since it's a capacitor input filter, the DC voltage regulation won't be all that great. When unloaded, the DC will be a good deal higher than the nominal operating voltages, especially since this puppy uses RC filters.

Still, given that, the voltages you're seeing seem too high. Did you use the correct power xfmr? The schemo calls for a 290-0-290 secondary, and so you shouldn't be seeing much more than 400Vdc. If you have the right xfmr, how's your line voltage? If that's higher than nominal, that will also cause above normal DC.

[Tweeker]

One option for testing the power supply alone is using a ~5100ohm power resistor as a dummy load. ~2500 ohms for two channels.

[ray_moth]

There's probably no problem here. You probably switched off a bit too soon!

The 5Y3 is a directly heated double diode rectifier tube, which means it warms up very much more quickly than the indirectly heated tubes in your amp. It is probably giving full DC output within two or three seconds of switching on, whereas the 6V6s don't start drawing current for about ten to twelve seconds. While the PS is unloaded, the CRC filter will cause B+ to rise to about 1.4 x the AC RMS voltage from the power tranny secondary. It should drop to a sensible voltatge when the 6V6s start conducting.

[krzanik]

Thanks for the help. I was freaked out by the rapidly rising DC voltage and turned off the amplifier too soon. After about ten seconds the voltage began to decrease to around what it should be. They are still a little hight but I'm going to tweak the power supply a little more to get the the voltages about right. I guess this oscillation in DC voltage is something you just learn about with experience. Thanks for everyone's help.

[Sherman]

Originally posted by krzanik
... After about ten seconds the voltage began to decrease to around what it should be. They are still a little hight...

Tubes are pretty forgiving when it comes to B+ voltage. If your voltage is within 10% of target I wouldn't worry much about tweaking it further. In your case with a target of 300V if your B+ settles down to 330V you should be fine. If it is less than 330V then you can almost consider it to be dead-on accurate.

[ray_moth]

You could try reducing the first C in the filter (the smoothing cap) to something less than 12uF. This would drop the B+.

[krzanik]

Hmm. That sounds like a pretty appealing option. It might allow me to avoid changing the resistors between caps on the power supply. I'll see what capacitors I have laying around. One other quick question. When I started up the ampifier it sounded good at first and then after about 30 seconds some hum started which increased untill it was a high pitched squeel. I assumed that this had something to do with my power supply voltages being about 50v too high and once they were closer to there design values this would go away. Any experience with this?

[Svein_B]

high pitched squeel might indicate that the NFB is positive rather than negative. Try temporarily disconnecting the * resistor. If that helps, you should probably reverse the secondary or primary of the OPT to get the phase right.

Svein.