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Power supply problem

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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.
 
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
 
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.
 
Ex-Moderator
Joined 2004
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.
 
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.
 
krzanik said:
... 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.
;)
 
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?
 
I've been going over the amplifier and it seems to be built according to the schematic. I am using 6V6GTs instead of 6AQ5s. The really weird thing is I can hook up the negative feed back to either channel channel and it works fine. But if I try to attach BOTH at the same time it makes a terrible squeeling noise. The output transformers are hooked up exactly as the manufacturer recomends. I'm stumped. If anyone has any suggestions let me know. Thanks.
 
Ex-Moderator
Joined 2004
The output transformers are hooked up exactly as the manufacturer recomends.
The transformer manufacturer cannot know in advance which way around the primary (or secondary) should be connected to ensure the feedback is negative, unless he also designed the circuit you're using AND you can tell which end of the primary is which (you often can't).

Negative feedback reduces gain, so the amp should be quieter/less sensitive and more stable with the FB connected. Positive FB increases gain, so the amp should be louder/more sensitive and less stable with the FB connected. If there is enough positive FB, it will cause oscillation.

The phenomenon you describe is most unusual but I suppose it's possible that your FB is positive and is just on the point of instability with one amp; with two amps, it could be that there is just enough cross-talk somewhere to set them both off squealing at each other.

Have you tried reversing the connections to primary or secondary (whichever is convenient)?
 
Hi RayMoth,

Well, this is all pretty new to me so maybe I should explain a little more. The output transformers I bought are from Triode electronics. They came with a data sheet that lists the following:

Primary:
Power RED
Plate BLUE

Secondary:
8 ohms GREEN
4 ohms YELLOW
Ground/0 ohms BLACK

The schematic I' using http://www.angelfire.com/vt/audio/se6aq5schem.gif indicates that the negative feedback will be connected to the 8 ohm (indicated as green on the data sheet for my transformers) leg of the secondary. The other end is connected to the negative end of the cathode bypass cap on the 12AX7.

From what your saying it sounds like I'm getting positive feedback (the output does get louder and I get a little more back ground hum) when I connect one of these wires. Before I blow anything up I want to make sure I understand your suggestion. I should take the RED (power) from the primary and switch it with the BLUE (plate) from the primary? So that the B+ voltage will be going into the BLUE (identified as plate on the datasheet) and the RED wire (identified as power on the datasheet) will be connected to the plate? I trying to spell this out very clearly becuase I'm afraid I might be misunderstanding you and I don't want to damage the output transformers.

I can't help wondering why the manufacturer provided a data sheet which indicates what each wire on the primary and secondary is (e.g. 8 ohms GREEN, Plate BLUE) if this isn't necessarilly correct. It makes it confusing to know how to hook it up.

Well let me know if I have this correct and I'll make the changes, cross my fingers, and fire it up.

Thanks
 
Ex-Moderator
Joined 2004
I can't get an image from your link, just a blank screen. Anyway, it doesn't matter. As I said before, the transformer manufacturer cannot know what circuit you're using, how many inverting stages you have in your feedback loop and, therefore, which way around the primary winding should be connected to ensure your feedback will be negative.

Forget the red and blue colors of the primary leads, they mean nothing. If you had no NFB loop from the OPT secondary, then it wouldn't matter which way around the primary was connected, it would work just as well either way. However, since you do have a feedback loop from the secondary, it most certainly does matter. Connecting the primary one way around will give negative FB and the other way will give positive FB.

Bottom line, I suggest you try swapping over the primary leads, i.e. take the red lead to the plate and the blue lead to B+, as you suggest, and see if that fixes the problem. Do it with one channel only at first, leaving its FB loop in place but disconnecting the FB loop in the other channel (for safety's sake) . If it works for the first channel, you can then do the same thing for the second channel and reconnect its FB loop .

If I'm wrong, the worst that can happen is instability and that won't damage anything providing you switch it off as soon as you hear any oscillation.
 
Ray Moth,

Thanks for your help. I switched the plate and power wires as you suggested and the amplifier sounds great. The negative feedback reduces hum noticably. I'm really excited. This is my first (succesful) amplifer from scratch. Thanks for all your help. Without this forum I'd be lost. If you ever find yourself in Austin I owe you a beer!
 
hammond AO-40 amp

hello krzanik
tryed to email you but since i am a newbe i cant email members until i have a certain # posts.
i have a AO-40 amp chassis
and
AO-33-3c and AO-32-1 chassis's
finding your link has helped me gain some insight as to what i am up against , as i want to convert this AO-40 to 2ch. hifi use.

it looks like you may have (or are paveing) paved the way already.
without imposeing on you any pointers appreciated.
thanks
paul

p.dow@knology.net


:)
 
I messed around with that AO-40 for a bit and had some limited success. I ended up removing all the parts and buying a couple of Hammond PP output transformers to make a Push Pull (PP) 6V6GT amplifier. I'll reuse the power Xformer and the chokes as well as almost all the tubes. I got some very nice old 6V6GTs with the amp. I've already used a couple of the 6V6GTs in a single ended amp I just finished. At less than a hundred dollars for the amp I figure I got a decent, though not great deal. If you can figure out how to make use of the seperate treble and bass output xformers (maybe use the bass xformer as a subwoofer amplifier?). good luck. If you find a use for those output transformers, let me know as I would like to put mine to some use.
 
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