Grounded Grid power supply issue.

Chops54

Junior Member
I wonder if anyone can advise me. I own a pair of Transcendent Sound Grounded Grid preamps which I'm very happy with but after my son asked if he could have one of them I decided to build myself a replacement in two boxes using tag strip and point to point wiring. The power supply is a bit unusual in that it's a balanced supply using a pair of small transformers to give +200 volts and -200 volts. Each psu uses a doubler circuit to achieve around 240 volts which is regulated to 200 volts by a pair of 100 volt zeners. There's a 5k shunt on the psu output for the zeners. My problem is the shunt is getting too hot and I thought maybe the zeners were causing an issue so I lifted one leg of each zener pair and I now have something like 560 volts after each shunt. Obviously the shunts are getting hot because the zeners are having to ditch so much voltage but I don't understand why I have so much voltage in the first place. I can post a circuit diagram if a mod will allow it but I don't want to take liberties even though it is already in the public domain. Any help will be appreciated.
 
Further investigation tells that one transformer is putting out a different voltage to the other. I thought at first I'd wired them wrong putting the primary windings in parallel so I swapped everything round and put them in series and that's when I discovered the differing outputs. At one point I was measuring 220 volts on the secondaries but in series I'm down to 108 volts on one and 90 volts on the other. Very strange but I'm sure the issue is with these transformers. So, I've opened up a spare GG preamp and pulled the feeds to the preamp board so the psu is unloaded. They measure out at 208 volts pos and 208 volts neg so that's good. Next I've lifted a zener on the positive half of the power supply and the output has jumped up to 440 volts. That's a good 120 volts lower than my diy build and I'm also thinking that once the valves start to conduct they'll pull the power supply down even further. There's 240 volts spare there to play with. Anyway I'm going to get hold of two new transformers and see where that takes me.
 
I don't think anybody is going to care if you post your power supply schematic. We can assume that the GG preamp pulls 10-15mA, then go from there in terms of what your power supply is doing.
 
Thanks Paul. I'll attach the circuit diagram.

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The 22uf cap at the end of each half of the psu is not there. It's actually on the preamp board instead. I've had the heater supply transformer fail on both my pukka preamps and I replaced them with a small toroidal. I assume the preamp itself will not pull any HT current until the valves are warm.

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I'll post another pic of the finished power supply tomorrow.

Si.
 
What size of caps did you use in the voltage doublers? A 120V winding into a voltage doubler shouldn't make more than about 350V. I would also wonder about how you have the transformer primaries wired. They must be in parallel (the side going the power cord/fuse/power switch). If you have these in series, then you will get different output voltages on the secondaries.

The circuit only draws about 8ma, and let's say the negative doubler makes -350V. That leaves 150V across 15K of dropping resistors, which is 10mA, indicating 2mA of Zener current. That all makes sense. If you had safety diodes (which you should) to ensure that the cathode voltage of V2A/V2B never goes below 0V, then it would make sense that your power supply would be rather immediately toasty.
 
Apologies, I've been tied up with family matters and other things, laziness being one of them I'm afraid.

I've ordered two new transformers and I reckon that'll cure the problem. I'll report back as soon as.
 
Covid has really slowed things down. My transformers are finally in the uk after their journey from Canada. I should have them for the middle of next week. Only taken a month :(
 
The new transformers have been sitting here for a while and I finally managed to galvanise myself into action. Dimensions are slightly different so a couple of extra holes needed drilling but it all went together ok. The good news is I now have the correct HT voltage.

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Here's the other half of the two box preamp. The volume pot has appalling channel balance so will need replacing. I think I'll treat it to a decent pair of output caps too. Apart from that it sounds pretty good and I have to say I'm amazed at how quiet it is 👍

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