Output transformers Edcor GXSE10-6-8K

BL

Well I think I figured out the PROPER way to attach a file.

BL

I think your post is visible. I'm afraid I am not able to be helpful. Hopefully someone who can will answer soon.

When the B+ continues to drop as you've described it means that something is drawing too much current -- likely something shorted or partially shorted to ground.  

Whatever it is will be getting abnormally hot by the time your B+ drops below 200 volts so that may be one way to find it.

Otherwise you need to figure out what part of the circuit is drawing too much current.  If you lift one leg of the 6.8K 2 W resistor you will be turning off the input stage.  If when tested with no power feeding the input stage you still have the issue, then you know it is the output stage.

If it looks to be the output stage then I would lift one leg of the cathode resistor, which is 150 ohm, 5W.  This will turn off the output stage and any current draw from the tubes.  If by some chance this doesn't put the fire out then you have a short or partial short circuit somewhere between the 1K 5W and the transformer leads that connect to the circuit board and ultimately the plates of the output tubes.  It could be a bad transformer.

You can lift one of the primary lead wires from each output transformer one at a time to see which of the two channels is causing the problem.


Measure the voltage drop across the 150 ohm 5 W cathode resistor and using ohms law figure out how many milliamps the output tubes are drawing. Typically 25mA per tube, so 50mA across the resistor. If it blows past 50mA, remove one output tube at a time to see which one is causing the issue.

Above all remember that the thrill of hearing something sound good that you made with your own hands and the long term satisfaction that comes with it is directly proportional to the amount of suffering that you put yourself through trying to get it to work.

Greetings and welcome. Can you post a clearer photo? I can’t tell some of the fine details. Looks like maybe one of your diodes might be reversed but maybe it’s just the photo.  

It looks like you aren’t using the two 20uf caps in series like the schematic shows, so what single cap are you using instead? And it looks like you have a resistor inserted at that point that is outside of the schematic specs?

This doesn’t really explain why you are experiencing a problem only in the left channel but I like to start at the foundation and work my way through a circuit.

Had to double check to see if I posted in the correct forum for help.

BL

BJzen, thanks for the response.

Double checked solder - joints good/no bridges

As stated above the issue is in the power section , left channel. It is at a slow pace pulling too much current. At the point that B+, node after the ceramic 1K 5w resister in the power supply string, gets to ~225v I shut it down. The 1K 5w ceramic resister gets very hot, the 6.8k 2w does not or the 220k 1w bleeder resister.

The power tubes have been swapped out with different ones and the problem still exist. The OPT transformers have been swapped left to right and the issue still exist in the same channel. I even swapped in a spare OPT transformers grasping at straws. No love.

As far as the 22uf 500v  cap being an issue, electrically it shouldn’t be. In most instances 2 series caps are used to increase the voltage capacity if you don’t have one with the required voltage capacity. 2 250v caps in series equals 1 500v cap. You are correct that the capsitance is halved of 2 caps of equal capacitance.

In this case (22uf vs 10uf) the only increase in current would only be a slight increase in inrush which would stabilize very quickly. Anyway that part of the circuit is PS filtering for the preamp tube. And as stated above the 2 power supply resisters after the 1K are not getting hot. Just the 1K.

I write all the above down not to be argumentative, but to state what I believe to be true in my mind. I am hoping there are some of you out there that are a lot smarter than I (not a high bar ) and can explain to me where I might be wrong or can take a look at the provided picture and see something wrong.

If I’m not learning I’m going backwards.

I do appreciate all the input. The ultimate goal is to admire the glow while listening to good music.

BL

BL, no offense taken. I'm here to learn also.
Your thinking is correct. My question is how was it executed? Double check that! I often find that when we stray from the norm with a well thought out direction sometimes the execution is flawed leaving us in a cloud of smoke.
You got the right guy on it.
Hope you see those tubes glowing soon. Good luck.

I'm stumped.

From SD;
Then check both solder pads for those leads with your meter to make sure they are not grounded.

Shows no continuity, not grounded together. Blue to ground = infinity / Red to ground ~250k which is about the same as the good channel.

Everything put back to "NORMAL". The B+ goes to below 250vdc before I turn the amp off. (Smoked 1k 5w resisters, replaced)

Pull power tube on offending channel. With power tube in the good channel the B+ stabilizes at about 350vdc.

Pulled power tube in good channel and place in bad channel and B+ craters with 1 power tube installed. Bad channel.

I don't know what else to do.

BL

Oh that’s a frustrating situation. I don’t envy you.

The transformer you are using is spec. I’ve built several of these circuits with that transformer- certainly not an issue.

This is a wild guess but have you checked the tube socket on that bad channel? I have had cheap ceramic sockets go bad.

Otherwise, in this situation I’ve just gone through one piece at a time and checked every part, sometimes replacing as I go.

Good luck and keep us posted.

My only builds were point to point, wire only.
Can you post a snippet of the schematic circuit area you believe is defective?
Or point me to the schematic link of the pcb.
If you can I will take some time to review and see if I can come up with a sequence of steps to help narrow it down.

I first looked for differences between your schematic and the schematic I drew from the Decware designs before I did my point to point builds.
The first difference I noticed was in your schematic there is a voltage divider on each output tube consisting of two 1K 1/2W resistors. For some reason in my design, which I thought I used a lot of the ZKIT design to get values from, has the resistor to ground in that divider a 330K 1/2W, not 1K.
That is quite a bit of a difference. It makes no sense why one channel would work and the other one not because of this but it is a difference.
I am still looking but I would check the BOM to see if that divider should have different values than 1K || 1K.

2019 schematic has errors.  2021 looks correct.

The reason the B+ waits until the tubes warm up before it drops is because the tubes do not conduct current until the heaters get hot enough to emit electrons.

 

SO

Back to the board, got to be something on the board. Hooked everything back up to the board per-spec and ran it again. B+ cratered again. Problem has to be between where B+ enters the board at the Blue lead of the OPT and Pin 7 of the power tube. Right? I thought maybe the board was leaking B+ where the circuit trace crossed a ground.

So I cut the trace on the board right before Pin 7 of the power tube. Jumpered B+ from the Blue lead of the OPT and the 1k resistor to Pin 7 of the power tube. That way bypassing the board trace to Pin 7. Should work fine right? NOPE. B+ cratered.

So I pulled the board up again and pulled the tube socket. Check the tube socket again, checked board for solder bridges underneath again and the board looks good. Got to be the tube socket. Right? I don't have another circuit board mount tube socket because I don't use circuit boards.

Decided the next best thing would be to swap tube sockets left to right and right to left and see if the problem followed the tube socket. I cleaned both of tube sockets first with contact cleaner and then re-installed them in the others place.

Don't forget to jumper the cut trace.

Start the amp up and B+ craters as I expect with both power tubes installed. Pull the power tube out of the right channel, the ?good? channel, and the B+ stays good on the ?bad? channel. Pull the tube on the "Bad" channel and reinstall the tube on the ?good? channel and turn on the amp and B+ craters.

So the problem followed the tube socket. Bad tube socket, first one I've ever had and I've done a LOT of builds. Mostly point to point or my own turret boards.

Ordering a new tube socket........... and a spare.

We'll see what happens next. What a PITA.

BL

I am actually working on this for a friend who thought it would be a neat project.

People ask if they could “build” a tube amp. My answer is almost always “Yes, if you can follow directions and it all goes together correctly and the parts are good. If one of those things aren’t good you better be able to troubleshoot electronics.”

It’s easy til it’s not. More to come.

BL