First power up on 124 build. Bias circuit resistor fried.

[mcshaner2k]

First power up on my dumble 124 build.

Immediately the 3.9k bias circuit resistor starts frying. The first resistor after the 60v transformer tap. I'm using a prefab bias/rectifier board. I used a 3.9k instead of the 3.3k on the schematic.

I've gone over the schematic many times and can't figure out why this is happening. Wondering if anyone has any suggestions. Thanks!

[martin manning]

Measure the resistance to ground from the anode of the bias rectifier (the non-banded-end). Then reverse your probes and measure again. What values do you get?

[Structo]

Did you remember to connect the bias cap so the + goes to ground and the - goes to the bias pot?

Same with the diode, the cathode (non banded side) connects to the - end of the cap.

This provides the negative voltage necessary for the power tube bias.

[mcshaner2k]

Thanks for the response.
With my positive probe on the anode and negative to ground. It's jumping around from .300m to 32k. Then It resets at 32k every 5seconds or so.

With the negative probe on anode and positive to ground it starts at about .300m then jumps to 32k and climbs from there.

Not sure what all this means.. Ha

[mcshaner2k]

Did you remember to connect the bias cap so the + goes to ground and the - goes to the bias pot?

Same with the diode, the cathode (non banded side) connects to the - end of the cap.

This provides the negative voltage necessary for the power tube bias.

Thanks for the help.
Yes, I confirmed both the bias cap + is to ground and non banded end diode is connected to the - of bias cap.

[martin manning]

I was looking for a short or low resistance to ground, but it looks like you have something like the bias trim pot resistance plus the fixed resistors, and the diode does not appear to be shorted. What is the wattage rating of the first resistor?

[mcshaner2k]

It's rated at 1watt.

Could a bad cap cause this to happen?

[martin manning]

Yes. If you lift the cap's ground you should get stable resistance readings to ground, then see what happens when you power it up.

[mcshaner2k]

I tried another cap with the same result:(

[martin manning]

Hmm... Disconnect and check the AC voltage on the bias tap just to be sure it is correct, it should be 60VAC or so. You measured resistance to ground before, but do the pot and resistor to ground check out? This circuit is too simple to cause trouble!

[mcshaner2k]

I'm getting 60vac on bias tap with it pulled from the board.

Yes, the pot and resistor to ground are ok. I bypassed the board and built a point to point circuit using different components and a 25k pot I had laying around. I'm getting the same result. I'm wondering if I have a bad batch of diodes. It's the only components on this build I purchased from tayda. Also I have not confirmed the other boards are working which house the same diodes.

I'm using the red with blue stripe 60vac bias tap but there's a red with yellow stripe I have tied off, schematic shows this as a bias wire. I was getting 4v From this. Some layouts show it going to ground. I tried grounding it and it blew the fuse. It's classictone transformer.

http://www.classictone.net/40-18004.pdf

[martin manning]

Red/yellow is the center tap and it should be grounded if you are using a full-wave rectifier set up with two diodes (and diodes in series only count as one diode). That is what the PT you are using is designed for. If your HT rectifier has four diodes, and you grounded the red/yellow, then you created a short. Sounds like maybe you have the wrong transformer, or at least it is wired incorrectly... What layout are you using, and where did you get the boards?

[mcshaner2k]

Ok I feel like we're getting somewhere now!

I'm building the #124 with the exception of the prefab boards. I was completely unaware of the difference in rectifier circuits.

This is the layout I used for the most part. https://tubeamparchive.com/download/file.php?id=11313


Boards from here. I used his schematics to build the boards. http://www.agbamplifiers.com

Using this layout to reference the wiring on boards. I'm not using the precision power supply board.
https://tubeamparchive.com/files/102_ad ... v8_161.pdf


I appreciate the help!

[martin manning]

Ok, rectifier type and transformer match, so no problem there. If you can't ground the red/yellow CT, then you have a wiring error, or perhaps your rectifier diodes are shorted. I'd replace all of them with good quality 1N4007 or UF4007's if you are at all suspicious. I also suggest building a lightbulb current limiter, and going over your wiring carefully.

[mcshaner2k]

I should have built the light bulb limiter on the last build..

I know the schematic calls for a 3a slo blow. I used a 1a thinking it would be safer on first start up. I'm thinking this could be the problem all along now..