power transformer bad?

[Chuck H]

Are your PT and OT oriented with the laminated steel perpendicular? Are you using a choke? How is the choke oriented and what is it's location? Maybe you could sketch a transformer layout showing the layout and steel orientation for us?

[Tom Phillips]

... Maybe you could sketch a transformer layout showing the layout and steel orientation for us?

Or...post photos of the beast!

[TarheelTechinTraining]

here is a pic from above. If a shot of the guts would help I'll send it. Thanks for all the help. I've made a few pedals but this is my first amp build.

[Chuck H]

I think you have EMF inducted into your OT from the power supply transformers due to proximity. The choke specifically. You can try moving the OT over on two of it's screw holes. I realize this may require fussing with other things occupying space inside the chassis but there's no better answer I know of. If you temporarily lengthen the OT leads (wire nuts) you should mount it temporarily as shown and make sure it works before drilling extra holes.

[Tom Phillips]

...If you temporarily lengthen the OT leads (wire nuts) you should mount it temporarily as shown and make sure it works before drilling extra holes.

That's a good experiment. I recommend that you add enough wire that you can move around the OT and listen for a change in the hum while you do the move. Include rotating the OT too. Don't bother to drill new mounting holes until you decide on a final location and orientation.

[TarheelTechinTraining]

OK fellas I'll give it a try. Chuck , you said my cathode followers hated me. What can I do about this?

[Chuck H]

OK fellas I'll give it a try. Chuck , you said my cathode followers hated me. What can I do about this?

The specified max heater to cathode differential for 12ax7 tubes is 200V (IIRC) so your amp would be over that spec. NBD historically with high voltage Marshall type circuits but for a time there were a lot of rusky preamp tubes failing in the cathode follower circuit of Marshall type amps even below that spec. I haven't read much about it in the last five years so maybe those troubled designs got it worked out. Just the same, I try not to exceed the spec.

What can you do about it? You can float the filaments some measure of +VDC instead of at ground (0VDC) to decrease the differential. It's not hard to do but if you're not having any trouble you probably don't need to worry about it. I'm just funny that way.

[TarheelTechinTraining]

You fellas are the bomb!! What you fellas thought was the OT is the choke. I did as y'all suggested and ,viola, no hum. Thanks for the help. Now how about some more "brown". Any suggestions?

[Tom Phillips]

Glad you were able to fix the hum.
Can you post an "after" photo? Gut shots are always appreciated too.

[TarheelTechinTraining]

[Chuck H]

That's pretty bomb for a first build (or even a second, or third!). Other than the transformer interaction it's layed out pretty well. You've clearly done your homework. My first scratch build probably wasn't an amp you'd keep and use. In fact it's pretty much like a beat up Mr. Potatohead doll missing half it's parts and sitting in my closet awaiting further desecration But this is an amp you'll keep and use forever. A big, loud high voltage Marshall monster. Tony Iommi much!?! Get down Charlie Brown.

Speaking of brown... You asked about a browner sound. The first thing I might try is reducing the preamp voltages. You'll still have the big, harder clipping of the EL34's at high voltage but the preamp tone will be rounder and more mid focused. It's as simple as increasing the value of the resistor in the HV rail after the choke. The same wattage rating as what you have should work fine.

[TarheelTechinTraining]

Increase the resistor by 20% or 25% ? Also should I increase only V1 or all pre-amp tubes including the PI?
Thanks so much for the compliment. I've slowly worked on it for at least a year. I plan on eventually leaving it to my grandson. He better be a musician.

[Chuck H]

I plan on eventually leaving it to my grandson. He better be a musician.

Indeed! I use to fantasize that my children and grandchildren would all be musicians and that I would exploit them for my own evil purposes. Think Partridge Family but with crunchy Marshalls It hasn't worked out yet.

Reducing preamp voltages can be done in all the ways you mentioned. Any stage reduced in voltage will make the amp a little "browner". As far as I know the term "brown sound" stems from the lower voltages of brown out conditions altering amplifier tone. Ergo, lower voltage = brown.

You can experiment a lot here and choose what you like. As long as the tubes are operating in their safe zone there's no rules for tone. You could, for example, decide that you'd like to hear what it sounds like if only reduce voltage on the third node. By increasing resistance between the second and third node and then decreasing resistance to the fourth you can achieve that. In order to have the fourth node end up at exactly the voltage you had you would need to adjust for the net current reduction with the preceding nodes because they are usually in series, but it's easier to just maintain the same net resistance in the rail and it will be close. The preamp nodes don't draw much current. This is just one example of how you can experiment.

More along the lines of what I was suggesting, and an easier place to start, would be to increase resistance overall after the choke. This is done easily enough as I described in the last post. 25% or 30% won't make a lot of difference usually. More like double the value. Like I said, the preamp doesn't draw a lot of current so you need relatively substantial resistance if that small current is to drop voltage across said resistor.

Between the choke and PI node most 800's use 10k for 50W models and two 10k's in series for 100W models. If you followed a 50W diagram you probably have a 10k there. Try adding another in series or just use a 22k (close enough). This will drop all voltages on the PI and preamp. If you have 20k between the choke and PI I wouldn't raise that any more (though you could). Instead I'd just go to the next node which is probably 10k and just increase that to 20k or 22k which will reduce voltages on the preamp but not the PI.