Thanks!

I just realized that I ordered .47uf caps instead of .047uf caps for the to ground cap of the Meier crossfeed that I plan to build into this amp. I think I'll finish building the amp on Tuesday night without the crossfeed, and add that part later.

Those CCS diodes are tiny. The cost per ounce on those things is astronomical!

More pics soon...

I'm planning to have the whole circuit powered up and making sweet music later tonight!

The second way is preferrable, assuming that it doesn't lead to any additional noise (which it did for Mains)

Just make sure that the CT on the trafo is the ground reference point.

Earth ground can make a great shield, but if it's implemented in such a way that it can carry current between two potentials, then it's not really a shield.

I'll figure it out tomorrow. At worst I'll just build another power supply.

I can tell when the music is loud enough to mask the hum that this thing is going to sound great. Can't wait to hear it cleanly!

Here are some pictures:

[Edited by Guitarsenal on 12-08-2005 at 12:13 AM.]

I think I may have damaged one of the bridge rectifiers when I wired one of the 12v rectifiers incorrectly. That might account for the higher than expected B+ that I measured before.

I think you've got to do some measurements on the circuit.

Check the B+ to see if it is >55V.

Check all PS caps to make sure they are installed in the right polarity.

Check your bridges. Do you really mean a 12V bridge?

The main thing we want to be careful of are the cc diodes.

It would help to see the underside to get an idea of the earthing and what the vol pot looks like.

Last night while checking a couple of things on the prototype for Batman, I broke the earth wire to the pot body.
Instantly I got (50Hz ) hum that exhibits itself just as you describe.
Touching the 12AU7 glass envelope makes it worse, touching a metal part of my Rio makes the hum go away, and touching the volume pot body makes the hum go away too.

I've had this sort of hum problem in the past, and the most frequent reason was the volume pot body, either needing decoupling from the case, or explicitly needing an earth back to the signal ground. The latter case being the more frequent fix.

Yes, the hum was worse with low Z headphones ( 32Ω ), and I found my high Z headphones ( 300Ω ) hid the hum quite well, but it was still there.
Fixing the broken earth wire to the pot body made the amp quiet again.

As the hum I experienced seems so similar to your description, I think we need to investigate earth wiring before suspecting components or the B+ supply.

Toasting the 79L12 will make the sound distort but the bridge probably has survived. As you now know, the pin out is different to the 78L12.
Maybe the circuit diagram ought to show the pin numbers to make this obvious ? .. lets see what Runeight thinks.
I don't think this problem will be the reason for the hum though as I found it much more difficult to induce hum around the opamp wiring compared to the pot and valve(tube), but then the opamp is running as a unity gain buffer so this is no suprise.

[Edited by mains_hum on 12-08-2005 at 12:40 PM.]

Also measure the plate voltage (pins 1 and 6) of the two halves of the valve and adjut the pot to get ~39-40V on the cathodes. My unit has some hum if the plate voltages are too high.

Look at the caps in the PSU, particularly the two caps that couple the B+ rectifier to the bipolar power supply. If these caps are in backwards you might get hum. Don't ask me how I know this

It really does sound like a ground loop kind of thing. I have the case grounded to the center tap on the transformer, so the pot body is grounded.

I'll do a little star grounding when I get home and see if that helps. Also, do you think I should run a ground wires to the connectors on the case that are using the case for their ground?

Batman and mains are a little better at tracking down grounding problems than I am. As many pictures as you can post of the wiring will help them.

[Edited by runeight on 12-08-2005 at 05:52 PM.]

This thing sounds great! Really big sounding. Sounds very relaxed, with plenty of reserve for big snare hits or kick drum. Very well defined bass. Nice high end too.

I like it. I don't have any tube amps to compare it to, just CMOYS (which also sound great).

Thanks for coming up with such a great little project!

Even though the hum problem is relieved, it's probably not really solved. If you feel like it, it might still be worth some further work to track down the problem. I'm suggesting thisn only because both Batman and Mains_hum are using theirs without the 120R resistors and are not having hum problems. So we know that the amp, when working right, is free from hum.

Let us know how it sounds after a few days of listening.

I think I'll buy a few extra tubes of different types and see how the performance of the amp changes with different tubes.

And I'm going to temporarly move that power switch and it's wiring and see if that helps.

The problem with building this into a small case is that I don't have a lot of choice about where the wiring goes.

I could rectify the filiment voltage too, but that would mean more parts in the small box...

The filament wires must be twisted and must sit against the chassis.
Also, they should connect to the valve base pins at right-angles when possible.

A picture of the underside would really help here.

Whatever tubes you get, ensure at least one is a NOS 12AU7 (RCA, GEC) or ECC82 (Brimar, Mullard).
You'll find they work different to the modern JJ, Ei, etc.
I don't mean for audio quality, just for setting 40V at the anode.
What you think about any differences in the sound between new tubes and NOS tubes is subjective, but still interesting to hear about all the same, so do say what you think after your tests.

The buffer is dead quiet. There is no hum before the tube warms up.

It's either a ground loop or dirty B+. I'm heading out of town for the weekend, so I'll have to figure it out next week.

Thanks for all of your suggestions!