18 Watt TMB Build issues

[Littlewyan]

Easiest thing to do is measure the DC voltage on the EL84 cathodes. They should both be exactly the same voltage (assuming you're using the same cathode resistor for both).

[Spunky99]

Measuring from PIN 3 to PIN 7 on the EL84 tube sockets, I get V4 = 413 and V5 = 414

[Littlewyan]

Measure from pin 3 to ground and from pin 7 to ground. Don't measure from pin 3 to pin 7.

[Spunky99]

I'm a dunce.....told ya...will do in an hour...

[guitar-rocker]

Looking at your voltage chart, and the photo of your board, it seems V2 is your PI. If so, the voltages written down on your chart don't look right, or perhaps are written in the wrong column? Pins 2 & 7 are the grids, 3 & 8 are tied together for the cathodes. 3 & 8 should be the same voltage. Can you look at your socket and check that out?

[Spunky99]

V4, 3 - .077, 7 - 418
V5, 3 - .078, 7 - 418

[Littlewyan]

Pin 3's voltage is too low those valves are barely turned on. Measure Pin 9 to ground on the EL84s. Also measure pin 2 to ground, we dont want any DC voltage on that one but we want DC voltage on pin 9.

Also check your LTPI tail resistor which I've circled in red in the attached schematic. I'm assuming your's is meant to be either 47K or 56K the two values normally used in 18W EL84 builds, however if you accidentally put 470K or 560K in then that would explain the crackly sound and high voltage on Pin 8 of your phase inverter which guitar-rocker rightly pointed out.

Looks like we may have more than 1 issue, but we'll get there .

Edit: Then measure from the 130R Cathode Resistor for the EL84s to ground.

[Spunky99]

Thanks Littlewyan,

measured to ground DCv
V4 Pin 2 - .620 Pin 9 - .621
V5 Pin 2 - .690 Pin 9 - .630

The schematic didn't attach, and I don't understand the "130R".

[Littlewyan]

Ok you need to trace the wires for your screen grids and make sure they're connected ok. Should be getting DC voltage on pin 9 thats close to the anode voltage. Dont worry about the 130R resistor, lets sort the screen grid voltage first.

With regards to the schematic it would probably be a bit more helpful if you could show us the one you followed when you built the amp. That way it'll be easier to point to different parts.

[Spunky99]

Sorry, replied too quick Littlewyan,

I verified I have a 56K resistor to ground.
This is the old layout I used to build with.

[Spunky99]

Littlewyan, now we are getting somewhere.....

I looked at Pin 9 on both EL84 and there is a jumper between them but no connect to the board.....Ha....

Let me connect to the 100 ohm resistor and see what happens.

[Spunky99]

Well, we have an amp..

It has a nice buzz but it is working.

I'd like to thank Littlewyan, guitar-rocker, sluckey, 10thTx, Richie, Colossal so much for the help....

The problem was all the square bends, the pin 9 to pin 9 loop was tucked high and out of the way so I didn't notice it wasn't connected to the board. Stupid loop to nowhere.....

Now I've got to figure out why it is so noisy.
I took voltage measurements on all the tubes and they are a little high. That is probably where most of the buzz is coming from. Any recommendations?

[drew]

If you're getting the buzz with the chassis just sitting out on the workbench, it might just need to have the open side of the chassis closed off. And/or change the wires from the input jacks to shielded cable, with the shield grounded at one end only.

[Spunky99]

Thanks Drew,

The amp buzzes in or out of the cabinet. I added some beryllium copper shielding to the cover panel and that made no difference. I think it is partially the plate voltage is too high on the EL84 and possibly some shielding needs some grounding and maybe some grounds removed from the pots.

What I need to know is from the voltages I provided, what plate resistor size should I change to in order to get the voltage close to 300v DC.

[Spunky99]

Here is a picture of the amp. It is missing the plexi panel. Does anyone know where I can get one cheap?
Swanson Cabinet