#183 B+....

[martin manning]

Did they clarify the dimensions and mounting centers for you?

[strat100]

just sent an email to clarify the sizes,will wait for a response

[strat100]

This is the info for the PT

[martin manning]

That's the document posted on their web site. Standard mounting centers for a Twin PT are 3.75 x 3.00, or 95 x 76mm. Inmadout's PT sketch shows 100 x 80, so not a "drop-in" fit. The dimensions of the lam's are shown at 100 x 82, which is clearly an error.

[strat100]

Here is the latest info i just got from them

[martin manning]

That makes more sense. They need to fix that on their site, and they should warn that it does not match the dimensions of the originals.

[mrdarwin]

Finally ordered the Plexi 100w lay down style.
Modified PSU with FWB and replace Bias resistor with a 6k8.
Now waiting the PT!

[mrdarwin]

Hi!
Received Plexi PT, and mounted it.
Enough A for heater, 482V on B+, put à 4k7 3w resistor instead 3k...
any suggestion?
Correct Bias current with 6k8 resistor...

Now I have a hum that was not present before... everything is grounded. Still looking for...

[martin manning]

Ok so you are close to the HT target now (+4%), and you have plenty of heater current available. Adjusting the dropping resistor to get the preamp voltages in line is the right thing to do, so you are good there. What idle bias voltage and current do you have, and where is the bias pot in its rotation? I'd say you want about 36mA per EL34 for the idle current. If you are at the extreme of the rotation (most negative voltage setting) there will be increased ripple on the bias voltage.

Where did the PT come from, and did it fit in your chassis easily?

[mrdarwin]

I ordered TAD plexi lay down style and it fit perfectly.
I cant get 36mA bias current...
Do I have to decrease or increase the 6k8 resistor?

[martin manning]

I suggest changing the 22k going to ground from the bias pot. Above I estimated 15k. Do you have ~98VAC to ground at the input (transformer) side of the 6k8?

[mrdarwin]

15k changed yet...
Trying with 6k8, now I can get 36mV bias voltage.
But hum is still here... B+ is 445V now

[martin manning]

Ok, I expected you would see B+ drop some. You must have had very little bias current before. On your rectifier board you have the four diodes wired as a full-wave bridge, then you added a ground wire, correct? Then, where is that wire grounded? It should go to the same place that your center tap was grounded.

Is the filament winding's center tap grounded?

Is the hum 50 or 100 Hz?

Can you tell me what the bias voltage on the output tubes is? Also, is the bias pot somewhere near the middle of its range?

Maybe post a picture of the power section too.

[mrdarwin]

The ground wire of FWB is grounded at the same place where the CT of HV Twin PT were grounded.
CT of HV plexi is connected between the two big psu capacitors, after the first 270k resistor, like in the 100w plexi... connected or not, hum still here.
CT filament is grounded, tried with a "virtual" CT with two 47R to ground, hum still here.
Hum is "thin", not dark and loud, so I suspect a 100Hz hum, but I have to measure it.
Output tubes biased to 30-31mA@442VDC

When I plug a jack into RETURN, hum still here... can I say the problems stays in the AMP part, not in the PREAMP?

I've got a Ceriatone ODS 50w, I can hear a same hum, but not so loud than the hum on my #183...

[martin manning]

Hmm... (sorry) all of your wiring seems correct. I was interested in bias voltage, on pin 5 of output tubes.

Is the plug you inserted into the return jack shorted, grounding the PI input? If it is and there is still a hum present then yes I'd say it is in the power amp.