A Collection of #124 Information (Schematics, Layout, etc.)

[Dino]

Thanks tried that with only power amp on and no noise but when i reconnect the preamp its back also whats the bias voltage supposed to be i think -55V but may bee wrong.....

Thanks for the help

[Structo]

Then switch the primary wires at pin 3 of the power tubes.

The bias depends on what plate voltage you are running.

Do you have 1 ohm resistors on the power tube cathodes to ground?

If not, that will be the easiest way to measure your bias unless you have a bias probe.

Bias formula is:

(W * .7) / V = I
W= power tube max watts.
.7 = 70% of max watts
V = plate voltage

Here W is multiplied by .7 to give us 70% of the tube's maximum plate dissipation.

Say you have 440v on your plates.

A 6L6 is rated at 25 watts (some are 30 but we'll stick to 25 for this example).

25 * .7 = 17.5
17.5 / 440 = .0397 amps or 40 ma.

So your high limit is 40ma
Most D style amps sound better at 55-60% max.

So in this example you would adjust the bias pot until you measured 31-38 ma or so. Listen.

Also be sure to check the plate voltage after each adjustment.
It will go lower the higher you set the bias.

This method does not take the screen current into consideration.

If you want to do that then measure the voltage at pin 4 and subtract that from the plate voltage before calculating.

[Dino]

Thanks for all the help that formula is very helpfull managed to fix the weird noise problem was my messy wiring now got it sort of working and im on the way keep you updated

[Deric]

The OD pots ARE labeled correctly for #124.

The layout makes it look like there is a jumper under the PAB relay that is not supposed to be there (I think ) where the red wire from the treble pot connects to the relay board. According to the schematic, the 2 sides of the relay are not connected.

[Structo]

The schematic shows both sides using the normally closed contacts.

I believe what it does is when PAB is switched on, the two right side lugs of the Bass pot are disconected.

At least that is how I wired mine.

[Deric]

Both sides of the relay are used but they shouldn't be connected to each other. The layout at the beginning of this thread shows a jumper from the NC contact on one side connected to the NO contact on the other side.

The schematic has N.C. next to the NO contacts on each side of the relay. I'm assuming that means No Connection not Normally Closed.

[Dino]

These are the diagrams i used scoured internet to find missing vales is this diagram correct or am i going mad.............. the noise is back

Thanks Dino

[ChrisM]

You might want to make a new thread insteadof "cluttering" up this one. I will delete this post later.

[Structo]

Both sides of the relay are used but they shouldn't be connected to each other. The layout at the beginning of this thread shows a jumper from the NC contact on one side connected to the NO contact on the other side.

The schematic has N.C. next to the NO contacts on each side of the relay. I'm assuming that means No Connection not Normally Closed.

OK I see what you are referring to.
Yes that is a mistake.

[talbany]

SOB :twisted: .. After doing the last revision Ratio Level I pulled an older file (gotta learn to delete each revision) already moved that jumper once.. anyway here it is again with all the revisions mentioned.. PAB and V1 filter..

Tony

[Structo]

Sorry to beat a dead horse but this layout does not have the Level and Ratio pot values correct.
It should have 250K on the Level and 100K on the Ratio.

[talbany]

Sorry Tom

Updated..

[grtamp]

Hi guys , sorry but I'm a bit confused about Ratio and Level pots .

If I understand the level pot( between od1 and od2 ) is 100k lin , and Ratio pot ( after od2 ) is 250K log ?

thanks all for help

[Structo]

No the Level pot is 250KA and the Ratio is 100KL

[grtamp]

Tom , thanks for the clarification!

Grtamp