Dumble 102 ODS testing

[martin manning]

For a start, dial it down to the most negative voltage on pin 5's, and you will need a way to measure the current flowing through the output tubes.

[desertmouth90]

-17V is much too high (not negative enough). You will need around -50V there.

Edit: above you had -50V available for biasing. Have you just not set the bias yet?

Bloody trim pot, I manhandled it abit and now I got -52v

Weird

[desertmouth90]

For a start, dial it down to the most negative voltage on pin 5's, and you will need a way to measure the current flowing through the output tubes.

I was thinking of buying a bias meter, but I have a multimeter. Each Half winding has a resistance of 18ohm

[martin manning]

ok, you can use that. Did you have a look at the document here: https://ampgarage.com/forum/viewtopic.p ... 60#p474160 ? You will be measuring current as shown at C in the diagram.

[desertmouth90]

ok, you can use that. Did you have a look at the document here: https://ampgarage.com/forum/viewtopic.p ... 60#p474160 ? You will be measuring current as shown at C in the diagram.

Got an A in maths at school, that was the last time I used it. Will go through this with my sister (she’s an electrical engineer)

[martin manning]

Well alright, great resource there! She should be able make sense of it, but I’m betting she hasn’t worked with valves before ;^)

[desertmouth90]

Well alright, great resource there! She should be able make sense of it, but I’m betting she hasn’t worked with valves before ;^)

No she has not, big gas turbines yes!

I will be able to work it out eventually.

It appears somewhere in the circuit the signal is getting pushed to ground.

Also appears that SPK2 Output is the main output for some reason… who only knows!?

[timrobertson100]

It appears somewhere in the circuit the signal is getting pushed to ground.

Also appears that SPK2 Output is the main output for some reason… who only knows!?

Can you please post new pics, including a closeup of your output socket wiring?

[desertmouth90]

It appears somewhere in the circuit the signal is getting pushed to ground.

Also appears that SPK2 Output is the main output for some reason… who only knows!?

Can you please post new pics, including a closeup of your output socket wiring?

As requested, two 22uf filter caps were out of the damn circuit… a rookie error from… er… a rookie!

FX loop and outputs changed to 12a from 12b. Will speak to mouser about this before my next order.

If you need anymore closeups of certain locations I can provide. Also tonight I will install 1R resistors to the Cathodes for biasing as apparently I forgot how to do maths.

Cheers
Clive

[desertmouth90]

Dont understand it, all voltages are perfect. 455v Screen Voltage - 457v Plate Voltage.

make it make sense haha

OT is wired up correctly - I checked with the manufacturer

[Richard1001]

I looked at your pictures and wonder what output transformer you use?

On most output transformers the primary winding would be red, blue, brown, with red being the centre tap. On your amp there is a red wire that goes to the anodes on one side and brown going to the anodes of the other tube pair. And then on the secondary side it looks like there is another brown wire going to ground (soldered on the speaker ground) It would be odd to have the same color brown wire both on the primary and secondary side of the transformer. I can't see which color wire is going to the B+

Maybe it would be worth checking the wiring.

[desertmouth90]

The Output transformer was custom wound by Michael at Modulus Amplification.

Hes also done my 100 power transformer and 50w transformers (OT and PT).

And on this OT he used Marshall wiring as follows:

https://modulusamplification.com/100w-o ... 3202-p.asp

although this is a custom 2k OT for 4 6L6s

Also just cut my foot on a Vintage fender pt (how very apt!) dont leave them on the floor where you are working again!

[desertmouth90]

I HAVE FINALLY DONE IT!

It’s unbiased but I have output!

I will get it biased. And the overdrive and internal pots need biasing.

The problem is the input is also 12b not 12a! I dunno whether to cry or be furious!!!!

How do I upload videos?

Thank you all that helped!!!!

[martin manning]

I HAVE FINALLY DONE IT!

It’s unbiased but I have output!

I will get it biased. And the overdrive and internal pots need biasing.

Great! Get the bias set properly, and dial in the internal OD entrance and PI trimmers. Those are not bias, they set the base OD level and the PI/Output stage balance. There is a lot of discussion on how to go about that on the forum.

[timrobertson100]

Congratulations!