Dumble 102 ODS testing

[desertmouth90]

Thank you guys,

I was full of mixed emotions, that Mouser sent me (a noob) the wrong parts. At first I couldnt believe I got it done, then I thought about how I could have got it done way sooner!

I was telling people around me for months: "When ive got this up and running".

Anyways ive calmed down now, I wrote to Mouser, see what happens.

This will be the first of many to come!

Thanks to all those that helped me, special mentions to Modulus in Scotland for the transformers and Andrea Antanello for the PCBs to start this journey.

Clive

[martin manning]

Check your Mouser order history and see what part numbers their records say you ordered. It's all too easy to choose the wrong part number by accident.

[desertmouth90]

Check your Mouser order history and see what part numbers their records say you ordered. It's all too easy to choose the wrong part number by accident.

Yeah I double checked, every time I ordered 12a they send me 12b... I messaged Mouser, not heard anything back yet.

for future builds I may move away from switchcraft as there are other Uk alternatives... still can't knock the quality though

[desertmouth90]

So a little update... in my noobish experience I wired up the bias pot back to front (Years of wiring up guitars means I got it wrong). I posted something about it on the Technical page and took a right drubbing because of the solder on the sockets too. They also said I fried my output tubes (which I don't think is the case fortunately).

It was because I used awful solder, I have since bought some much nicer lead free solder instead using cheap nonsense.

Bias pot wired up right, I am going to improve the grounding scheme and add bias probes inserts like on Ceriatones amps and then disengage the 100w 50w switch as I think its total nonsense to have this (Especially as I have a 50w in the works).

I am going to order 4 Belton Sockets for this amp and redo each of them with the much better solder and wire wound resistors.

[martin manning]

Most people use Pb-Sn lead-tin solder, since it’s easier to work with than lead-free. Solder containing lead is still sold in UK for hobby and repair use.

If you must go lead-free, there are different alloys available. K100LD and sn100c are recommended (quick search, ymmv). If you have found one that works for you that’s fine.

[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.

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.

So I wired up the bias pot wrong in the first place.

I have rewired it, but the output I get is a brittle sound. The pot ranges between -70 and -60 only.

I added 1ohm resistors to each pin 8 and changed the ground scheme to be neater and its just not working great.

I am worried I fried my 6L6GCs. although I tested filament continuity and they are fine in that respect.

[martin manning]

Do you mean you are getting -60 to -70V at power tube pin 5's? What are you measuring on pin 8's in DC mV with no signal? This will be numerically equal to the cathode current in mA.

It seems likely that you need to adjust the bias circuit to move the bias voltage range up about 20V (make it less negative) so you can get around -45V on pin 5's. The bias condition you have now could explain the thin sound, as the power tubes are essentially in cut-off.

[desertmouth90]

Do you mean you are getting -60 to -70V at power tube pin 5's? What are you measuring on pin 8's in DC mV with no signal? This will be numerically equal to the cathode current in mA.

It seems likely that you need to adjust the bias circuit to move the bias voltage range up about 20V (make it less negative) so you can get around -45V on pin 5's. The bias condition you have now could explain the thin sound, as the power tubes are essentially in cut-off.

Got the Bias working and the 1ohm Resistor mV readings... however the technical descussion were right, I toasted my JJ 6L6GCs.

I have ordered some TungSol 6L6GC STRs... Heres hoping I get this sorted!!!

[desertmouth90]

Dumble is running on full steam!!! sounds amazing just on its own.

https://youtube.com/shorts/Sw3XZmUvTiA?feature=share

Does anyone know the guidance on adjusting the trim pots inside the amp?

Thanks you to you all that helped me!

Clive

PS sorry about my playing haha

[martin manning]

Linked video doesn't work for me.

PI Trimmer instructions: https://ampgarage.com/forum/viewtopic.p ... 89#p416389

For starters, set the OD entrance trimmer at 25k as measured from wiper to ground.

[desertmouth90]

Re posted, thanks for pointing that out

[desertmouth90]

So my final uncovering was that I put the two of the trim pots the wrong way round. I will fire the amp up tomorrow and reshoot that video. I was wondering why I could drive the amp hard! was thinking I need a few overdrives ahead of it, but the clue is in the name haha

100k pot for gain is more like it!

[dbharris]

Glad to see you got the amp working!

You mixed up the 100K OD entrance trimmer and the 10K PI balance? Or you adjusted them incorrectly?

You may want to measure and post all of your voltages too.

-Dan

[desertmouth90]

Glad to see you got the amp working!

You mixed up the 100K OD entrance trimmer and the 10K PI balance? Or you adjusted them incorrectly?

You may want to measure and post all of your voltages too.

-Dan

They were the wrong way round, Yes I will. Also noticed I have biased it super cold at 16mV(mA) at the 1ohm Resistors... Way too low. I misinterprated my friend when he said 16 he meant Watts.

Clive

[dbharris]

Glad to see you got the amp working!

You mixed up the 100K OD entrance trimmer and the 10K PI balance? Or you adjusted them incorrectly?

You may want to measure and post all of your voltages too.

-Dan

They were the wrong way round, Yes I will. Also noticed I have biased it super cold at 16mV(mA) at the 1ohm Resistors... Way too low. I misinterprated my friend when he said 16 he meant Watts.

Clive

I suggest you use this very handy calculator to help you bias your amp.

https://robrobinette.com/Tube_Bias_Calculator.htm

-Dan