Dumble 102 amp testing

[desertmouth90]

Tested my new amp build with a Variac and bulb limiter. All voltages across the board are pretty much spot on.

The amp is unbiased (need to work that bit out).

Fired up the amp and there is no signal. Only a very loud hum.

[Helmholtz]

The amp is unbiased (need to work that bit out).

What do you mean with "unbiased"?

[desertmouth90]

I need to bias it to the correct negative voltage using the bias pot

[Helmholtz]

Make sure you have sufficient negative voltage (e.g. -50V) at socket pins 5 before (!) fitting power tubes.

[Stevem]

What you did should never be done to an amp that does not have a cathode biased output stage.

As posted above be for installing the output tubes always set the negative voltage as high as it will go, then pop in the output tubes.

Please post a schematic with the voltages on it that you observed.

And be 3 or 4 real good gut shots .

[desertmouth90]

I promise not to do that again.


Got the voltages around 380v they need to be 430v but couldn’t get the voltage on the Variac high enough because of the current limiter.

I’m buying a speaker cable too.

Images attached.

edit: Schematic - https://www.aaelectronicspcb.com/_files ... 72a1c8.pdf

[dbharris]

Are you using a guitar patch cable for the speaker load? You can damage your amp doing that. You really need a speaker cable.

The voltages will be low until you remove the current limiter. You should keep bias as far negative as are you can set it, remove current limiter, and then fire up the amp to bias it. The tube bias calculator app is very helpful for calculating where you need to be.

It doesn't look like you have bias test points on your chassis with 1 ohm current sensing resistors. How do you plan to measure bias current?

Safety is paramount when doing this. Please take time to read how to do this safely and ask questions before you start if you aren't sure. 1 hand in your pocket and a non-conductive screw driver are best practices.

-Dan

[maxkracht]

Are you using a guitar patch cable for the speaker load? You can damage your amp doing that. You really need a speaker cable.

Not sure why that would cause an issue unless it's a really cheap or damaged patch cable. Even if it's somewhat thin wire, it's only a foot. Cheap or damaged speaker cable would pose the same risk...

[dbharris]

Are you using a guitar patch cable for the speaker load? You can damage your amp doing that. You really need a speaker cable.

Not sure why that would cause an issue unless it's a really cheap or damaged patch cable. Even if it's somewhat thin wire, it's only a foot. Cheap or damaged speaker cable would pose the same risk...

Hi Max,

Damaged speaker cable would definitely pose the same risks. If only setting idle bias, a patch cable would likely be fine. But I would be worried if instead for an extended time I was running a sine wave into the input to set the PI balance with the Level up a good bit in a 100W amp. Maybe it wouldn't fail even then, but I'd be inclined not to risk it. Just my opinion

-Dan

[desertmouth90]

Speaker cable has been ordered.

Do you know of any guides to biasing that I can use? the ones on youtube are abit sub par.

I will get this biased in the next couple of days then I might add some probes to the design to bias easier.

I have confidence after a dim light bulb test that I have no shorts so this thing is on track!

50w version is coming up next

Thanks all,

Clive

[martin manning]

I put a document together a few months ago, just posted it to the Tech section here: https://ampgarage.com/forum/viewtopic.p ... 60#p474160

[maxkracht]

If only setting idle bias, a patch cable would likely be fine. But I would be worried if instead for an extended time I was running a sine wave into the input to set the PI balance with the Level up a good bit in a 100W amp.

Fair point. Certainly better safe, and should use the correct cable when possible. Still think odds of failure are low, assuming the cable isn't damaged and it is at least moderate quality.