300B DIY Budget Tube Amplifier Build Series

[StepheK]

Yeah, that is what I thought, I hope I did not destroy the 300b tubes with the heater center tap grounded, they are so expensive!

I would be looking at all the wiring, something is WAY off. And most tubes are pretty tough, I highly doubt they are the problem.

 

[ngoyette]

I would be looking at all the wiring, something is WAY off. And most tubes are pretty tough, I highly doubt they are the problem.

Okay, I have traced a the wiring several times and it all looks good, but I will do it again as something is not right

 

[ngoyette]

Okay, I have traced a the wiring several times and it all looks good, but I will do it again as something is not right

Since both channels look the same, I would suspect that there is a common issue between the channels? I plan to trace the wiring today, hope I find something!

 

[StepheK]

Are the voltages on the driver tubes correct?

It's hard to guess what is wrong, as an example someone sent me one of my designs they DIY build for me to "sort out" for them. They said they build it exactly like the video, but when it got here, right off the bat I saw they used a 22uf electrolytic cap for the coupling cap instead of a .22uf film cap.

In your case, either some part/parts got destroyed from how you changed the wiring, you subbed some way wrong part into the design or there is a wiring error you duplicated on both channels.

 

[ngoyette]

I think there is something wrong with the Audio Analyzer Suite, I tested my Dynaco which had great numbers before, is now showing terrible numbers (I did not take a picture when it was good back a month ago). But if you look at the input voltage (1.348Vrms) at 0.496W at the time of failure and I look at your video, you show 1.1Vrms at 6.3 W, this does not make sense.

I know that my PC recently had a Windows upgrade (Running Win 10 Pro), I wonder if something got mess up. I have not found a way to default WaveForms or the Audio Analyzer Suite yet, maybe I have to delete and re-install?

 

[StepheK]

I think there is something wrong with the Audio Analyzer Suite, I tested my Dynaco which had great numbers before, is now showing terrible numbers (I did not take a picture when it was good back a month ago). But if you look at the input voltage (1.348Vrms) at 0.496W at the time of failure and I look at your video, you show 1.1Vrms at 6.3 W, this does not make sense.

I know that my PC recently had a Windows upgrade (Running Win 10 Pro), I wonder if something got mess up. I have not found a way to default WaveForms or the Audio Analyzer Suite yet, maybe I have to delete and re-install?

Worth a try, you are measuring with the volume wide open etc right?

 

[ngoyette]

Worth a try, you are measuring with the volume wide open etc right?

On the Dynaco MIII there is no volume control, I am still trying out different settings, but I think it is time to remove the program and start from scratch.

 

[ngoyette]

Worth a try, you are measuring with the volume wide open etc right?

On the Dynaco MIII there is no volume control, I am still trying out different settings, but I think it is time to remove the program and start from scratch.

On the Dynaco MIII there is no volume control, I am still trying out different settings, but I think it is time to remove the program and start from scratch.

 

[ngoyette]

I don't understand how an input voltage of 1.348 Vrms is equal to 0.496 W, If I increase the input voltage above 1.6 Vrms I start to clip, which is I believe close to the max input for a Dynaco MIII? Does anyone explain the how that works?

Attached is Stephe results which shows her input voltage of 1.172 Vrms with a Power reported at 6.38W?

 

[ngoyette]

I figured it out, I had the scope probes on 10X instead of 1X!
Same test on the Dynaco but with the probe set to 1x, much better

 

[ngoyette]

This is the 300b amp with the correct settings on the probes, it looks like I need a little work to bring the THD down, not sure what exactly the issue is compared to Stephe amp, I know I put my input on the back of the unit instead of the side, maybe the longer distance is affecting the signal.
BTW, the amp sounds amazing, even my wife commented that it sounded very clear.

 

[ngoyette]

I noticed the last rectifier you used was a Sovtek 5AR4. This rectifier needs to have the proper amount of resistance in the plate circuit to operate within its parameters. The Sovtek and JJ 5AR4's are sensitive to this requirement. In addition, you may want to add two 1N4007 diodes in each plate circuit too.

Below is an example of what I am referring to. The resistors in each plate lead will be different for every power transformer. These resistors are necessary when there is not enough resistance in the secondary winding of the power transformer secondary. A 5AR4 needs approximately 112.5 ohms per plate lead of resistance to operate safely with 375VAC on the plates of the rectifier tube. An Edcor power transformer with 375VAC per plate lead has about 23 ohms. The Hammond may be similar. If so, you need to add let's say a 75 ohm/3W resistor to each plate leg. The 62/3W in the diagram below would become 75/3W. These resistors will add to the transformer secondary DCR to give you an effective circuit DCR of about 110 ohms.

The additional diodes eliminate the peak inverse voltage which puts a lot of stress on the rectifier tube(s).

If you buy other 5AR4's such as the TAD 5AR4 for double the price of a JJ or Sovtek, you can just plug it in, but I would still recommend adding the resistors in circuit because these are always needed for long tube life and reliability. The 1N4007 diodes are strictly for the JJ/Sovtek rectifiers that have known issues with arcing/flashover.

This is only aimed at the rectifier circuit, you still may have other issues that are unresolved regarding the fuse blowing, but this rectifier issue will blow fuses too.



View attachment 3132170

I put in the 75 ohm 3W resistors and the diodes and the unit is working great. The only thing I noticed is the B+ takes about 15 minutes to come up to full power, which is now 420V, Stephe amp is 435V,

 

[StepheK]

I put in the 75 ohm 3W resistors and the diodes and the unit is working great. The only thing I noticed is the B+ takes about 15 minutes to come up to full power, which is now 420V, Stephe amp is 435V,

If it sounds good, just stop here and enjoy it