Tubelab SSE - High B+ voltage

[DanS31]

Hi everyone,

First, a big thanks to Tubelab and the community for this great resource. I recently built a Tubelab Simple SE amp, and, combined with all of the info on the Tubelab website, the information and guidance on this board were super helpful.

My amp is working - no smoke or explosions, no shocks when touching it, and sounds great! But I have some questions about my checkout measurements:

My power transformer is the Edcor XPWR035 and i'm in the US. Also, the amp is tube rectified (Sovtek 5AR4 -- I took out the FRED diodes). My B+ measurement is 480v. The cathode voltage (measured at R17/R27) is 40v. My cathode resistor is 560 Ohms. This gives 0.071 amps current. I then calculated dissipation as 31.24 watts.

I'm using EH 6CA7 Big Bottle/el34 tubes. I believe they have a maximum dissipation listed as 25 watts. I haven't noticed any red plating or other issues so far, but I'm assuming bad things will happen if I keep running that much power through them. I just ordered some higher Ohm resistors to switch in for R17/27.

My question: Is 480v high for a B+ voltage with a tube rectifier? I haven't seen many threads here reporting such measurements. Is it anything to worry about? If so, are there any changes I should make? Would different rectifier tubes have an impact? I could install the FRED diodes, but my understanding is that SS rectification would result in higher B+.

Thanks in advance for your help!

Best, Dan

 

[wa4htz]

Hi Dan. I believe you are measuring it incorrectly. Don't measure B+; measure the plate voltage at the plate. If you used the screw terminals, this easy. Just put the positive meter lead on the terminal from the output xformer which goes to the plate. Put the negative lead on the cathode pin or the wire on the cathode side of the 560 Ohm resistor. This give you the voltage across the tube and is the voltage you should use in your power calculation. All the tube cares about is the voltage across it and the current through it. That is the tube actual dissipation. I can't tell you the voltage drop across your output xformer but you already know you have 40v less across the tube from the drop in your cathode resistor.

Hope this helps.
Ken

 

[DanS31]

Hi Ken,

Thanks for the kind reply. Very possible I'm doing something wrong here...

Let me try to explain what I did. I was following Ty_Bower's post #5 in the "Simple SE checkout for dummies" thread.

I measured B+ with the red lead at r4 and the black lead to ground. This gave 480v. I also measured the voltage at the cathodes of the power tubes by attaching the red lead to r17/r27 and the black lead to ground. This gave 40v. To compute the current going across the cathodes, I do V = I*R which in my case is 40 = I*560 ==> I = 0.071.

I didn't measure the plate voltage -- I just, as mentioned in Ty_Bower's post, "assume it will be fairly close to B+ (probably within 10 volts)." I then subtracted the cathode voltage of 40 from the (assumed) plate voltage of 480, which gives 440. I multiplied that voltage by amps to get watts: 440*0.071 = 31.24.

I haven't had a chance to take any measurements since reading your post, but will try to do so tonight or tomorrow at some point. But just to make sure I understand:

If I do the measurement you suggest (red lead to the screw terminal between the opt and the plate of the tube, black lead to the c12/c22 side of r17/r27), I just need to multiply that voltage by my measured current of 0.071, and that will give me the power dissipation of the tube. Is that right?

Thanks so much,

Dan

 

[DanS31]

Hi everyone,

I have a few additional related questions:

1. Is the B+ voltage of 480 anything to be worried about w.r.t. some of the capacitors (specifically c1 and c2) having a 500v rating? Is running close to the max value bad, or it only matters if you go above that level?

2. If this B+ voltage is indeed a little bit higher than expected/common, does it lend itself to certain set ups over others? For instance, maybe some tubes would be especially good or bad in such a setup. Or maybe using a Cathode Feedback or Ultra Linear setup, etc.

Thanks!

Dan