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300B TSE B+ mystery

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Hi,

When I bench tested my new 300B build, I had a B+ of 418 volts. Now that I have it built and in its chassis, the B+ has dropped to around 360 volts. The only change that I can think of is that I added a CL-90 to the power input.

I am using the 660 volt leads from an Edcor xpwr 131. I'm using a Triad c-14x choke.

C4 is a 47 uf 500 volt cap

c5 is a 150uf 500 volt cap in parallel with 100uf motor run cap

The amp sounds fabulous, except that I hear a strange crackle for a second as it powers up. No hum, lots of punch. Sounds more engaging than my TSE 45.

I'm using JJ 300B tubes which are huge.


Could the CL-90 be causing the drop in B+ and also the noise. Should I be using a different current limiter?

I'm happy to live the 360 B+, just don't understand why the change.

thanks, Jacques
 
Voltmeters or ammeters? Ammeters are supposed to be wired instead 10R resistors, not across them.

In any case you can make simple circuit with battery and resistor corresponding to your tube bias current and compare what your panel meters show in comparison to multimeters.

Chinese panel meters are not precise and I remember I had to adjust mine for 80mA current using above mentioned circuit and multi meter as reference.
 
Ohms law depending on what battery you have and your bias. I used 1,5V battery and 0,5w 30ohms trimpot to set resistance exactly to 18,75R for 80mA current across it, then connected panel meters (not simultaneously) inline and set them to be spot on for 80mA.
 
What rectifier tube are you using?

With a 5AR4, 330-0-330, CLC config, I would expect about 410 - 420 volts dc loaded, which is what you report you had on the bench.

Check the bias by all means, but I am doubtful that it could be so far out of adjustment as to drop 60 volts, without the tubes glowing red hot.

Is it possible you have made a measurement mistake, e.g. DMM set for AC when you are trying to measure DC?

You might want to check the voltage at the + lead of each capacitor and then at the bleeder resistor - maybe something is leaking to ground. If you have an aux cap, disconnect it and see if that makes any difference.

Win W5JAG

edit: the crackle sound on startup is not normal, and is a symptom of something ..... Long shot - a bad solder joint on one of the capacitors in the B+ or B- chain. You might be hearing some arcing as the voltage tries to get between the two layers of the board. Do you also smell a whiff of ozone? If so, you have lightning somewhere ....
 
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Are you sure about that? I made quick sim in PSUD, with 330-0-330V and 184mA total current for all tubes, B+ is 370V. If 300B's are biased to 100 mA each, then B+ drops to 360V.

A generic PSUD sim will show a lower voltage, but we don't have enough information to model it - we don't know the actual line voltage, or exact parameters of his specific components. Adjust the line voltage up 4%, and see what you get .....

The real world rule of thumb I have always used, and ( for me ) seems to hold up well for the typical 117VAC PT, being used at contemporary U.S. line voltages, is that with a 5AR4 rectifier, CLC configuration, loaded B+ will be about (1.25)(VAC).

(330)(1.25) = 412.5, so I would expect loaded B+ to be somewhere within 10 or 15 volts of that, either way, assuming the first C is not an unusually small value.

I don't know how you model or sim a start up crackle.

Win W5AJG
 
Ok, so I completed the first step in my process of eliminating potential problems. I did calibrate the panel Ammeters so they read the same when given the same current. Per zxx222's suggestion I used a 1.5 volt battery and a 22 ohm resistor to generate 69 ma per ohm's law. When I measured with my cheap multi-testers I got a reading of about 62 to 64 ma.

I decided to ignore these results and set the meters to about 69ma with my current generator.

One panel meter read about 64ma and the other about 60ma, so they weren't too far off.

Next I will be disconnecting the aux cap and see if that makes a difference. I am now more concerned about the crackle at startup than in the drop in B+. I'm going to look at the connections for the power supply caps.

Am I correct that the voltage regulators and diodes are not part of the B+ chain?
 
Are you sure about that? I made quick sim in PSUD, with 330-0-330V and 184mA total current for all tubes, B+ is 370V. If 300B's are biased to 100 mA each, then B+ drops to 360V.

This is right about what my TSE 300B was running with those same PT specs. At ~90mA bias, B+ was around 370 and change. At 74mA, B+ was in the 380V range and the amp was much happier.
 
Hi Guys,

Thanks for your help .

I replaced C4 with a 22uf capacitor and managed to get rid of the crackle. The two panel meters are now calibrated to each other.

I have the bias set to about 68ma and the B+ is 360 volts.

Would a different set of speakers cause the B+ to change?

It sounds great. The output transformers I'm using have multiple taps, I'm currently using the 5,000 tap and I'm tempted to try the 3,500 tap.

I wonder if I will hear a difference.

thanks again, Jacques
 
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