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Old 28th January 2010, 01:27 PM   #1
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Default Did I blow the tubes ?

So, I assembled the TubelabSE for 300B.

I am using a transformer 360-0-360 VAC, with 40 ohm resistance.

To lower the voltage I am using a 10 uF for C4, choke 93 ohm, and 5R4WGB that has a bigger drop than the 5AR4.

Checked filament voltages: OK

Only with rectifier I have B+ 510 V

Connected the drivers, I manage to have one at 175, but the other does not go over 165 V. Swapped the tubes problem changed side, so I guess the tubes are pretty different. Put both at 165 V.

Connected the 300BC.

Now they conduct heavily, the B+ goes to 274 V and does not vary much does not matter what I do with the trimmer. I have 1.2 V on R18 and R29.

I added two 300 ohm resistors (20W) between the transformer and the board, less B+.

Now B+ 211 V and voltage on R18 and R29 to 1.1 V.

So did I blow anything ?

Please help,

Davide
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Old 28th January 2010, 01:45 PM   #2
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Another thing that I did, was to electrically isolate the heat sink from the semiconductors, The voltage on R14 and ground (Bias) is only around -10 V. And this makes sense looking at the tube curves, but I don't understand what is the cause and what is the effect.

D.
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Old 28th January 2010, 01:48 PM   #3
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C6 and C7 are rated 450 V, C4 630 V, C5 800V.

Load is 5k ohm


D.
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Old 28th January 2010, 02:58 PM   #4
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And the winner is ... R5 it was showing 1.5 M ohm instead of 270 ohm.

Replaced it with an identical one, worked for few minutes, then again showed high resistance. I think I am going to buy a resistor with higher watt rate.

D.
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Old 28th January 2010, 03:04 PM   #5
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Something doesn't sound right. With the drivers, I had a similar issue with getting close to the "recommended 175 Volts". To correct that, I increased the driver current by dropping the CCS reference resistor values. As designed per the spec sheet, the CCS, it will hold ~7mA. I increased the current setpoint to 12mA. Well within spec of the 417a driver tube. You can find the value by looking at the curve of the CCS. I don't have them available to me now otherwise I'd check on it.

Now on to the output tubes. Once you set up the driver voltage, you need to set your bias voltage on the 300B. When you start out (before you insert the 300B tubes) it needs to be as negative as possible. What reading do you have at this point? Recalling from memory I was able to get to something like -140Volts. You should be okay if you can get it to -100. If you can't then I would start to trouble shoot the bias circuit. You might have a wrong value or a bad connection.

As far as the tube(s) are concerned, It is hard to say. If you didn't overload them for long, they probably are fine. They aren't bullet proof but are able to handle more abuse than semiconductors.

As far as the 300Ohm resistance, that seem like alot prior to the rectifier. Follow the recomended practices for the rectifier tube on the spec sheet. If more voltage drop is needed you can add more resistance after the choke.
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Last edited by SGregory; 28th January 2010 at 03:06 PM.
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Old 28th January 2010, 03:45 PM   #6
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You are blowing R5? That resistor doesn't dissipate that much...it is acting more like a fuse. I suspect you have a shorted cap...possibly C6. I actually put 450V caps in the C6 and C7 positions because I was concerned about the voltage across them before B+ comes up and starts drawing current though the bias circuit. My parts list shows:

C6,C7 - Capacitor - 100uF - Aluminum Electrolytic - 450V, 105^C, 650mA - 40x18mm, 7.5mm - Nichicon - PT Series - Mouser - 647-UPT2W101MHD - $3.31 - 2 - Driver bias supply filter (360V < B+ < 400)

EDIT: you already have 450V caps here, so nevermind about that.

If you are drawing enough current through the bias circuit to fry that resistor, then I would look more closely at the bias circuit. Like Gregory said, check the actual grid voltage going to the output tubes. Also double-check R6. On my amp, this resistor dissipates far more than oany of the others in the bias circuit. I put a 7W part there.

I wouldn't worry about the 5842 operating point. Have 16 of them on hand and the NOS Ratheon ones are all over the map.

Last edited by rknize; 28th January 2010 at 03:51 PM.
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Old 28th January 2010, 07:56 PM   #7
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Did you put in c6 backwards???? Remember that in this position the positive terminal goes to ground. Same for c7.

In either case check D2 and D3. I found that they are quite easy to toast. (Don't ask me how I know)
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Old 29th January 2010, 11:49 AM   #8
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So It looks like I did not blow anything beside R5. I bought today a new resistor that actually looks 3W that actually looks much bigger than the one mouser sent me labeled as 2W. This did not blow...yet.

I am running with 5k of impadence at 384 V and 66 mA of Bias. Tomorrow I am going to have a matching session in the shop where I got the drivers to find two that perform alike.

Let's see...

Few questions: What would you upgrade the cathode bypass caps with ?
And what about the PSU caps for the bias ?
What size of choke do you recommend ?


Thanks,

Davide
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Old 29th January 2010, 12:55 PM   #9
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Davide,
What is the voltage drop across your R5?
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Old 29th January 2010, 01:41 PM   #10
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This is what puzzle me, I have never seen more than 2 V, so the power dissipation is nothing. Maybe the resistors I had were just defective.

Now I saw another problem, I think that the 5V DC regulator goes onto temperature protection. As all of a sudden the current went to zero. I switched off and on after one minute and it was fine. Although this is again not good for the tubes. I'll buy a bigger heatsink tomorrow and try again

Let's again hope that tube are safe, they run for one minute with high voltage and no heaters :-(

D.
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