Check the power supply voltage by measuring DC voltage between the chassis plate
- Thread starter Colman
- Start date
Any metal on or touching the chassis plate. Any of the RCA jack ground tabs, any of the center lugs of any of the terminal strips, the IEC power entry module earth lug, stuff like that.
Uh oh! I checked the B socket and it measured an acceptable 6.25 V. After 10 minutes, I went to check the chassis. I unplugged the red probe from B2. I forgot I still had the black probe clipped on B7. As soon as I touched the red probe to terminal 21, there was a big spark at terminal 21! I'm unharmed but it really woke me up! Do you think I damaged the Moreplay?
I rechecked all the voltage values (pg. 59 MP Manual) with the chassis upside down. I got a couple of bad values. Terminal 4 read 0 instead of 7-9, and terminal 6 read 252 instead of 110-140. All other measurements were good. I powered off the MP. After 15 minutes, I turned the chassis right side up and powered up. Turns out the bad measurements are a separate issue: the tube in socket A doesn't light up and there is no sound from the right channel. The left channel is fine. I swapped tubes and remeasured. This time, terminal 4 is 6.54 and terminal 6 is 111. There is no sound from the left channel and the right channel is fine. So the tube is blown.
Both 6V6 tubes were working fine before. I'll just get a replacement pair. It will take some time before they get delivered.
Also, after performing the voltage test I'm sure that my MP survived the spark undamaged.
Questions:
1) How do I test filament power?
2) Can anyone recommend particular 6V6 tubes or their equivalent?
Both 6V6 tubes were working fine before. I'll just get a replacement pair. It will take some time before they get delivered.
Also, after performing the voltage test I'm sure that my MP survived the spark undamaged.
Questions:
1) How do I test filament power?
2) Can anyone recommend particular 6V6 tubes or their equivalent?
hmbscott
New member
1) Short answer: filament power = V^2/14, in watts. The answer should be close to 2.84W. So measure the filament voltage and apply that formula.
The long answer: Power is a bit tricky because you need to know the hot resistance of the filament. and you measure the filament voltage. With those two values you can calculate filament power. The formula is P = V^2 / R, where P = power in watts, V = filament voltage, R = the hot filament resistance. The nominal hot filament resistance can be calculated from data on the tube datasheet if it gives heater voltage and current. The formula is R = V/I. The Radiotron 6V6GT sheet I just looked up states V = 6.3 V (AC or DC), and I = 0.45 A, so the nominal heater hot resistance is 14ohms. Note that you cannot just measure the heater resistane when it's cold because it will be much lower and thus give you the wrong heater power.
2) Recommended tubes: I have the following NOS tubes, all of which I like, some better than others. In order of least to best sounding IMO:
#4 NOS 6V6 JAN (metal body tubes made several companies) This is a nice tube and cheap. $15 ea. Pin 1 should be grounded because the metal body is connected to pin 1. It sounds better than the stock tubes. Great value.
#3 NOS Bendix Jan-CEA-5992 This is an expensive sought after tube that is compatible with the 6V6. At $175 each its not all that for the money IMHO, sorry I bought them. They sound fine, but not $175 fine.
#2 NOS Sylvania 6V6GT JAN "Brown Base". $40 ea This tube and the next one are sonically equally beautiful. Much nicer than the stock tubes IMO
#1 NOS RCA 6V6G gray glass. $40 ea This is a "coke bottle" shaped tube (6V6G) that looks cool and sounds beautiful. It's my favorite tube and my daily driver. I included a pic.
The long answer: Power is a bit tricky because you need to know the hot resistance of the filament. and you measure the filament voltage. With those two values you can calculate filament power. The formula is P = V^2 / R, where P = power in watts, V = filament voltage, R = the hot filament resistance. The nominal hot filament resistance can be calculated from data on the tube datasheet if it gives heater voltage and current. The formula is R = V/I. The Radiotron 6V6GT sheet I just looked up states V = 6.3 V (AC or DC), and I = 0.45 A, so the nominal heater hot resistance is 14ohms. Note that you cannot just measure the heater resistane when it's cold because it will be much lower and thus give you the wrong heater power.
2) Recommended tubes: I have the following NOS tubes, all of which I like, some better than others. In order of least to best sounding IMO:
#4 NOS 6V6 JAN (metal body tubes made several companies) This is a nice tube and cheap. $15 ea. Pin 1 should be grounded because the metal body is connected to pin 1. It sounds better than the stock tubes. Great value.
#3 NOS Bendix Jan-CEA-5992 This is an expensive sought after tube that is compatible with the 6V6. At $175 each its not all that for the money IMHO, sorry I bought them. They sound fine, but not $175 fine.
#2 NOS Sylvania 6V6GT JAN "Brown Base". $40 ea This tube and the next one are sonically equally beautiful. Much nicer than the stock tubes IMO
#1 NOS RCA 6V6G gray glass. $40 ea This is a "coke bottle" shaped tube (6V6G) that looks cool and sounds beautiful. It's my favorite tube and my daily driver. I included a pic.
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@andre, I retested A2+A7 and B2+B7. The values are 6.28 and 6.25 respectively. They are the same as before. Since there is no change in the values, the spark did no damage to my MP, thank goodness! Please excuse me, I know nothing about electronics, but I am learning along the way as I build more and more kits. Also I am using the one-hand rule and wearing electricians gloves when testing circuits to ensure my safety and to give me peace of mind, ha--ha!
@hmbscott, Applying the formula V^2/14, values are 2.81 W and 2.79 W. Those are good tubes you recommended. I looked around the web and there are no NOS that looked appealing. Not now anyway. I'll keep an eye out. I decided to buy a pair of new JJs. Not too pricey, and they should do the job.
@hmbscott, Applying the formula V^2/14, values are 2.81 W and 2.79 W. Those are good tubes you recommended. I looked around the web and there are no NOS that looked appealing. Not now anyway. I'll keep an eye out. I decided to buy a pair of new JJs. Not too pricey, and they should do the job.
@andre, things check out. Thanks for the suggestion.
@paul, I'm on pg. 28 of the MP Upgrade Manual: High Voltage Circuit Testing. My values for 6U/16U are 127.6/128.6. But B+ Reg TP is 185.9. The manual says it should be 175-180 V DC. Is my value within tolerance or is it too high?
@paul, I'm on pg. 28 of the MP Upgrade Manual: High Voltage Circuit Testing. My values for 6U/16U are 127.6/128.6. But B+ Reg TP is 185.9. The manual says it should be 175-180 V DC. Is my value within tolerance or is it too high?
That is OK, and actually a few extra volts in that spot isn't such a bad thing.
You can pop a pair in and just check the plate connections on the upgrade board to verify that the plate voltage hasn't deviated all that much.
