Two different preamps (one octal and one regular) in a Tweed Delux "clone"
- Thread starter Jerry garrcia
- Start date
Jerry garrcia
Tele-Meister
@andrewRneumann @printer2
So I’m alive with sneakers on indoors. Sweden is like in Japan. You don’t wear shoes indoors except if there is a party and you have the nice ones on.
I think it was a good try.
B+1 = 466 VDC
B+2 = 465 VDC
B+3 = 462 VDC
No tubes inside. To high? Won’t the voltage drop when the tubes are in place?
So I’m alive with sneakers on indoors. Sweden is like in Japan. You don’t wear shoes indoors except if there is a party and you have the nice ones on.
I think it was a good try.
B+1 = 466 VDC
B+2 = 465 VDC
B+3 = 462 VDC
No tubes inside. To high? Won’t the voltage drop when the tubes are in place?
andrewRneumann
Tele-Afflicted
Yes... as long as the heaters are hot and the tubes are installed you should see a lot lower. The problem is when starting cold. I'd go ahead and install all the tubes.
There are ways to avoid bringing the full 465V onto the caps. We might be able to install what is known as a "bleeder" resistor in the power supply. This might load down the power supply just enough to keep it below 450V even when cold or no tubes installed. Another option would be a standby switch to keep HT off the caps until the heaters are warmed up. Or possibly a well placed Zener diode that drops 20V.
Jerry garrcia
Tele-Meister
Will go with the tubes now.
andrewRneumann
Tele-Afflicted
And connect the speaker.
I was hoping your mains voltage was like 230v instead of 240v and that would keep you below 450V. Once you get this going, you should consider upgrading to 500V caps.
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Jerry garrcia
Tele-Meister
Still alive. Just got Amp curfew from my wife for tonight. She’s on call tomorrow so a lot of time to search for problems then.
Something is strange.
The voltage drop at B+1/2/3 isn’t as expected once amp is running with tubes in, major background hum and caps are emptying them self after the amp is turned of. B+ drops to <2VDC after it is turned of.
Grounding issues?
Shortage?
If anybody has a search pattern for the problem it is very much appreciated. I’ll have the whole day tomorrow.
I’ll include pictures of B+1/2/3. Before the tubes was installed the caps crept their current after amp was turned of.
2 sets of pictures depending on preamp switch. Caps devoltage them self in both preamp settings
Jerry garrcia
Tele-Meister
Speaker connected
andrewRneumann
Tele-Afflicted
Don’t worry about caps losing voltage after shutdown. That is to be expected because the tubes continue to conduct even after you turn off the switch. The fact that they held voltage with no tubes means you don’t have a short. Also the fact that your voltage is too high would indicate no short. Progress!
So… B+1 is staying up near 428 even with all tubes installed?
It is time to take some voltage readings on those power tubes. Fill out your chart with voltages for the power tubes. All voltages measured to chassis please. Remember to measure heater voltage (pins 2 and 7) to chassis in AC mode on your DMM.
So… B+1 is staying up near 428 even with all tubes installed?
It is time to take some voltage readings on those power tubes. Fill out your chart with voltages for the power tubes. All voltages measured to chassis please. Remember to measure heater voltage (pins 2 and 7) to chassis in AC mode on your DMM.
With the tubes hot they will continue to draw current from the capacitors when the power is turned off and drain them.
If you could, a voltage chart on the tube pins will help in troubleshooting now. First measurement of concern is the cathode resistor on the output tubes. We want to figure out how much current they are drawing from the power supply.
With the voltage and resistances we will do some Ohm's law.
430V - 366V = 64V 70V / 5600 ohms = 10.4 mA
366V - 260V = 106V 106V / 22000 ohms = 4.8 mA
So this tells us you have about 1mA per triode/pentode, looks about right. Subtract 10.4 mA - 4.8mA = 5.6mA going to the screens, which looks reasonable. So on first power up it seems you have a running amp.
To figure out more we can use the pin voltages. Also before you fire it up again, drain the power supply filter caps and then measure the resistance across the OT primary. Use your volt meter to make sure there is no voltage across the primary and then you are going to measure from the plate of one 6V6 to the first power supply node.Then do it for the other side. Then along with getting the voltages when the power is back on 'Carefully' measure the voltage across each side of the primary. We will use the voltage across the primary and the resistance of the primary to calculate the current through each tube. Also get the voltage drop across the sag resistor.
Not sure how much hum you have, but if you have a alligator clip jumper you could try shorting out pin 3 to pin 1 of a volume control to kill any signal coming from the preamp section. I will just leave that for now until we have the numbers. Your looking good so far.
If you could, a voltage chart on the tube pins will help in troubleshooting now. First measurement of concern is the cathode resistor on the output tubes. We want to figure out how much current they are drawing from the power supply.
With the voltage and resistances we will do some Ohm's law.
430V - 366V = 64V 70V / 5600 ohms = 10.4 mA
366V - 260V = 106V 106V / 22000 ohms = 4.8 mA
So this tells us you have about 1mA per triode/pentode, looks about right. Subtract 10.4 mA - 4.8mA = 5.6mA going to the screens, which looks reasonable. So on first power up it seems you have a running amp.
To figure out more we can use the pin voltages. Also before you fire it up again, drain the power supply filter caps and then measure the resistance across the OT primary. Use your volt meter to make sure there is no voltage across the primary and then you are going to measure from the plate of one 6V6 to the first power supply node.Then do it for the other side. Then along with getting the voltages when the power is back on 'Carefully' measure the voltage across each side of the primary. We will use the voltage across the primary and the resistance of the primary to calculate the current through each tube. Also get the voltage drop across the sag resistor.
Not sure how much hum you have, but if you have a alligator clip jumper you could try shorting out pin 3 to pin 1 of a volume control to kill any signal coming from the preamp section. I will just leave that for now until we have the numbers. Your looking good so far.
Jerry garrcia
Tele-Meister
Thanks for still giving guidance. Will do readings and check for bad solder joints tomorrow. Will then update the chart. The hum was quite profound.
I might still need the anti-popping resistor at S2. When the speaker was turned of, used a power extension wire with a fuse in it, it was quite a pop when turned of. Volume at 100% on both channels. Haven’t plugged in guitar, just the speaker.
Jerry garrcia
Tele-Meister
Thanks for positive input. “Measuring across each side”?
For the OT resistance primary? Between B+1 to pin 3 of V5? Or to ground?
B+1 and pin 3 of each tube.
Jerry garrcia
Tele-Meister
I measured the resistance but the voltages readings created quite a noice. Not appreciated by tha family and dog who all was asleep before I powered it up with the speaker. Like the shielded wire made a difference… Have to do it tomorrow unfortunately. Not a fast process.
That is why I suggested shorting out the pot to see if it is in the front end. Try (during the day) pulling the input tubes. If it still hums try the PI tube.
andrewRneumann
Tele-Afflicted
You can turn those down. I had you have them up to 100% for some resistance checks. They won't affect the voltages.
Jerry garrcia
Tele-Meister
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Jerry garrcia
Tele-Meister
@andrewRneumann @printer2
Now updated the chart with voltage readings.
With the volume at 100% a major hum, also more hum with volume at 0%, but less than 100%. Between 10-90% the hum is not volume shifting dependent and manageable lower. Measurements with volume at 90%. not much hum.
All voltage measurements are done with the preamp switch in the same position. Didn’t shift the switch to the right preamp tube for measurement. Don’t really know in what position the switch equals to 12ay7 or 6SJ7 preamp tube.
The 6SC7 is the PI tube? How much will that tube theoretically affect the sound?
Hasn’t understood how the two volume knobs now work. In the same way as in a regular 5E3 circuit?
Phuu. Quite scary with the voltage measurements with short stifted probes in that tight chassi. Glad I wasn’t drunk
Now updated the chart with voltage readings.
With the volume at 100% a major hum, also more hum with volume at 0%, but less than 100%. Between 10-90% the hum is not volume shifting dependent and manageable lower. Measurements with volume at 90%. not much hum.
All voltage measurements are done with the preamp switch in the same position. Didn’t shift the switch to the right preamp tube for measurement. Don’t really know in what position the switch equals to 12ay7 or 6SJ7 preamp tube.
The 6SC7 is the PI tube? How much will that tube theoretically affect the sound?
Hasn’t understood how the two volume knobs now work. In the same way as in a regular 5E3 circuit?
Phuu. Quite scary with the voltage measurements with short stifted probes in that tight chassi. Glad I wasn’t drunk
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andrewRneumann
Tele-Afflicted
Yes, you should be awake, alert, not drunk, not angry, and very patient when working with voltages that can kill you. It’s good to be a little scared.
I didn’t check your preamp tubes, but your power tubes looked reasonable given the voltages you have. It doesn’t seem like our sag resistor (10W) is doing much sagging though.
Turn off amp, unplug, confirm caps are discharged (did you know they can build some charge back up even after discharged?). Measure resistance for the HT center tap (where it connects to the 270R / 10W) to chassis. Make sure it reads 270.
It would be nice to take a voltage reading there, but it’s not going to be accurate because the current there is not nice and smooth.
I didn’t check your preamp tubes, but your power tubes looked reasonable given the voltages you have. It doesn’t seem like our sag resistor (10W) is doing much sagging though.
Turn off amp, unplug, confirm caps are discharged (did you know they can build some charge back up even after discharged?). Measure resistance for the HT center tap (where it connects to the 270R / 10W) to chassis. Make sure it reads 270.
It would be nice to take a voltage reading there, but it’s not going to be accurate because the current there is not nice and smooth.
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Jerry garrcia
Tele-Meister
There might be a problem there. When measuring HT CT to chassis R=0 ohm.
I measured the 270R leg (that attach to CT eyelet on board) connection to ground and there is continuity. I measured CT/270R connection to R17/R18 connection (wire connection under board) and continuity there.
I still have the R17 and R18 in those positions according to @printer2 layout. But have the connection under according to your layout.
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andrewRneumann
Tele-Afflicted
Yes that is a dead end wire. I believe @printer2 ‘s layout had it removed. I left it in just to make life easier on you. It shouldn’t be connected to anything.
