It is working
I added a 270 ohm cathode resistor on the 6L6s and I can make music.
Gain is a bit low due to v1 (first 12au7) not working.
Anyway here are the voltages (add dc unless noted):
B+ 414
on the 6L6CG pins:
3 416
4 386
5 11.7 (leaky 0.047 coupling cap?), 1.4 (leaky but not as bad)
8 34.2
126mA (34.2/270 cathode resistor)
48 watts across the plates (416-34.2)*.126 = 48
I could drop the cathode resistor some. How about 180?
v2 12au7 pins:
1 260
2 0
3 84.5
~3mA (84.5/27000)
6 131.7
7 0
8 7.4
~3mA (7.4/2200)
v1 12au7 pins:
1 338
2 0
3 0
6 334
7 0
8 0
Something is wrong with this tube. Plates have voltage but nothing on the cathodes. I need more time to diagnose.
[minutes later]
tube heaters are wired through the 6l6 cathode pins. What a mess. I'll fix this tonight.
Thanks for everyone's help. I'm ordering new coupling caps and tweak with the 6L6 cathode resistors (individual bias?).
rick
I added a 270 ohm cathode resistor on the 6L6s and I can make music.
Gain is a bit low due to v1 (first 12au7) not working.
Anyway here are the voltages (add dc unless noted):
B+ 414
on the 6L6CG pins:
3 416
4 386
5 11.7 (leaky 0.047 coupling cap?), 1.4 (leaky but not as bad)
8 34.2
126mA (34.2/270 cathode resistor)
48 watts across the plates (416-34.2)*.126 = 48
I could drop the cathode resistor some. How about 180?
v2 12au7 pins:
1 260
2 0
3 84.5
~3mA (84.5/27000)
6 131.7
7 0
8 7.4
~3mA (7.4/2200)
v1 12au7 pins:
1 338
2 0
3 0
6 334
7 0
8 0
Something is wrong with this tube. Plates have voltage but nothing on the cathodes. I need more time to diagnose.
[minutes later]
tube heaters are wired through the 6l6 cathode pins. What a mess. I'll fix this tonight.
Thanks for everyone's help. I'm ordering new coupling caps and tweak with the 6L6 cathode resistors (individual bias?).
rick
Free heater Supplies
The front end tube heaters wired in the cathodes of the output tubes was a fairly common trick used in PA Amps of that vintage. Saves a bit of wasted power/heat in cathode resistors of the output tubes (That is the front end tube heaters were used as common cathode resistors).
About 75mA idle current in each output tube means 150mA through the front end tube heaters, ideal for the 12A?7 series of tubes. Usually there would be 2 x 12A?7 heaters in series and therefore developing 24 V (12 V each tube heater) of bias to the output tubes. Commonly a resistor was also added in series with the two heaters to allow a bit of fine tuning of bias. This also meant that in order to keep output tube anode dissipation within limits the B+ was limited to around 350 Volts (to give Vak of say 320V). If you keep this scheme then the heater closest to the output tube cathode connections will have the highest DC voltage on it and should be used for the tube with the concertina splitter to minimize heater to cathode voltage in that tube.
This trick of using tube heaters for cathode bias resistors works well - as long as you don't try to use the heaters of the output tubes themselves, if you try to do that it doesn't work at all as there is no heater current to get tube current to get heater current (the ultimate circular arguement).
If you delete this scheme you'll need to check that the heater supply has enough capacity to cope with the front end tubes (the heater supply for the outputs will be 6.3 Volts so each "rewired" front end tube will draw an extra 300mA from this heater supply)
BTW the 1/2 12AX7, 1/2 12AU7 mentioned earlier is a 12DW7 and were used a lot in AMPEG Guitar Amps. Modern production 12DW7 are available (made by EI).
Cheers,
Ian
The front end tube heaters wired in the cathodes of the output tubes was a fairly common trick used in PA Amps of that vintage. Saves a bit of wasted power/heat in cathode resistors of the output tubes (That is the front end tube heaters were used as common cathode resistors).
About 75mA idle current in each output tube means 150mA through the front end tube heaters, ideal for the 12A?7 series of tubes. Usually there would be 2 x 12A?7 heaters in series and therefore developing 24 V (12 V each tube heater) of bias to the output tubes. Commonly a resistor was also added in series with the two heaters to allow a bit of fine tuning of bias. This also meant that in order to keep output tube anode dissipation within limits the B+ was limited to around 350 Volts (to give Vak of say 320V). If you keep this scheme then the heater closest to the output tube cathode connections will have the highest DC voltage on it and should be used for the tube with the concertina splitter to minimize heater to cathode voltage in that tube.
This trick of using tube heaters for cathode bias resistors works well - as long as you don't try to use the heaters of the output tubes themselves, if you try to do that it doesn't work at all as there is no heater current to get tube current to get heater current (the ultimate circular arguement).
If you delete this scheme you'll need to check that the heater supply has enough capacity to cope with the front end tubes (the heater supply for the outputs will be 6.3 Volts so each "rewired" front end tube will draw an extra 300mA from this heater supply)
BTW the 1/2 12AX7, 1/2 12AU7 mentioned earlier is a 12DW7 and were used a lot in AMPEG Guitar Amps. Modern production 12DW7 are available (made by EI).
Cheers,
Ian
> on the 6L6CG pins: 5 11.7 (leaky 0.047 coupling cap?), 1.4 (leaky but not as bad)
FIX THAT! You are burning-up the 6L6, and you can't diagnose the driver with all that leakage (several mA) going the wrong way.
That may be why the 12AU7 "Plates have voltage but nothing on the cathodes."
> tube heaters are wired through the 6l6 cathode pins. What a mess. I'll fix this tonight.
If that is what you really have (there seems to be doubt), DON'T "fix that"! And get rid of that 270 ohm resistor: your cathode current is not 34/270 but much higher.
The finest Fisher I ever met used this trick. So did others.
If you are abandoning the mike-amp tubes, run the 6L6 cathode current through the 12AU7s that are left. DC heat at zero cost.
25 watts in the plate is not a bad value for push-pull 6L6GC (don't try this with original 6L6 or 6L6G!). I would not go higher, except with the heater-resistor trick you are forced to run 140mA-160mA to keep the little tubes happy. Assuming your B+ stays around 415V and the G2 voltage forces about 25V grid bias (two 12AU7 heaters) to give 75mA per tube, you will be around 30 watts dissipation per plate, an OK value. Hotter than it needs to be, but the DC heat is nice when it is cheap. (It does mean the amp takes twice as long to warm-up.)
FIX THAT! You are burning-up the 6L6, and you can't diagnose the driver with all that leakage (several mA) going the wrong way.
That may be why the 12AU7 "Plates have voltage but nothing on the cathodes."
> tube heaters are wired through the 6l6 cathode pins. What a mess. I'll fix this tonight.
If that is what you really have (there seems to be doubt), DON'T "fix that"! And get rid of that 270 ohm resistor: your cathode current is not 34/270 but much higher.
The finest Fisher I ever met used this trick. So did others.
If you are abandoning the mike-amp tubes, run the 6L6 cathode current through the 12AU7s that are left. DC heat at zero cost.
25 watts in the plate is not a bad value for push-pull 6L6GC (don't try this with original 6L6 or 6L6G!). I would not go higher, except with the heater-resistor trick you are forced to run 140mA-160mA to keep the little tubes happy. Assuming your B+ stays around 415V and the G2 voltage forces about 25V grid bias (two 12AU7 heaters) to give 75mA per tube, you will be around 30 watts dissipation per plate, an OK value. Hotter than it needs to be, but the DC heat is nice when it is cheap. (It does mean the amp takes twice as long to warm-up.)
Caps are on order.
Since I pulled the 12ax7 mic driver, the heater to the 12au7 wasn't on. Oops! The wiring seemed original and once Ian described the design, I could visualize it from memory.
I only have one Fisher tuner (purchased new March 31, 1959) and didn't look at how the heaters were wired. I just fixed the leaky coupling caps.
Thanks again everyone. Once I get parts in and installed, I'll try to post my results.
rick
Amp works
I got the amp working.
I installed new 0.047uF coupling caps, removed the 270 ohm resistor, added *all* the 12ax7 mic tubes to get the 12au7 heaters working and the amp works. I'll bypass the 'presence' plug but I feel confident the amp now works.
I measured tube pin voltages and they seem good.
Thanks again for everyone's help. Not sure what I'll do with the amp but I did learn a few more things.
rick
I got the amp working.
I installed new 0.047uF coupling caps, removed the 270 ohm resistor, added *all* the 12ax7 mic tubes to get the 12au7 heaters working and the amp works. I'll bypass the 'presence' plug but I feel confident the amp now works.
I measured tube pin voltages and they seem good.
Thanks again for everyone's help. Not sure what I'll do with the amp but I did learn a few more things.
rick
Just for reference,etc..
Here's the schematic of the Bell 6060 amp that uses the heaters-as-cathode-bias scheme,The first amp I've seen to use it.
Kind of a neat 'trick'.
http://www.users.qwest.net/~ptaylor/Audio/Bell6060B1.gif
http://www.users.qwest.net/~ptaylor/Audio/Bell6060B2.gif
Here's the schematic of the Bell 6060 amp that uses the heaters-as-cathode-bias scheme,The first amp I've seen to use it.
Kind of a neat 'trick'.
http://www.users.qwest.net/~ptaylor/Audio/Bell6060B1.gif
http://www.users.qwest.net/~ptaylor/Audio/Bell6060B2.gif
Bell 6060
Sorry to BUMP this thread up but I found this thread about a month ago and I just had to post something about it. The Bell 6060 that is. As some of you may recall from some old lost post from about a year and 1/2 ago I used to own one of those from 1965 to 1986, had to leave it behind in Florida.
Also I've been looking for a schematic of it besides the one in my brain. Left Sam's in FL. too! Soo thanks DigitalJunkie!
Yeah, a neat trick to use DC filaments on the low-level stages at the same time. Mine was a little different in that it used 6.3 VAC on the driver stage, DC on the phono eq-amp and tone-amp stages with a 50W aluminum heat-sink mounted 15k or 18k resistor from 435V source to the output stage's cathodes, with hum (bias) adjustment pots connected to EL34 grids via the 270k grid resistors. And aaaahhh there's the 5V3A, a 5U4GB butt with more balls...bigger too!
Wayne
Sorry to BUMP this thread up but I found this thread about a month ago and I just had to post something about it. The Bell 6060 that is. As some of you may recall from some old lost post from about a year and 1/2 ago I used to own one of those from 1965 to 1986, had to leave it behind in Florida.
Also I've been looking for a schematic of it besides the one in my brain. Left Sam's in FL. too! Soo thanks DigitalJunkie!Yeah, a neat trick to use DC filaments on the low-level stages at the same time. Mine was a little different in that it used 6.3 VAC on the driver stage, DC on the phono eq-amp and tone-amp stages with a 50W aluminum heat-sink mounted 15k or 18k resistor from 435V source to the output stage's cathodes, with hum (bias) adjustment pots connected to EL34 grids via the 270k grid resistors. And aaaahhh there's the 5V3A, a 5U4GB butt with more balls...bigger too!
Wayne
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