I got the 845 tubes.
With the 845 installed, the grid voltage does not change and it reads -88 VDC. It was getting red so I turn off the amp.
Here is how the grid voltage is affected by the driver tubes:
- without any tubes, grid voltage is -392 VDC, unaffected by bias adjustment.
- with back tube 6CG7 (the tube that has the grid wire connected), grid voltage is -30 VDC at highest bias setting.
- with back tube 6CG7 + middle and/or front 12AU7 tubes, grid voltage is -88 VDC.
- with just middle and/or front 12AU7 tubes, grid voltage is -392 VDC.
If only the 845 installed, without back driver tube 6CG7, then the 845 runs just fine. The grid is still -392 VDC.
With the 845 installed, the grid voltage does not change and it reads -88 VDC. It was getting red so I turn off the amp.
Here is how the grid voltage is affected by the driver tubes:
- without any tubes, grid voltage is -392 VDC, unaffected by bias adjustment.
- with back tube 6CG7 (the tube that has the grid wire connected), grid voltage is -30 VDC at highest bias setting.
- with back tube 6CG7 + middle and/or front 12AU7 tubes, grid voltage is -88 VDC.
- with just middle and/or front 12AU7 tubes, grid voltage is -392 VDC.
If only the 845 installed, without back driver tube 6CG7, then the 845 runs just fine. The grid is still -392 VDC.
Sounds like the output tube is biased through the driver tube. IE. the driver tube is configured as a cathode follower and DC coupled to the output tube. The bias adjustment on the amp actually controls the bias on the driver tube which in turn control the bias of the output tube through the driver's cathode. If this is the case, you have a good design there.
-88 volts sounds about right for a 211. What is your B+ voltage to the output tube? 1000-1200 volts?
Changing the bias supply will involve changing the resistor values to the driver tube, changing the driver tube to a beefier unit as it will have to swing more voltage for the 845, and probably adding another gain stage to get the voltage swing up. It will be easier for you to get a pair of 211's and plug them in to your amp.
Of course this is pure speculation on my part as I have not seen your amp and its circuitry, but it sounds like you have an Ongaku clone amp.
Looks like Sy is slipping up....
Daniel
The grid voltage without tubes inserted was something like 300V, so there's plenty of range to accommodate either 211 or 845.
I've suggested to Man that he spend some time tracing the circuit out- there are undoubtedly other problems which will have to be overcome- while he had it over my place, one of the power resistors snapped off its lead while we were measuring it! There were also no bleeders, making this a very dangerous unit to work on.
For reference, here's the schematic for the Ongaku: http://www.drtube.com/schematics/an/ongaku.gif
I've suggested to Man that he spend some time tracing the circuit out- there are undoubtedly other problems which will have to be overcome- while he had it over my place, one of the power resistors snapped off its lead while we were measuring it! There were also no bleeders, making this a very dangerous unit to work on.
For reference, here's the schematic for the Ongaku: http://www.drtube.com/schematics/an/ongaku.gif
By the way he described his amp it sounds like the driver (6CG7?) is DC coupled to the output tube. In this case depending on wether the driver tube is plugged in or not, there will be WIDE voltage variation on the grid pin of the output tube. Driver tube has current going through it so voltages will very depending if it is plugged in or not.
No driver tube Bias will go to it's max. Driver tube plugged in, Bias will lower depending on how much current is going through the driver.
Daniel
No driver tube Bias will go to it's max. Driver tube plugged in, Bias will lower depending on how much current is going through the driver.
Daniel
It is, and there are several unmarked adjustment pots. At least two of them should vary the grid voltage of the driver and thus the output stage bias.
After the amp has been running and now the Grid voltage is -58 VDC with bias all the way up. The range is from +23 to -58 VDC.
This is not the 211 range either. The plate voltage is 1098 VDC.
I am thinking that the amp requires another driver tube instead of 6CG7. Do you guys know another 9 Pin tube that would bring the bias up?
This is not the 211 range either. The plate voltage is 1098 VDC.
I am thinking that the amp requires another driver tube instead of 6CG7. Do you guys know another 9 Pin tube that would bring the bias up?
Given the plate voltage of 1098VDC, -58 bias on a 211 is just about right. There is no doubt this amp was designed for a 211 tube.
If you do not plan on redesigning the amp, stick some Chinese 211's in it and enjoy it!
-55V is typical for the 211 at around 1kV, are you sure this not an Ongaku clone in which case the right tube would probably be a 5687..
Is there a way to verify that the back socket uses the 6CG7 and the middle and front sockets use 12AU7?
The back socket only lights up the 6CG7 and not 12AU7
The middle and front sockets light up 12AU7 and not 6CG7
The 6CG7 is getting 349 VDC plate voltage. The GE data sheet specified that the plate voltage is 300 VDC.
The back socket only lights up the 6CG7 and not 12AU7
The middle and front sockets light up 12AU7 and not 6CG7
The 6CG7 is getting 349 VDC plate voltage. The GE data sheet specified that the plate voltage is 300 VDC.
Obviously the driver tube is DC coupled to the output tube grid and (as you probably noticed) the bias range is greatly dependant of the driver tube type, which is unknow. (There are too many other possible alternatives to the 6CG7 as a driver tube in this circuit,and sometimes Japanese used strange or unusual tubes in their circuits). In this situation you can't rely on the bias range to determine whether the original tube was a 211or 845 , no way: you're stuck. As there are too many unknows you should try another approach and check the primary impedance of the OPT which is usually lower for a 845 (3 to 6 k range) and higher for a 211 ( 7 to 10 kilohms). Do you have the spec's of the TANGO OPT ? Maybe it can be wired for different primary impedances ? Once you have determined the output tube type you can work backwards and select a driver tube giving the correct bias range (about -40 to - 70V for a 211 and -100 to -200 for a 845). Please note that the (grid) bias must be allways measured with all tubes in the circuit and WRT the cathode (heater) of the output tubes and should never be a positive value.Probabilities are great that your amp was designed for a 211 but more investigations would be needed to ascertain this. The 845 is also harder to drive and requires much more grid swing (150 to 200 Vpp) than a 211, the driver topology and performances would need to be checked to confirm this. (oscilloscope and audio generator needed)
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.