Howdy all,
I just finished up construction of a scaled down Model T clone (2x6550). The changes I made were to use 35uF MePo Panasonic caps instead of the normal 22uF electrolytics in the PS. In fact, all of the caps in it save for the cathode bypass caps are non electrolytic. The other change I made was to use a cascode based on Ted Weber's designs available on the Weber website and using a 6CG7, but with a 250uF cathode cap rather than the 25uF. The power transformer is a Hammond 373BX while the output is the same old iron from the Bogen MX60A that this amp is built in. Since there are no ultralinear taps, it is currently triode configured.
Anyway, what happens is that the plate voltage without tubes is hunky dory, around 480 volts and no ripple. When you put any of the tubes in however, it drops to around 240. i had chance to scope it briefly and noticed a semi periodic spike in the idle voltage, kinda like it is oscillating at a frequency my Fluke cant really pin down.
Any help in diagnosing this would be much appreciated, and I will be able to run over and take measurements in a little bit to get more specific details in a bit,
Thanks!
I just finished up construction of a scaled down Model T clone (2x6550). The changes I made were to use 35uF MePo Panasonic caps instead of the normal 22uF electrolytics in the PS. In fact, all of the caps in it save for the cathode bypass caps are non electrolytic. The other change I made was to use a cascode based on Ted Weber's designs available on the Weber website and using a 6CG7, but with a 250uF cathode cap rather than the 25uF. The power transformer is a Hammond 373BX while the output is the same old iron from the Bogen MX60A that this amp is built in. Since there are no ultralinear taps, it is currently triode configured.
Anyway, what happens is that the plate voltage without tubes is hunky dory, around 480 volts and no ripple. When you put any of the tubes in however, it drops to around 240. i had chance to scope it briefly and noticed a semi periodic spike in the idle voltage, kinda like it is oscillating at a frequency my Fluke cant really pin down.
Any help in diagnosing this would be much appreciated, and I will be able to run over and take measurements in a little bit to get more specific details in a bit,
Thanks!
Put one preamp tube in and the B+ drops to half? Or did you mean any power tube?
You have a periodic spike in your B+? I bet it is a ripple spike. Set your scope up and just touch the end of the probe with your finger. That will put 60Hz on the screen. Adjust the sweep so you get two complete cycles across the screen. We don't care that it is not clean sine wave, the frequency is what we want.
Now, DO NOT ADJUST the sweep speed, but do adjust the vertical if you need to, and scope that periodic spike. It is at 60Hz or more likely 120Hz? (twice the cycles across)
If it really is something other than mains related freq, then either disconnect your NFB or reverse the OT primary leads.
You have a periodic spike in your B+? I bet it is a ripple spike. Set your scope up and just touch the end of the probe with your finger. That will put 60Hz on the screen. Adjust the sweep so you get two complete cycles across the screen. We don't care that it is not clean sine wave, the frequency is what we want.
Now, DO NOT ADJUST the sweep speed, but do adjust the vertical if you need to, and scope that periodic spike. It is at 60Hz or more likely 120Hz? (twice the cycles across)
If it really is something other than mains related freq, then either disconnect your NFB or reverse the OT primary leads.
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