Hi Folks, I've just fixed a VOX AC30 S1 guitar amp which had, in the past, blown the heater fuse and had It's ECC83 (V1) replaced. I put this down to maybe a heater/cathode short.
This time V1 has gone again and when tested it shows the grid voltage having know effect of anode current - A new ECC83 has fixed the problem.
However, the AC30 S1 schematic shows V1(b) grid being driven directly from the anode of V1 (a) which is 99.5v. its cathode is 100v which is correctly biased. When I look at the datasheet for the ECC83 it shows max positive grid voltage is zero volts. Is it therefore safe to run the ECC83 with such a high positive voltage on it's grid and am I looking at another valve failure in the future?
Your advice would be appreciated.
Regards Geoff.
This time V1 has gone again and when tested it shows the grid voltage having know effect of anode current - A new ECC83 has fixed the problem.
However, the AC30 S1 schematic shows V1(b) grid being driven directly from the anode of V1 (a) which is 99.5v. its cathode is 100v which is correctly biased. When I look at the datasheet for the ECC83 it shows max positive grid voltage is zero volts. Is it therefore safe to run the ECC83 with such a high positive voltage on it's grid and am I looking at another valve failure in the future?
Your advice would be appreciated.
Regards Geoff.
At R68 how much is the V?
If it is 100 v and the pin 1 of V1 is at 99,5 v it is ok
The 83 is biased correctly
Walter
If it is 100 v and the pin 1 of V1 is at 99,5 v it is ok
The 83 is biased correctly
Walter
That's grid voltage relative to the cathode voltage. Has nothing to do with absolute voltages.
The heater supply is grounded though, so you have around 100V cathode to heater voltage, but even this is whithin datasheet limits (180-200V according to different major manufacturers). Maybe the tubes from some shady manufacturer can't handle this?
This raises an issue of the grid voltage being present when the amp starts from a cold start. Before the tubes conduct, the supply voltage would be present on the anode/grid connection. The 100v on the cathode would lag being generated until the tube warms up and begins conduction. There would be a large (+) on the grid relative to the K for maybe 10 seconds during that period. Let me say I haven't looked at the schematic so I can't say for sure how this circuit is layed out but it sounds like a cathodyne inverter. Going by that assumption. Also it depends on if the PS is SS or converted to SS, as opposed to tube rectifier B+ rampup time.
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