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Hi folks,
new project idea: GU-50 in the framework of an old Dynacord Eminent / Imperator / ... amp.
Basis is a HiFi-Favorit 2 after meltdown. Meanwhile i would not dare to restore the old PCB with its severe burning traces close to the anode connections of the EL34.
Data: 80W, 2xEL34, 740V on the anodes under full load. ECC81 catodyn PI. Transistor preamp.
PT: sec voltages 315V, 245V, 45V, 60V, 6.3V, currents unknown
60 V bias - sufficient for at least 3xEL34, 1xECC81, 2 light bulbs 0.3A each (and someone has reported having successfully added an additional valve)
45 V - transistor stage. Takes roughly 90 mA, estimated from the schematics. 58V after rectifier (-> schematics)
315 V -> 420V after rectifier. lower part of anode voltage, G2 supply, supply of ECC81.
245 V -> 330V after rectifier. 2nd part of anode voltage.
Very preliminary testing with the Ohms meter - at low voltage no damage in isolation noticeable, and resistances plausible. My old Philips PM2505 can measure that low...
I think i should also do the resonance test before powering up the PT.
OT: M102b, primary impedance 8k. Apparently no isolation problem.
Schematics. Just the power stage. I plan to use a bridge rectifier instead of the old half wave rectifier to obtain a larger bias voltage and will probably need to take care not to put too much load on the winding.
Check what's going in in the PSU. I know, not fully realistic, as in reality the secondaries are isolated against each other. 100 mA fairly constant in the 45V winding. Looks good?
What about the 320 mA peak? The average should be somewhere around 180 mA.
A commercially available replacement for that PT is rated at 260mA for all three windings in the stack, take 2/3 of that ... mhmm?
What about the stacking order? I needed to reverse it in oder to be able to handle the constraints of the G2 voltage of the GU50.
PI stage: originally a concertina at 340V. After a little bit of tweaking it can drive the grids up to the onset of grid current. Quite surprisingly i had to increase the cathode/anode resistors significantly and had to reduce the idle current through the stage for lower distortion and (much) larger output swing.
(Just noticed that the problem is not the onset of grid current in the tube - the asymmetry also develops on a pair of load resistors with power stages decoupled).
new project idea: GU-50 in the framework of an old Dynacord Eminent / Imperator / ... amp.
Basis is a HiFi-Favorit 2 after meltdown. Meanwhile i would not dare to restore the old PCB with its severe burning traces close to the anode connections of the EL34.
Data: 80W, 2xEL34, 740V on the anodes under full load. ECC81 catodyn PI. Transistor preamp.
PT: sec voltages 315V, 245V, 45V, 60V, 6.3V, currents unknown
60 V bias - sufficient for at least 3xEL34, 1xECC81, 2 light bulbs 0.3A each (and someone has reported having successfully added an additional valve)
45 V - transistor stage. Takes roughly 90 mA, estimated from the schematics. 58V after rectifier (-> schematics)
315 V -> 420V after rectifier. lower part of anode voltage, G2 supply, supply of ECC81.
245 V -> 330V after rectifier. 2nd part of anode voltage.
Very preliminary testing with the Ohms meter - at low voltage no damage in isolation noticeable, and resistances plausible. My old Philips PM2505 can measure that low...
I think i should also do the resonance test before powering up the PT.
OT: M102b, primary impedance 8k. Apparently no isolation problem.
Schematics. Just the power stage. I plan to use a bridge rectifier instead of the old half wave rectifier to obtain a larger bias voltage and will probably need to take care not to put too much load on the winding.
Check what's going in in the PSU. I know, not fully realistic, as in reality the secondaries are isolated against each other. 100 mA fairly constant in the 45V winding. Looks good?
What about the 320 mA peak? The average should be somewhere around 180 mA.
A commercially available replacement for that PT is rated at 260mA for all three windings in the stack, take 2/3 of that ... mhmm?
What about the stacking order? I needed to reverse it in oder to be able to handle the constraints of the G2 voltage of the GU50.
PI stage: originally a concertina at 340V. After a little bit of tweaking it can drive the grids up to the onset of grid current. Quite surprisingly i had to increase the cathode/anode resistors significantly and had to reduce the idle current through the stage for lower distortion and (much) larger output swing.
(Just noticed that the problem is not the onset of grid current in the tube - the asymmetry also develops on a pair of load resistors with power stages decoupled).