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Old 8th November 2007, 02:58 PM   #1
hal3101 is offline hal3101  Norway
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Default Strange 45 behaviour - help

Trying to convert my to stage 6c45-300B amp to a 3 stage 6J5-(RC)-45-(RC)-300B amp and run into some problem regarding the 45 driver tube tube. I have some weird noise problem on the output. The 45 tube was the problem and is now isolated from the input tube and the output tube to try to locate the problem.

The 45 tube is cathode biased and has a dropping resistor (1.5k) from B+ and a decoupling cap(10uF) to get the B+ to the 45 tube. I have tested with different anode resistors from 2k to 4k but the result is always the same. Cathode resistor adjusted from 2k to 1250 ohm - same result. Grid leak resistor is 60k. Filament is DC and has a ripple of 200mV. I am also using a hum pot of 50 ohm.

On my scope I can see that the filament voltage is a smooth sinus curve with AC-200mV - no spikes.

When I power the 45 tube I can see some wierd noise on the anode/cathode (ac-1.4V) of the tube. Looks like a square wave on the top but on the bottom it is like a triangle (point). The frequency of the noise is 50Hz.

The really strange thing is the voltage measurement drop on the anode resistor (2k) compared to the cathode resistor(1250ohm). B++ is 410V and the anode voltage is 350V (drop is 60V/2K= 30mA) . Cathode voltage is 62V and the cathode resistor is 1250ohm (62/1250=49.6 mA) . This does not add up. Why is the current in the cathode different from the anode ???

If I adjust the anode(2k-4k) and the cathode resistor(1250-2k) the measurements stays around this area - 350V and around 60V.

I have - Changed the 45 tube, changed the socket, checked all the soldering points, measure again all the resistors etc..

The 45 tube is located near a power choke - can the 45 tube pick up som swiching noise from the choke and give these wierd measurements ??

Any tips are welcome.

TorH
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Old 8th November 2007, 11:58 PM   #2
kevinkr is offline kevinkr  United States
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The choke may be radiating a magnetic field that the 45 is picking up, or your power transformer.. With full wave rectification noise pickup should be 100Hz not 50Hz if this is the source. Power transformer may radiate 50Hz, 100Hz, and 150Hz..

Also you should try to limit the voltage across the 45 plate to cathode to about 250V, at which point the cathode voltage should be around 45V with a plate current around 30 - 35mA.

For good performance with a 45 you need at least 3X RP so this plate resistor should be at least 5K. Use fixed bias since you probably don't have the voltage headroom or use a > 30H choke load instead of the resistor.

200mV is a lot of ripple for a dc filament supply - you might be better off heating with 2.5Vac..

You might have grid current in answer to the question about the discrepancy between the cathode and plate currents - or a measurement error - or that cathode resistor is not what you think it is.
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Old 9th November 2007, 09:38 AM   #3
hal3101 is offline hal3101  Norway
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Yes - it should be 100Hz and not 50hz. I think I will try with plain AC filement first and compare the result. The move the 45 away from the choke.

The plan was to add a choke - I am testing with a plate resistor first to see if everthing is stable and no noise.

I am confident that the grid is only connected to ground via a 60k resistor and nothing else. I do not understand where this current can come from. (I could be a leaking resistor from the first stage (6J5) but this is disconnected.

Thanks.

TorH
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