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Old 19th September 2012, 02:33 PM   #11
TheGimp is offline TheGimp  United States
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No oscillation.

I suspect this is where it is designed to operate.

The schematic shows Va=446V, and Vg2=430V at the 470R resistor so with the increase in line voltage G2 is at 450V.

Looking at the RCA datasheet with the anode at 450V, they have Vg2 at 350 with Ia=95ma(2 tubes) at Vg1=-30V and Vg2=400 with Ia=116mA (2 tubes) at Vg1=-37V.

Extrapolation for Vg2=450 is certainly going to require g1 at -45V or more to get Ia down below 45mA.
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Old 20th September 2012, 02:59 AM   #12
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Hi "Gimp"

It just occurred to me, something my father told me way back when, probably 1950 or so: pentodes and beam pentodes tend to burn out and melt the screen grids if they are overloaded without a proper load in the plate circuit. Melted and destroyed screen grid wires would not control the plate current properly and you would not be able to turn the bias on the output power-tubes off enough to stop the plate-circuit from overloading.

If you ever got the plate to glow red, the screen-grid would have been white-hot and would have been melting. The grids were made of nickel wire which melts somewhere in the 1600 deg C region. The beam pentodes were carefully assembled so that the screen grid was in the electrostatic shadow of the control grid. If that alignment was ever off the screen would catch the beam and dissipate like "the crazy" and most certainly melt.

I suggest that because you have had the plate hot enough to glow red, the screen grid has already been damaged. With a damaged screen grid the control grid can no longer control the tube current very well. You may actually already have output power-tubes that are damaged and do not work correctly.

My suggestion is to turn the tube bias "way low" remove the power tubes and measure the voltages of the empty sockets from those measurements you should be able to tell if the bias conditions of the amp are such that the amp even has a chance of working correctly.

I have been doing this sort of amplifier stuff since about 1948, so I date myself.

Good Luck

Hans J Weedon
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Old 26th September 2012, 04:06 AM   #13
TheGimp is offline TheGimp  United States
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Hans,

Yes, without a proper plate load the screen acts as a plate. However, this is not the case for the amp in question.

I have abused many tetrodes and pentodes without warping the screen grid sufficiently to change bias conditions. Given that the tube is operated with a proper anode load, short term over-dissipation is not extremely detrimental to tube operation in my experience.

What I experienced is very difficult to see in ambient light conditions and requires turning off the lights to see it well. It is confined to the junction of the radiating fin and body of the plate. The attached picture is of a 6P1P with worst over-dissipation than I was seeing with the 6L6GCs.

Should a tube be driven to the levels required to warp the screen, screen current would increase dramatically. This is not the case as under bias conditions Vk=470V, Vg2=470V) current through the screen resistor is calculated at 4.9mA with a cathode current of 40mA.

This is very close to the values for 450V operation in class AB1, and screen dissipation is only half max spec.

I did find one new 12AX7 that was quite noisy (Groove Tube branded). However this does not seem to be a contributing factor.

I am convinced that operating with the plate and screen at 470V is the most likely cause of the over-dissipation, and increasing the magnitude of the bias is the correct fix.
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File Type: jpg Small Red Plate_1.JPG (140.8 KB, 21 views)
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Old 26th September 2012, 05:16 AM   #14
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Hi Gimp.

I really appreciate the picture of the glowing tubes. It does however remind me of something my dad showed me in 1945. In Norway the Germans had confiscated all Radios in 1942 and put them in storage. My dad had an RCA143 type Radio before WWII that we got back when the war was over in 1945. When we plugged it in all seemed to work OK, except the the 2 #42 output pentodes were glowing red with a blue shine to it, just like your output tubes.

My dad explained that the tubes were probably OK but they had for some reason become gassy. I seem to see that the internal screen grid supports and the screen grid wires are glowing red with a blue shimmer. are you sure that your output tubes are not gassy? I have worked a lot with power tubes and in my experience the only anodes that are allowed to glow dark red are transmitter output power tubes with large distances to the glass bulb. With a red glowing plate there is a greater risk that the heat from the plate may overheat the glass and the tube will implode.

There are specs on the maximum allowable glass bulb temperature, usually around 150 to 200 degrees C. With an infrared surface thermometer measure the maximum bulb temperature to avoid bulb implosion. If the bulb temperature is not exceeded with a dull red plate, I believe you are OK.

Also I may remind you that in a beam pentode the screen-grid and the control grid are carefully aligned to minimize screen-grid dissipation. if this alignment for some reason or other got disturbed due to overheating, the screen-grid current may be much higher than the specs state. Just a thought that is all.


Hans J Weedon

Last edited by HJWeedon; 26th September 2012 at 05:26 AM. Reason: minor corrections
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Old 26th September 2012, 12:44 PM   #15
TheGimp is offline TheGimp  United States
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What is glowing in the right tube is the cathode due to the angle of the picture. The tube on the right in particular is showing signs of being gassy, and the condition is probably not helped by the red plate which could evolve trapped gas.

Back to the 6L6GCs, I measured the screen resistor values, then measured the voltage drop across each and calculated the screen current. As stated above, it was only 4.9mA so no excessive screen current is indicated.
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