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Old 11th July 2011, 11:43 AM   #1
needsp is offline needsp  United Kingdom
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Default Problem with Legacy pentode driver stage

Hi

I need some advice, and this is, without doubt, the best the place to get it! Iím confident where triodeís are concerned, but know only the basics when it comes to pentodes

My friendís 300B SE ampís developed a fault, and I offered not only to repair but to upgrade them. As part of this process, I decided to try the pentode driver stage from the Legacy amp, shown below. I used a 6P15 (which is I believe, a SV83 in disguise!). The HT in the amp is 475 V. I decided to stick slavishly to the operating conditions shown, and so introduced a simple RC network to drop the driverís HT to 420 V, to avoid exceeding the plate voltage rating of the valve. But, as the power supply in the amp is unregulated, I decided to use a regulator to put 160 V on g2 (this is functioning correctly). But the circuit didnít operate as shown. I had to lower the cathode resistor to 240R to increase the current and thus lower the plate voltage to 270 V. The article I got the circuit from- writen by Thorsten Loesch- also talks of the driver running at 16 mA- which would have the 12k plate load resistor contravening Ohmís law for a startÖ.

As Mr Loesch has forgotten more than I know, I can only think that he drew the circuit to operate with a different pentode. Did I do the right thing to lower the cathode resistor in value? Would an alternative set of operating conditions be preferable? Any suggestions would be much appreciated.

Thanks

Paul Needs
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File Type: gif Legacy300BAmplifier.gif (8.6 KB, 348 views)
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Old 11th July 2011, 12:35 PM   #2
euro21 is offline euro21  Hungary
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SV83 vs EL84
"SV83 is identical in construction to the 6p14p (which is equivalent to EL84)."

Russian 6P15P-EV = EL84/6BQ5 ??
"The 6p14pev is a drop in replacement for the EL84/7189, the 6p15pev is not(but close)."

6P15P, 6P15P-V, 6P15P-EV
Tube Tester Files - 6Ō15Ō - 6Pi15Pi (SV83)
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Old 11th July 2011, 12:58 PM   #3
needsp is offline needsp  United Kingdom
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Hi euro21

I'd read about this, and opinions differ, but I thought that SV83 was a 6P15P, and 6P15P had identical characteristics to 6P14P, and that the only difference was that in 6P14P g3 was internally connected to the cathode, but in 6P15P it was not, and that 6P15P had an internal screen connected to g3, which was connected in turn to pins 1 and 6 (not connected in 6P14P). So the two valves were interchangable so long as one connected pins 1 and 6 of the valve base together. Am I wrong? I just checked the characteristics I have for both types, but they were measured at widely differing g2 voltages

Paul
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Old 11th July 2011, 01:06 PM   #4
Yvesm is offline Yvesm  France
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Hi Paul,

Your schemo looks great to me except for the value of the cathode resistor.
According to the data sheet, an SV83 with 150 V on the screen should be biased at around 3.5 volts for a 16 mA plate current, this means a value of (R = U/I) 3.5 / 16 = 218 ohms.

Yves.
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Old 11th July 2011, 01:47 PM   #5
TheGimp is offline TheGimp  United States
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6P14P and 6P15P produce different plate curves on a curve tracer. Thus, they are not the same.

See the attached file in the first post for plate curves, or check the data sheets:

6P43P Strange Plate Graphs
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Old 11th July 2011, 01:53 PM   #6
needsp is offline needsp  United Kingdom
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Hi Yves

That looks about right as 240R is giving a plate current of 12.5mA. But if I drop the cathode R a litle to set 16 mA, the plate will fall to ~ 230 V.

Hi euro 21

Just took afternoon off to try a 6P14 (see the power you have over strangers?!) With a cathode R of 240R the plate drops to 200 V, with 211 V across the plate resistor so plate current has risen to 17.6 mA. Clearly I was wrong about the the equivalence of 6P14P and 6P15P!

But my original point remains. I could use a suitable piece of wet string to set the current through the plate resistor at 16 mA, but 16 mA through 12 k would still drop 192 V, putting the plate at 228 V, not 270 V, for an HT of 420 V!

And if the SV83 is an equivalent of either valve, it must be the 6P15P- as it has an internal screen and no internal g3 to k connection

So- should I drop the value of the cathode resistor to give 16 mA plate current, and live with predicted plate V of 230 V; or

fit a 6P14P, increase the cathode R to 470R, and hope for, and live with, 16 mA/plate 230 V; or

never agree to do favours for friends!

Thanks

Paul
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Old 11th July 2011, 02:12 PM   #7
TheGimp is offline TheGimp  United States
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You will not have 16mA thorugh the anode resistor. The 16mA thorugh the cathode resistor includes the g2 current. So you have less current thorugh the plate resistor, less voltage drop across the resistor, and a higher anode voltage.
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Old 11th July 2011, 02:46 PM   #8
needsp is offline needsp  United Kingdom
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Hi Gimp

I had thought it was always the plate current that was quoted in these characteristics, not the cathode current! Anyway, in my circuit with a 6P15P and 240R cathode resistor the cathode current is about 14.3 mA- so 1.8 mA g2 current. So I suppose it's more than possible that the plate current of a 6P14P with a 470 R cathode resistor might be 12.5 mA, with a g2 current of 3.5 mA

So is this the consensus- a SV83 has a screen and no internal g3-k connection, like a 6P15P, but has characteristics close to a 6P14P?

I will of course be trying a 6P14P with a 470R cathode resistor- soon

Thanks

Paul
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Old 11th July 2011, 02:54 PM   #9
TheGimp is offline TheGimp  United States
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Looking at the Svetlana data sheet for SV83, 6P14P and 6P15P data sheets it looks to me like the 6P15P is a closer match to the SV83 than the 6P14P is.

Changing the bias should allow you to use the 6P14P, but you might see slightly lower gain.
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Old 11th July 2011, 04:32 PM   #10
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I built the legacy driver stage. I recalculated the plate load and bias resistors. I am using a lower B+ with a 10k plate load but running a higher current. I find I can interchange both 6p14p and 6p15p in this setup as the higher bias current pushes both into a similarly linear portion of the curves.
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