Randy -
OK. Well not all 'new' tubes are 'good'. That circuit is very straightforward, and should have
4.5V on the cathode with a 120 ohm cathode resistor. (4.5 / 120 = 37.5 mA) which is the specified
operating point for the 6EH5 in Class A service. So if you put 140 VDC on the Plate, slightly less
on the Screen, and ground the Grid via ~ 500k ohms, the tube should idle at 37.5 mA. The only
external thing that can affect this is if a DC bias is applied to the Grid. One possible source is
a leaky coupling capacitor (C185) putting a small DC bias on the grid. The only internal source of
trouble is grid emission or gas. Grid emission is when small particles of cathode material flake
off and stick to the grid and continue emitting, driving the grid positive. This causes the tube to
draw more than it's idle current, heating it up, which heats the grid further, which causes it to
emit more, which ... You get the idea. The problem presents as essentially normal operation to
start, then the grid heats up and starts emitting, shifting the bias, etc....... Since this is a
Class A amplifier, the idle current should not change when the stage is driven, so the voltage on
the Cathode should stay at 4.5 VDC for the constant 37.5 mA cathode current.
The 6CA5 is close, but not an identical tube. It's designed for a slightly lower operating current,
but 37.5 is still within it's range. You could try raising R44 to 150 ohms with a 6CA5, but I don't
think that's the problem.
I know that sometimes I have to go through 3-4 'new' 6EH5s to find a 'good' one. Almost as
susceptible to this malady as the 12BA6s.
I get tubes wherever I can find them! :-) Antique Electronic Supply is a good reliable source,
although a little higher in price. N9TEW is another one. Vacuumtubes.net is good.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Randy WB4SPB wrote:
Thanks Garey. I have tried 3 tubes altogether, including my own spare and a second I purchased
online. The original is a 6EH5, the spares were 6CA5. I believe (hope) there is no material
difference. All behave the same way.
The one I ordered turned out not to be a new tube by any stretch of the imagination. Where do you
like to buy tubes?
73,
Randy WB4SPB
----- Original Message ----- From: "Garey Barrell" <[email protected]>
To: "Randy WB4SPB" <[email protected]>
Cc: <[email protected]>
Sent: Friday, February 10, 2012 5:30 AM
Subject: Re: [Drakelist] R4B audio distortion
Randy -
Sounds like a classic case of a 'gassy' (grid emission) tube. Replace it!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
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