Randy, I have had excellent luck with www.radiotubesupply.com/ Roy seems to know his business and has always had prices in line with the market. I have purchased used tubes from him when availability or price was way out of line. I have always been pleased.
73, Ron WD8SBB --- On Fri, 2/10/12, Garey Barrell <[email protected]> wrote: > From: Garey Barrell <[email protected]> > Subject: Re: [Drakelist] R4B audio distortion > To: "Randy WB4SPB" <[email protected]> > Cc: [email protected] > Date: Friday, February 10, 2012, 2:54 PM > 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> > > > > > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist > _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
