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Scott 299B Output tubes glowing Red

170.20.11.9
Posted on May 6, 2016 at 08:43:48
onekid
Audiophile

Posts: 79
Location: New York
Joined: April 21, 2002
 I installed a NOS set of Amperex 7189A's and balanced them using the transformer shunt method. I adjusted the bias so I was reading .99VDC. One tube seemed to be glowing very red from the inside center of the tube. I thought perhaps the coupling caps were starting to leak. So I replaced them with some Russian PIO's for the 7189A's and the 12AX7 preamp tubes. I used Audiocap Thetas for the AF-Amp 12AX7's. Now it seems the same tube is glowing a bit more red from the inside as well another tube from the other channel.

 

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RE: Scott 299B Output tubes glowing Red, posted on May 6, 2016 at 10:06:02
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Check the bias supply and the associated circuitry.

Also, the 7189A pins out differently than the 7189. Check how the sockets are set up. The 299B schematic shows 7189s, not 7189As.

Save money in the future. The Russian 6p14p-ev, AKA EL84M, is an affordable, tough, and good sounding true 7189 equivalent. Leave the very costly NOS to "audiophools".


Eli D.

 

RE: Scott 299B Output tubes glowing Red, posted on May 6, 2016 at 10:06:02
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 There may be an issue with the tube itself.Did you move it to another socket to see if it moves with it?
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: Scott 299B Output tubes glowing Red, posted on May 6, 2016 at 14:28:06
onekid
Audiophile

Posts: 79
Location: New York
Joined: April 21, 2002
 Nope not yet. I will try that

 

RE: Scott 299B Output tubes glowing Red, posted on May 6, 2016 at 14:31:38
onekid
Audiophile

Posts: 79
Location: New York
Joined: April 21, 2002
 I actually have a set of Sovtek EL84M as well as Russian 6p14p-ev as well. I don't have anything connected to pins 1, 6 or 8 so I assume the 7189A's should be ok.

 

RE: Scott 299B Output tubes glowing Red, posted on May 6, 2016 at 16:32:28
DaveV
Audiophile

Posts: 513
Location: SC
Joined: December 26, 2009
 It could be that you need to adjust the DC balance pot for each channel because one tube in each pair is working harder than the other to get that .99 VDC. That could make one tube glow red because it's drawing too much current while the other tube is just coasting.
The .99 VDC is the combined reading for both tubes and it doesn't tell what EACH tube is doing.

In looking at the schematic I don't know how Scott expected you to adjust the DC balance with one cathode resistor per pair of outputs.

Maybe try the hum test if you have high efficiency speakers or low impedance headphones to connect to the 16 ohm speaker terminals.

With the volume all the down down and no signal, listen for a hum in each channel and adjust the DC balance pot until the hum goes away on each channel. It works on my Scott 299A.

 

RE: Scott 299B Output tubes glowing Red, posted on May 6, 2016 at 18:31:14
onekid
Audiophile

Posts: 79
Location: New York
Joined: April 21, 2002



I checked the bias and it was a little high. Channel A measures 1.012V and Channel B 1.007. Don't know if this helps but here is a picture of the one tube in CH A that is glowing more than the others.

 

RE: Scott 299B Output tubes glowing Red, posted on May 6, 2016 at 20:31:15
DaveV
Audiophile

Posts: 513
Location: SC
Joined: December 26, 2009
 Since you've changed the coupling caps and you can get .99 vdc at the test point for each output pair, I'd think that the bias rectifier is OK right now but for how long if you didn't replace it? But that's another issue.

I'd be looking at the DC balance setting but that's hard with the way the 299B is currently wired with one 18 ohm resistor in the cathode lines of the output pair.

The overall bias voltage per output pair is adjusted with the bias pot and -15 volts on pin 1-2 is an approx voltage but the actual bias voltage needed to have each tube draw the right amount of current depends on the tubes and unless the tubes in each channel are well matched, the bias voltage will be different for each tube in the pair.

The DC balance pot in each channel shifts the bias voltage down on one tube and increases the bias voltage on the other tube in the pair so that hopefully they will each draw about the same current.

The .99 volts as measured from the test point is the voltage across the 18 ohm resistor in each output pair cathode line which seems right when converted to current with that value resistor BUT if one tube is doing all the work, it may glow red because it's over conducting.

You have the right voltage figure but you don't know how well both tubes are sharing the work to get it.

You can try the hum test to see if you can hear hum on each channel with the volume all the way down.

If that works for zero hum then see if an output still glows red but make sure you still have around .99 to 1 VDC VDC at the test points and not more.

 

Also, might be a good idea to check grid stopper and screen resistors, posted on May 7, 2016 at 07:05:07
FenderLover
 If this amp has them. Be sure the voltage applied to the screen is stable and in spec. Grid stoppers are in spec.

Good luck!

 

RE: Scott 299B Output tubes glowing Red, posted on May 7, 2016 at 09:32:37
onekid
Audiophile

Posts: 79
Location: New York
Joined: April 21, 2002
 I checked the DC balance of the tubes. Here are the results.
Channel A:
25.4mA (Glowing tube)
25.4mA
Channel B:
25.2mA
24.9mA
All large can capacitors were replaced a few years ago. Selenium rectifier replaced with silicon bridge and corrected dropping resistor installed. Just to give you a little background on the amp.

 

RE: Scott 299B Output tubes glowing Red, posted on May 7, 2016 at 11:34:41
onekid
Audiophile

Posts: 79
Location: New York
Joined: April 21, 2002
 I moved it around and the glow followed in each location. I was however able to set the DC balance very equally with it.

 

RE: Scott 299B Output tubes glowing Red, posted on May 7, 2016 at 14:32:50
DaveV
Audiophile

Posts: 513
Location: SC
Joined: December 26, 2009
 If it glows red in different locations then I guess it's something with that tube.

If your B+ is around 420 VDC at the plate of the outputs try increasing the bias voltage a bit so your reading closer to 24 ma per tube and see if it still glows red.
The 25.4 ma per tube is running them a bit on the hot side and I doubt if you'd hear a difference at 24 ma per tube.

Did you try another set of tubes in the 299B?

 

Eli, you happen , posted on May 9, 2016 at 11:04:37
Oface
Audiophile

Posts: 1497
Location: columbia, south carolina
Joined: May 3, 2003
 to know any reputable sellers from the Ukraine or anyone you have done business with when buying these 6p14p-ev's?

Thanks....

 

RE: Eli, you happen , posted on May 9, 2016 at 15:29:28
Jim McShane
Dealer

Posts: 5910
Location: Chicago
Joined: January 13, 2003
 I'm not Eli, but the current production version of the 6P14P-EV is the Sovtek EL84M. Nice sounding, TOUGH tube!

 

I did it right and rebuilt the 299B with individual, posted on May 10, 2016 at 17:42:08
gkargreen
Audiophile

Posts: 1562
Location: DC
Joined: February 5, 2005
 10 ohm cathode resistors, so I can easily measure the cathode current and adjust each one correctly...

 

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