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6BL8 phase splitter advice needed

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M

mstamper

Member
Joined 2015
#1
I have a 1959 Lafayette LA-250 stereo amp that uses 7199 phase splitters and EL-86/6CW5 output tubes. The amp as purchased had weak/leaky 7199s. To save money, I rewired the sockets to accept 6U8/6GH8A/6BL8 phase splitters. I tried all three tube types. The 6BL8 sounded the best. I took voltage readings on the 6BL8s and compared them to comparable pins in the 7199 (according to the SAMs expected values for the LA-250). All were comparable except for pin 3 (pentode grid 2). The expected value was 30vdc. I measured 47vdc. For reference, the pentode plate (pin 6) measured 48vdc; expected was 50vdc. Why is g2 high? Do I need to increase the value of the 1.1 meg resistor to g2? Should I reduce the value of the 10uf capacitor to g2? Or do I have no problem? I do not have a distortion analyzer, yet I am aiming for minimal distortion. Here is the schematic section showing the phase splitter circuit:

Lafayette LA-250 KT-250 Improved Driver Circuit | Audiokarma Home Audio Stereo Discussion Forums

Any advice would be sincerely appreciated.
 
#2
Why is g2 high? Do I need to increase the value of the 1.1 meg resistor to g2? Should I reduce the value of the 10uf capacitor to g2? Or do I have no problem?
Click to expand...

Voltages look OK. The most important is that the plate voltage is close to original (50 Vdc) since this voltage determines the operating point of the phase inverter.

Ug2 is high(er) since the Ig2 of 6BL8 seems to be lower that of 7199, but there is no need to make any changes to screen resistor.

10 µF at the screen is OK. I don't see any need for modifications. Both 7199 and 6BL8 pentodes have quite equal Gm and therefore no need to adjust GNFB.
 
#4
mstamper,

I’m unclear about what you have actually changed. The schematic shows component values for the 7199 circuit. Have you retained all those R and C values and simply rewired the sockets for using 6BL8 in place of 7199?

The link says “....Improved Driver Circuit”. Is this not the standard schematic for this Lafayette LA-250?

I have an LA-250 in storage and would like to make sure I get the 7199->6BL8 conversion right.

Thanks and good wishes.
 
#5
The original (1959) Lafayette LA-250 used 7199 drivers but used different resistors, at least in the pentode section. When I rewired the socket to accept 6BL8s, it just "motorboated". I then located the 1960 SAMs for this model. The schematic showed very different resistors for the pentode section of the 7199. I not only rewired the socket to accept a 6BL8; I made all of the alterations necessary to conform the the SAMs schematic, which I attached earlier. The result is the the 6BL8 conversion seemed to work great. I was able to successfully set the bias without motorboating or other issues. I recommend that you either get the SAMs folder for this model, or follow the new schematic I have uploaded. Perhaps your LA-250 looks like the SAMs. If so, it is probably a later model.
 
#7
There could be a problem with the 6BL8.
The g1 of the pentode 7199 is far away from the anode of the triode.But at the 6BL8 they are next to each other.Since the phase at those pins is the same there is a risk of HF coupling and oscillation.
Mona
 
#11
These tubes will happily autooscillate without RC networks to cut high frequency or capacitor to ground from G1 pentode or triode ... as drawn in all schematics . So extra care should be taken using them , definitely not as triode design .
 
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