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Does anyone know where the Zener or bias circuit is on this amp. At idle when not keyed, tubes are fine. But shortly after it is keyed with very little drive, the tubes start to glow. Put a new set in and same thing. Its gotta be bias. Just not sure where to look. Any help would be greatly appreciated.

Manual Display Page

Crusher: If the Bias has not been has not been changed this should give you your answer....See page 19
All the best
BJ

__________________
Yaesu FT-950 + FT-817 / Kwd TS-450sat
Yaesu FT-450at Mobile
Antennas are 95% of your signal,
"A dime in the antenna is worth a dollar in the transmitter any day"

There are mods out there to put a zener in the L4B, and it does work well google Drake L4B mods.

I have done several of the mods including the amp I have in the shack. I think BJ has also done the mod to his L4B.

WB4HFN Home Page*** Drake Techni

Here's a version that's pretty common...See the Page EBS Bias for Drake amps
I am still looking through my notes, but I think the last one I used I got from W4EMF???...
Crusher I'll keep looking
However if the bias circuit is Stock using the large pair of resistors in the PS cabinet....they are a common failure point...they develop small cracks and/or distort the internal windings because they get so damn HOT....Then they change value of course...
Last one I worked on and left Stock I moved them off the top of the transformer to the side of the case and mounted a small fan to blow air across them for cooling....That version is still working well...
The reason I did that was by the time I made all the other mods to the under side of the RF deck...there simply was zero room for another board...without moving more wiring and terminal strips...Guess I got lazy!...Those Pics I made are still on this board...
If she ever craps out...I'll open it up and do it like the other one...
Hope this help
All the Best
BJ

__________________
Yaesu FT-950 + FT-817 / Kwd TS-450sat
Yaesu FT-450at Mobile
Antennas are 95% of your signal,
"A dime in the antenna is worth a dollar in the transmitter any day"

Hey just thought of something!....Check those resistors in the glass tube at the bottom of the plate choke...if they go it can cause some weird things to happen!
BJ

__________________
Yaesu FT-950 + FT-817 / Kwd TS-450sat
Yaesu FT-450at Mobile
Antennas are 95% of your signal,
"A dime in the antenna is worth a dollar in the transmitter any day"

LOL, this amp has the bigger oversized transformer. Sits at about 3100-3200v. When i got it this time, the C clip on bottom of plate choke had cracked and fallen off. One 50k resistor got so hot, that it melted solder and fell off after copper wire let loose. Those Bleeders get hot. Should have put a fan on thecase to keep cool. Whenthe tubes are in standby, they are fine. but as soon as footpedal is pressed, the tubes will start to pull half an amp. one time, it surged up to 1 amp. Its gotta be the bias.

Did you check the plate choke...this sound more like self oscilation...that amp sounds like it went the way of a thermonuclear melt down....never seen one do that....
Yes with the big HV it must have had the bias changed...is there a standard relay in it or has it been changed to a vacuum relay?
Most I've seen that have the std relay use the extra pole to switch the bias....See if you can follow it back to the mod??
There's got to be something extra mounted somewhere...
Good luck....they must be using that poor amp on AM and driving the living Hell out of it
BJ

__________________
Yaesu FT-950 + FT-817 / Kwd TS-450sat
Yaesu FT-450at Mobile
Antennas are 95% of your signal,
"A dime in the antenna is worth a dollar in the transmitter any day"

The part about being used on AM and driving the hell out of it, you got that right. The bias is original, when they went to this big transformer, it only has 1 power setting now. All or nothing. LOL, at one point the guy was doing a 1500-1800 carrier and doing about 3000 pep. (The driving the hell out of it part). This ones gonna get about 15V of bias on it, not the 8,5V like the article stated. And man is he long winded. 1 minute keys are not uncommon. Yeah I figure CT to diode string. I plan on using 22 6A10 diodes in a string and go that route. Install a fuse. I had installed a soft start from Harbach about 6 months ago or so and its ran fine. But the abuse is catching up. Made sure I cleaned the blower and oiled it while I was at it. Looks like bias is original, never changed.