The AM Forum

I have a 32V-1 that I'm having a problem with the mod bias, the manual calls for it to be set to 55ma on the 700 volt setting. I cant get mine to go below 125ma. I've tried 2 different sets of 807's with the same result. The pot doesnt seem to make much difference at all when adjusted. Has any one else had a problem like this? There is a fixed resistor inline with the 1000 ohm pot, so I'm looking that direction. Thanks for all of the help in the past, and in the future. Any suggestions would be greatly appreciated.

Do the 807's show a lot of blue color in the envelope and are the plates getting red? If not, perhaps it's a meter shunt, maybe you could put another meter in the cathode circuits to double check. This is just shooting from the hip, no schematic here.   

Well I replaced the 5Z4 LV rectifier and now when I bring the HV in it pegs the meter, If I take the 807's out  there is no current draw. I did replace the 807's another set with the same result. Any ideas ?

Hi Ray

Does the  v 304 v(vr -75) glows  .its the source of  min bias for the 807,yes the 1 kilo ohm pot is in series ,with 750 ohm value ,it can be that the resistance is broken or changed its value,or the pot is damaged.,measured the min bias at grid of the 807 (by pulling out the 807 ,you can measured at the control grid  hole at the socket) of the 807 or the connection at the tap on the pot,it must be around - 32 volt.

and its true maybe the meter shunt has changed,it must be 25 ohm

I looked at the 32V-2 schematic diagram ,I hoped it have the same schematic basic


Gito

It looks like the new 5z4 is bad.
You really should solid state ALL those rectifier tubes to offload the LV transformer.
Mine has been solid stated for 30 years without a single problem, plus I run it at over 700 volts.

The bias circuit is simple, just get the meter out and measure various spots in the circuit, check the value of the resistors and the pot.

Brett


 

And 73 mag. has a great article on making a ss equiv. for the 5z4.  Essentially put 250 ma fuses in each leg of the AC HV feeding the rectifier anodes along with a common 50 ohm, 2 watt or better, voltage dropping resistor on the output to the filter caps.   

The rational for the light fuses is that the main LV primary fuse has so many other functions and loads that if a filter cap shorts, etc, the main fuse won't even see it, hence melting down the transformer AC windings.

I ss'd both the HV and MV xfor's years ago and now think I'll do the 250 ma fuse mod.  Makes sense.
Those 32v series MV transformers are worked way too hard in the stock tube circuits anyway.

ps.
there was an errata in the August issue, pin outs wrong on the 5z4 in the orig. article. 

I never used any resistors when solid stating things, and have had no problems for 30 years or so.
I use the 32V3 as an exciter for the big rigs, I sold the 756 pro 3 for motorcycle money, so it has and gets a lot of use, lots of sitting there warmed up, and sometimes I use it as a standalone rig, no problems at all with the thing, very well built.

But its been solid stated since I got it, no resistors, no fuses...

Brett

Well the culprit was the 750 ohm wire wound resistor thats inline with the mod bias adjustment. I replaced it and all was well again. I also replaced all of the resistors in the audio section and I'm going to change all of the caps too. So hopefully I'll be on the air with the 32V soon. Thanks again for all of the replies and help!

Oh, forgot to mention that when I ss'd the HV rectifiers on the 32V2, I also disconnected the filament winding leads from the 5R4 tube sockets; didn't want HV back into the LV transfomer filament winding.  Then just put a 470 ohm resistor across the 5 volt winding just to keep down 'strays.'