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Title: DIODE IN B+ LINE FROM MOD XFORMER TO PA Post by: W2PFY on June 20, 2018, 08:09:27 PM I have seen many examples of a diode in series with the B+ line from the secondary of the mod transformer to the PA tube. It was used in some HF mobile rigs and there is some mention of using a diode in very early BC transmitters back in the 1930's. What is the purpose of this single diode? Does it allow only positive pulses from the audio wave to the PA and at the same time eliminate the possibility of the negative part of the cycle from approaching 100 percent negative? Is the negative part of the cycle eliminated?
I know of circuits using three diodes for negative cycle loading and then there's the three diode keep alive circuit that I believe Steve WA1QIX developed? Will the single diode application cause distortion? Any thoughts on this will be appreciated. I plan to build the QIX circuit for my BC-610D but will this cheap & dirty single diode do harm or good? I am not an engineer so you smart fellers out there must have an answer ;D ;D ;D Title: Re: DIODE IN B+ LINE FROM MOD XFORMER TO PA Post by: KD6VXI on June 20, 2018, 09:42:36 PM That diode is there to prevent a collapsing field from destroying the mod xformer. The inductive kick can be quite large, even on the CB radios.... I believe I measured about 60 volts coming back when I tried removing it.
On xmitters that are full bore TX constantly, it's not an issue. --Shane KD6VXI Title: Re: DIODE IN B+ LINE FROM MOD XFORMER TO PA Post by: DMOD on June 20, 2018, 09:55:07 PM I prefer dispensing with that diode and instead use a soft knee circuit to damping the ringing.
A soft knee circuit such as this also "loads" the transformer on negative voltage excursions such that the secondary does not see an open circuit. Phil - AC0OB Title: Re: DIODE IN B+ LINE FROM MOD XFORMER TO PA Post by: ka1tdq on September 22, 2018, 01:05:55 PM I'm bringing an Elmac A-54H back to life and it now has a pulse. I replaced the germanium diode with a new 1kv/3A one. Everything else is the same, including the 33k resistor on the secondary of the mod transformer.
I'm hearing ringing from the mod transformer. Maybe the ringing you mentioned in this post Phil? Elmac put a "PA on/off" switch on the front panel to short out the secondary windings for off times, and the pitch varies when I change the switch position. I haven't tried modulating the rig yet. I had to remove lots of weird "mods" done to the rig. Too much to go into, but I really don't know what this guy was thinking. Jon Title: Re: DIODE IN B+ LINE FROM MOD XFORMER TO PA Post by: ka1tdq on September 23, 2018, 09:28:28 AM The problem was just unknown mods in the audio chain. When I pulled the mic preamp tube, the problem went away. There was so much stuff added/changed/rewired in the audio section that it's impossible to figure out what the guy was trying to do. Some things I could follow what he was doing, but other stuff was just plain incorrect. Regardless, when he was done the result was the mod transformer going into the stratosphere.
Jon Title: Re: DIODE IN B+ LINE FROM MOD XFORMER TO PA Post by: DMOD on September 23, 2018, 01:01:42 PM I'm bringing an Elmac A-54H back to life and it now has a pulse. I replaced the germanium diode with a new 1kv/3A one. Everything else is the same, including the 33k resistor on the secondary of the mod transformer. I'm hearing ringing from the mod transformer. Maybe the ringing you mentioned in this post Phil? Elmac put a "PA on/off" switch on the front panel to short out the secondary windings for off times, and the pitch varies when I change the switch position. I haven't tried modulating the rig yet. I had to remove lots of weird "mods" done to the rig. Too much to go into, but I really don't know what this guy was thinking. Jon This guy has some suggestions as well for upgrades: http://www.crompton.com/hamradio/oldradio/a54.html |