Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Sep Tue 19, 2017 11:41 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 11 posts ] 
Author Message
 Post subject: 6GY6 transmitter question
PostPosted: Sep Thu 07, 2017 1:20 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17468
Location: Warner Robins, GA
I have a two tube 6GY6 transmitter that I use often.

Recently had to replace the 6GY6 tube due to grid leakage.

Schematic.

Attachment:
6GY6 transmitter schematic.jpg
6GY6 transmitter schematic.jpg [ 96.15 KiB | Viewed 262 times ]


Parts list

Attachment:
Parts list.jpg
Parts list.jpg [ 133.44 KiB | Viewed 262 times ]


B+ at the output of L3 with the transmitter operating is 204.6Vdc.

Is the B+ too high or is it ok?

I can measure the plate and screen voltage along with the cathode current of the 6GY6 when I get home today if that is necessary.

Reason I ask is because I want to be sure if the grid leakage was being caused by the B+ being what it is or if it is something that just happened.

Also I would like to install a meter to measure the 6GY6 cathode current which will make it easier to adjust the antenna tuner as I would then adjust for lowest current. As it is now I either have to use an oscope or S meter on a receiver to peak the antenna tuner.


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Thu 07, 2017 4:18 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12074
The 6GY6 has a 300v limit, doubtful voltage was the sole factor... Current should not be a issue either though if in doubt measure current to see if you're running near limit(screen is only rated 1W)... My bet is probably heat/cool cycles... I've seen a couple new 6GY6 shorted out of box...

Contrary to popular belief here on ARF, tubes do fail occasionally... Running 24/7 I've had more issues with tired audio amp tubes than the outputs... The 6AB4 don't seem to have more than 18 months in them... The 6C4 & 12AU7 probably double that, maybe a bit longer... I reworked my 9KC6 Tx to use TV tuner RF amp tubes(6ES5, 6FQ5, 6GK5, 6HA5 etc) for audio & VFO, have plenty of those... If the 6v versions run short in my next lifetime, can always series up the 3v flavors...


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Thu 07, 2017 4:51 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17468
Location: Warner Robins, GA
And to think people say TV tubes are useless........


Thanks for the info. Looks like mine probably just failed from use as there was a time I'd often leave mine on 24/7 for days at a time. Too lazy to turn it off :lol:

If the current was too high then the cathode voltage wouldn't be right and I think it was the modulation that suffered if the cathode voltage wasn't set just right. Modulation seems good to my ears.

At some point hopefully soon I do want to build a proper antenna tuner inside the transmitter so that I can mount the antenna using an insulator directly to the transmitter case which will also hopefully eliminate being able to receive the transmitter at nearly every 1 MHz interval when close to the transmitter. Would also probably tune the antenna better as well. The current tuner is one that I made using the original trimmer cap with the kit, a coil from an RCA 103 radio with a ferrite antenna rod I put inside as it was needed to make the coil be the proper inductance along with a variable cap from an EICO FM tuner to tune the antenna. Oddly enough when I built the tuner I ran the wire from the last variable cap to the terminal on the coil where the antenna connects and the tuner didn't work until I installed a two turn coil of wire between those two connections. I'm assuming the tuner is tuning the antenna to its best match, but I really have no clue whether it is or not. I feel like it could be much better and perhaps it could with the tuner inside the transmitter case given there won't be any losses from the wiring given the wires will be shorter going to the tuner as it will be next to the 6GY6 tube.

I would love to get a miliammeter for the cathode current so I could easily adjust the antenna tuner, but I'll have to measure the voltage across the cathode resistor to figure the current and select a meter where said current is lower on the scale. Would prefer a smaller meter or one of those round ones like some communications receivers used that were secured by a ring pressed on the rear of the meter.


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Mon 11, 2017 11:26 pm 
Member

Joined: Apr Thu 14, 2016 8:25 pm
Posts: 251
Location: pensacola fl
To measure the current use a 50uA d.c. meter movement and series resistor from the cathode to ground. the resistor is to calibrate the meter to read as you wish it to if you have made your own scale for it in mA. This will lower the cathode resistance just a bit so you can increase R8 to compensate. The series resistor might be like 60k for 3 volts cathode equals 6.38 mA as an example. I rounded off the numbers here but a 75k multi turn pot for the resistor to the meter would allow calibration control. 60k in parallel with 470 ohms is close enough to 470 to work fine in this case. By the way tv sets ran 6GY6/6GX6 tubes near max and they did have problems but not like other tubes had. They used them in color demodulator circuits and also some sound detector and sync-agc-noise inv as well.


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Tue 12, 2017 12:02 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17468
Location: Warner Robins, GA
Will try that.

I will have to measure the voltage across the cathode resistor without the antenna tuner connected so I know the maximum current.

Can then select a meter with an appropriate linear scale.

EDIT:

Just measured the voltage across the cathode resistor.

The 470 ohm resistor reads 455 ohms which is lower and I've noticed to be the case for a lot of the 1/4 watt resistors Radio Shack sold in their large resistor packs.

The voltage across the resistor is 3.546Vdc for a current of 7.79mA.

So I will need a meter that has a 0-10 scale. Maybe I can find a milliammeter with the proper scale.


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Wed 13, 2017 8:09 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 12074
The meter shown at beginning of LM386 thread is I believe a 50ua... Anyway I took the lazy route and paralleled my resistor sub box across it with resistance set to minimum, then moved to higher values till it read approx 3/4 scale... Selected similar value, done...


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Wed 13, 2017 9:40 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17468
Location: Warner Robins, GA
Cool


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Thu 14, 2017 6:23 pm 
Member

Joined: Apr Thu 14, 2016 8:25 pm
Posts: 251
Location: pensacola fl
You can measure the current directly but if you wish to use the meter to measure other things as well like output or swr then you need a confusing switching system. If you measure the voltage at the cathode with the 50uA meter that has a series resistor simple switch allows it to measure more things. Use a pot in series to make the meter read where you want it to. You can even print out a paper scale to place inside the meter on top of the existing scale. You can use the right resistor for your meter to make 50Ua=5mA or 50mA.


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Thu 14, 2017 6:34 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17468
Location: Warner Robins, GA
Will only be using it to measure the cathode current of the 6GY6.

Now as I adjust the antenna tuner will current or voltage be the parameter that will change the most?

Reason I ask is if current changes more than the voltage across the 470 ohm resistor then I want a meter that is in series to measure the current directly, but if the voltage changes more than the current I would want a meter with series resistor in parallel with the 470 ohm resistor.

Looking for the most meter movement I can get which will make peaking the antenna tuner easier.


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Sat 16, 2017 5:05 am 
Member

Joined: Apr Thu 14, 2016 8:25 pm
Posts: 251
Location: pensacola fl
Actually they will follow ohms law. If the current through the 470 goes up so will the voltage drop across it and they should track so if you have the mA meter movement with the proper scale or add a shunt it should read the same way the voltage meter across the resistor will read. In both cases the tuning will be seen there as a dip in both. I would go with the easier meter to work with in this case if all you want to read is the current through the tube. There is one thing that will have a negative effect on the reading to a degree is the fact that both the plate current and the screen current flow through this resistor. This being the case reading the plate current will have a greater effect on meter deflection change with tuning. With pentodes and more so dual control pentodes as the olate current changes so does the screen current. The change in cathode bias adds to both control grid bias and this changes both screen and plate current. Measure screen current and cathode current as well as plate current and see how they interact. Basically the electrons emitted by the cathode will go to the path of least resistance so if the plate is tuned to resonance then plate current will be at a dip so screen current will increase. You nay have a more apparent reading if you read plate current and look for the dip since the cathode dip will not be quite as much since the screen will carry more current making the dip more shallow.


Top
 Profile  
 
 Post subject: Re: 6GY6 transmitter question
PostPosted: Sep Sat 16, 2017 6:36 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 17468
Location: Warner Robins, GA
Oh ok I'll get a meter to measure current and put it in the plate circuit between the choke and filter cap.

EDIT:

I've realized that adding the ground lift switch in the initial build was a very good thing.

My R-390 hums more with the ground connected and my RCA K-80 console in the same room hums more with the ground disconnected.


Top
 Profile  
 
Post New Topic Post Reply  [ 11 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: Yahoo [Bot] and 4 guests



Search for:
Jump to:  
-->






















Privacy Policy :: Powered by phpBB