Yup, checking bias on this thing, they have the center tap going to relay. Then one pin goes to a resistor to case for cutoff or idle bias. The other side of relay when activated goes to a SK536 which is an 11V 50W diode. Using diode function on meter, it is open, other way I get continuity. It measures about 1.5M one way. The other way it is open. So it tests good. Funny thing was the nut to tighten it was loose. Took it off by hand. No wrench needed. I may just string a bunch of 1N5408's together for about 11 volts and see what happens. Just tryin to get it up and running before it gets modified or put back to stock.
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572B input impedance?
- Thread starter Crusher
- Start date
Toll_Free
Well-Known Member
Please explain this to me, because it makes no sense.
MOST amplifiers nowadays, in GG, have their grids tied straight to ground.
Cathode current = grid current + plate current. Monitor grid current there, subtracting plate current.
Like it's been done since, oh, I dunno, the 40s. Might wanna take a look at some amps that have the grids directly grounded. The lifted grids, run through a choke and cap provide a very limited amount of negative feedback, as well as raising drive requirements a LOT, depending on the cap size.
Removing the caps on the grids and directly grounding them helps with stability at VHF as well as increases gain. Seems like win-win to me.
--Toll_Free
Toll_Free
Well-Known Member
Resistor cutoff is the preferred method, since if you lose a bias supply you have sky high ZSAC when the amp is keyed or not. With the resistor cut-off method, you get the tube cutoff, you don't have 100 or so volts across the contacts of the relay (like the SB220, etc), etc.
I'd be careful with some of the 'info' in this thread. Nothing would kill you so far, but a lot of 11 meter 'voodoo' being repeated here.
1N5408s are crap for bias. They have a large excursion from 0 volts to their zener voltage (app .65) in current.... MEANING they will NOT hold your zener voltage constant..... MEANING, your bias will fluctuate like using a voltage tap resistor circuit.
6A10s are MUCH more suitable for creating bias blocks. And if the amp is going to be used on AM for any appreciable amount of time, try throwing in about 18 diodes. Even for 11 volts (stock), you'd need more than 11 diodes, 11 would give you about 7 volts of bias..., give or take depending on the diodes used.
BIG diodes, not necessary if the zener works. Strings of diodes are good for people that blew the zener, or those that know what they are doing, can monitor ZSAC and have a second receiver to monitor IMD with, and understand classes of operation.
Better thought might be to keep the zener in it.....
You've put a LOT of work in an amp you could have done a C check on the tubes... Usually bad tubes will have bent a element inside, and the C in, Cinternal and Cout won't be within tolerance... USUALLY. A high pot tester would have been good, too.
Might run into tuning problems with all the extra C you've put into the tank circuit..... That's almost 2.5 times the original amount of Cblock... You've got to be able to tune that additional C out somewhere, so either the tank circuit needs to be realigned, or you will have LOTSA Q in it, meaning sky high voltages and currents (bandswitches going out to lunch are common)...
Just my dollar eighty five worth.
--Toll_Free
The taps don't appear to have bben moved, in the orginal schematic they had a 1000pf as a blocking cap. I'll have to look around, might have a 470 pf laying around. Thanks for the tips. And yes, I have lots of 6A10's laying around. I had to use 45 of them in a string for a bias supply for a 35B box. Then had a switch that I could switch in an additional 14 more. Depending how I wanted it. This was on the B- supply.
Toll_Free
Well-Known Member
If a 1000 was OEM, leave it as a 1000. The Q would have changed would have been the biggest problem.
--Toll_Free
I was wrong, not sure what input was tuned for, but it was way, way off. Like a 9.99 SWR. Should have read what it actually measured in ohms. But it was way off. Pulled PI network out and putting my own in. Put a 50 resisitor just after input and tuned. Best I could get it was a 4.33swr. Might be part of problem.
Crusher, it is that 11 meter VOODOO that toll free is fond of saying. Only bad thing is 11 meters used to belong to the amateur bands oh way back in years.
Glad you are making progress.
I loved my ha-10. it was just BIG and HEAVY and HEAVY and Oh wait, did I say a P.I.T.A. to keep moving around?
The upside was it was built like a tank with everything shielded in copper... I had zero TVI etc. It was a great box I just hated it because it wouldn't fit where I wanted it to go. If it had fit the cabinet I would have kept it.
Mine had 572b tubes in it.
The upside was it was built like a tank with everything shielded in copper... I had zero TVI etc. It was a great box I just hated it because it wouldn't fit where I wanted it to go. If it had fit the cabinet I would have kept it.
Mine had 572b tubes in it.
Yes, this has 572! As well. Also the PS is set up on 220V not 110 like the original schematic that I was liiking at that had 110V supply. Should be able to fire it up and see what happens later when I get home from work.
well i have amp up and running somewhat. With only 2 watts in, had a 500 carrier out. input is stillaround 2.1 so i stillhave some work to do. alsorear left tube when i loaded up amp and just barely touched plate tuner, it arced internally. Also have another problem. using a foot pedal, but when i unkeyed radio, amp was still putting out power. Sounds like an oscillation problem. First want to get 4 new tubes and tune input better. What site areyou talking about. Could use afew pointers as to what is the best way to calm amp down some. Maybe better/ more bias to calm tubes. not sure as to what plate voltage is, so not sure as to where it is running.
Agreed. I could believe it if the tubes were ceramic pentodes maybe but never for glass triodes.