Re: [Amps] input matching question on the YC156?

[craxd <craxd1@ezwv.com>]

Peter,
On the bias circuits of most amps I've done had a relay that switched 
full negative supply voltage on G1 when the amp was idle. When it was 
keyed, it applied an adjustable regulated supply. For screen voltage, I 
always killed it with a relay so it was only present on transmit. About 
the only other way is to pick it up off the anode voltage supply 
somewhere like you mentioned. On these I used the same relay to switch 
in the screen and bias. It was triggered by the antenna relay. Some I 
used B- switching and some I didn't. Someone mentioned earlier about 
cathode bias switching which is another way to hamper the tube from 
operating on idle. This works especially well in triodes. Back on the 
screen, I used a current trigger to switch out the screen voltage and 
throw G2 to ground in and over current mode. Negative screen current was 
dealt with by a resistor at the socket connected to G2. It also had a 
MOV and a safety zener string which was a shade over the running screen 
voltage. The zener string and the MOV was there to clamp anything over 
the correct voltage to ground. On any I've built for myself, I've never 
used a timer. I just always made sure to watch the clock but that's not 
kosher for any commercial amp. I always let mine run 10 minutes before I 
ever keyed it down. That's overkill but I always stuck with it. I also 
had each supply voltage metered so if there was any problem, I'd see it. 
On large amps, with expensive tubes, I like a meter for each supply. One 
using a single meter and a switch, I find most are too lazy to go 
through it and visually check everything, including myself.

There's all kinds of different circuits to obtain the same result. Power 
bipolar transistors, or FET's could be used in place of relays for the 
screen and bias switching. They can be used for the cathode bias 
switching too. It's really up to the home brewer or customer on what 
they want.

Will Matney



Yes, it depends on the kind of tube.

I run several different homebrew amps:
Two with glass envelope tetrodes in highµ, those are instant on with
HT and heater applied the same time.
On 23cm I use a TH-338 triode with all voltages supplied at the same
time, but bias switching and coaxial relays disabled for 3 minutes.
I built two with the tetrode YL1056 for 2m and 70cm, there the screen
is grounded and screen voltage applied to the cathode. A string of diodes
from anode to ground makes shure I have screen voltage if the anode
voltage failes.
The tube is 3 minutes preheated and disabled for that time.
Screen and anode current is electronically controlled as well as
presence of anode voltage to disable screen voltage if absent.
At present I am building one with two of those tubes for HF
There is one with the 8171, same practise as the YLs.
Another two with 4CX3000/5000A, but currents controlled by relays.
All amps with ceramic tubes switch off without blower pessure after 30
seconds and screen voltage is not switchable  without anode voltage.
I never had the idea to switch the B+, that is switched on the primary
of the transformer only

73
Peter

----- Original Message -----
From: "craxd" <craxd1@ezwv.com>

>> Yes your correct. I mentioned doing this because I had success with it
>> using 4CX250B's. But, when you get up into the big tubes, there's a
lot
>> of current with the anode voltage to deal with. If the control
voltage
>> was monitored through a safety circuit, and if it failed (went open)
>> then a secondary negative voltage could be applied or a relay opened
to
>> prevent B+ or the cathode being connected. An error lamp would show
this
>> and the amp would be shut down until repaired. What works for one
tube
>> don't necessarily work for all. Most all this type of switching I've
>> seen has always been in home brew amps. Most all commercial amps
have
>> been built one after the other, the same for years. I hate to say
this
>> but they look at it, if the amp blows due to no bias, it's our
problem
>> not theirs. Plus, anything that will run the cost up and come out of
>> their pocket wont happen. I set down once and tried to think of
every
>> scenario I could for failures in amps. This was about #2 on the
list.
>> The first was having screen voltage applied before B+. That's
another
>> point, that if bias is missing in a tetrode, the screen can become
toast
>> quickly. So, screen voltage should never be applied before plate
voltage
>> or have screen voltage without the proper cutoff bias. You really
think
>> about it, in a triode without bias, thats like a big diode shorted
>> across the B+ and in a tetrode, like a boosted diode. Just like
turning
>> a valve wide open in a high pressure hydraulic line. It gets hot
pretty
>> darn quick!

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