Brenda Ann wrote:


It's a Stancor,

Have a number? some of us have old references that might list the specs...

best regards...
--
randy guttery

A Tender Tale - a page dedicated to those Ships and Crews
so vital to the United States Silent Service:
http://tendertale.com

"Brenda Ann" wrote in message
...

"maxhifi" wrote in message
news:U5fsi.22399$_d2.19727@pd7urf3no...
change it to a bridge rectifier across the whole 250V winding, and
everything should work fine!


*repeatedly kicks self in arse for not thinking of that in the first
place.... * It did clear up almost every issue I was having. Next thing I
think I'll do is add negative feedback.



you can also delete the capacitor feeding the grid of the second half of the
12AT7 - the one on the plate of the first half is already blocking DC.

"Brenda Ann" wrote in message
...

"maxhifi" wrote in message
news:U5fsi.22399$_d2.19727@pd7urf3no...
change it to a bridge rectifier across the whole 250V winding, and
everything should work fine!


*repeatedly kicks self in arse for not thinking of that in the first
place.... * It did clear up almost every issue I was having. Next thing I
think I'll do is add negative feedback.



Before you do this, I have redrawn your schematic - I added and changed some
components which I think might work a little bit better.

You can also delete the capacitor feeding the grid of
the second half of the 12AT7 -- the one on the plate
of the first half is already blocking DC.

I was going to suggest that, then realized it was part of the biasing. Which
you have altered in the following schematic.

I'm curious though, why anyone would design the circuit as it is, when
electrolytic caps are relatively cheap.

"Timothy Twerp was most astounded
When what he thought was -- wasn't grounded.
In fact, one could say he was really transfixed
With his thumb on pin 3 of a live 6L6."

Carl Kohler, ca 1958

You're right about the power transformer. 250VCT means 125 - 0 - 125. You
could use it with a full wave bridge if your load current isn't too high.

Measuring the inductance of the modulation transformer won't help much.
What you need to measure is the turns ratio. Then you need the load
impedance which is determined by the operating conditions of the RF
amplifier you are modulating.

If you want to keep the power supply as it is, a better output tube would be
a 6Y6. You will need to use a resistor to drop the screen voltage though.

--
Jim Mueller

To get my real email address, replace wrongname with eportiz. Then replace
nospam with sacbeemail.

"Jim" wrote in message
...

"Brenda Ann" wrote

As others in the other group have mentioned, you're pushing your luck
with 600V diodes. They have over 700V on them (or they would have if
things were working right). Replace them with 1000V diodes. This also
means you need to replace those capacitors with 1000V capacitors,
preferably AC rated.

As someone else said, you have a major problem with the power supply.
The output voltage should be much higher.

--

I think I have kenned out what that problem IS. It's not a 250-0-250..
it's a 125-0-125 on the secondary.. which would output pretty much what
I'm getting.


This is my transformer. From Mouser

http://www.hammondmfg.com/pdf/5c0030.pdf #261G6
261G6 45250V C.T.@ 130 ma 6.3V @ 2A 1 4.00 2.50 2.63 3.56 2.0



What is the modulation transformer you're using?

If it's a mains transformer it could be going into saturation. Proper
modulation transformers are designed to accomodate a d.c. bias through the
windings which normally requires an airgap in the core. In an ungapped
core the dc bias will drive the core somewhere up the BH curve and the
effects will be exactly as you describe - clipping on one of the peaks

"William Sommerwerck" wrote in message
...
You can also delete the capacitor feeding the grid of
the second half of the 12AT7 -- the one on the plate
of the first half is already blocking DC.

I was going to suggest that, then realized it was part of the biasing.
Which
you have altered in the following schematic.


Sorry for an ignorant question, but can you please explain how it works? As
drawn, I can't understand how the tube is biased properly, in either
section - won't the grid at zero volts, and the cathode at zero volts set
the tube to an undesierable operating point, or is there something here I am
missing?

I'm curious though, why anyone would design the circuit as it is, when
electrolytic caps are relatively cheap.

"maxhifi" wrote in message
news:8ezsi.26512$fJ5.13616@pd7urf1no...
"William Sommerwerck" wrote in message
...
You can also delete the capacitor feeding the grid of
the second half of the 12AT7 -- the one on the plate
of the first half is already blocking DC.

I was going to suggest that, then realized it was part of the biasing.
Which
you have altered in the following schematic.


Sorry for an ignorant question, but can you please explain how it works?
As drawn, I can't understand how the tube is biased properly, in either
section - won't the grid at zero volts, and the cathode at zero volts set
the tube to an undesierable operating point, or is there something here I
am missing?


Just clarify my question - I do understand grid leak biasing, but how is it
possible with the compnent values given in the original schematic? In an AA5
the grid resistor is in the M ohms

I'm curious though, why anyone would design the circuit as it is, when
electrolytic caps are relatively cheap.

"Brenda Ann" wrote in message
...
What is the modulation transformer you're using?

If it's a mains transformer it could be going into saturation. Proper
modulation transformers are designed to accomodate a d.c. bias through
the
windings which normally requires an airgap in the core. In an ungapped
core the dc bias will drive the core somewhere up the BH curve and the
effects will be exactly as you describe - clipping on one of the peaks

I don't have exact specs on it. It's a Stancor, an actual modulation
transformer with isolated secondary so that the RF final could be run off
a
different B+ supply if desired. I bought the transformer from a ham who
had
pulled it from an AM rig that used sweep tube outputs, and he had assured
me
that the match to a 6L6 would be good on both primary and secondary.
Perhaps
I can get a mH reading on it with my LCR meter, and someone can do the
math
for me.. I haven't used that math in many many years, and it's left me,
I'm
afraid.


Not withstanding the other suggestions, you may also want to try swapping
the connections on the modulation transformer - see attached sketch.

Figure 1 gives the worst case condition when both Modulator and PA dc bias
currents add to increase core flux density. In Figure 2 the dc bias currents
are opposing and help and reduce the flux levels in the core.

Also I would very much recommend putting resistors and decoupling capacitors
in the cathodes of all stages - particularly if you do use a bridge
rectifier to increase the B+ supply voltage as suggested. See maxhifi's
sketch newamp.jpg for details.

Just a final thought - is the modulation transformer intended for single
ended or push pull operation?