The AM Forum

Hi
I am finishing up my Viking 2, and was going to order a driver transformer replacement. I have read in the thread to use a Hammond P-T124E or a P-T124B. I noticed that the impedance is not the same. I have tried to find the impedance of the original with no luck. I tried to do the math to figure the proper impedance and was lost. I need a little help here on witch one to use. Also I have read that this stock circuit is lame. Is it necessary to do a radical rebuild or can it be done simply?
73 Johnathan N8QPC

Johnathan,

I believe the original was a 2.8 to 1 turns ratio and a 3 to 1 replacement should be fine.  I used the AES equivalent of the Stancor A-53C to replace mine several years ago and it has worked fine; as I recall it was around $15.  The impedance ratio is the square of the turns ratio so a 3 to 1 turns ratio gives you a 9 to 1 impedance transformation ratio.  The B suffix transformer would be the proper choice.

As to mods, that is up to the owner/user.  I prefer to keep mine true to the original design but there are a lot of possible mods out there.

Rodger WQ9E

The stock 6au6 driver tube is lame. When you start to pound on it it goes belly up because it cant deliver any real power, just voltage. so if the mod tubes go into AB2, and want some driving power, everything goes to hell.

Best mod out there to me is putting a 6AH6 in for the mic amp (direct plug in), drilling a new socket hole for a 6C4 in between the mic amp tube and the driver tube socket with a internal set and forget volume control. Then you widen the hole out of the driver tube from a 7 pin mini to a 9 pin mini and wire it for a 12B4 tube. very low plate impedance so it makes a excellent class B driver tube and can deliver good power when required to do so.

alternative - you can wire a triode connected 6AQ5 in which will work well but not as well as the 12B4.  make sure you get the driver xformer that will pass 30 or so ma of current, the bigger multi tap job.

use decently sized coupling caps and wire the first grid resistor on the mic amp tube to be 5 megohms if you re going to use a D-104.

OR... you can use the 12AX7/ 6C4 mod on the AM Window. It only requires the slight enlargement of the mic amp 6AU6 hole. This has been in the Viking Bud since 1998 with no problems.

I also used the 3:1 Standcore replacement from AES in '98 and it's still rockin".

 Viking Audio Mods.  (http://www.amwindow.org/tech/htm/viking2.htm)

Hi Guys

     Thanks for the help. I was all worried about input and output impediance, but unless I am wrong the turns ratio is what matters. When you guys say AES that is Antique Electronic supply? If so I will order a  P-T124B today. I am of modest means so I wanted to make sure I got the right one. I didn't know what the driving impediance of 807's was and I was lost. Rodger mentioned a Stancor A-53C is that the stock transformer?
     As soon as I get the little peice of iron I will be ready to key it. I have a solid 100 wattt carrier but no audio.Again thanks.

73 Johnathan N8QPC

Johnathan,

Looking at the AES web page, their P-T156 is listed as equivalent to the Stancor interstage unit so it should be fine and is quite a bit less than the Hammond transformer.  It has the proper turns ratio and should work fine.

Rodger WQ9E