The AM Forum

Hello, this is my first time posting.

I bought a Viking II at a hamfest this year. I noticed there's a audio mod done to it.
It's the "Audio Conversion for Johnson Viking I and II" mod from AMWindow website.

I have some questions, if anybody can help.

1. According to the schematic there's two 10 uf 25v caps and two 8 uf 450v caps. Are they electrolytics?

2. If they are electrolytics, the schematic doesn't show any polarity. What's the positive and negative polarity on the schematic?

3. I have all the parts in the audio mod schematic, accept three are missing, the 10 uf 25v attached to pin 8 of the 12AX7. Missing also, is the two 8 uf 450v caps. Why would these be missing? I would think they're suppose to be in the circuit.

Peter

The 10 UF's are cathode bypass caps.  The higher the value the more low end (bass) you are going to get.  If you go to high then you might let HUM into the audio chain.  Typical values are 25 to 50UF. 

The 8's  at 450 Volts are extra filter sections added to the B+ for the plate supply.  I guess this is to lower the ripple on the supply and stiffen the B+.   

The polarity is there on the schematic, as you can see one side of the cap goes to Ground.  That would be the - or negative lead of the capacitor.  The Positive side is the side that goes to Pin 8 and or the voltage side such as the plate lead or B+ for the 8's.

C

Welcome aboard, Peter!

Nothing like jumping in with both feet!

Cheers, Bill

The 10ufd 25V are cathode bypass caps.  Negative side of the cap goes to ground.  Same is true for the 8ufd 450V caps, neg to ground.  The 8ufd caps are decoupling caps.  You can use any cap that is close to those values.  For the 8ufd caps you can use anything, 10-20ufd.  Same is true for the cathode bypass caps, 10-25ufd.

Fred

Yea, what he said! ! ! ! ! ! ! ! ! !

(Hey, Phred, we agree again on something! ! )  :o  ;D

Without the decoupling caps you could easily get "motorboating", oscillations,or other anomallys in the audio. They are VERY important to have in there. However the value is not real critical as long as they are there.

Thanks for the reply, and Bacon you answered my third question. Will have to order the missing parts after I look over the rest of the transmitter.

Peter

that circuit is pretty much good for anything with AB1 modders. However if you are running the modders AB2 and need a little tickle of grid current, I swap the 6C4 for a 12AU7 with both sections in parallel. (An 'AU7 is basically 2 6C4s in one bottle.)