[AMRadio] TH/TL & More

JACK C. SHUTT w9gt at verizon.net
Mon Feb 25 11:43:53 EST 2008

Wow Barrie ....6C21s modulated by 304TLs.  I actually had the idea to build a transmitter using the same line-up.  I don't know if I will ever proceed with it, but thought it would be really cool and have "strap" to spare.  I have a pair of new 6C21s and several new(?) 304TLs.  The big challenge would be to find mod iron to handle the chores.  The 304s will run nicely at low plate voltage, but then current is really high and mod iron very low Z.  Anyway...will be interested in how your rig progresses.  Like I say, I don't know whether I will proceed with mine...I don't need TVI/RFI neighborhood problems!  Those 6C21s sure are neat looking though!
  73,  Jack, W9GT

Barrie Smith <barrie at centric.net> wrote:
  I'd like to thank all that responded to my question regarding the TH and TL designators. I did know the high mu and low mu difference, but I had always thought (due, I suspect, to seeing TH tubes as RF amplifiers in so many magazine and handbook articles) that the TH was the RF amp, and that the TL was the audio amp.

Now, I'm faced with another challenge.

I am planning to rebuild a transmitter I first built in the early 1990s. It used a push-pull pair of 6C21s, modulated by a pair of 304TLs. It worked great, and, according to on-the-air reports, sounded good. But, it was really big and heavy, overly complicated, and, due to my wanting it to look like an "old fashion rig", caused RFI/TVI for blocks.

I grew tired of neighbors marching down the street, bearing torches and chanting evil things. So, I tore it apart.

Now, I want to rebuild it. I have spent several hours the last couple of days hipoting 450TLs, 450THs and 304TLs. One of my 6C21s has lost it's vacuum, so I'll be using 450TLs in the (well-shielded) final.

Now I'm thinking that I may use 450THs as the modulators, instead of 304TLs. The hobby is supposed to be fun, and I think that would be fun. Four "tall tubes".

Well, there we go, getting big again. Anyone see anything wrong with that?

I ran the 304TLs in AB1 the last time I built the rig. I'd like to run AB1 this time, as well, unless something has changed in the last 15 years.

I don't see any specs for the 450TH in AB1, just AB2. Anyone have any numbers for that tube in AB1 they'd care to share?

I don't find any amateur web-sites with much "how to do it" information on building modulators for AB1. The handbooks (that I have) all feature class B. Are there any sites I could visit with such information?

Barrie, W7ALW, DN36au,
QRV 6M, 432 & 1296 EME
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