[Central-Electronics] Central Electronics 20A Power Modification

Wed, 08 Jan 2003 13:22:41 -0500

    Chuck,

    Thanks for the nice reply. I looked at tubes like 
you suggested, and it sounds good except for one 
thing. 
The peak rf grid voltage to the final amp on a 20A 
(stock) is about 15 volts. This means that with AB1 
bias, you need to run the tubes at -15 volts bias to 
get the maximum power out, and efficiency. Both the 
6AG7, and 7591 need more bias than this, so efficiency 
and power output suffer. Going to 40 meters is even 
worse. 
   If a low Gm tube like a 6L6/5881 was put in there, 
the bias might need to be -25 to -35 volts (depends on 
the G2 screen voltage). The tube would be plenty 
capable, but that 6BA7/12BH7 driver could not drive it 
with enough grid RF voltage swing.
   I was thinking more about this, and replacing the 
6AG7's with a single EL34 / 6CA7, or two 9-pin 6BQ5's. 
These tubes have high Gm, and the RF drive available 
should do 
OK. I chose the 7591 because I have 8 of them.
   The 7591's (not 7591A) that I have seem bullet 
proof. No oscillation, or emission sag. They just put 
out an extremely linear AM / SSB signal. 
   I am currently running an external 200V P/S with 
meters so I can read the plate current. My 20A B+ is 
currently 375V loaded (400V at 25% RF output), so I am 
running the 7591's at 6OOV (10 watts AM), and 575 
volts (50 watts carrier output). The max plate rating 
of the 7591 is 550 volts, so yes that is a little 
high. 
    I added a tuned input to my linear (increased 
linear gain from 10 to 13 Db), and now I can run 200 
watts AM output, or 800 watts SSB. 
  Thus far, I am very happy with the mod's, and I 
wanted to share this with the group.

73,
WD5JKO
Jim Candela

--- Original Message ---
From: "JM/CO" <[email protected]>
To: "Jim Candela" <[email protected]>, <central-
[email protected]>
Subject: Re: [Central-Electronics] Central Electronics 
20A Power Modification

>This is a very interesting, and valid fix for this 
situation, although
>running them at 500V is pushing it a bit.  My only 
problem here is in your
>choice of tubes. 7591 and 7868 types ( which are 
identical, electrically )
>are not being currently manufactured by anyone in 
their original envelope
>style. Groove Tubes has a 7591 available, but it's 
envelope is the same size
>as a 6L6-GC, and I question the actual electrical 
characteristics. Original
>7591-A tubes are rare and their price is very high. 
They were designed for
>audio use, and tend to have a rather short life, 
because they run very hot.
>I would think that the 5881 would be a much better 
choice. A very robust
>tube, it is the MIL spec 6L6. These are only slightly 
larger than the 7591,
>and are a lot more available. There are plenty of the 
MIL spec ones out
>there, mostly made by Tung-Sol ( who developed this 
type ),and the Russians
>still make them.
>Chuck N1LNH
>----- Original Message -----
>From: "Jim Candela" <[email protected]>
>To: <[email protected]>
>Sent: Tuesday, January 07, 2003 9:42 PM
>Subject: [Central-Electronics] Central Electronics 
20A Power Modification
>
>
>>
>> I originially shared this with the AMradio list...
>>
>>
>> Hi all,
>>
>> I have been working on a 20a for some time now, and 
I'd like to share
>> something here with the group.
>>
>> The 20a has low RF output from the 2 X 6ag7 RF 
linear amplifier output.
>Mine
>> was about 3 watts AM carrier, or maybe 12 watts PEP 
on 80 meter SSB.
>Getting
>> a 600L linear amplifier is next to impossible, so I 
took another approach.
>>
>> I gave a 4 times 572 G-G linear that needs about 80 
watts in for 800 watts
>> out (10 db gain). If only my 20A could put out 
more...
>> Using my linear on AM with the 20a gives me 30 
watts, and heats the ham
>> shack. I did this for a while, and people laughed 
at me for such a silly
>> lash up.
>>
>> Here is what I have done:
>>
>> 1. replace 6ag7's with 7591's, and rewire tube 
sockets  (could use 7868
>with
>> socket type change)
>>
>> 2. Add extra B+ supply (200v from small 150 vac 
transformer, bridge, and
>> cap) in series with existing 300v for ~ 500v 7591 
Plate voltage
>>
>> 3. Remove 10k resistor across 6ba7 tank circuit. 
This provides more drive
>to
>> 7591 grids
>>
>> 4. Adjust 7591 grid bias for -22 volts, and screen 
grid volts for 380
>volts
>>
>> The results have been stunning:
>>
>> 50 watts CW output (B+ at 475V), or  ~ 60 watts PEP 
SSB (B+ idles at 550v)
>>
>> 10 watts AM with plenty of headroom for > 100% + 
modulation. At 10 watts
>> out, each 7591 dissipates 15 watts (19 = max), and 
the % efficiency is 25%
>> (40 watts DC plate input for 10 watts rms output). 
The efficiency doubles
>at
>> 100% modulation (sine wave).
>>
>> 15 watts AM at 100% +/- modulation, and 150 watts 
from linear amplifier.
>> Efficiency a little higher here.
>>
>> The 7591 is about the size of a 6V6, yet has higher 
Gm (gain), and plate
>> dissipation. It fits nicely in the confined space 
of the 20a.
>Unfortunately
>> the 7591 can be pricey, as the audio nuts have bid 
the price up. Look on
>> EBAY, they are there. The 7868 is the same, and 
cheaper, but requires a
>> socket change.
>>
>> If any of you are interested in this, and want more 
details, just contact
>> me, and I'll share it with the group.
>>
>> Regards,
>> Jim Candela
>> WD5JKO
>>
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