Here's some info I've collected in the past few years re: conversion of Drake T-4X(x) transmitters for the use of 6146's in the finals to replace the 6JB6 sweep tubes.  I have not edited this info, or activated the links, so what you see is what you get.

I would be happy to include more info if anyone has it, on this subject.  Thanks to those who have written on the subject on the various email reflectors, whose work I have copied here.

There is a published mod for the Drake B line to convert the 6JB6s to 6146.

K5DKZ has kits listed for sale for the conversion.

After he read the comments shown below, he has added:

    There were some comments about the 15k AGC resistor and the 100 ohm screen resistor.  The screen resistor is there for safety.  It is not needed for proper operation but could save the tubes in case component failure resulted in excess screen current being drawn.  The 15k AGC resistor is wired across the 1meg AGC resistor that was installed by the factory.  At least that is what Drake did in my T4X.  The 1meg was a design mistake by Drake that was fixed in later models.  A quick solution to the problem was to simply wire a 15k across the 1meg and settle for slightly less than 15k.  With a 1meg resistor there was no AGC.
Frank Kamp, K5DKZ,  July 14, 2002


On Tue, 14 Nov 2000 22:40:55 -0600, Lee Bahr wrote:
>Too bad the article is just a tease! No mention as to what value and
>where the ALC resistor goes and no mention as to where the 100 ohm
>screen resistor goes! Does some other screen resistor get removed and
>replaced with the 100 ohm job? The auther (sic) is not telling the whole
>story. There is more to know before attempting the mod!

True, some details appear to be left out. But it's not rocket science,
either. The ALC resistor is given in the evilBay ad as 15K. Now look


The position of the screen resistor is shown. Note the 15K resistor
added in the AGC circuit Note also that the K5DKZ mod is described at
being for a T4, and the GIF schematic is for a T4B. The schematic for
my T4XC shows a 15K resistor at R29 in the ACG circuit, but it is in
series with the cap instead of parallel to it. Assuming that the web
page is for a T4 or T4X, and that the schematic is correct for T4B, it
looks like the added 15K might be a variation of a change made by Drake
in later versions. I haven't compared all the schematics yet, so I'm
just speculating. Between Hamanuals and BAMA, I should be able to look
at all the T4xx variations sometime over the next few days. There is
enough info provided between the two sources, however, for someone with
a fair amount of knowledge to successfully do the mod (IMHO).
Jim Shorney
Looks like the 100 ohm 1 watt screen resistor is in series with the feed
thru cap C56 and RFC3. The Bama schematic shows R8 in the AC-4 changed
from 3.3K to 6.8K, I guess, to achieve -90 volts bias. Looks like the
15K 1/2 watt AGC resistor is in series with C11 which is a .047 mfd
changing the time constant. Maybe this is the missing links!
Lee Bahr w0vt
I looked at the various schematics tonight. The T4X and T4XB do not
have the 15k in series with the .47 cap. Only the T4XC has the
resistor. The rest of the plate circuit of the ALC amp looks to be the
same. R8 is the designation of the 3.3k in the AC3; in the AC4 is is
labeled as R7.

The only difference from the BAMA schematic is that the 15k is in
series with the cap instead of parallel to it. I wonder which would
work better...?
There is a published mod for the Drake B line to convert the 6JB6s to 6146.
You can find it in the Drake archives at this site.

Date: Wed, 28 Oct 1998 00:35:28 -0800
From: Kelly Taylor <ve4xt@mb.sympatico.ca>
Subject: Re: [Drake] 6146 Drake equip?

My understanding is that the mod is not only electrical, but mechanical. 
While nothing is impossible, it requires mounting new sockets and 
recessing them from the main chassis, as otherwise, the 6146 is too tall 
to fit inside the RF cage.

The 6146 also is capable of a higher plate voltage, or at least I seem to 
remember my old HW101 having 800v on the plates. So the temptation would 
be to mod the power supply as well, although that's not, strictly 
speaking, necessary.

I think Drakemod covers this.
73 all.
From: msix@nmia.com
Date: Fri, 30 Oct 1998 09:28:52 -0700 (MST)
To: drake@qth.net
cc: drakelist@baltimoremd.com
Subject: [Drake] Drake PA Tube Conversion

Been reading the mail. Some thoughts...

1. The PI nets in the sets don't care what the PA tubes are or how many
as long as the plate current is roughly the same as the originals.

2. Given the socket and chassis mods needed, 6146s seem like a poor

3. Would think some other short 6V or 12V 9-pin sweep tubes would be much
easier as it would only involve rewiring the socket at most, which is
easily restored later.

4. Seem to recall Dexter Francis suggested a candidate.
Perhaps someone can post a list of potential substitutes?

5. When these sets came out, 6JB6s sold for $8-9. In today's dollars,
that's about $34-38. If they are now selling for $20-25, they're cheaper
than back when...

6. Same rationale for those fearing modification will lower resale value.
A T4XC and power supply cost what? - maybe $379 plus shipping - say $400
new. In today's dollars, that's around $1800. They now sell for maybe
$200-250 which would be like $45-55 in 1970. What resale value???

73 - John W7ZFB
1400 Catron SE Albq, NM 87123
From: "Roush, Richard J" <roushr@uf9422p01.DetroitMI.ncr.com>
Cc: "'drake@qth.net'" <drake@qth.net>
Subject: RE: [Drake] 6146's In Drake Xmtr

Great idea on the cooling fan for the T-4XC, Bill. I have been using a
4-inch "Muffin" fan for 20 years and enjoy at least a 300%, or more,
increase in the life of the finals. The "Muffin" fan sits directly on top
of the cabinet on 1/4" legs above the finals, it is blowing upward, and is
wired into the power supply so it turns on/off with the rig. I run all
modes, including FSK. 

The higher the CFM rating on the fan, the nosier it is, but a happy median
can be found by starting with a high CFM rating and slowing it down by
adding some capacitors in series with the AC until it's still pushing enough
air, but the high pitched wine is gone.

Hope this helps you other Drake owners, 73 - Rick, KA8BDD 

> ----------
> From: GoJolly[SMTP:gojolly@humboldt1.com]
> Sent: Friday, October 30, 1998 5:51 PM
> Subject: [Drake] 6146's In Drake Xmtr

> Been reading the posts about modifying the Drake Xmtrs to use 6146's. I
> certainly wouldn't want to do that to mine -- that would sink the resale
> value for one thing. Also, I see it a rather pointless. I put nice new
> guaranteed finals from International Components in my T4xc at a cost of
> $42. I take care of them by 1) being careful about tuning up and 2) I
> have
> an ordinary 7" household fan, the plastic kind that costs about $10,
> sitting on top of my R4c directed down onto to the xmtr cabinet. I run
> this on low speed whenever I use the rig. This keeps the finals nice and
> cool. A few weeks back I was talking to Tom the guru at Drake service
> about this and he said that his experience was that a fan would increase
> tube life by 300%. I hasten to point out also that back in the late 60's
> when I had my first set of Drakes I did not use a fan and worked 20 ssb
> all
> the time for several years and did not see any degradation of the finals.
> CW would be harder on them of course. Hence the fan.

> Bill K6ERQ

Date: Sat, 31 Oct 1998 23:26:07
From: GoJolly <gojolly@humboldt1.com>
Subject: [Drake] Drake PA Fan

Saw all the discussion re what kind of and where to put fans. Very good
info. I have to say that my plastic house fan sitting ontop of the
receiver next to the xmtr does cool the whole rig and have never found any
feeling of heat on the rig anywhere when the fan is running. Of course it
is a blow fan, but the rig is cool. The drawback is that it looks a bit
odd to see a stupid plastic fan ontop of my gear. 

In view of all the comments about muffin fan mountings I am going to work
on that, sounds much neater.

Bill K6ERQ

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