[Collins] 30S-1

Mon Sep 20 18:09:37 EDT 2010

A lot of capacity in screen and plate supplies has been the subject of much 
discussion in the amp world these days. Almost new industrial caps are 
selling for pennies on the dollar in surplus shops and fleabay.  I picked up 
a couple of 2400uF 105C CDE caps from a swap forum for $4 each a few years 
ago and recently a bunch of 1200uF 105C 500V CDE for a bit under $2 each. 
Less than 2 year old date codes from unused industrial products.

They should make for an extremely low impedence screen supply and do away 
with complicated and expensive regulators. The ESR is extremely low.

About 10 days ago I hooked up a string of the 1200uF to a 2x 3CX800A7 amp. 
With the stock 220uF caps the no load to full load HV sag was 362V. With the 
470uF I replaced them with the sag was 250V, and 56V with the 1200uF. PEP 
average was higher and I suspect the IMD was improved. The amp had a quality 
step start from the factory.

Some builders are using a string of 2400 to 5600uF on BIG tubes with handles 
and 5-8KV. That calculates to some serious energy storage requiring extreme 
safety provisions.

The only other concern I have with these caps is the effect on the tube 
longevity by increasing the average power considerably.

Carl
KM1H

----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
To: <collins at mailman.qth.net>
Sent: Monday, September 20, 2010 3:45 PM
Subject: Re: [Collins] 30S-1

> What Collins called a screen supply in most any rig greater than 1 KW
> was a misnomer. It really was a cathode supply and had to carry the
> total of the plate and screen currents because the plate supply was
> grounded, it didn't float. Whether 30S-1 at 1KW or 821A-1 at 250 KW.
>
> That being said, I found while in the Army that a MARS 30S-1 had lousy
> punch on high speed CW, that was cured by adding an old 350 mfd 300 volt
> electrolytic to the screen supply filter. I based that on Ray Swanson at
> Cedar Rapids having the same problem with the 208U-10 with Swedish
> silkscreens where they wanted them to run quality CW. The symptom showed
> up on the scope, that at key closure, the screen voltage takes a yoop,
> dropping enough to cut the output more than in half for about the length
> of a dot at 22 wpm. Then on dashes it rises to normal power after that
> delay. In the 208U-10 (10 KW output linear) it took a few thousand
> microfarads, maybe as much as a few 10,000, but 350 mf did the 30S-1
> just fine. This poor voltage regulation has to have contributed to less
> than perfect transient intermod distortion on SSB as well. I don't know
> that 100 or 200 may do as well, I had a 350 and it worked. I wasn't
> being paid a proper engineer's salary at the time to allow me to go to
> town and acquire a variety of capacitors to see which was enough.
>
> Before you go wild with the variable "screen" supply, try my capacitor 
> mod.
>
> Remember too, that the tank circuits are designed to handle 1 KW input
> power, not 2 KW output power and to supply a load impedance to the PA
> tube for that 1 KW input rating. Cranking up the plate current requires
> a lower impedance plate load, which means lower inductance and greater
> input tuning C in the PI output network which is more than a little
> inconvenient to achieve. That then means more circulating current (where
> the circulating current is at least Q times the input or output current)
> to heat the coils and bandswitch contacts. Its on the order of a compact
> linear so the coils may not have a lot of extra dissipation capability
> for half again (2.2 times the heat) or twice (four times the heat) the
> current.
>
> 73, Jerry, K0CQ
>
> On 9/20/2010 12:28 PM, Alan Chandler wrote:
>> The 30S-1 is one of the lowest IMD amplifiers ever produced for amateur 
>> use.
>> It uses a grounded screen grid and operates the cathode at about -200 
>> volts
>> as the screen supply.  The RF drive is supplied to the cathode so the RF
>> adds about 100 volts to the cathode which results in a total screen 
>> supply
>> of about 300 volts when fully driven.  The Eimac 4CX1000A maximum screen
>> voltage is 400 so adding 125 volts to the screen supply will exceed the 
>> maximum rating under RF drive.  Typical AB1 operation for the 4CX1000A is 
>> with 325 volts on the screen which is about where the 30S-1 actually 
>> operates.  Increasing the screen voltage does increase the gain of the 
>> tube, requiring less drive power but it reduces the available plate 
>> voltage swing before the screen current rises to the level that exceeds 
>> the screen dissipation (35ma at 325v is just under 12 watts,
>> the maximum screen dissipation.)  and the tube is in saturation. The net 
>> result can be more output power but with a substantial increase in IMD. 
>> You will typically get about 1 to 2 dB increase in power (0.2 to .04 S 
>> units) and about 10 dB higher IMD (2 S units).  Be kind to operators on 
>> adjacent frequencies and leave the screen supply where it was designed to 
>> be operated.
>>      I have just about finished setting up a variable screen supply and 
>> will
>> make power and IMD measurements with a 4CX1000A and 4CX1500B in a 30S-1 
>> later in the fall.  I started the project in the Spring but good weather 
>> forced yard work to take precedence.  When I complete this, the results 
>> will be sent to The Signal.
>>
>> 73, Alan - K6RFK
>> ______________________________________________________________
>>
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