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Author Topic: Hammarlund S-200SX & SP-400 Mods  (Read 1227 times)
N8FVJ
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Posts: 354




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« on: October 26, 2017, 08:04:12 AM »

The Hammarlund SP-200SX & SP-400 receive up to 30mHz vs the SP-200X standard upper limit of 20mHz. Weak signals in the higher bands are masked by noise due to the 6L7 mixer and the low gain 6K7 RF amps. I built socket adapters to use 6SG7 RF amp tubes that are very close to 6BA6 performance. And, the noisy 6L7 was changed to a 6SB7Y mixer that works well into the 6 meter range. No tuning the new tubes are required.
A 3.3pf 750NP capacitor added to the oscillator section with stabilized the upper frequencies during warm up. I also regulated the B+ for SSB stability, but not necessary on AM/CW. These simple changes allows high performance on the 15 & 10 meter bands.
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KM1H
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« Reply #1 on: October 26, 2017, 06:35:51 PM »

Im glad to find someone else that enjoys making worthwhile improvements to the oldies. I started learning those "tricks" with a HQ-129X in 1956 and really got going when I worked at National Radio 1963-69 and havent stopped yet at 76.

I have a BC-779, 794, and 1004, plus a SP-400X and did the 6SG7/6SB7Y mod to the ones that went to 20 mHz about 15 years ago.

That mod was written up in CQ around 1957 by Bill Orr, W6SAI for a NC-240D which was the first radio I converted with those tubes and have done many others since for myself and customers. My HRO-60 received a 6GM6 in the first RF and a 6BY6 as the mixer in both conversions; it is hot even up on 10M AM.

The SP-400X got a pair of 6SG7's with the 2nd one set for a bit over unity gain as was done by National in the HRO-60 using 6BA6's.
The mixer is a 6SL7 in a Pullen circuit with a 6C4 under the chassis as an oscillator buffer.

All wound up with the TC cap as used in later production SP-400's

I dont like changing sockets but I did it with a 6X8 oscillator/mixer in a HQ-129X and a 6SG7 in the RF and first IF. A HQ-140X wound up with a 6J6 Pullen and 6C4 buffer plus 6BZ6 in the RF. Im partial to good performance right thru 10M and 6M for my NBS-1, NC-183 and 183D.

Carl
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N8FVJ
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Posts: 354




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« Reply #2 on: October 27, 2017, 07:47:13 AM »

Im glad to find someone else that enjoys making worthwhile improvements to the oldies. I started learning those "tricks" with a HQ-129X in 1956 and really got going when I worked at National Radio 1963-69 and havent stopped yet at 76.

I have a BC-779, 794, and 1004, plus a SP-400X and did the 6SG7/6SB7Y mod to the ones that went to 20 mHz about 15 years ago.

That mod was written up in CQ around 1957 by Bill Orr, W6SAI for a NC-240D which was the first radio I converted with those tubes and have done many others since for myself and customers. My HRO-60 received a 6GM6 in the first RF and a 6BY6 as the mixer in both conversions; it is hot even up on 10M AM.

The SP-400X got a pair of 6SG7's with the 2nd one set for a bit over unity gain as was done by National in the HRO-60 using 6BA6's.
The mixer is a 6SL7 in a Pullen circuit with a 6C4 under the chassis as an oscillator buffer.

All wound up with the TC cap as used in later production SP-400's

I dont like changing sockets but I did it with a 6X8 oscillator/mixer in a HQ-129X and a 6SG7 in the RF and first IF. A HQ-140X wound up with a 6J6 Pullen and 6C4 buffer plus 6BZ6 in the RF. Im partial to good performance right thru 10M and 6M for my NBS-1, NC-183 and 183D.

Carl

Very nice post Carl. The SP-400x is my favorite AM ham band receiver. Others are close and even excel in some areas, but the SP-200 & 400 series audio clarity just works for my ears. I have older version SP-400 with the 6F6s vs the newer 6V6s.
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G3RZP
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« Reply #3 on: October 27, 2017, 08:43:52 AM »

Although not remote cut off, the 717A and 713A are slightly lower noise that the 6SG7 - 2.7k enr as opposed to 3.2k For the 6SG7. By feeding the screen through a dropping resistor so as bias is increased, the screen volts rise, you get a similar effect to remote cut off. The 717A and 713A are 'doorknob' tubes and are a bit better for internal inductance.

I cannot find anywhere any results for the Pullen mixer in terms of enr, gain compression and IMD performance. Pullen in his book (Conductance Design of Active Circuits) says very little about its performance.
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KM1H
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Posts: 2479




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« Reply #4 on: October 28, 2017, 07:00:12 AM »

Quote
Very nice post Carl. The SP-400x is my favorite AM ham band receiver. Others are close and even excel in some areas, but the SP-200 & 400 series audio clarity just works for my ears. I have older version SP-400 with the 6F6s vs the newer 6V6s.

The 6V6 was a drop in swap by Hammy and the distortion is supposed to be lower but I havent measured it.

Quote
Although not remote cut off, the 717A and 713A are slightly lower noise that the 6SG7 - 2.7k enr as opposed to 3.2k For the 6SG7. By feeding the screen through a dropping resistor so as bias is increased, the screen volts rise, you get a similar effect to remote cut off. The 717A and 713A are 'doorknob' tubes and are a bit better for internal inductance.

That didnt work in my HQ-129X in 1956 so I went back to a 6SG7 for 20M and lower where the AVC worked better on AM.
Collins used a 6AK5 (the 713A/717A was a horizontal 6AK5) and Hallicrafters a 6CB6 for years as a front end tube and neither had good signal handling.

Carl
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G3RZP
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Posts: 8123




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« Reply #5 on: October 29, 2017, 06:15:18 AM »

You only need the lower noise above 15 MHz anyway, except if you live somewhere like South Georgia, where you can be limited by galactic noise on 7MHz. Plus you are so far away from anything that overload isn't much of problem. But it is long way to the nearest bar for a beer!
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KM1H
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« Reply #6 on: October 29, 2017, 03:28:36 PM »

When megawatt SW BC was all over the place and intermodding each other and serious QRO hams, especially on AM, overload was a given. Since I started converting many boatanchors to Pullen mixers plus strong RF amps; and SW BC is only a shadow of itself the oldies are quite useable.
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KB7TT
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Posts: 16




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« Reply #7 on: October 29, 2017, 04:31:17 PM »

And if that SP400 uses a gimmic capacitor to couple the BFO into the mixer AND you pull out the two twisted wires they use to couple it and replace with a 10pf. you will see how amazingly well that receiver copies SSB... Too little injection with a gimmic capacitor.
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G3RZP
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« Reply #8 on: October 30, 2017, 05:41:05 AM »

That applied to many radios....
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N8FVJ
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Posts: 354




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« Reply #9 on: October 30, 2017, 07:52:24 AM »

Two RF amps are better than one. Although tubes such as the 6BZ6 and 6GM6 are quieter, a stock Hammarlund SP-600 with a pair of 6BA6 RF tubes provides great sensitivity. The 6SG7 is a little quieter vs the 6BA6.
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N8FVJ
Member

Posts: 354




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« Reply #10 on: October 30, 2017, 07:56:00 AM »

A 300 ohm to 75 ohm TV balun works well with the SP-200 & SP-400 series receiver. The nominal antenna input is around 200 ohms. 50 ohm coax looks like 200 ohm impedance to the receiver.
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KM1H
Member

Posts: 2479




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« Reply #11 on: October 30, 2017, 03:05:49 PM »

Quote
Two RF amps are better than one. Although tubes such as the 6BZ6 and 6GM6 are quieter, a stock Hammarlund SP-600 with a pair of 6BA6 RF tubes provides great sensitivity. The 6SG7 is a little quieter vs the 6BA6.

Sticking to the SP-200/400 for a minute they run a pair of noisy tubes balls to the wall into an even noiser mixer. For general SWL use most wouldnt notice the handicap bu that radio was never designed as a DX digger. Aviation plus commercial and military point to point were the primary use and back then most was below 12 mc.
The extra tuned RF amp helped image rejection a small bit. I use a pair of modified RME DB-22A's with a single 6GM6 and 3 more external tuned circuits; gain is set just to overcome circuit losses.

The SP-600 also needs help on the higher bands and a 6GM6, 6BA6, 6BY6 front end makes a heck of an improvement as it did on a HRO-60 which has more efficient plug in coils.

Most of the TV baluns I tested didnt even make it to 10 mHz as a balun; I wind my own strictly for HF and MF.
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