Anyone done the factory SP600 Nuvistor upgrade? 6DS4

[ke7trp]

While  Searching for the manual for my SP 600 I came across this document about replaceing the first tube with a Plug in 6DS4 Nuvistor.  It states that you can get up to 6DB of signal to noise improvement on certain Freqs. 

I know I have some of those buggers around.. Just cant find them.. Searched for 30 minutes through 2000 tubes.. LOL. 

Anyone have first hand experience doing this?

Clark

[ke7trp]

Edit.. I am a dope.. I did not go far enough on the Upgrade document!  the schematic is right there.. Going to build this over the weekend and give it a shot.

C

[KM1H]

The commercial product was called the Nuvistaplug.

The biggest problem is that you no longer have any AGC control on that stage and the nuvistor isnt known for strong signal handling ability.

There are several other better, IMO, choices to improve performance. Any number of miniature tubes with the 7 pin 6GM6 and the 9 pin 6EH7 being the hottest but with good AGC control. A 6DC6 or 6BZ6 will do almost as good and reduce the chance of having to tame oscillations.

A 6BY6 is a lower noise and higher signal handling mixer than a 6BE6 and is a drop in.

Carl
KM1H

[ke7trp]

The big plus of this to me is that it plugs in.  I really hate to modify an old RXer like this.  Thanks for the input!   I might try the 6BY6

C

[KM1H]

Any of the above will work on an adaptor as well.

Not having AGC will ruin the performance anyway in GC use and with a decent antenna.

Carl
KM1H

[WD5JKO]

Here is an interesting web page where the 6BE6 and other similar tubes are used as a micro power AM broadcast transmitter and modulator combination. Some of the items mentioned, and ways to increase dynamic range might translate to useful ideas to apply in a receiver.

http://www.wa2ise.com/radios/amxmit.html

Here is a more to the point link that is direct to the topic of this discussion:
http://www.wa2ise.com/radios/12ba7.html

Jim
WD5JKO

[KM1H]

The problem with the SP-600 is that it is not easy to make changes in the RF section.

I modified a HRO-60 last year and it has a similar tube lineup and is very easy to work on. After the basic restoration was done and aligned to spec it went to 75M for a battle condition test. While it was better than most any other receiver it did overload at times with the RF gain on full. A 6BY6 made a noticable improvement but I didnt take real measurements. A 6GM6 helped a bit more in the first RF and later I replaced the 2nd 6BE6 mixer which is used above 7mHz with a 6BY6.

I plan to do a full lab style evaluation and see what the real numbers look like in original config and with the incremental mods. The 6GM6 was measured for 10dB SNR on 10M and the AM sensitivity more than doubled over the 6BA6 and outperforms my R-390A there. Then I ran out of time and had to do customer work and the radio got put on an operating bench.

Carl
KM1H

[KE5YTV]

Hi, I am a newbie and would also like to upgrade my sp-600. I just want to make sure that I understand what Carl is saying. I plan to replace the first RF mixer 6BE6 with a 6GM6. And the second RF mixer 6BE6 with a 6BY6. And they are both drop in replacements? Thanks for any help from the Guru's.

Mike
KE5YTV

[WD5JKO]

Hi, I am a newbie and would also like to upgrade my sp-600. I just want to make sure that I understand what Carl is saying. I plan to replace the first RF mixer 6BE6 with a 6GM6. And the second RF mixer 6BE6 with a 6BY6. And they are both drop in replacements? Thanks for any help from the Guru's.

   Mike,

    I too have an SP-600, and have done the 6GM6 mod for the 1st RF. That had a noticeable improvement in sensitivity as seen on 10 meters, and 6 meters. Adding more RF gain however can lead to overload when the band is crowded, such as in 49 meter SW, and the 75 meter ham band. There was some good discussion about this on the following thread:

http://amfone.net/Amforum/index.php?topic=22392.0#lastPost

    I linked to two web pages earlier in this thread where various tube substitutions were discussed. Several more tube types were mentioned that expands the list of tubes to try.

   Keep in mind that the 6BA6 replacements all have to be remote cutoff or semi remote cutoff pentodes such as 6BZ6, 6GM6, 6HR6, and others. The 6BE6 replacements all have to be Heptodes or Pentigrid converters such as 6BY6, 6CS6, 5915, and possibly others such as the 6BA7 with socket change to 9 pin.

Here is a thread talking about IF amp tube subs, pinout considerations, etc. on a Star Roamer receiver:
http://www.antiqueradios.com/forums/viewtopic.php?t=103727&view=next&sid=7cefcfbed008be46056543773eedefe1

   Any substitution should be proceeded by looking up the tubes, checking pinouts, DC and AC characteristics. At minimum the circuit alignment will need a tweak, and possibly some modification to the under socket wiring will be needed. As Carl said the SP-600 access to the RF stages is a bear.

   If your primary bands you use on the SP-600 are 160-80 meters, then increasing the RF gain will NOT increase the sensitivity in practice since atmospheric noise will mask any improvement in the receiver noise figure. In fact modifications such as adding the 6GM6 to the Rf stage 1 will decrease the receiver dynamic range, and possibly make the receiver more prone to overload. I have not tried any of the 6BE6 substitutes, but if these increase the mixer dynamic range, then that will be a good thing.

Jim
WD5JKO

[N2DTS]

I don't know much about receiver design, but suspect you don't need any sort of rf amp below 20 meters unless you use a very poor antenna and a very lossy mixer.

I suppose many receivers were designed for poor antenna's (short whips, indoor antenna's, etc) and have way too much gain for use with a good antenna.

In the homebrew RX, I want to try a 6BY6 as the mixer, but don,t have one on hand.
Its hard to imagine better performance, but its worth a try.

The homebrew has no gain before the mixer, and just 2 IF stages, and hears as well or a little better than the flex 5000 does. Very weak, or very strong signals seem fine, very low noise (that is where it tops the flex), and no sign of any overload on really strong signals, but the transmitter does overload it.


Brett

[KM1H]

Hi, I am a newbie and would also like to upgrade my sp-600. I just want to make sure that I understand what Carl is saying. I plan to replace the first RF mixer 6BE6 with a 6GM6. And the second RF mixer 6BE6 with a 6BY6. And they are both drop in replacements? Thanks for any help from the Guru's.

Mike the 6GM6 is a pentode and only used in the first RF amp. The 6BY6 is a converter tube and used in place of both 6BE6's.

Of course the sensitivity improvement is only above 20M but that is where I spend a lot of time. A S-38 with IF filtering is fine for 75

Carl
KM1H

[KE5YTV]

Thanks Carl. This seems to be a better idea than the nuivistor conversion.


Mike
KE5YTV   

[KE5YTV]

  Thanks to everyone for the help. It's a great help for a beginner to be able to ask the experts. I learn something on this board everyday! 73's



Mike
KE5YTV

[ke7trp]

I dont need more gain.. I need less noise.   I run the RF gain down at 4 or 5.   In fact, On the low bands, Going past 5 does not do much.  I can see it on the new band scope but I cant hear it on the air.

So to recap:

Dont do the Nuvistor upgrade since carl said you will have NO AGC. If i can ever find those 6DS4s I would make a plug in for fun.

There are no direct plug in tubes that will lower Signal to noise.

Clark

[WD5JKO]

I dont need more gain.. I need less noise.   I run the RF gain down at 4 or 5.   In fact, On the low bands, Going past 5 does not do much.  I can see it on the new band scope but I cant hear it on the air.

   Clark,

   I am attaching a document about noise across the radio spectrum. Section 3, Figure 12 highlights the issues pretty well for HF. For example, excluding man made noise, evening atmospheric noise at 1 Mhz is about 50 db stronger than at 10 Mhz. So trying to improve S/N ratio at 160-40 meters is probably best done with the antenna itself. The exceptions might be with a Beverage antenna, or a loop antenna where extra receiver gain without extra noise would be useful.

   I always like a receiver to have enough gain such that the sum of the man made + atmospheric noise just raises the S-Meter off the minimum peg. More gain than that just works against you. With my Icom R75, on 75 meters, switching on the attenuator still results in S5 or greater. No need for a preamp here, except maybe during the daytime when the atmospheric noise usually drops..that is until the XYL and kids start turning on the blenders, XBox, etc. 

   The 6GM6 mod to the SP-600 1st RF stage helped me on 10m, and overload on the lower bands is rare since I cut down the RF gain control anyway. This simple mod is a good bang for the buck. Keep in mind that the cathode & G3 pins often swap going from a 6BA6 to some of the alternatives. Check pinout and radio schematic and make sure that is OK. I presume the two pins are tied together anyway making the concern go away.

Jim
WD5JKO

[Pete, WA2CWA]

If you really want to get rid of front-end noise, get a receiver that doesn't have an RF amplifier. I often recheck my  Squires Sanders SS-1R antenna connection because the receiver is so quiet (lack of bacon grilling background noise), I forget this isn't your typical communications type receiver design.

[KM1H]

Altho a 7 pin to 7 pin swap is the easiest, pay attention to the tube manual specs. The 6GM6 as an example should run at a 125V Ep and E sg to take full advantage of its performance and not prematurely fry it. If you want to run it isolated from the RF gain control then use a 56 Ohm cathode resistor.

One of the reasons that the SS-1R failed to catch on was its severe overload and image problems when used on non 50 Ohm antennas as well as multiband antennas. There are very good reasons for an isolating RF stage in the real world.

Carl
KM1H

[Pete, WA2CWA]

One of the reasons that the SS-1R failed to catch on was its severe overload and image problems when used on non 50 Ohm antennas as well as multiband antennas. There are very good reasons for an isolating RF stage in the real world.

Carl
KM1H

Since I only use coax connected antennas designed for the frequency area of operation, I have none of those issues. Actually Squires Sanders created an accessory product for the SS-.. receiver series, called an antenna matcher, for users who required the use of non 50 ohm type antennas. I suspect the high price tag curtailed many ham purchases of the receiver rather then "non 50 Ohm antennas as well as multiband antennas", and, by the 70's, transceivers were the rage.

[KM1H]

Having to use the "antenna matcher" to fix a design problem was of little help as the word of mouth damage had already been done. The price didnt help either.

Drake, Collins, and a few others had no problem with seperates right thru the transceiver era.

I remember when Sam Harris, W1FZJ/W1BU, had a SS-1R for evaluation. It looked great in the lab at work but at home it fell apart on a tribander. He then tried it as an IF for VHF/UHF converters and it sounded like an aviary. His comments arent permitted to be repeated here

Carl
KM1H

[N3DRB The Derb]

if you want less noise on 75 talk to God n tell it to crank the lightning down. 

[Pete, WA2CWA]

Having to use the "antenna matcher" to fix a design problem was of little help as the word of mouth damage had already been done. The price didnt help either.

Drake, Collins, and a few others had no problem with seperates right thru the transceiver era.

I remember when Sam Harris, W1FZJ/W1BU, had a SS-1R for evaluation. It looked great in the lab at work but at home it fell apart on a tribander. He then tried it as an IF for VHF/UHF converters and it sounded like an aviary. His comments arent permitted to be repeated here

Carl
KM1H

Not hanging a tunable RF stage on the front end requires "special" considerations for the types of antennas you might want to connect. It clearly identifies this in the manual. I've used my SS-1R on various types of antennas including dipoles, Vees, tribander, and even a GAP "multiband" vertical and experienced no issues of birdies or images. I've also used the Ameco CN-50 and CN-144 converters, SSB Electronic LT-6S transverter, and several homebrew VHF/UHF converters with the receiver and never experienced any birdies or other weird signals other then the typical crud I also hear on the 756 PRO II or the Flex 5000. The SS series was only in production from roughly 1963 through 67/68. I guess the CB market was far more profitable in the those latter 60's years and early 70's.

[KM1H]

Hanging anything on the front end, including converters, was a gamble. Sam used Tapetones and Ive no idea how close to 50 Ohms they were at the 10M output. This was also the SS-1R very early production as Sam headed to Arecibo soon after.

Using any antenna or converter on his S Line or R-390 worked fine.

Carl
KM1H

[KE5YTV]

  Tonight I finally replaced my 6BE6's with the 6BY6's. Wow, what a difference!! It really lowered the noise floor. If there is anyone besides me who hasn't done this already, I highly recommend it.


Thanks guys,

Mike
KE5YTV

[ke7trp]

What was involved Mike?  I just got my Sp600 up and running with the HiFi mods.  I have to take it apart AGAIN as now the switch for AM/CW is not working.

C

[KE5YTV]

All I did was replace both 6BE6's with the 6BY6's. It is a drop in replacement. It made a big difference for me in quieting down the noise level.

Mike