Quote:
Originally Posted by socrates63
You need tube adapters to use the 5687. The octal socket is the standard socket. Mikhail can sell you different adapters.
PFKMan, thank you so much for your straight-forward summary on the tubes. I had been thoroughly confused for the longest time with the various tube types. Now I think I can go back and read Earl's posts and make some sense out of them.
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Sorry about the confusion. But all these tubes have been discovered over about a two and a half year time period. Plus, whatever tube type I am using at any given time I usually learn more about and pass on more information as time goes along.
Since you seem interested I am going to give you a rough outline of the tube progression in SP amps from my perspective. Some of the ideas preceded others but I was unable to enact them as quickly.
Back when I had my original PPX3 I used three 6cg7's like everyone else who had the amp. But it was clear how much better the nos 6cg7 tubes sounded. I wanted a MPX3 and after I bought one I was dismayed at how much nos 6sn7 tubes cost. My response was to sell the MPX3-6sn7 and have Mikhail build me a MPX3-6cg7. I very much liked the MPX3-6cg7, especially for rock, but I missed the airiness of the 6sn7's. Thats when I was thinking and got the idea to use the 12sn7gt tubes. So Mikhail built me one .... and the MPX3 12 volt amps were created using the 12sn7gt tubes. That satisfied me for awhile but after I heard some tube amps using the 300B tube I realized the SP amps could sound better if they had a more powerful output tube.
I looked around and discovered the little 5687 while messing around on duncanamps.com. Mikhail, wasnt real thrilled with the 5687 idea at first. But he did go ahead and build me the amp .... and this amp became the SLAM PPX3. About this same time I also noticed the loctal tubes and I had read some posts on Audio Asylum about people using tube socket adapters. I got a friend (Mike K) to build me some octal to loctal adapters and before you knew it I had loctal 7/14n7's in all my MPX3 amps. In the amps that were 12 volt capable I found I could use the 14n7 .... which to this day is my favorite gain period. I also figured out the 7/14af7 would work and that tube eventually became the featured gain tube in my Supra's.
Armed with the knowledge about tube socket adapters I had Mike build me some octal to 9 pin adapters to use the 6cg7/6fq7. With this adapter I used the 6cg7 or the 12 volt version of the 6cg7, the 12fq7, in the 12 volt amps for gain applications. Whenever I wanted a change to a faster, more dynamic sound character I moved to this tube. I was then hooked on the adapters and started trying to get Mike to make 5687 adapters. But the adapter needed a compensation network to keep the amp from oscillating and he couldnt figure out how to fit the network inside the adapter. I pestered Mikhail and he eventually came up out with his 5687 adapter .... and before you knew it .... I had 5687's on the outputs of most of my MPX3's.
The 7/14n7-5687 combination was awesome and I sometimes wonder why I just didnt stop here. But I had also noticed the 6bl7gt/6bx7gt tubes. I sent Mikhail a couple of these tubes but he never had the time to worry about trying them out. I finally used them several months later and it was clear this was a very special tube even used sub-optimally. He finally did listen to them at my urging and again he was impressed with the tubes potential. I had a Supra I was using 5687's with and I was very taken by the amps huge soundstage when I put in the 6bl7/6bx7gt tube. So, I sacrificied one of my old chassis Supra's and had the amp optimized/ reegineered for the 6bx7gt tube. Well .....WOW!!! What a great sounding amp. This is still the my favorite amp to this day.
From here I had one of my MPX3's fitted with a Supra power transformer, a two position high/low voltage switch and a four position bias switch to optimize the amp for the 6bl7/6bx7gt's. Again, this amp was the best MPX3 yet .... IMO. Also at this time Mikhail began to branch out with the adapters and he made some octal to 9 pin 12ax7/12t7/12au7 adapters that he eventually used in the ES-1. So suddenly this series of tube types became an option in the SP amps as well. I did get some of these adapters and have been trying the 12au7 military equivalent, the 5814a, in my amps with very good results.
Furthermore, recently when I was messing around on the duncanamps website again and I noticed the 6gu7. Hmmmm same pin out as the 6cg7, same heater current, very closely aligned specs .... this should work as a 6cg7 substitute .... and the 6gu7 did work .... exceedingly well. So now, for those with the old PPX3-6cg7 or MPX3-6cg7, who feel the amp is a little to closed in at the top, the 6gu7 does a wonderful job of correcting for that flaw. Upon further research the 6gu7 turned out to be a 6 volt version of the highly regarded and somewhat expensive 12bh7 .... but the 6gu7's cost $5-6.
So thats the present, but the future holds even more potential tubes. I am getting Mikhail to develope an octal to octal adapter for me (and you) so I can use some of the large number of inexpensive power pentodes for output tubes as .... "triode strapped pentodes". If this works I have the 6ey6/6ez5/6k6gt/6v6gt/6w6gt and 6y6g already purchased and ready to try. Then we see if these pentode types can compete with a good triode. In case the high output impedence of these tubes wont function well in an OTL design with low impedence headphones, I already have a Supra back at SP to be reworked adding a transformer coupled output. I know the tubes will work with a transformer coupled output. The final question will be how good do the tubes and/ or the transformer output sound. Triode/OTL is very hard to beat but you never know until you try.
If the pentodes dont sound good singly with the adapters something tells me a FOUR tube, push/pull, class A, triode strapped pentode output stage may show up in a SP amp at some point.
Hopefully I didnt confuse anyone yet again.
