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Dynaco ST 70

Blackdog61 said:
@TubesGlo sent me some Groove Tubes EL34Ms to try which are supposedly based on Mullard Xf2s. Replacing the Nesstones I immediately noticed more detail and a slightly forward upper midrange. I find the Nesstones to be more musical in my system. I’ve had a quad of EH 6CA7s for at least two years and eventually found them to be a bit harsh on top with my Canton monitors. The excellent bass they have was never heard by me with those speakers so they got pulled.
Well, several months ago I got some Vandersteens and finally decided to try those fat tubes. Now I hear it! The bass is much deeper and flatter; really the frequency response seems much flatter and the soundstage is wider and deeper. The top end is very smooth and detailed.
Component matching is SO important.
Waiting on some skinny RFTs next.

Also, I have been using 6GH8As with adapters on the Dynakit Parts 7199 board and ordered the 6GH8A board. I feel like that will complete the amp.
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Similar to what I experienced comparing the 6n3c-e to my xf2's. I hate admitting they sound better than the " holy grail" of el34's.
 
Got the RFTs in. First impressions?
Full, round, smooth, lush, together. They seem to handle dynamics well.
I’m glad they sound good because they weren’t cheap.
 
I know they're very good el34's. I popped my xf2's in again for a listen yesterday and fell in love with them all over again. It's good to have options if for nothing other than the fun of listening to something different. The 6n3c-e offers things the xf2's don't have and vise versa. Glad the RFT's are working out. I regret not hearing the 6ca7 EH before prices skyrocketed.
 
I bought the RFTs from an eBay seller who seemed to be legit. The tubes are used but don’t seem to worked much. The history of RFT is pretty interesting in that as I understand it, the company was owned by Telefunken and was stranded in East Germany. RFT became a lower cost option for TF, Mullard, Siemens and others as those brands simply rebranded RFT tubes. The quality is good though. Mine are branded Mullard and I’m guessing are from 80s production. Cost wasn’t much more than a quad of the New Sensor stuff, but they are still used.
They sound very warm and smooth.
 
I think you told me you installed the bias balance kit from Kevin as did I. As long as the tubes in each channel aren't too far out of whack it works great. I've purchased a couple single xf2's and tried them with known matched tubes and had to crank that bias adjust all the way in one direction or another to balance them out. If you're used RFTs line up anywhere near center of those pots when adjusting I'd say you did really well.
 
A note on the EH 6CA7. I had one go open on me. It lit up and didn’t do anything weird but the bias on that pair was wrong. I bought another pair. They look to be well made but I just don’t know.
 
TubesGlo said:
I think you told me you installed the bias balance kit from Kevin as did I. As long as the tubes in each channel aren't too far out of whack it works great. I've purchased a couple single xf2's and tried them with known matched tubes and had to crank that bias adjust all the way in one direction or another to balance them out. If you're used RFTs line up anywhere near center of those pots when adjusting I'd say you did really well.
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Yea. Most tubes bias with the slots in the bias pots close to vertical and these are fine. They biased right up just fine. When I drop in the new driver board I’ll roll in some different tubes to see if I can get a little more on top. Still, these are good sounding tubes!

Because my ST-70 came to me without the front octal sockets or bias resistors, I opted to install Kevin’s individual bias kit. Although I still want to use matched pairs, they aren’t absolutely necessary.
 
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Blackdog61 said:
Yea. Most tubes bias with the slots in the bias pots close to vertical and these are fine. They biased right up just fine. When I drop in the new driver board I’ll roll in some different tubes to see if I can get a little more on top. Still, these are good sounding tubes!
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Glad you're liking them. I bet they compare pretty close to xf2's. I've heard good things about them. They're are very few new production tubes that can compete in every way with NOS.
 
Blackdog61 said:
Yea. Most tubes bias with the slots in the bias pots close to vertical and these are fine. They biased right up just fine. When I drop in the new driver board I’ll roll in some different tubes to see if I can get a little more on top. Still, these are good sounding tubes!

Because my ST-70 came to me without the front octal sockets or bias resistors, I opted to install Kevin’s individual bias kit. Although I still want to use matched pairs, they aren’t absolutely necessary.
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And that was one of several things I never liked about the VTA board, the cheezy little individual bias adjust pots with led indicators. I like Kevin's setup much better. I'm stupid about checking bias so often. I bought some digital led readouts that show me each tube's bias setting real time. I tapped off pin 8 of each tube socket and ran them out the back of the amp with quick disconnects and mounted them in a panel along with an AC voltage indicator tapped off my bucking transformer. The panel sits under the amp cover behind the amplifier. The pic doesn't show the detail but I can easily read the digits. Gives the same reading my DMM does without having to mess with the probes constantly.
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I’m going to run these RFTs for a bit but my impression is that they sound a bit like they look; skinny.
The bass is there but it’s a bit light. The midrange is very full and lush but the response at the top and bottom isn’t really there.
The soundstage is a bit narrow.
 
That's what I read a lot. The EL34's tend to have a great midrange. That's what I found, but overall kind of lacking in low end in these ST-70's. Including the VTA ST-70 I have.
 
This thread motivated me to give my amp some love so I bought the Dynakit Parts 6GH8A circuit board and am waiting for some Russian PIO caps before installing it. I have been using those tubes with adapters for about two years. Curious to revisit the 7199s, I dug them out, finding them noticeably warmer, like a wet blanket. The amp sounds more like a sixty year old tube amp.
The JAN Phillips 6U8A are best. Clear and lifelike with a tight, detailed low end.
The RFT EL34s sound nice and are staying in. I feel like the new board will complete the amp.
 
I went ahead and stuffed the board with the yellow caps Kevin sent and well, I liked them in the 7199 board. Kevin doesn’t recommend the Russians anyway and I trust him.
The new board is in and I installed 100 ohm 1w grid stoppers while the amp was open.
It’s playing music with JAN 6U8As (no adapters!) and RFTs biased to about 48ma each.
This amp sounds really good!
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Incidentally, Kevin’s 7199 and 6GH8A boards are virtually identical, with the same values and placement. The differences are in the pinouts, so some might want to just make those changes with jumpers or whatever.
 
TubesGlo said:
There's plenty of references on forums where these tubes have been used successfully. Jim McShane was selling these 12 or so years ago burning them in while testing at upwards of 500v on the plates without issue. He recommended following the spec sheet for the 6l6gc. Now this is only for the 6p3s-e
(6n3c-e) with the coin base. These are extreme duty mil spec tubes vs the non e civilian version where the max plate voltage is indeed only 250v. I've been running my tubes at 410-420v on the plate 4 hours a day on average every day for 4 months. Google 6p3s-e and 6n3c-e ST-70, plenty of info.
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You convinced me and they are in the amp now, biased to 40ma. I didn’t think they were going to work because the left channel wouldn’t go below 47ma, but eventually both channels dialed in. They aren’t really even warmed up and I’m streaming from my phone, some ECM with acoustic bass, cello, piano, trumpet and percussion which I’m familiar with. The tone is very fat and full in the lower mids, yet detailed. Extension at the extremes is good and very musical. The RFTs are a leaner sounding tube in this amp.
 
Blackdog61 said:
You convinced me and they are in the amp now, biased to 40ma. I didn’t think they were going to work because the left channel wouldn’t go below 47ma, but eventually both channels dialed in. They aren’t really even warmed up and I’m streaming from my phone, some ECM with acoustic bass, cello, piano, trumpet and percussion which I’m familiar with. The tone is very fat and full in the lower mids, yet detailed. Extension at the extremes is good and very musical. The RFTs are a leaner sounding tube in this amp.
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Since our amps are set up very similar I knew you'd like them. After a few listening sessions bring the bias right up to 50ma per. These are rugged tubes and can easily handle the plate voltage and the bias current. Meassage me your thoughts after you've lived with them a bit.
 
My take so far is that the 6P3S-e sound a little more like what people think of with tubes. The highs are just slightly smoothed out. The low end is tight and deep but as the volume gets pushed the mid bass becomes fatter. I’m enjoying these immensely but don’t have any illusion that my system is as neutral as it was with the RFT 34s. Very smooth and plenty of detail delivered in a polite way with a full, fat middle. Very listenable.
Oh, bias dropped in the first few hours so I’ve been cranking the pots. Stable at 42ma now.
 
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