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-   -   Re-tubing stock ST70 (http://www.diyaudio.com/forums/tubes-valves/179362-re-tubing-stock-st70.html)

indianajo 18th December 2010 01:15 AM

Re-tubing stock ST70
 
I have a totally stock 1961 dynakit ST70. I put it away 10 years ago because it was getting weak and the guitar shop was charging $25 for imported tubes. Now there is the internet! cool. I am going to buy some 6CA7's probably from tubesandmore because they are the only source of something else I need, and I only want to pay one shipping charge.
So which ones, JJ's ($57) (hungary?), stocker electroharmonics($57) russian, or winged C electroharmonics.($144) . They don't mention country of origin on ruby, groove tube, or Valve art, so I guess they are Chinese, whatever their features. I'm sure not buying any milk from China soon.
A guitar amp builder on chopship's thread said JJ's fail a lot. Agree? disagree?
My 25 year old GE's have the big bottle like the winged C, but I don't know if that means anything. Nobody cares what my amp looks like.
I precharged the caps, ran it at 70 VAC for a minute, then heated it up to night. I had put a test point out on point "C" the first B+ cap after the choke. I'm getting 410V idle and 400V with signal so I guess my 5AR4 is okay. The B+ can was installed in 1983, a cardboard tube from cornell-dublier, oddly enough, 20-15-15-15@525. I can't get more than 1 v bias current on either output pair, with new electrolytic caps on bias I installed this week. Putting 1/2 V pp in on both channels at about 800 hz quasi sine, I get about .5 v pp out on 8 ohm 225 W resistors, a little less on one channel. So I'm pretty sure it is output tube time. That is why I put it away.
Is the $144 tube set really better? This is not a guitar amp, no overdrive.
I was only getting .6V pp at the end of the .1 caps to the grids on the 6CA7's, with 330 V on the preamp B+ socket on the front, so I think the 7199's are probably weak, too. No recourse on that, except I'm getting ready to transistorize a Hammond H and it has 3 7199's in it. A little more signal in might have helped, but .5 v out is so much lower than the stocker 16.6 v pp that the scope confirms my ear, the tubes are tired. The 0.1 output grid coupler caps are film type installed in the seventies. The bias rectifier is selenium stack, installed in the seventies.

6L6 18th December 2010 01:31 AM

I have a Alan Kimmel driver board on my ST70

I have used JJ, Sovtek and (St.Petersberg) Svetlana (now Winged C). I have had no problem with any of them. I would seriously look at the EH and the new 'Mullard'.

My only experience with Chinese tubes in this amp is the 5AR4, which I believe I had 3 failures before I totally and completely swore off Chinese tubes for the rest of my life.

As for 7199, I can't comment, my amp uses a completely different circuit.

Eli Duttman 18th December 2010 03:05 AM

IJ,

Good current production tubes for the amp include the SED (=C=) EL34, the EH 6CA7, and the "reissue" GEC KT77. Of late, JJ's Octal production has been garbage, so stay clear.

Like it or not, the 7199 is fast going the way of the Dodo. I strongly suggest you think about Triode Electronics' replacement driver board populated with 2X EF86 and an ECC99. You retain the OEM topology but employ available types that are, in fact, superior to the 7199.

What is it that only AES has? Check in with Jim McShane. Jim's prices are fair and his service is 2nd to none. The man burns tubes in and matches them under real world conditions. All too often, factory "matching" is a bad joke.

DigitalJunkie 18th December 2010 03:14 AM

I'd suggest replacing that selenium bias rectifier with a modern silicon part. Bad things happen when those go south.

Original Burnedfingers 18th December 2010 11:59 AM

I would recommend replacing the stock driver board with something more current that would allow you to source the tubes easily. I have used the SDS Labs 6AU6/12Au7 driver board and found it sound quite well. I am also very fond of the Mapletree Driver board that uses a 6sj7 6sl7 combination. I would also suggest modifying the bias circuit to allow for individual tube bias adjustment in addition to the other mods already covered.

Output tube replacement is a difficult choice. My personal favorites are the
6bg6ga's and I also like the Russian 6n3cE tube. The 6n3cE is a more hefty version of the 6n3c. The 6n3cE is a very balanced sounding tube and it is cost effective.

kstagger 18th December 2010 01:11 PM

if the amp is totally stock and original, I would also be worrying about the electrolytics. SDS (via Triode Electronics) has a nice board that slips under the chassis. It will effectively replace the can-cap which you can keep in place for visual purposes.

My old Dynaco 70 had the SDS driver board (EF86/12AU7) and the power supply board. The sound afterwards was much more punchy and less soggy than the original. The end effect was a better amplifier, but (not to be a spoil-sport) still not a 'top leaguer".

indianajo 18th December 2010 03:37 PM

Quote:

Originally Posted by Eli Duttman (Post 2402534)
IJ,
Good current production tubes for the amp include the SED (=C=) EL34, the EH 6CA7, and the "reissue" GEC KT77. Of late, JJ's Octal production has been garbage, so stay clear.
What is it that only AES has? Check in with Jim McShane. Jim's prices are fair and his service is 2nd to none. The man burns tubes in and matches them under real world conditions. All too often, factory "matching" is a bad joke.

Thanks for all responses.
Mr McShane doesn't say where Genelex (not GEC) tubes are manufactured. "Doesn't say" means china to me until proved otherwise by your experience. Electroharmonix (russia) has a winged C 6CA7 at tubes&more, don't know who is selling "SED" or where they come from.
I've been busy coping with the trash tube era by putting 100 ohm resistor between the rectifier cathode and the first capacitor, and 30 gauge wire between the transformer and plate on the ST70 and the organs. This consumes phenolic headers, as does replacing the selenium rectifier with a 1N4002. Headers are also useful for putting Metal Oxide Varistors on the main voltage coming in to transistor devices to snub lightning strikes, which we have a lot of here. Mouser, Newark, triodeelectronics don't have them, only tubesandmore has them and they call them "terminal strips" which is what everybody else calls screw barrier strips. My caps are fine, and are available new from FP at T&M & triode. I bought an "EF86" from triode, it came in as a "6*32n" also CT21 and =C= and since I can't yet manufacture a tube grounding ring where the ittermittant socket has been removed where it goes, I haven't tried it out. Are these "6*32n" things as good as Mullard & GEC EF86's which if genuine, are very expensive? Who is =C=? I thought winged C refered to the plate on the 6CA7, which looks like a black gutter with wings standing up in my GE (US) 6CA7's.
I've just bought the best speakers I've ever owned, and am wanting to try the up to stock ST70 versus the DJoffe bias mod ST120 and the stock Peavey CS800S to see what they all sound like. As I said, I've got a whole organ full of 7199's to loan if it is a problem, and the loan woud be temporary since the ST70 consumes so much power silent. The ST70 was my main music box for 30 years, but time marches on.

Eli Duttman 18th December 2010 05:02 PM

The "reissue" Genelex tubes are made at New Sensor's Saratov, Russia, plant. New Sensor has acquired the rights to many famous brands and they have been doing a good job of making the tubes so labeled.

The "Flying C" (=C=) logo is used by the Svetlana plant in St. Petersburg, Russia. New Sensor owns the Svetlana brand in the U.S. So, SED is how =C= tubes are marketed here.

The "6*32n" things are older Russian EF86s, which (IIRC) were made at St. Petersburg. They are not good. OTOH, New Sensor currently makes the EH EF86 and the TungSol EF806, both of which are good. The TS is better and, at 2X the price of the EH, should be.

Stop fussing over MOV mounting. Buy a high quality computer surge suppressor power strip and kill 2 birds with 1 stone. You get the power outlets and the MOV Lightning protection.

kstagger 18th December 2010 05:51 PM

Just a quick word on the EF86 - I used to be an EF86 "fanatic", using the darn thing just about everywhere - driver tube for SE amps and triode connected in a preamp.

The Russian EF86 was just plain nasty triode connected. Very harsh! The EH was much better! The still common Phillips/Mullard/etc is worth the hunt while the GEC CV4085 was the top of the bunch.

In amplifiers with loop feedback, differences started to shrink into near insignificance.

indianajo 18th December 2010 06:30 PM

Thanks for the clear responses. There is no point in buying a 7199 replacement PCB for the ST70 if it uses EF86 and the only decent EF86's are NOS and priced for rich people. With the importers buying up trademarks like Sovtek and EH and muddying up the works by sourcing them from somewhere else leaves a bad taste in my mouth. I can get a whole Hammond organ for $100 with 3 Hammond brand 7199 and an EF86 in it. The "EF86" I got from Triode was inferior you say, not even marked that way, so what is the point. Lots of scams going in tubes.
As far as power strips go, the only "surge suppressor" ones available in my county are from R**** S*****. I don't think I have ever bought anything from R**** S***** that wasn't junk in some way. The strips are made in China anyway, the MOV's I use are from blown up motor drives and are US origin. Cutting up used old junk doesn't hurt the balance of payments. I didn't buy the Russian 6CA7s in 1983 because I had just come off a term of defending the Fulda Gap and the Berlin wall wasn't down yet. Russia hasn't shot any missiles over the seventh fleet lately, I may go with them.


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