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Dynaco ST-70: tubes

P

pontoon

Member
I have been reading many threads here on tube choices for this amp and just thought I'd add another one! Well, I've narrowed it down to a few choices and thought I'd get some advice.

I just got this amp and it's pretty dusty. The person I bought it from said the power tubes are gassy and I'd need new ones. The power tubes have no writing on them. The other tubes are Realistic.

I turned it on for a few minutes and it sounded very distorted. The rectifier tube flashed so I turned it off.

I'm hoping that new tubes will get this thing working. I don't intend to do any mods or anything, at least for now.
 
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So I'd like to focus on the lower priced tubes for starters. I think I would like a tube on the warmer side. Seems like the SED Winged C would have been good, but the prices have gone up a lot and they are now $50 each.

So I've been considering these:

Electro-harmonix 6CA7 - seems like these are pretty warm sounding

Electro-harmonix EL34

Russian 6P3S-E

Russian 6P3S

What are the difference between these two Russian ones? The ones with the E are more expensive and are basically as much as the electro harmonix ones. Are they worth it? If anyone has links to specific Russian ones for sale that they recommend, I would appreciate it! There's a lot of talk on different versions but I'm not sure I'd be able to tell.

Or should I just splurge on Gold Lion KT66's?
 
I am running JJ's in mine and they sound good to me. As for the rectifier, I would replace it also, but the real question is if the can cap is good, I would replace it if it was mine and has not been replaced before, also the bias filter caps need replaced as well as the bias rectifier if not been replaced already. Not trying to say you need to rebuild the entire amp but those parts are the most common fail items and no need to damage your new tubes.
 
+1 on the caps. If your amp has the original caps they need to be replaced, on the board and underneath.

You should also replace the square selenium rectifier that rectifies your bias voltage. Bad caps can hurt your amp. Bad tubes won't. More than likely your tubes are fine.

This link shows pics of replacement caps on the board: http://www.antiqueradios.com/forums/viewtopic.php?t=89664

And this site shows a pic of the caps and selenium rectifier you need to replace underneath: http://www.angelfire.com/ma2/AdamVon/dynaresto.html In fact, this page will get you going safely. These are not mods, just replacement of caps that are past their sell-by date.
 
Ditto on the power supply capacitors and the power tubes grid coupling caps too. But, it is worth it in the end.
 
Ok, thanks. I keep reading bad stuff about the JJ's but seems like a lot of people like them as well. How hard would it be for a novice to learn to replace a capacitor? Or should I leave this to a professional?
 
here are a couple pics. definitely needs a cleaning.
 

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I use those green Scotch brite pads,,, steel wool will leave little pieces of metal wire all over the amp...

Regards,
John
 
pontoon said:
So I'd like to focus on the lower priced tubes for starters. I think I would like a tube on the warmer side. Seems like the SED Winged C would have been good, but the prices have gone up a lot and they are now $50 each.

So I've been considering these:

Electro-harmonix 6CA7 - seems like these are pretty warm sounding

Electro-harmonix EL34

Russian 6P3S-E

Russian 6P3S

What are the difference between these two Russian ones? The ones with the E are more expensive and are basically as much as the electro harmonix ones. Are they worth it? If anyone has links to specific Russian ones for sale that they recommend, I would appreciate it! There's a lot of talk on different versions but I'm not sure I'd be able to tell.

Or should I just splurge on Gold Lion KT66's?
Click to expand...

I'd like to go so far as to say "good tubes sounds the same". Then the question would be "which tubes will work fine in my st-70".

What might be considered is that filament current for el34 is 1.5A, and which
stock dynaco transformer it's unwize to use 6550 or kt88. EL34 kt77 kt66 and 6ca7 will fit the socket.

Then it's up to local distributor, and i do not mean price i mean "testing/matching" tubes ( st-70 need matched pairs ) and warranty. Buying cheap is often buying several times at a higher price.

And yes, replace rectifier and the big capacitor. Also replace the selenium rectifier if it's still there ( i think dynakitparts.com have ready made kits for this purpose).

If the driver tubes are original 7199 that are hard to fins. I have some new that also measures new, it's not impossible to get but alternatives ( an adapter that make 6GH8 possible ) might be needed, a bad 7199 will sound bad.

As regards to JJ : EL34 is an excellent tube. It also has the property that it
does not strengthen putin's economy, which concerns us in europe.

Congrats to a fine amp, when brought back to it's original shape it will keep competition behind. Then there is upgrades ( tubes4hifi for instance)
 
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The EH6CA7 gold is a very nice all around performer for me,also-the tubes in there right now look like RFT early 70's issue to me.[also a very nice tube in my book]
 
These amps were designed to be built as kits. No pro knowledge needed just to get it running well and safely. Download the assembly manual from the second link in post #5 above.

Get a drop in replacement for the can cap here: http://store.triodestore.com/40ufat52twis.html.

The other replacement caps are quite cheap, as is the one diode you'll need to replace the selenium rectifier.

There's another tutorial here: www.tuberadios.com/temp/st70.doc

p.s. peterh is right, dynakitparts.com even have kits of the parts you need.
 
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Pontoon,

I love the Gold Lion KT-66's!!! However, I would not recommend them until you get your rig stable. Get a cheaper set of EL34s.
+petercapo

But wait, Is this a stock ST-70? You might want to investigate some other changes first even before a tube change. Can cap, selenium rectifier are a couple of good places to start and if you can make a good solder joint, you can do the work. If your shakey with an iron, get some one to do it for you!

Don't forget to unplug the amp and, short out the cap prior to unsoldering. Stored voltage will give you a jolt! :stupid:
 
Rebuild the amp, and get the tubes tested. They may be OK, or maybe not. Often the "it needs a tube" fix is not the tubes at all. No point spending money on a set of tubes that you may not need though. It definitely needs the caps replaced for it to have much hope of working properly and reliably.
 
thanks for all the info. i'm going to have to do a little research. looks like the parts can add up. still, i think it'll be a good deal for a tube amp if it all works out. they've always been out of reach.

and then there's the issue of the pre-amp. to test it i was taking the pre-amp signal from a sansui 2000x but hoping to get something else...
 
I was one who was having issues with the JJ KT77s roasting on his ST-70. I'd rebuilt the unit but I left the original sockets in. The JJs might have been sensitive to old connections but I'm not certain because I switched to Tung-Sol reissue EL34Bs and a NOS Mullard GZ34 when I replaced the sockets. Haven't had any problems since.
 
The good thing about the ST-70 is that it was sold as a kit, so most parts are not that hard to find except maybe the 7199's tubes. But you can buy adapters to use 6GH8's, you can replace all the tubes, if needed, and caps in the amp for around $200. Check out Antique Electric Supply.
 
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