6CA7EH in ST70

knowledge base for the classic Dynaco ST70

Postby kevco » Thu Jul 30, 2009 6:29 am

So...After many home improvement projects and family events, I was finally able sock away enough money to retube my DiyTube ST70. I decided to try the 6CA7EH. I found that my first choice of retailers (Tubedepot) was out of them, so I purchased a fully matched quad from Thetubestore.com at a cost of $80 delivered. Tubes were date stamped 08 12. My initial impression of sound quality was very good. They are punchier and much fuller sounding than Ruby EL-34STR's of dubious age that came in the amp when I purchased it. Bass response is far greater. I have also noted greater gain in relation to the position of the volume pot on my pre-amp. I have only had time to give them a bias and listen for about a half hour so far, but I am very pleased. I also have noted no unusual glowing on the plates. A quick question: Should I bias these at 500mV per tube as stated in Shannon's manual? Would that be the ideal bias for these tubes? Is there a safe range to stay within? Are there any options for "running them soft", or is that not bias related? Thanks again, all!
Last edited by kevco on Thu Jul 30, 2009 9:57 pm, edited 1 time in total.
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Postby Ty_Bower » Thu Jul 30, 2009 7:53 am

Peer through the three round holes in the plates, and verify you don't see any glowing spots on the screens. You should see the cylindrical cathode sleeve glow a dull orange (normal) and you will likely see blue stripes on the inside of the plates (also normal). You don't want to see anything that looks like a burning piece of thin wire.

Biasing these guys to 500 mV (50 mA idle current per tube) should be about right. The 6CA7EH is a good sounding tube. I felt similar things about mine - nice and punchy, with plenty of heft on the bottom end. It's curious how similar these tubes appear compared to the old US made Philips 6CA7.

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Postby kevco » Thu Jul 30, 2009 10:04 pm

I'm curious Ty, how do those original Phillips 6CA7's fare? I read an online account of them being notoriously unreliable in Marshall heads and not dealing well with high plate voltages....but those are not issues for home audio users. They can compete with Mullard El-34's price wise...but so can a microphonic 12AX7!
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Postby kevco » Fri Jul 31, 2009 9:51 am

I dialed em in at 500mV after about a 15 minute warm up today and sat back to listen to a favorite CD. No visible glowing on the screens. Sound continues to wow me with bass punch, great mids and perhaps slightly less air at the top than the Rubys...but this is all so subjective. I am certain though that overall gain in relation to the volume pot on my pre is greater. I suppose that this may be due to age of the Rubys. After about an hour of listening, I rechecked bias and it had drifted down about 15mV per side. I'll continue to monitor it as they burn in. That's it! I had a pretty hard time finding subjective information about choice of tubes, everybody has their favorites...I hope this thread helps somebody else make an informed decision.

I should also mention, the only visible quality issue: On one of the tubes, the heater element sits slightly higher in the cathode sleeve, making look as if it glows more brightly than the other three. I'd say it protrudes about 1/2 mm. higher than the others. Anybody see this as problematic?
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Postby Dynacophil » Sat Dec 12, 2009 5:22 pm

Another pic showing EH in action...

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try the 6P3S-E tubes !

Postby bronco » Tue Dec 29, 2009 4:36 pm

kevco wrote:
burnedfingers wrote:Try 6BG6GA's

They have the punch the 6L6's have and the low end you are wanting.

Don't these have a protruding anode that needs to be cabled and capped? I'm not a fan of having that kind of voltage sitting on the tops of my outputs. Makes reaching over them a little scary. I'm pretty new to the game, so maybe I'm not aware of all the ways these tubes can be used...
I also took a cursory look at Ty's Russian 6P3S-E's and the price can't be beat if you don't mind direct shipping from Siberia. They seem to be a close replacement for 6L6's, 5881's...which I would definitely try if I was interested in going totally over to tetrode and dealing that change. My main interest lies in the option of drop in tetrode comparison to the EL-34's and being able move back and forth without major bias issues. For that purpose the EH's seem to be a winner. Anybody know offhand if the original US made 6CA7's were tetrode?


I have rolled a slew of tubes in my Curcio 3-tube driver boarded ST-70: Mullard XF2, Seimens, Groove Tubes, Ruby, EH, Matsushita. I was never entirely satified with the sound with any of these. Then I read some of Michael Samra's posts on the Tubes Asylum, and trusting his judgement, tried a quad of cheapie 6P3S-E. You know what was my favorite, and not by a small margin? The lowly, cheapo Russian 6P3S-E ! Best balance of sound, from deep tight lows to sparking highs and beautiful mids in between.

If they were rare and extremely expensive, they would be all the rage in audio. I was able to get these to bias correctly 45-50ma without changing any resistors in the bias circuit. For less than $50, definitely give these a try!

I would be curious as to how these compare to the 6CA7 tubes, as these would be more similar to them than to an EL34. Anyone try both of these for comparison?
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Postby kevco » Tue Jan 05, 2010 10:31 pm

Actually, funny enough I have a quad on order for $27...How can you beat that? Looks like I'll be the one writing about that comparison soon enough. I'll report when they arrive.
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Postby burnedfingers » Wed Jan 06, 2010 5:34 pm

I just ordered a quad of them to try.
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Postby tomlang » Thu Jan 07, 2010 11:42 am

Suggested vendor(s)?
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Postby kevco » Fri Jan 08, 2010 8:31 am

I purchased my quad from BSA_79 on Ebay. $27 plus $10 shipping from Ural in the Russian Republic, where they make the cool but very unusual Ural motorcycles. As I have not yet received them, I can't comment on the seller, but he has 100% good feedback and is a top rated seller. Will report on tubes on arrival and compare to 6CA7EH. He also sells in bulk lots of up to 100 tubes.
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Postby Vince » Fri Jan 08, 2010 10:46 am

I saw one of those bikes in a local parking lot several years ago. Very interesting as is the story about how they got the design!!! Vince
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Postby burnedfingers » Sat Jan 09, 2010 6:37 am

Suggested vendor(s)?

I ordered mine from ebay russhamma (171)

$20 for 4 tubes and $9 shipping

ebay item 220531049378
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Postby kevco » Sat Jan 23, 2010 12:12 pm

Got my 6P3S-E's today. They biased quickly and seem to be pretty closely matched. I noted immediately that the balance pot on my driver board sits much more neutrally than with my 6CA7EH's on the right channel. One of the "quad" of 6CA7's always drew enough more current than the others that the balance pot had to be turned all the way to one extreme to keep an even balance. Tube quality is seemingly good and construction is uniform with the exception that one tube leans just slightly on it's base. The compactron type bases make the tubes sit lower than other tubes with standard octal bases. They remind me of the later model 7591 tubes looks wise. Gettering is very heavy for tubes of this size and they really have a look of military construction. Sound-wise, I'd say they compare favorably against the Electro-Harmonix with the exception that bass response is somewhat lighter. That may change as they burn in. All in all, I'd say that that the Russian 6P3S-E is a fine choice for ST70 users on a budget, and I'm going to let them burn in for a while and see what happens. Anybody have any thoughts on what these should be biased at in the long term? I have them at 40mA per tube for now.
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Postby tomlang » Sat Jan 23, 2010 1:15 pm

Kev, that is good to hear. Did you order just 4 of them? Reason I ask is I think I would have been skeptical and would have probably ordered 6-8 thinking one or two would be not so good or worse.

What is the packaging like for shipping?
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Postby kevco » Sat Jan 23, 2010 11:37 pm

Yes, I ordered only 4. Matched up just fine. Packaged in original light cardboard sleeves, stapled closed, obviously never opened, with a spec sheet (in Russian of course) wrapped around each tube. Then entire bundle of tubes was wrapped in closed cell foam wrap and placed in a small cardboard box. Box taped shut securely and sealed in a plastic shipping envelope. Payment was sent Jan 5, tubes arrived Jan 22. I would have no hesitation buying from this seller again. Note: these aren't miracle tubes, def not as open or bassey as my 6CA7's but plenty good for the money.
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