6CA7EH in ST70

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6CA7EH in ST70

Postby kevco » Sun Jun 21, 2009 5:01 am

I would like to say hello, as I'm new here, and this is my first post. I was wondering if anyone has direct experience using the new(ish) Electro Harmonix 6CA7 tube as an output in the ST70. I am planning on purchasing a new quad of power tubes and this particular tube has really caught my eye for a couple of reasons: If I understand correctly it is a tetrode, rather than a pentode and behaves more like a 6L6 tonally. My listening room is small and bright, and I was hoping to pick up a little more bottom end. I have been informed by a reliable source, that this tube would be a good option, but that there are some grid alignment issues that need to be addressed by EH. Their price makes them an attractive option at less than $20 each, matched. Finally, I just like that fat bottle look. Anybody used them? Worth a try, or should I go after those Winged C's?
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Postby Dynacophil » Sun Jun 21, 2009 7:41 am

Hi
I have them since 2 month in my VTA-mod-ST-70, and I had no reason to throw them out yet :)

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Postby mesherm » Sun Jun 21, 2009 8:58 am

I used a quad of them in a modified ST70 also and was quite pleased with both the sound and the looks.
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Postby Ty_Bower » Sun Jun 21, 2009 9:25 am

I have a couple quads of the 6CA7EH. The ones I have are clearly beam tetrodes. You can see the beam formers inside. They also have some minor grid issues. You can hear faint mechanical jingling coming from the grids while the amp is on. There might be a couple tiny spots on the screen where it glows a little bit. I'd also recommend you keep a watch on the overall dissipation, and keep it under 25 watts. Aside from these minor issues, they're decent tubes. They have a good sound that I like. From what you've written it appears you've already picked up on most of their details.

If you like beam tubes, you might also consider the Russian 6p3s-e (on the far right in the photo). Their prices have gone up recently, but you can still find a quad for less than $40 shipped. I'm not sure how they would fare in a ST70, as I've never tried them in one. I have run them wired ultralinear, with 450 volts B+. Keep the overall dissipation under 20 watts and they will be happy.

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Postby burnedfingers » Sun Jun 21, 2009 11:28 am

Try 6BG6GA's

They have the punch the 6L6's have and the low end you are wanting.
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Postby kevco » Sun Jun 21, 2009 8:44 pm

Great things to think about! Thanks everybody who gave an opinion. I honestly can't wait to compare these to my El-34's...
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Postby Sal Brisindi » Sun Jun 21, 2009 10:29 pm

I have a quad of EH 6CA7's in my ST-70 for the past 2 1/2 years and I am very happy with them.

Back then, they cost $45 a quad but the price jumped at least 50 percent in 2 years.

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Postby kevco » Mon Jun 22, 2009 11:27 am

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?
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Postby TerrySmith » Mon Jun 22, 2009 5:41 pm

The EH 6CA7's I have are date coded 03 02 and the screens light up like the inside of a toaster. These are supposed to be reverse engineered from the Sylvania 6CA7. They do sound good though, hope the problem is fixed. Judging by the prices the Sylvania's must be the holy grail of American 6CA7/EL34 types.

Anyone tried the GEC KT77 reissue?
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Postby Dynacophil » Tue Jun 23, 2009 1:30 am

Hi

I hope to complete a set of Sylvanias 6CA7 within the next week :)
I started it by buying a pair of as new PhilipsGEC 6CA7 her in germany cheap not knowing that those are original Sylvanias....

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Postby burnedfingers » Tue Jun 23, 2009 4:59 am

quote


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.

Scary? Why scary? I've never gotten shocked by touching or brushing over the top caps on my amps. The top caps I use are insulated.
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Postby kevco » Tue Jun 23, 2009 11:25 am

burnedfingers wrote:quote


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.

Scary? Why scary? I've never gotten shocked by touching or brushing over the top caps on my amps. The top caps I use are insulated.


I also didn't mention that I have a pair of lunatic kittens running around and would hate to see them roasted...:)
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Postby TomMcNally » Tue Jun 23, 2009 11:29 am

You really don't want an amp with plate caps anywhere
"the public" could get near it. Under a Dynaco cage
would be fine. The amp in this picture sits a few feet
from me, but no one will get near it when it's on but me.

No fear. Then again, the stuff I work on runs about
10,000 volts at 3 1/2 amps on the plate. Respect, not fear.

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Postby mesherm » Tue Jun 23, 2009 11:40 am

Anybody know offhand if the original US made 6CA7's were tetrode?


Philips had the patent on the pentode design and in their design the screen grid alignment to the grid was different.
US companies came up with the "beam tetrode" to get the advantage of pentode operation and increase the power without infringing on the patent. The beam forming plates replaced the supressor grid and winding the screen grid in the control grid's "shadow" decreased the screen current so the beam tetrode was more efficient.
So all 6CA7s were technically tetrodes.
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Postby EWBrown » Mon Jun 29, 2009 9:39 am

This has been covered in several other postings, it is a good idea to use "stopper" resistors on the screen grids, 100 ohms, 1 watt, connected between the SG and the OPT's UL tap might also cure the glowing screen grid issues. The 100 ohm resistor also gives a convenient way to measure the actual screen grid current for each tube.

IIRC, there was additional suggestion to connect a UF4007, cathode towards the screen grid, in series with the 100 ohm resistor, but I have not yet tried that approach.

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