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Old 1st February 2008, 07:21 PM   #1
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Default 6CA7 Mikael Abdellah's KT88 SE amp?

I like the sound of the EL34's in this amp, so I've been tube rolling somewhat. The JJ EL34's sound better to me than the Electro Harmonix EL34's so far.
I'm interested in buying a pair of the Electro Harmonix 6CA7's to try in this amp. These are the big bottle designs.

Parts Express 6CA7

I was wondering if someone could explain what the difference is between these and an EL34? I see a lot of data sheets labeled 6CA7/EL34. Are they identical?

Thanks
Glenn
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Old 1st February 2008, 11:09 PM   #2
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in theory the 6CA7 is the american designation of the EL34 and they are equivalents. However the 6CA7 is a beam tetrode while the El34 is a true pentode.

I'm sure there are various pentodes labeled 6CA7 and beam-tetrodes labeled as EL34 - so its not a rule set in stone! There have been so many variations of the EL34 made through the years.
http://www.jogis-roehrenbude.de/EL34...ory-Seite4.htm
http://www.jogis-roehrenbude.de/EL34...ory-Seite5.htm

I've heard the EH 'big bottle' 6CA7s are beam tetrodes.

on a side note, I would check the amount of current going through the EL34 so you don't go over max. plate dissipation.
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Old 2nd February 2008, 12:05 AM   #3
m6tt is offline m6tt  United States
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retension pins when you switch back. although they sound pretty good after burn-in, the pins on mine were a bit larger than standard octal pins. Removing them from the sockets is still quite difficult.
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Old 2nd February 2008, 02:55 AM   #4
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The 6CA7 is an electrically equivalent beam tetrode to the pentode EL34 as was posted earlier. However, since 1988 when I started in tubes, I have never seen one "mislabeled" and as such assume that these tube follow their proper construction technique of 6CA7 = beam tetrode and EL34 = true pentode. It also seems that a 6CA7 has a larger envelope, but I do not think the plate dissipation is increased. Check your manufacturers specs to be sure.

On to the finer points, I haven't heard any current production EL34 come close to the Siemens and Mullards of old. In fact, I'd reckon that of all current production output tubes, the EL34s are on the bottom as far as audio goes. (they sound great in my guitar amp though!) I have heard some great 6l6, 6550 variants in current production...

I will also say that if you want to tube roll, you can get the Siemens EL34s on ebay for $140-$200 matched new quad. These are better than new production. You should consider it. It's what I have in my restored EICO HF87 and it's so good it makes old guys get giggly.

Almost lastly, I'll add that in general I prefer the sound of the beam tetrode tubes for hi-fi, so you may prefer the EH 6CA7 over your EL34s, but I'd say they are unlikely to rival the old tubes recommended above.

As far as parts-express goes, I think they have great service, but I wouldn't use them as a tube supplier, it's easy to beat their prices for better matched tubes.
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Old 2nd February 2008, 09:41 AM   #5
ja2dhc is offline ja2dhc  Japan
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Recently I purchased a pair of EL-34B made by ShuGuang in China when I visited ShangHai in last December.

The attached photo show a comparison to Matsushita's 6CA7
which I purchased in 1963 when I was a high school student.
Matsushita had technical joint from Philips, Holland at that time. Therefore the construction of Matsushita's 6CA7 in the photo is considered to be the same as the original Philips'.

I confirm that two tubes in the photo are manufactured as pentode and both tubes' constructions are nearly equal each other.

'73 de JA2DHC
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File Type: jpg 6ca7.jpg (60.6 KB, 1173 views)
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Old 2nd February 2008, 01:28 PM   #6
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Thanks everyone for the replies.
I have considered the Siemens EL34s as you say because they aren't too outrageously priced on EBAY. I'm just a little hesitant to use nice old tubes in this design, due to running in SE mode at close to the 25W plate dissipation. Why burn good tubes? I've considered re-biasing, but I also run KT88's, which would be biased a little too cold if I changed the cathode resistor value. A switch for changing cathode resistor values would be kind of cool

I've got a set of 6L6GC's coming for this amp also. We'll see how they sound.
That's the nice thing about this amp, with a little tweaking you can use a lot of different types of power tubes.
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Old 27th February 2008, 01:00 PM   #7
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Okay, continuing with the tube rolling theme.
I would now like to try a KT66 in this amp. My question is, do I need to add a grid stopper resistor before trying this? The data sheet suggests a value of between 10k & 50k. Doesn't this resistor control gain of the stage (along with the grid leak resistor?) I know this resistor has to be soldered right at the tube socket. Can I leave this resistor in the circuit if I want to go back to the KT88's & EL34's?
The only thing I don't like about this, is that it's another component in the signal path.
Sorry for all the questions, but I'm still learning
Thanks in advance.
Glenn
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Old 8th April 2008, 01:31 PM   #8
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Okay, I finally got a pair of the EH 6CA7's to try in this amp. Very nice sounding to me. I actually prefer the sound of these over the EH EL34's. They are less "edgy" than the EL34's. Very similar sounding to the JJ 6L6's I've also tested.
Overall, I'm quite impressed with these tubes.

The other change I've made to this amp is that I'm now running 510 ohm cathode resistors instead of the 560 ohm ones I was previously running. This gives me around 60mA of plate dissipation, where before I was around 50mA.

Glenn
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Old 8th April 2008, 06:01 PM   #9
jduffy is offline jduffy  United States
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Porkchop, do you have some pics of your rig? Love to see both externals and internals if you have them.

I'm interested to see your layout etc. One day I hope to build this amp myself.
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Old 8th April 2008, 06:09 PM   #10
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A few pictures here:
http://webpages.charter.net/porkchop...8_SET_amp.html
Here's the wiring:
http://webpages.charter.net/porkchop...REO_WIRING.jpg
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