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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
 
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-Story/EL34-Story-Seite4.htm
http://www.jogis-roehrenbude.de/EL34-Story/EL34-Story-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.
 
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.
 
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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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.
 
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 :D
Thanks in advance.
Glenn
 
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
 
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.

They will probably hold up better than most new tubes... also, it's what they are made to do - you are still looking at 2000 hours minimum for quality output tubes. Think of it like toothpaste and a movie, it's meant to be used, and a good one is always better than an ok one. Buy them, use them, experience them.. cause someone else will and no matter what, eventually they will be gone. I personally bought a lifetime supply and plan on using them.

porkchop61 said:
A switch for changing cathode resistor values would be kind of cool :
So do it. It does work.

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?
I am impressed with the Shuguang KT66. It's not the best in appearance, the structures don't always line up, glass envelopes don't match... in general the construction appearance seems down compared to the newer russian tubes, but it is a mighty fine sounding tube, completely unlike the cheap 6L6 they make that everyone sells for $5-$10. It has a nice sound and is worth listening to. Plus it's HUGE and looks cool. Which is worth something. To some.

As for the grid stopper, its purpose is to prevent radio frequency oscillations in high gain stages. Its on almost all stages in a guitar amp for this reason, but in hi fi, the output tube is generally the only one run to the max on gain, hence the common resistors on the output tubes. Yes they have to be at the socket, and no they don't reduce gain if calculated correctly (also they come after the grid leak, not before). At audio frequencies, the grid is an almost infinite input resistance, so there is no voltage divider network to reduce the gain, the signal passes through the resistor unimpeded. At radio frequencies where the capacitance of the tube starts to play in, there is a measurable resistance and thus a voltage divider network that reduces gain. Hence the reduction of gain at radio frequency and any oscillation. A little time with google will help you figure out how to calculate these, but some circuits use as high as 470K ohms! 1 to 10k is common in hi fi output tubes, but you could essentially go higher... it's not a super narrow field. And no, it wont hurt to use one on a different style tube. The worst thing that would happen is you roll off the high end of your amp, something you would notice and then go back and change to a lower value and problem solved. You won't break anything.

PS - your amp looks great!
 
Thanks for the tips.
Yes, I really like the looks of the Shuguang KT66's, but looks aren't everything :)
I haven't put the grid stopper resistors in yet, but I'll probably get to it this weekend.
I'm really impressed by the sound of an SE amplifier. This is my first, but I do antique radio repair as my other hobby, and most of those are SE design (although not hi-fi by any means).
This amp is very, how do I say, musical. Must be the harmonics that aren't present in solid state stuff. At least with an SE design, you don't get any crossover distortion :D Also, tube matching isn't a big deal either.
Glenn
 
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 have an amp set up like that. With two, maybe 3 resistors, you can try all those octal output tubes that conform to this same basic pinout. It's fun to try them out. 6V6, EL34, 6CA7, 6L6 (even old metal can ones), 6550 etc etc
 
Yeah, that's another thing I've been thinking of adding.
Nice thing about this hobby, you can tweak things as much or as little as you want.
I wonder how people like THD make these guitar heads that take a bunch of tubes without any changing of components. They must have some auto bias circuit?

"Class-AB amplifier with foot-switchable overdrive/boost, foot-switchable master volume control and the ability to use almost any preamp and power tubes in any combination, including 6L6, EL34, 6V6, 6CA7, 8417, 6550, KT66, KT77, KT88, KT90, KT100, EL84 (with Yellow Jacket adaptor), 6K6, 6F6, 12AX7, 12AY7, 12AU7, 12AT7, 12AZ7, 12DW7, 12BH7, ECC83, ECC82, ECC81 and many more, giving the user a huge pallet of available sounds simply by changing tubes."

Glenn
 

chrish

Member
2003-10-20 2:43 pm
Sydney
Saw a post a while back about switcheable cathode bias resistors for TubeLab's SimpleSE. Cant quite find it now, but essentially you want to have a resistor in the circuit at all times. This means fixing the highest value resistor in the circuit and having a switch that selects a second resistor to parallel that fixed one for the lower values. This way there is always a resistor in the circuit if the switch breaks before makes or if it fails etc.

Cheers,

Chris
 

chrish

Member
2003-10-20 2:43 pm
Sydney
Porkchop,

That was pretty neat the way you laid out your wiring and chassis placement on your link. What program did you use? I have wondered about using a simple cad program for this type of thing (maybe sketchup as I have a mac). Problem is it looks like it would take ages to model all of the components required. Have tied a google search to see if there are libraries of standard components for valve amps, but have not had much luck.

Amp looks great! I hope you enjoy using it :)

Chris
 
Thanks for the kind words everyone. I did have fun making this amp. It was very simple to make compared to other guitar amps I have made (push pull).
Like this:

[IMGDEAD]http://webpages.charter.net/porkchop/tube_stuff/amp&guitar_small.jpg[/IMGDEAD]
[IMGDEAD]http://webpages.charter.net/porkchop/tube_stuff/november/IMG_0249.jpg[/IMGDEAD]
[IMGDEAD]http://webpages.charter.net/porkchop/tube_stuff/november/IMG_0247.jpg[/IMGDEAD]

As I said, it sounds quite different from push pull designs. I really like the signature it gives to the music.

chrish-
I actually used a high-end CAD system at work (after hours :)) to do the layout. I heard Sketchup fro Google is good also, but I haven't tried it yet.

Glenn
 
porkchop61 said:
Yes, I was thinking of adding a two pole multi-position rotary switch that would allow me to change out bias resistors. Of course you would not switch this while the amp is operating. I have a two pole six position rotary switch now for selecting between UL/Triode/Pentode modes. Similar implementation.
Glenn

I do it all the time won't hurt anything. Almost all of my amps have 2 bias positions - one I lovingly call girlfriend/TV mode and the other, audiophile mode. You can run the tubes cool for non critical listening and hot for the good stuff.

As long as you aren't removing the resistor entirely, just adding an R in parallel or in series, there is no thump, no noise, nothing. You can switch it back and forth all day while the music is playing and it won't hurt a thing.