Hello,
I'd like to ask if a well designed Pentode EL34 PP monoblocks (+-30w) can sound excelent, really hi-fi? This can sound as good as a triode design?
What you can say about the sound quality? The Pre-amp will use 6ns7 tubes.
Thank you
I'd like to ask if a well designed Pentode EL34 PP monoblocks (+-30w) can sound excelent, really hi-fi? This can sound as good as a triode design?
What you can say about the sound quality? The Pre-amp will use 6ns7 tubes.
Thank you
Guijs said:
I am a newbie.
I'm told that EL34 Class A PP will put out 15W in pentode. Half that in Class A triode. In Class AB Pentode you may get 30W.
One option to consider (atleast I am) is a EL84 PPP amp. using 4 EL84 tubes. using auto bias or SS bias they can be biased to run well.
What do the experts think.
i got a Audio Innovations S500 Class A 25w ULPP , what i can say is this amp sound very good !! The mid not as good as the SE type amplifier , overall better than the SE amplifier . Most of the SE amplifier high not extend , low not that punch !! it depend on ur taste . nothing in between . Somehow u hv to use different brand/type of valve to tune the sound ...
Some of the differences between EL34 PP in pentode-mode and triode-mode:
- Pentode-mode is more efficient than triode, giving greater voltage gain and higher power output.
- Pentode-mode is more critical than triode-mode regarding the optimum plate load impedance. This means that the variation of load impedance with frequency, which happens with most speakers, has a more marked effect on the sound of a pentode amp.
- Pentode-mode will produce more of the higher-order odd harmonic distortion (5th, 7th, 9th, 11th), which can be unpleasant. As a result, pentodes can sound more "strident", whereas triodes are more "mellow" (a generalization, I realize).
- Pentode-mode requires a stable voltage for the screen grids unless the operation is in class A. Without this, non-linearity is a risk. The stability of the plate voltage is less important.
- Pentode-mode has much higher output impedance than triode (about 15x). This will give a very low damping factor and will not work well with most types of speaker, unless a lot of negative feedback is applied (26dB or more) to reduce the output impedance. Adding so much NFB incurs its own problems and will almost certainly require compensation networks, to prevent peaks in the frequency response and instability.
- Pentode-mode is more efficient than triode, giving greater voltage gain and higher power output.
- Pentode-mode is more critical than triode-mode regarding the optimum plate load impedance. This means that the variation of load impedance with frequency, which happens with most speakers, has a more marked effect on the sound of a pentode amp.
- Pentode-mode will produce more of the higher-order odd harmonic distortion (5th, 7th, 9th, 11th), which can be unpleasant. As a result, pentodes can sound more "strident", whereas triodes are more "mellow" (a generalization, I realize).
- Pentode-mode requires a stable voltage for the screen grids unless the operation is in class A. Without this, non-linearity is a risk. The stability of the plate voltage is less important.
- Pentode-mode has much higher output impedance than triode (about 15x). This will give a very low damping factor and will not work well with most types of speaker, unless a lot of negative feedback is applied (26dB or more) to reduce the output impedance. Adding so much NFB incurs its own problems and will almost certainly require compensation networks, to prevent peaks in the frequency response and instability.
ray_moth said:
fantastic. this is well put.
given the above obviously you would recommend triode mode.
how much power does the EL34 put in PP Class A Triode. Can we run the tube in Class AB triode to get more power? How much power can we expect from the EL34? Are the below rating correct?
EL 34 PP Class A Triode = 10W
EL 34 PP Class AB Triode = 20W
EL 34 PP Class A Pentode = 20W
EL 34 PP Class AB Pentode = 30W
what would you recommend EL34 PP or EL84 PPP in the above 4 configurations. I'm told they deliver the same power.
EL 84 PP Class A Triode = 5W
EL 84 PP Class AB Triode = 10W
EL 84 PP Class A Pentode = 10W
EL 84 PP Class AB Pentode = 15W
EL 84 PPP Class A Triode = 10W
EL 84 PPP Class AB Triode = 20W
EL 84 PPP Class A Pentode = 20W
EL 84 PPP Class AB Pentode = 30W
An alternative is U/L but lets not complicate this by adding a 3rd option.
Both the damping factor and distortion issues of pentode mode can be dealt with by using O/P trafos that incorporate tertiary cathode feedback windings. Loop NFB requirements are GREATLY reduced, possibly to zero. TANSTAAFL, such O/P "iron" is expensive and the drive voltage requirements of the "finals" are high.
EL34 triode-mode PP is what I do at present. I would recommend it in preference to other modes (UL, pentode) if the power is enough for you. With class A you can expect only about 8-10 watts.
A limitation with class A is that, to get the tubes biased properly so there is room for the gid voltage to swing far enough either side of the quiescent point and still stay in the linear portion of the plate curves, you need a low B+, no more than 300v plate-to-cathode. This means that you need more plate current to get the power, which means, in turn, that you will have a lower bias voltage and, consequently, cannot apply so much drive to the grids.
Class AB1 can push out 16 watts if the B+ is high enough, at least 400v plate-to-cathode.
Paralleling will halve the required load impedance and double the available power.
A limitation with class A is that, to get the tubes biased properly so there is room for the gid voltage to swing far enough either side of the quiescent point and still stay in the linear portion of the plate curves, you need a low B+, no more than 300v plate-to-cathode. This means that you need more plate current to get the power, which means, in turn, that you will have a lower bias voltage and, consequently, cannot apply so much drive to the grids.
Class AB1 can push out 16 watts if the B+ is high enough, at least 400v plate-to-cathode.
Paralleling will halve the required load impedance and double the available power.
Hi Guijs,
Have a look at the Eico HF-87a schematic. 35W / CH with EL34, I use mine with 6CA7's now. One of the better amps I've heard over the years. There is the Dynaco ST-70 to consider too. Updated driver boards are a must with these.
The output transformers have the final word on sound quality. If you use cheap iron, you will not get good sound out of it. Of course everything makes a difference, but the iron is important. Buy good tubes, you don't need to go NOS crazy.
-Chris
Have a look at the Eico HF-87a schematic. 35W / CH with EL34, I use mine with 6CA7's now. One of the better amps I've heard over the years. There is the Dynaco ST-70 to consider too. Updated driver boards are a must with these.
The output transformers have the final word on sound quality. If you use cheap iron, you will not get good sound out of it. Of course everything makes a difference, but the iron is important. Buy good tubes, you don't need to go NOS crazy.
-Chris
Ray, let's expand things a bit:
Triodes need very stable plate voltage supplies. That means regulation or very bulky passive supplies. Pentodes just need tight screen regulation, which is at 4-5 times lower current and (often) lower voltage.
Pentodes have much higher gain, which allows a good degree of feedback.
Pentodes have much lower input capacitance, hence higher bandwidth before the application of feedback- this also simplifies compensation.
Bringing ultralinear into the mix may make life more complicated, but it's a damn good compromise.
Chris, have you noticed the handy little unused 50V winding on the HF87 power transformer? Perfect for a conversion to fixed bias and cathode follower drive. And the "super" 6BG6s work like a dream in that amp.
Triodes need very stable plate voltage supplies. That means regulation or very bulky passive supplies. Pentodes just need tight screen regulation, which is at 4-5 times lower current and (often) lower voltage.
Pentodes have much higher gain, which allows a good degree of feedback.
Pentodes have much lower input capacitance, hence higher bandwidth before the application of feedback- this also simplifies compensation.
Bringing ultralinear into the mix may make life more complicated, but it's a damn good compromise.
Chris, have you noticed the handy little unused 50V winding on the HF87 power transformer? Perfect for a conversion to fixed bias and cathode follower drive. And the "super" 6BG6s work like a dream in that amp.
Hi SY,
Ahhh, now you're going to make me look at the schematic.
No, I didn't notice the extra winding. I was mostly concerned with doing a restoration. I'm going to have to get another HF-87a to work on for this. That way I a) Don't mess up a nice one, and b) I can compare the conversion to a known quantity. If I don't like it, I can always bring it back to original.
Are there any details on thisyou can point me to Stuart? Thanks.
-Chris
Ahhh, now you're going to make me look at the schematic.
No, I didn't notice the extra winding. I was mostly concerned with doing a restoration. I'm going to have to get another HF-87a to work on for this. That way I a) Don't mess up a nice one, and b) I can compare the conversion to a known quantity. If I don't like it, I can always bring it back to original.
Are there any details on thisyou can point me to Stuart? Thanks.
-Chris
Thanks for the help guys.
So I don't need to worry with the pentode design, because this can be as good as a good triode.
So I don't need to worry with the pentode design, because this can be as good as a good triode.
Hi Guijs,
If you want the power, you will be in pentode mode. You can use ultralinear. This is a great compromise as SY pointed out, and my personal favorite. In triode mode you either need a different tube, or more of them. That gets expensive.
-Chris
If you want the power, you will be in pentode mode. You can use ultralinear. This is a great compromise as SY pointed out, and my personal favorite. In triode mode you either need a different tube, or more of them. That gets expensive.
-Chris
Hi Chris,
That's what i'm thinking.... A triode with this power will be much more expensive.
At the moment I have just a Fostex FE108ex fullrange so I don't need a lot of power. But I have another projects for the future lika a Line Array and then I will need power.
But know i'm convinced that I did right choosing this mode.
Thank you
Guilherme
That's what i'm thinking.... A triode with this power will be much more expensive.
At the moment I have just a Fostex FE108ex fullrange so I don't need a lot of power. But I have another projects for the future lika a Line Array and then I will need power.
But know i'm convinced that I did right choosing this mode.
Thank you
Guilherme
Hi Guilherme,
Look at the 6L6 family while you are at it. There is a lot to be said for tetrodes with beam construction. I found the 6CA7 (tetrode) sounded better than the EL34's I tried.
-Chris
Look at the 6L6 family while you are at it. There is a lot to be said for tetrodes with beam construction. I found the 6CA7 (tetrode) sounded better than the EL34's I tried.
-Chris
Anatech,
Are you sure? In my RCA Tube Manual the 6CA7 is given as identical to the EL34 (i.e. pentode); also I serviced a Dynaco using 6CA7s about a year ago and I could swear they are the same.
Ray has summarised well in his post #4, but I would like to add that UL is a very good alternative and not complicating things; its operation is about 80+% triode except for higher maximum output. Regular readers would have seen my argument in favour of this in other threads, so I will refrain from boring honourable members unless specifically asked. But I must warn that trying to lower the output impedance of a pentode output stage simply by enough feedback (especially global) can be problematic as mentioned, not just because of stability requirements. It is not the same as having an output stage with inherently low internal impedance such as triode or UL (almost). In a Proc. AES article long ago Prof. Matti Otala analysed this for transistor output stages, but the basics are universal. The theory is a little lengthy to quote here, but I can look up the date if someone is really interested.
The cathode feedback version (which is just another form of UL) mentioned is my own personal preference despite the larger drive requirement, because of certain distinct advantages. But I have not seen commercial output transformers for that (I design my own) so that is not an option for this thread - unless someone can broaden my horizons.
Are you sure? In my RCA Tube Manual the 6CA7 is given as identical to the EL34 (i.e. pentode); also I serviced a Dynaco using 6CA7s about a year ago and I could swear they are the same.
Ray has summarised well in his post #4, but I would like to add that UL is a very good alternative and not complicating things; its operation is about 80+% triode except for higher maximum output. Regular readers would have seen my argument in favour of this in other threads, so I will refrain from boring honourable members unless specifically asked. But I must warn that trying to lower the output impedance of a pentode output stage simply by enough feedback (especially global) can be problematic as mentioned, not just because of stability requirements. It is not the same as having an output stage with inherently low internal impedance such as triode or UL (almost). In a Proc. AES article long ago Prof. Matti Otala analysed this for transistor output stages, but the basics are universal. The theory is a little lengthy to quote here, but I can look up the date if someone is really interested.
The cathode feedback version (which is just another form of UL) mentioned is my own personal preference despite the larger drive requirement, because of certain distinct advantages. But I have not seen commercial output transformers for that (I design my own) so that is not an option for this thread - unless someone can broaden my horizons.
Sorry, to follow on: I am also a 6L6 fan (especially 6L6GC) despite some liberties taken by certain manufacturers these days. The anode dissipation is actually 30W compared to 25W for EL34 and heater current only 0.9A as to 1.5A for EL34; I have used many over the years and they are most reliable if the brand is.
anatech said:
Sure. When you turn the amp over, you'll see the two little wire stubs (blue) sticking out of the transformer. They're good for about 50VAC. In a simple conversion, you get rid of the output tubes' 235 ohm cathode resistors and 50u bypass caps, going straight to ground (or through a small sense resistor for measuring idle current). Then build a little bridge rectifier/cap for the 50V winding, which will net you about 68-72V for biasing. Set up a bias string with a pot in the middle- plan on a 25-45V adjustment range to accommodate most EL34 and 6L6 varieties. Return the grid leak resistor to the potentiometer wiper. Voila, a fixed bias amp, and if you beef up the power supply a bit, you'll get an extra 10 watts out as your reward for the extra part count.
Other things to do are replacing the 6SN7 plate resistors with matched 30K0 resistors and replacing the 18K long tail resistor with a 7mA CCS. And while you're at it, lose the input pot and just have a fixed grid-leak resistor. This will increase the sensitivity, so plan on increasing feedback a bit and diddling the compensation.
At that point, you'll have the original circuit well-optimized and can think about cathode followers and AB2.
SY said:
I can't deny that. Stability is important, at least for class AB1, which is what I use. My PSU has an SS bridge rectifier, followed by CLC filter, consisting of 235uF - 5 Henry - 470uF. It's a pretty stiff PSU.
(Class A operation doesn't impose a fluctuating load, so stability is less of an issue).
I can't deny that. Stability is important, at least for class AB1, which is what I use. My PSU has an SS bridge rectifier, followed by CLC filter, consisting of 235uF - 5 Henry - 470uF. It's a pretty stiff PSU.
(Class A operation doesn't impose a fluctuating load, so stability is less of an issue).
ray_moth said:
Did you compare this to other modes using the EL34? (Class AB Triode for example) or other tubes like PPP EL84 (should have the same power as PP EL34)?
SY said:
Triode Class AB gives about the same power as Pentode Class A right? in this case which configuration would you prefer?
How much power will the EL34 PP / EL84PPP deliver in U/L Class A? 12W? 15W? more?
Mullard's EL34 datasheet will give you everything you wanted to know about power, class of operation, distortion, etc.
Which would I use? Probably ultralinear AB, but there are good arguments for other approaches.
Which would I use? Probably ultralinear AB, but there are good arguments for other approaches.
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