EL34 Pentode in Push-Pull!

[Guijs]

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

[navin]

Originally posted by Guijs
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 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.

[TJ]

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 ...

[ray_moth]

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.

[navin]

Originally posted by ray_moth
Some of the differences between EL34 PP in pentode-mode and triode-mode:...

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.

[Eli Duttman]

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.

[ray_moth]

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.

[anatech]

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

[SY]

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.

[anatech]

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