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Old 20th March 2012, 11:59 AM   #1
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Default Eico pentode vs triode VA

I am planning to build an EL34 amp based on the Eico HF-89 or HF-50 designs.

I looked over the schematics and the HF-50 uses an EF86 pentode voltage amp, while the HF-89 uses a 12AX7. The pentode circuitry is more complex but the input sensitivities are comparable. The HF-50 seems to have an extra 3 db of negative feedback, possibly allowed by the greater pentode gain.

I have always enjoyed the Citation V amp with pentode inputs.

Any thoughts on the relative merits of these approaches?

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Dan
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Old 20th March 2012, 12:44 PM   #2
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A triode amp has less gain than pentode, but THD is also higher and noisier than triodes. Remember that triodes are naturally NFB given Miller capacitance. I dont know how this affects sound, Im not musician. only Electronic Engineer and DIyer.
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Old 20th March 2012, 01:51 PM   #3
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I'm a firm believer that you design the input stage to meet the drive needs of the output stage.

What output power are you designing for? Are you running the EL-34's ultralinear or in pentode mode?

What supply voltages do you have avalaible and are you running fixed bias or cathode bias?
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Old 20th March 2012, 02:53 PM   #4
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Default More details

>What output power are you designing for? Are you running the EL-34's >ultralinear or in pentode mode?

Around 40W, limited by OPT power handling, runiing in UL.

>What supply voltages do you have available and are you running fixed bias or >cathode bias?

Around 470 VDC and Fixed bias.
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Old 20th March 2012, 05:14 PM   #5
DF96 is offline DF96  England
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The signal level at the input stage is low, so distortion will not be an issue there. The ECC83 and EF86 are both low noise valves, so not much difference there either. The EF86 will give more open loop gain, so more loop gain for a given closed loop gain. The extra loop gain means it needs an anode compensation network, so you could get lower distortion at low frequencies but higher distortion at high frequencies when compared to the triode version. It will also have a higher output impedance, but the phase splitter should be a fairly easy load to drive as they have used a low mu valve (6SN7) so little Miller effect.

Finally, the input capacitiance will be greater for the triode version due to Miller effect, but this is unlikely to be a problem unless your source has high output impedance.

Not much to choose between them, really.
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Old 20th March 2012, 06:38 PM   #6
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If the winding matches the 4 ohm tap that is roughly 20% cathode feedback. Typically the 6SN7 driver would swing about 70 volts peak to peak but with the 20% cathode feedback you'll need more and by quick calculation on the order of 110 volts peak to peak. I'm assuming you'll have a 350 volt swing on one side of the primary so that adds about 20 additional volts on each side so 55 volts 0-peak or 110 peak to peak.

I would use a solid state constant current sink in the common cathode of the 6SN7 long tailed pair and bias the plates about 250 to 300 volts above the cathode voltage. Since they are not driven differentially the gain of your driver will be about 10 best case. That still requires a relatively low output swing from your input stage.

Either of your input stage choices will still give you a fairly low open loop gain. I'd consider making the change to a 12AX7 or 6SL7 long tailed pair that drives a cathode follower witrh an active load. I'd still use a solid state sink on the long tailed pair. You could use a 6922 input stage with an acitve load and get a much higher open loop gain and operate it with a plate voltage of around 90 volts which will give you a better bias point for the long tailed pair.

This is more complicated, but will be a much more linear input stage.
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Old 20th March 2012, 06:48 PM   #7
DF96 is offline DF96  England
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According to Mullard an ECC83 LTP phase splitter can directly drive an EL34 output without a cathode follower (see 5-20 design). The 6SN7 would certainly benefit from a CCS tail; then you could equalise the anode resistors.

If you boost the open loop gain too much you have to throw it all away again at high frequencies with a compensation network.

20% cathode feedback? I can't see that, unless you mean the feedback to the first stage.
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Old 20th March 2012, 10:57 PM   #8
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Default Thanks for the comments

Thanks Tom and DF86,

I like the triode for its simplicity and am inclined to go that way. I would think that the standard 6SN7 setup would drive the 30-40 watts I want to see since the original worked ok to 50 watts. I was told that the HF-89 OPTs are better than that on the HF-50, so less VA gain and less negative feedback was necessary to achieve low distortion.

Is the only advantage of the CCS to eliminate the need for an AC balance control and setting the AC balance? I somehow dislike the historical anachronism of adding SS parts to tube circuits, and nothing becomes obsolete as quickly as SS parts.

Best,
Dan
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Old 21st March 2012, 01:35 AM   #9
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Sorry about all the stuff regarding cathode feedback. It was a copy/paste error from another reply I was writing for a different topic.

The balance of the driver is directly related to the matching of the plate resistors and the impedance of the cathode resistance. As the resistance approaches 2 megohms the differences tend to be insignificant. Also as the tubes age the bias stays at a constant current. It is my preference, but many good amplifiers have been built without the current source. Parts going obsolete isn't really an issue here and the choice of transistors is not that critical provided you have the necessary voltage rating and heat sinking if necessary. I've yet to have a solid state CCS fail and I've used a lot of them.
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Old 21st March 2012, 10:47 AM   #10
DF96 is offline DF96  England
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The CCS ensures AC balance. In this circuit it won't stabilise current as the valves age much more than the tail resistor already does. Roughly speaking, LTP PS balance error varies like 1/mu if a resistor tail is used so the higher the mu the better. 6SL7 or ECC83 can be used with equal anode resistors and a resistor tail. 6SN7 needs either a CCS tail, or unequal anode resistors.

This is because differential mode gain is roughly mu, and common-mode gain is roughly (Rload/2xRtail).
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