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Advice on refining SE EL84 pentode Amp
Advice on refining SE EL84 pentode Amp
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Old 30th December 2017, 12:58 PM   #21
artosalo is offline artosalo  Finland
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Quote:
As I mentioned earlier I don't have any 2W resistors of the correct value for the anode to anode negative feeedback, which is the mod I'm intending to try first.
As you know the 100k feedback resistor dissipates some 0.4 W and 150k about 0.27 W, so you manage without 2 W resistors, at least when tweaking briefly to see if the result is what you want.

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Any suggestions for potential improvements?
Cathode negative feedback is one possibility.

Putkivahvistinrakennusohje SE/84 Brachyera - Osa 2 | AudioVideo.fi

Last edited by artosalo; 30th December 2017 at 01:01 PM.
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Old 30th December 2017, 01:28 PM   #22
LeftHandFool is offline LeftHandFool  United Kingdom
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Thanks for the advice. I'll try some values this afternoon. The postman has been and gone but brought me no resistors, so I'll use the 1W parts until they do.

Regarding the cathode negative feedback - if I'm reading the schematic right, I disconnect the EL84 cathode resistor/bypass cap from the 0V and reconnect at the top of the OPT secondary?

I'm using the 8ohm tap, but I notice in the schem it's connected to a seperate tap. Is this essential?

Matt.
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Old 30th December 2017, 01:39 PM   #23
artosalo is offline artosalo  Finland
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Here you can see the marked colors of the taps.
Click the image to open in full size.

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I'm using the 8ohm tap, but I notice in the schem it's connected to a seperate tap. Is this essential?
Yes it is essential. It is maybe 16 ohms tap. (I assume you have Hammond OPT datas)
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Old 30th December 2017, 02:07 PM   #24
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by artosalo View Post
Here you can see the marked colors of the taps.
Click the image to open in full size.



Yes it is essential. It is maybe 16 ohms tap. (I assume you have Hammond OPT datas)
Hi,

Thanks.

Yes, I have the datasheet. The circuit in the schematic must be running a 5k primary, for 8 and 4 ohms on the yellow and green wires respectively.
I'm using 10k, so the white wire is actually 32ohm and the yellow wire is the 16ohm tap.
Can I use either?

What effect is the negative cathode feedback actually having on the circuit. I'm aware that negative feedback reduces gain and affects impedences, but I'm a little fuzzy as to exactly how. More reading on the subject is evidently in order.
It's good to know that something will work, but it's always better to know a little of the why .

Matt.
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Old 30th December 2017, 02:43 PM   #25
artosalo is offline artosalo  Finland
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You should also check if the 5k load impedance is better. 10k is at high side taking into account the operating point of the output tube.

You can try 16 and 32 ohms taps.

The negative feedback methods suggested to you will reduce distortion, extend frequency range, improve signal to noise-ratio and increase damping factor.

To accomplish all the above some amount of gain is traded off.
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Old 30th December 2017, 06:52 PM   #26
LeftHandFool is offline LeftHandFool  United Kingdom
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Hi,

I'll look into switching to the 5k tap, although I did design the circuit (such as it is) specifically for a 10k load. Measured voltages are somewhat higher than anticipated, so I'll take a look at the loadlines and see how a 5k load might affect things.

Single-Ended Pentode Power Amp.png

Above is the latest version of the schematic, you'll be pleased to see some changes have been made, (fixed the grid stopper, for one thing). The 220k resistor is doing a passable job of taming the mids. 470k and 330k were a little much.
I'll have a listen to some favourite pieces of music this evening and see how I like it.
Hopefully I'll have time next week to try out some negative cathode feedback as well.

Thanks for your help. I appreciate your taking the time to answer what are likely trivial questions, especially on a circuit I suspect may not be your cup of tea.

Matt.
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Old 30th December 2017, 07:17 PM   #27
ArcticBrew is offline ArcticBrew  United States
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I agree with Artosalo about trying a 5k OT tap.

If my calculations are correct, based on your most recent schematic, a 100k resistor for plate to plate feedback should provide approximately 10% feedback. This is the value that Schade says is optimum.

If you put resistors in series to get the desired value the power rating will be the sum of individual resistors power rating.
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Old 30th December 2017, 09:30 PM   #28
PRR is offline PRR  United States
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NFB monitors the Output of the amplifier and tries to make it "same" as the Input (only larger).

You really want this. Generally you un-bypass the driver cathode, and add a resistor from there over to the speaker output. If it howls real loud, reverse the OT primary leads.

No-NFB-at-all reeks of $9.98 table radios and a large class of snarly guitar amplifiers.

The real simple path is to wire the pentode as triode. Triodes have internal NFB. You will get a dmaping factor over 4 which is enough to tame the bass boom and rising highs.
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Old 30th December 2017, 11:34 PM   #29
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by ArcticBrew View Post
If you put resistors in series to get the desired value the power rating will be the sum of individual resistors power rating.
Thanks.

So if I put, say, a trio of 33k, 1W resistors in series, I'd have the equivelent of a 99k, 3W resistor?

I'll recalculate my loadline for a 5k load when I get a minute. I'd been expecting a lower B+; 10k seemed like it would offer a more linear response with the expected voltages, but since the consensus says 10k is on the high side I'll certainly try halving it.

What difference would you expect switching to a 5k load to make?

Also, is the 100k anode to anode resistor optimised for a 5k load?

Matt.

Quote:
Originally Posted by PRR View Post
NFB monitors the Output of the amplifier and tries to make it "same" as the Input (only larger).

You really want this. Generally you un-bypass the driver cathode, and add a resistor from there over to the speaker output. If it howls real loud, reverse the OT primary leads.

No-NFB-at-all reeks of $9.98 table radios and a large class of snarly guitar amplifiers.

The real simple path is to wire the pentode as triode. Triodes have internal NFB. You will get a dmaping factor over 4 which is enough to tame the bass boom and rising highs.
Thanks.

I omitted any kind of NFB not because I didn't want it but because I don't know enough to correctly implement it, hence this thread!

I'm quite fond of ten dollar radios and snarly guitar amps, but there's a time and a place for such novelties and I recognise this is neither .
I'm listening to a few well played favourites as I type, it seems what I originally took for strident upper mids is actually an aggressive treble response, which is not tremendously pleasing.

I'm thinking that the general consensus is to switch to a 5k load, and apply global negative feedback? But how do I know how much NFB to apply?

Regarding triode strapping the EL84 - I'm all for trying it, but the datasheet quotes (from memory) 2W max dissipation in triode mode, whereas I'm dissipating over 3W at peak power. Is it safe to switch to triode mode without making other modifications?

Matt.
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Old 31st December 2017, 12:09 AM   #30
rdf is offline rdf  Canada
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Quote:
Originally Posted by kstagger View Post
As an aside I've always wanted to build an EL84 SE amplifier using an EL84 driver, ie an EL84 being driven by another EL84.
Possibly implementation, I found trying that many moons ago that the EL84s at hand were too microphonic for driver duty.
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