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Advice on refining SE EL84 pentode Amp
Advice on refining SE EL84 pentode Amp
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Old 29th December 2017, 09:10 AM   #11
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by ArcticBrew View Post
If you want to try local feedback look at RH84 designs. Connect a 100-150k resistor from the plate of the EL84 to the plate of the 12AT7.
Hi Steve,

Thanks for the advice.

First up, my schem with the value of the driver valve corrected:

Single-Ended Pentode Power Amp.png

Is this is the RH84 design you're referring to:

RH84 amp schem.png

I haven't seen feedback from anode to anode like this before, but it has the disinct advantage of being easy to implement so I'll give it a go, perhaps using shef's suggestion of starting at 470k and walking it down. I don't own an oscilloscope (although it's becoming increasingly obvious that I'll need to get one sooner rather than later), so I'll have to judge it by ear. Not something I've tried before but it should be fun, maybe a bit like tuning the soundboard on a guitar .

Matt.


Quote:
Originally Posted by ArcticBrew View Post
P.S. If you decide to revisit your EL34 plans let me know and I will post the working schematic.
I suspect my EL34 project is shelved indefinitely, but not permanently. I'll give you a heads up when I re-start. Better get this one working first!

Last edited by LeftHandFool; 29th December 2017 at 09:31 AM.
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Old 29th December 2017, 09:28 AM   #12
LeftHandFool is offline LeftHandFool  United Kingdom
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Thanks for the response, I'll try the anode to anode feedback from the RH84 design and see how I get on with that.

Quote:
Originally Posted by Shef View Post
Adding the el cap to 84's G2 will make it operating better though not as good as with being regulated.
The 'el cap' - I'm afraid you've lost me there!

Quote:
Originally Posted by Shef View Post
The UL would be better alternative, does OPT have the UL tap?
Unfortunately not.

Quote:
Originally Posted by Shef View Post
What you probably need more is the proven schematics, with both stages' operating points selected deliberately to mutually cancel the distortions. The B+ will matter in such.
I've designed the circuit myself, so there are no proven schematics. The one above is all we have to go on.
Interesting how similar it is compared to the RH84 design, although perhaps not that surprising given the comparative simplicity of the circuits.
I suspect the main difference is that the chap who designed the RH84 circuit knew what he was doing . Seems like he had a slightly controversial presence, one wonders what you have to do around here to get yourself banned!
I'll up the B+ before I go any further, see how that affects things.

Thanks again,

Matt.
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Old 29th December 2017, 09:49 AM   #13
LeftHandFool is offline LeftHandFool  United Kingdom
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Thanks for the advice. As mentioned above, I'll up the B+ before I proceed. Also, the ECC82 was a typo, it's an ECC81. I've posted the corrected schematic above.

Quote:
Originally Posted by 6A3sUMMER View Post
A few quick and easy things to try:

3. Less power but more damping factor, and smoother frequency response
across the low, mid, and high frequencies when driving a loudspeaker: Remove the 1.5k Ohm EL84 screen resistor, and connect the EL84 screen to the plate through a 100 Ohm resistor. (Triode Mode).
And distortion at the lower power will be less than the same power in pentode mode.
Will switching to triode mode not drastically reduce the power output? Or blow something up?
I've calculated output at 3.26W, but the datasheet states 1.95W max for triode connection. That doesn't sound like a good fit, unless I'm missing something?

Matt.
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Old 29th December 2017, 11:46 AM   #14
LeftHandFool is offline LeftHandFool  United Kingdom
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Default Progress...

Morning all,

I've spent an hour or so tinkering this morning. I've upped the B+ as suggested, here's the schematic with the measured voltages:

Single-Ended Pentode Power Amp.png

Checking the parts drawer, I've no high value resistors at 2W or above. I (briefly) auditioned 1W 470ok resistors anode to anode, keeping the volume low, but didn't want to push my luck.
It had a marked difference. I lost some volume, but the mid range was definitely tamed somewhat. I was listening through an appalingly trashy pair of speakers form a midi hi-fi I rescued from a skip (handily 8ohm and entirely disposable), so I won't form any firm opinions just yet.
Suffice to say I'll order a selection of 2W resistors and audition them through my actual speakers when they arrive.

Matt.
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Old 30th December 2017, 07:05 AM   #15
artosalo is offline artosalo  Finland
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Your amplifier's performance is at present, with pentode connected output stage and no negative feedback, similar to a low cost tube radio output stage from 1950's.

What do you want to achieve ? Real performance (low THD, flat frequency response, high S/N etc.) or just something that pleases your ears.
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Old 30th December 2017, 09:17 AM   #16
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by artosalo View Post
Your amplifier's performance is at present, with pentode connected output stage and no negative feedback, similar to a low cost tube radio output stage from 1950's.
You say that like it's a bad thing .

Quote:
Originally Posted by artosalo View Post
What do you want to achieve? Real performance (low THD, flat frequency response, high S/N etc.) or just something that pleases your ears.
I'm firmly in the latter camp at present. I'll leave real performance to people who know what they're doing...

Seriously though, I'm not out to set any records here. It's my first valve stereo project, I'm just out to learn a little more and have a little fun. Hopefully I'll have a working amp at the end of it, which pleases my ears.

As an aside, I've owned a couple of 'decent' amps over the years, but my favourite of all was an old Philips valve amp.
High end it was not, but it's controls were rather charmingly labelled 'Voluum', 'Hoog' and 'Laag' (if memory serves), and it made the Abbey Road sound fantastic.

Matt.
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Old 30th December 2017, 12:08 PM   #17
artosalo is offline artosalo  Finland
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Quote:
I'm firmly in the latter camp at present. I'll leave real performance to people who know what they're doing...
You have got suggestions how to improve the circuit, but it still remains basically the same. I don't see any suggested modifications applied.
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Old 30th December 2017, 12:27 PM   #18
scott17 is offline scott17
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If the 1K resistor going to the grid of the EL84 is supposed to be a grid stopper, then it's on the wrong side of the 270K grid leak resistor.
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Old 30th December 2017, 12:42 PM   #19
LeftHandFool is offline LeftHandFool  United Kingdom
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Quote:
Originally Posted by artosalo View Post
You have got suggestions how to improve the circuit, but it still remains basically the same. I don't see any suggested modifications applied.
True.

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.
I ordered some yesterday but I don't expect they'll arrive until the new year.
The updated schematic was just to show the new voltages with the increased B+, as previous posters had remarked it was on the low side.

Any suggestions for potential improvements?

Matt.
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Old 30th December 2017, 12:43 PM   #20
artosalo is offline artosalo  Finland
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If that 1k resistor is just at the tube socket, it works as it should.
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