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PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread
http://www.diyaudioprojects.com/Forum/viewtopic.php?f=9&t=1232
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Author:  gallileo1982 [ 26 Feb 2019, 03:06 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

ILoveHiFi wrote:
ERSE Pulse X is recomended for low cost and top sound quailty, at this price point you get most worth of your money while being close to absolute best caps.
Any geniune metalised polypropene capacitors also work well for hifi, in genrall I think this is best bargin most quality per dollar.

I tried ECWFE and is really good, it beats a polar nichicon fine gold in all areas when used as input capcitor.
RS components you can but like $1 of goods but still get free shipping also fair price.



Hi,
thank you for recommendation, however ERSE Pulse X are not available in Europe and it doesn't pay to ship few items from US. So I'm rather looking for some European brand.
BTW, I'm still a noob in electronics, so could you please confirm if my understanding is correct - INPUT cap is green marked and COUPLING ones are red marked?
What's the role of 10uf cap (orange marked)?



Thank you in advance!

Author:  ILoveHiFi [ 26 Feb 2019, 16:41 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

The only capacitor that have influnece on sound quality is the 0.33uF 470pF and 100uF

the 0.33uf is the couppling capacitor between gain stages to prvent dc signals and allow ac signals (music)

The red ones are for regualting a voltage refrence so these can be cheap ones

orange capacitor is power supply capacitor for b+, This can be polar capacitor to save money

Author:  gofar99 [ 27 Feb 2019, 17:26 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi, For best linearity the second 0.33 needs to be similar to the coupling capacitor. It acts as a matching (close if you ignore the Ra of the SRPP stage) impedance for the gird of the second tube. I like to use the same capacitor in each location.

Good listening
Bruce

Author:  ILoveHiFi [ 27 Feb 2019, 18:31 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

gofar99 wrote:
Hi, For best linearity the second 0.33 needs to be similar to the coupling capacitor. It acts as a matching (close if you ignore the Ra of the SRPP stage) impedance for the gird of the second tube. I like to use the same capacitor in each location.

Good listening
Bruce

Is the second EL84 acting like a grounded grid ampflier with marked red circle capacitors and 0.33uF to Ground.

The SRPP feeds top EL84 and the voltage at top cathode will then go into the bottom el84 acting as grounded grid amp. Thus push pulling with single ended input.

If feels weried and I get confused with output stages drawing like this, I'm so used to the scheamtic drawings with the triodes or transistors next to each other.

Author:  gofar99 [ 27 Feb 2019, 20:46 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi, Well I never thought of it that way and I don't know if anyone else has mentioned it that way either, but sort of. The second tube does get its drive (cathode to grid signal) from changes on the cathode and thus needs to have the grid at a fixed reference. Thus the reason for matching (not exact but close enough) the impedance that the tube sees to ground with respect to the impedance the driven one sees. The circuit is based on an ancient one that used a resistor in the cathodes and not a CCS. It was workable but rather non-linear. (actually rather dismal behavior) The CCS is a huge improvement. The original circuit also just put the lower tube's grid right on the signal ground. Making the impedance mostly equal (part resistive and part via the capacitor for frequency adjustment) improves the linearity even more. The result is an amplifier that is very wide band, highly linear with a full power distortion level typically in the 1-2% range. All with nearly no negative feedback. That is there only to insure stability well above the audio band. With no NFB the response is quite significant out to the 75-90KHZ range with the components used. The NFB brings it down to the 30-35K range and away from an inherent transformer resonance point at 70kHZ. The primarily limitation of the design is that it only operates in class A mode. The CCS prevents it from going AB. This naturally limits the output power but IMO that is easy to solve, just build the bigger versions. They go up to KT120 with 40-45 watts RMS.

Good listening
Bruce

Author:  ILoveHiFi [ 27 Feb 2019, 22:17 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Yeah I think we are on the same page with the schematic design.

Author:  gallileo1982 [ 07 Mar 2019, 05:45 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi guys,
Thank you for detailed explanation. I will order some caps soon and compare.
Anyway after few weeks of listening I have concluded, that mid and high ranges are very clear and detailed, however bass is quite thin (but still very well controlled).
Do you think that bass character depends on some parts quality (e.g. thin signal wires) or it's just typical for this design?

Author:  ILoveHiFi [ 08 Mar 2019, 01:55 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Thinn bass is typicall for tube amps. However some amps lack bass due to poor schematic design having cutt of frequency at 10hz or above. This one has cut off at arround 3hz.
Thinn wires don't make any diffrence as impedance between connections is high.

Author:  gofar99 [ 09 Mar 2019, 10:56 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi, these amps are not bass shy. Check that the NFB is properly phased. Temporarily disconnect it and see if the sound goes up or down. If it goes down then it is backwards and will have poor bass, if it goes up then it is correct. If it is correct, then something is amiss elsewhere.

Good listening
Bruce

Author:  gallileo1982 [ 11 Mar 2019, 12:39 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi Bruce,
I checked as you suggested.
Disconnected right channel NFB and it has gone noticerably louder, so apparently NFB is connected properly.
However with NFB disconnected I noticed that bass was really stronger. I also compared to left channel (with NFB connected), playing sample file 60Hz sound and in L heard almost nothing, while in R it was quite loud.
That's weird....

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