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PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread
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Author:  amigafan [ 02 Oct 2018, 11:17 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Thanks for the nice words. Yes, the frequency response is surprising transformer data taken into consideration.
I am considering adding an active Baxandall tone control, I have a little difficulty hearing the high notes (my age).
It would also be nice to be able to regulate the bass.

Br Stig Hansen

Author:  gallileo1982 [ 14 Dec 2018, 05:00 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

gallileo1982 wrote:
Hi Bruce,
Thanks for the scheme.
I'm collecting parts and found cheap soviet tube 6N2P, which is described as equvalent of 5751. Parametres look the same, but there is some small difference of the heater pins:
5751 has three heater pins, 6N2P only two (no mid-tap). Does it mean, that 5751 can have only half heater working (for one triode) and 6N2P only whole heater?
As Podwatt uses only 1/2 of 5751 each channel, does 6N2P can be used as well?
Please have a look on the attached when you have a while.
Thanks!

Hi, The 5751 can be used on either 12 volts with the power applied to pins 4 and 5 (no connection to 9) or 6 volts if you tie pins 4 and 5 together and apply power between them and pin 9. The other tube is strictly 6 volts between Pins 4 and 5.
If you look at the schematics for the amp you will see that both triodes in the tube are used.



Hi Bruce,
You already replied to my question regarding heater wiring in russian tube 6N2P, however I'm still not sure how to apply it to EDCOR XPWR236 power transformer with two 6.3V wires and center tab.
Should I connect one 6.3V to pin 4, second 6.3V to pin 5 and center tap to common power ground? Did I understood properly?

Author:  gofar99 [ 14 Dec 2018, 08:53 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi, The 6N2 needs 6 volts between pins 4 and 5. It sounds like the transformer you are using is a 6-0-6 type as opposed to a 3-0-3. If it is the 3-0-3 then use the two three volt ones and do nothing with the "0" wire. If it is the 6-0-6 the you would use one of the 6 and the "0" to pins 4 and 5 on one of the 6N2 and use the other 6 and the same "0" to the pins on the second one for the other channel. I would split the EL84s the same way. Two on one side and two on the other. In that arrangement I would attach the DC voltage feed from the power supply (about 60 or so volts) that protects the tubes to the "0" on the transformer. It will give a uniform lift voltage to all the tubes.

Good listening
Bruce

Author:  gallileo1982 [ 14 Dec 2018, 09:54 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi,
I have exactly the version like on the picture, so 6-0-6. I'm going to follow your suggestion, so use one 6V output for each channel (for 6N2P and EL84).
Does it matter, if 6v AC will be sticked to pin 4 or 5 and same with 0?
Another question - where on the Power Supply schematics should I take 60V from? And to which 0 (BLK/RED or WHITE/BROWN) attach it?

Thanks in advance for clarification.

Author:  gofar99 [ 15 Dec 2018, 17:50 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi, The 60 volts (approx) is DC and comes from a tap on the power supply. In most configurations it consists of a 220K resistor from the filtered B+ to a 100K resistor that goes to the signal ground. There is a 1.0 to 2.2 uf poly capacitor from the junction of the two resistors. It also goes to the signal ground. The junction then is connected to the heater circuit. I have not found that it matters which side of the heater circuit it is attached to. Just be sure the heater circuit is not grounded in any way.

Good listening
Bruce

Author:  gallileo1982 [ 16 Dec 2018, 07:37 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

gofar99 wrote:
Hi, The 60 volts (approx) is DC and comes from a tap on the power supply. In most configurations it consists of a 220K resistor from the filtered B+ to a 100K resistor that goes to the signal ground. There is a 1.0 to 2.2 uf poly capacitor from the junction of the two resistors. It also goes to the signal ground. The junction then is connected to the heater circuit. I have not found that it matters which side of the heater circuit it is attached to. Just be sure the heater circuit is not grounded in any way.

Good listening
Bruce


Just to make myself 100% sure, please confirm it should be as on the picture (red remarks)?
Sorry if I ask obvious questions, but this is my first tube built and I wouldn't like to fry it...

Author:  gofar99 [ 16 Dec 2018, 09:26 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi, That was a file that was corrected later see the attachment with the correct placement of the one connection

Goofd listening
Bruce
Attachment:
Opera Zdjęcie_2018-12-16_131332_diyaudioprojects.jpg

Author:  gallileo1982 [ 16 Dec 2018, 10:41 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Bruce, thank you so much for the clarificatrion.
Last question (I hope) from my side - I will probably need DC 6v or 12v as power for delayed anode current relay switch.
Can I connect the Greatez bridge as below (to have 12v from both 6v)?

Author:  gallileo1982 [ 22 Jan 2019, 03:35 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

Hi all,
I finally finished my Poddwatt building.
Except an issue with positive feedback (due to wrong Edcor wires coloring) and wrongly soldered A+ (again, Edcor marks the wire as B+...) it works properly from very beginning, what makes me really happy :)
However I'm still struggling with EL84 bias set up.
Could someone please answer my questions:
1) how the bias should be measured properly? (with signal or not?, what volume pot position?)
2) where it should be measured? Schematics says "between TP1/TP2 and TP3/TP4. Value closest to 41mV I can measure only between TP2 and TP3? Is that correct?
3) where should be multimeter ground connected? To "global" amp ground or to TP2 or TP3? When I switch TP2 and TP3, voltage switches from positive to negative and so on.
4) Even when I succeed to set the bias between TP2 and TP3 on 41mV, it's swinging for about +/- 5mV. Is that correct?

Author:  ILoveHiFi [ 22 Jan 2019, 04:11 ]
Post subject:  Re: PoddWatt EL84 Push-Pull Stereo Tube Amp - Support Thread

So the lm317 is set to a constant current source with about 1.25v accross 15r resulting in v=ir 83.33mA of total current draw.
You want both valves to draw same current so the voltage accross 1R resistor (TP1 TP2 measuring point) should be 41-42mV (1R*0.5*83.33mA) = (1R*41.665mA).

Ultamatley voltage accross TP1 TP2 should be same as TP2 TP3 as same current.
The amp should warm up say about 30 minutes after cold start its fully warm and you adjust so tubes are same current. This way afterlong operating hours or burn in both tubes have similar current.

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