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10W+10W EL34+ECC83 single-ended Class A Stereo amplifier Kit wlx
10W+10W EL34+ECC83 single-ended Class A Stereo amplifier Kit wlx
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Old 15th March 2017, 05:47 AM   #31
jazbo8 is offline jazbo8
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10W+10W EL34+ECC83 single-ended Class A Stereo amplifier Kit wlx
@phi112358 - Please keep the discussion on topic, your OT post above has been removed.
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Old 22nd April 2017, 06:54 PM   #32
phi112358 is offline phi112358
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im going to use this as an output stage afaik. This 12ax7 el34 SE 10w*2 amp will live between my the SE DAC which outputs via both headphone and 2*rca jacks at an estimate at 1v or equivalent to your average home CD player, and my pair of full range speakers which are 8" 35w. The source (DAC) will be fed from a DC lab psu. On the psu there is an option of 1.5A or 3A. Looking at its design, the 3A mode uses two transistors where the 1.5A mode does not use them. Both are running through a rectifier and filters(of course) . If there are any theoretical advantages to one mode or the other, I would like to hear any thoughts about them. (The power source for the DACs is using a multi tap transformer (~12 taps).

The el34 12ax7 se amp will be using a sovtek 12ax7 and 2 jj el34s and fed from a massive 220v>240v and 6v transformer, and I will post info on OTs asap. I'm looking for the best schematic available for that use Will post OT info ASAP

Thanks
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Old 2nd February 2018, 10:07 PM   #33
wishmasterX is offline wishmasterX  Czech Republic
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Hi, Guys ...
I´m little confused about negative feedback connected to secondary on output transformer ... I have SE EL84, schematics comes from old Stereo Czechoslovakia amplifier from year about 1966, in original version, there´s OT for "Hi-Fi" use, NFB with 2K7, I use OT from old televisions ... i like to build new amp with the same tubes and I like to use toroidal trans from Toroidy.pl, TTG-EL84 ... I still need NFB for this OT ?
Thank´s for your opinions ...
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Old 3rd February 2018, 07:08 AM   #34
artosalo is offline artosalo  Finland
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It would be much easier to give a correct answer if you showed the schematic.
In general, if you want to make Hifi, you need some sort of negative feedback.
In this context Hifi means a certain measured performance, that has nothing to do with "good sound".
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Old 15th October 2018, 11:30 AM   #35
josnio is offline josnio  Portugal
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Good morning, after a few months of using this amplifier with 8ohms speakers I intend to use it with a new ones with 4 ohms impedance. Is it enough to connect to the correct transformer output?
I have these: TTG-EL34SE - Tube output UL transformer [3,2kOhm] EL34 / 6L6 SE - Shop Toroidy.pl

Will I have to change the value of R7? Will I expect greater output power? Any change in the supply voltage? Grateful for the attention
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Old 15th October 2018, 12:04 PM   #36
quassar is offline quassar  Romania
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It depends, if you use EL34 or 6L6 then the transformer you are talking about will do good. no need to re-bias, if the primary impedance is the same on the new transformer, but it is not bed to check it after you change the transformer. Not all the transformers are born identical.

You can search for a transformer that has 0 - 4 - 8 ohm output.
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Old 15th October 2018, 01:07 PM   #37
josnio is offline josnio  Portugal
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I junt want to use the amp with other speakers (4ohms) mantain everything the same. My tubes are EL34.

I was doing some measurements and got: I over R1 (artosalo drawing) 72mA with R1 390 ohms, Vplate (EL34) 358V. can someone tell me what power are the el34 to dissipate? Are they working in a safe zone?
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Old 15th October 2018, 11:51 PM   #38
6A3sUMMER is offline 6A3sUMMER  United States
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josino,

If R1 is the EL34 self bias cathode resistor, then:

390 Ohms cathode resistor, and 72mA (0.072A)
390 x 0.072 Amps = 28 Volts
358 Volts - 28 Volts = 330 Volts Plate to Cathode

330 Volts x 0.072 Amps = 23.76 Watts plate plus screen dissipation; the EL34 plate is rated for 25 Watts.
Recommendation: Use Good EL34 tubes.

The Toroidal transformer you referred too has both 4 ohm and 8 Ohm taps. Use the 4 Ohm tap for the 4 Ohm speakers.

Last edited by 6A3sUMMER; 15th October 2018 at 11:54 PM.
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Old 16th October 2018, 08:26 AM   #39
josnio is offline josnio  Portugal
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Thank you very much for the quick reply. I had an idea what these calculations would be .... I'm using JJ EL34, Do you have any (make / model) that suggests?

Well, i was wrong, i use EL34EH from electro Harmonix. I did some measurements today with the oscilloscope and got about 5W rms (charge of 8ohms, input 450mv and 355V in EL34) in UL mode. Is it typical value? Will I get less in triode mode, right? If switching output transformers can increase the value too? Thanks for help
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Old 17th October 2018, 06:06 PM   #40
6A3sUMMER is offline 6A3sUMMER  United States
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josino,

A current schematic of your amp would be helpful, including voltages, resistances, transformer impedance and screen tap %.

You list an input of 450mV. Did you increase the input level until you got too much distortion, and then back it down to 450mV where the distortion was reasonable? That is a measurement for 'undistorted power'.

When you get 5 Watts out for 450mV input (with the volume control turned to maximum) are you asking "is that normal"? If you convert the 5Watts into 8 Ohms into a voltage, then Vout/Vin is a 'measurement of Gain'.

Undistorted power, and Gain are two different things.
You can have lots of gain but no power, you can have lots of power but no gain, or you can have both.

5 Watts at OK distortion levels may be perfectly normal for a single ended ultra linear mode EL34 that has about 24 Watts plate plus screen dissipation. That is 5/24 = 0.208, or 20.8% efficiency.
What is you amplifier's distortion percent at 5 Watts, either THD, or 2nd and 3rd harmonics?

Last edited by 6A3sUMMER; 17th October 2018 at 06:17 PM.
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