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EL84PP, a simply circuit
EL84PP, a simply circuit
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Old 26th September 2021, 02:42 PM   #1
waltube is offline waltube  Italy
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Default EL84PP, a simply circuit

Hello!

This is the last circuit I have developped around a push-pull of EL84.
The intention was to make a simply solution with a very good OT trafo.
In attach the schematic: it use a 6922 with the first section at gain and the second as cathodyna.
Then the output stage with fixed bias and a pentode connection ( my preferred).
The OT trafo is a double C core, laminated 0,1 mm, with 39:1 ratio.
The secondary is one fixed at 6 ohm nominal; the ratio is very high this because the intention was to fix the bias point in full class A until the max power.
The second image is the proto of the OT trafo and it is possible to see one chassis this because the entire project is a mono amp so there are two chassis.
The third photo is the pcb with all components on board included the tubes.
At the test the output power is 10 w.
The Vdc is 300 volt and bias current is 40 mA each 84; the filter caps are 1000 uF total.
The diagram of the freq. answer is without feedback that in this project is 9 dB. The power is 1 watt/8 ohm.
At -3dB the cirve is good; 15 Hz to 85 kHz
The other diagram is the response with feedback; - 1 dB at 10 Hz and 120 kHz ( around).
The third response is with a 0,22 uF in parallel to 8 ohm; also a good response.
The feedback is applied without any compensation cap and this is a good results.
The lasts are the wave taken at the 1 ohm resistor of the 84 so it show is this circuit works in class A.
One is at 1 watt, the other is around 10 watt, close to the switch in AB.
Last is the square wave 20 kHz, a good results ( I have taken other screenshot at differetn frequencies, always fine).

NOTE: this is a project Simulation FREE!! and Solid state FREE!! ( there is only the diode bridges)



Walter
Attached Images
File Type: jpg EL84_PP_schema.jpg (245.0 KB, 751 views)
File Type: jpg IMG_1164.JPG (116.2 KB, 746 views)
File Type: jpg EL84-PP_board.jpg (444.1 KB, 689 views)
File Type: jpg EL84-PP_board_2.jpg (456.3 KB, 668 views)
File Type: jpg EL84PP_risp_1w_8ohm_noFB.jpg (122.6 KB, 684 views)
File Type: jpg EL84PP_risp_1w_8ohm_9dB_FB.jpg (121.9 KB, 129 views)
File Type: jpg EL84PP_risp_1w_8ohm_9dB_FB+0,22uF.jpg (124.9 KB, 98 views)
File Type: jpg EL84PP-1w_classA.jpg (49.0 KB, 117 views)
File Type: jpg EL84PP-10w_fine_classA.jpg (50.8 KB, 98 views)
File Type: jpg EL84PP_140mVin_square_20kHz.jpg (43.3 KB, 112 views)
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Old 28th September 2021, 06:04 AM   #2
waltube is offline waltube  Italy
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I forgot to put a photo of the chassis, one channel

Walter
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File Type: jpg IMG_1165.JPG (110.7 KB, 280 views)
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Old 28th September 2021, 06:44 AM   #3
Drewaudio is offline Drewaudio  New Zealand
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Nice!

Where did you get the output transformer?

Andrew
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Old 28th September 2021, 05:30 PM   #4
waltube is offline waltube  Italy
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Hi Andrew

the OT are custom made from a little company near me. They wound only trafo on C nucleus laminated 0,1 mm; for 300B (also s.e.) , EL34/KT88 and 84
The idea is to offer the kit.

Walter
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Old 28th September 2021, 07:01 PM   #5
waltube is offline waltube  Italy
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A little correction:

They wound for me only trafo on C nucleus laminated 0,1 mm

Walter
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Old 29th September 2021, 06:00 AM   #6
6A3sUMMER is offline 6A3sUMMER  United States
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waltube,

A C-Core transformer, but enclosed with traditional E-I End Bells?
Right?

That is a great idea!
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Old 29th September 2021, 09:27 AM   #7
waltube is offline waltube  Italy
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Hi 6A3,

yes. I found two types of bells that fit good , one for this OT ( around 15 watt at 30Hz) and other for 300B s.e. and EL34/KT88 around 40 watt at 30 Hz

Walter
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Old 2nd October 2021, 11:26 AM   #8
waltube is offline waltube  Italy
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A pair finished.

Walter
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Old 2nd October 2021, 03:17 PM   #9
Windcrest77 is offline Windcrest77  United States
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EL84PP, a simply circuit
Nice!
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Old 2nd October 2021, 06:54 PM   #10
6A3sUMMER is offline 6A3sUMMER  United States
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waltube,

Congratulations,
Looks great!
I bet it sounds great too!

With a square wave that looks that good, with only a resistor for negative feedback and no comp cap across it . . .
That means the output transformer is Superb.

i would like to have a pair of those transformers, and 'matching' end bells.
Too expensive, especially with shipping.

Last edited by 6A3sUMMER; 2nd October 2021 at 07:02 PM.
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