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Old 4th April 2011, 07:38 AM   #21
H.Michi is offline H.Michi  Japan
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Aforementioned residual noise level 0.004mV is incorrect.
It should be 0.004mV(A),based on JIS norm.
Internationally it is less than 0.01mV.
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Old 18th April 2011, 02:48 AM   #22
H.Michi is offline H.Michi  Japan
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Further touch up of 71A/6DJ8 sound processor

I have made a trial to the said sound processor by increasing HT voltage for improving distortion characteristics through a) changing a series resister in PSU, and b) grid bias voltage fine tuning for the best operation point of the
71A. Though, I was having a feeling of 2.5K ohm load impedance was too heavy.

Measures taken:
1) Replacement of a series resister 8.2K ohm with 5K ohm at HT circuit in PSU.

2) Replacement of a load resister 620 ohm with 1K ohm terminated at the secondary 600 ohm output coil of NP-8 with 1K ohm. This means primary impedance of NP-8 has assumingly become 4K ohm from 2.5K ohm.(1000/600 x 2.5K ohm = 4K ohm)

3)Grid bias voltage deepened.
Operating conditions of 71A have become similar to the power output stage.
By lowering grid bias voltage as well as setting the plate current to 18mA, distortion rate came down to 0.16% from 0.4% for 1V output. Additionally, its required input voltage downed to 1.02V from 1.3V for 1V output.
As a consequence, it has become a true "0dB ampifier" , and I can say the sound is smoother and nicer after this modification done.

Parameters:

Plate voltage: 151V
Plate current: 18.0mA
Grid bias: -30.1V

Output voltage vs. distortion vs. input voltage
1.0V 0.16% 1.02V
0.5V 0.05% 0.52V
0.1V 0.004% 0.105V
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Old 24th May 2011, 08:42 AM   #23
H.Michi is offline H.Michi  Japan
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Default Updated schmatics of 71A/6DJ8 sound processor

I have made minor modifications to the PSU and the Amplifier as follows:

 B+ voltage is to be always supplied to both tubes:71A and 6DJ8, for which the built in toggle switch was removed. So selection of the operation is only done by the filament voltage.
 Addition of a resister 3K ohm (5W) & a decoupling capacitor for lowering the operation voltage of 6DJ8 in the amplifier.
 Addition of a pair of Oil paper caps 0.47 micro F (1Kv) to the cathode of 6X4 (an idea adapted from the recent thread ).
 Addition of another bleeder resister 60K ohm (25W) in parallel in the PSU.

So far, I am fully content with the sound of either 71A or 6DJ8. However I do concern on the flicker noise level appeared namely in the 6DJ8. I suspect that this flicker noise level would influence badly to those of high order harmonics, though it is not audible noise. Is there any idea to overcome this problem? Never the less, I understand that choke input circuit is good for sounding but weak at the flicker noise characteristics.

Measured the residual noise level by VTVM(1mV and 3mV range)
71A: 0.04mV ~ 0.3mV
6DJ8: 0.1mV ~ 3mV
The inherent gain of 6DJ8 amp was of 12dB, while 71A was of 0dB.
Attached Images
File Type: jpg 71A-6DJ8.jpg (119.2 KB, 576 views)
File Type: jpg PSU.jpg (133.4 KB, 563 views)
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Old 10th August 2011, 09:21 AM   #24
H.Michi is offline H.Michi  Japan
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Default Updated schematics of 71A/6DJ8 switchable sound processor

I have made further touch up to the said sound processor as follows.
Now I am quite contented with the sound of both 71A and 6DJ8-SRPP.

1. Removed a load resistor 1K ohm terminated at the output coil of NP-8.
2. Some changes have been made in operating point of 71A:
*Plate voltage: 175V
*Plate current: 15.7mV
*grid bias: -40.3V

3. Input voltage vs. Output voltage vs. Distortion%
1.00V vs. 1.42V vs. 0.02%
2.00V vs. 2.90V vs. 0.028%
3.00V vs. 4.30V vs. 0.048%

By removing a load resistor of 1K ohm, improving distortion characteristics and an anmprlifier gain became 3.0 dB from 0 dB.
Attached Images
File Type: jpg 2011y08m09d_205641001.jpg (170.7 KB, 498 views)
File Type: jpg 2011y08m09d_210244369.jpg (110.6 KB, 488 views)
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Old 30th August 2012, 02:17 AM   #25
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by H.Michi View Post
Now I am quite contented with the sound of both 71A and 6DJ8-SRPP.
Hi Michi,

Do you have any more modifications ?

what schema of 71A is the best sounding for you ? best soundding has not always best measuring

Do you try 71A with plate choke ?
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Old 31st August 2012, 06:43 AM   #26
H.Michi is offline H.Michi  Japan
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Quote:
Originally Posted by tcqanh View Post
Hi Michi,

Do you have any more modifications ?

what schema of 71A is the best sounding for you ? best soundding has not always best measuring

Do you try 71A with plate choke ?
Tcqunh,

Thank you for your interested in my 71A pre-amp.
In the meantime, I have replaced Tango NP-8 in use with a new FM15K(200H)-150x2 .
First impression was of expansion of bass range and smoother in Mid and High range.
This output transformer is specially ordered, which is using Finemet core.
http://www.hitachi-metals.co.jp/prod...f/hl-fm9-e.pdf

This output transformer has a primary impedance of 15K ohm in series connection and 3.75K ohm in parallel connection. Secondary impedance is of 600 ohm.
Max current: 15mA/15K ohm
Primary inductance: 200H/15K ohms/5Hz

I feel this output transformer performs better.

I have not tried plate choke circuitry since I do prefer low impedance output design.

Hiro
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Old 1st September 2012, 12:49 AM   #27
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by H.Michi View Post
This output transformer is specially ordered, which is using Finemet core
This output transformer has a primary impedance of 15K ohm in series connection and 3.75K ohm in parallel connection. Secondary impedance is of 600 ohm.
Max current: 15mA/15K ohm
Primary inductance: 200H/15K ohms/5Hz
your OPT looks so good

Quote:
I have not tried plate choke circuitry since I do prefer low impedance output design.
plate choke is really economic solution with quite good sound quality. I can do one pair of this choke for myself (do it myself ) only with 10-15 usd = 1 pair of Ricken Ohms Resistor 2W

Zout is lower than 1.7K ohm at 135V, 17mA when using plate choke. I think this is not problem in case of interconnect cable <= 1m.
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Old 11th September 2012, 11:06 AM   #28
H.Michi is offline H.Michi  Japan
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Default The last version of 71A pre-amp

[QUOTE=tcqanh;3145060]Hi Michi,

Do you have any more modifications ?

what schema of 71A is the best sounding for you ?

For the past 18 months, I have been playing in developing or upgrading the 71A pre-amp on try and error basis. Now I would conclude this project as I have got an favorable results in all respects.

followings are the highlights:
*Change OPT from Tango NP-8 to Finemet core OPT,FM15K(200H)-150x2
*Load impedance from 2.5K ohm to 3.75K ohm

For the next project, I am considering to try 26 as it has been said in the community of its specific sound properties.
Attached Images
File Type: jpg 71A Pre-amp-1.jpg (191.8 KB, 342 views)
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Old 11th September 2012, 11:26 AM   #29
tcqanh is offline tcqanh  Viet Nam
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Join Date: Apr 2011
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Quote:
Originally Posted by H.Michi View Post
For the past 18 months, I have been playing in developing or upgrading the 71A pre-amp on try and error basis.
why do you choose the bias 97V/ 10mA instead of higher voltage and more current ?

any change in your taste, audio equipments?

what are your listening system ?

for me, the 71A pre amp is nearly done....
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Old 12th September 2012, 04:28 AM   #30
H.Michi is offline H.Michi  Japan
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Quote:
Originally Posted by tcqanh View Post
why do you choose the bias 97V/ 10mA instead of higher voltage and more current ?

any change in your taste, audio equipments?

what are your listening system ?

for me, the 71A pre amp is nearly done....
My taste become a sort of feeling warm and mild rather than wider in dynamic range and friquency responce, may be from age.

For the detailed information about my listening system:

Single-ended triode Amps using zero-bias transmitting tubes

Good luck for your preamp.
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