71A

[H.Michi]

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.

 

[H.Michi]

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

 

[H.Michi]

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.

 

[H.Michi]

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.

 

[tcqanh]

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 ?

 

[H.Michi]

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/prod02/pdf/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

 

[tcqanh]

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 🙂

http://www.hitachi-metals.co.jp/prod/prod02/pdf/hl-fm9-e.pdf

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.

 

[H.Michi]

The last version of 71A pre-amp

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.

 

[tcqanh]

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....

 

[H.Michi]

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.

 

[tcqanh]

For the detailed information about my listening system:

Single-ended triode Amps using zero-bias transmitting tubes

Good luck for your preamp.

Thank you Michi,

It seems that you like 811A very much, also the other transmitting DHT. I have some pairs of 811A but I hesitate on building the ampli with this tube. Many friends in my country don't like their sound, agressive and harsh...

 

[tcqanh]

First I used plate choke of 50H, bass was a little bit weak. Now I exchange to 100-150H choke (I have 2 manual LCR meters, one show 100H, the other show 150H at 120Hz scale 🙂), so bass become better. But I feel that bass is not tight very much when compared to my solid state preamp.

midrange and treble are fanstastic. high frequencies get better, sparkle with 150H plate choke.

here is my schematic

 

[H.Michi]

Thank you Michi,

It seems that you like 811A very much, also the other transmitting DHT. I have some pairs of 811A but I hesitate on building the ampli with this tube. Many friends in my country don't like their sound, agressive and harsh...

Yes, Mr. Sisido also mentioned that the sound of 811A SE amp once he built it carelessly was disaster. The energy was shifted for high and mid range and poor in low.
Finally he succeeded for adopting custom ordered OPT(EX-Tango#11268) and Inter-stage transfomer(#11248).
The 811A is a very sensitive tube and its characteristic is similar to pentode, so that it has a large r(p).
The best way of changing characteristic be quiet is use suitable cathode NFB.
It`s a kind of "The taming of the shrew", you can enjoy a process of how to control a difficult tube and finally get well balanced sound.

 

[tcqanh]

...some updates of Filament circuit and operating point for 71A. Dynamic and bass become better much, very low AC noise 🙂

 

[Noodle88]

Hi H. Michi,
Have you trying the 71a preamp with Tamura A-4714 line output? I'm planing to built 71a preamp with it.
Regards,
Cheah

 

[H.Michi]

I have not tried the 71A preamp with Tamura A-4714 opt.
I built a 71A main amp employing A-4714 in mid 90s.
The line-up was 1/2 12AX7 - 71A - A-4714(6 ohm output).
I am sure you will get a good result for your 71A(single) preamp with A-4714(600 ohm output), though the gain should be less than 0 dB.
Hiro

 

[Noodle88]

Hi Hiro,
I'm thinking of adding a Lundahl LL1674 1:4 step up transformer in front of the 71a tube as first option. 2nd option will be adding a 01a tube driver stage for the 71a. What do you think?
Cheah

 

[Noodle88]

Hi Hiro,
I'm planing to add a Lundahl LL1674 1:4 step up transformer in front of 71a as option one. Another option would be adding a 01a driver stage for the 71a. What do you think?
Cheah

 

[nvduybom]

71A Preamp

I have plans do 71A preamplifier, but it's delay for a long time.🙁
The schematic was designed by Matsunami-Japan, I have done a few projects in his books, very good results .
I use Hashimoto HL-20K-6 with parrallel primary for 5K:600 Ohm .

 

[Noodle88]

Thanks for sharing nvduybom,

I'm looking for a higher gain from the 71a pre. Looks like i need to add a DHT driver stage. 01a? 26?
Anyone could help?

Regards,
Cheah