01a > 71a DHT Line Preamplifier.

by noodle88 Fri May 10, 2013 3:58 pm

After playing with my 45 DHT preamp for more than 2 years, my hand start to itchi. As a DIYer, I always explore new possibilities and try out new ideas.

By reading the post from DIYHIFI forum, I start to realise that the implementation of my 45 preamp may not be optimum.

My 45 preamp
S&B TX102(+6db) TVC > 1:4 step up trans > 45 > Tamura A4714 line output transformer.

Many may say the TVC is a difficult load for most source. More over the TVC is follow by a 1:4 step up transformer, this will make it even harder to drive.

After doing some research, I decided to built a 71a preamp as below:

Input > 71a > Tamura A4714 > Slagle AVC > output

With this, there will not be any complex load in front of the 71a tube. Who knows it will sound better.

Cheers,
Cheah

by mthoi Sat May 11, 2013 12:23 am

Cheah - A 71a would be an interesting project. Keep us updated on the progress.

I can offer my 76 preamp (cap coupled - dact/76/6SN7) or 26 preamp for sonics comparison.

Have you tried the TVC after the 45's OPT?

by noodle88 Sat May 11, 2013 9:40 pm

Hi Mthoi,

Thanks for your offer, Why not we arrange one day to exchange note between my 45 and ur 76 & 26.

As my 45 preamp is not a DIY unit, I do not mod it. But I will try my spare pair of S&B TX - 101 TVC after the Tamura A-4714 on the 71a preamp.

Cheers,
Cheah

by noodle88 Sun May 12, 2013 10:20 pm

Hi,

Finally I decide to get a partial 71a DHT preamp kit from Thomas Mayer ( Vinylsavor ). I like his style of making amplifier, all passive supplies for B+ and filaments, using LCLCLC filter in B+, using transformer couple through out his design. These old school style of construction enable the amp to last. He is very helpful and provide me with technical support for this project.

01a > 71a DHT Line Preamplifier. Image16

01a > 71a DHT Line Preamplifier. Image16

01a > 71a DHT Line Preamplifier. Image18

Above are some high quality parts that supply by Thomas.

As 71a has a MU of 3, Tamura A-4714 line output (5k: 600 ohm) is a 2.9:1 step down. So the total gain of this preamp has only 1.03 . As my poweramp only needs 0.25 VRMS drive into full power, using a 2 VRMS output CD player as source will enable this preamp to have enough gain and headroom to drive my 300B power amp.

Cheers,
Cheah

by noodle88 Thu May 16, 2013 10:01 am

Hi,

Some photos of the power supply.
01a > 71a DHT Line Preamplifier. Image19

Above are part of the LCLCLC filter for B+. Lundahl Chokes are being used. ASC oil cap for B+. Aluminium bar for sub-chassis.

01a > 71a DHT Line Preamplifier. Image21

01a > 71a DHT Line Preamplifier. Image21

Power supply in Aluminium casing.

Cheers,
Cheah

by noodle88 Fri May 17, 2013 10:10 am

Hi,

01a > 71a DHT Line Preamplifier. Image23

Added filement supplies for 71a. To ensure this preamp free of hum, separate DC heater is supplying each DHT tube. LCL filters are being used.

01a > 71a DHT Line Preamplifier. Image24

01a > 71a DHT Line Preamplifier. Image24

Cheers,
Cheah

by noodle88 Mon May 20, 2013 5:43 am

Hi,

01a > 71a DHT Line Preamplifier. Image25

Fitted the B+ transformer and 2 heater supply transformers. These transformers are very nicely made and also supplied by Thomas Mayer.

01a > 71a DHT Line Preamplifier. Image26

01a > 71a DHT Line Preamplifier. Image26

All power transformers are suspended with rubber dampers.

01a > 71a DHT Line Preamplifier. Image27

4 pcs of 6ax4 tv dampers are used as B+ rectifier. CMC Teflon tube sockets is being used. A piece of ebony is used to hold those tube sockets.

Cheers,
Cheah

by mthoi Mon May 20, 2013 6:04 pm

Cheah - That looks like good progress. And some serious quality components in the build.

And you probably burned the mid-night oil doing the project since you posted at 5+am.

We should meet-up to compare notes when you have it fully assembled. Reg.

by noodle88 Tue May 21, 2013 5:43 pm

Hi Mthoi,

I will surely invite you over ones my 71A preamp is completed. But I need few more months. Very Happy

All these photos are just the PSU section, I still have the signal section to complete.

Cheers,
Cheah

by vt4c Tue May 21, 2013 8:45 pm

Cheah,
Nice work! Love to hear your work one day

by noodle88 Wed May 22, 2013 9:45 am

Hi,
Why not we(DIY Kaki ) meet up this coming KLIV to have tea and exchange ideas ?

Cheers,
Cheah

by lhgin Wed May 22, 2013 1:52 pm

Cheah,

When you import the transformers and parts from Vinylsavor, did you get any trouble from the Malaysia customs? I don't like importing expensive components due to taxes. Sad

By the way, is the frame part of the kit too? I wonder if I can get the frame material locally.

I like your style, going all out! Razz

Btw, there are some silver OPTs available from Vinylsavor too. Evil or Very Mad

Thanks!
Ken

by lhgin Wed May 22, 2013 3:49 pm

noodle88 wrote: Many may say the TVC is a difficult load for most source. More over the TVC is follow by a 1:4 step up transformer, this will make it even harder to drive.

Yes, it is. With weak driver/source, the bandwidth will suffer. Usually the input impedance for tube power-amp will be around 50-100k. In order to have low enough distortion, the secondary of the TVC will have to have around 5-10k output impedance. With that, the primary impedance will be 5-10k, and lower if a step up is required. That is not a friendly load for weak source.

Ken

by vt4c Wed May 22, 2013 9:16 pm

Good idea! Very Happy

noodle88 wrote:Hi,
Why not we(DIY Kaki ) meet up this coming KLIV to have tea and exchange ideas ?

Cheers,
Cheah

by noodle88 Wed May 22, 2013 11:02 pm

lhgin wrote:Cheah,

When you import the transformers and parts from Vinylsavor, did you get any trouble from the Malaysia customs? I don't like importing expensive components due to taxes.

By the way, is the frame part of the kit too? I wonder if I can get the frame material locally.

I like your style, going all out! Btw, there are some silver OPTs available from Vinylsavor too.

Thanks!
Ken

Hi Ken,

Those equipment show on the photo above are custom made by a Englishman locally. Those are running on switching power supply. But this round I planed to follow the old school way which all passive filter will be used. Better reliability, maybe?

> So far so good, no problem with custom at all. Thomas(vinylsavor) is very helpful and parts that he supply are very high in quality, may not be able to get them locally. He price them reasonably. So, for me worth paying tax. Very Happy

> I don't think we can get those Aluminium Bar locally, those bar are not soft like local aluminium.

>I'm planed to use Tamura A-4714 line output transformer for this project, so I didn't opt for those silver OPT

Cheers,
Cheah

by noodle88 Wed May 22, 2013 11:08 pm

vt4c wrote:Good idea!

noodle88 wrote:Hi,
Why not we(DIY Kaki ) meet up this coming KLIV to have tea and exchange ideas ?

Cheers,
Cheah

Hi!

Please set a date !

Cheah

by noodle88 Thu May 23, 2013 10:48 am

Hi !

It's turn to hook up all the wires. This process may take some time as I'm a believer of cables are directional. Very Happy

01a > 71a DHT Line Preamplifier. Image30

01a > 71a DHT Line Preamplifier. Image31

The power supply section is 70% completed !

My initial planing for signal section

01a > 71a DHT Line Preamplifier. Image32

01a > 71a DHT Line Preamplifier. Image33

Cheers,
Cheah

by vt4c Thu May 23, 2013 9:25 pm

Hi Cheah,
For me, I'll be back in M'sia on 06/07 and will only make it on Sunday (07/07)

noodle88 wrote:

Hi!

Please set a date !

Cheah

by noodle88 Sat May 25, 2013 9:24 am

Hi,

Here's the signal section of my 71A preamp. I used a nice black aluminum casing for this. The rare Tamura A - 4714 is sitting in the middle shows that they are the main cast. 4 Pcs of ASC X386 oil cap is used for filtering and once again Teflon UX-4 tube socket is being used.

01a > 71a DHT Line Preamplifier. Image34

01a > 71a DHT Line Preamplifier. Image34

Preamp with ST type 71a tubes.

01a > 71a DHT Line Preamplifier. Image35

Globe type 171a tube fitted.

VT4C - 7/7 is fine for me, will confirm time with you when the date is approaching. Will share some tweaks with you when we meet up.

Cheers,
Cheah

by noodle88 Tue May 28, 2013 11:37 am

Hi !

01a > 71a DHT Line Preamplifier. Image36

Here's the signal section top plate.

01a > 71a DHT Line Preamplifier. Image37

In order to keep in put signal path short, input RCA jack is place in front of the Tamura A4714. All 4 ASC oil cap painted metallic silver to match the colour of A4714.

01a > 71a DHT Line Preamplifier. Image38

01a > 71a DHT Line Preamplifier. Image38

Ones again, Lundahl plate and filament chokes are being used. It's time to wire up the signal section !

Cheers,
Cheah

by noodle88 Wed Jun 05, 2013 10:25 am

Hi !

01a > 71a DHT Line Preamplifier. Image39

Wiring for the signal section has completed 80% ! No exotic hook up cable is being used. For me, I'm very particular in wire direction, I always make sure wire direction for both channel is identical. So, all wire direction has been marked and place correctly.

01a > 71a DHT Line Preamplifier. Image41

01a > 71a DHT Line Preamplifier. Image41

01a > 71a DHT Line Preamplifier. Image42

Cheers,
Cheah

by vt4c Wed Jun 05, 2013 9:02 pm

Cheah,
Very nice!

Do keep me posted on the date. Thanks Very Happy

by noodle88 Wed Jun 12, 2013 7:12 pm

Hi !
The signal section is completed!
01a > 71a DHT Line Preamplifier. Image47

Time consuming part is to wire up the Slagle AVC and Seiden Volume Switch. To make life easier, mono Seiden switches are being used. 0.28mm Triple insulated copper is used between AVC and volume switch, where else the output is wired with teflon insulated solid core silver which supplied by Thomas Mayer( Vinylsavor ).

01a > 71a DHT Line Preamplifier. Image46

01a > 71a DHT Line Preamplifier. Image46

Test No. 1 : Rectifier heater - OK !

Cheers,
Cheah

by Wikin Sat Jun 15, 2013 7:58 am

Lots of work ahead Cheah, nevertheless thumbs up to you !!

cheers

by noodle88 Mon Jun 24, 2013 1:15 pm

Wikin wrote:Lots of work ahead Cheah, nevertheless thumbs up to you !!

cheers

Hi Wikin !

Thank you for reminding me. I'm well aware that making a good sounding preamp is not easy. It would be much much harder to built a all new preamp to sound better than my 45 pre which oredy set a very standard. It's not easy but not impossible as well !

That's why I took a short cut to seek for Thomas Mayer's (Vinylsavor) help. He is very helpful in making this project a successful one.

Cheers,
Cheah

by noodle88 Mon Jun 24, 2013 4:11 pm

vt4c wrote:Cheah,
Very nice!

Do keep me posted on the date. Thanks

Vt4c, which day do you planed to visit KLAV2013 ?

Cheah

by vt4c Mon Jun 24, 2013 9:06 pm

Cheah,
It will be on Sunday. Thanks. Will PM you my contact number.

by noodle88 Tue Jun 25, 2013 3:55 pm

Hi !

01a > 71a DHT Line Preamplifier. Image53

01a > 71a DHT Line Preamplifier. Image54

01a > 71a DHT Line Preamplifier. Image55

The 71A DHT Preamp is up and running !!

Current test equipments:
Source : Marrantz CD60
Preamp : 71a DHT Preamp
Power Amp : Conrad Johnson Premier 11a
Speaker : TAD TSM-300

How does it sound?
It plays good music and sound promising !

Does it hum?
No, dead quiet.

Micro phonic ? Does it sing along the music?
Low level of micro phonic, doesn't sing along with music. May be improve further by isolate tube base from chassis.

More fine tunning need to be done, will update u guys.

Cheers,
Cheah

by mthoi Tue Jun 25, 2013 5:29 pm

Cheah - CONGRATULATIONS!!

We are all in anticipation to listen to your latest baby...

by noodle88 Tue Jun 25, 2013 7:07 pm

Hi Mthoi,

Feel free to give me a callif you don't mind to listen to it at current position.

Cheers,
Cheah

by noodle88 Sat Jun 29, 2013 3:08 pm

Hi!

After few days testing, I decided to move the 71A pre to my main system last night.

01a > 71a DHT Line Preamplifier. Image10

I do have a practice that always power up my system with volume off.
Then slowly crank up the volume, its dead quiet from volume step 1-15.
Starting from step 16-20, some high frequency oscillation noise 'E...e...e....' Can be heard from speaker. The noise getting louder as volume increase!
It's strange, I never had this before.
Due to John's made 300B amp grounding system is a bit complicated, i suspect some mismatch between 300b amp and 71a pre.

I email Thomas Mayer and tell him what's happening. Within an hour, I got his reply.
After a few email and troubleshooting, the noise stop !
I'm so grateful that Thomas could response to my problem and solve it so quickly.
Now I'm a happy man !!

Cheers,
Cheah

by sph Sun Jun 30, 2013 9:11 am

Looks AWESOME! I envy you guys for having the knowledge to embark on such projects.
I'll be a happy man too if I could achieve 50% of what you have accomplished.

by noodle88 Sat Oct 12, 2013 10:54 pm

Hi!

01a > 71a DHT Line Preamplifier. Image17

01a > 71a DHT Line Preamplifier. Image20

It's been more than 3 months finish building this 71A line preamplifier. I had made a few changes into it.

1) All signal wire changed to silver wire which supply by Thomas.

2) Cerabase Compact as footer for signal section.

3) Lundahl 1:4 step up transformers are added.

As the total gain of this preamp is close to 1, it acts as a buffer. In order to have enough gain and head room to drive my poweramp, a pair of step up transformer needed.

How does it sound ?
This preamp is very revelling, any changes in up stream it will show. Very quite, hum free, ultra low microphonic.
It plays clean music, well balance from hi to low.
This preamp is very different from all the preamp I own/ built, it sounded very natural. The more I listen to it the more I like the Thomas's preamp concept.

Cheers,
Cheah

by sss333 Fri Oct 18, 2013 11:52 am

noodle88 wrote:Hi!

After few days testing, I decided to move the 71A pre to my main system last night.

I do have a practice that always power up my system with volume off.
Then slowly crank up the volume, its dead quiet from volume step 1-15.
Starting from step 16-20, some high frequency oscillation noise 'E...e...e....' Can be heard from speaker. The noise getting louder as volume increase!
It's strange, I never had this before.
Due to John's made 300B amp grounding system is a bit complicated, i suspect some mismatch between 300b amp and 71a pre.

I email Thomas Mayer and tell him what's happening. Within an hour, I got his reply.
After a few email and troubleshooting, the noise stop !
I'm so grateful that Thomas could response to my problem and solve it so quickly.
Now I'm a happy man !!

Cheers,
Cheah

So what the main cause of the noise while volume getting high?? grounding??

by noodle88 Sat Oct 19, 2013 9:13 am

Hi sss333,

The noise is due to some grounding issue in the poweramp. Nothing wrong with the 71A line preamplifier, its dead quiet even though playing with 99db speaker.

Cheers,
Cheah

by noodle88 Thu Oct 31, 2013 10:04 pm

Hi!

I have move my source, phono and preamp to side wall. Living only my pair of 300B power amp in front.

01a > 71a DHT Line Preamplifier. Image21

The result is well worth the hassle !

In order to do so , I have to run a pair of 10 meters unbalance interconnect from preamp to poweramp. As this 71a preamp is being transformer couple, no noticeable different in sound quality sported.

Cheers,
Cheah

by noodle88 Thu Nov 07, 2013 11:10 am

Hi!

As I'm enjoying good music from my system, an idea suddenly strike my mind. 'How about modding the 71a preamp to accommodate other DHT tubes with 5v heater'. If I like the sound of this new preamp, I'll built a new preamp signal section and use back the same 71a PSU. With this I can save about 50% of the cost.

Looking into tube data sheet and my collection of DHT tubes, 12a and 01a are being spotted.

So happen that the Lundahl line output transformer LL1660 for my 10y / 801a preamp project just arrived. OK ! I can use it for the 12a preamp.

Stay tuned!

Cheers,
Cheah

by noodle88 Sun Nov 10, 2013 1:06 pm

Hi!

Just modded my preamp to accommodate 12a.

01a > 71a DHT Line Preamplifier. Image24

This design is done with 2 * 3.9 uf Mcap silver/ oil pre channel as ultra path capacitor.

01a > 71a DHT Line Preamplifier. Image27

It's seems that all DHT preamp sounded good when done right !

Cheers,
Cheah

by noodle88 Wed Nov 13, 2013 10:16 pm

Hi!

After listen to the 12a for a week, I modded this preamp to use filament bias. Meaning to say, there wouldn't be any capacitor in its direct signal path.

Although the B+ still a bit lower than targeted and this opt is not optimised. The result is stunning !!

Consequently, I did ordered a pair of LL1660 line transformer which optimized for 12A.

Cheers,
Cheah

by vt4c Wed Nov 13, 2013 10:36 pm

Cheah,
Great stuff! Very Happy

by noodle88 Fri Nov 22, 2013 12:18 pm

Hi!

A week ago, I did push up the 12a plate voltage to 155v. This preamp sounded even more stunning !! The voice is more focus, bass goes lower and sound open up even more.

I did invited a few good friends of mine for an audition. They concluded that the current combination performed way better than the previous 45 pre + 300B power combo.

All these have strengthen my trust towards passive filtering and Thomas Mayer's building concepts.

Ones again I would like to thank Thomas for his help and guidances to make this project a successful ones !

More to come ! Stay tune !

Cheers,
Cheah

by seng Sat Nov 23, 2013 4:44 pm

Great stuff cheah!
This is the first time ( after a few sessions) I listen to your westministers being driven to its full glory. I would be happy if my jbls can achieve this level of sonic pleasure.
Keep it up!

by noodle88 Mon Nov 25, 2013 9:11 am

Hi Seng,

Thanks for your encouragement , I'm sure your system can sound as good as mine if not better. Just give me a call whenever help needed.

There will be more preamp to come, with better tubes !

Do come for another audition when my UX-201a preamp is ready.

Cheers,
Cheah

by noodle88 Thu Nov 28, 2013 10:51 am

Hi !

Some update on my preamp.

I have modded my preamp to accommodate UX-201a tubes.
201a have a plate resistance of 11000ohm, so LL2745 being used here. I wired these line output transformers with silver wire in 5.6 : 1 step down.

01a > 71a DHT Line Preamplifier. Image39

01a > 71a DHT Line Preamplifier. Image40

I like the glow of these tubes! They also sound wonderful !.

01a > 71a DHT Line Preamplifier. Image41

Cheers,
Cheah

by dixchen Mon Dec 09, 2013 5:29 pm

So Cheah, finally got hold of Thomas's gear huh, someone told me you got a new 71A preamp and I was wondering where it came from. ( haven't been treading in this forum for more than 1 year now ) By the way, nice that you have documented the build with photos. I was too lazy to do mine earlier.

So which DHT do you prefer best amongst the three?

I think it's time you go for his full transformer coupled phono stage. This was the final word in my system that made me stop looking for a better sound. Laughing

by noodle88 Tue Dec 10, 2013 7:57 pm

Hi!

Received a pair of LL1660 from Thomas, its gap for 10mA which is optimize for my 12a preamp.
These line transformers can also be used with the famous WE 101d !
01a > 71a DHT Line Preamplifier. Image50

01a > 71a DHT Line Preamplifier. Image50

Will update the difference between LL1660 gap 18mA and 10mA ones done.

Dixon - Hey, long time didn't hear from u. You should have documented all your built so that we can benefit from there.

Cheah

by dixchen Thu Dec 12, 2013 2:31 pm

noodle88 wrote:Hi!

Received a pair of LL1660 from Thomas, its gap for 10mA which is optimize for my 12a preamp.
These line transformers can also be used with the famous WE 101d !

Will update the difference between LL1660 gap 18mA and 10mA ones done.

Dixon - Hey, long time didn't hear from u. You should have documented all your built so that we can benefit from there.

Cheah

Yeah I have been busy with my 2nd newborn boy beginning of the year as well as finishing off the phono stage at the same time that took me 3 months to build and ' tune ' as well.

Didn't document it cause building it was already very tiring, spending hours and hours on it only at night but nevertheless satisfying at the end. One of the LCR RIAA coils he sent me had an internal defect that caused one channel to be muted. The debugging process was killing, I was pretty unlucky cause the mounting screw was right above the output signal wire inside the coil. What happens when you screw that to the chassis? Dead short! Shocked

His recently mentioned new all silver 211 -> trans coupled -> 211 he is working on wouldn't happen to be yours would it? Laughing

Anyway good luck and have fun with your quest on your preamp adventure, lucky I had my fair share of fun with these DHT's for the past 2 years, given the very little time I have today, I will never be able to finish any of it! Rolling Eyes

by noodle88 Thu Dec 12, 2013 6:04 pm

Hi !

>Didn't document it cause building it was already very tiring.

Tough u should feel exciting rather than tiring.

I do hope that all silver 211/211 poweramp is mine.

by noodle88 Tue Dec 17, 2013 3:29 pm

Hi!

Over the weekend I have sometime, so I built up this optimized version of 12a line preamplifier.

Here's the signal section

01a > 71a DHT Line Preamplifier. Image55

Matching with my earlier power supply

01a > 71a DHT Line Preamplifier. Image56

With the LL1660/ 10mA, bass is lower and got more detail. But it's too early to make judgment as its need some time to burn in. Very Happy

Cheah

by dixchen Thu Dec 19, 2013 8:10 am

noodle88 wrote:Hi!

Over the weekend I have sometime, so I built up this optimized version of 12a line preamplifier.

Here's the signal section

Matching with my earlier power supply

With the LL1660/ 10mA, bass is lower and got more detail. But it's too early to make judgment as its need some time to burn in.

Cheah

interesting, which model lundahl trans were you using before this?

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