Oct 5, 2016

E140 SE A2 amplifier Part 3

Tubes selection made, now it's time to mix up all these good things.


Before publishing amplifier schematic let's have a look to the driver limitations.

High frequency: 12B4A & Miller's effect

The input capacitance of the tube C, in conjunction with the source impedance Rg of the stage, forms a simple low pass RC filter with an upper -3dB cutoff frequency equal to:

           f = 1/(2π x Rg x C)     where     C = Cgk + Cga x (Gain12B4A +1)

The -3dB point with a 220K Rg resistor will be: f = 1/(2π x 220K x 39pF) = 18.8kHz
As I can neither reduce the load (90K _ See part 2) of the previous stage without lowering the E80CC gain nor increase Rg, some local NFB will help to keep a good overall bandwidth.
A 4 to 6 dB loop fine tuned with an FFT analyzer will give the desired bandwidth and distortion figure, definitely set by listening test.

Low frequency: inverted interstage transformer

Tamura IT is used with inverted primary / secondary coils.
In that way E140 grid current in the secondary cancels a portion of 12B4A current flowing in primary coil.
Unlike all Shishido's amplifiers working A2 all the time, this one can swing the E140 A1 where no grid current appears. This is why I have carefully chosen a 12B4A operating point where anode current do not exceed 10mA.
With such a primary current the transformer low frequency response is very good.
High frequencies are also very good without roll off with this Tamura transformer. I believe it is the way the transformer is wound that prevents attenuation when inverted connected.

Amplifier schematic



Power supplies

This amplifier requires both high and low voltage.
Low voltage is required to bias E140 grid through interstage transformer. Needs to be very steady and adjustable, a LM317 is the simplest way to achieve good regulation and very little ripple. It requires just a few parts and can be easily implemented using PCB's.



High voltage supply is a bit more complicated than usually mainly because the power stage high voltage (240V) is lower than the ones needed for previous stages (270/260V). It imposes to split the supply in two just behind the first choke keeping in mind the necessity of well calculated cells to insure good transient response. In that way I had to consider both ripple attenuation and time constant factor.

I choose choke input filter for low, almost perfect sine wave ripple and good regulation. Moreover it is less stressing for the power transformer than capacitor input filter.
Unlike capacitor input filter the ripple is not a function of current. It just depends upon voltage, choke and capacitor.
It can be calculated from formula:

           Vripple = η VHV    where    η = 1,2/LC   (L in Henries and C in µF).

In that case it will be 0,00048 x 260 = 0,125 Vcc or 0,044 Vrms and represent a 60 dB attenuation of the ripple behind rectifier (44 Vrms). Very efficient!




Last post, if there is, will talk about parts selection, amp construction and tests....However there is so little people interested in this blog topics that it will probably be the last one.

Anyway I thanks those who took time to read my publications.

Amplifier is now complete. Sold to a German amateur.

Sep 7, 2016

E140 SE A2 amplifier Part 2

Class A2 amplifiers need a more elaborated driver than the usual A1.

Technical approach

To source the E140 grid properly it imposes a low impedance driver stage. It is the heart of this project. I could have used a simple cathode follower directly coupled to the final triode, but to have a steady E140 operating point I had to use a well regulated high voltage supply, which I did not want. A medium power triode used as voltage amplifier loaded by a step down transformer was the Shishido's way. I bought two Tamura A-8713 (20K/600) line out trans. Not too expensive they accept 20mA unbalance primary current with very decent bandwidth.
Though providing a quite low impedance path to the E140, the IT voltage ratio (1,73 10-1 or -15,3dB) commands a tube capable of a very large anode swing with low distortion. Not so many candidates. Keeping in mind the necessity of a current not exceeding 10mA trough IT (it roughly corresponds to twice the E140 grid current at maximum positive swing) the 12B4A is the tube to go. At 20K load its linearity is excellent and is able to deliver 350Vpp with low distortion, giving a driving voltage behind IT up to 60Vpp.
I do not need that much.

Back to SFR E140 triode

after a careful study of the E140 characteristics the best operating point for full power is

                                  Vak 240V, Ia 40mA, Vg +10V, Rload 5K.

It permits an anode swing of 330 Vpp with a grid swing of 40 Vpp.
The output transformer have a 4 10-2/-28dB voltage ratio, thus 330 Vpp or 117 Vrms will give 4,68 Vrms/8 ohm or 2,7W. With a dummy load the amp puts out 3,8W before clipping.
Enough for any sensitive speaker.



12B4A point of view

To get 40V pp on E140 grid the 12B4A triode must have an anode swing of
40 x 5,78 (ITvoltage ratio in that way) = 230 Vpp
The operating point will be set at

                   Ia 10/11mA with Rload 20K, Rk 3.3K, Vak 225/235V, V+ 260/270V

On the characteristics below we can see that this tube is up to the task.




First stage

this was the trickiest choice I had to make for this amplifier.
I needed a high voltage capability tube mainly because most of the A2 circuits I studied used some NFB and that I kept in mind the use of a local feedback loop between power stage and driver to:
1- get a better damping factor.
2- cancel amplitude distortion when tubes reach their extremes.
3- keep a good overall bandwidth.

To make a long story short I finally choose the E80CC among half a dozen contenders
( 5687, E182CC, 12BH7A, 6CG7...).
The E80CC is renowned for its sonic qualities and very low distortion. Many professional audio devices used this tube primarily intended for computer use.

The 12B4A needs 40 Vpp (14,1 Vrms) on grid to deliver 230 Vpp. Even with a 6dB feedback loop between the last stages the E80CC will swing the 12B4A with low distortion. Below a 150K loaded tube with a 32 Vrms (90 Vpp) output @ 1.4 Vrms (4 Vpp) input shows the high gain capabilities.



In facts DC load differs from AC. AC's one takes the next stage grid resistor Rg2 in account. In that case if I make Rg2 = 220K the AC load is about 90K. As seen below it does not change a lot the gain capability but slightly increases distortion (Philips data sheets give an output voltage of 20 Vrms (56 Vpp) @ 3,4% distortion under 250V/100K, we should be very close).
The main problem with high Rg will be Miller's effect. I will point out the incidence on 12B4A bandwidth in the next article.



Just two makers for this very fine audio tube, Philips and Tungsram.



Next episode: complete schematic, power supply and more...








Aug 8, 2016

E140 SE A2 amplifier Part 1

A while ago I found some peculiar tubes that I bought for a decent price.
These E140 tubes, like the E60M, where manufactured by SFR and it was a reason for me to buy some.

SFR tubes are among the best constructed I ever saw in my life. Needless to say I knew nothing about these triodes and at that time I was not sure I could use them for an audio project.
They remained a while on a shelf till I decided to find some useful data's. Searching the web brought me very little information, not a surprise (all SFR production was intended for military use and records or data's are really unobtainable). But little was enough to catch my interest. I found that this tube was similar, at least by its shape, to a Philips transmitting triode, the TC04/10. Data's for this transmitting triode were easy to source.



I spent a lot of time plotting points to make a decent Excel Ia_Va/Vg graph that I could compare to the Philips data's.
The tubes are the same (almost) and at first sight not ideal candidates for audio use. High µ, high ρ, 10W power handling. I was a bit frustrated.
However these kind of transmitting triodes reminded me the mail I had with the late Nobu Shishido (I bought his book but did not understand a single japanese writing and he has been very helpful in translating some parts). In the last card I received from him he explained me how to use a line out transformer to drive tubes in A2 mode with low impedance and grid current.


At that time I was just wondering about the benefits using tubes with grid current while avoiding large NFB amount to temper HF ringing and keep good overall bandwidth. Moreover in the 90' I still could find plenty of good tubes I could use A1. It was simply not my way thinking HiFi.
I was wrong.

I have today a more open mind and I decided to give a try to Nobu's approach. It was challenging as I never used tubes that way. I was in "Terra incognita". I took some time to read more about A2 modulation and decided to start this project whatever the result.
A careful look at the E140 _ TC04/10 curves shows up the very good linearity with or without current grid.



A few maths later I realized that this project was feasible and I ordered parts to build an amplifier.
In this peculiar amp grid current do not flow all the time (unlike Shishido's amp) and I cannot take advantage of permanent current cancellation.
The E140 is biased such a way that the amp works A2 for a small portion of grid swing, then switch to A2/A1.
Many would think that this abrupt change will impart some sonic alteration.
Believe me it's not the case. I have breadboarded one unit and made a try with my cumbersome work, this amp sings very, very well even with the cheap parts I had on hand!
Encouraged by the results I am now waiting for high quality Hashimoto irons.

A small batch of triodes is a good idea when initiating such a project...


And to sort tubes to get matched pairs whenever possible is a good one too.


Next step, driver requirements. Stay tuned.

Jun 15, 2016

6J5 Line preamplifier Part 3

Back on the air...
Normally there should not be a part 3, but I recently found a few Visseaux 6J5G and put them to good use in my line preamp.
While listening to CD's and LP's I know very well I was so surprised by the overall qualities that I called my wife for a neutral and objective appreciation. It did not take a long time to get the certitude that what we were listening at was the most realistic of all we have heard till today. These tubes are among the very best, not to say the best, of all the 6J5 I plugged in my preamp. More important, when used with the MC1/60 amplifier the combo brings music reproduction to a truthful level hard to beat.

Visseaux was a French manufacturer established near Lyon that made high quality tubes (each being individually tested) till the 50's before becoming an ITT subsidiary.


Construction is typically Visseaux with copper alloy fasteners, grey mica and black anodes.

Amazingly the tubes I found bear the Radio Technique logo, which is very surprising RT being a Philips subsidiary. Could it means Philips bought tubes from Visseaux and relabeled them ?
This would have been very unusual because the tube history told us the contrary, Philips sold hundreds of thousands of tubes that where relabeled by famous companies such as Siemens.


A snapshot of my main sytem.

6J5 line along with a tetrode based LCR phono preamp ( I will post one day when power supply will be definitely set) and its Entré ET100 (Soltear Electronic Japan fed with special Tamura's I suspect to be TKS83 relatives) step up transformer companion.
Garrard 301 armed with JVC 7082 on a very heavy plinth.
DL103, AT33 Mono and the incredible Entré EC30 to go with this arm.
MC1/60 amplifier.
Modified Klipsch La Scala (Tractrix wooden horns + JBL 2470's, AlK Xover ).


Super rare vintage Entré EC30 pure boron cantilever cart riding some good vinyl




For this report we listened and appreciated









May 3, 2016

6J5 Line preamplifier Part 2

High voltage requirements

Needless to say that a good supply, free of ripple, is essential in SE stage that have very bad CMRR. This generally dictate a large capacitor bank at the expense of transient response due to great time recovery.
As I prefer a punchy response it needs a good calculation of the filtering cells to keep ripple at least 10-5 (- 80 dB) below last stage voltage. Means 240*10-5 or 2,4 mV rms while keeping the smallest possible time constant.
Using a input choke of large value greatly helps reducing the following capacitors value. Furthermore behind the choke the ripple is very smooth and quite sinusoidal, generally 2 cells are enough to have an almost perfect continuous voltage.

Main supply



To calculate the ripple behind the choke we have to know the ripple factor   η = 1,2/L*C
 (L in Henries and C in µF) where C is the first filtering cap.
An estimation of this cap can be done if we consider a time constant not more than 15 millisecond. The choke having a DC resistance of 800 ohm, C will be 15 10-3/800 = 18,7 µF, the closest usual value is 16µF.
Then η = 0,00125 and the ripple on the first cap will be ηV = 0,00125*265 = 0,33Vpp or 0,117 Vrms. Network time constant is 13 millisecond.
A second cell with 1,2 Kohm and 16µF will leave a small 25mVpp / 8.8mVrms ripple with a 19 millisecond TC.
A last cell with 2,4 Kohm and 10 µF will leave 1,6 mVpp / 0,56mVrms and a 24 millisecond TC.
These 0,56mV AC represents a -113dB attenuation below the 240V feeding the 6J5, way better than the -80dB expected, while keeping a very fast recovery power supply. I did some tests with pulse trains and supply is rock solid.

* Time constant is a very important factor in audio, it determines the capacity of an RC system to respond to an AC signal modification and is essential in the ability to follow the sound envelope or ADSR. It is too often neglected when calculating a power supply.
In the case of AC circuits, the RC time constant tells you which signals the circuit passes through and which signals the circuit filters out. An RC circuit acts as a high pass filter which passes high frequency signals and blocks low frequency signals.
When we apply an AC current to a capacitor we are basically sticking some charge onto the capacitor and then taking it off. If we do this at a low rate, smaller than the time constant, then the capacitor has time to discharge. But if we put the charge there rapidly, at a speed much greater than the time constant, the capacitor doesn't have time to discharge. In this way, we can pass high frequency signals through the capacitor.
Remember :
-1 A given capacitor charges 99,9 % at 5 RC.
-2 As a rule of thumb, TC should increase from the first to the last decoupling cell.


No more technical considerations...

Construction

Like for the line preamp just a few parts but of high quality.
An RCore transformer for low overall magnetic loss, high efficiency (just bettered by the toroidal) and slim design to fit my chassis.

A rectifier, good capacitors (German F&T dual 16µf and a couple of Siemens 8µF MP/JS + 2µF Sprague PIO to make my 10µF last decoupling cap), a few resistors, a filtering choke and quality sockets.
Add some hardware and plenty of time....and you are done.


In colour with Neotron 6J5G




For chassis I used 2U professional grade steel ones, 3 layers blue gray paint, very well suited for tubes with lots of openings for optimal air flow. They provide good shielding and are super rugged. I just add some screen printed front and rear plates for better appearance and way better WAF.




Listening report

The 6J5 and its relatives are too numerous for an extensive test. I will give my impress upon the most interesting ones I have in stock. Anyone wanting to build this simple BUT "great performer on any kind of music" preamp will easily find a wide range of readily available tubes.
All the tubes listed below are up to the task, some are more involving or more waaahou !! on a specific material, but a very few simply made me listen to the music instead of searching the preamp qualities and / or shortcomings.
Most need a few burning hours to deliver there full colour.

6J5V Radio Technique
Best of all. Just say evident whatever the music. Delightful triode, vivid, natural with lots of air, detailed to the extreme but never tiring. This is the 6J5 "just sit and listen". These PTT tubes were intended for very long life service, each tube came with a numbered biographical sheet to keep records for 15 years!

6J5G Neotron
Would be my second best tube. A bit more brilliant with slight grain on voices, still stays very detailed and involving. Due to incestuous relationships between tube makers Neotron's were possibely made by Visseaux (with slight construction differences). I have some tubes bearing both Visseaux and NT labels. So who made what ?

L63 / CV1067 GEC Straight Bulb
Nice sound, would say polite, lacks the details of the RT's or Neotron's, no grain but a smaller sound stage that make music more intimate.
By comparison the L63 GEC ST (see part 1) have a bigger and wider sound but are surprisingly very slow tubes that give the feeling of wrong tempo.

6J5WGT Raytheon
I did not like much this tube despite some remarkable ability on voices. Muddy sound with grain, tight and punchy bass but not as accurate as the other ones. Huge sound stage, lush presentation BUT if you like Billy, Ella and Louis or country singers with hoarse voices, this tube gonna make you cry.

Last of the batch 7193 National Union
I had this tube for decades and it is the kind of weird thing you don't want to exhibit. It is a mistake, and I regret not having foreseen the holes for the anode / grid wires. This tube is a killer by many aspects, it has the CV1067 GEC qualities with a much more authoritative presentation and a wide but realistic sound stage.

I was willing to test the 6P5G that bear an excellent reputation, unfortunately impossible to source a NOS pair at a decent price.

Like in every tube based device the rectifier plays an important role in final restitution.
Here, due to the very few parts used its impact is more than evident and the choice made will determine the preamp character. I tried all the ones that could fit, directly or indirectly heated. On the dozen I listened to, the very best for neutral but vivid presentation are Mullard GZ30/CV2748, Mazda GZ32 and STC/ Brimar 5Z4G, with a slight margin for the GZ30 in terms of bass resolution and voices transparency.




Apr 14, 2016

6J5 Line preamplifier Part 1

The 6J5 is a general purpose not to say an ubiquitous small signal triode.
Its linearity is well known and appreciated by many DIYer's. It is commonly said that what goes in is what comes out. It is the successor of the glorious 27 / 56 family of tubes and has been declined in numerous shapes (G, GT, Metal) and constructions (i.e.the weird E11488) to meet industry or military requirements. The British denominations is L63.
The electrical characteristics of the 6J5 are identical to the 6SN7 with just a single triode in the glass enveloppe.
Using such a tube when seeking for high fidelity audio reproduction is a very good choice.
Complete data here

Below 6J5G / L63 GEC ST shape tube in white military box.


and fancy Marconi L63


A simple line preamp, theoretical approach.

Building a very simple but very good sounding line preamp as always been challenging.
Very simple means just a few parts of the highest quality for utmost sound
reproduction. The simplest would be nothing more than an attenuator which I tried with some disappointments. No gain, lacks of dynamic and random noise. Something was wrong in my approach of simplicity and I decided to use an active element to reach my goal ( that clearly appeared to be an impedance matcher with gain). Obviously an active element in signal path means alteration and I had to decide which tube would be up to the task.
Playing with triodes for decades taught me that the ones that could be used are not so numerous. For the indirect I would go to the 27 (76, 6C5, 6J5) family of tubes or the Bi / REN904 and PTT100, for the direct the RS242 and relatives, the 841 and some post tubes like Aa and PTT0. All these tubes share excellent linearity, medium µ and exceptional sonic qualities. Unfortunately most are expensive or too scarce for the average amateur I am and I decided to go for the 76/6J5.
These are very cute 6,3V triodes, affordable and easy to source. A good point for future sonic character comparisons of different brands.
Having a pair of Hirata's NP206 (20K/600 ohm) on hand I started to think about the best operating point for the lowest distortion.
My first choice was for the 76 but its higher ρ did not perfectly match my transformers, thus I went for the 6J5 (which I did not regret…).

Some drawings and calculations...

A line stage in my system must have a gain of 2/2,5 (+ 6/7dB) to accommodates my amps sensitivity. Loading a 6J5 with 20K will roughly give a gain of 14.5 (+ 23 dB) with a fully decoupled cathode and at operating points Va 240V_250V / Vg -8V / Ia 8mA.
With a 20 Kohm load distortion appears to be very small at usual grid input voltage. Most DAC's have a 1.5 to 2V rms output asymmetrical mode, means a maximum of +/- 2.8 Vpeak on grid. Right in the linear region.
The transformer voltage ratio is 1.73 10-1 (-15,3 dB) multiplied by 14,5 it gives me an overall gain of 2.50. Just what I need!



Line stage drawing. Very, very simple, the components choice will determine the qualities of the preamp.



Construction.

As I said previously parts must be of very high quality.

First of it, main attenuator. Might better not consider the usual potentiometer but prefer a stepped attenuator. Just 2 resistors in the signal path making a precise voltage divider.
Good rotary switch and quality resistors give better tracking and balance than the usual plastic or carbon pot.

One exception the ALPS RK40 "Black Beauty". Having both on hand choice was not easy and I picked up the stepped attenuator randomly.

Tubes. I will tell you in a next post, when preamp will be completely finished, my impress upon the different 6J5 / L63 I have on hand.

Transformer. Here a Tango, but any good transformer with 20K primary handling 15 to 20 mA will be fine. Some good ones from Hashimoto like HL20K-6 (I love Japanese trannies).


Resistors and capacitors. Plenty of choice, I used Takman metal film resistors with some vintage Sic Safco low ESR professional caps.

Tips. Short leads, star grounding plus some good oil caps just next to the transformers. Especially important to keep a good transient response when PSU is on a separate chassis. In that case I always split in two the last decoupling capacitor. Can really see the difference with an FFT analyzer.
Some feedback can help in the very low end, and just for once I prefer this preamp with a small amount (2/3dB). Better sound focus and tighter bass.


Next step power supply considerations, complete preamp in its new suit plus some listening tests ....


Mar 23, 2016

EL84 Push Pull amplifier, the little giant

The EL84 is NOT a DHT, but still a fascinating tube for many triodes lovers (I belong to this group) who recognize its sonic qualities.
Great manufacturers like Dynaco, Scott, Leak, Loyez just to name a few, sold thousands of amplifiers using this cute little pentode and for many serious listeners of that time these amps were by far superior for music reproduction to their more powerfull concurrents using EL34, 6550 or KT88.
Yes this tube is very involving, it as the speed ( that usually lacks to the bigger tetrodes and pentodes) the naturalness and ease so many people do like in the very best triodes. It reminds me the 10, the E130 or the RS242, it makes music living like and it's power limitation is it's only drawback. Remember that in the 60's there were not so many speakers with 97/99dB sensitivity to perfectly match a 10 watt amplifier.



In the past 20 years I made half a dozen amplifiers using this tube, always with great pleasure, and that's why I decided to make a post about it.


Also because I always keep an EL84 amp to drive my beloved Wharfedale Super 12 RS/DD. Magnificent full range speakers especially on open baffle.


The amp I will talk about is not a design of my own, it belongs to Harman Kardon. I just made some modifications for safer use and better sound reproduction (I built one exactly like the original for comparison).
Why the HK amplifier ? I found during all these passed years making electronic stuff that simplicity is usually the best way to fidelity and the HK20 design is really simple and very smart.

It operates with a single 6CG7 as driver and phase splitter, two EL84 and a rectifier.
You can't do more simple and when you add the qualities of the 6SN7 family tubes you are right on the way for a good sounding amp.


HK 20 original schematic



As you can see, very simple with some tricks to prevent the use of a decoupling cap in the first stage. V1a bias is taken through R10 in the cathode of the phase inverter V1b. The current of the two sections flows in this resistor, the result is a smaller value that it would have been if directly grounded the usual way. This helps to lower the effect of degenerative feedback thus giving a higher gain to the first stage. Elegant, efficient and easy to calculate if you want to change the tube (I tried the E80CC instead but prefered the 6CG7 after a long listening comparison). For a perfect cathodyne balance a single R11 resistor in parallel with R8_R9_R10 will give the exact value of R7.

To calculate R11 with (R8 + R9 + R10) = Req   use formula   R11 = Rreq X R7/Req - R7

My own HK20

I was not comfortable with EL84 operating voltages.
370V on anode and 350V on screen make the tube work very hot and very hard. I am not sure the usual EL84 will last very long under such a fire.
Even the professional and super strong Mazda 7320 reach their limits, furthermore when bread boarding an amp for tests I had some hum that I could not cancel. So I decided to modify the whole amplifier supply and to pay great attention to time constants, I also preferred the EL84 UL to the pentode mode, I reduced NFB to get decent THD (1,5% @ 10W, mainly third) and a more mellow sound. I suppressed R1 potentiometer (always a soucre of trouble) and replaced it by a voltage divider to match the output of my line preamp.
Now with such a supply and mods the amp is dead silent, punchy, open with a wide soundstage.
Very detailed heights, lush mids, rock solid bass make this amp an all purpose companion.

Power supply revisited.



Inside this little giant, AB resistors and Aerovox V161 caps give a vintage look without compromising sound qualities.
Reliable Siemens capacitors to smooth out the ripple, way better than the usual electrolytics (MKV and MP J/S are my preferred) and Hashimoto OPT.



Philips Holland killer tubes, Mazda GZ32 and shielded 6CG7




The extra socket was for a buffer stage intended to be used with a 600 ohm symmetrical input transformer. I finally did not populate.



Personal EL84 review

I am lucky enough to have different brands in stock to make a comparison.
I wont write a lot just give my impressions, for an extensive test read  Vacuum Tube Valley issue 8

Mullard soft and round sound, not my taste.
Philips RTC the very best with this amp, very well balanced.
Philips ( Amperex ) Holland top of the line with Philips RTC.
Mazda 7320 strong and powerfull sound, can be tiring.


TFK the worst nothing else to say.
Tungsram excellent performer, lacks a bit of punch in low end, the best on voices.
Russian Reflector 6P14P/K (and only this suffix, means vibrations proof. NOT EB or ER) a real surprise, very good sounding, on par with Philips RTC.


Some great CD's to listen with