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Oh no - YAWE91B300BA (Yet Another WE 91B 300B Amplifier)

Posted by Thorsten (M) on January 19, 2005 at 11:03:30

Folks,

First, this Amp is a bit of joint project between DIY HiFisupply’s Brian Cherry and myself. I had wanted to build a 91 Style Amp for a while, having previously reviewed the Billie Kit from DIY Hifisupply I figured this would be a good starting platform.

While I talked to Brian about getting a Kit from him we got talking about the 91 Circuit and it turned out he was also looking to possibly produce a version of his 300B Kit based on the 91 Circuit. We had some outstanding "trade in kind" anyway and I ended up with a LDP Kit, the older version of the chassis. Therefore, at least the bits where specific DIY HiFisupply parts are mentioned keep in mind I used them because easily get them and was convinced the quality was good, other alternatives exist.

Our starting point was Joe Roberts seminal Article in Sound Practices on the abridged 91, you find a similar design with the Angela 91 Amplifier.

I also studied the design of the old, WE 91 based Le Maison de l'Audiophile 300B Amplifier, pictures of these Monoblock Amplifiers are on a Japanese 2nd hand sales site:

http://www.hifido.co.jp/

Final influences on the implementation must be attributed to the only current commercial 91 Type Amplifier that has any profile to be worth noting, the Reimyo PAT-777 300B Single Ended Amplifier.

A review of the PAT-777 with some interesting internal shots can be found here: http://www.dagogo.com/HarmonixReimyoPATCAT.html. Another with also quite detailed Photos (which can also be found on the net after registering) was in the german "Image-HiFi" Magazin (http://www.image-hifi.com.

That was where I started my own work on the 91. Some may ask "Why on earth the 91?". I have previously argued extensively for penthode Drivers (review this link for example), recently the very high profile Yamaoto Amplifiers used penthode drivers and quite frankly, to my ears I they sound best. I had some WE 310A's around plus some ideas what could be done with a 91, so off I went.

One change I requested from the beginning to the original kit was to have somewhat universal Output and Mains transformers, so that it will be possible to convert the Amplifier to a wide range of valves. I ended up taking well longer to finish these Amp's than I had intended, so the first 91's on the Lady Day Chassis where actually build over in HK by DIY HiFisupply. Still, my ones are now playing and very well too....

Okay, here is what it looked like at the beginning:

I wanted the Amplifier to be as universal as possible, initially I intended to actually make even the driver stage chassis sub-plate exchangeable, this last feature was dropped eventually as the whole thing clashed with too many other requirements. I am still able to change the driver as block quite easily, but it will take a little more time than I wanted.

My next steps towards universality where an Ohmite Brown Devil 1K5/50W adjustable cathode resistor. With that you can cover anything, from 45, 2A3, 300B to 205D (or 10Y at high current/low voltage operation) in terms of cathode resistor, just set the desired value. Right now I have 880R for the 300B. I also included a 5R/25W Brown Devil resistor to be looped into the Output valve heater supply to adjust the Heater voltages to correct levels for the DC Heater supplies.

Further I specified an Output Transformer with multiple secondary tapings to allow a use as Universal Output Transformer very much like the Tango U808, XE20S and the Hammond 125ESE by simply adjusting the secondary connection. This is arguably a compromise solution, but one we have seen to work well for many users and it was doable without adding significantly to the cost.

The primary inductance is okay for this kind of Job, a while back I measured around 20H @ 50Hz with 80mA DC Current on the original output transformers (the ones without multiple secondary taps). I agree that for a nominal 7K primary transformer 20H appear low, though the lower DC current should give use a little more inductance anyway.

By comparison Tamura's F-5004 is specified as 7K SE Transformer with only 23H rated primary inductance and their F-7004 equally 7K with 25H primary inductance and the F-2005 as 7K with 21H primary inductance. The Tango XE20S is specified with 18H/80mA and 5K maximum primary impedance and the U808 with 21H/100mA and 5K maximum Impedance. Bottom line, I think this particular "universal" output transformer is in good company.... ;-)

BWT this Output transformer is now standard for the LDP Kit AFAIK at no extra cost. My pair slightly deviates from the final Spec (in terms of tap arrangement), but are quite fine otherwise. Here is what the side of the Output transformer looks like with a big label stuck to it's side listing the various tap assignments is I can remember them.

A screw terminal (electrical type) strip is used to make changing the output impedance easy:

The last part of making the Amp universal is to allow for different Anode and Heater Voltages. One could handle Anode Voltages lower than 420V using a big adjustable dropper resistor (another Ohmite Brown Devil maybe), but this strikes me as inelegant as does adding a cheap silicon chip regulator. The chassis had no space for a big Valve regulator, so the sensible way was to get the mains transformer with multiple secondary taps. I specified as follows:

Heater windings:

0-5V/3A                          (Rectifier Heater)
0-6V-9V/1A                       (Driver Heater)
0-2.5-8V/4A                      (Output Heater)

High Voltage 

400V-300V-0-300V-400V/0.2A(DC)   (Main +B Winding)
0-25V/0.2A                       (Boost/Buck)
0-25V/0.2A                       (Boost/Buck)

All the various voltages where again terminated in electrical screw terminal strips. With the PSU as build and these windings I can cover anything from around 320V +B for a 45 to >>450V +B for high voltage 300B operating conditions.

If we allow for different rectifiers too (GZ34 vs 274B vs 5R4 vs 5V4 vs 5U4 vs 5Y3) and variations in series resistance we have a lot of variation for different output Valves. Heaters allow for anything from 2.5V AC or DC (designed for up to 2.5A for the 2A3) to 7.5V/1.5A AC or DC for a type 50.

Anyway, here a look inside the chassis with all the various features to make the Amp universal in view.

The terminal block in the lower right-hand corner holds the HT Connections, on the left-hand side are the various heaters, you can also see the two adjustable "brown devil" wire wound resistors.

The actual PSU including my heater DC Supply was build on a larger and mirror imaged PSU PCB (compare first picture for the original PCB). This allows me to include bypass capacitors for the HT Filters and a CLC filtered Heater DC supply with an added common mode choke and keep everything reasonably neat.

A further fundamental change to the standard Kit was to substitute chokes designed for below chassis mounting, which allowed the can on top of the Amplifier to be used to house a number of large size film capacitors which where used in the sonically most critical positions in the circuit. These Capacitors where a motley assembly of "what I had around", including some 33uF ICW "Clarity Cap" for the 310A Cathode, some 50uF Ansar "Supersound" for the 300B Cathode and a 7.5uF and a 15uF DIY HiFisupply "Obbligato" capacitor per amp. All fitted fine into the can and make a neat looking "block" capacitor.

For the rest a schematic as build might help, here goes:

Note that some of the resistors in there are to compensate slightly high +B and Heater voltages and need to be considered as required to be adjusted by the builder, these are the 2R7 in the driver supply, the 180R after the rectifier and the 0R47 in the heater line for the rectifier.

Here is what the actual Amplifier circuit looks like:

The coupling capacitor is a Silverfoil/Mica type made for DIY HiFisupply (otherwise 0.1uF/500V Silver Mica are virtually impossible to find), probably to my ears the best type of capacitor for coupling.... Resistors are Kiwame. Small Filter Caps around the 310A (Screengrid & Anode) are SCR/Solen, I had them at hand and they fitted well.

Past that the circuit is largely orthodox, in principle unchanged since 1935 saw the WECO 91A Amplifier. There is something to be said for things that last. I applied a few changes compared to the Sound Practices. These derived from a mixture of reasons, one was to use what was available from the DIY HiFisupply Kit efficiently, others came from the investigations into the l'Audiophile and Reimyo designs.

The power supply uses a small value film capacitor as first reservoir capacitor (as also seen in the Yamamoto A08 and the Reimyo Amp), filtering is via "bulk" electrolytic capacitance, as two sets of electrolytic capacitors where present I placed a modest value resistor between them.

This gives added filtering, especially for high frequency hash that can ride straight trough normal filter chokes. As at high frequencies normal electrolytic capacitors become inductors and fail to filter such noise as well I added some low inductance "yellow box" 1uF Bypass capacitors, basically what I had around.

I also added an extra RC filter cell for the driver stage over and above what is normally fitted. I find often that perceived dynamics (especially at LF) improve if the driver stage gets decoupled from the main +B by two RC cells instead of the commonly found one RC cell.

Another round of stomping heavily on noise, especially higher frequency components which are often not so much perceived as noise, but as a subtle degradation of the sound was applied in the heater supply. If I want I can wire the Amp for AC Heaters, as where used in the Original, by Joe Roberts and in the Angela 91, as well as by l'Audiophile. But truth be told, to me at least the subjective sound quality suffers with AC on the heaters and I am not even so much talking about the steady state 100Hz hum.

So I added a DC Heater supply following what I find good practice. I used a bunch of common cathode 30A Dual Schottky Diodes made into a nice "bridge" pack followed by CLC Filtering. This time I did not get special chokes made, I used two fairly small value (2.9mH) torroidal Chokes normally used in interference suppression. They are not as effective as many times this value, but they do manage to knock down the noise quite a few db even at 100Hz. A common mode choke is placed between the 300B Filament and the DC supply, this again tends to help quite audibly. In the end I have around 18mV noise across the 300B Heater which on the scope is a very nice looking pure sine wave, no higher frequency "buzzy" sounding components.

Oh yes, a final note on the power supply, the primary of the toroidal mains transformer has a so-called DC Trap, two 1,000uF/16V electrolytic capacitors back to back with a bridge rectifier across to protect the capacitors from over voltage.

The electrolytic capacitors literally act as coupling capacitors (aka DC Blockers) between the mains which can have between several 100mV up to as much as 2V DC offset (on my mains anyway) which in turn make the toroidal transformers mechanically noise, make them run hot and generally dimish the quality of the sound.

The DC Trap circuit is basically this one here:

Overall all these measures lead to Amplifiers that are preternaturally quiet. Depending on the Valves used in each Amp I measure between 0.2...0.3mV wideband noise, unweighted. This translates into around 80db unweighted Signal/Noise ratio referenced to 1 Watt output. Most of this noise is very low frequency "shot noise", the 100Hz component is even further down, on my current speakers no noise is audible with the ear to the cone.

By reference a rather popular "High End" 300B Amplifiers from a major US maker has 14db more noise even though it too uses DC Heater supplies and massive capacitors in LC filtering of the +B. Reimyo on the other hand manages a similarly quiet amplifier, perhaps part of their success in sound quality?

The next stop is the addition of what one may call "optimised WE Cathode Bypass", this is seen on many older WE Amp's and helps to further reduce power supply noise and actually sounds a lot better n my experience. The key factor is the 15uF Capacitor between the +B line feeding the output transformer and the output valve cathode. Together with 50uF Cathode cap it simply cancels wideband any power supply noise/modulation out, by around 20db, the subjective result is a cleaner sound with again improvements to perceived dynamics.

The odd looking circuit between the cathode of the 300B and the 300B gridleak resistor is also original WE, it is a form of positive feedback that overcomes the limited LF frequency response cause by using fairly small value (but high quality) cathode bypass capacitors. In the old days large value capacitors where too large and expensive, today the same holds true for high quality types, hence this circuit still has use. Note it only works with penthode drivers or in specific configurations with interstage transformers, no point adding it to a triode driver amplifier.

A last trick is implemented in a pair of 1nF capacitors between the 300B Anode and 310A screengrid. This is actually positive feedback at high frequencies. This helps to overcome the HF rolloff inherent to the classic 91 Circuit, where usually 20KHz is around 3db down. In my Amp's I measure -3db somewhere at around 75KHz with this positive feedback in place, without -3db is around 20KHz as expected.

As build the amplifier now measures 20Hz - 20KHz with better than +0db/-1db, I cant measure the -3db point at low frequencies, my generator only goes to 15Hz. Again, not particularly remarkable, unless we consider the performance of the traditional 91 Circuit with the LF and HF positive feedback deleted (more like -3db @ 20Hz & 20KHz).

Output power into correctly matched load is around 8W before visible waveform distortion (meaning around 5 - 10% THD). Nothing unusual for a Zero Feedback 300B Amp really all in all, merely confirming good operation.

And after all that, a pair of pretty plain looking amplifiers play in the system.

Look at them and you would never guess. Oh yes, when they are playing sound is great, refined, airy.... [add long lists of audiophile adjectives]. Definitely my new "reference" Amplifiers.

Ciao T

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