Western Electric 91A Amplifier Clone

[radium7]

My next project is Western Electric 91A Amplifier (Monoblocks). I had two circuits in mind WE91A and JE Labs 300B DX Amp. Both have lot of good reviews, different design and different sound signature. I finally decided to build WE91A. I know it's frequency range is slightly narrow, but I like to hear the sound of this famous amplifier which powered lot of theatres those days . I decided to use the original WE91A circuit and not any other improved or altered one, because i like the vintage sound. I want to keep my amp close to the original design, So I take care in selecting the components. In the original WE91A all the capacitors are paper in oil type, so I collected pio capacitors first and there is no electrolytic capacitors (power stage, cathode bypass, coupling everything are pio).
Just like the original WE91A all resistors are Allen Bradley

Resistors-Allen Bradley

Power resistors are Reidon 50W type

PIO capacitors
Power stage- ASC capacitors
Cathode bypass- Russian PIO capacitors
Coupling capacitors –Jensen Copper foil in oil capacitors

Hookup wires- Cardas

Tubes
6SJ7—i got (Ken-Rad, RCA, Westing house, NEC, etc.,)
5U4G/GZ34— Mullard, RCA, BEL.
300B—Eletro Harmonics


Transformers
The power txr and choke are custom made (Hand winding). I had a very good transformer winder, the power and choke made by him gives very good results. I choose first quality copper and iron and do not compromise in quality.
For the output transformer I had both Tamura and Hasimoto transformer in hand.

Now designing the layout and chassis and will update soon.

 

[radium7]

The output transformers are japanese Tamura and Hasimoto. Japanese OPTs are the best OPTs that i had used sofar. I will choose any one of this for WE91A.

 

[anilva]

Looking forward to your build. I am a big Hashimoto fan. I have a 45/2A3 SET with Hashimoto outputs. Next build is a 300B of Genelex. Viren ji is going to do for me. Again with Hashimotos.

Cheers.

 

[Nikhil]

Fantastic build plan radium7!

Can you share where you got the tube sockets from?
I am looking to change the tube sockets in one of my preamps.

Regards.

.

 

[shafic]

Great initiation.
Best wishes!!

 

[radium7]

Thanks. tube sockets i bought this from www.theaudiocrafts.com

 

[sadik]

This is gonna be an awesome built...

 

[har297]

Tagging along for this awesome build...

 

[radium7]

Thanks

 

[sunilj]

Any progress on this project

 

[sunilj]

Hi radium7,

Did this project complete ? Photos, opinions ?

 

[radium7]

Working on few other projects, not able start it. soon i will try to complete it post the details.
Now i'm building The Christiansen "DG" 300B Amplifier, ECL82 tube based Single ended Amp, 6V6 Tube Single ended Amp and JE labs Preamp.
I will post all details soon.

 

[srinisundar]

Wow.. Great Projects in hand. Please do post the updates..

 

[Kannan]

that is among the best set of components one can build on

 

[Hari Iyer]

I will suggest good quality wire wound resistors for reduced noise.

 

[radium7]

Sir, Thank you for your suggesion. I use wire wound resistor in power stages and in cathodes of power tubes, where more power need to be handled and high stability required. My opinion Cracked carbon or carbon film resistors are best for anode load, cathode, grid and grid bleeding stages in tube amps. Carbon film resistors are suitable for tube preamps because they are silent than cracked cabon resistors. Metal oxide resistors can be used in power stages(low power ). Metal film resistors are prefered for all solid state circuits (suitable for phono preamps and DAC).

Resistors make lot of difference in the sound signature of a circuit. AB resistors have lot of disadvantages- poor tolorance, poor stability, temperature sensitive. noisy, etc., but the warm tone in vintage style delivered by those resistors can't be beaten by other type resistors. We can avoid its disadvantages through carefull design, proper layout and construction of the amp.

Resistor Noise: Generally some poor quality resistors always make noise, besides that all other resistors any type or any brand they will also make nosie when they are overloaded beyond their power handling limit.

Resistor noise is due to poor quality and power handling capacity of resistors.
Resistor type and brand will influence the sound signature of amp or preamp.

In the begining i didn't concentrate on resistors, used local ones in my amps and preamps. when i started using different type and diferent brands (particularily imported) i find the difference.

The resistor brands i used in my circuits so far:
AB, Arcol, PRP, TKD, Takman(Both carbon film and metal film), Mills, Reidon, Ohmite, Amtrans, Vishay Dale, Kiwame, Welwyn.

I won't tell it will make a very big difference or day and night difference, its 1% or 2% only, but its magical.

 

[Hari Iyer]

Thanks for the details. Of all resistor type wire wound resistors are lowest noise approximately equal to zero. Carbon composites are the most noisy and are used in grid stoppage as there is no PD across it. But increasingly DIY are not using them due to value drift over time. I have used ww in grid stopper and anode ckts too other than cathode bypass. The small amount of inductance prevent RF & EMI noise in the anode and prevent them from coupling with the audio signal and removes clutter in the mids and highs.. They are safe to use till 60 to 70 KHz after which rolloff can creep in. In cathode by pass having a inductive ww actually blocks the HF from passing through the resistor and allows it to bapass through the capacitors. I shall even go ahead and add ferrite beads in the cathode bypass resistor to block HF pass through it. Even input resistors benefit from ww with the reduced input noise, RF& EMI blocking. All the above imo.

 

[radium7]

fine

 

[sunilj]

Working on few other projects, not able start it. soon i will try to complete it post the details.
Now i'm building The Christiansen "DG" 300B Amplifier, ECL82 tube based Single ended Amp, 6V6 Tube Single ended Amp and JE labs Preamp.
I will post all details soon.

The Christiansen design is the one with the 'Maida' regulator ? Nice choice. Did you buy them pre-populated ? Let us know how it goes.

I've got ABs and don't find anything negative about the sound. You're spot on about implementation though - get it wrong and you'll have problems.

 

[radium7]

Yes, Christiansen design is with Maida regulator. I'm building this amp for a FM, he send me all populated boards. I have to assemble it and to tune the amp. Tomorrow will post photos.

ABs are one of the best resistors, no doubt in it. i have used it and using it, there is no issue for me.