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2A3 with Robin Hood outputs

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Posted on February 16, 2013 at 15:04:02
Paul Joppa
Industry Professional

Posts: 7353
Location: Seattle, WA
Joined: April 23, 2001



A couple months ago, Mike asked me for a good operating point to use with the Robin Hood output transformers. I've always been interested in entry-level designs to get people started with SETs. So the idea of a simple, low-cost 2A3 amp built around those most-affordable MQ outputs was very attractive to me. I wound up making a complete design. I've sent Mike a PDF with design notes and parts selection notes, but for those immediately curious here's the schematic.

I am hoping a few will build this and report on this forum - I have not built one myself, but will participate on the forum and revise the design after there is some experience with it.

 

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RE: 2A3 with Robin Hood outputs, posted on February 16, 2013 at 16:58:01
xcortes
Audiophile

Posts: 1221
Joined: July 23, 2003
 thanks paul, interesting to see you deviating from your typical parafeed/ss rectified deisgns.

talking about ss vs tube rectifiers. how easy is to go from one to the other? let's say from the uf4007 used in most bh amps to the 5y3/5r4 proposed here? in addition to the sockets and filament heaters, that it.

thanks

 

RE: 2A3 with Robin Hood outputs, posted on February 16, 2013 at 19:55:36
Paul Joppa
Industry Professional

Posts: 7353
Location: Seattle, WA
Joined: April 23, 2001
 The tube rectifier will lose 30-50 volts or so, and it requires filament power. So a different power transformer would be needed in order to change rectifier types in an existing design. At Bottlehead, I have the flexibility to specify custom power transformers according to my own design ideas, but that's not the case for roll-your-own enthusiasts.

Traditional design has series feed, tube rectifiers and center-tapped high voltage windings. As a matter of fact, though I specified a specific power transformer, there are several transformers with identical or very similar specs listed in catalogs. This kind of parts flexibility seemed to me important in a design for those who either want to start building on their own, or want to experiment further. A traditional circuit will be more compatible with a vast majority of available books, articles, web sites, etc.

While this circuit is designed with "learning tool" partially in mind, it's also designed to the best of my ability to sound good - that is, the operating points and parts specs are optimized for sound quality by my standards. There are many roads through the forest!

 

This looks like a fun project, posted on February 17, 2013 at 02:33:41
Mossback
Audiophile

Posts: 1881
Location: Washington, the State
Joined: November 17, 2001
 Thank you for taking the time and effort to provide a fun learning tool for us average hobbyist. I look forward to trying this project.

 

RE: This looks like a fun project, posted on February 17, 2013 at 03:22:06
D Mike
Audiophile

Posts: 128
Location: Michigan
Joined: October 24, 2005
 Can you also post the operating voltages for the two tubes? This always helps me when building a circuit and some substitutions have to be made for whatever reason.

Pardon my lack of knowledge about these things but, what is the purpose of Rb2/Cpsu2?

Thanks for working this up and sharing.

 

voltages and currents, posted on February 17, 2013 at 10:38:38
Paul Joppa
Industry Professional

Posts: 7353
Location: Seattle, WA
Joined: April 23, 2001
 Sure. The target is 375v B+ (more or less), leaving 360v on the 2A3 plate with about 60v bias at 35mA. The driver operates at about 4mA plate current, with half the supply voltage at the plate (185v).

When Mike posts the PDF, there is a short section with circuit description which should answer more questions. I'll try to collect any new questions that come up and provide an update in a few months.

 

RE: voltages and currents, posted on February 17, 2013 at 10:58:54
D Mike
Audiophile

Posts: 128
Location: Michigan
Joined: October 24, 2005
 Thank you for the reply.

I answered my own question about the resistor and cap tied to the 2A3 cathode resistor. It just threw me off when I first saw it drawn that way.

 

Text, posted on February 17, 2013 at 16:06:27
Paul Joppa
Industry Professional

Posts: 7353
Location: Seattle, WA
Joined: April 23, 2001
 Until Mike gets a more flexible web site, here's the text that accompanies the circuit:

Circuit

This circuit is intended to be an affordable 2A3 amplifier using the Magnequest RH-40 output transformer. The approach is kept as simple and standard as possible, both to keep the cost low and to make it relatively easy to understand.

The RH series transformers have a 5000 ohm primary impedance, which calls for a higher voltage and lower current compared to the operating point quoted on the data sheet. The plate to cathode voltage is set at 300 volts, which is commonly taken to be the maximum specification. A current of 35mA matches the tube to a 5K load with good linearity and tolerance of speaker impedance variations. Plate dissipation is only 10.5 watts, which should assure a long life for the tube. Output power should be slightly over 3 watts, depending on the particular tube.

The driver is a 6J5, a standard for many decades and widely available at moderate cost. Its gain is relatively low, so the amp will have an input sensitivity around 2.5vRMS for full power. It operates at 4mA, with 185v on the plate and 6.4v bias.

The last capacitor of the power supply is split between Cpsu2 and Ckp. This allows Cpsu2 to serve as an “ultrapath” capacitor.

Each transformer or choke has its core grounded to the chassis; this is best done to a single point near where the power cord ground is located. This precaution seems to reduce the chance of hum being introduced.

The circuit suggests a 5Y3 rectifier, or a 5R4. It is possible the voltage will be too low (the circuit has not been tested yet) and if that is the case, a 5V4 or similar may be substituted to bring the voltage up.

Notes on Parts Selection

T1 6K3VG power transformer, 117v to 650vCT/40mADC, 5v/2A, 6.3v/2A (Allied)
T2 6K86HF filament power transformer, 117v to 2.5v/3.0A (Allied)
T3 RH-40 output transformer, 5K:8/16 ohms, 40mADC (Magnequest)
Lpsu C-3X filter choke, 10 henry 500 ohms 50mA (Triad)

Resistors are 5% or better. Power and voltage ratings are the minimum, higher is OK.
Rin,Rg 249K or 270K grid resistors
Rstop (2 places) 220 ohm grid stopper, carbon composition or carbon film or metal film. Any value 100 to 2200 ohms should work well.
RL 47K 3W driver plate load. Noninductuve wirewound is best but metal film is nearly as good. Avoid metal oxide or any carbon type – they are noisy.
Rkd 1.6K 0.5 watt driver bias. Preferably metal film or wirewound; less critical than RL
Rkp 1.6K 10W power bias. Preferably wirewound but even metal oxide is acceptable; less critical than RL
Rhum 25 ohm 3 watt hum balance rheostat, wirewound. Can be up to 50 ohms. Radio Shack sells a suitable one.
Rb1,2 (2 places) 470K 1W bleeder, rated 500v working voltage or more. May need to get 2W or 3W types to achieve the voltage rating. Important safety feature, do not skimp on ratings!

Cc 0.1uF 630v coupling cap. Important sonically - polypropylene or better. Avoid caps with a reputation for leakage current.
Ckd 100uF 10v driver cathode bypass. Quality electrolytic. 16v maximum.
Ckp 160uF 100v output cathode bypass. Quality electrolytic. 160v maximum.
Cpsu2 47uF 450v output filter capacitor. Quality electrolytic or film. Film can be higher voltage. This cap is important sonically. Should be close to 1/3 the value of Ckp.
Cpsu1 47uF 450v power supply input capacitor. Electrolytic. Can be 22uF to 220uF.

 

RE: 2A3 with Robin Hood outputs, posted on February 27, 2013 at 08:41:29
Jim_R
Audiophile

Posts: 110
Location: Rocky Mountains
Joined: March 25, 2009
 Thanks Paul, this will be a fun project for sure. I like the choice of driver tube ;-). I'll have to lookk this up, but isn't 47 uF on the high end for loading a 5y3?, or is that why an electrolytic is specified?

Put a Quickie in front of this and you could have a really sweet low cost rig.

-- Jim

-- Jim Rebman

Audio is in the hear and now.

 

RE: 2A3 with Robin Hood outputs, posted on February 27, 2013 at 15:03:59
Paul Joppa
Industry Professional

Posts: 7353
Location: Seattle, WA
Joined: April 23, 2001
 The 5Y3 spec is 20uF for a 50-ohm source resistance; the specified power transformer has 450 ohms resistance each half so the peak current is well controlled.

 

RE: 2A3 with Robin Hood outputs, posted on February 27, 2013 at 18:38:27
Jim_R
Audiophile

Posts: 110
Location: Rocky Mountains
Joined: March 25, 2009
 Hi Paul,

Thanks for the explanation -- not questioning your design, just used to seeing something more like 10-20 uF here but then I didn't realize that the PT's output impedance was so high. All makes sense now.

Thanks,

Jim

-- Jim Rebman

Audio is in the hear and now.

 

RE: 2A3 with Robin Hood outputs, posted on August 5, 2013 at 20:32:51
Dr. Exotica
Audiophile

Posts: 365
Joined: January 22, 2002
 Hi Paul.

This looks like a fun project to get experience with 2A3's without going all in on a Horus, etc. I have a question - if one were to convert this into an integrated with a line-in input (e.g., in my case, either a DAC, or later on a phono stage such as the Reduction or the Eros), would it be necessary to add additional voltage amplification before the 6J5?

Thanks for any insight.

 

RE: 2A3 with Robin Hood outputs, posted on August 6, 2013 at 12:13:05
Dr. Exotica
Audiophile

Posts: 365
Joined: January 22, 2002
 Also, as a follow-up, I was trying to understand all the math associated with operating points, as well as resistor and capacitor sizing. Rather than just copy as design, I am hoping to understand and learn from the design. Apologies for the simpleton questions ...

For the driver stage, my understanding reading Paul's description is that the grid bias is -6.4v, the plate current is 4mA, and the plate voltage is 185v. Plotting the load line yields an operating point near 135-140v at 1mA. Is this correct?

Another question that I have is how to interpret "the amp will have an input sensitivity of around 2.5vRMS for full power". Does this mean that the grid will swing 2.5v (e.g., from ~5.15v to ~7.65v)? I am guessing not...

 

RE: 2A3 with Robin Hood outputs, posted on August 17, 2013 at 07:41:51
johnsonad
Audiophile

Posts: 759
Location: Central California
Joined: June 9, 2008
 Paul, if you're feeling even more generous than you have been already, would you consider a design using a 45 and the Robin Hoods?

 

RE: 2A3 with Robin Hood outputs, posted on February 1, 2024 at 15:18:39
triode3
Manufacturer

Posts: 413
Location: Midwest
Joined: August 23, 2005
 I know this post is now 11 years old, but I was going through my MQ transformer boxes just today and spotted a pair of RH-40s that I had put aside with a few other MQ irons for future projects... I think I was going to do a poor mans Seth or something else, but I just ran across this post.

Did anyone ever build this? I might as well give it a go, unless there is a "better" RH40 based design out there? It looks nice on paper, and I have two separate power transformers (MQ R-160s) that I could make monoblocks with them (unless I was going to do a Bugle amplfieir)....

Opinions?

 

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