SJS SE 300B with Interstage Transformer Mk2

[DVDHack]

Following the thread I started on the "Opinions on Walton Audio 300B Design" which started a very lively debate, I have decided to pursue a different design.

I will be building this design by SJS Electroacoustics




I welcome any comments on the design. I'm particularly interested in the Power Supply as I'd like to change the 470uF capacitors to something smaller - as I want to use a quality unit and many do not go to these values.

Please keep all discussions on the topic of design and construction of this amplifier.


Many Thanks


Ralf

[ThorstenL]

Konnichiwa,

Originally posted by DVDHack
I welcome any comments on the design.

Main one, experiment with lower current for the Driver valve (10 - 12mA rather than the > 20mA used. The result is somewhat higher 2nd Harmonics and MUCH lower upper harmonics which when mixed with the output stage distortion result inmuch lower high order products and somewhat lowered 2nd harmonics.

Seconly, note the 100uF Electrolytic capacitor in the fixed bias supply is actuallu IN THE SIGNALCURRENT loop. It acts as coupling capacitor in the circuit, so you retain a coupling capacitor, though not in the usual place and hence invisible, PLUS a much lower quality one.

Also, you have a massive Electrolytic in the Driver Cathode, not good at all. You may wish to experiemnt with more modest values of Film Capacitors (start with 100uF). Thgis can with the primary inductance of the Interstage Transformer lead to a peakerd up lower LF response which with the right "tuning" can extend the LF response of the Amplifier to something ebtter than possible with just the output transformer.

Originally posted by DVDHack
I'm particularly interested in the Power Supply as I'd like to change the 470uF capacitors to something smaller - as I want to use a quality unit and many do not go to these values.

Well, you could use one 50H Choke and one 470uF Capacitor. Or you could use two LC Cells, with easily obtained 10H/100R Chokes (make sure the first one is suited to choke input and the second one offers as high a resonance as possible) and 50uF Oil/Paper or Film Capacitors.

In the first case you get around 15mV hum on the +B supply and around 0.65mV Output Hum (assuming NO other hum sources). In the second case you get 2.5mV hum on the +B supply and 0.1mV Output Hum (assuming no other sources).

The downside is that the supply with the two 10H/50uF cells has a slightly poorer load step response, which may be audible. If you inchrease the seond choke to 20H and use two 50uF Capacitors things improve notably and hum is even lower (0.2mV on +B line).

For the 300B Heater Supply make sure the transformer has an electrostatic screen and I would recommend using a suitable Choke in the filter circuit, instead of the Resistor. Maybe 100mH/1.5A/1...2 Ohm should be easily doable using normal winding machines and Iron.

Such a chokje has an impedance of 63 Ohm @ 100Hz, compared to 1 Ohm @ 100Hz for the 1 Ohm Resistor. This gives around 30db lower noise on the heater (which is also the cathode) than the simple RC circuit. Using the proposed LC filter the noise on the heater is around 0.4mV with the RC filter around 15mV and has a waveshape that is quite rich in higher harmonics, which might cause an audible "greying out" of the sound.

Anyway that would be my suggestions that exclude fundamental re-designs to eliminate some of the subideal approaches taken.

Sayonara

[DVDHack]

Does that 100uF Cap and Resistor network need to be there at all - is it just setting the Voltage?

I want to change those 470uF caps to 220uF, can I do that? Going to try and model this stuff in that PS Application.

That 2200uF Cap is there to avoid the Bottom end roling off I think, so I'm happy to change it to a smaller bypass value - I'll determine the size once I decide if I'm running a Sub Woofer on this system, thinking I'll get away with less efficient speakers by eliminating Bass load and get beter sonic performance.

I'll probably add a switch to allow me to change the bottom end response by putting additional capacitance in place - then its suitable with non sub situation as well.

[arnoldc]

hi,

Originally posted by DVDHack
[B]Does that 100uF Cap and Resistor network need to be there at all - is it just setting the Voltage?

it is there to make sure you get accurate voltage to bias the output tube.

I want to change those 470uF caps to 220uF, can I do that? Going to try and model this stuff in that PS Application.

better. i simulated it in the past and i was getting weird results, so i dumped that section and made a new one. however, my IT has not arrived. i'm stuck.

That 2200uF Cap is there to avoid the Bottom end roling off I think, so I'm happy to change it to a smaller bypass value -

i feel the same way. in my 5842 dc'ed to 245 i also have a high value, but i don't have a scope yet to try to figure out how low i can get it to.

cheers.

[DVDHack]

You can calculate the capacitor you can get away with

C = 1 / (2 * pi * F * R)

That is for the -3dB point

[SHiFTY]

You may need those 470uF caps on the power supply, due to the choke input there is a lot of ripple. But 470uF/450V is a pretty common value these days due to computer power supplies. Also make sure to use very, very good chokes otherwise they will buzz mechanically. Most chokes do when you put ripply DC through them.

In my opinion, choke input is wasted on this design. Single ended amps have fairly constant current draw, so the better regulation from a choke-capacitor (L-C) input filter is not really noticeable over plain CLC filter. And you would need at least 3 chokes and 3 rectifier tubes, and a custom power transformer! Ouch.

The 2200 6.3V bypass cap looks huge to me, but it might be due to the I.T. i suppose... I use about 470uF on the first stage in my R-C coupled design and thats huge overkill.

And why the 437A? As far as I know its a radio frequency tube a bit like a 12AT7. Oh hang on, its made by Western Electric Must be amazing

Heh don't mean to rain on the parade but why not try a more conventional design like the Flesh and Blood for your first amp?

[arnoldc]

Originally posted by DVDHack
You can calculate the capacitor you can get away with

C = 1 / (2 * pi * F * R)

That is for the -3dB point

yes, that's what my spreadsheet say. but i just want to compare actual and computed.

cheers!

[ThorstenL]

Konnichiwa,

Originally posted by DVDHack

Does that 100uF Cap and Resistor network need to be there at all - is it just setting the Voltage?

The resistor network sets the bias voltage, the Capacitor closes the current loop for audio signals. Both are needed to have the circuit function as intended, BUT given the DC resistances around it the capacitor could be a much lower value Film Type, I'd try 10uF film of good quality.

Originally posted by DVDHack
I want to change those 470uF caps to 220uF, can I do that?

Yes, but hum is going up. Plus there are few readily available
non-electrolytic of that kind of value anyway and you really do not want ANY electrlytic capacitors in signal current loops if avoidable.

Originally posted by DVDHack
That 2200uF Cap is there to avoid the Bottom end roling off I
think

Not DIRECTLY. It reduces the increase in aprarent anode impedance. If the value is too small the effective anode impedance of the Valve rises with lowering frequency, which MAY lead to LF rolloff in conjunction with the load.

If you operate such a stage with an unloaded interstage transformer as load, the cathode capacitor, cathode resistor, psu capacitor and valve form a damped resonant circuit, whose resonance usually falls below 10Hz and can be used to boost
the LF response a little by correct tuning extending the LF response.

For that we "preemphasise" the drive signal to the 300B to offset some of the LF rolloff caused by the (loaded seconadary and thus usually well damped) Output Transformer. If the LF lift ends up too much, a "damping resistor" of a few 10's of KOhm can be used to finetune the degree of lift, the cathode capacitor tunes the frequency of the lift with a given IT.

Ideally we want a non-electrolytic capacitor in the cathode, of high quality. As these are rarely found in large enough values (exceptions are the Rubycon SWR series which includes a nice sounding 100uF/35V Mylar Cap), so electrolytics are used. Electrolytic capacitors have rather high levels of various distortions, these are best minimised by using ovsezized Units, which sadly again limits the choice rather more than one likes.

I think Film Cap's like the mentioned Rubycon are a better choice.

Originally posted by DVDHack
so I'm happy to change it to a smaller bypass value - I'll determine the size once I decide if I'm running a Sub Woofer on this system, thinking I'll get away with less efficient speakers by eliminating Bass load and get beter sonic performance.

Actually, a lower value cathode bypass cap in the circuit you intend to use will extend the LF and possibly even lead to a bass boost.

Originally posted by ShiFtY
You may need those 470uF caps on the power supply, due to the choke input there is a lot of ripple.

Ripple cleanup is best done using a second LC cell with a choke wound for minimal shunt capacitance.

Originally posted by ShiFtY
But 470uF/450V is a pretty common value these days
due to computer power supplies.

Only as Electrolytic Capacitors, which are a poor choice for Amplifiers.

Originally posted by ShiFtY
And why the 437A?

Very high transconductance allows very low drive impedance. It can be substituted with a range of similar valves including triode wired pentodes, just look for a high transconductance and > 4W Anode dissipation, plus a Mu > 40.

Originally posted by ShiFtY
Heh don't mean to rain on the parade but why not try a more conventional design like the Flesh and Blood for your first amp?

The F&B sounds okay but not overly exciting (non of the 2-Stage 6SN7 Driver Circuits do to my ears) and our friend does want build his first SE Amp, he wants his ONLY SE Amp....


Sayonara

[angel]

As far as the WE 437 is concerned, you'd probably wan't to go for the 6S45P-e instead, which is more readily available, and is a pretty close equivalent. It'll run you something like USD 10 ea.

As for Torsten's suggestion to halve the bias current, I think this depends on the current capacity of the interstage transformer. If necessary, you could use a PP-to-SE interstage transformer, and a rheostat to find the appropriate flux density.

Most people I've seen tend to comment that the 6S45P-e performs ideally at 40mA and above. Granted, these people usually go for a capacitor instead of an interstage, but again, I think that's because not all SE-to-SE interstages like 40mA DC. A couple of the Lundahls do, however.

[ThorstenL]

Konnichiwa,

Originally posted by angel
As for Torsten's suggestion to halve the bias current, I think this depends on the current capacity of the interstage transformer.

Actually, the suggestion to lower the Current have more to do with making the Driver stage more linear. Lower current means slightly increased 2nd harmonics (around 6db) but DRASTICALLY DECREASED higher harmonics. The difference in sound is quite profound. Valves like 6S45 or WE437A at somthing like 40mA tend to sound hard and steely when used to drive a 300B and show a quite substantial number of higher harmonics coupled with a fairly substantial levels.

Note that the "low current/high voltage" operation is for use with Inductive or active loads ONLY, not generally for resistively loaded stages.

Sayonara