Request for suggestions. Akai M8 mono amp schematic validation.

[xpl1c1t]

Dave,

I'm taking your advice, I realize that the gain is definitely too high for anything but phono cartridges, even then..

So i'm considering doing away with the 12ax7 entirely and using the EF86/6267 tube in triode mode as the preamp.

Seeing the Phillips specification sheet makes me wonder; rather than insert a volume pot into the signal path, since voltage and current output are directly related to the supply voltage, a potentiometer regulating this supply would be effectively equivalent. I'm unsure about how this would affect performance.

For reference, here's a chart of the relevant EF86 triode characteristics

[planet10]

I do know the EL86 can work, that is what we have at the front of our RH84 monblocks.

Changing the supply voltage to change volume is a can of worms.

dave

[xpl1c1t]

The M8 chassis has 4x octal sockets, so i figure i can build it up according to the following schematic. (I enjoy the concept of transformers and their simplicity, plus relatively inexpensive Edcor products fit in each position, so i figure why the hell not - i think i can make everything fit.)

The tubes are;

2x EF86 (triode mode)
2x EL84 (triode mode)

for a total of about 5 watts

[planet10]

I has 4 noval 9-pin sockets, To implement that circuit you would need to replace all the iron, what is the point in that?

dave

[xpl1c1t]

The push pull design is simply appealing. I read that distortion decreases and low volume power would be more readily available, both things I am interested in attaining. Just the output transformes are replaced, the input and interstage transformers are added. The power transformers should hold up, they are rated for 60MA.

6v4 - ~1.5ma max
2x EF86 ~2.5ma max each
2x EL84 ~25ma max each

That is pushing the limits of the power transformer, but I do not intend to be running at full volume ever.

[planet10]

If you want to do a push-pull amp, then you need to start with a different donor.

A pair of push-pull EL84 draw ~ 80 mA.

A pp will decrease 2nd order harmonics below the 3rd, not necessarily a good thing, and the output transformer is then running thru the kink in the magnetic curve, increasing distortion levels at low power.

If you are using the M8 as a donor, you have the means to build a mono SE amp. If you want a PP amp, time to start from scratch... a shame, because tarted up, the M8 can be a fantastic little amp.

dave

[xpl1c1t]

The one i have intact sounds great indeed, and though the gain is pretty high and there is some noticeable distortion and granularity from the unreplaced capacitors and components, I agree fully that it has hooked me on tube amplifiers. Thus the reason for the last weeks of study and postulation for what i want to achieve and what through my studying has seemed most idealistic.

The phillips datasheets, which have been absolutely the best help in decision making, have the following data (attached)

At the rectified voltages from the 250V Main, which should be between 300-250v after the initial CLC filter, these datasheets point in the direction of a anode current between 20-26ma.

The top part of this clipped datasheet depicts the triode connected characteristics (which i'm interested in).

[xpl1c1t]

Heres the current WORK IN PROGRESS schematic.

Throw some bones if you are interested.

[xpl1c1t]

The internet still amazes me.
Thanks to forums like these and the abundant audio related information sites I have learned a plenty over the last two months without touching and idiots guide manuals (though I have ONE on the way, i dont suppose it will change this schematic very drastically).
Dave/Planet10, thank you for noticing how flawed my initial design way, you must have laughed seeing the first circuit I posted. And for seeing me through my variations on the theme.




Plate choke and interstage, and my reasons for using them;


Plate choke to; maximize output signal swing, linearize the EF86 loadline.

2:1 Interstage to allow; stepped-down input impedance (4x lower for 2:1 step-down), full bandwidth transmission of ~40VRMS stepped-down to ~20VRMS, phase splitting!

The plate choke on the EF86 triode will be 120 Henries.
Since the EF86 will have a internal resistance of ~16k, the following calculation solves for a -3db rolloff at frequency X.
X = 16,000 / (2 * pi * 120) = 21Hz

It will be composed of two series-attached Hammond 155C chokes and I'll be taking the following gentleman's advice. (from here)

Originally Posted by Gary P

3rd, you can mount the chokes on standoffs, bottom to bottom and wire the chokes with one winding backwards. This makes the pair of chokes hum buckers. In my test setup the pair of chokes picked up 20dB less hum than a single choke.

Additionally, the choke's series capacitance will be reduced by half.

My decision to not run B+ through the primary of the interstage was simply because I cant find a suitable transformer with a primary impedance of 120H+ to allow for the rolloff that these chokes will supply.

Which brings me to the one gripe I have with the circuit, the coupling capacitor between the EF86 anode and the interstage transformer. Blocking DC on the interstage seems a good thing; but how will this play out in terms of the resonance between the cap both the plate choke and interstage primary?

I'm aware of the equation for determining the resonant frequency of an LC circuit;
f = 1/2pi*sqrt(LC)
For the 120H choke, i'll have a 45Hz resonance with the .1uf cap in the schematic (not my final choice)

If the interstage primary is about 25H (likely if I choose an Edcor), the 20Hz resonance occurs with 2.5uf cap. Pointedly, i should increase the inductance or increase the capacitance, eh? Thats a big cap for signal though...


(Any recommendations for solid interstage transformers would be helpful)

[xpl1c1t]

Pretty awesome how the M8 power supply has perfect specs for this application.