Akai M8 Circuit Cut-Down

[thefragger]

This is for my big dual-mono project.

I've tried my hand at cutting-down the circuit. How does it look? (top is cut, bottom is original; I assumed the switch 'SW1' was in the position reverse of what's shown)

I'm not totally sure what to do with 'SW3' and what's up with the potentiometers on the heater circuits?

 

[ducati_EL34]

This is for my big dual-mono project.

I've tried my hand at cutting-down the circuit. How does it look? (top is cut, bottom is original; I assumed the switch 'SW1' was in the position reverse of what's shown)

I'm not totally sure what to do with 'SW3' and what's up with the potentiometers on the heater circuits?

The pots on the heater are to eliminate hum.

SW3 is the tape speed selector switch. I would simply remove it from the circuit.



.-.-.-

 

[thefragger]

The pots on the heater are to eliminate him.

SW3 is the tape speed selector switch. I would simply remove it from the circuit.



.-.-.-

Rock on, thanks.

Do I remove that 50p cap or ground the one leg out?

Do the two wipers connect to each other (as denoted by the 'X') or are they just left disconnected?

 

[RWood]

The wipers should either be grounded or they can be elevated (cathode of the power tube or a dedicated, filtered B+ node).

One of the hum pots was for the lower-voltage filament winding that fed the EF86, and since you aren't using that tube you can disconnect it entirely. The other pot is for all the other tubes.

RWood

 

[nerdorama]

There is an "X" at the cathode of the EL84 on the original schematic which you've removed from your modified schem. I interpret that as the connection for the "X" on the hum balance pots to give an elevated voltage for hum reduction.

John

 

[thefragger]

There is an "X" at the cathode of the EL84 on the original schematic which you've removed from your modified schem. I interpret that as the connection for the "X" on the hum balance pots to give an elevated voltage for hum reduction.

John

Yea, I noticed that just not on the bottom schematic.

What I did was modify the right schematic and the left one is untouched--even in the original, there's no 'X' like there is at the cathode of the EL84 like there is in the left channel... :scratch2:

There is a node there at the elbow, and to me that demonstrates that it's at lease a three-way connection, and not a compression artifact or some kind of corruption of the image, when coupled with the 'X.'

 

[battradio]

How much gain do you need , with both sections of the 12AX7 in the circuit and one before the the volume control , a CD player is going to over drive the first section .

 

[thefragger]

How much gain do you need , with both sections of the 12AX7 in the circuit and one before the the volume control , a CD player is going to over drive the first section .

Oh hell; that 500k pot is coming out, too. This will just be a power amplifier, no volume nor tone controls.

There will be a preamp before the RCA jacks. What's the suggestion, go with a lower gain tube?

 

[battradio]

All you need is 1 section of the 12AX7 , get rid of the volume control and everything in front of it and use a 470K ohm resistor to ground and a 1K ohm resistor as a grid stoper .

 

[thefragger]

All you need is 1 section of the 12AX7 , get rid of the volume control and everything in front of it and use a 470K ohm resistor to ground and a 1K ohm resistor as a grid stoper .

Since this is for a mono amp, I would be leaving half of the tube disconnected. Would it make sense at this point to add a 3PDT switch underneath to select the 'stronger' side of the tube that I would want to use, or parallel the side, or swap out a lower-gain tube?

Just trying to explore my options. Paralleling a tube looks like there are some benefits to be had there from what I'm reading... :scratch2:

 

[dcgillespie]

Batt is right on here. Not only would you only need the one half of a 12AX7 section before the output tube, but I would also suggest:

1. Remove the network at the output tube plate beginning with C19 in its entirety.

2. Remove the network at the output tube plate beginning with C14 in its entirety.

The primary purpose of the former network was to supply the audio signal to be recorded to the recording head, while the latter network helped shape the proper eq response in record mode.

3. Remove C30.

4. Reapply the NFB network consisting of C18 and R21.

5. Remove R24.

These changes will basically leave a two stage audio amplifier with negative feedback around both stages to provide the flattest frequency response, and an appropriate sensitivity level for basic power amplifier service.

Good luck with your project!

Dave

 

[dcgillespie]

Or simply use a 6AV6, which is the equivalent of one section of a 12AX7.

Dave

 

[thefragger]

Thanks for the suggestions, Dave!

Well now, this is awkward, because I already built up the chassis with 9-pin sockets in mind.

Curious how people are simply pulling the EF86 tube, locking the switch in the 'record' position, and playing with these amps as they are with fantastical results... :scratch2:


Question: When the switch 'SW1' is actuated (moved to the 'left' of its displayed state) R21 and C18 are out of the circuit.

 

[thefragger]

This is a list of mods that are commonly used to make the amps suitable for Hi-Fi use: http://www.audiokarma.org/forums/showpost.php?p=1100906&postcount=9

It looks like this is what's actually happening at the input, re-reading the mods...

[/img]​

 

[dcgillespie]

If operating this amp in record mode is what some folks are doing, that's certainly fine, but it's not "hifi". In that mode, much of the NARTB recording eq circuitry is left in tact, producing anything but a flat response to a standard line level signal. Having an equal response to all audio frequencies strikes at the essence of what high fidelity reproduction is all about to begin with.

If you are looking to have the amplifier operate as it normally would in record mode, then basically all of my suggestions should be disregarded. Sorry for the confusion!

Good luck with it!

Dave

 

[thefragger]

If operating this amp in record mode is what some folks are doing, that's certainly fine, but it's not "hifi". In that mode, much of the NARTB recording eq circuitry is left in tact, producing anything but a flat response to a standard line level signal. Having an equal response to all audio frequencies strikes at the essence of what high fidelity reproduction is all about to begin with.

If you are looking to have the amplifier operate as it normally would in record mode, then basically all of my suggestions should be disregarded. Sorry for the confusion!

Good luck with it!

Dave

10-4 Dave, I appreciate your input. I'm curious about how these amps are being run without R21 and C18 (the NFB) in the circuit when the record switch is actuated.

Have any suggestions for a 9-pin tube that might work for the first stage?

 

[battradio]

R1 & R2 the 1 meg and 10K to ground is the volume atenuator that alows them to inject a normal level signal into the circuit with out overload into the radio input (100 to 1 ratio )
The best fedelity would be to get rid of al the extra stuff that Dave and i sugested .

You could use the EF 86 wired as a triode of just use 1/2 of the 12AX7 perside or put a nice piolet light in the exta chassis hole .

 

[thefragger]

Ok, thanks guy.

Just to clarify, this is my build log: Dual-Mono Akai M8 Amp Build

Thanks for helping me along

 

[battradio]

http://www.audiokarma.org/forums/showthread.php?t=460792&page=2
Are you going to paint the caps , a bronze color woul look nice with the black

 

[jaymanaa]

Just my 2 cents but I'd ditch the entire circuit and start from scratch with the iron and the holes you have. I'm not a big fan of paralleling tube sections due to some stuff I read in an old Sound Practices Magazine. I do however use just one section of two 6DJ8s in my main 2A3 amp. It's handy with that tube as I wired one socket for T1 and the other for T2. Then I can use tubes with one weak or noisy section instead of tossing them out. I like your idea of a switch too, but it it would be an extra "thing" in a fairly low signal crucial part of the amp. I'm not sure if it would have negative effects or not (added capacitance or resistance maybe).:scratch2: I'm anxious to see what you come up with as I have a small mountain of these amps.:yes: