M49 Oliver Archut alt version

[Spencerleehorton]

Just waiting for some other caps and resistors.
With my toroidals I’m getting 20vac and 142vac so only need to adjust B+ nodes, rather than two 6k resistors I’ll try 7.5k and see how close I can get to 120v.

Got bigger case for this to go into now and will also put some tape inside the M49 body to cut down any boom or ringing of the body.

 

[RuudNL]

That is a LOT of µF's for the filament voltage!

 

[Spencerleehorton]

Ruud, I’m open to other designs which keep it quiet if you wanted to pitch in as so far this is the only design which keeps it quiet.

 

[Khron]

Ruud, I’m open to other designs which keep it quiet if you wanted to pitch in as so far this is the only design which keeps it quiet.

Capacitance multipliers?

Ahem...

 

[Spencerleehorton]

Khron, are you able to explain, possibly with a schematic or diagram?
Just putting capacitance multipliers doesn’t give me much of a clue?

 

[Khron]

I didn't think it would be that difficult to highlight two words and do a quick Google search, but ok:

https://sound-au.com/project15.htm

 

[RuudNL]

The M49B is sensitive to hum/noise on the filament voltage. The M49C has much less problems with this.
If you really want an M49B, connect a 1µF capacitor in parallel with R6 and hum and noise are gone with the wind...
(+ on cathode.)

 

[Khron]

The M49B is sensitive to hum/noise on the filament voltage. The M49C has much less problems with this.
If you really want an M49B, connect a 1µF capacitor in parallel with R6 and hum and noise are gone with the wind...
(+ on cathode.)

Not in parallel with R7?

 

[RuudNL]

No, because it is the AC voltage between cathode and grid that matters.
If you would put the capacitor in parallel with R7, the grid voltage would be grounded for AC, but the 'modulation' would still be on the cathode.
I did this modification many times for people who experienced hum/noise on their M49b and always with success.

 

[Khron]

Sounds like the same trick would work for U67's as well, then..?

 

[RuudNL]

I think so, yes.

 

[Spencerleehorton]

There seems to be some confusion as I have built the Oliver Archut alt version which uses cathode bypass 22uf with 2.2k resistor.

 

[RuudNL]

This is the same as the 'C' version, so hum/noise on the filament voltage should be no issue.

 

[Spencerleehorton]

Which is what i thought, but when i build the Gyraf G7 PSU i get hum?

the 0v and Heater gnd dont show connected to chassis?

 

[RuudNL]

Usually a LM317 should be good enough. In case of doubt, add a 4.7 or 10 ohms series resistor (>1 Watt) an a 1000µF capacitor for extra filtering.
At 300 mA you will get a voltage drop over the series resistor, so you have to compensate this by adjusting the output of the LM317 a bit higher. But now you have a very 'clean' filament voltage.
470µF-10K-470µF filtering for the +HT looks more than adequate to me!

 

[Khron]

Which is what i thought, but when i build the Gyraf G7 PSU i get hum?

the 0v and Heater gnd dont show connected to chassis?

View attachment 111169

I'd say they both should get connected to chassis, but at a single point.

 

[Spencerleehorton]

Think I have just realised what is wrong!!

 

[Spencerleehorton]

I'd say they both should get connected to chassis, but at a single point.

So connect them together then put to a single point or keep them separate and put them to a single point?

 

[Khron]

So connect them together then put to a single point or keep them separate and put them to a single point?

The latter - there's quite a bit more current running around in the heater supply than the plate supply - you probably wouldn't want to end up with the former modulating the latter through a ground loop.

But then again, unless you allocate separate ground pins in the connector / wires in the cable, they'll "meet" at the connector...

Are there no suggested board layouts for the PSU available as part of the G7 project?

 

[Spencerleehorton]

not that ive found yet, but ive gone back to percervia with the gyraf g7 PSU, so far voltages are good but still a hum, hum goes away when i switch off PSU and sounds great until the voltage gets too low, so its definetly the design at fault i think, could be a dodgy LM317?
I'll try what Ruud has suggested with adding in a 10R and another 1000uf on the H+ and see if hum goes.