DIY Audio Projects Forum

Thank's,

I'll have a look at those, never seen them before.

Regards
M. Gregg

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What is the sound of one hand clapping?

Whilst building the board,

a thought that came to mind was the reference voltage for the heater lift above ground.

The specified resistance values give something like 45/50v lift, however looking at the cathode voltage of the 5687 this would seem to be about 5.5v (in circuit) <<this isn't from testing the board its not powered up yet its based upon quoted Anode current on the internet.

So I'm going to look up the max heater to cathode voltage of the 5687.
Does anyone have any thoughts?
There are several circuits on the net but they are all based on the original circuit the same as I'm building:
https://diyparadise.com/w/simple-5687-based-preamp/
I assume this is the original M7..maybe. It seems to match values from internal pictures of the M7 shown on the internet.
The original uses the white coffin type 5 watt wire wound resistors.
The problem with over voltage lift on the heater is heater emission or flashover.
So the potential difference in theory would be about 40V between heater and cathode.
NB I'm not going to connect direct to one side of the heater supply just using a couple of resistors to create a false centre tap and lift the whole supply.

I'm wondering if this heater reference is based on the cathode follower M7 circuit but I don't have the cathode voltage of the 5687 in that circuit. My guess is its probably higher than the Original M7 circuit. I might float a polypropylene cap from the heater to ground then power the circuit and look at the resultant charge voltage on the cap to see what's happening.

Regards
M. Gregg

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What is the sound of one hand clapping?

Max heater to cathode voltage is quoted as 90V,

So I guess its OK, its interesting to note Max plate voltage is 300V.

All the above from the spec sheet, it still seems a high lift for the heaters.

Regards
M. Gregg

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What is the sound of one hand clapping?

Slight mod to main board,

to give more flexibility to mod input wiring.
The Caps are supported using lacing and spacers.
Next step is to power up the main board and check voltages.

Regards
M. Gregg

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What is the sound of one hand clapping?

Becasue cathode to heater voltage and biasing is no secret.
The ideal cathode to heater votlage would be 0.

By changing the resistor divider ratio to set the heater bias votlage you can get close to zero.
But if one valve has hv on the cathode arround 1/2vcc, the other is close to ground.
Then you wanna bias half way in between, which is 1/4vcc

Yes agreed

But its interesting in the links that others trying to copy the M7 are using 40V difference between cathode and heater, that makes me wonder of they have the heater bias set from the other M7 which has a cathode follower output.
The interesting thing is that I would expect this with SRPP but this circuit isn't SRPP.
So whats going on for so many people to "get it wrong", or is it correct to the original M7?

Here is the same heater bias with the cathode follower:
https://www.google.co.uk/search?q=audio ... vtM:&vet=1

And this from a different redraw:
https://www.google.co.uk/search?q=audio ... vIM:&vet=1

If you look at the circuits here on an M77 its the reverse situation high voltage on the cathode but standard heater CT to ground?
http://www.dms-audio.com/audio-note-m77-diy-clone

Its very strange for so many people who must have researched this circuit to all get it wrong
Am I missing something obvious here??

It looks to me like the cathode follower needs the heater lift but the "original M7" I expect to see about 5.5V on the cathode.
Which would make the others wrong, but both should work with a heater to cathode max allowed at 90V (spec sheet).

LMAO I could use a couple of LEDs in place of the bottom half of the voltage divider and get regulated 5V and bias the heater to 5.5V
Or a 5V regulator with a diode on the ground leg. Its all a bit nuts, I'll just measure the cathode and look again.
Is there some reason they don't connect the heaters to Ground via the usual couple of resistors
I wonder if it has a sonic impact on the circuit...I wonder if you can get some sort of modulation from ground noise.
Maybe the 5V reference isn't such a mad idea.
In the past I have floated heaters with a high voltage polypropylene capacitor that just charges from the heater leakage.

Regards
M. Gregg

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What is the sound of one hand clapping?

Allot of people only know the coppy n paste.
They can't do anything for *****.
Looking at schematics for big and famous companies I can too always spot many mistakes.
So its no supprise that hobbie dudes online get it wrong.

When you bias up the heaters then you obviously cannot directly connect the heaters to amp ground anymore.
Because its connect to the bias voltage with refrence to amp ground using the amp b+
Now your using a large capacitor 100uF or larger to ground the heaters to amp ground.

OK,

power up main board<<guess what in my stupidity I normally fit 1 meg resistors across the input but following the circuit (which doesn't use them) and just testing the board I had no volume pot connected. Its obvious the tubes ran flat out (with the grids floating) but limited by the anode resistors. So no damage done.

B+ readings
218.5V

Voltage across anode load was wildly different.
164V V1 / 170V V2.

Cathode voltage across the 2K
6.72V 6.74V

I immediately realised what was wrong so I put a 1Meg across the input on each channel.
New readings
B+ = 234V
Voltage across the Anode load
138.0V V1 / 140.2V V2

Cathode voltage across 2K
5.57V 5.65V so a slight difference in tubes.
So the input will keep the 1meg tantalum resistors I just prefer it.
It will probably get the 22K series resistor as well but we will have to see.

So I was expecting about 5.5V on the cathode its pretty close.
The main board is now fitted in place.
Next step is a tidy and clean of the connections, then look at the heater reference "lift" about 6 V should do it better slightly higher than lower.
I just can't bring myself to rely on the pot wiper to hold the grids down.

Regards
M. Gregg

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What is the sound of one hand clapping?

Thinking about the heater ref,

I will probably leave the 100K and 20K divider so if anything fails the most it can go up to is 40/50V with a max spec of 90V.

Then fit something like LED's or a Zener across the 20K to give about 6/7V reference voltage for the heaters.
That way there is no direct ground.

Regards
M. Gregg

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What is the sound of one hand clapping?

Heater Ref,

OK I tested some LED's I had left over from the indicators and series connected them Four gives about 7.5V which is approx. 1V above cathode voltage so I have fitted them and tested the main board which seems ok at DC levels. I was going to sleeve the LED's with heat shrink but the indication of B+ on is quite useful because you can see the system discharge.
I also wired the 22K tantalum onto the volume control and connected the ground with stranded silver wire.

So the next step is to wire the balance control.

Regards
M. Gregg

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What is the sound of one hand clapping?