Help needed from someone who understands tube amplifiers / tube heaters

[MatthewMachinist]

Hello team

I am in the process of working with a pcb designer to design pcbs for the Group DIY Reddi DI project.

Attached are some images of the board.

My question relates to the HH 6.3v DC supply for the tube heating. My understand is that pins 4 and 5 are for heating.

The pcb designer has made is so that both 4 and 5 are positive. Presumably he is assuming that the current will return via the ground pin.

I was thinking along the lines that pin 4 should connect to positive and pin 5 to ground.

If anyone can let me know I would appreciate the help.

 

[Tubetec]

Some tube heaters can be run both in parralel at 6.3 and series 12.6 v ,depending on how you wire them ,

 

[MatthewMachinist]

Tubetec

Does that mean the pcb is correct?

I just checked a tube with my multimeter and there is continuity between pins 4 and 5, but there is no continuity between 4/5 and 9. This makes me think the pcb is wrong.

Cheers.

 

[moamps]

....This makes me think the pcb is wrong...

It is if ECC88 is used. Also heater "ground" should not be mixed with audio "ground".
There are some other small flaws, your pcb designer is obviously a beginner. And it would be good to post schematic to which pcb refers.

 

[MatthewMachinist]

Thanks for the help Moamps, attached is a schematic

 

[beatnik]

The capacitors are too close to the tube, it will work but the heat will shorten their life.

+1 about keeping the heater ground away from audio ground

This circuit is so simple it kinda doesn't make sense to put the effort in a pcb design. I would use point to point wiring instead.

 

[Walrus]

The LM317 will need a large heatsink, and the output attenuator pot is on the wrong side of the 300 Ohm resistors.

 

[abbey road d enfer]

the output attenuator pot is on the wrong side of the 300 Ohm resistors.

I'm not so sure about it. It's wired as a variable shunt on the xfmr secondary.
I wouldn't speculate if it has advantages over a potentiometer arrangement, but it works.

 

[abbey road d enfer]

This circuit is so simple it kinda doesn't make sense to put the effort in a pcb design. I would use point to point wiring instead.

I understand the OP's desire to have a clean PCB, that maybe he intends to put for sale, but considering the number of mistakes and misunderstandings*, a point-to-point arrangement would provide ease of rectifying the mistakes much easier than with a PCB.

*e.g. "The pcb designer has made is so that both 4 and 5 are positive."
Actually, none should be grounded on the PCB.

 

[MatthewMachinist]

The reason I am designing a pcb is to make the project more beginner friendly. Point to point is a bit intimidating for someone who has never done it before.

Ever though no one has said it explicitly I think I need to go back to the designer and say that I want a separate pad leading away from pin 5, which is to be hooked up to the negative 6.3v power supply.

 

[MatthewMachinist]

What would be a minimum distance from the tube to the capacitors, I'm thinking we should increase it by about 10mm

 

[abbey road d enfer]

Ever though no one has said it explicitly I think I need to go back to the designer and say that I want a separate pad leading away from pin 5, which is to be hooked up to the negative 6.3v power supply.

Please note that heaters do not care about polarity; actually most are designed to operate on AC current.

 

[abbey road d enfer]

What would be a minimum distance from the tube to the capacitors, I'm thinking we should increase it by about 10mm

That is not enough, particularly if the circuit is in an enclosed box. There is a reason why tube equipment is bulky.
Many tube equipment have the capacitors away from the tubes.

 

[ruffrecords]

I notice there are no thermal reliefs on the pads connected to the ground plane. They are going to be difficult to solder.

To help with heat it is sometimes better to mount the PCB so the tube is horizontal. That way there is more surface area causing convection.

Cheers

Ian

 

[Walrus]

I'm not so sure about it. It's wired as a variable shunt on the xfmr secondary.
I wouldn't speculate if it has advantages over a potentiometer arrangement, but it works.

If the pot is turned right down, it will present a short circuit to the transformer.
Is that not a problem?

 

[abbey road d enfer]

If the pot is turned right down, it will present a short circuit to the transformer.

It's understood.

Is that not a problem?

It would be a problem if it exceeded the output capabilities of the tube, but here the tube is like a current generator, so the voltage collapses as the current stays more or less constant.

 

[moamps]

The reason I am designing a pcb is to make the project more beginner friendly. Point to point is a bit intimidating for someone who has never done it before. Ever though no one has said it explicitly I think I need to go back to the designer and say that I want a separate pad leading away from pin 5, which is to be hooked up to the negative 6.3v power supply.

Here are some of my suggestions that not everyone may agree with.

Both terminals 4 and 5 should be connected by separate wires to the heater power supply. Pin 9 can be connected to audio ground although this is not necessary in this case. Heater power supply can be elevated for some voltage above ground potential.

Mounting holes should be insulated from the audio ground. The audio ground is connected via a "ground lift" switch to the chassis safety ground.

The potentiometer on the transformer's secondary needs to be moved after the 300 ohm resistor, unless someone intentionally wants to use it as a saturator (with guitarists you never know what’s on their mind).

A 10mF capacitor in the heater power supply should be rated for 16V.

Tell the circuit designer to pay attention that some tracks are at high voltage, and that some current is flowing through some others.

Capacitors C1 and 2 can be rated for 16V and should be rated for 105degC. R2 and R4 may have a common value, eg 1k2.

In the end, be a little inventive, replace that design with a Russian junk of tube with some old beautiful vacuum tube from RCA, GE, TFK, ITT etc. eg. ECL84 (6DX8) or PCL 84 pentode (under €5 on ebay) in a triode connection.

 

[abbey road d enfer]

The potentiometer on the transformer's secondary needs to be moved after the 300 ohm resistor, unless someone intentionally wants to use it as a saturator (with guitarists you never know what’s on their mind).

As I already commented, this may be intentional; who knows?

 

[Gus]

The circuit and PCB need work.

Is this a copy of something?

To add and repeat what others posted

PCB track layout the distance between the tube pins traces?

8VAC for a 6.3VDC supply using a 317? this part needs help

>16mA SE in the primary? Are you making a power amp?

The circuit and PCB looks very beginner

If you are going to sell this do you have liability protection if someone hurts themselves?

 

[industrialarts]

To add to the noise - I am +1 for the point to point wiring, even though I understand the designer wanting a PC board. A solid bus bar of something like 14g copper wire would anchor most of the components that are not tied to input and output, maybe one terminal strip inside. Heatsink the LM317 to the case.

my 0.02