Ultra High Gain Preamp - Valve Wizard’s

[kuDo]

Hello,

I have been preparing and building the Valve Wizard’s Ultra High Gain Preamp the past weeks.
After reading Merlin’s books I was so curious as to how it would sound that I had to try.
I know there is an other forum post on this design but there have been no sounds posted.

Years ago I ordered a couple of Merlin’s universal PCBs. I thought this would be a good occasion to put them to use.
I used two of them. One for the input stage and one for the tone stack.

The PSU I build on separate perforated boards from Tube Town.

I decided that I would supply the preamp tubes heaters with DC this time.

Below you will find a couple of pictures.

[kuDo]

Yesterday I finished wiring up all the parts.
And today I switched it on for the first time using the variac.

At first I seemed to have made a mistake. As I was only hearing a loud humming sound, when I amplified it using my tiny lm386 chip amp. After investigating a little it must have been the wire connecting the preamp to the chip amp.

So I am happy to say it works. There is plenty of distorted sounds to be had.
So far I have been feeding sounds using my looper pedal, sinds I have sold my Ibanez Kiko10p guitar past week.

[kuDo]

There is however I little bug still.
There seems to be a 150hz hum present. Which is off course unwanted.

The hum is there even when the input is grounded.
I have been experimenting a little. By undoing the wiring for the heaters and re-wiring it for 6.3volts power supply.

Initially The heaters were supplied 12.6volts dc.
But I have now tested it with a 6 volt battery.
The 150hz hum is gone, only to make room for a 50hz version and a high frequency hiss. Both however are more quit then the 150hz hum from de dc supply.

What could cause the 150hz hum? It seems to be a multiplication of the 50hz from the net.
Currently the heater DC supply is floating. Should it be grounded?

If any of you have any thoughts on the matter, please enlighten me.

Happy new year all!!

[pdf64]

The heater circuit shouldn’t be left floating. It needs some sort of DC reference to prevent the heater to cathode voltage exceeding the limit, eg 180V.
And the AC impedance between the heater circuit and circuit common needs to be low, eg not much higher than 100 ohms.

[kuDo]

The heater circuit shouldn’t be left floating. It needs some sort of DC reference to prevent the heater to cathode voltage exceeding the limit, eg 180V.
And the AC impedance between the heater circuit and circuit common needs to be low, eg not much higher than 100 ohms.

Aah, great! It worked.

First I tried referencing the AC side of the heater supply to the screw that attaches the toroidal transformer to the chassis. That already helped. But then I disconnected it again to try referencing the other side of the rectifier.

I have now referenced the DC side of the heater supply to the input ground. That did the trick. It is now without the 150hz/50hz hum it had earlier.

How is it that, thát little connection made it hush?

This circuit doesn’t have a cathode follower so if I am correct there is no need for elevation. The 180volts is not being exceeded.

Thanks sir!

[sluckey]

It's not a matter of elevating the heaters. It's a matter of referencing (connecting) the heaters to ground.

[kuDo]

It's not a matter of elevating the heaters. It's a matter of referencing (connecting) the heaters to ground.

Okay, that’s clear. but why is that responsible for the disappearing of the hum?
It had already had a DC supply? A mystery to me.

[LOUDthud]

Okay, that’s clear. but why is that responsible for the disappearing of the hum?
It had already had a DC supply? A mystery to me.

There is parasitic coupling between windings inside the transformer due to the way it is wound. Usually the 6.3VAC heater winding is the last thing added over the top of a high Voltage winding. Other Voltages may be next to it because the 6.3VAC won't take up a full layer. Although the capacitance between the windings is only 50pF to 100pF, the AC Voltage is quite high, so significant Common mode Voltage will be on the 6.3VAC to ground unless it has a center tap that's grounded. (Or you could just ground one side like an original 5E3.) Take the ground off the 12.6VDC and measure the AC Voltage to ground with your DVM. Caution: If you touch it, you'll need to change your underwear. This big AC Voltage will cause hum if it's anywhere near your preamp.

[kuDo]

Okay, that’s clear. but why is that responsible for the disappearing of the hum?
It had already had a DC supply? A mystery to me.

There is parasitic coupling between windings inside the transformer due to the way it is wound. Usually the 6.3VAC heater winding is the last thing added over the top of a high Voltage winding. Other Voltages may be next to it because the 6.3VAC won't take up a full layer. Although the capacitance between the windings is only 50pF to 100pF, the AC Voltage is quite high, so significant Common mode Voltage will be on the 6.3VAC to ground unless it has a center tap that's grounded. (Or you could just ground one side like an original 5E3.) Take the ground off the 12.6VDC and measure the AC Voltage to ground with your DVM. Caution: If you touch it, you'll need to change your underwear. This big AC Voltage will cause hum if it's anywhere near your preamp.

Thank you for the explanation. So by connecting the two separate supply loops, they have the same base. After that there is no big alternating difference between them anymore to be amplified, right?

[kuDo]

This is the PSU that I used for HT and DC heaters.

The heater supply's LM317 might need a little adjustment. Currently it is at 10.9v dc where it should be 11.4v dc at minimum for it to be optimal. 12.6v would be the goal.

[kuDo]

This is the amp at the moment.

I used the pointers Merlin gave in his new power supply book.

Maybe I will put this preamp in a dedicated housing. But before I decide, I wil wait until I have used it with a guitar and see if it is worth the effort.

[sluckey]

The heater supply's LM317 might need a little adjustment.

I think your LM317 circuit is flawed. Try this instead...

[martin manning]

The LM317 is being used as a current limiter. That's nice for a gentle start-up. I don't see any problem using it as shown, as long as there is only one ground reference on the DC side. Note that the regulator will limit to 300 mA for the two heaters in parallel, but the voltage may not be exactly 12.6 due to manufacturing tolerance. Measure the voltage across the 4.1 ohms and calculate current to verify that you are getting what you want.

[R.G.]

N.B.:
Hum at the power line frequency (that is, 50Hz or 60Hz) is caused by the mains leaking into the audio path. This can be poor grounding, capaictive coupling, magnetic field coupling AC wall socket ground or neutral shifting, that kind of thing.
Hum at twice the power line frequency (100 or 120Hz) is caused by the rectifiers. There is always 2x power line frequency in full wave rectification. This forms the classical sawtooth ripple voltage on the B+, but is also part of the hum caused by ground blips caused by poor grounding of the first filter cap. It's possible, but very rare to get capacitive leakage of the ripple voltage coupled into very high gain stages as well.
Hum at three times the power line frequency (150 or 180Hz) is caused by something clipping the AC mains sine wave. This can come from magnetic saturation of the incoming AC mains in the power transformer being coupled magnetically into the signal path.
Technically, rectification produces 2x, 3x, 4x, 5x, ... hum components, but it's unusual to see much beyond the first few components as the size of the harmonic decreases very rapidly.
A buzzy sounding hum is usually caused by pulses happening at 1X or 2x the power line frequency. These are most commonly from fluorescent lights or dimmer hash being coupled through the AC power lines, but can sometimes be caused by the "slam off" nature of silicon solid state rectifiers. The cure for this last cause is snubbing the rectifiers or changing to soft recovery diodes. I notice that the schematic includes "UF..." specced diodes, which is good; changing to 1N... diodes could cause rectifier buzz, so be sure to use the UF ones.

[kuDo]

Measure the voltage across the 4.1 ohms and calculate current to verify that you are getting what you want.

It is due to the fact that I picked a standard transformer, which might not entirely match my calculations. The output voltage of the transformer winding for the heaters might have lowered a little.
A thing I need to get use to is that in practice things just might be a little different from my basic calculations
Maybe I should have used a variable resistor.