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Problem with the high hum in a tube amplifier for a guitar

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tubes humming

Hi Frank,

I'm sorry for not paying attention! Now I see the problem, didn't understand it before(the 5% at mid position).. I'll try the 100k to ground before I change the pots (don't have any in stock) Thanks,

Thomas
 

ground loop for tube amplifier

Hi Frank/Bob!

I think I've got all the pots wrong! I thought that 'A' was Linear and 'L' was Logarithmic...wondering who decided to change these pot codes:?:


I guess thats why the Eq's are very sencitive:D

Thomas
 

diagnosing tube amp problems

Now I just have to wait untill the pots arrive..In the meanwhile I'll try to figure out how the footswitch is working.

My question is what is the purpose of the 0.1uf in the "red" circle going from ground/-3v and then again to ground/-3v??

Thanks

Thomas
 

hum eq changes affect the humm

That is a mistake - pin 5 of the 7420 and the capacitor terminal should go to pin 14, Vcc.

If pin 5 is gounded the output on pin 6 will always be high.

/Ram
 

    T

    Thomasdj

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tube hum in right channel

Hello Thomas,

that's obviously a drawing error. The Pin 5 wire most likely should be connected to Pin 14 (Vcc). Pin 8 would also work, but then the 0.1u capacitor has no purpose, it makes sense as a supply bypass only.

Regards,

Frank
 

    T

    Thomasdj

    Points: 2
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grounding tube amps

Hi Frank/Ram

Thanks! just had to be shure :D

Thomas
 

how to use a scope to find amp hum

On a side note, the "digital" design of the footpad is both ingenious and silly - probably the result of many failed iterations.
But as long as it works, it is fine. Today the same thing would probably be overengineered with an MCU... :wink:

/Ram
 

    T

    Thomasdj

    Points: 2
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ground star tube amp

Hi Ram/Frank,

I'm a a newbee in eletronics so guess I'll have to stick with it as is..

I actually have heard some bad thing thing about this footswitch (unstability etc.)

What can I do to make the design more stable? I've attached the recieving part of the circuit ("Rotary" is the manual control voltage, "red circle" is the footswitch control voltage).

Thanks

Thomas
 

relay problems in tube amplifier

Wow! It is getting hilarious... :wink:

They generate a three-level analogue voltage in the footswitch, which mimics the one
that is generated by the rotary switch, and then feed it to an LM3914 - a dot/bar
LED driver that is used to switch something. The CH2 voltages are different though...?
Where do the signals from pins 1 and 17, of the LM3914, go?

It is understandable why the footswitch operation can be unstable.
I haven't dug deeper into it but one thing that comes to mind is the generation
of the "Low-V supplies" - page 5 of the "3ch Solo Head" schematics.
I can imagine that those voltages could fluctuate a bit depending on their load conditions.

Seeing this I would actually prefer an MCU version... :D

/Ram
 

    T

    Thomasdj

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+tube amplifier +hum

Hi Ram!

Your right! offcourse it's the load conditions! Don't know anything about MCU's so I gues I will try to put a -3v voltage regulator to supply the switching. The 1.8v (2v) are supplied via a 1N750 zener..so guess that's ok

The rest of the circuit is as the 3ch dual rec schematic shows (the missing part in the schematic, switching matrix).

Thanks

Thomas
 

grounding network tube amplifier

-3V LDOs are not that common but e.g. LT1964 is available in an adjustable version that might do the trick.

You should probably add a +3V LDO also.
The zener is probably fine.

The supply voltages throughout the the schematics are rather inconsistent - I've found 1.5, 1.8 and 2V.
And in the footswitch there is suddenly a +5V!
Probably just typos...

/Ram
 

    T

    Thomasdj

    Points: 2
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v2b hum problem

I've also noticed the 1.5/1.8/2v issue..however I think it's just typos because the switching matrix works with 1.5 voltage.. I'll make the PCB for the footswitch tomorrow and try the regulators

Thanks

Thomas
 

start grounding tube amp

Hello Thomas,

I would basically keep the antique electronics, as far as it's operational at all, but I think, it is. An acceptable reason to change it could be laziness. You can eliminate a PCB design and use a processor on a breadboard wiring as a footswitch replacement (if you know to program it). It could still use the analog voltage signaling or a serial interface, implying some changes at the amp side, too.

But it would be a different amp.

Regards,
Frank
 

    T

    Thomasdj

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hum from tube amp

Hi Frank

A friend of mine knows how to program a chip, but I'll use the original circuit! I am pretty sure that the only part missing is a -3 voltage regulator. This will make the circuit stable, and I already have the parts (except from the 74564 which I substitute with 74HC564, guess that doesn't matter).

I'll make the PCB tomorrow and let you now how it works (if it works)..

Thanks

Thomas
 

tube amp input impedance

I think HC types also could improve the stability, as they are more relaxed regarding Vcc - 2-6V - whereas
"old" TTL requires 4.75-5.25V.

/Ram
 

    T

    Thomasdj

    Points: 2
    Helpful Answer Positive Rating
how fix grounding problem in a tube amp

Hi All,
Just to let you know that I am still around and lurking on the Forum. :D Yes, the circuit should work fine as it is, if you have got the chips, use them (with the VCC pull up correction already noted).

The 7406 is an open collector hex inverter device so it works as a 3-bit DAC in this configuration, that technique was a common way of implementing an up to 6-bit DAC (or longer if you cascaded two or more devices) in the early days before we had such a plethora of off the shelf precision DACs to choose from. Stabilised voltages are critical for a DAC to work reliably as has already been noted by Ram, so use a stablised supply to at least feed the 10k / 1k5, 3k5 and 5k6 voltage select resistive divider that make up the DAC even if you dont use it for all the circuit supply. For a precision DAC precision resistors would be used and the output voltages would have then been buffered by a unity gain op-amp before letting them loose on the rest of the circuit.

Good luck with the rest of the project Thomas.
Regards.
Bob.
 

    T

    Thomasdj

    Points: 2
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amp input jack hum

Hi everybody!

I'm now testing the footswitch circuit (the channel select part) ..I couldn't find any -3v regaulator so thought I would give the circuit a try without the regulator.

The channels are shifting almost too perfect! however, the LED's are off all time (at the footswitch circuit only). I measured the voltage between pin 19 of the 74ALS564 and ground (0v) when channel 1 is selected; -170mv DC. Which resistors should I tweak, the three 1k5 at top right?

The negative supply voltage is 3.2v - 3.5v and the positive supply is 1.8v.

Thanks

Thomas
 

tube dc hum

Hello Thomas,

I don't understand, what is working now.

1. The "ground" is identical to -3 V, so you have arround 5 V supply at the TTL/HC/whatever IC (564 pin20, 7420 pin 14)

2. The switch operation should be such, that 564 pin 2,3,4 have high idle level and
go to low each, when the corresponding switch is pressed.

3. 7420 pin 6 has idle state high and goes low, when any switch is pressed.

I think, the 1k resistor at pin 11 should be a pull-up to positive supply, not a pull-down to ground. Then the switch state would be latched 100 ms after pressing the switch. Now it is latched on release, but depending on the different logic IC threshholds possibly too late.

May be the second drawing error with the schematic.

Regards,

Frank
 

    T

    Thomasdj

    Points: 2
    Helpful Answer Positive Rating
hum in tube amps

Hi Frank,

It's not necessary that the 1k should go to positive..the switching feels right. The signal goes low when a switch is pressed (and the amp does change channel) but the related LED's doesn't light at all (564, pin 17/18/19).

Thanks

Thomas

Added after 2 hours 8 minutes:

The three LED's cathode goes to ground(-3v), so my guess is that the pin 17/18/19 at 564 should go positive when a switch is pressed..

however, I measure -170mv at pin 17/18/19 when the related switch is selected.
 

interference tube amp

O.K., I didn't understand clearly that the switch is operatíng. But then, the LEDs should also show some light.

The voltage measurements should be better referenced to logic ground (actually -3V) to my opinion. So -170 mV are actually around 3 V referenced to logic ground.
This means, you should have a positive voltage drop at the LED.

What's the voltage at 564 output in idle state?
 

    T

    Thomasdj

    Points: 2
    Helpful Answer Positive Rating
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