# Good small valve amp for keyboards?

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### Re: Good small valve amp for keyboards?

Posted: Sun May 19, 2019 9:49 am
ef37a wrote:
Arpangel wrote:Hugh! That's fine, I can make up a lead, but can I be a pain and ask you the value of the resistors I would need?

Phoarrr! How long is a piece of string, or wire in this case! Not knowing the actual output level of the kbd and the input sensitivity of the amp? Pretty d difficult. But...

Ten k Ohms (10,000) in the hot wire and 1k2 (1200 Ohms) from the junction of the hot pin and 10k to earth, jack sleeve will give just under 20dB of attenuation, i.e. a reduction of about 10 times. Making both resistors 10k will halve the signal, -6dB.

Ideally the resistors should be at the destination plug not the source but if the cables are short, under 3m not really a problem.

N very B. If you make the attenuator up as a jack to jack cable LABEL it! Otherwise in a years time you will pull it out the bag and wonder WTF's gone wrong with sommat!

Dave.

Thanks a lot Dave, Ok, a 10k in-line to the hot pin, and a 1/2k plus a 10k from hot to the screen terminal? I've looked for a circuit diagram for this but can't find one..?

### Re: Good small valve amp for keyboards?

Posted: Sun May 19, 2019 11:53 am

You want the first circuit on the left, the L pad. R1 is the 10k and R2 the 1k2, can you visualize putting them in a plug? If not I am sure someone here can do you a drawing? I am not that PC cute!

The resistors are necessarily going to have to be very wee, does not matter HOW wee, they consume no power to speak of. The sadly missed Maplin used to do a Lucky Bag of Rs and you would have found something in there to use. The values can be widely varied. Don't make R1 less than 10 k but 12, 16, 18, 22k are all possible then keep R2 such that the ratio is about 9/10:1 for about 20dB loss. 20dB might be a bit much actually so I would fix on a value for R1 then "tack in" a guesstimate and give it a blow.

(btw, if you want to get technical, the loss is given by..R2/R1+R2 then log the result and times by 20 to get it in dBs)

Dave

### Re: Good small valve amp for keyboards?

Posted: Sun May 19, 2019 6:36 pm

You want the first circuit on the left, the L pad. R1 is the 10k and R2 the 1k2, can you visualize putting them in a plug? If not I am sure someone here can do you a drawing? I am not that PC cute!

The resistors are necessarily going to have to be very wee, does not matter HOW wee, they consume no power to speak of. The sadly missed Maplin used to do a Lucky Bag of Rs and you would have found something in there to use. The values can be widely varied. Don't make R1 less than 10 k but 12, 16, 18, 22k are all possible then keep R2 such that the ratio is about 9/10:1 for about 20dB loss. 20dB might be a bit much actually so I would fix on a value for R1 then "tack in" a guesstimate and give it a blow.

(btw, if you want to get technical, the loss is given by..R2/R1+R2 then log the result and times by 20 to get it in dBs)

Dave

Dave, much appreciated, I'm OK at soldering, no problem, but my maths especially relating to electronics is virtually non existent! But, the way I see that diagram is this, there is a resistor R1 connected "in line" to hot or tip of the plug, then R2 goes between cable screen and shield on the plug.
I think I may have some 10k resistors in my toolbox, I'll experiment with values for R2.
Alternatively, I may just use my Behringer mono MiniMix, it'll enable me to match levels, and also have control over the balance between my i Pad and my Buchla into the Cube.

### Re: Good small valve amp for keyboards?

Posted: Mon May 20, 2019 7:41 am
"then R2 goes between cable screen and shield on the plug. "

No. R2 goes between the hot pin of the jack to screen pin on the plug. I.e the JUNCTION of the hot pin and R1.

Dave.

### Re: Good small valve amp for keyboards?

Posted: Mon May 20, 2019 9:59 am
ef37a wrote:"then R2 goes between cable screen and shield on the plug. "

No. R2 goes between the hot pin of the jack to screen pin on the plug. I.e the JUNCTION of the hot pin and R1.

Dave.

All is now clear!

I'll have a go I've got nothing to loose but a few mins rummaging around for bits!

:thumbup:

### Re: Good small valve amp for keyboards?

Posted: Mon May 20, 2019 10:15 am
Excuse the crude drawing, but it should be something like this:

Insert and solder one end of both the 1k2 and 10k resistors into the centre pin of the destination jack plug.

Then take the other end of the 1k2 and solder to the screen barrel of the destination jack plug.

Wire the unbalanced cable in the usual way to the source jack plug.

Place some sleeving over the centre wire at destination end and solder to the free end of the 10k resistor. Use sleeving to protect the junction and prevent shorting against the barrel.

Solder the cable screen to the destination barrel.

Swear loudly because you forgot to put the jack plug body and cap over the cable first....

:lol:

Remake destination end and then label the cable in some reliable way to indicate that (a) it introduces 20dB attenuation and (b) which end is the destination end (the one with the resistors in it).

As Dave says, the resistor values aren't too critical. Anything above 10k is fine for the series resistor (the red one above), but the shunt (green) resistor should be about 1/10 of it's value for 20dB attenuation.

A higher value for the shunt will give less attenuation, and a smaller value will give more. If the shunt and series resistors are equal, you'll get just 6dB of attenuation.

### Re: Good small valve amp for keyboards?

Posted: Mon May 20, 2019 11:24 am
Hugh Robjohns wrote:Swear loudly because you forgot to put the jack plug body and cap over the cable first....

:lol:

One of the worst feelings in the world. :)

### Re: Good small valve amp for keyboards?

Posted: Mon May 20, 2019 12:40 pm
And yet one so commonly experienced! :)

### Re: Good small valve amp for keyboards?

Posted: Mon May 20, 2019 3:24 pm
Thanks Hugh.

Re "forgetting" Lost count how many times I have put a 15A rubber plug on a T lamp and forgotten that the outer must go on first! Quick snip with the side cutters fixed that one.

Been a loooong time since I was on a lighting deck, do they still use 15A plugs?

Dave.

### Re: Good small valve amp for keyboards?

Posted: Mon May 20, 2019 10:26 pm
ef37a wrote:Been a loooong time since I was on a lighting deck, do they still use 15A plugs?

Dave.

It's still pretty standard for dimmers to terminate to 15A sockets.

### Re: Good small valve amp for keyboards?

Posted: Tue May 21, 2019 7:30 am
Hugh Robjohns wrote:Excuse the crude drawing, but it should be something like this:

20dB instrument attenuator.png

Insert and solder one end of both the 1k2 and 10k resistors into the centre pin of the destination jack plug.

Then take the other end of the 1k2 and solder to the screen barrel of the destination jack plug.

Wire the unbalanced cable in the usual way to the source jack plug.

Place some sleeving over the centre wire at destination end and solder to the free end of the 10k resistor. Use sleeving to protect the junction and prevent shorting against the barrel.

Solder the cable screen to the destination barrel.

Swear loudly because you forgot to put the jack plug body and cap over the cable first....

:lol:

Remake destination end and then label the cable in some reliable way to indicate that (a) it introduces 20dB attenuation and (b) which end is the destination end (the one with the resistors in it).

As Dave says, the resistor values aren't too critical. Anything above 10k is fine for the series resistor (the red one above), but the shunt (green) resistor should be about 1/10 of it's value for 20dB attenuation.

A higher value for the shunt will give less attenuation, and a smaller value will give more. If the shunt and series resistors are equal, you'll get just 6dB of attenuation.

Many thanks, that's very clear.
Leaving the cover off of a DIN plug is the worst feeling, but DIN plugs are the work of the devil anyway...

### Re: Good small valve amp for keyboards?

Posted: Tue May 21, 2019 9:51 am
The trick with DIN plugs is to use a socket in a vice to plug the pin assembly into, heat shunts it a bit and gives all round access.
Then, put wee bits of silicone sleeving over each wire as you go and run it down to each pin. Then if you make a "whisker" it does not short.

I used to LOVE making up cables of all sorts, 15 pin VGA high density are lots of fun. I found it very "zen" !

Dave.

### Re: Good small valve amp for keyboards?

Posted: Wed May 22, 2019 8:12 am
ef37a wrote:The trick with DIN plugs is to use a socket in a vice to plug the pin assembly into, heat shunts it a bit and gives all round access.
Then, put wee bits of silicone sleeving over each wire as you go and run it down to each pin. Then if you make a "whisker" it does not short.

I used to LOVE making up cables of all sorts, 15 pin VGA high density are lots of fun. I found it very "zen" !

Dave.

I used to like making leads too, but not as I've got older, I tend to avoid it now, buying them instead. It's like I don't want to waste time making leads when I can be making music, when I think about the time I must have spent making leads, days, months, it's scary, and it's this feeling that time is ebbing away and I must make valuable use of every minute.

### Re: Good small valve amp for keyboards?

Posted: Tue May 28, 2019 11:52 am
OK, all is amazing with this little Roland Cube.........however (there's alaway a "however")
I'm getting some truly mind blowing sounds out of this thing with my synth, especially using the distortion setting on the amp, but "I think" the COSM technology used to create the overdrive is also giving me gating effects, brief pumping in between notes. It's not the end of the world, but I'd prefer it if it wasn't there. I'm guessing, that if this was a proper valve amp then that gating effect wouldn't be there? If this is the case I may need a valve amp.

### Re: Good small valve amp for keyboards?

Posted: Tue May 28, 2019 1:46 pm
Arpangel wrote:OK, all is amazing with this little Roland Cube.........however (there's alaway a "however")
I'm getting some truly mind blowing sounds out of this thing with my synth, especially using the distortion setting on the amp, but "I think" the COSM technology used to create the overdrive is also giving me gating effects, brief pumping in between notes. It's not the end of the world, but I'd prefer it if it wasn't there. I'm guessing, that if this was a proper valve amp then that gating effect wouldn't be there? If this is the case I may need a valve amp.

Well now! Valve amplifiers can suffer something called "Blocking Distortion" which might sound similar? Won't happen in any competent design mind.

The Cube might object to the continuous nature of a synth's signal? Guitar signals are spikey and short lived. Pretty sure the Roland will be a class D amplifier and such amps can be modified to be very loud for (say) guitar signals but shut down for relatively small amplitude sines. The very potent Blackstar ID range can kick some serious A with a guitar but you struggle to get more than a few watts out of them on the bench with a sig genny!

Two consequences to that.
The amps have a reputation for delivering "valve like" stage volumes and they don't go wrong!

You could ask Roland if there is a firmware update?

Dave.

### Re: Good small valve amp for keyboards?

Posted: Tue May 28, 2019 4:37 pm
The Elf wrote:...and all of these points are the reason why I strategically run my keys through a Line 6 Helix!
:lol: :beamup:

### Re: Good small valve amp for keyboards?

Posted: Wed May 29, 2019 7:35 am
OK, lots going on here, my synth is noisy, also not at home yet, so haven't been able to attenuate the input to the amp. It's being hit with a full on synth output, and is responding accordingly. I think when I put my little mixer in the chain things may improve regarding the gating effect.
Sounds blooming damn amazing though, noise and all! The first time I've really explored amps and synths, it's going to be the way forward, micing up the amp.