Hi guys,
I am going to start a new project a subminiature Fender Champ clone, but with tone control (more similar with the 5F2).
Regarding the valves, the 12AX7 will be replaced by a 6N21B and the 6V6GT by a 6P30B-R. I like to use a pot on the cathode of the valve, so I can change the bias of the first stage. With the bypass cap, I won't loose signal level when I mess with the pot.
I've done a first schematic. Please tell me what do you think?
Cheers,
Pedro
I am going to start a new project a subminiature Fender Champ clone, but with tone control (more similar with the 5F2).
Regarding the valves, the 12AX7 will be replaced by a 6N21B and the 6V6GT by a 6P30B-R. I like to use a pot on the cathode of the valve, so I can change the bias of the first stage. With the bypass cap, I won't loose signal level when I mess with the pot.
I've done a first schematic. Please tell me what do you think?
Cheers,
Pedro
Attachments
Running DC through the wiper of a pot is generally considered A Bad Thing(tm) (yes, there's plenty of trim pots out there).
Typically, this is done with a switch (yes, that's the pentode not input, so adjust values to suit) to avoid scratch pot nastiness. The size of the bypass cap also has a huge effect on tone.
I can only recommend reading a pile of Rob Robinette (e.g. Amp Voicing) to get some ideas of what sort of messing around can be done.
Typically, this is done with a switch (yes, that's the pentode not input, so adjust values to suit) to avoid scratch pot nastiness. The size of the bypass cap also has a huge effect on tone.
I can only recommend reading a pile of Rob Robinette (e.g. Amp Voicing) to get some ideas of what sort of messing around can be done.
A CRO or a suitably isolated audio "sniffer" will allow you to find where the hiss is coming from.
If it's a single note (or two) it's likely oscillation, if it's noise it's likely to be just that.
The likely option is that you've made a radio receiver and you're picking random noise up (likely from a bad joint or a missing resistor-to-earth - that's my usual cause of broadband noise). Pentodes are high gain/high impedence so they're ideal noise receivers - make sure there's always a low impedance to ground above 10kHz.
Finally RF oscillation can also sound like "noise" if it intermodulates something else in the amp.
Hi,
Thanks for the comment. It makes perfect sense.
I preferred the way I was doing it before, which was having a 330ohm resistor in series with a 10kohm pot, so when the pot was set to 0, wouldn't damage the valve. On top of that, I would get all the values in between.
Maybe I will get back to that.
Cheers,
Pedro
Thanks for the comment. It makes perfect sense.
I preferred the way I was doing it before, which was having a 330ohm resistor in series with a 10kohm pot, so when the pot was set to 0, wouldn't damage the valve. On top of that, I would get all the values in between.
Maybe I will get back to that.
Cheers,
Pedro
As long as you stay within spec of the pot, there is absolutely nothing wrong with running DC through a pot.
In fact, that's how many other devices and appliances work (like a lab power supply for example).
The only problem with audio, is that crackling sounds can (sometimes) be heard, so that's the mean reason why it's not advisable. Also some people don't pay attention to the specs and (max) limits of those kind of things.
In that case you run the risk of damaging things.
Hi,
@bremen nacht
Please disregard my previous comment. The volume goes indeed to 0 (no sound), but as soon as I rotate the volume knob by 1 or 2mm the volume goes high very very fast.
The amp is outputting an insane volume. I wasn't expecting this ....
I've biased the valve using my favourite tool designed by Giuseppe Amato. https://www.vtadiy.com/loadline-calculators/loadline-calculator/
I based my design on his tool since it has already 6P30B (by the way the values are spot on with the ones I've measured). However, I have a 6P30B-R which is the same valve but can go up to 7W of max dissipated power instead of 5.5W. So I went crazy and biased it at max dissipation, which gave me about 3.5W of power. Maybe it will be better to change the cathode resistor to something of higher value, so the valve can last longer.
To be honest, I've never thought that 3.5W could be this loud, but this thing rocks !!!
Cheers,
Pedro
@bremen nacht
Please disregard my previous comment. The volume goes indeed to 0 (no sound), but as soon as I rotate the volume knob by 1 or 2mm the volume goes high very very fast.
The amp is outputting an insane volume. I wasn't expecting this ....
I've biased the valve using my favourite tool designed by Giuseppe Amato. https://www.vtadiy.com/loadline-calculators/loadline-calculator/
I based my design on his tool since it has already 6P30B (by the way the values are spot on with the ones I've measured). However, I have a 6P30B-R which is the same valve but can go up to 7W of max dissipated power instead of 5.5W. So I went crazy and biased it at max dissipation, which gave me about 3.5W of power. Maybe it will be better to change the cathode resistor to something of higher value, so the valve can last longer.
To be honest, I've never thought that 3.5W could be this loud, but this thing rocks !!!
Cheers,
Pedro
First, check that the volume pot is actually Log and not Lin (I've done that before).
Second item: inject a few mV of signal and measure the signal levels. Then you can work out where you want to dump some gain. I have built a similar amp (with an 807, so 8W and the opposite of subminiature
) and I ended up adding a master volume to make it useable.Thanks for the reply. I am using a 1Mohm log potentiometer. I think you are right, I may end up having to add a master volume too.
Another option would be to use a 6N16B instead, which has a gain of 25 instead of 70, but I don't want to do this since I would be getting far away from the 12AX7/AT7 kind of sound. I know that 6N17B is still far from that, but the next iteration will probably have a 6N21B.
Cheers,
Pedro
Another option would be to use a 6N16B instead, which has a gain of 25 instead of 70, but I don't want to do this since I would be getting far away from the 12AX7/AT7 kind of sound. I know that 6N17B is still far from that, but the next iteration will probably have a 6N21B.
Cheers,
Pedro
Maybe an idea, but I have pretty good experience with those little 100V output transformers, like the Visaton TR 10.16 in small guitar amps.
They work pretty well for output powers < 8-10W or so.
Unfortunately those DON'T work for single ended, you will heavily saturate the transformer because of the DC.
But it's not to difficult to select a nice and very simple push-pull stage.
I often use something like a LND150 as a simple phase inverter.
That way it won't cost much more. Obviously just another triode can also be used as well.
Although strictly not sub-miniature, this works great with something like a EL91.
Which are tiny as well, same height as the 6P30b, only 19mm diameter instead of 11mm of the 6P30B.
Heater current is only 200mA compared to 400mA of a single 6P30B.
Another candidate would be the EL95, although this one is about 54mm high, same 200mA heater current.
Capable of more output power and also a bit more linear.
Btw, obviously totally personal and subjective, but I think the AA764 Champ circuit is a little nicer.
Better is even the AA764 Vibrochamp.
The regular AA764 uses the same amount of tubes, just in a slightly different configuration.
They work pretty well for output powers < 8-10W or so.
Unfortunately those DON'T work for single ended, you will heavily saturate the transformer because of the DC.
But it's not to difficult to select a nice and very simple push-pull stage.
I often use something like a LND150 as a simple phase inverter.
That way it won't cost much more. Obviously just another triode can also be used as well.
Although strictly not sub-miniature, this works great with something like a EL91.
Which are tiny as well, same height as the 6P30b, only 19mm diameter instead of 11mm of the 6P30B.
Heater current is only 200mA compared to 400mA of a single 6P30B.
Another candidate would be the EL95, although this one is about 54mm high, same 200mA heater current.
Capable of more output power and also a bit more linear.
Btw, obviously totally personal and subjective, but I think the AA764 Champ circuit is a little nicer.
Better is even the AA764 Vibrochamp.
The regular AA764 uses the same amount of tubes, just in a slightly different configuration.
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