Feeler: Classic Solo

[ding]

Here is the Mouser BOM for the main board only. Its missing the XLRs. I think everything is in there but its always good to double check. If anyone spots something off or has a preference for XLRs let me know and I'll fix it.

http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=264ebdcaf7

 

[kosi]

ordered a pair of Sowter 1948c !
not sure about the output TX...

 

[madriaanse]

Hi Ian,

When implementing the 10K gain pot, are both C1 and C6 needed, or could I just make C1, say, a 1uF and jumper C6? How about jumpering C2 if using as a transformer coupled mic pre only?

Thanks,

M.

 

[ruffrecords]

Hi Ian,

When implementing the 10K gain pot, are both C1 and C6 needed, or could I just make C1, say, a 1uF and jumper C6? How about jumpering C2 if using as a transformer coupled mic pre only?

Thanks,

M.

You must retain C1 to provide dc isolation. With a 10K load you can safely reduce C1 to 1uF. You can jumper C6 but I would advise against it. Most tubes will have a tiny but non zero grid current. If this flows directly through the pot wiper it may cause crackling. C6 will prevent this .

Cheers

ian

 

[madriaanse]

Hi Ian,

When implementing the 10K gain pot, are both C1 and C6 needed, or could I just make C1, say, a 1uF and jumper C6? How about jumpering C2 if using as a transformer coupled mic pre only?

Thanks,

M.

You must retain C1 to provide dc isolation. With a 10K load you can safely reduce C1 to 1uF. You can jumper C6 but I would advise against it. Most tubes will have a tiny but non zero grid current. If this flows directly through the pot wiper it may cause crackling. C6 will prevent this .

Cheers

ian

Thanks Ian,

i might implement a 10k stepped attenuator to prevent crackles. C2 can also be  jumpered if you're just using a mic input transformer, right? Can you tell I'm a big fan of taking caps out of the signal path?

Best ,

M.

 

[ruffrecords]

Thanks Ian,

i might implement a 10k stepped attenuator to prevent crackles. C2 can also be  jumpered if you're just using a mic input transformer, right? Can you tell I'm a big fan of taking caps out of the signal path?

Best ,

M.
[/quote

There is no reason to suppose a stepped attenuator would be any less susceptible to crackles than a regular pot. However, the chances of it happening are small so I suggest you try it and see. It is easy enough to put the cap back in circuit if you get crackles.

You can definitely jumper C2 for a mic pre. I also jumpered it when I built one as a DI with no ill effects.

I am not sure why you are keen to remove caps from the signal path. In this design, their minor sonic signature pales into insignificance compared to that of the tubes, but it is your build so go for it.

Cheers

Ian

 

[madriaanse]

I am not sure why you are keen to remove caps from the signal path. In this design, their minor sonic signature pales into insignificance compared to that of the tubes, but it is your build so go for it.

Cheers

Ian

Thanks Ian,

It's more the principle of it. I like squeezing every last bit of sonic performance from my mic pres and I'm a big fan of stripping down circuits to their bare minimum.

Best,

M.

 

[ruffrecords]

Thanks Ian,

It's more the principle of it. I like squeezing every last bit of sonic performance from my mic pres and I'm a big fan of stripping down circuits to their bare minimum.

Best,

M.

In which case keep C1 at 4.7uF as with a 10K pot load this will minimise the phase shift at low frequencies.

Cheers

Ian

 

[madriaanse]

Awesome! Thx! Can't wait to build this thing.

M.

 

[bernbrue]

Hi Ian,
I've got the pcb almost stuffed and found a nice enclosure as well. Is there any special trick to solder the 1mm tube pins? They do not fit precisely into the mounting hole. Should I use some pressure to put them in?
Best
Bernd

 

[anjing]

Hi Ian,
I've got the pcb almost stuffed and found a nice enclosure as well. Is there any special trick to solder the 1mm tube pins? They do not fit precisely into the mounting hole. Should I use some pressure to put them in?
Best
Bernd

Hi Bernd,
    I usually put the pins in first, it's easier to insert them that way. It's not so bad if they are not a 100% even, the tubes will still sit firmly. You might need to heat the pins a little longer than the other components to solder.

Regards,

Pierre

 

[bernbrue]

Thanks, Pierre. That helps a lot.
Best
Bernd

 

[ruffrecords]

Hi Ian,
I've got the pcb almost stuffed and found a nice enclosure as well. Is there any special trick to solder the 1mm tube pins? They do not fit precisely into the mounting hole. Should I use some pressure to put them in?
Best
Bernd

There does seem to be some variation between PCB manufacturers and PCB batches in the finished size of these holes. If you are using the recommended knurled pins then sometimes it can need considerable force to fit the pins. I use a small Philips head screwdriver for this. I have never had any problem with these no matter how mucj force I have used.

There are two alternatives. The first is to use pins without the knurling. These usually fit very easily but it is hard to get the pins to solder so thay are a;ll vertical to the PCB. To get around this you can fit all the pins in the holes and then plug in a tube to hold them all in place while you solder them.

The second alternative is for me to make the holes sightly larger. This should make it easier to fit knurled pins but it means it is possible they will be loose in some PCBs.

Its the usual set of engineering compromises.

And as Pierre says, it is best to fit the tube pins first - it does say this in the documentation!

Cheers

Ian

 

[bernbrue]

Hi Ian,
No problem, I managed to solder them in and it was really easy. I did a lot of tube stuff and always soldered the tube sockets last. This one is a bit different. I really like the pins holding the tube really tightly in place.
Cheers
Bernd

 

[ruffrecords]

Hi Ian,
No problem, I managed to solder them in and it was really easy. I did a lot of tube stuff and always soldered the tube sockets last. This one is a bit different. I really like the pins holding the tube really tightly in place.
Cheers
Bernd

I am pleased you like the idea. I saw it on another product shown here on groupDIY several years ago. I was uncertain about using these pins because I was not sure if the tubes would fall out but they seem to grip very well. The advantages of the pins are reduced PCB area and reduced overall height.

Cheers

Ian

 

[bernbrue]

Hi,
Pcb is stuffed and enclosure prepared. Everything fits nicely and gasps for some current.
Cheers
Bernd

 

[bernbrue]

 

[ding]

Are those 250v on c1 & c5?

 

[bernbrue]

Yes and voltage rating  might be too low. The WIMAs I ordered are as big as a package of cigarettes and don't  fit. I'm going to measure the voltage and might change them.
Bernd

 

[ruffrecords]

Yes and voltage rating  might be too low. The WIMAs I ordered are as big as a package of cigarettes and don't  fit. I'm going to measure the voltage and might change them.
Bernd

You might get away with it. When the tubes start to conduct, the HT voltage drops to about 285V and there will be about half this across the two output capacitors which is well within 250V. However, as the tubes start to conduct, you never know if the top or bottom triode is going to conduct first. If the top one goes first then the voltage across these caps could easily exceed 300V. WIMAs can be incredibly big, not to mention expensive. I tend to use Panasonic types:

http://uk.farnell.com/panasonic-electronic-components/ecwf2w475ja/cap-film-pp-4-7uf-450v-rad/dp/1854925

Cheers

Ian