Line stage

[6B4G]

Hi guys I just found preamp schematics that I built some years ago, I was wondering if any good changes could be made to make it sound even better. I have added some comments in red and I was wondering if they would be ok. The transformer is a PA211 or the PA522 European and US version ( secondary 300V-0-300V 15mA, 12V 800 mA) powers 2 X 12AX7 and 2X 12AU7

[6B4G]

I re uploaded the file as per errors in some cap values I found

[Geek]

Hi,

I'd suggest modding it for a mu-follower rather than a SRPP. The reason being is SRPP likes to operate into one load-Z as optimized (why they're used as output tube drivers efficiently), while a line is anything but constant-Z. A mu-follower doesn't really give a hoot what it's driving, as long as it isn't too heavy.

And for a 12AU7, I'd increase that 220R series resistor with the output to 470R or even 1K, lest capacitive loads make yourself a nice little VHF transmitter

Cheers!

[Gingertube]

1H is way to low an inductance for the current draw this thing is likely to exhibit. If you want to put in a 20H OK - else it is a waste of time and effort (In general the lighter the load current the more inductance you need).
Leave the "mf" metal film resistors, going to "CC" Carbon Composition would be is a huge backward step particularly with respect to noise.
12AU7 sound better with the current pushed up even a little more, change those 680R bias resitors to 560R or even 470R - you want greater than or equal to 8.5 mA.
By all means try it as a mu-follower (see Geek's suggestion) but be prepared to change back. The SRRP is reputed to have a "better" harmonic profile, that is, it produces mostly low order even harmonic distortions.
Cheers,
Ian

[6B4G]

Thank you guys all info is well accepted.
As I said earlier on I have built this preamp and it sounds quite OK
but I think it can sound better.
I didn't mention that this is the VTA SP6 preamp from http://www.tubes4hifi.com/
that I bought and built at least 8 years ago.



Regards
Greg

[Geek]

I think Bottlehead had a similar one, but it went through revisions over the years?

By all means try it as a mu-follower (see Geek's suggestion) but be prepared to change back. The SRRP is reputed to have a "better" harmonic profile, that is, it produces mostly low order even harmonic distortions.

Yes!

Like wine, each has their own distinct character. I've tried both, but like mu (so I speak biased). Many like SRPP. Just a different flavour.

Oh, for some adventure, check around Broskie's site (http://www.tubecad.com) for a creature known as SEPP

As for the choke, Ian is quite right. I missed that There's a little 30H Hammond that's great for these.

Cheers!

[6B4G]

As for the choke, Ian is quite right. I missed that There's a little 30H Hammond that's great for these.

Is there any benefits to adding the choke would it be quieter?
would the values of the resistors change to the rest of the ps?

I also would like to change the 12X4 with a 6X4 due to having heaps of them and none of the others
I have extra 6.3v 2A transformers to power the tubes.
If I run 12au7's or 6AU7's with 6v and a 6X4 I need no more than 1.2A

The original schematics call for a 50k vol pot and a 50k balance pot, I was reading up on the perfect volume control and it stated that the
balance pot should be 2.5 X the vol pot, does that sound correct?

[Geek]

Is there any benefits to adding the choke would it be quieter?

You'd be creating a choke-input supply if you built it as shown. Your voltage will plummet, but will be better regulated. So as an alternative....

would the values of the resistors change to the rest of the ps?

What I would do is not put the chole where you have now, but just swap it in for the 1.5K/1W resistor shown on the schematic.


As for quieter or better or what have you, that's up to your ears. If you have no noise now, I'd not bother. But a C-L-C and a C-R-C do sound different due to storage mechanisms (chokes store energy, resistors do not) as well as supply impedance.

Cheers!

[6B4G]

Thanks for the info.

Greg