valve newbie; 6H1H pre-amp.

[Mark25]

All,
A few years back my previous valve amp, an AMC CVT 3030 integrated, expired. Although i was aware that it had some good qualities, i have not bothered with tubes since, preferring the cast iron grip afforded by active SS amplification. However, recently a good (generous) friend of mine, Bas Horneman decided it was time for me to get back into valves, and threw me a 6H1N to play with. Here's the outcome:

An externally hosted image should be here but it was not working when we last tested it.

Here's the circuit;

An externally hosted image should be here but it was not working when we last tested it.

and the PSU's;

An externally hosted image should be here but it was not working when we last tested it.

It plays music, accompanied by hum!
Here are some of it's vital signs;

B+ 230 vdc
Va 145v
Ra 20K ohm
Rk 680 ohm
Gain 15

Here's the problem(s);
I have about 50 mV hum at the output. I'm aware that my PSU for B+ is not state of the art, but assumed that if it was good enough for 30w push-pull EL34's (AMC), it would support a pre just fine. The 390 uF cap certainly gives a marked improvement over a 100uF cap, (which gives 200mV hum on the o/p). The amp itself seems to give a PSRR of 0.66, the ohms law effect of Ra, i guess.

I guess the way ahead for me is to look into a more sophisticated type of psu, pi filter, choke i/p, cap i/p, etc ?????? Beyond the names and topology, i don't really understand the difference between these types of psu's BTW.

Also, the 390uF resovior cap is taking about 10 secs to stabilise at B+, i'm wondering if that is stressing my 4.9 va isolation transformers too much?

Any comments/suggestions welcome.

Regards Mark

 

[moamps]

Try this simple CRC filter...
(btw, PushPull apps have much higher PSRR)

Regards,
Milan

 

[Bas Horneman]

Also you could change the 1k resistors in the LT Heater supply to 50R or so..

 

[Mark25]

thanks

Thanks moamps, i implemented your cct, but with a 2W 680 ohm R, it's all i had. Pre-amp now gives 5mV hum on o/p, that's a factor of 10 decrease - great! Although the hum sounds like it's higher in frequency now, more of a buzz than a hum, i'll check the F later.

Bas, how did i know you'd say that? i'll get round to it one day, thanks again for the valve -> fun.

Mark

 

[Bas Horneman]

You could also try a HV snubber...

 

[Mark25]

You could also try a HV snubber...

What's that then? Don't forget i'm a newbie!

 

[Bas Horneman]

http://64.154.92.195/forums/bottlehead/messages/54580.html

And just in case you miss his followup link.
http://www.siteswithstyle.com/voltsecond/Rectify_1/rectifiers_and_xfmrs.html

 

[Mark25]

right, lots of theory there, especially in the followup link. For SS stuff i normally refer to;

http://131.109.59.51/images/pdf/Calculatin_%20Optimum_Snubbers.pdf

and have not had much success, are Snubbers more important in valve cct's?

What effect do they have on the sound?

I'd like to keep my cct as simple as poss.
Mark

 

[Bas Horneman]

That was the science part....

Most of the time 2 * 10R resistors WW with little 0, 01uF caps....do the trick nicely...

What effect do they have on the sound?

My suggestion would be to try it out for yourself...And generally I do find any changes in psu for a valve circuit more audible than in the psu for an opamp...with the massive feedback of the op amp..all detail is blurred into a homogenous cesspool anyway..and changes are hard to detect.

😉

For SS stuff i normally refer to;

Written by....a tube kinda guy!...Jim Hagerman
http://www.hagtech.com/

 

[Mark25]

OK, normally i'd wait for the snubber until the cct was finalised, but your general quick fix proposal can be applied at any time so i may give that a go.

The thing i'm having trouble understanding with tubes is:

Why isn't dual mono psu's an absolute must, being that the PSRR of the amp ccts are so low?

 

[Bas Horneman]

Why isn't dual mono psu's an absolute must, being that the PSRR of the amp ccts are so low?

Well why would you want the ripple rejection of your circuit to be high? Just build a low ripple PSU? Or go PP.

A little RC network with 1k res 3,3uF PP cap provides enough decoupling between channels.

Then again there is no law preventing you from going dual mono ....is there? But then I would also suggest going for 2 tubes instead of 2 tube halves in one envelope....etc, etc.

 

[Mark25]

.............
A little RC network with 1k res 3,3uF PP cap provides enough decoupling between channels..........

This is obviously what i'm missing here, so what's the sonic difference between this network and dual mono?

I favour the simplicity of dual mono, and have had such success in SS with this approach that it biases me in that direction.

Thanks for your help thus far.

Mark

 

[Bas Horneman]

This is obviously what i'm missing here, so what's the sonic difference between this network and dual mono?

Very little really...some even prefer not to have dual mono...some won't have it unless it is dual mono....Depends a lot on how you have voiced your system. But if dual mono works for you ....go for it..

 

[Mark25]

....................
My suggestion would be to try it out for yourself...And generally I do find any changes in psu for a valve circuit more audible than in the psu for an opamp...with the massive feedback of the op amp..all detail is blurred into a homogenous cesspool anyway..and changes are hard to detect.
....................

is that an offer of a SS vs. valve pre-amp shoot out?

 

[pedroskova]

OK, normally i'd wait for the snubber until the cct was finalised, but your general quick fix proposal can be applied at any time so i may give that a go.

The thing i'm having trouble understanding with tubes is:

Why isn't dual mono psu's an absolute must, being that the PSRR of the amp ccts are so low?

After your second cap, split your B+ into L&R channels with 2 inexpensive Hammond chokes followed by 10-20uF motor run caps and you'll have pseudo dual mono with plenty of isolation. If you want more, follow this up with a couple of VR tubes per channel. 🙂

 

[Mark25]

been reading

I've read a few (quite old) valve books and KWY's valve begginers links, but cannot find what's wrong with using 1k Ohm resistors for the pseudo centre tap heater supply. I can't even see where the 100 ohm norm is explained.

pedroskova: I guess VR tubes are voltage regulators? The simplicity of dual mono supplies would appeal to me more, unless there are some sound reasons to take another approach.

I can't think of a logical reason why someone would not prefer dual mono supplies for tubes, but accept that having both your L and R amplification devices within the same tube may offer some advantages, eg matching of parameters.

I did read that choke input supplies offer better regulation than capacitor input type, and are therefor more suited to PP type ccts. So in a way, that sort of suggests that C input type may be better for my class A SE triode. Although the only technical backup for that i could find was that the voltage drop of a C i/p is less, i'm a bit low on B+ anyway, so that's one reason to stick with C i/p, i couldn't find anything about sound of each tho.......

All the beginner info didn't seem to answer the questions i have, i must have the wrong approach! I'm supprised nobody said my resisitor values are wrong and i'm operating my valve in a bad sounding area of its operating charchteristic

Cheers Mark

 

[Bas Horneman]

I've read a few (quite old) valve books and KWY's valve begginers links, but cannot find what's wrong with using 1k Ohm resistors for the pseudo centre tap heater supply. I can't even see where the 100 ohm norm is explained.

Does that mean you don't accept my guru status? ;-) 😡

 

[Mark25]

Does that mean you don't accept my guru status? ;-) 😡

Of course not Bas! You're the man for this stuff, what i really mean is; i have "ideas" why 1k is bad, but don't want to sound too silly, or even worse -> like an SS

EDIT: and as you know, i don't just "accept", anything!

 

[pedroskova]

Re: been reading

pedroskova: I guess VR tubes are voltage regulators? The simplicity of dual mono supplies would appeal to me more, unless there are some sound reasons to take another approach.

I can't think of a logical reason why someone would not prefer dual mono supplies for tubes, but accept that having both your L and R amplification devices within the same tube may offer some advantages, eg matching of parameters.

...well then, you might as well make monoblocks. Depending on the quality of your transformers and whether or not you include chokes, you'll find your chassis getting pretty crowded and heavy. VR tubes are also known as "glow diodes". They're easy to use and are great eye candy...

I did read that choke input supplies offer better regulation than capacitor input type, and are therefor more suited to PP type ccts. So in a way, that sort of suggests that C input type may be better for my class A SE triode. Although the only technical backup for that i could find was that the voltage drop of a C i/p is less, i'm a bit low on B+ anyway, so that's one reason to stick with C i/p, i couldn't find anything about sound of each tho.......

If you need the extra voltage, start with a small cap(~3uF) in front of your choke. You'll still get many of its benefits while not losing B+ volts.