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Tube biasing using CCS, based on SOHA II, not working as expected. - Page 3

post #31 of 53
Quote:
Originally Posted by KimLaroux View Post

 

Now I'm curious about using a linear reg on the tail... never seen that done, wonder why?

 

I'm starting to understand now... I think.

 

As for the long tail pair, is not the goal of such circuits to provide both an inverting and non-inverting output? I don't need those two signals for my output stage. So how would such a topology provide any benefits in my case?

 

Do you mean like an LM317, or some newer (and better) version? Unless you have a negative voltage supply the voltage overhead is kind of a killer in gain tubes. If you have a negative supply (which makes the cost of the long tail pair {LTP} look small) just build an LTP.  In power tubes people who really care grid bias or use CCS on a LTP... In the case of gain tubes most people who want to use SS parts just use LEDs or rectifier diodes. Impedance is not as low (theoretically) but low enough for our purposes.

 

The long tail pair allows you a few neat things in a SE circuit. 

Gain is always lower (usually half) of what you would get from a common cathode amp. Its hard to get gain low enough in a hybrid IMO so buying the next few lines while reducing gain kills 2 birds with one stone. 

Input capacitance follows gain, which is lower, which means more bandwidth, which is nice.

If you care about phase (its audiophile BS) you can make a non-inverting gain stage this way.  

The long tail pair allows neat feedback arrangements if you are into that sort of thing. 

 

The downside of a long tail pair is that it requires many more parts than a simple common cathode or even cathodyne. But it often pays you back in performance. 

 

I have never been a fan of current mirrors - they just seem weird to me. I would stick with plate resistors, but to each his own. If you stick with plate resistors I'm pretty sure you could convert the amp to LTP without adding any parts. Break the connection between the plates and just leave one hanging out with a 15Kohm resistor to B+. Break the connection between the grids and ground one. Done!


Edited by nikongod - 8/16/13 at 11:07am
post #32 of 53
Quote:
Originally Posted by nikongod View Post

 

I would stick with plate resistors, but to each his own. If you stick with plate resistors I'm pretty sure you could convert the amp to LTP without adding any parts. Break the connection between the plates and just leave one hanging out with a 15Kohm resistor to B+. Break the connection between the grids and ground one. Done!

 

Yep.  It's what I would do too.  The current mirror in the SOHA 2 is to facilitate tube swapping of compatible 9 pin types to fix an operating point for the plate, especially with a really low B+ of 60V.  If you're not doing that (and shouldn't), you don't need it.  Resistors work.

post #33 of 53
Thread Starter 

Oh, I thought the current mirror was necessary in this topology. I obviously never heard of LTP used as SE amps before, so I'm totally in the dark here.

 

And you make it sound so easy =P

 

I want to try it, as it seems a simple thing to try at this point. Could you clarify which plate gets the resistor to B+, which grid gets the input, which plate gets the output? And do I ground the other grid directly, without any resistors?

post #34 of 53

Easiest to just use a picture.  From tubecad...  Ignore the values of the items, they don't apply to you.  You may want to read the tubecad link I posted.  It will help you.

 

post #35 of 53
Thread Starter 

Yeah I read it, and now reading that http://www.tubecad.com/2007/05/blog0107.htm

 

It's the first time I see such a topology, it's quite interesting. I think I'll read some more before I jump into it.

 

In the picture you posted, the right hand tube's grid is biased up. Should I not do something similar?

post #36 of 53
Quote:
Originally Posted by KimLaroux View Post

Yeah I read it, and now reading that http://www.tubecad.com/2007/05/blog0107.htm

 

It's the first time I see such a topology, it's quite interesting. I think I'll read some more before I jump into it.

 

In the picture you posted, the right hand tube's grid is biased up. Should I not do something similar?

 

Just to be fair, Alex Cavalli talked about it too.  http://www.cavalliaudio.com/diy/soha%20ii/main.php?page=design/ipdesign

 

 

 

As for the biasing.  It will depend on the operating points that you work with.  Yes, if you use the circuit as depicted, as the plate voltages are different.  It's used to select it's operating point.

 

The current mirror of the SOHA 2 forces the plate voltage, which means you don't, as when you fix the plate voltage and the cathode voltage, the bias comes automagically.  Alex floated the 2nd grid with a resistor to ground, to allow for variances.

 

You could set the tubes up with 2 resistors (one on each plate).  You will need trial and error to pick the DC point, since tubes vary.  That was the beauty of the current mirror, it became part of the solid state circuitry and sort of auto-configuration of different tubes and different variances.

post #37 of 53
Thread Starter 

Right... so I did some maths and got there:

 

Everything seems fine until I need to get that 3.3 V on the grid. I calculated what I could get using combinations of resistors I have and I can't get it close enough. What's more, this added voltage divider pulls more current trough the 18 K plate load resistor, affecting the voltage drop across it. Even a 1 M resistor in there adds 2 volts across it. Is this of concern or should I just ignore it?

 

I'm tempted on using trimmers or diodes to get the 3.3 V I need there. My only concern is current. Is there a minimum current I should aim for in this divider? I'm thinking the grid is mostly voltage-driven and therefore doesn't need much current, so it should not really be a problem. Anyone can confirm?

 

That simple problem suddenly makes the current mirror so easy to build ;)

post #38 of 53
Thread Starter 

Playing around some more, I got this:

 

 

The maths tell me the plate would sit at 103 instead of 106 V... dunno how that will turn up in practice. 

 

I'm ready to try this circuit now. Any comments before I do?

post #39 of 53

It looks fine.

 

If you need negative feedback, you can apply it through that bias point as well (with a resistor) from the output (with caps and all).

 

I think you'll like this topology much more than the typical grounded cathode configuration.  I know I do.

post #40 of 53

You may need to play with the plate resistor values some after you build it and test.

 

What tube are you using, btw?  I haven't checked the resistor values and voltages.  I'm not sure I will, but good information to have if something comes up.

post #41 of 53
Quote:
Originally Posted by holland View Post

I do know that if you load a tube with a CCS on the tail of a single triode and try to get output from the plate with an output transformer, you won't.

 

Here's a transformer coupled SE with CCS bias. It's necessary to bypass the CCS or you won't, in fact, get any gain. It's all about disassociating the AC and DC conditions. Gain is a quite respectable 20dB but is 6dB down @50 Hz with this transformer. Parasitics and coupling coeff. are not much better than guesswork.

 

 

w

post #42 of 53

Kim:

Can you tweak your operating point to use 3.4-3.5V at the grid of the "non driven" triode to use a pair of red LEDs to set the bias voltage? 

You can use the same 2 LEDs for both channels without crosstalk, and this will save you the trouble of matching them, but since moar LED is moar better(er) I guess you will need 4 LEDs. 

 

Quote:
Originally Posted by wakibaki View Post

 

Here's a transformer coupled SE with CCS bias. It's necessary to bypass the CCS or you won't, in fact, get any gain. It's all about disassociating the AC and DC conditions. Gain is a quite respectable 20dB but is 6dB down @50 Hz with this transformer. Parasitics and coupling coeff. are not much better than guesswork.

 

 

w

 

How does C1 affect the impedance to ground as seen looking out of the cathode of U3? 

Congratulations for solving a problem that does not exist in the OPs project. 

post #43 of 53
Thread Starter 
Quote:
Originally Posted by holland View Post

You may need to play with the plate resistor values some after you build it and test.

 

What tube are you using, btw?  I haven't checked the resistor values and voltages.  I'm not sure I will, but good information to have if something comes up.

 

12AU7. Can you explain to me how different value plate resistors would improve or degrade the operation of the circuit? I'm just curious, really. The only thing I can think of is higher value would improve PSRR, true?

 

Quote:
Originally Posted by nikongod View Post

Kim:

Can you tweak your operating point to use 3.4-3.5V at the grid of the "non driven" triode to use a pair of red LEDs to set the bias voltage? 

You can use the same 2 LEDs for both channels without crosstalk, and this will save you the trouble of matching them, but since moar LED is moar better(er) I guess you will need 4 LEDs. 

 

I actually thought of using LEDs, but then discarded it thinking LEDs need too much minimum current. But I may be wrong... how much current would they need at a minimum?

 

I'll have to bench test the LEDs I have in stock to find their voltage and current ratings... 

 

Thanks for the tip of using the same set for both channels, but then on which channel do I power them from? Would I need to power them from B+ in that case?

post #44 of 53
I'd try to avoid running LEDs under 3mA as their Vf can be a bit variable in low current situations. But I have seen some amps running them at ~1 mA but they require more matching iirc.

I just saw this thread as I've been away from the forum for a few days. I too would urge you to try the amp without a CCS, I've rebuilt my amp with a higher voltage input stage (about the same as yours) and a chunkier output stage that can also push about 5W RMS into 8R. Due to me wanting to add some more tube distortion to the design I designed the output stage with a fairly input impedance (~150k) and am driving it with a CCDA (check it out on tubecad). It is definitely one of my favourite valve topologies that I've ever heard (I'm using feed forward in the gain stage with 12au7s) and all the basing is resistors.

In the end it depends on what effect you're going for and I'd be interested to see your general plan for the output stage too. WRT plate loads, I'm using 20k in my amp. I generally glance at the datasheet graph, check the principle in the sim, and then build and tweak it. I know that some people calculate plate loads more precisely than this, but given the variation of tubes I found it best to use something universal/wide tolerance for tube variations like some CCS or resistor biasing, or to tweak it in circuit if ultimate performance is required. There are some very different philosophies here I suppose.

Glad to see you've been able to get started on this again. smily_headphones1.gif
post #45 of 53
Thread Starter 

Oh, fun!

 

 

White wire hooked directly to the 160 V rail. One 220K resistor. Two red LEDs in series. Only 0.73 mA going trough the LEDs, is that playing with fire? It seems to be working fine... Note those are old LEDs, not the super bright sort you get these days.

 

edit: The LEDs are quite dim at 0.73 mA. Next value down I have is 150 K, which pushes 1.1 mA trough the LEDs, making them much brighter. I get 3.32 V with this one. Not sure which would be a better choice... since I don't have the datasheet of those.


Edited by KimLaroux - 8/17/13 at 12:50pm
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