#26 pre amp

[andyjevans]

I want to try out battery bias with 26 - not in a preamp but the first stage of my SET. The preamp stage before it is a 10Y into a Hammond 126C OPT which I'm going to put on the same chassis as the amp. So I can use the secondary of the 126C for the battery bias to the 26.

So fill me in here as a battery bias beginner for what I need to do
- Do I need one battery or two, one per channel?
- If one battery, do I use it via a 1 meg resistor to each grid or what?
- If two batteries do I again use a resistor or just connect it straight to the grid? What value resistor
- What kind of battery is it best to use?
- Anything else I should know?

Just occurred to me that with a transformer secondary, the grid and earth are connected so that would drain the battery, so I need a cap somewhere?

andy

[nicoch58]

can help?

[andyjevans]

Hi nicoch46

That's helpful. The first diagram is new to me - I didn't know you could place the battery in series. How does that sound?

The second diagram is the one I need I think. I can see you don't need a cap here since it's open circuit when the tube is off. So presumably no resistor between battery and transformer if the diagram is to be believed.

That looks like you'd need one battery for each channel?

andy

[Salas]

Originally Posted by coolzero

Salas, I found the problem BTW. The SSHV was not controlling the voltage because the IRF 840 got opened for some reason. After I replaced it, everything came back to normal. As you said, I checked the current on anode CCS which is at 9ma and SSHV is at 40ma. Is this the optimal to operate? Now the pre amp is more silent without any speaker rumbling, but I felt the overall gain reduced significantly as well as it lost the liveliness of the music it had with anode chokes. I assume this is because the reduced gain hence any advice to fine tune and increase the gain?

Having 31mA more, gives good reg's output impedance. I consider 20-25mA extra as adequate and always must allowed, so your excess is nice. As for best anode load I consider anode choke, electronic gyrator, CCS, in subjectively descending order. A CCS gives more gain than a resistor but sometimes less than a choke.

[nicoch58]

I haved send PM,
sorry never try , you are the pioneer !

glad to help a bit

[andyjevans]

Here's my quick first draft of a schematic where the 26 is the second stage with battery bias - please make any comments or corrections.

I'm not sure if this comes out landscape or portrait - the original is landscape but sometimes my attachments get turned around. Can anyone explain what to do?

andy

[coolzero]

Originally Posted by Salas

Having 31mA more, gives good reg's output impedance. I consider 20-25mA extra as adequate and always must allowed, so your excess is nice. As for best anode load I consider anode choke, electronic gyrator, CCS, in subjectively descending order. A CCS gives more gain than a resistor but sometimes less than a choke.

Thanks Salas. Even I prefer the anode chokes but when it comes to noise, it is terrible compared to others as I observed. I have pretty decent anode chokes done by EP but still when it comes to the larger output coupling cap (in order to get the optimal low frequency), it is bit of problem to use those chokes. It seems like that there is no way out to have both the larger caps and chokes together without this compromise.

[nicoch58]

[coolzero]

[QUOTE=andyjevans;2356900]

Andy here's what I have tested so far which was recommended by Kevin.

- Do I need one battery or two, one per channel? One per channel.

- If one battery, do I use it via a 1 meg resistor to each grid or what?
- If two batteries do I again use a resistor or just connect it straight to the grid? What value resistor?
Refer the schematic attached.

-Just occurred to me that with a transformer secondary, the grid and earth are connected so that would drain the battery, so I need a cap somewhere? Probably you could refer Kevin's article in PF. (http://www.positive-feedback.com/Issue16/kennedy.htm). I have seen a very informative discussion on this topic in one of the DIY site which I couldn't find the link (I think it was a Philippines or Malaysian site. Remember "ArnoldC" was active there so it could most probably a Philippines). Check if you could locate it.

- What kind of battery is it best to use? I tried few of different types but Alkaline came to top comparing the sonics.


Hope this info. helps.

[andyjevans]

Thanks for the tip about alkaline batteries. I assume you mean disposible rather than rechargeable? Like the common Duracell types?

Kevin recommends one battery per channel.

andy