(DIY ECL86 tube amp) Some advice needed, again... :embarrassed:

[pedcab]

Hi Guys

So I'm gathering parts to build this 12AX7 x ECL86 thing here:



but two things are bugging me:

1- I've managed to get my hands on a bunch of precision VITROHM 0.1% wire wound resistors. Can I use these or will they bring unwanted inductive phenomena to the system?

2- How do I determine the ratio of the speaker coupling transformer? I once built a valve amplifier and used an automotive ignition coil as an output transformer, which was all but perfect for the job I must say...

Sorry for the probably silly questions  but this time I want to start build only when I'm sure I have all the right parts, so I don't have to take shortcuts as usual...

Your comments would be greatly appreciated.

[nigel]

Hi Pedro,

1- I've managed to get my hands on a bunch of precision VITROHM 0.1% wire wound resistors. Can I use these or will they bring unwanted inductive phenomena to the system?

Well ... I'd question why you'd want to use 0.1% resistors anyway. The tolerances of the valves are huge, so I don't see any point in using better than 5% resistors in this type of circuit. 1% maybe appropriate sometimes, when you need things carefully balanced, but not in this case. I'd say keep the posh resistors for something special that needs them

2- How do I determine the ratio of the speaker coupling transformer? I once built a valve amplifier and used an automotive ignition coil as an output transformer, which was all but perfect for the job I must say...

I'm going to respond with what is probably a silly question - do you mean that already have a transformer and need to work out the ratio, or that you need to know what sort of transformer to look for? 

One other observation - if you're going to use an ECL86, then you already have a triode & a pentode in that one valve, so couldn't you do away with the 12AX7 ?

[LeeS]

Primary impedance for the output trafo should be between 6000 ohms and 8000 ohms for an ECL86.

I'd have to agree with Nigel and go with something tried and tested, such as this :

http://www.r-type.org/static/2-3.htm

[pedcab]

Hi guys!

Thanks for your fast replies!

Hi Pedro,

Well ... I'd question why you'd want to use 0.1% resistors anyway. The tolerances of the valves are huge, so I don't see any point in using better than 5% resistors in this type of circuit. 1% maybe appropriate sometimes, when you need things carefully balanced, but not in this case. I'd say keep the posh resistors for something special that needs them

Hum, that's nice to know. I imagined valve circuitry to be much more sensitive to component tolerances thus my idea of using these resistors I've got... I'll follow your advice and use "normal" components. But I'm still curious... Would the use of wire wound's influence the circuit by placing inductive loads where they're not supposed to exist, or by any other factor that escapes my imagination?

I'm going to respond with what is probably a silly question - do you mean that already have a transformer and need to work out the ratio, or that you need to know what sort of transformer to look for?  

No Nigel, I don't have a transformer and I need to source for one, so I'd like to know what to look for...

One other observation - if you're going to use an ECL86, then you already have a triode & a pentode in that one valve, so couldn't you do away with the 12AX7 ?

Well, I don't have a proper answer to that ... I guess I chose this circuit for simplicity and I didn't know that the ECL86 could be "self-sufficient"...
So if there's no need for a separate preamp stage what do you reckon to be the purpouse of the 12AX7 in this particular circuit? (sorry for my ignorance)

Primary impedance for the output trafo should be between 6000 ohms and 8000 ohms for an ECL86.

I'd have to agree with Nigel and go with something tried and tested, such as this :

http://www.r-type.org/static/2-3.htm

That's also a very interesting design, although I don't have any rectifier valves... Could I use solid state rectification instead?

[pedcab]

Meanwhile I've found a couple more ...



( from http://www.bonavolta.ch/hobby/en/audio/ecl86.htm)




(from here http://www.jogis-roehrenbude.de/Leserbriefe/Stefan-Gloeden-ECL86-Amp/The_Small.htm)

[Wout]

Or you could go for a RIAA preamp with 1 ECL86 per channel.
Options, options ...

[reinderspeter]

Well, I don't have a proper answer to that ... I guess I chose this circuit for simplicity and I didn't know that the ECL86 could be "self-sufficient"...
So if there's no need for a separate preamp stage what do you reckon to be the purpouse of the 12AX7 in this particular circuit? (sorry for my ignorance)

Ignorance is something that can be worked at so no problem!

An ECL86 is a two valves into one: a ecc83 (12ax7) like triode plus a el84 like pentode. The schematic you show in your first post uses both of these sections. So all you need to build a stereo amp is two ecl86's.

[reinderspeter]

From all the schematics you show I'd choose this one:



nice and simple.

[pedcab]

Or you could go for a RIAA preamp with 1 ECL86 per channel.
Options, options ...

That's also a suggestion woth considering, but I'd like to build a versatile amplifier and not only for LP playing... I guess I could follow your suggestion but setup the RIAA section in a switchable manner though...

Ignorance is something that can be worked at so no problem!

An ECL86 is a two valves into one: a ecc83 (12ax7) like triode plus a el84 like pentode. The schematic you show in your first post uses both of these sections. So all you need to build a stereo amp is two ecl86's.

That's an interesting approach Peter...

I allready drilled a steel plate to fit two valve sockets but I guess I'll leave some extra room for if I later end up improoving the design to stereo (right now I'd prefer mono for simplicity at this early stage of valve amp experimentation) I can use the same chassis...

[pedcab]

First stage: complete

The resistors are all metox, although a couple of them are uncoated... Any comments on using metox resistors in audio circuits?

Just need to go to Lisbon and get some capacitors I'm lacking and the transformers. I have a feeling that the step up 275V power transformer will be a pain to find arround here...

To simplify stuff regarding the transformers I was thinking on feeding the valves' heaters' with a separate 6.3V regulated supply (I've got plenty of small 12V transformers and lm317's). What do you guys think?

There are still a couple os questions in my mind though.



Cheers!







 

[reinderspeter]

Hi Pedro,
I'm not sure I understand what you are doing.
You DO NOT need the 12ax7 at all.
You can wire it all to one socket and use both the pentode and the triode half of the ecl86.

[pedcab]

Hi Pedro,
I'm not sure I understand what you are doing.
You DO NOT need the 12ax7 at all.
You can wire it all to one socket and use both the pentode and the triode half of the ecl86.

Hi Peter!

Thanks for your head's up

I'm only trying to build the circuit of my first post... And I'm using the triode 12AX7 because I wouldn't know how to recalculate the circuit to build it usind the triode section of the ECL86 

Anyway, will this aproach of mine have any negative influence on overall sound? Why do you guys reckon the original designer included the 12AX7 instead of using just the ECL86?

I'd like to go for this circuit because I have the book with a full description and it is somehow more attractive to me than a single valve one... But hey, If I'm wrong I wouldn't mind going on other direction!

What do you say...

[reinderspeter]

Hi Peter!

Thanks for your head's up

I'm only trying to build the circuit of my first post... And I'm using the triode 12AX7 because I wouldn't know how to recalculate the circuit to build it usind the triode section of the ECL86 

Anyway, will this aproach of mine have any negative influence on overall sound? Why do you guys reckon the original designer included the 12AX7 instead of using just the ECL86?

I'd like to go for this circuit because I have the book with a full description and it is somehow more attractive to me than a single valve one... But hey, If I'm wrong I wouldn't mind going on other direction!

What do you say...

An ecl86 is two valves in one bulb. both of the valves you see drawn in the schematic are inside that bulb. So the original designer didn't include an extra 12ax7.

http://www.mif.pg.gda.pl/homepages/frank/sheets/020/e/ECL86.pdf

Using an extra valve will work too but you are using only half of them both. At the same time the heater of the unused parts is used and this can cause premature degradation.

[pedcab]

An ecl86 is two valves in one bulb. both of the valves you see drawn in the schematic are inside that bulb. So the original designer didn't include an extra 12ax7.

http://www.mif.pg.gda.pl/homepages/frank/sheets/020/e/ECL86.pdf

Using an extra valve will work too but you are using only half of them both. At the same time the heater of the unused parts is used and this can cause premature degradation.

Ah! Now I get it!  

I don't know why but when I first read the circuit descrition I got the idea that a 12AX7 valve would be required. I've just been reading it again and what it says is:




So thank you Peter for your insistence on correcting me...  

Now I'm stuck with two holes in the chassis   Guess I'll have to build a new one or go for stereo instead...

[reinderspeter]

Ah! Now I get it!  

So thank you Peter for your insistence on correcting me...  

Now I'm stuck with two holes in the chassis   Guess I'll have to build a new one or go for stereo instead...

 

All you have to do for stereo is the same thing twice. Easypeasy
If your power transformer can take the extra current (and it probably can) you can use the power supply for both channels.