Single Ended EL34 amp

[Ianm2]

I dont mean to take the gloss off a great build , its lovely, but I htink the choice of output loading is incorrect, youd' be better with 6k5-10k, the distortion from 2k5 is rather high, you wouldn't lose many watts.
just a kind remark

[The Shadow]

Originally Posted by Ianm2

I dont mean to take the gloss off a great build , its lovely, but I think the choice of output loading is incorrect, youd' be better with 6k5-10k, the distortion from 2k5 is rather high, you wouldn't lose many watts.
just a kind remark

Thanks Ian I'll investigate upping the loading

I'm still learning so don't worry. I spent so much time fretting about the voltage amplifier stage I never even thought about the output.

I'm always open to advice and tips from more experienced people. After all it would have been even further off the mark had I not listened to Paul

I'll post back in a trice and let you know how it turns out.

Steve.

[andrew ivimey]

Hello Steve, I like the looks of your amp very much (very neat and simple- great) and if it sounds good then ...

I think Ian might be on the right track though. I would have gone straight for 5k output transformers, not as far as 10k. I can't remember what HT it will take safely but it is more than 400 volts, I think? The only el34 SE thing I've built used 5k and was good, but not brilliant. The front end was different to yours. I wonder, you should be able to get away with 6SN7. Many would say it is a better sounding valve; less gain sure but better tone. might be worth a try, when I get home I will look to see if I'd recommend different component values, (but right now I'm grabbing a sandwich and finding it hard to be back at work. )

good luck.

[acorn]

Hello Steve


Nicely built Steve I know what effort you put into that Amp, because of the ones I have built by hand I am now making a Test amp but there is a lot of metal work cutting iron is time consuming and tedious, why do I do it simply because its a challenge and probably my ego anyway thats a good idea of yours about posting our projects on the Web site as I am not sure how to do it a few pointers would be very welcome.

The 6SL7 valve was a good choice has these series valves give far less distortion than the newer ECC type valves.
Any how I like looking at other peoples efforts and pick up some tips.

Regards Acorn

[The Shadow]

Thanks for the comments on the amp folks. They're much appreciated.

Acorn:
To get pictures, schematics etc up to my website you need to email the pics to me and put a few notes about what you want me to write. I'll then make the page and upload it with an announcement on the BB that your work is up.
I'm hoping that Peter can make the bit where members uploads are announced a sticky as Greg suggested. I've PMed Peter with the idea but as yet I haven't received a reply.

Ian
You've made me think now.
I tried loading the output to 5k but was disappointed with the resulting sound. Volume and life had been sucked out and the amp clipped prematurely. I did not take it any further and returned the amp to a 2k5 load.

Investigating further by looking back through my notes I found that I had thought I had worked out roughly which output transformer to use before I had gone and gotten myself bogged down and overly preoccupied with the front end.

About 2 months ago I had used the EL34 data from Duncan's amp pages which for Class A SE triode operation gives a Va of 375V and a Zout of 3k for 6W at 8% distortion.

I set my HT at 380V and chose the Hammond 1627SE OPTX which at 2k5 impedance seemed to give the best match to the output impedance of the EL34 when connected as a triode. I assumed that was that for the output stage and gave it no further thought. I then started some serious head scratching and forum posting about the front end.

The methodology had seemed logical so I hope I haven't made a silly mistake but the amp sounds lovely with these transformers. The power transformer barely gets warm after 12 hours of continuous use and the chassis sits there at just above room temperature.

The question on my mind now is did I do the output design right? My ears and the way the amp runs as a physical entity seem to say I didn't make a bad job but I'm a bit confused now.

Steve.

[Paul Barker]

No complaints about that circuit.

Since the el34 is one of the few true pentodes which doesn't have the screen grid internally connected to the cathode you can further improve it by connecting the screen grid to ground, which puts it more negative relative to the cathode when you use autobias.

[James D]

When Paul says screen grid he means supressor grid (g3)... it's his Yorkshire accent that does it

[Greg.]

And of course, If the sound is right for you, what more is there worth considering. Don't let the suggestions of others mess you about if you really are happy. If you want to try alternatives then the suggestions of others can then be helpful. Simply, you are obviously confident in what you've achieved. Don't let anything spoil that confidence, but if you do want to explore further, so beit. Take all the advise you can get.

Best wishes,

Greg

[The Shadow]

Thanks once again chaps. I feel a bit more reassured about the basic design now.
I'll try grounding g3 to see what happens.

Andrew: I had thought about the 6SN7 as the front end but was concerned that it would not have enough gain in single valve mode.

Mind you my PreII has a lot of gain with those transformers so reducing it via the power amp might be feasible. It's definitely worth thinking about.
Ah, but then I could arguably get the same effect and better sound by converting the PreII to cathode follower. AAAGH where's me washboard.

This is what it's all about! I get it now!
This hobby doesn't half keep the mind active.

[acorn]

Hi Steve


I agree with Greg if it sounds good to you it is usally is put to together allright, try the suppreser grid G3 to ground by all means and probably a larger smoothing cap after the choke may tighten up the bass a little and the 47uF change to 220uF would improve the bass even more I have done this type of upgrade to my amp and it does really work well, any how if you do decide to implement the upgrade let us know how it sounds and thanks for that other info about posting pictures of projects.

Acorn