6C33C SE-OTL Project

[Ray P]

Following on from the initial thread on this subject;

http://www.audio-talk.co.uk/phpBB3/view ... f=5&t=7256

I'm starting this new thread to document the build/development of this amplifier.

To start the ball rolling, here's an initial sketch to show the direction we're heading in;

Output will be around 1W into 8ohm speakers but both of us use high efficiency speakers with 15ohm drive units so it'll be nearer 2W and we'll optimise (feedback, output cap) the amp to use with those drive units.

[Nick]

Cant help thinking a smaller grid resistor would be a good idea, IME that valve needs no excuse to run away with itself. I would suggest 47k, I know that will affect the driver, but just my thought.

[Ray P]

Cant help thinking a smaller grid resistor would be a good idea, IME that valve needs no excuse to run away with itself. I would suggest 47k, I know that will affect the driver, but just my thought.

That's one of the specific areas we'll be exploring Nick. The diagram is just a starting point.

[Ray P]

Wolfgang, my stateside buddy on this project is very much the brains of the partnership... He#s now aware of this thread and will welcome any observations/pointers.

Here's his initial comment on Nick's observation;

"Nick has a good point but if we reduce the grid leakage resistor from 470k to 47k it will reduce the combined resistance of the plate resistor (20k) with the grid leakage resistor (470k) to 14k for AC. That means for AC operation that the load resistance is lower than that of its DC operation (20k). The load line for the 6SN7 would change and gain of the pre-stage would be reduced which I try to avoid at this point..."

Pre-stage gain is important because of the need to use feedback to lower the output impedance of the amp so there is something of a balancing act to perform.

Ray

[IslandPink]

Cant help thinking a smaller grid resistor would be a good idea, IME that valve needs no excuse to run away with itself. I would suggest 47k, I know that will affect the driver, but just my thought.

I can see there being some potential for distortion with grid current, but can it run away, with a current source in the cathode ?

[Nick]

I can see there being some potential for distortion with grid current, but can it run away, with a current source in the cathode ?

Whats to stop it. at least with a resistor instead of a current source there will be some degeneration to pull it down if it wants to float up, not so with a current source, The current may stay the same, but the voltage can go to wherever it wants.

[IslandPink]

Ok... so grid dissipation exceeded rather than overall dissipation ?

[Nick]

Ok... so grid dissipation exceeded rather than overall dissipation ?

No probably not, but I can see the grid side of the valve charging up and taking the valve up with it. I must have repaired just about all the Almarro A318B in the UK over the last 5 years. They all fail because of cathode component overload and bias instability. In every case its because they had too high a grid resistance to earth. Its fine with new valves, but as they age they seem to be prone to grid current and that leads them to runaway. Now they don;t have a CCS in the cathode, so they shouldn't run away in this case, but I can see the voltage across the CCS increasing as the grid rises up. I have not seen any S318B after fitting smaller grid resistors.

But it was just a suggestion.

[IslandPink]

I agree, a very good suggestion. I would not want more than 100k on the grid leak with one of those - I checked Romy's amp and the bass channel uses a 50k grid leak -
http://www.goodsoundclub.com/Forums/Sho ... ostID=4973

[simon]

First time I've seen Romy's system, quite something!

[Ray P]

Wolfgang, my partner on this project (and the one with the technical nouse) has done some calculations and developed an initial schematic with part values (100K grid resistor) and is pushing ahead with an initial build to see how it behaves/sounds.

Further updates when there's something to report.

[Ray P]

Wolfgang has been progressing the protoype build, initially building and testing the 6SN7 pre-stage and now hooking up a 6C33C on his workbench;



He's been experimenting with the NFB and measuring using a dummy 16ohm load (we're building this especially for our 15ohm Lowther drive units);



Good looking sine-wave on the output. Output impedance is the region of 6ohms and power around 1.5W. He should get to listen soon and then start to optimise, particularly the NFB, for best results.

[RhythMick]

A left-field thought from a complete amateur, but would a choke help in that position? Low DCR but high ac inductive reactance?

Probably. Just me and my love affair with chokes...

[Wolfgang]

Hi guys,
I thought it’s about time to register with this forum. This OTL- project on which I am working together with Ray could become even more interesting in combination with the impedance multiplier circuit. As it is right now these OTLs only work with 15ohm or higher impedance speakers. A first listening test with only one channel was extremely pleasing so let’s see where all this will finally lead up to. It's amazing what SQ a simple OTL based on two tubes can produce.

Wolfgang

RhythMick,
there are chokes in the B+ psu for the 6C33. Don't know if this answers your question?

[RhythMick]

Welcome Wolfgang

I meant would a choke be useful in place of the 470k, following the earlier discussion about the following valve going into runaway. A choke would give low DCR for the grid but high ac impedance.

Just a thought and could well be wrong.