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Vacuum Tube OTL power amp!!

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Just some more progress.

Tx's and caps mounted.



Regards
M. Gregg
 

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Just for interest,

:snail:

PSU wired giving +/- 180V off load.
Ultra fast soft recovery diode bridge.
PSU for preamp & driver tubes.
Polyswitch fusing.

Also a picture of the relay select board in mounting position.


Regards
M. Gregg
 

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Hi Alastair,

I am starting the build for the pre/driver section. I will look at the cathode resistors for the 6C33c's. I expect the B+/- to drop with current draw not quite sure what that will be at full idle. I thought I would get more done, work is a bit demanding at the moment.

I thought about a crowbar circuit to help with protection, then again it may effect freq response. Are you using AC or DC for the pre/driver tube heaters?

Regards
M. Gregg
 
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All my heaters are currently AC. They are biassed to around 35V to reduce hum....

One thing that Ive been thinking of trying is a simple 'Hum bucking' circuit in each of the O/P stage rails supplies, a simple MOSFET affair, nothing special, but hum currently is pretty low so I haven't bothered....
--The 'Futterman' has pretty crap PSRR, so thats why we use such big caps in the rails...

Yup--I think a little tweaking when you have it all together may be needed on the cathode-resistors, as it occured to me the 6C33's I'm using do have a number of hours on them!
--I wouldnt be surprised if they have a thousand hours on one or two!

Ive been messing round with my breadboard. I tried a CF driver between the PI and the 33's but there wasn't that much benefit over the cap-coupling, just a little better control of transient reovery at near full O/P. (No 'blocking' when near/at clipping) I dont think there is any real benefit in doing this for the current amp, as there appeared little if any change in sound at normal levels.
Not enough benefit to warrant another hole in the chassis and another tube per channel anyway!

Ive even tried replacing the first gain stage with a globe #27 triode, which sounds quite nice, apart from some microphonics but again the difference is subjective.
6J5/6SN7 is a great performer IMHO for this amp....
 
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One thing that Ive been thinking of trying is a simple 'Hum bucking' circuit in each of the O/P stage rails supplies, a simple MOSFET affair, nothing special, but hum currently is pretty low so I haven't bothered....
--The 'Futterman' has pretty crap PSRR, so thats why we use such big caps in the rails...

Alastair,

What about something like the electronic choke e.g.

post #15 / #17 in this thread.

http://www.diyaudio.com/forums/tubes-valves/183476-electronic-choke-2.html

Follow it with some film caps?

Regards
M. Gregg
 
Hi Gregg,--

Yes, I was thinking on the likes of --Oleg's circuit, --about halfway down the page. Just something nice and simple but effective....

They seem to call these circuits 'electronic-chokes'--But, They aint IMO, a 'Ripple-Filter' would prob be a better description....

A Choke has the ability to store a relatively large amount of energy, while the ripple-filter has no energy storage capacity of itself at all, relying on the preceding caps to supply the energy--Not that it matters, with what we are doing here....

I believe a couple of caps, say, an eletrolytic of say, 470uF and a nice film of 50uF should work well on its output. The Film cap could be mounted physically close to the output stage....
--Probably the electrolytic aint needed seeing the ripple-filter will be effectively removing all the crap from the PSU anyway, but thats just me....
 
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Hi Alastair,

Yes Oleg's circuit looks interesting. Are you going to give it a try?

I have ordered the IRF710 fets.
I have also fitted a NTC CL60 inrush suppressor for the Tx's

I will also fit the neon protection for the DC coupled stage.(grid cathode)

Managed to find some 4.7M resistors 2W 1.kv flash over.

Regards
M. Gregg
 
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Joined 2010
I believe a couple of caps, say, an eletrolytic of say, 470uF and a nice film of 50uF should work well on its output. The Film cap could be mounted physically close to the output stage....
--Probably the electrolytic aint needed seeing the ripple-filter will be effectively removing all the crap from the PSU anyway, but thats just me....

I would think that their is enough "C" in the PSU so just a film cap after the (ripple filter) should be OK. Only thing is you need a minus supply version for the other leg..?

It would be interesting to see how low the value of C-filter-C could be in this circuit and still keep ripple down?

Regards
M. Gregg
 
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Relay input board connected.
Just for interest.
Cables
PTFE cable silverplated.
Jupiter silk/silver. << built into a screened cable.
Only because its a short run.




Regards
M. Gregg
 

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Hi, would like to ask whether the vacuum tube OTL power amp worth to build? How is the sound performance if compare to normal output transformer type?? Does anyone has any good schematic diagram for OTL power amp, can share out, thinking to try to make one.

Hi dear, I just finished a layout for a Totem-Pole OTL with very low Harmonic Distortion levels (0.079% at 20 W rms, 8 Ohm and 1Khz). The design has not been tested in a practical implementation. It is symmetrical and uses that many 6C33C tube stages that you can afford (or have the power supply for).

I've opened a Thread on this this morning, and hope receiving comments and suggestions for further improvements.

Attached you'll find a ZIP with the LTSpice model and the library.

CU Gary
 

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Disabled Account
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Just some more progress,:snail:
Just for fun.

Driver/preamp board.

I wanted to compress it into a small area away from the 6c33c's.
So point to point wiring.

Just connections for the IRF710 to do.

Feedback is via PTFE home made screened cables.
Bias supply cap is Rubycon HV.
Coupling cap is Jupiter, Amtranz.
Takman resistors.

Heaters should be DC for the pre/ 6SN7's if all goes well!
and AC for the 6SL7.

Heater cables run across the back of the board to shield them from the pre circuits.
The board only provides screen no signal transfer. star Gnd.

Still on going, lots to do!


Regards
M. Gregg
 

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Disabled Account
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Regards the 6SL7,

Neon protection for the DC coupled stage.(grid cathode) has been fitted!

Just for interest the top plate shields the tubes from the Heater cables.

Alastair,

What voltage do you have on the anode of the driver tube?

(6SL7) in my circuit. So I can check the bias point, thank's.

Regards
M. Gregg
 
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