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Vacuum Tube OTL power amp!!
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Old 29th August 2011, 10:54 AM   #141
M Gregg is offline M Gregg  England
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Just some more progress.

Tx's and caps mounted.



Regards
M. Gregg
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Old 29th August 2011, 07:17 PM   #142
Alastair E is offline Alastair E  Wales
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Looking good, Coming along really well!
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Old 14th September 2011, 05:42 PM   #143
M Gregg is offline M Gregg  England
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Just for interest,



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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Old 14th September 2011, 06:18 PM   #144
Alastair E is offline Alastair E  Wales
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Coming along really well!

I think though, with 180V you may need to increase the value to the 6C33C Cathode-Resistors, up to say, 220-300 ohms....

Check the current, and set to around 180-200mA....
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Old 14th September 2011, 06:27 PM   #145
M Gregg is offline M Gregg  England
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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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Old 14th September 2011, 07:23 PM   #146
Alastair E is offline Alastair E  Wales
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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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Old 15th September 2011, 07:08 AM   #147
M Gregg is offline M Gregg  England
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Quote:
Originally Posted by Alastair E View Post
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.

Electronic choke

Follow it with some film caps?

Regards
M. Gregg
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Old 15th September 2011, 06:15 PM   #148
Alastair E is offline Alastair E  Wales
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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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Old 15th September 2011, 06:48 PM   #149
M Gregg is offline M Gregg  England
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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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Old 15th September 2011, 08:47 PM   #150
M Gregg is offline M Gregg  England
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Originally Posted by Alastair E View Post
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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