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Old 25th July 2011, 06:57 AM   #141
M Gregg is offline M Gregg  United Kingdom
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Alastair,

Couple of questions:

How many watts are you dissipating in R9/R10?
What voltage do you have across them?


The reason I ask is Mills resistors seem to be 12 Watt and you are using 17W. (I did not want to parallel if not required).

I know you have used standard WW. I just like Mills Resistors.

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M. Gregg
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Old 25th July 2011, 10:49 AM   #142
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Well, you're looking at around 200mA and 20V at 120V supply, so be in the order of 4W....

Your P-diss will be somewhat higher, being a higher value 150 Ohm and 150V supply, but will prob be in the order of 6-8W...

A 12W resistor will be fine, Not sure of the Mills construction--I'm going to look them up now....

I used 17W as those are what I had around, They get hot--but not excessively so...
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Old 26th August 2011, 03:06 PM   #143
M Gregg is offline M Gregg  United Kingdom
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More slow progress.. work is getting in the way..LOL

I can start the internal build now the chassis is strong enough.

I will add more vent holes as I build, so it's still pulling the air from the front and around the top via the tubes and cathode resistors.

Inputs mounted! Lots to do.

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M. Gregg
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Old 29th August 2011, 10:54 AM   #144
M Gregg is offline M Gregg  United Kingdom
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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   #145
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Looking good, Coming along really well!
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Old 14th September 2011, 05:42 PM   #146
M Gregg is offline M Gregg  United Kingdom
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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.


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M. Gregg
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Old 14th September 2011, 06:18 PM   #147
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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   #148
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Alastair E View Post
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....
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   #149
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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   #150
M Gregg is offline M Gregg  United Kingdom
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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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