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Old 2nd March 2012, 08:23 PM   #1691
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Default Zobal on 1660

George & Salas,

I put the zobal on, 100n with 20k trimmer. The square wave looks pretty good up to about 5k with the trimmer at 4.64k. I make up a 100n & 4.7k, same number as George. Will report after a serious listening.

Rod,

I have the filament resistors re-done with some single stand-off strips which were given by my best friend Johnny. I left a little more room for air circulation. I'll check the temperature and will post the picture later.


Albert
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Old 3rd March 2012, 03:42 AM   #1692
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Rod,
With this set up, the temperature is top at below 100c on couple in a group. I'll leave it to run for sometime and see if there is any other way I could make it better. The voltage after 3 hours are 11.14/9.71v and 11.15/9.74v.
Do I have to tune them back to 11.5v/10v respectively for better operation ??
Albert
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Old 3rd March 2012, 09:07 AM   #1693
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hi Albert, 100 deg C. sounds reasonable to me.

Is the "Ground" side of the resistors connected to chassis just there? If so, make sure it's the only position. Actually, you can drain a lot of heat into the chassis, using a strip of EMC-type (sticky backed) copper foil between chassis and the resistors, soldered at a few points.

11.15/9.74V is fine for running, just check the bias is where you expect in terms of anode-current. You can fine-tune to, say, 11.2V when hot, if you prefer.
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Old 3rd March 2012, 09:10 AM   #1694
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Quote:
Originally Posted by andyjevans View Post
I don't know exactly why but this made me think of Spinal Tap - "It's in D minor - the saddest key of all".....

Maybe your CLASH logo......!

Andy
Not to mention that this design will help you "get all the way to 11" again, without using caps.....
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Old 3rd March 2012, 02:17 PM   #1695
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Rod,

Is the "Ground" side of the resistors connected to chassis just there? If so, make sure it's the only position.
It's grounded to the star through the brown wire.

Actually, you can drain a lot of heat into the chassis, using a strip of EMC-type (sticky backed) copper foil between chassis and the resistors, soldered at a few points.
I don't have the foil, but will ask and see if my friend has it by chance.

11.15/9.74V is fine for running, just check the bias is where you expect in terms of anode-current. You can fine-tune to, say, 11.2V when hot, if you prefer.
I thought I'd let it breaking in for sometime then to adjust it. It's time to enjoy. Will report.
Thanks

Albert
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Old 3rd March 2012, 04:05 PM   #1696
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Talking of bias, I just had a look at a 26 preamp I built with filament bias and two Hammond 156C anode chokes in series - pretty much the circuit I published earlier on this thread. I was using slightly starved filament - 1.35v and 950mA. bias resistor was 10 ohms as usual with 9.5v on it, so total voltage to be supplied was 9.5 plus 1.35 = 10.85v. Supply to my Coleman board is 15.8v from a 18v secondary, 100uF, Hammond 158ZC choke 2A, 60mH, 10,000uF cap.

With this setup I'm getting 178v on the top of the 156Cs and 148v on the anode. However, this is passing just 4.5mA per tube with the 9.5v bias. Not at all what the curves say!! The chokes have a resistance of 7K.

andy
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Old 3rd March 2012, 06:09 PM   #1697
mogliaa is offline mogliaa  United Kingdom
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Andy, filament starvation increases anode resistance and curves and to be graphical the anode curves are displaced to the right.
Im traveling at the moment but will try to plot the 26 curves with filament starvation on my return.
Have done it for VT67 recently
Cheers,
Ale
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Old 3rd March 2012, 06:16 PM   #1698
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With the Hammond anode chokes a bit more anode resistance isn't a big deal - plenty of inductance. It would be more so with a LL1660. I must say it sounded great at this operating point - very detailed.

andy
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Old 4th March 2012, 01:14 AM   #1699
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have tried some rectifier tubes that I have on hand, 5R4, 5Y3, 5AR4, CV378, 83 and 5U4G RCA. Love the sound that 5U4G present, more balance.
I have the RCA Cunningham on, will try Reytheon, Syvenia and see if I can hear the different.
Thanks guys for the guidance.
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Old 4th March 2012, 01:32 AM   #1700
Salas is offline Salas  Greece
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5R4GY RCA I liked in my 6V6 line pre with least liking 5R4WGB Raytheon. Maybe you got one 5R4GY to test also? Very nice with still more midbass but retaining enough of 5R4GY's open sonic was 5AR4/GZ34. Here pictured with an RCA 5U4G which gave a rather more closed in result than 5R4GY but with richer midbass.
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