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Old 18th June 2008, 12:44 AM   #1
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Default Oddball Sweep tubes in PP

When using oddball sweep tubes for PP close matching is obviously out of the question most of the time. Is it therefor advisable to use fixed or combination bias for the purpose of adjusting current balance?

Also most have pretty low screen voltage ratings so I presume that UL and triode mode are not often used. So that leaves pentode (with good screen regulation) or screen drive. So what is a good link for a "Screen drive for dummies" type write up?

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Old 18th June 2008, 01:03 AM   #2
SY is offline SY  United States
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I'd start by poking around Tubelab's site. And go to David Berning's site, go to "support," and download the schematics for his ill-fated Audionics amp, the EA 230, the and EA2101. Study them well, there are some very nice design tricks.
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Old 18th June 2008, 01:50 AM   #3
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There isn't much screen drive stuff on my web site currently. I have a schematic for a 80 watt P-P amp using 6AV5's, but I never developed it into a complete amp. Pictures of the prototype are on the web site on the 6AV5 testing page. I did experiment with a big amp using 6LW6's and screen drive, but never made it work right. I discovered that the 275 volt screen rating can be ignored so I proceeded to wire up a triode PPP amp using 4X 6LW6's. I had the B+ up to the limit of my power supply which is 550 volts. I had similar issues making it work right. I got sa pair of surplus Plitron toroid OPT's that are rated for 400 watts at 20 Hz. I have discovered that they are very sensitive to current imbalance. The DC offset at idle needs to be under 10 mA. I believe that DC shifts caused by mismatched tubes may be causing distortion at high power levels. I have not had time to work on any of this stuff lately.
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Old 18th June 2008, 02:05 AM   #4
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Default Re: Oddball Sweep tubes in PP

Quote:
Originally posted by mashaffer
When using oddball sweep tubes for PP close matching is obviously out of the question most of the time. Is it therefor advisable to use fixed or combination bias for the purpose of adjusting current balance?
I always use fixed bias. It gives better performance. In using these sweep tubes, I've found that the balance isn't all that far off anyway.

Quote:
Also most have pretty low screen voltage ratings so I presume that UL and triode mode are not often used. So that leaves pentode (with good screen regulation) or screen drive. So what is a good link for a "Screen drive for dummies" type write up?
Le Renard Main Schemo

Screen Regulator

Main PS
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Old 18th June 2008, 03:19 AM   #5
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Default Oddball sweep tubes in PP.

Another candidate for tubelab, the 6CB5A ?

Looks like a fat EL34 with an anode cap.

Jeff
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Old 18th June 2008, 04:01 AM   #6
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Default Re: Oddball sweep tubes in PP.

Quote:
Originally posted by vinylkid58
Another candidate for tubelab, the 6CB5A
Jeff
Ah-ha, you've discovered one of my secret output tubes. DuKane amplifiers also used them.

Victor
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Old 18th June 2008, 06:09 AM   #7
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Default Re: Re: Oddball sweep tubes in PP.

Quote:
Originally posted by HollowState


Ah-ha, you've discovered one of my secret output tubes. DuKane amplifiers also used them.

Victor
Are you using them for HiFi or guitar amp duty?

I found a Dukane 1A475 schem. that uses 4 x 6CD6GA's. Seems there is interest in these PA amps for conversion to guitar. With 75 watts, I can understand why.

Jeff
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Old 16th May 2009, 04:35 PM   #8
JLH is offline JLH  United States
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Quote:
Originally posted by tubelab.com
There isn't much screen drive stuff on my web site currently. I have a schematic for a 80 watt P-P amp using 6AV5's, but I never developed it into a complete amp. Pictures of the prototype are on the web site on the 6AV5 testing page. I did experiment with a big amp using 6LW6's and screen drive, but never made it work right. I discovered that the 275 volt screen rating can be ignored so I proceeded to wire up a triode PPP amp using 4X 6LW6's. I had the B+ up to the limit of my power supply which is 550 volts. I had similar issues making it work right. I got sa pair of surplus Plitron toroid OPT's that are rated for 400 watts at 20 Hz. I have discovered that they are very sensitive to current imbalance. The DC offset at idle needs to be under 10 mA. I believe that DC shifts caused by mismatched tubes may be causing distortion at high power levels. I have not had time to work on any of this stuff lately.

If I recall correctly, the B+ was 550V on that screen drive 6AV5 amp. What voltages did you end up using for the screen grid and grid bias supplies? Only asking because no values are given in your schematic. Thanks.

Rgs, JLH
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Old 4th June 2009, 01:17 AM   #9
rman is offline rman  Canada
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hi Mike.

John Broskie recently put an article on Tubecad Journal about a technique developed by Alan Blunheim that keeps the idle current through push pull tubes perfectly matched. It uses cathode bias and cross coupling resistors. the only downside is you need to increase the B+ voltage by the amount of the desired grid bias. It is called the garter circuit. Check it out.

Rolf.
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