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Using a regular OPTX in an UL schematic

66.74.191.120
Posted on November 2, 2009 at 23:27:06
Audiodyssey
Dealer

Posts: 512
Joined: November 7, 2004



Ok, I found a schematic I am happy with. It is from an AudioExpress article "King Size Quarter Horse Power Stereo Amplifier." The scheme calls for an Ultra Linear OPTX. I have a pair of OPTX without the screen taps. Am I correct in assuming that I can build without the UL's? Would the only modification necessary be applying voltage to the screen grids? I beg your pardon, but HOW do I determine the voltage amount, if this is at all possible? Are there other things to consider?
"I KNOW you can hear it, but are you REALLY listening?"
RE: Using a regular OPTX in an UL schematic, posted on November 12, 2009 at 19:59:25
Michael Koster
Industry Professional

Posts: 585
Location: Bridgeville, CA
Joined: October 24, 2007



How to determine screen voltage? I like to draw the load line. Let's assume your OPT is about 5K ohms anode-anode Zpri. That gives 1250 ohms load on each anode in class AB. Also assuming 550V B+ and about 50mA idle current.

I choose a G2 voltage that results in peak current on the load line at 0V g1, in this case the curves for g2=300V look about right. This load line would give you about 80W power output.

For higher B+ or lower Zpri, the g2 voltage would be increased to get the 0V g1 point up to a higher anode current.

(550-650V B+ is about what to expect with a 750-0-750 PT and a 5R4 with L-input filter, assuming reasonable DCR components. Your B+ current at full power will be a little less than 300mA)

Cheers,

Michael

RE: Using a regular OPTX in an UL schematic, posted on November 12, 2009 at 21:10:40
Audiodyssey
Dealer

Posts: 512
Joined: November 7, 2004
 Michael, I really appreciate the thoughtful answer. This information settles my mind about my component choices. This does lead to another question: I have heard that chokes need a certain current requirement to function properly...if that is so, will 500mA chokes be suitable?
"I KNOW you can hear it, but are you REALLY listening?"
other things to consider?, posted on November 5, 2009 at 07:32:41
grhughes
Audiophile

Posts: 1871
Location: Central US
Joined: February 17, 2004
 Allen Wright commented on the input stage tube phase splitter as one "he" doesn't like. This design uses about 20 db of feedback. You could use the Ampex phase splitter with very little modification and it would be more symmetrical.
http://recordist.com/ampex/schematics/351man/351big.gif

In the playback amplifier (2V4=12ax7-ECC83), after 2R36 (250K volume control) on the above GIF schematic, notice 2C18 (.02 ufd) and 2R39 (1 meg ohm) which, and both cathodes and plates have the same value loads. You could use this phase splitter and return feedback to the same point as in the Voss and Ellis article. Using a diff amp, I feel (which is modern practice) will cause the gain to fall way too far since feedback is going to require the type of gain that a 12ax7 can deliver.. Ray
"I think the journey should be just as enjoyable as the destination." GRH
RE: other things to consider?, posted on November 11, 2009 at 08:04:11
Audiodyssey
Dealer

Posts: 512
Joined: November 7, 2004
 Thanks a lot! You have been very helpful.
"I KNOW you can hear it, but are you REALLY listening?"
It depends if you're going for max power or low distortion, posted on November 4, 2009 at 07:29:06
grhughes
Audiophile

Posts: 1871
Location: Central US
Joined: February 17, 2004
 If you go triode the max power you can expect from a pair of KT88 is about 25 watts in triode mode. Triode operation only requires a 100 ohm, 2 watt resistor from plate to screen of each output tube. The other way for increased power is to apply screen voltage from a regulated power supply a la the RCA tube manual (50 watt Hi-fi amp). The other way of course is to find these Triad transformers or Acrosound or find one made by a current company like Hammond or Sowter and have them wound that has screen taps. Ray
"I think the journey should be just as enjoyable as the destination." GRH
How about Triode Mode?, posted on November 3, 2009 at 11:04:34
gusser
Audiophile

Posts: 841
Location: So. California
Joined: September 6, 2006
 Just connect the screens to the plates through 100 ohm resistors. Less power than UL, about 1/2, even less than Pentode but most claim much smoother midrange.

RE: How about Triode Mode?, posted on November 3, 2009 at 11:16:08
Audiodyssey
Dealer

Posts: 512
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 Actually I thought about doing this and wasn't sure how to do it. So thanks, I'll be sure to at least try it.
"I KNOW you can hear it, but are you REALLY listening?"
I can't read the type number..., posted on November 3, 2009 at 02:54:46
Allen Wright
Manufacturer

Posts: 4729
Location: Schaffhausen
Joined: January 31, 2002
 ...of your output tubes, but if they are KT88 then you can run the screens at the same voltage as the anode supply.

Regulating the screen voltage will make it sound better, but that's a more advanced projectthan just getting an amp built.

Second point - that circuit is a rippoff from the GEC KT88 application note circuit circa 1955, and I've never liked the sound of it because the phase splitter adds an extra tube to the "lower" half compared to the "upper" half. Build it , listen to it and then maybe you'll be interested in finding some better phase splitter circuit.

Regards, Allen

RE: I can't read the type number..., posted on November 3, 2009 at 17:50:53
Audiodyssey
Dealer

Posts: 512
Joined: November 7, 2004
 Allen, yes this is for KT88's.
I was looking at this schematic for a power supply, and thought it may work for this amplifier. This guy states he's using 720VCT, does he mean 1440VCT???
Does my first choke need to be a swinging choke in this configuration? I have 2 Hammond filter chokes rated for 5Hy @ 500mA that I could use. Also, can anyone hint at the regulated portion at the bottom of the schematic; would this IDEA work for a regulated screen voltage supply? If so, what screen voltage should I set it up for assuming 650V B+ ?
Does my idea suck, or will this configuration work????
Thanks everyone.
"I KNOW you can hear it, but are you REALLY listening?"
RE: I can't read the type number..., posted on November 3, 2009 at 08:08:03
Eli Duttman
Audiophile

Posts: 6155
Location: Plainsboro, NJ
Joined: March 31, 2000
 Allen is correct about regulating g2 B+. Also, open loop linearity will be better if g2 B+ is less than anode B+.

The fact that the intended O/P "iron" has lots of magnetic headroom is good. Full pentode mode requires more total NFB than UL mode does. If 100% of the NFB is global, LOTS of magnetic headroom is needed or the core will saturate.

Eli D.
Here's the PS Schematic, posted on November 2, 2009 at 23:33:46
Audiodyssey
Dealer

Posts: 512
Joined: November 7, 2004



Power supply schematic. BTW, The OPTX's I have are rated for 120+ watts.
"I KNOW you can hear it, but are you REALLY listening?"

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