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8th May 2003, 02:50 PM  #1 
diyAudio Member
Join Date: Dec 2001
Location: Earth

Output transformer Impedance
Currently i am using a pp el34 with a 6k 30 w transformer. The el34 are run in pseudo triode mode giving total power of 12 W. I find that is that the amp is clipping at load notes. I taught of upgrading the power by adding an extra pair of el34. I know my power wouldn't be double (24 W) but be less. BUt how much less by using the same output transformer of 6k. What are the bad points or good points utilizing the 6K impedance to 8 ohm. would there be extra requirement of the driver stage e.g extra voltage swing, by the way the driver output impedance is around 45K should be okay for driving the el34s. The transformer i spec for the usage is 350vac at 250va ,8 vac 125 va so hence the extra room for the additional el34 or additional drivers. Plus i like the output tubes to be bias into A if preferably. So is the extra el34 worth it ? I find that adding additional el34 is cheaper than going for kt88, 6550 as there are more than double the price of one el34. Plus what are the bad points of paralling output tubes? Thanks for the help guys

8th May 2003, 02:55 PM  #2 
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Join Date: Dec 2001
Location: Pittsburgh, PA, USA

Parallelling will get more power only if your power is limited by output current; i.e. if you have voltage swing to burn. What is the peak voltage swing of the output stage right now? Is the limitation in the driver stage or in the output stage?

8th May 2003, 03:36 PM  #3 
diyAudio Moderator

If you're only getting 12W, my guess is that you've got a lowish B+. Get it up to 500V and you'll get closer to 20W from that single pair of EL34s into a 6K pp load. Paralleling them will probably not help much with that sort of plate loading, but would allow you to get the same power at full Class A (if that's your religion). Put into perspective, the differences we're talking about are pretty minor, ~3dB more output.
I assume that by "pseudotriode," you mean "screen tied to plate?"
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8th May 2003, 03:37 PM  #4 
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output swing is around 30 peak to peak i guess the limitation is in the output stage input still fine. Hence from your post i conclude that by adding more tubes i get more current but if the driver stage is not enought than is still can't get the power i need rite. But i think the voltage stage is fine.More concern with output impedance ratio and how it effects the loading of parallel tubes

8th May 2003, 05:09 PM  #5 
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Join Date: Dec 2001
Location: Pittsburgh, PA, USA

Ok. 6k pp load. The impedance multiplication is 750, so sqrt(750)~=27.
You will take the AC voltage impressed on the primary side and see 1/27th of that voltage at the output. So for 20W into eight ohms you need to swing nearly 500Vpp. Adding extra tubes won't help you swing more voltage into this load, just supply more current. Now, if you have a 16 ohm tap you can attach your 8 ohm speaker for a 3k pp load, and the multiplication is ~19. Now you need swing only ~340v for 20W output. At some point you will need to add additional tubes to the output stage because you won't be able to supply enough current into this lower load => current limiting your output instead of voltage. I'm not that familiar to know what the load lines look like off the top of my head. If anyone sees a problem with my math feel free to correct. 
9th May 2003, 04:06 AM  #6 
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thanks for the replies i understand further now but i do have some things to clear up
tiroth you said 6k pp load. The impedance multiplication is 750, so sqrt(750)~=27. how do you get 750 i taught impedance for one of the el34 should be 6k/4 that should be 1500. Your explain is fine elsewhere and informative too thanks SY if i up the B+ to 480 i think should get more than 12 around 1518 watts but i might not have enough current to stay in A rite? hence add more tubes to remain in A? Do you happen to know the formula about calculating the required current into the lowered load as tiroth as mention, tiroth do chirp in if you know how? tiroth what you mean by At some point you will need to add additional tubes to the output stage because you won't be able to supply enough current into this lower load => current limiting your output instead of voltage. how is the load lowered? triode is screen tied to plate that what i taught it way call correct me if i am wrong. 
9th May 2003, 05:27 PM  #7 
diyAudio Member
Join Date: Dec 2001
Location: Pittsburgh, PA, USA

Nick: I don't know what transformer you are using, sorry if I made a wrong assumption from your words. Is it 6k:8 or 6k:4? Does it have multiple taps?
Whatever it is, this (nominal 6k) is the "load". If you change to a 3k pp or a 1.5k pp you are "lowering the load". This means you get more voltage output at the speaker, but you need more current to drive it. To figure how much current is needed simply take Vpp/load. (this is the Vpp on the primary, NOT the loudspeaker) 
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