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Old 11th March 2008, 06:16 AM   #1
Switch is offline Switch  United States
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Thumbs up what to do with a 5K output transfomer

Hi guys. Boring times round my house lately, but i have a new pair of Hammond 1615 output transformers and no ideas.

They are PP, 5K load, 15W. primary current is 100mA per side.

I'm looking for ideas as far as output stages, I can rig whatever i can find into a preamp and phase splitter stage.

so far, i've got some EL86 on hand, but they'll give much more than 15W...

Thanks for your suggestions!
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Old 11th March 2008, 06:23 AM   #2
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You could use EL84's or 6V6's in ultra-linear and put 8-ohm speakers on the 4-ohm taps? 10K a-a should work out OK. There are more interesting options that use less common tubes but I don't have any off the top of my head.

If you want to go "boutique" you could also use 2A3/6A3/6B4G's in push-pull. 5K is the textbook impedance for these tubes and 15W is a reasonable OPT size.

Another option: use the transformers in single-ended parafeed (just one 5K primary). A bigger tube like a 6L6 or EL34 in triode might be cool. I'm not an expert on parafeed though, maybe I'm wrong?
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Old 11th March 2008, 09:43 AM   #3
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Look for the cheapest 300b, 2a3 or 6B4G you can lay your hands on - Russian, Chinese, anything. You can replace with better at a later point. But even cheap DHTs like these will out-perform conventional indirectly heated tubes.

Probably the easiest to implement is the 2a3 since it's happy with 2.5v AC. The 300b is reasonably happy with 5v AC (with a pot or some sort of centre tap), and the 6B4G is the only one that may possibly need DC filaments.

Depends on your speakers sensitivity - high sensitivity, possibility of hum so better with 2a3. Low sensitivity, may need 300b for power and hum less critical.

I've used that Hammond as above and it's a very nice sounding OPT.
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Old 11th March 2008, 11:59 AM   #4
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The suggestions above are very good. If you already have plenty of 6CW5/EL86s and haven't any other plans for them, that's what I would use. True, they are capable of nearly twice the output power your transformers are rated for, but you can select operating points that will be easier on the valves and better suit your present transformers. If you keep B+ below 180 volts you will be using only a percentage of their capabilities and not straining the valves at all. They should last a very long time.
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Old 11th March 2008, 12:40 PM   #5
ilimzn is offline ilimzn  Croatia
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EL86/PL84 is a good suggestion, datasheet gives a plate voltage of 250V, screen voltage of 200V inro 5.5k PP, for 18W at 4% distortion. This is a bit 'rich' for your transformers but a lower operating voltage can reduce this, and do good for tube life.

Another possibility would be using something like an EL/PL36 or E:/PL504 triode coupled, but with a 4 ohm load connected to the 8 ohm tap. In general cross-connecting taps this way is best done so that the primary impedance halves, not doubles, as the former gives a higher primary inductance, something which is very nice for better bass. Something like this could give you about 10W output with very decent distortion figures and no NFB, at moderate B+.
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Old 11th March 2008, 02:52 PM   #6
Fuling is offline Fuling  Sweden
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5k would be a good plate-to-plate load for a pair of triode wired EL/PL36 in class A without fiddling with the secondary impedances. I measured 7,56W from one tube in SE triode with 2,5k plate load and 300V B+ (cathode bias), so a PP class A circuit should give 15W.
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Old 11th March 2008, 05:22 PM   #7
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Some other possibilities - 6Y6, 6W6, triode wired 6AU5 or 6AV5, even 6EM7 push-pull... all common as dirt and almost as cheap!
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Old 11th March 2008, 05:28 PM   #8
ilimzn is offline ilimzn  Croatia
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Sorry, I was unclear about that - strapping a 4 ohm load onto the 8 ohm tap should be done for EL/PL504, not for the EL/PL36.
2k5 SE seems a bit low for the EL/PL36 (what was the idle plate current?), but in class A PP it should be fine, and the extra power is welcome.
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Old 11th March 2008, 05:46 PM   #9
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Me, I wouldn't go that far afield. EL34s in triode, a little low and hot to match a 5K load; say, 380v and 60mA.  Bias is down to 25v or so, and you can just make your 15w rating wide open.  That's some yummy sound.

Aloha,

Poinz
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Old 11th March 2008, 09:42 PM   #10
Jeb-D. is offline Jeb-D.  United States
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I'm currently building a 6as7g amp using those transformers. Since it has 2 triodes within, It makes a single output tube push-pull.
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