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What to do with a 5k output transfomer?

Switch

Member
2006-11-23 8:14 pm
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!
 

sorenj07

Member
2006-05-04 9:15 pm
Berlin
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?
 
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.
 

tubewade

Member
2007-06-03 8:54 pm
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.
 
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+.
 
strapping a 4 ohm load onto the 8 ohm tap should be done for EL/PL504, not for the EL/PL36

I see. 5k would work for `504 too if they run on high voltage and low current BTW.

I agree that 2,5k is a bit low for PL36 (SE), I should of course have mentioned that I ran the tube as a power cathode follower during the test, but max output power should be the same in a plate loaded circuit.
 
Fuling said:

I see. 5k would work for `504 too if they run on high voltage and low current BTW.

Yes, probably, although no-one has, to my knowledge, done extended range triode plots for the 504 (pity!), so the data I have is based on the triode curves in the datasheet. I know for a fact PL36 and PL500 can take lots more than the datasheet would suggest, but also that the 509/519 has almost no reserve. The PL504 is somewhere inbetween as tube generations go, but tons have been used and based on how many good ones I got from a largeish lot of used ones in comparison to used 509/519s, I would bet they can takle a lot more voltage than the datasheet suggests.
 
A friend uses russian equivalents (or at least near equivalents) to EL504 in a triode wired PP class A amp. I don´t know if he drew his own triode curves before he designed the amp, but he runs them "high and cool" with a high p-p load and it sounds great.

Triode curves for 509/519:
http://www.tubes.mynetcologne.de/roehren/daten/el509bpt_as_triode.pdf

I have a paper somewhere with PL519 triode curves measured all the way up to 850V, so I guess they can take a lot mor than what they´re speced for.
 

tubewade

Member
2007-06-03 8:54 pm
I know this thread has been exhausted somewhat already, but I've been thinking about this some more. Much of your choice depends on how much you want to spend as the possibilities are endless. I tend to want to use what I have on hand or what will do what I want economically so suggestions along those lines would be 6BG6Gs in triode mode, or parallel 6AQ5/6005s. Both very inexpensive, yet fine sound.
 

Switch

Member
2006-11-23 8:14 pm
I decided on 6CU6/6BQ6 in triode mode. (Heheheheh....cheap tubes, free OPTs, sockets on hand, only need a PT and I'm ready to go...) Does anyone here have curves traced out? (I did my own, but ended up identical to triode curves for 6DQ6A, and don't trust them so much...want to make sure that either BQ or CU will work...)

The Gist:

6CU6/6BQ6/6DQ6 PP

-280V, 56Ma, 5K load (15.5W dissipation)
-fixed bias
-12AT7 LTP phase splitter
-CCS loaded tail

Preamp???

I don't know what kind of gain I'll need to drive these tubes yet. That math is for tomorrow. Any suggestions?
 

tubewade

Member
2007-06-03 8:54 pm
Great choice. I have interest in those valves and the possibility of using them in an amplifier the future, so I'll be very interested in keeping up with your project.

I have the tetrode curves for the 6BQ6, but no triode curves. Hopefully someone else having experience with them has plotted their curves and will share. Would the tetrode curves be of any value at all?
 

tubesrock

Member
2008-01-04 9:12 pm
Hammond output transformers are rated by wattage at full bandwidth (20-20khz). I've seen and heard circuits built for guitar amps using Hammond iron rated at 60W running 4 6L6GC's for 100W. The catch is the bandwidth is cut to frequencies unsuitable for Hi fi but fine for guitar (150-12Khz). Within the MAX DC current rating and target program content much more is possible with Hi Fi Hammond iron. Hope this helps.
 

Shoog

Member
2002-08-15 10:16 pm
Eire
I have had the idea of building a little amp using the PL84/EL86 in a self splitting arrangement with differential output stage. Since you have all the components to do this why not build one and tell me how it sounds :D Various discussions and builds of this topology suggest you get the sound of SE with the punch and control of PP.

I have read that the EL86 is an excellent sound valve - easily a match for the EL84.

Shoog