Hi
I have a pair of humongous (5"x5"x4"), never-used Douglas PP output transformers, the data sheet of which says they are intended for 4xEL34 or 4xKT88 with fixed bias in a 100W amplifier, (but no U-L tap).
Also given is the primary side anode load of 2 x 1.75k, and max primary current of 2 x 200mA.
The secondary taps are for 3.75R, 8R and 16R.
What are the likely problems if I use them with only a PAIR of EL34/KT88, and regard the 3.75R speaker tap as good for an 8R speaker?
Right now I can't afford eight KT88s or EL34s. It's a waste of such grand transformers, but if it's fudgable, I can add the extra 4 tubes at a later date. The mains trafo I currently have also can't do 2x100W on the HT, but could do 2x40W or so.
I'd also go for cathode bias rather than fixed, I think.
I have a pair of humongous (5"x5"x4"), never-used Douglas PP output transformers, the data sheet of which says they are intended for 4xEL34 or 4xKT88 with fixed bias in a 100W amplifier, (but no U-L tap).
Also given is the primary side anode load of 2 x 1.75k, and max primary current of 2 x 200mA.
The secondary taps are for 3.75R, 8R and 16R.
What are the likely problems if I use them with only a PAIR of EL34/KT88, and regard the 3.75R speaker tap as good for an 8R speaker?
Right now I can't afford eight KT88s or EL34s. It's a waste of such grand transformers, but if it's fudgable, I can add the extra 4 tubes at a later date. The mains trafo I currently have also can't do 2x100W on the HT, but could do 2x40W or so.
I'd also go for cathode bias rather than fixed, I think.
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Self biasing KT88s is (IMO) a challenge, as the I2R heating losses in the cathode resistors is quite substantial.
The thread about Mullard circuit KT88 monoblocks contains many hints relevant to your situation. Obviously, your "iron" rules UL mode O/P tubes out, but full pentode mode employing 300 V. regulated g2 B+ should do nicely. I strongly recommend the 6GK5 voltage amplifier, as your project will have a fair amount of GNFB and the issue of slew limiting becomes even more pressing.
The thread about Mullard circuit KT88 monoblocks contains many hints relevant to your situation. Obviously, your "iron" rules UL mode O/P tubes out, but full pentode mode employing 300 V. regulated g2 B+ should do nicely. I strongly recommend the 6GK5 voltage amplifier, as your project will have a fair amount of GNFB and the issue of slew limiting becomes even more pressing.
EL34s are a lot easier to come by here, so they might be a more likely choice.
I have other UL transformers earmarked for a quad of EL34s, so I thought I should try something a bit different, yet not excessively expensive.
(Tube TVs were never made or sold here, so we don't have a great stockpile of cheap tubes as you can find in other countries)
Since you didn't mention any obvious problem, does it mean you think it's OK to magnify the low anode-to-anode impedance by using a different speaker tap?
One thing I want to experiment with is an artificial UL tap, created by means of a MOSFET source-follower, resistively divided off the primary. But that's a matter for a different thread and some time away.
Would that be a fairly ordinary EL84 PP output, but buffered with source-followers to give extreme current drive?
Hw does such a configuration sound and perform? Certainly its an attractively economical option.
Other than potentially insufficient primary inductance, you should be fine. You have magnetic headroom for a NFB induced LF error correction signal, with just a single pair of EL34s as "finals". IMO, you have excellent chances of success.
Look at the TDSL EL34 info. page. The 475 V. anode, 400 V. g2, and -36 V. g1 conditions set seems promising. Reducing the g1 bias to -30 V. could work out nicely. Don't forget the shared 750 Ω g2 current limiter.
quoted from OP:
Also given is the primary side anode load of 2 x 1.75k, and max primary current of 2 x 200mA.
The secondary taps are for 3.75R, 8R and 16R.
So this would be 3k5 a-a? Very near 4k suitable for EL34 in single pair and just a bit higher than amps like W6m and Citation II for single pair KT88( delivering 2k8 to 3k2 a-a ). I don't see a big issue with the single pair use here.
cheers,
Douglas
Also given is the primary side anode load of 2 x 1.75k, and max primary current of 2 x 200mA.
The secondary taps are for 3.75R, 8R and 16R.
So this would be 3k5 a-a? Very near 4k suitable for EL34 in single pair and just a bit higher than amps like W6m and Citation II for single pair KT88( delivering 2k8 to 3k2 a-a ). I don't see a big issue with the single pair use here.
cheers,
Douglas
If it really is 2 x 1.75K then since the impedance transformation scales as the square of the turns ratio then I suspect it would be 7K A-A which doesn't really make sense in a transformer designed for PPP operation. If 3.5K a-a then it would actually be slightly less than 900 ohms per primary winding which does make sense with a pair of EL34 or KT88 in parallel on each winding.
I would seriously scrape the rand together for a quartet of EL34 for each channel and probably triode connect the outputs - you could get nearly 60W if you were aggressive about it. Another option would be cathode feedback providing the secondaries are symmetrical (i.e. really 0, 4, 8, 16 ohms) you can verify by applying some reasonable ac voltage across the entire primary and measuring the voltage between the 0 tap and the 3.75 ohm tap, and the 3.75 ohm tap and the 16 ohm tap - if they are equal you can try cathode feedback otherwise you can't. (Well you can but it gets more complex.)
1,75k X 2 is what it says.
PPP is somehing different through, is it not? Using a non-tapped primary. Was that a typo?
They are indeed symmetrical. I think they are actually marked 3.75 - 7.5 - 15.
I don't have a big mains transforner, so a stereo pair of EL34 pp would match my mains transformer OK.
Otherwise it's going to be 8xEL34 and a new transformer, which is going to get expensive.
Probably best to apply a signal to the primary and measure the output to be sure of the turns and impedance ratio.
I would agree with Douglas that this is suitable for a pair of bigger output tubes, if it is indeed a 3.5K a-a load. One is supposed to be able to get ~75 watts out of a pair of 6550 with a 3.5K load.
http://frank.pocnet.net/sheets/135/6/6550A.pdf
Thanks for the ideas. My internet has been down for a few days so no chance to reply sooner, but I had time to dream....
I think I'm going to hold onto these transformers for a while until I can afford 8 big tubes.
For now I have another pair of 'ultra linear' OPTs that look to be good for about 100W, which I think I will do OK with 4 EL84/6BQ5s per channel, although te amplifier it came from probably used 6550s (a Steinhart).
The EL84 is less than half the price of an EL34, and 4 EL84s would suit my present power transformer nicely
I think I'm going to hold onto these transformers for a while until I can afford 8 big tubes.
For now I have another pair of 'ultra linear' OPTs that look to be good for about 100W, which I think I will do OK with 4 EL84/6BQ5s per channel, although te amplifier it came from probably used 6550s (a Steinhart).
The EL84 is less than half the price of an EL34, and 4 EL84s would suit my present power transformer nicely
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