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Old 19th July 2003, 01:52 PM   #1
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Default Plytron/Amplimo 4070-CFB OTF's

Hi,

For a new amp project I am planning to use Plytron/Aplimo 4070-CFB output transformers driven by a pair of KT88’s. These transformers are quite expensive compared with UL transformers without cathode feedback windings. So I am wondering if these transformers are worth the extra expense.

Does have someone used these transformers in a DIY project? Curious about the results and if these transformers with the use of cathode feedback really are an improvement concerning the sound.

Cheers
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Old 19th July 2003, 06:36 PM   #2
Gunders is offline Gunders  Norway
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According to the datasheets those OPTs from Plitron should be fantastic, also I have considered to use those transformers myself, but unfortunately I found them too expensive.
But I have found a cheaper OPT which will be nice foran a EL84 amp, PAT 4000.

I would also like to hear if there are any here who have experience about the output transformers from Plitron.
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Old 20th July 2003, 02:58 AM   #3
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Hey-Hey!!!,
I have no experience with the Plitron output transformers at all. However, for the cathode feedback arangement, I have ( and am now listening to ) amps built around the ancient Dynaco A441. 15% tertiary winding and no taps in the main anode winding. The feedback level is enough to run pentodes without any other NFB to absolutely fantastic sound. I am building some more OPT with this winding config, and a seperate similar winding for offering additional screen feedback( Ultra-Linear style) and seperate voltage supply .

The only trouble you may have with the torroid core is lack of DC imbalance tolerance, causing saturation at level that make the less permeable cores of E-I constructed tx's yawn and ask if they should be excited. Perfect idle current and tube characteristic match will be very important I suspect.
regards,
Douglas
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Old 20th July 2003, 05:34 AM   #4
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I listen to an amp using transformers with a 10% tertiary winding made as replacements for a Mcintosh MA230. The sound quality is excellent and the output stage can be driven with "normal" circuitry.

Dynaco A441

I would like to get a pair of these. Any suggestions on a source? MagneQuest?
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Old 20th July 2003, 12:49 PM   #5
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Hey-Hey!!!,
Mike at Magnaquest is not planning a run of that TX anytime soon. Anticipated pricing on that TX is about $600 too, which is fairly reasonable. I have been pestering Mike for more than a year to wind some of them. Somebody quoted me a minimum run of 30 pairs and that is more than I can buy at once( by about 29 pairs ). So I am making my own, with improvements too. This source is a bit more reasonable and the primary z is lower impedance for a different output valve. BUT, it is available and without too much delay and has more Iron in it too.
regards,
Douglas
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Old 20th July 2003, 02:03 PM   #6
Gunders is offline Gunders  Norway
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Quote:
Originally posted by Bandersnatch


The only trouble you may have with the torroid core is lack of DC imbalance tolerance
I have heard it before, but I'm just wondering....
Plitron does also make some toroid chokes for use in tube power supplies, how have they solved the problem with DC current in these chokes?
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Old 20th July 2003, 03:15 PM   #7
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Quote:
Originally posted by Gunderz


I have heard it before, but I'm just wondering....
Plitron does also make some toroid chokes for use in tube power supplies, how have they solved the problem with DC current in these chokes?
Don't they also do SE OPT's too? Wouldn't they simply cut the core and gap it?
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Old 20th July 2003, 03:43 PM   #8
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Hi,

Thanks for input so far. Looks promising. Yes sensitivity for imbalance is also something I am worrying about. But that holds for any PP transformer isn’t it? I am working on a servo for the grid bias to solve this and to keep idle currents balanced automatically. Toriods have a very tight closed magnetic circuit, which has its own disadvantages. But on the other hand it is the configuration with the lowest leakage inductance and hence widest bandwidth. I do not know what steel they are using. Normally some “leakage” also relaxes the influence of the large non-linearity of the permeability of the steel.

Have found a specialised transformer maker here in the Netherlands, Automatic Electric Europe. They use special ultra thin laminated C-cores and amorphous cores for audio transformers also. They are on holiday till July 28. I will contact them when they are back. see what they can do. But it seems Menno v.d. Veen has patented the CCF topology combined with UL, called "Super Triode". Don’t know if this only holds for toroidal transformers.

dshortt9,

Sowter in Britain makes replacement transformers for the Quad II with a tertiary cathode winding. Maybe this is of help.

With 10% tertiary cathode winding I need roughly 70V_pp to 80V_pp extra driving voltage. You are using a “normal” driving stage you told. What driving tube are you using for it? And at what HV?

Cheers.
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Old 20th July 2003, 03:56 PM   #9
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Quote:
Originally posted by Pjotr
But it seems Menno v.d. Veen has patented the CCF topology combined with UL, called "Super Triode". Don’t know if this only holds for toroidal transformers.
Patents only hold for production runs, and only if they can be enforced. If you spec the Tx and take that to a winder, there's nothing stopping them making one for you.
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Old 20th July 2003, 04:04 PM   #10
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Thanks for the info on the Quad II but I'm looking to build a higher power amp.
My amp is built around a vintage Eico HF-89 chassis bought without the output transformers on e-bay. Here is the schematic:
http://sanacacio.net/hifi/hf89schematic.jpg
I used 6L6GC's instead of the EL34's that were originally used as they matched my CFB transformers better. I use a 1.8k feedback resistor instead of the network shown on this schematic. With 475V B+ and fixed 450V on the screen grids (no UL) I get about 55 wpc. The 6SN7 works well for this but I would use the newer JJ ECC99 in a future design. I love the sound of this amp on my ProAc 2.5 clones.

If anyone is interested in a group purchase of Dynaco A441's please post here.
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