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Old 19th September 2012, 07:42 AM   #2031
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Hi Ale,

Thanks mate, I am looking forward to getting to know it. There is still a lot of experimenting to be done, and even more listening. But at least for now I do not have any excuse to get on with actual work!

Cheers

Charlie
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Old 19th September 2012, 09:18 AM   #2032
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Quote:
Originally Posted by CSlee View Post
.. Andy I hold you fully responsible for getting me into DHTs and may never have money again because of it, but at least i'll be happy! Cheers Charlie
Well, my cunning plan is to use nothing but 4P1L tubes, which can be bought for around $2. Input, driver, and paralleled for output. Admittedly you have to heat them up, but just think - no more ridiculous prices for Mullard 12AX7 and the like! You can laugh in the face of tube collectors and those ridiculous ebay prices!! Just buy a box of 100 and never use anything else ever again!

Andy
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Old 20th September 2012, 08:08 PM   #2033
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Jamesdb was asking about putting the filaments of two 26 tubes in series (or maybe parallel?) which got a negative response from me and Rod. We've had this discussion before, and I think the general idea was use individual supplies. Anyone who's had success with combining them is welcome to describe their experiences!

Andy
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Old 21st September 2012, 09:29 AM   #2034
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There is one situation where the series-connected filaments, using my Regulators, might be a good idea, but beware of the prerequisites:

- Push Pull, cathodes connected together at one end, and Grounded.
- Fixed Bias, with separate bias pot for each side of the PP pair. The bias adjustment will be Vf higher on one side (i.e. a difference of 1.5V between them for a 26 DHT pair, or 2.1V for 4P1L)
- balance resistor might be needed across one filament, since the filament current is often spread 10% or more across samples of the same DHT.

The arrangement would make no sense for cathode bias or filament bias, and battery bias would be difficult to balance the PP pair.

I have not tried this yet, but can prepare the circuit drawing, if there's an interest in it.

The big advantage is that one transformer winding will do for the 2 PP filaments. With PP, there's little risk of crosstalk spoiling the sound.
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Old 21st September 2012, 01:46 PM   #2035
airegin is offline airegin  Italy
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Quote:
Originally Posted by andyjevans View Post
Jamesdb was asking about putting the filaments of two 26 tubes in series (or maybe parallel?) which got a negative response from me and Rod. We've had this discussion before, and I think the general idea was use individual supplies. Anyone who's had success with combining them is welcome to describe their experiences!

Andy

Nobody remember the amazing Kevin Kennedy's 26 Preamp ?
kennedy
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Old 28th September 2012, 03:43 AM   #2036
H.Michi is offline H.Michi  Japan
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Hello Rod,

Thank you very much for informative advice.
I now completed the first step of the 26 pre-amp partly modified from my 71A pre-amp. Frankly, I like the sound of 26 pre-amp.
I shall try "Filament bias method" as a next step adopting a new PSU.
Attached Images
File Type: jpg 26 pre-amp-1.jpg (972.1 KB, 618 views)
File Type: jpg 26 Pre-amp.jpg (149.2 KB, 599 views)
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Old 28th September 2012, 05:25 AM   #2037
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Hello Hiro, I am pleased to hear that the change to the 26 is a success!

The output transformer, with Hitachi FineMetal core looks very interesting. How does this trans sound?
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Old 29th September 2012, 11:26 AM   #2038
H.Michi is offline H.Michi  Japan
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Originally Posted by Rod Coleman View Post
Hello Hiro, I am pleased to hear that the change to the 26 is a success!

The output transformer, with Hitachi FineMetal core looks very interesting. How does this trans sound?

When I replaced from Tango NP-8 to Finemet core OPT, FM15K(200H)-150x2 in my 71A pre-amp, my first impression was of expansion of bass range and smoother in high range with less distortion.
The sound of 26 pre-amp just suits my taste.

Specification of FM15K(200H)-150x2:

Winding ratio 1 : 0.22
Inductance 212H (5Hz) 15K
216H (100Hz)
256H (1KHz)
Maximum current 15mA/15K
DCR primary 783 ohm x2
Secondary 43 ohm x 2
Frequency response 8Hz ~ 30KHz (-3dB) ( Drive with rp=15K ohm)
Attached Images
File Type: jpg 26 & 71A.jpg (162.7 KB, 548 views)
File Type: jpg 26 pre-amp-2.jpg (235.8 KB, 463 views)
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Old 29th September 2012, 02:40 PM   #2039
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Yum yum, that's a tasty transformer. Feeling jealous here - was it expensive?
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Old 29th September 2012, 09:47 PM   #2040
Magz is offline Magz  United States
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Well, I finished up my 26 preamp last night and man, does it sound great! I'm running the 26 directly into the AVC in a parafeed arrangement using the mu output of the plate ccs. Square waves look excellent at all volume settings, frequency response extends below 10Hz (the limit of my equipment), and even with only 3 hours running time it sounds clear, punchy and warm with rock-solid bass even at lower volumes. Unfortunately I had fly to Florida for work today, until next weekend, so details, pics, etc. will have to wait.

The AVC is the Bent Audio TAP, with full remote functionality, stacked 3x3 for about 150H inductance. Using LL1676 transformers on input for galvanic isolation, phase inversion and gain cut 2:1.

Stay tuned, more details in a week.
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