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PostPosted: 20 Jan 2021, 17:33 
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Joined: 05 Feb 2018, 11:49
Posts: 18
Perhaps you anticipated this being an EE, but thought I'd mention in case it's helptul.

The B+ voltage will be lower using a tube rectifier, but should still be in a good operating range for 6DJ8 tubes. If you want to increase the B+ voltage, lower the value of one or both resistors R1 until it's where you want it (maybe try 330 Ω). You can compensate for the decrease in ripple filtering by increasing the size of each capacitor C2 that follows.

Even better, use a lower resistance choke in place of one or both resistors R1. I did so in order to increase the B+ over 200V so I could use 6CG7s (typo on the choke value there; they are 0.45H or about one half Henry each): viewtopic.php?p=54046#p54046

Good luck, it's a fun project and great amp!


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PostPosted: 20 Jan 2021, 20:59 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Tube rectifiers are fine. The biggest issue with 6DJ8s is getting too much voltage. I have not tried it, but I suspect that the amp will work quite well with anode voltages down to about 75 volts. There would be a slight reduction in output power....but it will be way more than most phones need.

Good listening
Bruce

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PostPosted: 17 Apr 2021, 16:24 
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Hi I have a set of monitor headphones and now have an excuse to build a tube circuit.
But I am from UK and I can not source output transformers? On mouser and farnel they showing me signal transformers whose low frequency is shown as +-2db at 300hz can this be okay?

I have attached the pdf file with specs


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PostPosted: 18 Apr 2021, 14:27 
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Joined: 17 Apr 2021, 15:52
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Hi hope someone can help

Spent two days searching for audio transformers that would work for this headphone tube amp build.
Could someone please have a quick look at spec below and let me know if these will work?
I think they will work fine as multiple taps to try out. primary and secondary
has four taps each. Although the secondary has the larger number windings with primary the smaller number of windings. But I am assuming I can use in reverse.

Purchased from CPC manufacturer is called Carnhill.
Product number is A262A3E

SPEC SHOWN AS BELOW

Winding ratio: 1+1:6.45+6.45 Impedance:

Primary 150‡ parallel connected 600‡ C.T. series connected
Secondary: 6.25k‡ parallel 25k‡ C.T. series connected
DC resistance (+/- 15%):
Primaries total: 20.6‡
Secondaries total: 1143‡ Inductance, measured at 1kHz, 0.27V:
Primaries 125mH min. per winding
Secondaries: 5.2H min. per winding Proof voltage: primaries to secondaries: 1kV DC
Frequency range: 30Hz - 25kHz +/- 1.5dB. Power: 100mW @ 300Hz and 1mW @ 30Hz Distortion: less than 1% T.H.D. (ref.600 ohms)
30Hz - 25kHz measured at 0dBm Operating temperature range: 0 to +70oC
Storage temperature range: -25oC to + 120oC
N.B. Do not pass DC through windings


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PostPosted: 18 Apr 2021, 20:44 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
That is a microphone input transformer. It would likely not work particularly well as the output in Bruce's design.

You might try experimenting with their N35A002F transformer.

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PostPosted: 18 Apr 2021, 20:53 
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Location: Arizona, USA
Hi, I don't know where you are so I can't tell if there are any local types. But the ones you picked are not suitable. You specify no dc current. The ones needed do have to handle some. Even though you might not want to order from EdcorUSA look at the specs for the transformers I used. With that information look for others. Many will work well. They fall in a catagory of interstage matching transformers in the 2.5 to 5 watt range. The 5 watt size has a little better bass response. The EDCOR part number used was XSM10K-150. It is a 10K center tapped primary (goes to the tube anodes) with a 150 ohm center tapped output. They also come in a 300 CT and 600CT output when I last checked. Select the value that will match you headphones.

https://www.edcorusa.com/xsm10k150

Good listening
Bruce

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PostPosted: 19 Apr 2021, 07:30 
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Thanks for your help. I am based in UK. The only suitable one I could find for my headphones which are beyerdynamic 400 ohms. Is from a small company called sowter.co.uk Here is following spec below. Sorry for horrible format of this hope it makes sense.

But the big problem is, it is retailing at £74 which is way beyond what I was expecting. I have gone and purchased a coil winder as this is so interesting I may try and learn a bit more by winding one that fits the bill (or first some simple voltage transformer type ones). I am reading book Coil Design by Babani books no 160. Which is fascinating. But from reading this I am beginning to get the impression all parts I will need and tools etc to make this, will probably cost in same ball park. Any suggestions or any ideas really welcome. I have a desire to build this as the headphones are going to be used as monitor headphones to transcribe bass lines of songs etc. Plus valves circuits I find more intuitive and more interesting than transistor circuits. So I do have the motivation to build this. But be nice if I could just find 2 transformers of the right type and reasonable price. I have already sourced the valves at a good price. Thanks again for the advice really helps a lot.

9041 PP triode or AC coupled c/follower to line. 5CT:1

FEATURES Combination Mumetal and Grain Oriented (M6) laminated core for minimal distortion and a good output level. The Mumetal core ensures the LF response is maintained even at very low levels where the ear less sensitive. Multiple interleaved primary and secondary windings to give an unusually wide bandwidth. Additional centre tapped winding for negative feedback.
TYPE No 9041 9042
Voltage ratio 5CT:1 3CT:1
Bandwidth (-3 dB points, typical) 10 Hz to 120 kHz 10 Hz to 120 kHz
DC Resistance (Primary) 99 Ohms 72 Ohms
DC Resistance (Secondary) 31 Ohms 31 Ohms
DC Resistance (Feedback) 9 Ohms 5 Ohms
Total DC resistance ref primary 950 ohms 350 ohms
Max dc current per side of primary 30 mA max 30 mA max
Max unbalanced dc current 0.5 mA 0.5 mA
Ratio of negative feedback winding (% of secondary voltage) 20% 20%
Max output level at 50 Hz +23 dBu (11 Volts) +23 dBu (11 Volts)


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PostPosted: 19 Apr 2021, 08:09 
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Thanks Matt, that is price I can afford. The ideal nominal turns ratio for my 400 ohm headphones is 5:1 for tube output impedance of 10k.
Have I understood that bit correct? If so, then my goal I think is to get as close to that ratio as I can. With something that can handle the primary DC current

From my calcs the DC input/output power circuit can use up to p=vi where i=135volts/10kohms = 13.5mA so power being 135v*13.5mA = approx 2Watts.
Is this correct? So their spec of one you suggested (they showing 3.5 watts) is good yes??

So would this transformer work for a ratio close to 5:1. I am not quite sure on the specs etc. Don't fully understand much of this yet.

Maybe a turns ratio of 7.35 would work for my phones? eg if I just ignore all on primary except for taps 3 and 4 (1000 turns) and on secondary join all taps into one long winding (136 turns). Am I thinking along right lines here? which has nominal output of about 185ohms could my 400ohm phones benefit from that?

This is all new to me so I have no idea if the calculations above are correct.


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PostPosted: 19 Apr 2021, 08:23 
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Joined: 17 Apr 2021, 15:52
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Is it possible to add fixed value inductors to the top of last taps on secondary windings to make up the extra turns ratio to give me enough for my 5:1 optimal turns ratio? eg to simulate an extra 64 turns on output of using taps as a 1000 to 136 turns transformer. 136+64=200 1000/200 = 5:1

Or maybe? Instead of adding a fixed inductor how easy would it be to wrap another 64 tuns around the transformer then attach those to secondary to bring the ratio up to 5:1

I think I can answer my own questions as I've just realized I would have to dismantle the transformer to put on any extra windings. So not an easy or practical solution.


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PostPosted: 23 Apr 2021, 14:11 
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Joined: 01 Apr 2020, 16:49
Posts: 11
Hi Bruce,

A year ago I built a Forewatt and am still very happy with it. Now I’m planning to give my good old Beyer Dynamics DT880 a new partner (from around 1980, 600 ohm, still going strong).

I’m planning to use the same case as the Forewatt and the same power transformer (Toroidy), again placing the nice transformer and the tubes on top.

From the Forewatt project I still have parts and most importantly several power supply pcbs laying around.
I figure, if I drop the B+ voltage to around 140V by changing or adding resistors I’m fine?
Now, I could branch out from the pcb when I’ve reached the B+, but I can see no harm in using the LR8N voltage regulation part as well? Obviously I need a bit more leeway in the voltage for regulation then.
I will break the heater voltage lift, too.
And I can see heater and B+ share a common ground?

Happy to plan for a new project.

Regards


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