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Old 7th October 2012, 09:21 AM   #2101
mogliaa is offline mogliaa  United Kingdom
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Quote:
Originally Posted by tcqanh View Post
DC filament (CLC circuit, C totally 10000 uf, L 200mH) is used for 71A, with good filtered negative supply for fixed bias. I guess hum from my plate choke. I did place 2 HT chokes and 2 filment chokes a little bit near anode choke and B+. next step is isolating all that choke in separate chassis.

Op point of 71A is 130-150V/ 10-11mA. I feel the sound a bit thin.

Could you show me your 71A schema ?
Here you are: 71a DHT pre-amplifier | Bartola Valves
I have learned and experienced a bit since this build so I'd probably improve the circuit used, however it sounded really nice and was dead quiet.

I used filament bias so no fixed bias here. Choke placement can be problematics sometimes as they pick hum easily. Do you have supply and preamp on same chassis? Many of us around here will recommend splitting preamp and supply on separate chassis.
I have my HT and LT supplies on a chassis lying on the floor. It's far off from the preamp

Hope this helps
Cheers,
Ale
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Old 8th October 2012, 01:35 AM   #2102
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by Vinylsavor View Post
Hi!

Try LCL instead on the filaments and be amazed
Requires higher voltage from the secondary of the filament transformer due to choke input.

Best regards

Thomas
I just drop in my friend who is using 300B ampli Audio Kits serie 2. He change LM1085 regulator for CLC filament. Wow, the sound get more bass, richer tone, sparkle... It's worthy
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Old 8th October 2012, 01:49 AM   #2103
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by Joao@AltheaMusica View Post

Advantage of a 2nd stage will be to lower the output impedance, easier way to drive long distance cables. Old German studio gear - Klangfilm etc. - used to have output impedances of 60Ω .

BR, Joao
I've got a pair of OPT from L2 preampli of Audio Note kits like the photo below. This one has ratio 25V:1V . I did check this ratio by means of supplying some voltages on the secondary winding . My friend say it has 200H, but my manual Peak teach 2615 shows only 50H at scale of 120Hz. So I don't know how its impedance is, and how is real Henry. wonder if it is suitble for tube 26.
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Last edited by tcqanh; 8th October 2012 at 01:51 AM.
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Old 8th October 2012, 02:26 AM   #2104
tcqanh is offline tcqanh  Viet Nam
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correct:OPT from preampli L3 Audio Note kits

Last edited by tcqanh; 8th October 2012 at 02:28 AM.
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Old 8th October 2012, 08:05 AM   #2105
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Hi tcqanh,

If your plate choke or OPT is 50H you can always consider the 4P1L instead of the 26. Or 71A, though that will give you a lot less gain and I also think maybe not so good sound. You might like to try different solutions that work for the transformers you have available and then compare the results. The 26 really needs 150H and upwards to get a clean sound, or it will start getting a bit mushy.

Also, Thomas is right about LC rather than CL. With something like what I've been using, a 280mH choke input (or a small cap like 220uF), you get a nice clean supply with low ripple, and you can further refine that.

Last edited by andyjevans; 8th October 2012 at 08:12 AM.
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Old 8th October 2012, 08:42 AM   #2106
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Quote:
Originally Posted by andyjevans View Post
Hi tcqanh,

If your plate choke or OPT is 50H you can always consider the 4P1L instead of the 26. Or 71A, though that will give you a lot less gain and I also think maybe not so good sound. You might like to try different solutions that work for the transformers you have available and then compare the results. The 26 really needs 150H and upwards to get a clean sound, or it will start getting a bit mushy.
Andy is correct, the AN uses a 5678 and it has a plate resistance of 1.5 ~ 3k (depends on operanting point). If you will use that output transformer you should use one of the tubes he recommends - or you use IDHT 5687, but we discuss here DHT line stages.
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Old 8th October 2012, 09:55 AM   #2107
bravi is offline bravi  New Zealand
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Hi Magz,

Any advantage with using Music rails on the filament supply before Rod's regulator? Any difference in sound without it?

Cheers!

Ravi
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Old 8th October 2012, 01:50 PM   #2108
Magz is offline Magz  United States
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Quote:
Originally Posted by bravi View Post
Hi Magz,

Any advantage with using Music rails on the filament supply before Rod's regulator? Any difference in sound without it?

Cheers!

Ravi
I did not check with/without. I've used them before in an EQ and another preamp and they provided measurable noise reduction, so I just put them in this time without measurement. There is a thread on the "everything else" forum here with some results and measurements.

Bybee stuff is controversial but the music rails at least have been shown to do something measurable.
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Old 8th October 2012, 01:55 PM   #2109
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@Rod or Andy

How much voltage draws the AZ1 or in other words: how many VAC have to be the mains tx?
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Old 8th October 2012, 02:15 PM   #2110
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Hi Felipe,

With a genuine choke-input filter, the rms current in the secondary will only be a little higher than the dc current.

But it is best to have a good margin, so that the transformer runs cool, and so that you can experiment with higher anode current, or different DHT, etc.

E.g. For a Line amp with 100V on the anode, and (say) 35mA dc draw maximum in total (anode current, plus an allowance for divider or bleeder, or gas-discharge regulator tube) -

choose Choke-input filter, 160V rms trafo to get about 135V with the attached circuit.
The rms current in the secondary is about 37mA. Total Trafo VA is about 6VA. It would be better to choose something like 50VA to keep the regulation reasonable (ie loaded voltage versus unloaded voltage).

choose a transformer with an electrostatic shield, or a "split-bobbin" (dual-bobbin) EI construction. Cheap Toroidal not recommended, they will let the noise in to your amp.
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