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Old 6th October 2012, 07:07 PM   #2091
Magz is offline Magz  United States
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Now some Frequency Response Plots.

First is pink noise, at 0dB, -6dB, -12dB and -18dB attenuation at 1/24th octave resolution.

Second shows the impact of the lossy parafeed cap + resistor on the frequency response at -18dB. The plot with the 10dB hump at ~13Hz is using only a 1uF parafeed cap, while the flat plot also has a 3.3uF + 8,080ohm resistor in series added to the parafeed cap. The lossy parafeed (also known as DrP or damped resonance parafeed) really flattens out the hump and extends the FR much lower - my sims say 4.8Hz for the -3dB point.
Attached Images
File Type: jpg FR Plot 26 Preamp Pink Noise.jpg (225.3 KB, 348 views)
File Type: jpg FR Plot 26 Pink Noise 1uF vs 4.3LPF, -18dB.jpg (215.3 KB, 334 views)

Last edited by Magz; 6th October 2012 at 07:21 PM.
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Old 6th October 2012, 07:21 PM   #2092
Magz is offline Magz  United States
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So the B+ power supply is Hammond 374AXP - 5R4GY - LCLC followed by (for each channel) a 10uF cap - cascoded CCS + VR150-VR75 - cascoded CCS - 26 tube.

Filament supply (for each channel) is Hammond 10V 50VA split bobbin transformer - schottky bridge - 6mH, 2A Hammond choke - 33,000uF THSA Cap - Bybee Music Rail - 1000uF cap - Coleman regulator. Set at 1.5V.

The other small transformer at the front of the PS is a 10V 20VA Hammond for the control circuit power supply 9VDC.

Signal path is input - LL1676 transformer wired 2:1 (both sides in parallel) - 0.22uF Duelund CAST cap - 26 tube - 1uF Duelund CAST cap with 3.3uF Mundorf Supreme and 8080ohm TX2575 resistor in parallel - Bent Audio TAP autoformer volume control (~150H) - output.
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Old 6th October 2012, 07:25 PM   #2093
Magz is offline Magz  United States
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Listening tests so far (~4 hours total). Very detailed, very warm, rock solid bass.

I LIKE IT!

My listening is typically in the range of -24 to -6dB on the volume control, with -6db being LOUD. My amp has selectable gain of 20 or 27db - I'm using the 20dB setting.
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Old 6th October 2012, 07:30 PM   #2094
DimZ is offline DimZ  Greece
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Magz


Quote:
Originally Posted by Wavebourn View Post
What happened with 4P1L on epay? Only 2 sellers left...

One from Ukraine (selling matched pairs) and one from Czech Republc...
some still at ebay.de .... (but not for long I think ) quite pricey though ..

4P1L | eBay
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Old 6th October 2012, 07:31 PM   #2095
Magz is offline Magz  United States
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Finally, here are the frequency response plots at -6, -12, -18, -24dB using the TrueRTA Quick Sweep function, instead of pink noise.
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File Type: jpg 26 Frequency Response Quick Sweep.jpg (213.6 KB, 311 views)
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Old 6th October 2012, 08:31 PM   #2096
Magz is offline Magz  United States
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One more thing. If anyone is interested in the PSUD or FPD CAD (front panel designer) files, just PM me.
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Old 6th October 2012, 08:36 PM   #2097
Magz is offline Magz  United States
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Quote:
Originally Posted by Magz View Post
So the B+ power supply is Hammond 374AXP - 5R4GY - LCLC followed by (for each channel) a 10uF cap - cascoded CCS + VR150-VR75 - cascoded CCS - 26 tube.

Filament supply (for each channel) is Hammond 10V 50VA split bobbin transformer - schottky bridge - 6mH, 2A Hammond choke - 33,000uF THSA Cap - Bybee Music Rail - 1000uF cap - Coleman regulator. Set at 1.5V.

The other small transformer at the front of the PS is a 10V 20VA Hammond for the control circuit power supply 9VDC.

Signal path is input - LL1676 transformer wired 2:1 (both sides in parallel) - 0.22uF Duelund CAST cap - 26 tube - 1uF Duelund CAST cap with 3.3uF Mundorf Supreme and 8080ohm TX2575 resistor in parallel - Bent Audio TAP autoformer volume control (~150H) - output.

Oooops. Forgot a stage in the Filament supply. It's

Filament supply (for each channel) is Hammond 10V 50VA split bobbin transformer - schottky bridge - 2.2uF 10W resistor - 680uF cap - 6mH, 2A Hammond choke - 33,000uF THSA Cap - Bybee Music Rail - 1000uF cap - Coleman regulator.

Based on Rod's design, with a few minor deviations.
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Old 6th October 2012, 08:42 PM   #2098
Magz is offline Magz  United States
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More details.

The filament transformers and HT transformer are all snubbed with .022uF caps and 330ohm resistors, the HT from each side of the secondary to center tap and the filaments across the secondary.

Power switch is wired so that the HT cannot be applied unless the Filament is on.
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Old 7th October 2012, 03:34 AM   #2099
tcqanh is offline tcqanh  Viet Nam
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Quote:
Originally Posted by mogliaa View Post
I had a lovely 71a loaded with Mu follower gyrator. Filaments DC using Rod's regulator and was dead quiet
Go for dc filaments!
Ale
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 ?

Last edited by tcqanh; 7th October 2012 at 04:04 AM.
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Old 7th October 2012, 07:38 AM   #2100
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Hi!

Quote:
Originally Posted by tcqanh View Post
DC filament (CLC circuit, C totally 10000 uf, L 200mH)
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
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