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300B SET amp measurements without tubes installed
300B SET amp measurements without tubes installed
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Old 5th December 2017, 06:56 PM   #21
MEPER is offline MEPER  Denmark
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Thank you for the answer.
Something to look at. I will look at the 7805 data sheet and check the capacitor values. Maybe consult tentlab for a couple of DC filament supplies for 300B.
The choke I was looking at was this one:
Welter Audio Electronic Seit 1975
It is rated as 200 mA.
Each 300B draws approx 60 mA bias current.......but your calculation is 120 mA for each tube? .....do they draw double when full loaded?

DR3 choke......

The values of the two caps after the rectifier for the 7805 are 4700uF/10V and the cap connected to the output of the 7805 is 330uF/40V.

I was good I tried the "Duncan PSUD2" simulator.
I does not work at all just inserting a 10H/130 ohm choke.
The voltage drop is way to high. I must find another solution. First I will measure the ripple using a scope to see how bad the ripple is.
Hope my main problem is the AC on the 5V DC......and maybe the wires.
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Old 5th December 2017, 08:21 PM   #22
Rod Coleman is offline Rod Coleman  United Kingdom
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300B SET amp measurements without tubes installed
Try putting the choke in the place of the 220 ohm resistor, for modelling.

200mA choke is ok to try, if running 300B at 60mA idle.
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Old 5th December 2017, 09:45 PM   #23
MEPER is offline MEPER  Denmark
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It works better by changing the RC to a LC. 200 mV ripple goes down to 10-15 mV......approx.
The 8V ripple measured using a multimeter was wrong. I could see that using a scope. But the LabNation scope showed some stranges values probably caused by the high voltages.
It worked better when I measued the 5V DC. It looked worse than expected.
I have attached a picture of the scope screen. It is 50 Hz noise....1V pp.
How can it be 50 Hz?
It fits fine with what I hear in the speaker......it sounds like 50 Hz.
If it was the 7805 regulator ripple it would be 100 Hz?
Think the 5V DC is my main problem and I should concentrate solving this first.
Attached Images
File Type: jpg DSC_3677.jpg (703.4 KB, 122 views)
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Old 6th December 2017, 07:50 AM   #24
Rod Coleman is offline Rod Coleman  United Kingdom
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300B SET amp measurements without tubes installed
Looks like a problem. What does the supply-input to the 7805 look like? Measure on DC mode, if possible, as well as ac-coupling, and we can look at both. I suspect it will look bad.
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Old 6th December 2017, 05:25 PM   #25
MEPER is offline MEPER  Denmark
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I think the measurements from yesterday was not right. The probe had a bad connection so it picked up hum from the mains. I saw this also tonight.
I have made new measuremens which are more correct.
Measurements are:
Input to 7805 in both DC and AC mode for left channel.
To my eye it looks ok.
300B filament in both DC and AC mode for left and right channel.
In AC mode some noise is visible. Should it be less?
Attached Images
File Type: jpg DSC_3678_DC_7805_in_left.jpg (267.7 KB, 107 views)
File Type: jpg DSC_3679_AC_7805_in_left.jpg (301.8 KB, 106 views)
File Type: jpg DSC_3682_DC_300B_filament_right.jpg (269.1 KB, 103 views)
File Type: jpg DSC_3683_AC_300B_filament_right.jpg (232.0 KB, 105 views)
File Type: jpg DSC_3684_DC_300B_filament_left.jpg (287.2 KB, 12 views)
File Type: jpg DSC_3685_AC_300B_filament_left.jpg (978.1 KB, 18 views)
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Old 6th December 2017, 05:37 PM   #26
MelB is offline MelB  Canada
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MEPER If you have your 400V transformer A/C lines bundled with your 5V DC lines that run to your 300B's my first move would be to unbundle those and move them as far apart as I could.
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Old 6th December 2017, 05:48 PM   #27
MEPER is offline MEPER  Denmark
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It could be a good project and it can't be worse by doing that.
Maybe I should also disconnect the NFB wire for both channel until the problems are "solved".
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Old 6th December 2017, 05:57 PM   #28
MelB is offline MelB  Canada
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You don't need to unsolder anything. Just cut the ties and separate them and and measure the filaments again. See if it starts to look better if they are separated.
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Old 6th December 2017, 06:00 PM   #29
Rod Coleman is offline Rod Coleman  United Kingdom
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300B SET amp measurements without tubes installed
The output noise is reasonable, given that you have 1V peak-peak ripple voltage on the input.

Unbundling is worth trying now, yes. No need to disconnect the NFB now, IMHO.
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Old 6th December 2017, 06:02 PM   #30
MEPER is offline MEPER  Denmark
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OK.....just as a first "try" to check if it has any impact.

There is a delayed B+ "turn on" (45 sec).
How much is that needed?
If I could remove the 555 timer board then the 400V AC wires could be kept better aside and wires could be shorter.
Does it mean a lot for the lifetime of the tubes that you turn on heater first and then after 45 sec. turn on the B+?

OK.......next step will be to unbundle the wires. Now I also have documentation so a change can be verified.

Some results after the bundle has been broken and 400AC wire pulled away from the others.
It seems the noise signal is a bit cleaner as the scope now can measure 100 Hz.
All measurements are with probe on 300B filament.
There is a measurement with no power cord plugged in.
Then there is a measurement with only power cord plugged in but no power on trafo. It seems there is a bit of noise before the power trafo is turned on.
Chassis is connected to safety ground and cord has safety ground but no saftety ground in the installation (normal here not to have safety ground in the living rooms). So safety ground is "floating" in the power cord). Can that cause hum?
I measure with a headphone box connected as this has 2 x 8.2 ohm resistors. When I connected the headphone the noise signal was reduced a bit.......why? .....the headphone is a very small load compared to the two 8.2 ohm resistors.
It seems the hum is a bit less.....subjective when I took the headphone on.
Maybe a re-wire project can reduce it further.
Attached Images
File Type: jpg DSC_3686_wires_cut.jpg (280.5 KB, 36 views)
File Type: jpg DSC_3688_AC_300B_filament_no_power_cord.jpg (298.6 KB, 36 views)
File Type: jpg DSC_3690_AC_300B_filament_left.jpg (263.9 KB, 13 views)
File Type: jpg DSC_3691_AC_300B_filament_left_head_phone.jpg (342.3 KB, 12 views)
File Type: jpg DSC_3692_AC_300B_filament_only_power_cord.jpg (259.1 KB, 11 views)
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