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300B SET amp measurements without tubes installed
300B SET amp measurements without tubes installed
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Old 4th December 2017, 09:02 PM   #11
MEPER is online now MEPER  Denmark
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Some additional measurements (measued by using a multimeter only):
Short the input at the ECC83 input did not make any "hum change" compared to short the input at the RCA connector.


Measured DC on 300b heater to be 4.95V.
Measured AC on the 5V DC on the 300b to be 55 mV <-- can that cause hum?
Measured AC on the B+ to 8 V <-- is that much?


Could be interesting to look at is using a scope.
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Old 4th December 2017, 10:23 PM   #12
6A3sUMMER is offline 6A3sUMMER  United States
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I. 8V ripple on 300B B+ is far too much.

Example: 300B rp = 700 Ohm, OPT primary = 3000 Ohm, secondary = 8 Ohms.

Root (8/3000) = 1/19.4 the voltage ratio of the output transformer.
8V * (3000/(3000 + 700)) is the ripple across the 3000 Ohm primary
8V * 0.81 = 6.48V across the 3000 Ohm primary
6.48V/19.4 = 0.334V (334mV ripple)

II. 300B u = 3.85
The gain of the 300B and output transformer is 3.85 * (1/19.4) = 0.198
That is the gain from the 300B grid (relative to the 300B filament), versus the output at the 8 Ohm tap.

Now, suppose you have 55mV ripple on the 5V filament, and you do not either use a balance pot, nor at least use some matched resistors on the filament to connect their center to the bias resistor. There is no balancing of the 55mV ripple.
In that case, you would connect one end of the filament (+ or -) to the bias resistor to ground.
Then:
55mV * 0.198 = 11 mV ripple at the 8 Ohm tap.

Is the hum mostly 50Hz, 100Hz, or both?
That is also a clue to where the hum(s) is/are coming from.
Full wave rectification should have dominant 2x line frequency (i.e. 100Hz).
Half wave rectification should have dominant 1X line frequency (i.e.50Hz).

Ironically, AC filaments on a 300B will usually have dominant 2X line frequency.

Hint: Never rely on negative feedback to reduce hum, get the hum reduced first, and then the negative feedback will not have to "work so hard".
And by the way, the "hum error correction signal" that is fed back will go back through the amplifier, and will be distorted.
50 Hz hum will now have 100 Hz and 150 Hz 2nd and 3rd harmonic distortion.
Yes, those 100Hz and 150Hz distortions will be partially corrected by the 2nd time around of the negative feedback correction signal, but 2nd and 3rd harmonic distortion will now make them 100x2, 100x3, 150x2 and 150x3Hz.

For more on how negative feedback creates upper order harmonic distortions, look up some Norman Crowhurst documents from the 1950s.

I generally shoot for less than 500uV ripple on single ended amps, and less than 300uV ripple on push pull amps (and they do not have any global negative feedback).

I hope that helps.

Last edited by 6A3sUMMER; 4th December 2017 at 10:39 PM.
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Old 5th December 2017, 06:42 AM   #13
MEPER is online now MEPER  Denmark
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Thank you very much for the answer and calculations.
I need to look at the measuremets using a scope to be sure my measurements are correct. Also to check how the waveforms look like. To my ear there was some 50 Hz from the woofer. Also some "buzz" from the mid speaker.
The NFB was put there (by the maker) to give the amp a more solid bass response. I was told I cold just remove it if bass response was "too much". It seems a good idea to remove it.......at least until the hum problem is solved.
I have plans for a 10H choke just in front of the diode bridge. This should reduce the ripple? .....the one I am looking at has a resistance of 130 ohm as far as I remember so I will loose a few volts......but I guess it should be ok. It can be placed in front of the power trafo. Should it be turned 90 degree to the power trafo?
I noticed that the wire bundle you see on the picture consist of the 400V AC, 6.3 V AC, 2 x 5V DC. Can that cause a problem? ....can that cause some noise to be inducted into the 5V DC? the 400V AC is going direct to the bridge and after that a fast 220uF Jensen audio grade capacitor. There will be some current peaks in the 400V AC wire which could be transferred to the 5V DC wires?

By the way....I was not able to reduce the 55 - 65 mV ripple at the 5V DC by trimming the hum pot.
The amp as a soft start so after approx. 45 sec the B+ is put on. I will measure how the ripple at the 5V DC looks like before and after the 45 sec. It was yesterday I discovered how the softstart was implemented and that I can use it in the measurements to check where the hum is generated.
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Old 5th December 2017, 07:13 AM   #14
6A3sUMMER is offline 6A3sUMMER  United States
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The pot does not reduce the 55mV ripple. It merely applies a different amount/portion of the ripple to the 2 ends of the filament.
Suppose one end of the filament versus the grid voltage is more sensitive than the other end of the filament versus the grid voltage. i.e. 40mv to the less sensitive end of the filament, and 15mV to the more sensitive end of the filament (and so tends to cancel the effect, because it is the opposite phase).

Bundling the HV wires with other wires is probably not the best construction technique.
I would worry more about the 400V and 6.3V wires bundled together. The high gain ECC83 / 12AX7 might have some capacitive 'leakage' from the filament to the cathodes. That might be the cause of the higher frequency buzz in the midrange (capacitive 'leakage' amplitude increases as the frequency increases)

For a power supply, the order is transformer secondary, rectifier(s), choke.
It is not transformer secondary, choke, rectifier(s).
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Old 5th December 2017, 01:03 PM   #15
Rod Coleman is offline Rod Coleman  United Kingdom
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300B SET amp measurements without tubes installed
Quote:
Originally Posted by MEPER View Post
Some additional measurements (measued by using a multimeter only):
Short the input at the ECC83 input did not make any "hum change" compared to short the input at the RCA connector.


Measured DC on 300b heater to be 4.95V.
Measured AC on the 5V DC on the 300b to be 55 mV <-- can that cause hum?
Measured AC on the B+ to 8 V <-- is that much?


Could be interesting to look at is using a scope.
Firstly, be careful about connecting a scope to the filament. Please remember, the scope ground should be connected to safety ground, and the amplifier ground/chassis is (should) also be connected to safety ground. If you connect the probe-ground to one end of the filament, you will short-circuit the cathode resistor network, and the anode-current of the 300B will increase dramatically, and risk damaging the 300B.

You can safely measure the filament noise with the anode voltage switched OFF.

But in any case, 55mV at the output of a 7805 is not a good result, and with 95dB/W speakers, you will have difficulty getting it quiet.

The problem is likely to be that the input voltage to the 7805 is too low to achieve good supply-rejection (it needs over 8V when the mains supply is at its lowest), but it must also support the range of output current required by different 300Bs (1.2 - 1.4A).

So, what voltage do you have at the input of the 7805?

I see that the 2 7805 regulators are mounted close together on the same heatsink. Increasing the supply voltage will risk overheating the 7805, especially when the mains-supply is at high tolerance. The older Audio Note Kit-1 had problems like this. The The 7805 has a thermal protection for itself (it cuts out when near the thermal limit), but cutting the heating power while the anode voltage is still present is not a good idea.

Reducing the ripple at the input to the 7805s can reduce the noise, but bigger supply-capacitors means higher rms current through the transformer and more stress & heat on the rectifiers - so it should only be done if all the design considerations are understood.
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Old 5th December 2017, 02:46 PM   #16
MelB is offline MelB  Canada
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Rod he could replace the 7805 regs with some low drop regulators that have higher ratings. Like these:
MIC29300-5.0WT Microchip Technology | Integrated Circuits (ICs) | DigiKey
They only need 0.6 volts drop at 3 amps. They are pin compatible so will fit right in with no mods.
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Old 5th December 2017, 05:32 PM   #17
MEPER is online now MEPER  Denmark
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The 55 mV AC on the 300b filament is there without B+ is on. So it must be the 5V DC which is "noisy" by itself?
I measure 7.7V AC as input to the rectifier bridge and 8.5V DC on +- terminals of the rectifier bridge. 7805 gets what it needs to be able to regulate properly?
About my scope and measurement on the 5V filament. My scope is a LabNation SmartScope and connects to an Android tablet so no common safety ground (battery driven).
I see the problem using a real scope if the probe ground is the same as the amplifier ground......
I should be able to use the SmartScope as I use my multimeter.....i assume/hope....
Could a capacitor direct on the 300B filament reduce the AC noise?
What size would you recommend?
Something that has been done before?
About extra B+ filtering. In the moment it is as follows:
secondary : Rectifier bridge : 220uF capacitor : 220 ohm filter resistor : 220uF capacitor.

The idea was to insert a 10H (130 ohm) choke just after the rectifier bridge and keep the other as is. Would that work? .....how much less ripple can I expect?

About the wiring I have attached a picture of the big bundle of wires:
Twisted brown: 400V AC (B+)
Twisted white: 6.3V AC (ECC83 heating)
Twisted gray: 6.3V AC back from ECC83 and used to the 555 timer board for delayed B+ (45 sec).
Twisted white/red
Twisted white/yellow: 5V DC to 300B filament
Black: ground
Green: NFB wire back from OT to ECC83 for left channel (the one which has most hum).

Do you think I should spilt bundle and rewire some of it?
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Old 5th December 2017, 06:48 PM   #18
Rod Coleman is offline Rod Coleman  United Kingdom
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300B SET amp measurements without tubes installed
Yes, the 55mV in the dc filament supply output is caused by the ripple voltage on the chip's input. The capacitors are probably too small (they look too small) and so the ripple voltage will be too large, and the voltage will drop below the minimum value to maintain low noise. Electrolytic Capacitors that are too small will also have a short life, because they can't withstand the ripple current (the rms value of the pulsating current from the rectifiers: nearly 2.3A for a 1.2A dc rectified supply).

low dropout regulators like the MIC29300 that Mel suggests can reduce noise, but beware that they are not always a drop-in replacement: to get low dropout they use PNP output transistors, which greatly increase the open-loop output impedance. This means that the caps at the input and output must be as per the data sheet: No ceramics or other low impedance film caps, but bigger electrolytic values than for 7805. These old 1990s chips will oscillate if the caps are outside the specified range of C-value and ESR. For all the work involved in swapping the chips & fixing these capacitor problems, you still have a voltage regulated supply, which can't distinguish between controlling the filament heating (good), and destroying the differential part of the music signal, across the filament (bad). A current-driven filament supply avoids this flaw.

10H choke for extra B+ filtering? Yes, if it is rated for enough current (say 120mA for 1 300B, or 240mA for stereo pair) then it will improve the B+ noise, yes.

Search for the "Duncan PSUD2" application, to model the B+ supply, and see how much improvement in ripple you get.
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Old 5th December 2017, 06:56 PM   #19
Rod Coleman is offline Rod Coleman  United Kingdom
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300B SET amp measurements without tubes installed
Quote:
Originally Posted by MEPER View Post
About the wiring I have attached a picture of the big bundle of wires: <snip>
400V ac from the transformer to the rectifier, and the wiring from the rectifier to the B+ capacitor should be kept short. Very short - and kept right away from any signal wiring (the 300B filament wiring should be treated as signal). The voltage swing, and the current pulses in these wires mean that they will emit large electromagnetic fields. Keeping them short minimises the problem - having them long and close to other wires makes the problem much worse.

The 300B filament wiring should certainly be kept separate from all other wires.
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Old 5th December 2017, 06:58 PM   #20
Rod Coleman is offline Rod Coleman  United Kingdom
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300B SET amp measurements without tubes installed
Having the NFB wire bunched with noisy signals is a big problem, too. Run it away from other cables, and consider using screened cable.
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