....the question is about.....the 50hz hum is at the output of the vt25 .....but not through the pins into the tube.It's not easy. . I decided to start assembly a part of a 6b4g dht pre-amp project and see if it exists 50hz hum or not?.
The circuit of 6b4g is similar to the circuit of vt25.. The components of the both power supply are completely similar. There is also a filament regulator in dht pre amp.
5
5
Is amplifier "grounded" -via wire or small resistor or hum loop blocker- to the mains safety earth?
Is your scope mains has safety earth?
BTW measuring in tube amplifier without HV (no operation within tubes) is shooting in the dark.
Is your scope mains has safety earth?
BTW measuring in tube amplifier without HV (no operation within tubes) is shooting in the dark.
How about a schematic of the amp along with a test connection diagram with particular emphasis on the AC power and grounding. Draw it up and the answer may reveal itself.
Once I forgot to ground the secondary of an output transformer. With the amplifier switched off, I could hear pretty loud hum in the speakers.
To minimize hum, attention has to be paid to minimizing circuit wiring loop areas. This involves pairing source and return wires and keeping the distances between them as close as possible. With actual wires, twisting them together works. PCB traces should be laid out with this in mind. Wire loops can act as receiving and transmitting antennae.
So, there are some things to check:
So, there are some things to check:
- twist wires from AC power inlet to the power transformer
- twist the wire pairs from the transformer secondaries to the rectifiers
- twist the DC wires from the rectifiers to the filter circuit
- twist the positive and negative wires from the filtered DC power supply to the tube filaments
- twist positive and negative wires from the B+ filter to the tube high voltage circuits
- + and - signal wires should be twisted as well
good question .... i use voa psiphon vpn for internet serching.My son lives in Canada.so i chose the Canadian version in( v p n) This was for the search results in finding online shop and parts in Canada. My son buys the parts and sends them to me. The last time I came to this forum with ....vpn... automatically saved my land ...canada . But I tried to change it several times. I didn't succeed. I'd rather not have any land. Because I don't really have one. But my son has it now.
Messy wiring can transmit noise, too, so other parts of your amp, such as the power supply can broadcast noise that is picked up by your output circuit wiring.
I don't build tube amps anymore, but I pay attention to wiring when I build my solid state amps as the principles are the same.
I don't build tube amps anymore, but I pay attention to wiring when I build my solid state amps as the principles are the same.
Attachments
There is a simple way to test. We cut the paths and look at the scope screen . Does it disappear hum quickly? Or does it stay? The tube turns off completely after a few moments. I did this test many times. Tube DHTs turn off earlier than indirect heat tubes. So you have to be quick and repeat the test several times.
who said without h.v i do this test? please see the mosfet ripple filter....its work in hv circuit in vt25.
Has anyone connected the probe of scope in the x.1 to the anode of the dht tube ( vt25 or 6b4g 801 ) ....and taken a picture? If there is such a picture, please share it.
Not a good idea directly connect to the anode the scope input (without 10x or 100x attenuation).
All modern oscilloscope is SS device, usually with maximum 400V DC limit.
On the tube's anode, if even the Uak is 300V, the maximum swing peek may soon surpass this limit.
From your Hantek scope manual:
"RMS signal level including all DC components removed through AC coupling must be limited to 300V. If these values are exceeded, damage to the oscilloscope may occur."
I use for tube devices old (very old) tube oscilloscope (600V DC limit), BUT always AC mode with 10x probe (for "low" voltage devices 400V probe, above it 1kV one).
All modern oscilloscope is SS device, usually with maximum 400V DC limit.
On the tube's anode, if even the Uak is 300V, the maximum swing peek may soon surpass this limit.
From your Hantek scope manual:
"RMS signal level including all DC components removed through AC coupling must be limited to 300V. If these values are exceeded, damage to the oscilloscope may occur."
I use for tube devices old (very old) tube oscilloscope (600V DC limit), BUT always AC mode with 10x probe (for "low" voltage devices 400V probe, above it 1kV one).
I worked as a senior engineer at Sony and Panasonic for 30 years. Before LCDs tv ... I always worked with scopes on CRT TVs. The horizontal section had high DC voltage and a high voltage signal... I had learned how to take care of the scope so that it wouldn't be damaged by the high voltagen recent years, In recent years of crt tv i had a portable digital scope..
My request for a picture is not for a power amplifier. It is never difficult to fix hum problemsthe on power tube amp. But a pre-line with a DHT tube can be a real challenge. I wrote that the volume is closed. My presence must be checked in silence in the audio circuit. The DC voltage is usually not more than 200 volts for the these tubes I mentioned. Usually no one likes to show hum .
My request for a picture is not for a power amplifier. It is never difficult to fix hum problemsthe on power tube amp. But a pre-line with a DHT tube can be a real challenge. I wrote that the volume is closed. My presence must be checked in silence in the audio circuit. The DC voltage is usually not more than 200 volts for the these tubes I mentioned. Usually no one likes to show hum .
Last edited:
Solved😊
Most people who have practical information use it for Business. .
My problems are solved. Now vt25 is silent. There is no 50 Hz hum.
.
Most people who have practical information use it for Business. .
My problems are solved. Now vt25 is silent. There is no 50 Hz hum.
.
changes one thing at a time and looks at the response in the scope, but nothing changes. You have to consider all the things involved in hum reduction. After doing them, for example, we can see that .......shielding the tube.... has an effect.