My tube preamp (Audio Hungary Qualiton apr204) either wears fast the line stage tubes (6922) or all the 3 pairs i bought were defective (1 pair genalex gold lion, 1 pair electroharmonix and one NOS pair voskhod 6n23p) They lasted about 6 months with aprox. 2 hour per day listening. One day when i turn on the preamp, the 6922EH catastrophically failed with sparks and flashes inside the right channel tube and VERY LOUD noises heard from the speakers. The previous day before total failure i was hearing clicks, pops and squealing from the right channel. Fortunately my Power Amp and speakers survived.
I disassemble the unit and copy the line stage schematic from the pcb traces.
The schematic is this:
I have no experience with tube designs and i would like to know if my preamp drives those 6922 hard and reduces their lifespan.
Any suggestions for modding the schematic or was just bad luck with defective tubes?
Also i want to change the gain of the preamp...did someone know which resistor value to change?
Thanks in advance!
I disassemble the unit and copy the line stage schematic from the pcb traces.
The schematic is this:
I have no experience with tube designs and i would like to know if my preamp drives those 6922 hard and reduces their lifespan.
Any suggestions for modding the schematic or was just bad luck with defective tubes?
Also i want to change the gain of the preamp...did someone know which resistor value to change?
Thanks in advance!
Several faults :
1/ The second triode will get 250Volt between grid and cathode at power on. This kills the tube
Solution, a 1n4148 between grid and cathode will remove the dangerous voltage
2/ no gridstoppers ! A gridstopper ( 1-10k, preferably CC ) connects to the grid, it should be mounted
as close to the grid as possible.
3/ the NFB loop will forward any positive voltage surge to the grid of teh first tube. If (1) is installed
it is likely no problem but it won't hurt to protect the first grid with another 1n4148
4/ a "lead-out resistor" ( 100 ohm CC) between cathode and output coupling cap prevents oscillation.
1/ The second triode will get 250Volt between grid and cathode at power on. This kills the tube
Solution, a 1n4148 between grid and cathode will remove the dangerous voltage
2/ no gridstoppers ! A gridstopper ( 1-10k, preferably CC ) connects to the grid, it should be mounted
as close to the grid as possible.
3/ the NFB loop will forward any positive voltage surge to the grid of teh first tube. If (1) is installed
it is likely no problem but it won't hurt to protect the first grid with another 1n4148
4/ a "lead-out resistor" ( 100 ohm CC) between cathode and output coupling cap prevents oscillation.
Thanks peter,
So if i understand correctly the lack of the diode was the reason why at the first power on the tube arcs and flashes... i cant understand why a manufacturer does that critical mistake... Anyway, according to your corrections the schematic will look like below...
The anode leg of the diodes should be connected after the grid stoppers as the image above or before? Also by CC resistor you mean carbon composition resistor?
If i understand correctly, the 47K feedback resistor defines also the gain of the preamp?
Thanks!
So if i understand correctly the lack of the diode was the reason why at the first power on the tube arcs and flashes... i cant understand why a manufacturer does that critical mistake... Anyway, according to your corrections the schematic will look like below...
The anode leg of the diodes should be connected after the grid stoppers as the image above or before? Also by CC resistor you mean carbon composition resistor?
If i understand correctly, the 47K feedback resistor defines also the gain of the preamp?
Thanks!
Thanks peter!
I was browsing for some direct coupled triode schematics and i found some examples that the diode is connected prior to grid resistor... e.g. the examples below:
What is the difference of connecting the diode anode after the grid resistor as shown on my schematic versus those two examples?
Also can you comment about the two low values of input and output capacitors on my schematic (1μF & 0.22μF) ? Aren't too low values for full range sound? Will i miss some lower bass? My Power amp have 10KΩ input resistance...
The CC Resistors you recommend me to add must be some specific wattage?
Your opinion is very valuable and will help me to order the components...
Thanks in advance!
I was browsing for some direct coupled triode schematics and i found some examples that the diode is connected prior to grid resistor... e.g. the examples below:
What is the difference of connecting the diode anode after the grid resistor as shown on my schematic versus those two examples?
Also can you comment about the two low values of input and output capacitors on my schematic (1μF & 0.22μF) ? Aren't too low values for full range sound? Will i miss some lower bass? My Power amp have 10KΩ input resistance...
The CC Resistors you recommend me to add must be some specific wattage?
Your opinion is very valuable and will help me to order the components...
Thanks in advance!
Connecting the diode after the grid stopper increases the area for picking up feedback. But the effect is small
The caps is not low values. The input could be 68nF and still give a corner freq of 10hz.
The size of the output depends on the input impedans of the power amp, choosen value of 0.22 uF
used on an amp with 100k input impedanse will give a corner of 7hz
An amp with 10k input impedance and 1uF cap would give a corner of 15hz
Note that hunting anything below 20 is often a null task as recordings seldom has anything below 40hz
and most speakers has the same limitations. But low frequency's from rumble and record warps will
often modulate music in unwanted ways.
Excessive large output caps increase the risk that turn on and turn off "thumps" will affect poweramp
and/or speakers in negative way. Too large caps will not increase fidelity only decrease reliability.
The caps is not low values. The input could be 68nF and still give a corner freq of 10hz.
The size of the output depends on the input impedans of the power amp, choosen value of 0.22 uF
used on an amp with 100k input impedanse will give a corner of 7hz
An amp with 10k input impedance and 1uF cap would give a corner of 15hz
Note that hunting anything below 20 is often a null task as recordings seldom has anything below 40hz
and most speakers has the same limitations. But low frequency's from rumble and record warps will
often modulate music in unwanted ways.
Excessive large output caps increase the risk that turn on and turn off "thumps" will affect poweramp
and/or speakers in negative way. Too large caps will not increase fidelity only decrease reliability.
Oh, i see Peter!
So either way is correct... If you were me, which route you will choose... Diode before or after grid resistor?
For grid resistors 0.6 Watts are ok?
Thank you!
So either way is correct... If you were me, which route you will choose... Diode before or after grid resistor?
For grid resistors 0.6 Watts are ok?
Thank you!
Perfect! Many thanks Peter! I will order the parts (and new tubes 😪) and i will do the modification.
