Hi,
I have Tubemeister 18 head. I have one problem with it. When switching from clean to lead channel there is "pop" sound. I discovered, when I stay either on the clean or lead channel for a while, say 20 minutes, and then switch channels, the pop sound is much louder. And this pop sound is still heard when both master volumes for clean and lead are at zero. I have contacted HK support 3 times already, but recieved no response from them. So I do not have exact schematics, but earlier model Tube20 is quite similar (schematic attached).
What could be the problem? Leaking cap? Bad jfet?
Thanks for help.
I have Tubemeister 18 head. I have one problem with it. When switching from clean to lead channel there is "pop" sound. I discovered, when I stay either on the clean or lead channel for a while, say 20 minutes, and then switch channels, the pop sound is much louder. And this pop sound is still heard when both master volumes for clean and lead are at zero. I have contacted HK support 3 times already, but recieved no response from them. So I do not have exact schematics, but earlier model Tube20 is quite similar (schematic attached).
What could be the problem? Leaking cap? Bad jfet?
Thanks for help.
Does it continue to pop at the same volume after cycling the channels?
Essentially what I am getting at is it can be quite common for some amps to pop when switching channels, especially when first coming out of a standby mode.
For some of them, it is a static discharge that builds up on the channel switching relays.
Your amplifier is probably fine.
If you can narrow down exactly what is happening perhaps a capacitor or high value resistor could be tossed in somewheres.
It is a very common phenomenon.
Essentially what I am getting at is it can be quite common for some amps to pop when switching channels, especially when first coming out of a standby mode.
For some of them, it is a static discharge that builds up on the channel switching relays.
Your amplifier is probably fine.
If you can narrow down exactly what is happening perhaps a capacitor or high value resistor could be tossed in somewheres.
It is a very common phenomenon.
When I switch channels rapidly, the pop sound is there but very very quiet. This is not problem and I know it is normal. BUT when I stay on one channel for longer time and then switch channels, the pop is VERY loud, sounds more like "bam".
It seems like there is some capacitor charging up, which shouldn´t.
It seems like there is some capacitor charging up, which shouldn´t.
I did not look at the schematic that hard but maybe there are some capacitors that are left floating that need to be referenced to ground. It is beyond my skills to suggest any changes to the schematic at this point.
There is a slight voltage that develops when the "off" channel is left floating, either it be static or some small charge that accumulates in the off channel.
The longer it is disconnected, the more energy that builds up.
When you switch to that channel, the energy is released as a quick audible zap.
That is why I suggested that a high value resistor or some type of capacitor may be used to dispose of this unwanted charge. This is of course if you are confident that the amplifier is in fact wired properly.
There is a slight voltage that develops when the "off" channel is left floating, either it be static or some small charge that accumulates in the off channel.
The longer it is disconnected, the more energy that builds up.
When you switch to that channel, the energy is released as a quick audible zap.
That is why I suggested that a high value resistor or some type of capacitor may be used to dispose of this unwanted charge. This is of course if you are confident that the amplifier is in fact wired properly.
Leaky Cap(s)
The tone circuit drive coming off Tube 1 has 350 v. on it. The 150nF decoupling cap C16 is the most likely suspect. C17 through C20 at the common connection point is also "floating," which will put a DC potential of indeterminate voltage across these caps. If any one is leaky it could cause the pop. IMHO the design is flawed and should have included a DC resistor to ground at the junction of C16, C17, C18, and R26.
I suggest replacing C16 (.15uF) with a 600v rated capacitor and add a 2.2M ohm 1/2 watt resistor at the junction of C16, C17, C18, and R26 to ground.
If the above doesn't fix it another suspect would be C15. If leaky it would put DC voltage on the grid, which gets grounded by the Clean relay switch - ERGO POP!
I'd also suggest spraying all of the controls with contact cleaner if the amp is old. A number of them provide DC stabilizing and if they go open due to dirty contacts it also cause noise and pops.
WARNING: Make sure the amp has been unplugged from the AC power for a few minutes before trying to do any work. I also suggest taking a jumper wire and shorting the junction of C16, C17, C18, and R26 to ground since it could still hold a charge. If not confident in doing the work yourself take it to a shop that does amp repairs and provide them this detail.
The tone circuit drive coming off Tube 1 has 350 v. on it. The 150nF decoupling cap C16 is the most likely suspect. C17 through C20 at the common connection point is also "floating," which will put a DC potential of indeterminate voltage across these caps. If any one is leaky it could cause the pop. IMHO the design is flawed and should have included a DC resistor to ground at the junction of C16, C17, C18, and R26.
I suggest replacing C16 (.15uF) with a 600v rated capacitor and add a 2.2M ohm 1/2 watt resistor at the junction of C16, C17, C18, and R26 to ground.
If the above doesn't fix it another suspect would be C15. If leaky it would put DC voltage on the grid, which gets grounded by the Clean relay switch - ERGO POP!
I'd also suggest spraying all of the controls with contact cleaner if the amp is old. A number of them provide DC stabilizing and if they go open due to dirty contacts it also cause noise and pops.
WARNING: Make sure the amp has been unplugged from the AC power for a few minutes before trying to do any work. I also suggest taking a jumper wire and shorting the junction of C16, C17, C18, and R26 to ground since it could still hold a charge. If not confident in doing the work yourself take it to a shop that does amp repairs and provide them this detail.
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When all tone, gains and master controls are set to zero and even if I am not playing, after cca 20 min there is still loud pop, when switching channels.
I think I won´t solve this without proper schematic of the Tubemeister 18.
I think I won´t solve this without proper schematic of the Tubemeister 18.
Well try reverse engineering it from the Tube 20 schematic. From what I can see if you set the Treble, Bass, and Midrange controls all to ZERO (0) then the circuit in question is effectively "grounded. Does it still pop when try it with these Tone Control settings?
Sorry, I see you already tried that, which may indicate one of those controls is open due to worn or dirty contacts. How old is this unit?
It is about 1 year old. I don´t think it is the problem of controls, because all of them works just fine.
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