Hi, and thanks for taking the time to read this post. I am a neophyte, so please excuse my ignorance.
I have a Knight KG-400, which is a push-pull 32W integrated amplifier consisting of 4 6bq5(s), 1 GZ34, and 6 12ax7(s). The amp currently has no playback output, and some of the 6 12ax7(s) are not getting power. So I am stuck at pin 8 of the GZ34, which should be about 390V. With all the tubes in, I'm getting like 35V. If I remove tube 1, the 12ax7 phono tube, the voltage jumps to about 320V. In the current state, when I turn the amp on, I just get a slight hum - my dim bulb tester does not get real bright, nothing is burning, etc. Where is the voltage on pin 8, and how do I go about troubleshooting this?
I have a Knight KG-400, which is a push-pull 32W integrated amplifier consisting of 4 6bq5(s), 1 GZ34, and 6 12ax7(s). The amp currently has no playback output, and some of the 6 12ax7(s) are not getting power. So I am stuck at pin 8 of the GZ34, which should be about 390V. With all the tubes in, I'm getting like 35V. If I remove tube 1, the 12ax7 phono tube, the voltage jumps to about 320V. In the current state, when I turn the amp on, I just get a slight hum - my dim bulb tester does not get real bright, nothing is burning, etc. Where is the voltage on pin 8, and how do I go about troubleshooting this?
Sounds like a tube with an internal short. You might want to invest in some newer 12AX7s, or just replace the one that causes the problem with a new tube.
Remove the dim bulb tester (which is only for the initial power-up, but not for further testing),
install all rubes, and measure the DC voltage at pin 8 of the rectifier tube again. Should be 395 VDC.
Tubes V2, V3, and V4 must be in their sockets, since their filaments are powered by the HV secondary center tap.
Check capacitor C31 and resistor R66.
The filament of tube V1 provides bias for the output tubes. It must also be in its socket for normal operation.
You'll need to do further testing without the bulb tester. Install all the tubes and start with the DC voltages
on all the plates of V1 - V6. Exactly what do you mean by "the tubes are not getting power"?
install all rubes, and measure the DC voltage at pin 8 of the rectifier tube again. Should be 395 VDC.
Tubes V2, V3, and V4 must be in their sockets, since their filaments are powered by the HV secondary center tap.
Check capacitor C31 and resistor R66.
The filament of tube V1 provides bias for the output tubes. It must also be in its socket for normal operation.
You'll need to do further testing without the bulb tester. Install all the tubes and start with the DC voltages
on all the plates of V1 - V6. Exactly what do you mean by "the tubes are not getting power"?
Last edited:
Well, I tested the tubes AGAIN, and 2 12ax7(s) are testing bad. I have to order them. Thanks for the input guys...
After borrowing a good pair of 12ax7(s), the voltage on pin 8 is up to 426V. Tubes 1, 2, 3, 4 show no life (no illumination). Pin 1 Tube 1 is 34V. Pin 1 Tube 2 is 62V.
Turn out the lights, and look for a faint glow at the top or bottom of the plates.
jeff
Filament voltages across pin 4 on tube 4 -26, Tube 3 -17, and Tube 2 -9. Still not glowing, even with the lights out. The output tubes (EL84) are glowing blue at the base, so I don't leave the amp powered on for too long...
High B+ and V1, 2, 3, 4 heater voltages low means that the output tubes aren't drawing normal current. MIGHT mean an open output transformer winding - I bought one of these real cheap at a garage sale - turned out both output transformers were open.
When you remove that preamp tube you have also turned off all the output tubes. B+ increases but is still low. We don't know if you've already replaced the power supply electrolytics or anything about its recent history, so can only make wild stabs in the dark. A little detail goes a long way.
All good fortune,
Chris
Ok, thanks again guys for the input. The electrolytics have been replaced, and the amp was working, just with a noticeable hum. Now I am not getting tubes 4-1 to come up. The amp was gone through one by a skilled technician that hotrodded it with boutique capacitors and new filter caps. I got it with a hum, and have not been able to fix it. I will note that I changed out the filter caps once to see if I could get rid of the hum, to no avail. I don't believe I did anything to the output transformers, which were working as expected a few weeks ago. I will keep on it, and report back any updates...
Ok, some updates. I restored the amp back to where it plays an audio signal through the AUX input. However, almost all my voltages are high primarily pin 8 on the GZ34 - it's coming in at 424V. That is with a spot on 117V AC outlet. Also, there is noticeable hum, constant, changes with the balance control (changes channels), don't know if it's 60Hz or 120Hz (I've never dealt with hum before, and am having a hard time with the youtube signals). If I had to guess, it's 120Hz - I put 2 ceramic capacitors (1 on each leg) off the heaters tap to ground with no real noticeable difference. Open to any input, and thanks in advance...
And the Kiss of Death. The hard truth that everyone is trying to avoid saying is that the amplifier has been crippled by a Guru.
To get a handle on the scope of this project, could you discuss what test equipment is available to you (yours or borrow-able), if you have a safe (no kids, no pets, closed door) space to work, and how comfortable you are working inside the chassis? You know these details, but none of us can, without guidance.
A separate issue is why a "skilled technician" let you have an amplifier with noticeable hum. But that's just a personal beef.
All good fortune,
Chris
Hi Chris, so yes. I have been working on electronics for a couple of years, fairly proficient, and quite an interest. I have a safe workspace with a soldering iron, scope, LCR meter, multimeter, VTVM, isolation transformer variac, Audio Generator, DC power supplies, signal generator, etc. I am not real comfortable using the scope, just because I haven't used one ever before and do not want to damage it (Tektronix 2465). I am comfortable working inside the chassis as I always power it down before I take readings or any other adjustment. I also know how to discharge the filter caps.
This amp has not been cannibalized by random parts - the skilled tech who did the work with the capacitors (Sonic Caps) did a nice job - all the carbon resistors match the schematic, and the filter caps are Nichicon, not too far off original in terms of value. I bought the amp with the understanding that it needed work, and thought it was the filter caps. After changing the filter caps with no improvement, I put it aside for about 2 years -- now I am back on it. The person I bought the amp off of is the one that had it updated with new caps, and he said it was working for years without problem. I don't know if he drove it too hard or what.
There are two problems with the amp: 1) The voltages are high 2) There is a noticeable hum. The voltages come in high pretty much across the board, and pin 8 on the rectifier tube reads 424V when the schematic says 390V. That is at a controlled 117V wall voltage. I can dial back the voltage with a variac, although I hesitate to accept that as a long term solution. I don't know how to get this down, and I believe this coincides with the hum - the hum picks up when the rectifier warms up and comes up to temp. I almost believe it's the rectifier tube, or lack of filtering on the filter caps - however, I changed the filter caps, and the skilled tech had it working with similar existing caps in. I have two GZ34(s), both test good and perform similar in the amp. Another note - the power cord on the amp is a polarized two-prong cord.
Lastly, if I'm reading the schematic right, there is no center tap - I don't know anything about a virtual center tap outside of the fact it might do away with some of the hum. I plan on looking into this. The filter caps or 47uf when the schematic calls for 30uf I believe - could I increase the uf for better filtering? Also, I guess it's fair to say that the power transformer may be damaged?
This amp has not been cannibalized by random parts - the skilled tech who did the work with the capacitors (Sonic Caps) did a nice job - all the carbon resistors match the schematic, and the filter caps are Nichicon, not too far off original in terms of value. I bought the amp with the understanding that it needed work, and thought it was the filter caps. After changing the filter caps with no improvement, I put it aside for about 2 years -- now I am back on it. The person I bought the amp off of is the one that had it updated with new caps, and he said it was working for years without problem. I don't know if he drove it too hard or what.
There are two problems with the amp: 1) The voltages are high 2) There is a noticeable hum. The voltages come in high pretty much across the board, and pin 8 on the rectifier tube reads 424V when the schematic says 390V. That is at a controlled 117V wall voltage. I can dial back the voltage with a variac, although I hesitate to accept that as a long term solution. I don't know how to get this down, and I believe this coincides with the hum - the hum picks up when the rectifier warms up and comes up to temp. I almost believe it's the rectifier tube, or lack of filtering on the filter caps - however, I changed the filter caps, and the skilled tech had it working with similar existing caps in. I have two GZ34(s), both test good and perform similar in the amp. Another note - the power cord on the amp is a polarized two-prong cord.
Lastly, if I'm reading the schematic right, there is no center tap - I don't know anything about a virtual center tap outside of the fact it might do away with some of the hum. I plan on looking into this. The filter caps or 47uf when the schematic calls for 30uf I believe - could I increase the uf for better filtering? Also, I guess it's fair to say that the power transformer may be damaged?
I think you need to start at the basics, first do a DC conditions check. Get a notepad and write something like this -
HT/B+
C1
C2
etc
Also note AC @ C1 etc
Heaters.
V1
V2
etc
Then
V1
a
k
g
V2
a
g
k
Then note the DC voltage of each part of the circuit. This is just an example, it might give you & us an indication of where the fault is. You can then run a similar test but this time of AC conditions. EG input a 500mV 1khz sine wave then check with your scope or AC voltmeter each valves grid & anode remembering to not exceed your scopes specs.
Lastly, post a schematic & a few good quality pics of the underside of the amp, we may spot something.
Andy.
HT/B+
C1
C2
etc
Also note AC @ C1 etc
Heaters.
V1
V2
etc
Then
V1
a
k
g
V2
a
g
k
Then note the DC voltage of each part of the circuit. This is just an example, it might give you & us an indication of where the fault is. You can then run a similar test but this time of AC conditions. EG input a 500mV 1khz sine wave then check with your scope or AC voltmeter each valves grid & anode remembering to not exceed your scopes specs.
Lastly, post a schematic & a few good quality pics of the underside of the amp, we may spot something.
Andy.
Ok Andy, will do - probably won't get it up until this weekend. What do you mean by
V1
a
k
g
Thanks...
V1
a
k
g
Thanks...
I got it - I have an earlier alternative schematic that I will post - it's more clear than the one you find radiomuseum...
Yes, shorthand for the various electrodes. The main ones are anode, control grid (g1), screen grid (g2) & cathode all referenced to ground. It's also a good to measure control grid to cathode voltage on some circuits like cathode followers and long tailed pairs.`
Your looking for -
1) no cathode voltage.
2) A positive g1 voltage
Broadly speaking that is.
Your looking for -
1) no cathode voltage.
2) A positive g1 voltage
Broadly speaking that is.
The slightly high B+ is not unusual, given that power supply caps have been increased in value. Do you have hum when the amplifier is connected only to speakers and mains (no other wires, at all !, connected)? If not, the amplifier is not at fault; if so, how is the hum effected by the volume control?
All good fortune,
Chris
All good fortune,
Chris
Last edited: