I'm having a biasing issue in my Marantz 5 amp - I had a power supply cap go, and did 2 replacements. Now, I note then when I adjust one of the bias pots up (or down) the bias on both tubes goes up (or down). Used to be if one was adjusted one way, the other would go a little bit the other way, and this is how it still is in the other monoblock.
The only changes I made are 2:
replace a 20uf segment of C16 with a 20uf 500 v cap as the specs call for
replace the string of electrolytic at c15 with a 30uf 600v cap. I know the specs call for 6uf 600v.
Any thoughts on why increasing the bias at either bias pot is increasing it at both tubes? (hopefully not a short somewhere). It has me worried that something is amiss.
I've attached a schematic.
Many thanks!
The only changes I made are 2:
replace a 20uf segment of C16 with a 20uf 500 v cap as the specs call for
replace the string of electrolytic at c15 with a 30uf 600v cap. I know the specs call for 6uf 600v.
Any thoughts on why increasing the bias at either bias pot is increasing it at both tubes? (hopefully not a short somewhere). It has me worried that something is amiss.
I've attached a schematic.
Many thanks!
I checked carefully again. All is fine. Just replaced 2 caps, so not overly complicated and easy to check.
I agree it would be likely, and also easy to fix. But it’s not the case.
Can you take a look at the schematic to see where this could occur?
I agree it would be likely, and also easy to fix. But it’s not the case.
Can you take a look at the schematic to see where this could occur?
All I can think of is a leaky 0.1uF coupling capacitor.
To rule that out, lift one end of both (at the same time) and try adjusting the bias again.
In a circuit like this, it's normal for the other bias setting to go in the opposite direction,
because the B+ DC voltage varies with output tube currents.
But it's not normal for the other bias setting to go in the same direction.
Check that out, it indicates a problem.
To rule that out, lift one end of both (at the same time) and try adjusting the bias again.
In a circuit like this, it's normal for the other bias setting to go in the opposite direction,
because the B+ DC voltage varies with output tube currents.
But it's not normal for the other bias setting to go in the same direction.
Check that out, it indicates a problem.
Have you checked the B+ at the output transformer? Increasing C15 from 6uF to 30uF would probably raise the B+ quite a bit. It could be that the output tubes are running way too hot.
Grover, I am awaiting a new cap to put c15 back where it should be. But, can you explain how a high B+ would cause this issue? Also, can I just check it at the hot end of c15?
Rayma, I can try lifting one end of c7 and c8, same time as you suggest, but is there any danger of blowing something, or will I just be decoupling from audio input? And, if one of the caps went, you believe it would cause this? As far as I can tell the audio is really good right now.
Many thanks!
Rayma, I can try lifting one end of c7 and c8, same time as you suggest, but is there any danger of blowing something, or will I just be decoupling from audio input? And, if one of the caps went, you believe it would cause this? As far as I can tell the audio is really good right now.
Many thanks!
No danger at all. The audio will just not come through. But I think there may be a DC leakage problem.
The caps can still work but leak DC. It doesn't take much to cause a DC voltage drop across the grid resistors(s).
Try that (after discharging the power supply) and see if the biasing works normally now.
If so, those coupling caps are leaky and should be replaced. If not, no idea what else it could be.
Unless one bias pot is wired backwards.
If you increase first capacitor of CLC, the B+ will be also increasing, so bias can shift roughly.
Are you measured B+ (about 456V) and bias voltages (on both G1 about -36V) before and after?
BTW each simple biased PP amp shows such symptom: if you raise one tube bias, the other decreasing a little. After several attempts can be set the correct values.
Last edited:
I checked B+ at pin 1(350) and 6 (330)of 6cg7,
Pin 6 of gz34 (470)
The first(470) 2nd(404) and 3rd (404) parts of c16 - these last 2 are a bit low, I am curious as to why?
And from the bias caps which gives 47.
Now amp is for some reason humming also,unlike before testing.
Next step is Raymas suggesting of lifting c7 and c8. That might be tomorrow.
Anyone else have thoughts ?
Pin 6 of gz34 (470)
The first(470) 2nd(404) and 3rd (404) parts of c16 - these last 2 are a bit low, I am curious as to why?
And from the bias caps which gives 47.
Now amp is for some reason humming also,unlike before testing.
Next step is Raymas suggesting of lifting c7 and c8. That might be tomorrow.
Anyone else have thoughts ?
Testing with your DVM should not cause any problems, unless you directly probe the output tube grid.
That is a high impedance node and should not be probed directly, instead probe at the pot wiper.
That is a high impedance node and should not be probed directly, instead probe at the pot wiper.
Again, photos of the changes you made would help.
If you measure 470vdc at pin 8 of the GZ34, then section 1 of C16 should be lower, as indicated on the schematic.
If the amp is humming, then possibly the output tubes are out of balance. What do you get when you measure from EL34 pin 8 to ground? Should be 1.5 volts.
If you measure 470vdc at pin 8 of the GZ34, then section 1 of C16 should be lower, as indicated on the schematic.
If the amp is humming, then possibly the output tubes are out of balance. What do you get when you measure from EL34 pin 8 to ground? Should be 1.5 volts.