I have a buzzing Dared VP-845 amp and did bunch of tests as described below. These did not change the buzz at all.
Any help in nailing down the cause of the buzz would be greatly appreciated. I'm particularly interested in diagnosing the power and interstage tranny.
To be clear, the buzz of concern sounds like transformer buzz but it comes through speaker, and large enough to bother listening music. My power transformer gives mechanical buzz, but I don’t hear that when music is playing. It is NOT a hum that I can hear when I rotate the hum pot out of optimal position.
Here is what I did so far, all of which had no influence to the buzz.
1. Tried different 12AX, 6l6g and 845s
2. 845 bias currents adjusted.
3. Buzz is unchanged whether source is connected or not. Shorting the input signal to the ground didn’t change the buzz.
4. Tried to shield the input wiring.
5. Measured the DC and AC on the various stage of amp as recommended here to make sure they are low enough not to cause problems.
6. Padded one of the interstage tranny with silicon gasket since it vibrates a bit.
7. Shorted the grid of 6l6g and saw whether buzz is coming from tube stage before. No change.
8. Bypassed all of the caps in power section. No change in the buzz
9. Replaced the bridge rectifiers for 845 heater power supply.
For what it’s worth, the buzz does seem to change a bit over time as I turn on and off.
Thanks!
-John
Below is the main section schematic.
Any help in nailing down the cause of the buzz would be greatly appreciated. I'm particularly interested in diagnosing the power and interstage tranny.
To be clear, the buzz of concern sounds like transformer buzz but it comes through speaker, and large enough to bother listening music. My power transformer gives mechanical buzz, but I don’t hear that when music is playing. It is NOT a hum that I can hear when I rotate the hum pot out of optimal position.
Here is what I did so far, all of which had no influence to the buzz.
1. Tried different 12AX, 6l6g and 845s
2. 845 bias currents adjusted.
3. Buzz is unchanged whether source is connected or not. Shorting the input signal to the ground didn’t change the buzz.
4. Tried to shield the input wiring.
5. Measured the DC and AC on the various stage of amp as recommended here to make sure they are low enough not to cause problems.
6. Padded one of the interstage tranny with silicon gasket since it vibrates a bit.
7. Shorted the grid of 6l6g and saw whether buzz is coming from tube stage before. No change.
8. Bypassed all of the caps in power section. No change in the buzz
9. Replaced the bridge rectifiers for 845 heater power supply.
For what it’s worth, the buzz does seem to change a bit over time as I turn on and off.
Thanks!
-John
Below is the main section schematic.
Attachments
When you say you bypassed the cap,I assume you bridged them with know good ones.Im talking about the ones in the power supply.this means all caps.the hum could be from grid bias hum,high voltage hum or heater hum.Im assuming its also coming from both channels.If you missed bridging any now is the time to try this.
By bypassing, I meant that soldering + and - lead of known good caps to the circuit board traces connecting to + and - lead of power filter caps, respectively. And yes I tried this for all caps listed in the power section of schematic.
Yes, the buzz is coming from both channels, but buzz from each channels sounds a littl bit different in their pitch from each other. Some time left side is more prominent, some times not.
By the way, at one point in time, I used a 845 for which the plate was glowing a little. The tube might have been defective or its power dissipatoin rating might have been too low. But this was only for a short time period.
thanks for your help.
Yes, the buzz is coming from both channels, but buzz from each channels sounds a littl bit different in their pitch from each other. Some time left side is more prominent, some times not.
By the way, at one point in time, I used a 845 for which the plate was glowing a little. The tube might have been defective or its power dissipatoin rating might have been too low. But this was only for a short time period.
thanks for your help.
rdk845 said:
Well thats the only ideas that I had.There is something majorly wrong if the plate is glowing!The plate current must be to high.Is it possible that you are gettting some type of low freq feedback.You menson that the frequency of the buzz seems to change frequency some time so Im thinking the amp is squealing at a low freq and this may be overdriving the 845 causing it to glow red.Like someone else asked,has it ever worked right? If not it could even be a wiring problem.
Yes I bought it new and it used to have no buzz at all from speaker. The power transformer have been giving low mechanical buzz from new, but I never heard them from speaker until recently.
I don't recall whether the buzz started when the plate glowing incident happend, but it is possible. Although with new tubes there is no glow. Bias currents are set fairly low (~ 65ma) and they are stable. If a bad 845 demaged some part, what would they like be?
Is there any way I can diagnose whether low fequency feed back is there and if so, how would I fix it?
Thanks!
I don't recall whether the buzz started when the plate glowing incident happend, but it is possible. Although with new tubes there is no glow. Bias currents are set fairly low (~ 65ma) and they are stable. If a bad 845 demaged some part, what would they like be?
Is there any way I can diagnose whether low fequency feed back is there and if so, how would I fix it?
Thanks!
I'd suggest replacing the diodes in the 845 plate supply. If you were pulling enough excess current to red plate an 845, you might have poofed a diode in that FWB, or at least inflicted some serious damage, and it's just an accident waiting to happen. It would be helpful if you could o'scope and see what that noise is doing.
That would certainly account for the excess noise.
That would certainly account for the excess noise.
keithgreenhalgh said:
91K is connected to 8ohm tap. The line somehow got deleted on the picture. Sorry.
Miles Prower said:
That's a good point. I replaced both bridge rectifiers for 10V 845 heater since one of them had a diode gone bad. I checked all other diodes using multimeter in diode mod, they seemed fine, some resistance on the forward direction, infinity to the other.
Oscilloscope is a good idea, but I'm not sure where I can borrow one..
OK Heres what we know. When you first built it there was no hum so we can rule out a wiring error. All cap have been bridged so we can rule out the power supply hum.Wait a minute. If the hum is on both channels,the only thing common to both channels is the power supply. On the diagram of the output stage,there is a 100ufd 400volt cap.did you check this one too.But if it was bad there would only be hum in one channel. It has to be a problem in the power supply! Its the only thing common to both channels.One last thing you could try is pulling the 12ax7 anf the 6l6 to see if the hum goes away.
First of all, this amp is a commercial amp, so I didn't build it.
And yes, the buzz is coming from both channels, but they sound a little bit different in their pitch.
And since I test bypassed all power section caps, it is more likely a transformer or somewhere in the main section. Left side interstage transformer seems to be slight vibrating. Any way to check this transformer?
As far as main sections goes I grounded grid of 6l6g or shorted input wires ( which is connected to 12AX),to the ground and the buzz was always there. Any other way to check which stage could be at fault?
It's interesting you ask about that 100 micro /400V decoupling cap in main section, since the actual amp board differs from schematic here. Instead of 100 micro /400V for each channel, I see 1 100micro / 16 V capacitor connected to ground via 100ohm resistor, for each channel and lone 100micro /400V capacitor. I don’t know how this 100/400 cap is connected to the rest of main section, since board here is situated in a way that is difficult to detach and flip. All other part of amp seems to be same as the schematic, except in the power section for 845 heater (10V) , there are 2 8200 filtering capacitors instead of 1 per channel.
Any thought would be appreciated...
And yes, the buzz is coming from both channels, but they sound a little bit different in their pitch.
And since I test bypassed all power section caps, it is more likely a transformer or somewhere in the main section. Left side interstage transformer seems to be slight vibrating. Any way to check this transformer?
As far as main sections goes I grounded grid of 6l6g or shorted input wires ( which is connected to 12AX),to the ground and the buzz was always there. Any other way to check which stage could be at fault?
It's interesting you ask about that 100 micro /400V decoupling cap in main section, since the actual amp board differs from schematic here. Instead of 100 micro /400V for each channel, I see 1 100micro / 16 V capacitor connected to ground via 100ohm resistor, for each channel and lone 100micro /400V capacitor. I don’t know how this 100/400 cap is connected to the rest of main section, since board here is situated in a way that is difficult to detach and flip. All other part of amp seems to be same as the schematic, except in the power section for 845 heater (10V) , there are 2 8200 filtering capacitors instead of 1 per channel.
Any thought would be appreciated...
Left side interstage transformer seems to be slight vibrating. Any way to check this transformer?
If the schmetic is correct,it would be easy to check the transformer since you would not have to disconnect anything. We do not know the primary and secondary dc resistance but it should be the same on both sides.Also measure to gnd on both pri and sec and from pri to sec(this should measure open).Even if one transformer was bad,i don't see how this would cause hum on both channels. My money is on the power supply. One last option since it is a production amplifier and you have a schmetic is to take it to a repair shop and have them look at it.Since these people repair stuff of a living they could probably find the tble fast orthough it would cost more money.
If the schmetic is correct,it would be easy to check the transformer since you would not have to disconnect anything. We do not know the primary and secondary dc resistance but it should be the same on both sides.Also measure to gnd on both pri and sec and from pri to sec(this should measure open).Even if one transformer was bad,i don't see how this would cause hum on both channels. My money is on the power supply. One last option since it is a production amplifier and you have a schmetic is to take it to a repair shop and have them look at it.Since these people repair stuff of a living they could probably find the tble fast orthough it would cost more money.
The buzz you describe seems to me more like 60Hz than 120Hz. If that's the case, then either Miles is right about your bridge rectifier being wrecked or it's coming from the power tranny.
It could be picked up through poor grounding or possibly through induction between the transformers. The interstage tranny is particularly vunlnerable because it's followed by an amplifying stage but even the OP tranny could be affected. The orientation of the tranny cores is imortant to avoid inductive pickup. Although it's a commercial amp, that doesn't guarantee that transformer placement is ideal. You could try shielding with some pieces of iron or steel, if you have some handy, to see if it makes a difference.
I think with this type of buzz that making changes to the PS (e.g. bypassing the caps) could be a waste of time.
It could be picked up through poor grounding or possibly through induction between the transformers. The interstage tranny is particularly vunlnerable because it's followed by an amplifying stage but even the OP tranny could be affected. The orientation of the tranny cores is imortant to avoid inductive pickup. Although it's a commercial amp, that doesn't guarantee that transformer placement is ideal. You could try shielding with some pieces of iron or steel, if you have some handy, to see if it makes a difference.
I think with this type of buzz that making changes to the PS (e.g. bypassing the caps) could be a waste of time.
Thanks for the inputs. Here are some measurements recommended.
Yes. This does seem to be 60HZ. But there was no hum as new, so I'm thinking it is not transformer orientation. The diode seems to be O.K. but I could change that. Any way to check the power tranny?
Can you give me some direction on how to test the power transformer? The cable coming out of power tranny seem to be O.K. on eye, and resistances from gnd to solde points of these cables on pc board seem to be all megaohms.
Thanks again.
They are 0.3 ohm to ground for both of 12AX7 and 6l6g heaters. Onne of the 6.3V secondary seems to be connected to ground.SHiFTY said:
Actually the primary seems to be about 300ish and secondary is about 450ish ohm for both channels. Primary to Ground is mega ohms (I guess the 100/400v caps are charged ) and Secondary to ground is 20K and slowly increasing¡¦.keithgreenhalgh said:
ray_moth said:
Yes. This does seem to be 60HZ. But there was no hum as new, so I'm thinking it is not transformer orientation. The diode seems to be O.K. but I could change that. Any way to check the power tranny?
keithgreenhalgh said:
Can you give me some direction on how to test the power transformer? The cable coming out of power tranny seem to be O.K. on eye, and resistances from gnd to solde points of these cables on pc board seem to be all megaohms.
Thanks again.
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