Make sure you measure hum with the input shorted instead of open. Removing the RCA cable is one thing, shorting the input on the amplifier is another.
Regards, Gerrit
Regards, Gerrit
HI Francois, yes, I checked the voltages and find that B+ with all tubes on is a bit low at 352 when optimal is 365-380. All other tests noted in the manual seem fine.
I will look for more resources on testing EL84/6CA7. I did test pin 5 voltage (to ground) and they're around -37 which I think is a little too low.
Still hunting...I can't believe what an intractable problem this is.
Oh, I note that the hum sometimes also has a sort of rushing sound and little bits of high-pitched sound like air coming out of a balloon when you pinch the end of the balloon mostly closed. But those sounds are short-lived (and not quite as high pitched as the ballon but couldn't think of a better comparison.)
I will look for more resources on testing EL84/6CA7. I did test pin 5 voltage (to ground) and they're around -37 which I think is a little too low.
Still hunting...I can't believe what an intractable problem this is.
Oh, I note that the hum sometimes also has a sort of rushing sound and little bits of high-pitched sound like air coming out of a balloon when you pinch the end of the balloon mostly closed. But those sounds are short-lived (and not quite as high pitched as the ballon but couldn't think of a better comparison.)
When you hear the hum and switch off the power (or even better the heaters) is the hum immediately gone, or slowly fading out.
If it’s gone immediately it could be the heater supply picking up hum.
And you have to make sure whether it’s 60 Hz Or 120 Hz. 120 Hz. will be power supply noise, 60 Hz. is the heater supply (if you’re using AC for heating.
Regards, Gerrit
If it’s gone immediately it could be the heater supply picking up hum.
And you have to make sure whether it’s 60 Hz Or 120 Hz. 120 Hz. will be power supply noise, 60 Hz. is the heater supply (if you’re using AC for heating.
Regards, Gerrit
It appears to me that the two capacitors that provide an AC ground for the filaments are not connected on one side. Green with stripe to right but brown with stripe appears to be connecting to the center grounded tap of the strip. Might be just an illusion of the picture.
Good luck and happy hunting.
Good luck and happy hunting.
Trying to follow the black wires from the choke. Hard to tell in the picture. Do they both go through the hole to the top board? Almost appears that two black wires go to the ground tab from that side.
Hey Gerrit, It's so noisy that I really can't tell on a scope but when I put a multimeter on the speaker wires, it shows 60Hz mostly...but bounces around a bit.
Heater voltage seems to be about 6.37 VAC. Slightly above target of 6.3...same on both sides.
I don't have a way to turn off just the heaters but I'll try your test when I get home.
I note that the way the system powers up is that the auto-bias board warms up, the green lights indicating that things are working fine come on in about 20 seconds. It's at that time that I can first hear a very quiet hum, and then over the next 10-15 seconds the hum gets much louder and then levels off at some very annoying level.
Is it possible to have 120Hz and 60Hz from the two sources (rather than just as some kind of harmonic from the 60 Hz)?
What I'm noticing today (and I don't think this was the case before) is that when all tubes are running, B+ drops to 352 VDC with wall voltage at 119;7 VAC. Raising input voltage to 120.7, B+ goes to 355.7 but still below target of about 370 VDC. Any thoughts on that?
Maybe I have two different problems going on?
This -37 V value is in the ballpark. What matters is what the cathode current is. I believe those 10 Ohm resistors on the auto-bias board mean that the auto-bias board is trying to adjust the negative bias on pin 5 so that 400 mV is maintained over the 10 ohm resistor, meaning that you are passing 40 mAmps at each EL34 cathode. Is that what you observe?
Based on you description of the sound you hear I’m beginning to wonder if you got the phasing of the feedback wrong. There is a 50% chance of getting that wrong, and the sound is typically described as motorboating, howling or sqealing. Perhaps the action of the auto-bias board reduces the howl to more a polite air escaping from a balloon.
If you know where to disconnect the global negative feedback try the amp without feedback. It will be more sensitive (higher gain) and perhaps higher distortion, but the hum/squealing will be gone. The way to fix the problem then is to reconnect the feedback to the same place, but switch the primary wires from the output transformer in each PP pair of EL34s. (Since it is a UL PP the screen wire has to match the plate side correctly).
The reduction in B+ voltage could be sagging due excessive current draw with the oscillation. Focus on the noise first and then come back to the B+ level later.
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A simple way to determine if the power supply has excessive ripple: remove all four coupling capacitors to the output tubes.
Connect amp to speakers. Has the hum changed? If not, there may be excessive current being drawn from the power supply,
causing high ripple. Cause could be the "new" biasing circuit.
If the hum did stop, you likely have a marginal instability, causing oscillation.
Do check all the OT primary connections for wrong connections for the plate and screen.
Connect amp to speakers. Has the hum changed? If not, there may be excessive current being drawn from the power supply,
causing high ripple. Cause could be the "new" biasing circuit.
If the hum did stop, you likely have a marginal instability, causing oscillation.
Do check all the OT primary connections for wrong connections for the plate and screen.
Don't know if this is your problem, but I installed one of these autobias boards in some Dynaco Mk2's. The instructions for connections to the tubes and the main board had them backwards. Things were out of phase. I switched the tubes that the bias board and main board attached to and then it worked correctly. I found it when I disconnected the global feedback and the sensitivity went down rather than up.
One other thing: I don’t see the CCS for the phase splitters in the photos you posted. Please recheck the instructions on page 10 regarding the orientation of the SMT-334. If the phase splitters don’t function correctly it could also cause the noise you describe.
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Hey there, the SMT334s are soldered underneath the board to avoid tube heat, per an addendum to the instructions. I spent a VERY long time making sure the polarity was correct per the instructions!
I do know how to disconnect the negative feedback. In fact those little negative feedback boards at the back of the amp were optional addons and like an idiot I didn't test the amp without them first. I could try disassembling those from the amp. Or could probably just disconnect the feedback wire?
@Rossputin,
I didn’t read how the feedback is applied in your Miller Audio mod, or about the “negative feedback boards”, but if you disconnect the wire (that brings feedback to the front tube(s)) at the connection with the transformer secondary, you will definitely know if that is the source of the noise problem. If that quiets the noise reconnects the feedback wire and then reverse the output primary wires on each pair of EL34. Measure the primary resistances to make sure you have the correct screen wire mated with each plate wire. Alternatively, another way of remedy is to switch the wires from the PCB to pin 5 (control grid) on each EL34, but I’m not sure about your mod’s circuitry, so the first is probably better.
I didn’t read how the feedback is applied in your Miller Audio mod, or about the “negative feedback boards”, but if you disconnect the wire (that brings feedback to the front tube(s)) at the connection with the transformer secondary, you will definitely know if that is the source of the noise problem. If that quiets the noise reconnects the feedback wire and then reverse the output primary wires on each pair of EL34. Measure the primary resistances to make sure you have the correct screen wire mated with each plate wire. Alternatively, another way of remedy is to switch the wires from the PCB to pin 5 (control grid) on each EL34, but I’m not sure about your mod’s circuitry, so the first is probably better.
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This is a close to stock ST70 for comparison. The RCA inputs are on the front with very short connections to the driver board. I presume your leads are screened - where is the screening grounded?
How did you deal with the earth lead in the mains cable and where is the chassis earth? Should ideally be screwed to the chassis with a dedicated M4 connection + star washer, next to the power inlet.
Is everything grounded to that 3 way connector in front of the power transformer, in your picture? I think I would be tempted to improve the heater wiring a bit, hugging the chassis edges with the properly twisted wires. Then maybe consider lifting the circuit ground with a heavy duty diode and 10R resistor. Where is the last PSU capacitor? That is where I like to ground the different parts of the circuit.
How did you deal with the earth lead in the mains cable and where is the chassis earth? Should ideally be screwed to the chassis with a dedicated M4 connection + star washer, next to the power inlet.
Is everything grounded to that 3 way connector in front of the power transformer, in your picture? I think I would be tempted to improve the heater wiring a bit, hugging the chassis edges with the properly twisted wires. Then maybe consider lifting the circuit ground with a heavy duty diode and 10R resistor. Where is the last PSU capacitor? That is where I like to ground the different parts of the circuit.
Good comment from "oldhector". The amp shown is a VTA-70(not a dynaco but a tubes4hifi variant that uses a differenct driver).
It has it's own gremlin : as the chassies is stainless grounding via "simple clamping" is not reliable unless a very high pressure
is applied.
It has it's own gremlin : as the chassies is stainless grounding via "simple clamping" is not reliable unless a very high pressure
is applied.
HI Hector,
I'll do my best to answer your questions:
I used shielded two-conductor wire in my effort to move the RCAs to the back. The shield was connected (by an additional length of unshielded wire) to the chassis "star ground". However, in beginning to try to find the source of the hum, I removed the wires entirely and at the moment the RCAs are not connected to anything. It made no difference in the hum.
The power cable is shielded with the plug-end of the shield tied to the green ground wire and the amplifier end of the shield not connected to anything.
All ground connections within the amp are to the "star ground" in front of the power transformer. That includes the ground wire coming in through the power cord.
Not sure how I would figure out what the "last PSU capacitor" is...
I've thought about whether slightly different positioning of the wires could be an issue but when I have the amp on and am hearing the hum and I try physically moving wires around (obviously can only move them a short distance) nothing seems to change the level or tone of the hum.
Disconnecting the feedback circuitry from the positive speaker terminal made the hum louder. I was hoping that would be it but it isn't.