Interesting @nerdorama. If we could divert from OP topic a little; Do you know what the mechanism was that caused your hum problem? I thought the auto-bias board is agnostic of the phasing of the push-pull pairs, controlling one tube per section, as long as the cathode sensed and bias controlled grid connections go to the same tube. Even if the wires get crossed and the grid of the first tube in the push-pull pair is controlled by the current flow in the 2nd tube, I have perhaps wrongly surmised that the longish time constant of the auto-bias would not cause issues in the hum frequency range. I don't know what the time constant is, but I thought in the seconds range (to keep it from oscillating) rather than milliseconds range.
Just check the schematic. Lift the grid end of all four of the grid coupling capacitors for the output tubes.
This eliminates everything but the power supply and output tubes and transformer.
But the OT primary leads could still be miswired, so check those very very carefully.
"we need to reroute the green/white, the blue/white, blue and green wires from the OPT primaries to different tube sockets.
The reason is that we designed the layout of our CCA to look neat and orderly, which resulted in having to reroute those wires."
Maybe someone living near you would be willing to take a look, is anyone near Denver CO?
@Francois G
The connection diagram supplied by Audioamp with the boards had the grid leads swapped to the output tubes so the drive was out of phase. Not sure I had a hum problem as I never connected speakers until I worked this out. I wrote an email back to him but I don't think he has changed the document for the Mk3. The grid bias connection and the cathode sensing both come from the autobias board. I had to switch both pairs of wires to get the amp to work correctly.
John
The connection diagram supplied by Audioamp with the boards had the grid leads swapped to the output tubes so the drive was out of phase. Not sure I had a hum problem as I never connected speakers until I worked this out. I wrote an email back to him but I don't think he has changed the document for the Mk3. The grid bias connection and the cathode sensing both come from the autobias board. I had to switch both pairs of wires to get the amp to work correctly.
John
Still working on this. Hum went down slightly when shorting the inputs but it's still clearly there. I also twisted the AC-carrying wires coming out of the transformer and elsewhere in the circuit more tightly but that didn't seem to make a difference.
I note that B+ voltage is around 382VDC without the center 6SN7 but drops to 356 with the center tube in which seems like too much of a drop based on the instructions for the kit.
Based on a question from Francois, I also tested AC voltage there (with a meter, not an oscilloscope) and it shows about 35.5 VAC.
Wondering if that's too much...and is that what's called "ripple"? What are the usual causes of AC in what's supposed to be a DC line and can that cause my hum?
Thanks all!
I note that B+ voltage is around 382VDC without the center 6SN7 but drops to 356 with the center tube in which seems like too much of a drop based on the instructions for the kit.
Based on a question from Francois, I also tested AC voltage there (with a meter, not an oscilloscope) and it shows about 35.5 VAC.
Wondering if that's too much...and is that what's called "ripple"? What are the usual causes of AC in what's supposed to be a DC line and can that cause my hum?
Thanks all!
It seems that one of the decoupling caps (I don't know its value 47 mu?) is not grounded. 35 v ac is an insanely high ripple.
@Rossputin - where are you located? You have no location info listed. I sure would love to diagnose this if you were close by.
Suggestion - go back to basics. Do you have the original board and components? May be worth installing and seeing if hum is gone. Those original circuits are so simple to troubleshoot. May be faster to help determine source of hum.
Suggestion - go back to basics. Do you have the original board and components? May be worth installing and seeing if hum is gone. Those original circuits are so simple to troubleshoot. May be faster to help determine source of hum.
Do you experts think that 1N4004 diodes are a fine choice for the solid state rectification of the amp? That's what's in this circuit now. Any chance a different diode would reduce ripple and/or noise more? I see someone online talking about Vishay UF4007.
@skidave I'm in Denver. The original stuff is all way too disassembled (and I think some is gone). There was definitely no loud hum in the original. At this point with a variety of things I've worked on, the hum is diminished and seems not to have consistent 60Hz in it but there's definitely still noise there. I wonder if 35 VAC ripple at B+ could cause it? It's almost like white noise now. No smooth consistent curves at any frequency in the oscilloscope.
I guess what I'm really trying to understand is if 35 VAC seems out of bounds at B+.
I guess what I'm really trying to understand is if 35 VAC seems out of bounds at B+.
Did you scope the output? How much voltage is the hum?
For hum, 60Hz presumably.
120Hz, we say buzz.
For hum, 60Hz presumably.
120Hz, we say buzz.
AC component of B+ should not exceed 1Vac at idle. Something is not right for your power supply. Do you use solid state rectifier or tube?. Using solid state instead to rule out the issue of the rectifier tube.
Hey @jxdking
Solid state rectification. (Looks like two 4N1004 in parallel then a current limiter then another 4N1004.)
Just to be clear, when I put my meter on B+ and set it to VAC, it moves around a lot but if I keep turning meter on and off sometimes it settles at about 35.5 VAC. That millivolt measurement was at the speaker terminal, not at B+!
Solid state rectification. (Looks like two 4N1004 in parallel then a current limiter then another 4N1004.)
Just to be clear, when I put my meter on B+ and set it to VAC, it moves around a lot but if I keep turning meter on and off sometimes it settles at about 35.5 VAC. That millivolt measurement was at the speaker terminal, not at B+!
What signal tubes are you using? I guess it is just a variant of Bob Latino VTA mod. The tubes should be low Mu like 12AU7.
You may want to consider getting a VTA or similar board and starting over. Then you have individual bias for each output tube, modern preamp tube choices and bias doesn't drift that much. Sure, if you leave it on for 24/7 & 365.
Adding all that for auto-bias and may not be worth the skin in the game. I have 3 different versions of ST-70s, including a new Bob Latino kit. All are dead quiet. I check Bias once in a while.
Adding all that for auto-bias and may not be worth the skin in the game. I have 3 different versions of ST-70s, including a new Bob Latino kit. All are dead quiet. I check Bias once in a while.
Not starting over with this one...I'm determined to solve it. And I really believe that this Miller Audio kit is likely to have fantastic sound. Mr. Miller is definitely focused on the details, quality parts, etc. I'm just a bit frustrated with the lack of availability of schematics. Still, it doesn't turn me away from thinking my choice was fine. Just need to figure out the problem!
However, I also have another ST-70 (factory wired) that I have to decide what to do with. Because it's factory-wired my inclination is to do the minimum possible, which is probably replace the selenium rectifier and the cap capacitor and just run it 99% as original.
If I catch another ST-70 at a great price I'll probably try a Bob Latino kit on that one.
However, I also have another ST-70 (factory wired) that I have to decide what to do with. Because it's factory-wired my inclination is to do the minimum possible, which is probably replace the selenium rectifier and the cap capacitor and just run it 99% as original.
If I catch another ST-70 at a great price I'll probably try a Bob Latino kit on that one.
OK, this isn't highly scientific but I recorded sound from my speakers into my iPhone microphone and then ran the frequency analysis...
These are cheap small speakers and probably can't reproduce anything below 40Hz or so.
These are cheap small speakers and probably can't reproduce anything below 40Hz or so.
If you really have 1N4004 diodes you might test them. They don't have a high enough inverse voltage rating for an ST-70. I would want two 1N4007 in series, or UF4007. They might be damaged and could cause hum and your low voltages.
Check the diodes for leakage, since 1N4004 diodes are not suitable for this HV circuit, even in series.
At least use 1N4007 diodes. Paralleling diodes does not work, so use a single 3A diode instead.
At least use 1N4007 diodes. Paralleling diodes does not work, so use a single 3A diode instead.
Even a single pair of 1N4007’s is marginal for a Dyna ST70. Two series pairs are OK, as @nerdorama indicated, but the Miller PCB accommodates a single pair One could standup two diodes vertically in series, but I now use BY448TAP bought at Mouser in situations like this.
https://www.mouser.com/ProductDetail/Vishay-Semiconductors/BY448TAP?qs=RzxYCzJDjPWLX4xNWWzARg==
Trying to understand this “Ultimate Upgrade Kit” from Miller Audio. Upon closer inspection it is apparently the continuation of the kit developed by Erhard and apparently Erhard stopped selling this board and sold the rights to Miller Audio. On top of page 11 of the Miller Instructions you will see the board in the photograph has “Erhard” marked in the upper left.
https://milleraudiollc.com/wp-content/uploads/2025/01/Dyna_70_Auto-Bias_Assembly-Manual-v1.1.pdf
Are there any builders of either the Erhard or Miller kits that have built it and perhaps have advice to offer to the OP.
One aspect of the kit I’m trying to understand is how the “ideal”, according to the instructions, B+ is 370-380 Vdc. Miller suggest a Dynakit PA060 replacement transformer and those I believe have the same voltages as the OEM by higher current capacity. The thing is with SS rectification and 360-0-360 Vac primaries the B+ must be significantly higher than 370-380 Vdc. What is going on here?
https://www.mouser.com/ProductDetail/Vishay-Semiconductors/BY448TAP?qs=RzxYCzJDjPWLX4xNWWzARg==
Trying to understand this “Ultimate Upgrade Kit” from Miller Audio. Upon closer inspection it is apparently the continuation of the kit developed by Erhard and apparently Erhard stopped selling this board and sold the rights to Miller Audio. On top of page 11 of the Miller Instructions you will see the board in the photograph has “Erhard” marked in the upper left.
https://milleraudiollc.com/wp-content/uploads/2025/01/Dyna_70_Auto-Bias_Assembly-Manual-v1.1.pdf
Are there any builders of either the Erhard or Miller kits that have built it and perhaps have advice to offer to the OP.
One aspect of the kit I’m trying to understand is how the “ideal”, according to the instructions, B+ is 370-380 Vdc. Miller suggest a Dynakit PA060 replacement transformer and those I believe have the same voltages as the OEM by higher current capacity. The thing is with SS rectification and 360-0-360 Vac primaries the B+ must be significantly higher than 370-380 Vdc. What is going on here?
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