The issue with hum is solved. I changed the pots that control the voltage on the Coleman regulator to full size ones and mounted them with leads to the top of the chassis for easy access. Although the pot internals are electrically insulated from the case, the contact of the case with the pot induced hum into the circuit. Breaking the connection of the pot case with the top plate immediately got rid of most of the hum. Proximity of the pots to the power transformer and PS also induces hum. The pot that is physically closer still has a small amount of hum. I will connect the pots with shielded leads and put them on the side of the wooden case of the amp, towards the front, and much further away from the power transformer and power supply. That should completely eliminate the hum.
I installed the OPTs from the Decware SE84CS amp, and they sound better than the AliExpress OPTs with a more balanced presentation, and more bass. These OPTs are 9800 to 6 Ohm. This seems to be a mismatch, but the all the Decware Zen amps, including the SE84CS, use triode strapped EL84 or SV83 with OPTs of this spec. With these OPTs, there is noticeably less volume available. I am going to try a much larger pair of OPTs sourced from a Philips R2R amp next.
Although the AliExpress OPTs are not as good full range, they may be better than the Decware ones when used in a treble amp
Gain shouldn't be an issue once the amp is being used as intended. The crossover can provide it with much more than 2 volts if required.
I installed the OPTs from the Decware SE84CS amp, and they sound better than the AliExpress OPTs with a more balanced presentation, and more bass. These OPTs are 9800 to 6 Ohm. This seems to be a mismatch, but the all the Decware Zen amps, including the SE84CS, use triode strapped EL84 or SV83 with OPTs of this spec. With these OPTs, there is noticeably less volume available. I am going to try a much larger pair of OPTs sourced from a Philips R2R amp next.
Although the AliExpress OPTs are not as good full range, they may be better than the Decware ones when used in a treble amp
Gain shouldn't be an issue once the amp is being used as intended. The crossover can provide it with much more than 2 volts if required.
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It's absolutely unnecessary, and potentially a source of future mystery troubles.
This hum problem now was avoided, but later can cause instability.
I usually set this multiturn pot after 20 min tube heating, and remains stable for years.
hello Tiz, Yes, if possible, keep the pot on the board - if the PT is close enough to cause problems, a cable will present another opportunity for coupling of noise. In all cases any cable should not exceed 50mm, 2".
Power Transformer fields can be low-impedance magnetic (B Field), which will not be much affected by cable screens.
If the top-plate is coupling noise to the trimmer, it suggests that the PT is creating eddy-currents in the plate, which may cause other problems. Is there any scope to move the PT ?
Power Transformer fields can be low-impedance magnetic (B Field), which will not be much affected by cable screens.
If the top-plate is coupling noise to the trimmer, it suggests that the PT is creating eddy-currents in the plate, which may cause other problems. Is there any scope to move the PT ?
I had moved the pots because I wanted to try the many different 30s that I have on hand without having to go into the amp each time, and as a way to to be able to easily test and compare them. I’d rather avoid the hum and put up with going into the amp to adjust though. I’ll put it back where it was.
I changed the pot back to the original one and put it in its original position on the PCB. The hum seems to be unrelated to the position of the pot, as it was basically the same after moving it back.
Perhaps I'm expecting too much? Hum is barely audible from my listening position in my very quiet room on my La Scala speakers when I use EL84s in the amp, but not audible when using 6V6s. La Scalas are very sensitive at 105 DB/W/M. This is my first amp that includes a DHT. What is reasonable to expect from an amp with a DHT driver tube through a highly sensitive speaker like the La Scalas? Is complete silence achievable?
The other amp in rotation right now is a DIY KT120, triode connected, single ended one based upon the Alex Gendrano circuit. This amp is completely silent through the La Scalas.
hello Tiz - Does this mean it was not the pot that was picking up PT field?
IT's possible to get the hum on DHTs so low that they are quiet even in preamp use.
But the very oldest low-current filaments are harder work than (say) a 4P1L or a PX4, because the structure of these are more susceptible to pickup; Low-current filaments have higher impedance, which makes it easier to impose some voltage across them by means of PT fields.
Moving the source of the field is always the best solution, since other wiring and parts are often susceptible. Rotation of the PT is sometimes enough.
But some DHTs are less troubled by external fields, and these often make good drivers, too.
IT's possible to get the hum on DHTs so low that they are quiet even in preamp use.
But the very oldest low-current filaments are harder work than (say) a 4P1L or a PX4, because the structure of these are more susceptible to pickup; Low-current filaments have higher impedance, which makes it easier to impose some voltage across them by means of PT fields.
Moving the source of the field is always the best solution, since other wiring and parts are often susceptible. Rotation of the PT is sometimes enough.
But some DHTs are less troubled by external fields, and these often make good drivers, too.
tiz....inline with Rod's comment, at first you need to identify if the hum source is PS or anything else. You may like to try CCS/choke for the 30 plate. You can try DC heater for EL84/6v6. If it doesn't work you may put metal cover for the 30. And please double check grounding scheme and orientation/location of all the iron and tubes.
In my experience and report from other DHT builders it it possible to get completely hum free all DHT amps into 100+ db sensitive speakers. Not a big deal, you will be successful soon.
Regards
In my experience and report from other DHT builders it it possible to get completely hum free all DHT amps into 100+ db sensitive speakers. Not a big deal, you will be successful soon.
Regards
Thanks for the quick reply Rod. It appears that the pot wasn't the source of hum. If it is the tube itself picking up hum, the only thing nearby is the 6.3 Volt filament heater wiring for the power tubes. If that is indeed the culprit, would DC elevating those heaters help? DC elevation is easy, and I'll give it a go if it might help. I am considering using DC heaters on the power tubes if DC elevation won't help.
With respect to the proximity of the filament regulator PCBs to the power transformer, the PT transformer is top mounted, with a small part of it under the chassis, and it has end caps so the portion below the chassis has a hole where the wires go through, but is otherwise shielded. The PT is mounted with 4 silicone washers between it and the steel top plate. While this may be the issue, moving the regulators further away from the PT didn't make any difference to the hum. The large pots that were top mounted were much closer to the PT and had about 4 inches of wire going to them. Going back to the original pots on the PCB also made no difference to the hum. I just don't think the hum has anything to do with the regulators or the pots.
The B+ is super clean, and the amp has a star ground.
With respect to the proximity of the filament regulator PCBs to the power transformer, the PT transformer is top mounted, with a small part of it under the chassis, and it has end caps so the portion below the chassis has a hole where the wires go through, but is otherwise shielded. The PT is mounted with 4 silicone washers between it and the steel top plate. While this may be the issue, moving the regulators further away from the PT didn't make any difference to the hum. The large pots that were top mounted were much closer to the PT and had about 4 inches of wire going to them. Going back to the original pots on the PCB also made no difference to the hum. I just don't think the hum has anything to do with the regulators or the pots.
The B+ is super clean, and the amp has a star ground.
I have a few pairs of metal shields that fit the 30 perfectly, and I tried them a few days ago. They did not seem to affect the hum. DC elevation or DC heaters for the power tubes are probably my next step.
DC heating of EL84 output tubes is only a good idea if the EL84s are very close to the 'victim' DHT. Otherwise, they may make the problem worse. This is because the PT fields for the EL84 heat will produce bigger peak fields (in line with the current peaks into the reservoir caps).
Twisting (and maybe Shielding) the EL84 heater wiring can be helpful, and connecting the heaters to ground at a CT (or CT faked with resistors) should suffice for the purpose of minimizing interference.
Twisting (and maybe Shielding) the EL84 heater wiring can be helpful, and connecting the heaters to ground at a CT (or CT faked with resistors) should suffice for the purpose of minimizing interference.
Rod: Heater wiring for the EL84 is tightly twisted and there is an artificial centre tap present with a resistor from each leg to ground.
The PT came from an amp that had much more current drawn from the 6.3 volt winding. The amp had 9 tubes running rather than the 2 it has now. Although I don't have specs for the PT, hopefully that means that there is room to implement a DC supply for the power tubes.
In retrospect, I suppose that DC elevation won't make a difference since the AC will still be present.
I can try shielding the EL84 heater wiring where it is close to the 30s and the regulator. That is relatively simple, and appears to be the thing to try first.
The PT came from an amp that had much more current drawn from the 6.3 volt winding. The amp had 9 tubes running rather than the 2 it has now. Although I don't have specs for the PT, hopefully that means that there is room to implement a DC supply for the power tubes.
In retrospect, I suppose that DC elevation won't make a difference since the AC will still be present.
I can try shielding the EL84 heater wiring where it is close to the 30s and the regulator. That is relatively simple, and appears to be the thing to try first.
I tried shielding the twisted pair of heater wires that go by the 30 tubes, but that didn't make any difference. I tried elevating the heaters, because that was easy to do, but that made no difference as predicted. Based upon these results, and on your advice on DC heaters, I think I have exhausted the possibilities related to the heaters on the power tubes and their wiring.
I am left with two possibilities. One is that the PT is inducing hum into the circuit through either the top plate or through direct radiation, and the other is that my DIY raw supplies for the filament regulators are to blame. I am going to use two of the Coleman raw supply boards that came with the filament regulators and see what happens.
One thing I observed that may be relevant is that the hum volume increases briefly when I shut off the amp. This is not something that I have ever come across. Normally, whatever residual hum that I have had in an amp has immediately disappeared once I switched the amp off, and the music would continue briefly with no hum until it faded off to silence. In this case, the hum gets substantially louder for about 2-3 seconds after shut off.
euro21: At this point, the transformer causing the problem is one of the few things left that could be an issue. Tonight, I will remove the transformer and use leads to put it far from the chassis and see what happens.
I have built many amps over the years and never had to debug an amp this way. I have built on steel boxes before, but not on a free standing steel top plate that was going to be attached later to a wooden box. The majority of my builds have been on aluminum top plates or in Hammond aluminum or steel boxes.
It’s unfortunate that this troublesome build is also my first build using a DHT, although I don’t think the trouble has anything to do specifically with using a DHT.