Carl's DIY adventures

[carlman14]

Quick update: I hit my first major roadblock. I got some nice EL33s from Billington (which has been a pretty frustrating experience to be honest). I've been testing out the EL33s in my bottlehead crack which has been modified to take 6J5 tubes. To make it EL33 compatible, all I had to do was connect grid 2 to the anode with a 100R resistor and bam! EL33 compatible. The operating point isn't good (45V on the plate, 1.8V bias), but it serves as a good tester to make sure the tubes are working properly before I have my amp built.

Anyway, the EL33 seems to induce a very audible 120hz hum into the headphones. It happens with all 5 of the EL33s I have, but doesn't happen with any other input tube. I have used 6J5s for a long time in this amp, and also tried EL3Ns with EL3N -> 6J5 adapters, and there's no hum at all with any of those. So I'm pretty sure it's not the amp's fault. I ended up ordering a pair of EL33s of a different brand to see if they have the same problem. If they do, I will unfortunately have to rethink the amp and my tube choice.

 

[HiGHFLYiN9]

Pretty tubes, a shame they're problematic for your objectives.

 

[Tjj226 Angel]

I really doubt it's the tubes.

Have you made sure that the heater pins are grounded with a resistor divider?

 

[carlman14]

I really doubt it's the tubes.

Have you made sure that the heater pins are grounded with a resistor divider?

The 6.3V heater winding of the PT has a center tap, and that is grounded, yes. Also, the heaters are AC. So if the heaters were the problem, I'd expect the hum to be 60hz, not 120hz.

I'd like to think it's not the tubes too, but I'm running out of other explanations. The one difference between EL3Ns and EL33s is that the outer metal shell is grounded through the cathode on EL3N, but is grounded through a separate pin on the EL33 (pin 1, the same pin that is the metal shield on 6J5s). So I tried connecting pin 1 to the cathode to simulate how the EL3N is set up, and that didn't help either. It's confusing because I have no problem with the EL3Ns, and they're basically the same tube as the EL33.

 

[herdingunicorns]

So cool to see the progression

 

[Tjj226 Angel]

The 6.3V heater winding of the PT has a center tap, and that is grounded, yes. Also, the heaters are AC. So if the heaters were the problem, I'd expect the hum to be 60hz, not 120hz.

I'd like to think it's not the tubes too, but I'm running out of other explanations. The one difference between EL3Ns and EL33s is that the outer metal shell is grounded through the cathode on EL3N, but is grounded through a separate pin on the EL33 (pin 1, the same pin that is the metal shield on 6J5s). So I tried connecting pin 1 to the cathode to simulate how the EL3N is set up, and that didn't help either. It's confusing because I have no problem with the EL3Ns, and they're basically the same tube as the EL33.

Not grounding your heaters can cause all sorts of problems for all sorts of reasons. Plus its a very easy thing to check. Trust me when I say that you should never discount the heaters as the source of your problems and don't ask me how I know that lololololol.

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Now when you say "the outer metal shell is grounded", do you mean that the actual base of the tube is grounded?

If so, try lifting your tube out of its socket by a 16th of an inch to get the base to sit a little bit above the top plate. Maybe the base is making a ground loop with the top plate.

 

[carlman14]

Not grounding your heaters can cause all sorts of problems for all sorts of reasons. Plus its a very easy thing to check. Trust me when I say that you should never discount the heaters as the source of your problems and don't ask me how I know that lololololol.

I believe you! A while back I restored an old HH Scott LK-72, and it's heater was giving me problems until I grounded it. I have no idea why it wasn't grounded in the original design lol.

Now when you say "the outer metal shell is grounded", do you mean that the actual base of the tube is grounded?

If so, try lifting your tube out of its socket by a 16th of an inch to get the base to sit a little bit above the top plate. Maybe the base is making a ground loop with the top plate.

No, the base isn't grounded. The EL33 and EL3N don't have a "metal shell" per se, it's just what I've been calling it because it has the same symbol on the datasheet as other tubes with metal shells. It's actually a single thin wire that wraps around the tube that sits directly above the base. Internally it's connected to pin 1. Attached the pic from earlier with that wire highlighted.

Anyway, the heater should be ok with the grounded center tap. I'll try what you suggested, and I have a 6.3V transformer handy too that I can try swapping in. Other than that, who knows? Hum is my worst nemesis. I hate when I can't figure it out.

 

[carlman14]

Good update! After trying everything under the sun to quiet these tubes down, I finally did it! The solution was to heat them with DC. This is the first indirectly-heated tube that I've had to DC heat in order to quiet down. I'm just glad I found something that worked. Now, onward! I got a notification from Toroidy that my PT has shipped, and I'm about to submit my final chassis design to landfall systems. So implementation should start moving pretty quickly in a couple weeks.

 

[carlman14]

My chassis got delayed due to some unfortunate mishaps during it's production... So I have to wait another week for it. It will be a deep blue/gray color. Super excited to see it! Meanwhile, here's some other goodies that arrived that are going into the amp:

The electricity donut. Do not eat! This is the toroidal power transformer that will be underneath the chassis. I am also intending on building a mini steel enclosure for it just in case I need extra magnetic shielding. Something I learned through testing this thing is that toroidal transformers have a tendency to have high inrush current. I was popping fuses that didn't pop with other non-toroidal transformers. So my PSU will have 1 or 2 inrush current limiters in series with hot to limit the large inrush current.


Some nice NOS octal sockets. First time using NOS sockets.


And finally, the EL33 tubes! I found out that a lot of these older EL3* tubes have poor heater-cathode insulation, which can cause the hum I was hearing. Luckily heating with DC avoids this issue, and I can now run these dead silent! I am so pumped to see (and hear) these working in the amplifier.

 

[carlman14]

I finally received the chassis! Unfortunately Landfall has some improvement they need to do with their powder coating, so it took 3 different blues before we landed on one that they were able to do well. It's called casper blue. It wasn't my first choice, but it still looks really nice. I can finally start the build! I'm going to take my time with it, but it should be very fun. Stay tuned for updates!

 

[carlman14]

Slow but steady progress! Here's some updates.

Gyrators are calibrated to 150V and working correctly. These will be the anode loads for the input tubes.

The inside as of today. Gyrators are in place and most of the PSU is set up. The High/low HV switch is working and provides 260V in low mode (For EL33 and similar tubes), and 360V in high mode (for every other output tube). A nice little optimization without having switch between resistors on the output tube cathodes. I've been told several times that the cathode bias should be as simple as possible, so that's how it's staying! Included a pic of where the toroid PT is going to sit. My little internal chassis I made for it was a success and fits beautifully. It's made of steel, and it's purpose is to block any magnetic fields radiating from the PT into the signal. I don't know for certain if that's necessary for a toroid, but better safe than sorry.



And a sneak peak for how it's going to look when finished. The blue is very pretty and works well with the dark gray transformers.


On a side note, my SSD died last week, and since I didn't back it up like an idiot... I lost all my datasheets, chassis design files, load line drawings, diagrams, LTSpice schematic and models, etc. It sucked. I'm working redoing the schematic, but the distortion and frequency response graphs looked great. Distortion was between about 0.2% - 5% depending on load and output level. For headphones it will be on the lower end because their power requirements are so low. Frequency response looked great and stretched beyond human hearing in both directions before dropping off. I might post those later if I get everything in LTSpice set up in a reasonable time.

 

[HiGHFLYiN9]

Doh, that's very unfortunate. I recall Seagate had a pretty good data recovery software that I was able to use to recover someone's photos. Some were corrupt but we got a lot of them.

 

[carlman14]

Doh, that's very unfortunate. I recall Seagate had a pretty good data recovery software that I was able to use to recover someone's photos. Some were corrupt but we got a lot of them.

I can usually recover a faulty drive. I've used testdisk on linux before to recover lost data and fix corrupted partitions/sectors. However, this one seems really bad. I've run it through several data recovery programs, and none of them can find the corrupted partition. It's sad, but luckily most of my stuff is on my other drive (which is now backed up!). I mostly just lost files related to this project and save files for video games.

 

[carlman14]

It's been a long time since the last update, but it's finally done! I took a couple months off to focus on other things, but it's done! For a while there, I didn't think I'd be able to get it quiet. Turns out, having the PT under the chassis close to the entire circuit is asking for all sorts of trouble, and was inducing a lot of hum. Who would have thought? But I'm super proud that I managed to get it completely silent! To get it silent, I had to experiment with different brands of toroid transformers, and found the antek transformer to have the least amount of stray fields. I also had to shield it with a steel cover, also from antek. Then I had to rotate it around to find the point where the magnetic fields leaked the least into the circuit. Overall it was pretty difficult, and I probably wouldn't build another amp this way. Power transformers belong on top of the chassis or in a separate chassis.

Some final specs:

• Input tubes (single triode sockets): 6J5, EL33, 6C4 + many more (with adapter), and all equivalents
• Input tubes (dual triode): 6SN7, 6SL7, 12A/U/X7 (with adapter), and all equivalents
• Output tubes: EL33, EL34, EL38, KT66/77/88, 807 (with adapter), and all equivalents
• Input section: gyrator loaded, DC heated
• Output section: 5K:8/16/32 output transformers, AC heated
• B+: 250V or 370V depending on selection switch on the rear. 250V provides happy a operating point for EL33 output tubes, and 370V is for every other output tube. The 250V setting can also be used as an alternative operating point for EL34, KTxx, etc.
• Rear: RCA inputs, speaker taps, large heatsink for the maida regulator, impedance switch for 8/16/32 OPT secondaries.

And here's some listening impressions:
Mine vs Mogwai SE:

• I used as similar of a tube set as I could, but they differed slightly.
• Mine had more detail retrieval, most noticeably in the subbass. There was one occasion where I heard a bass note that was missing when listening through the Mogwai. It was subtle, but definitely there.
• I heard some slightly better detail retrieval in the mid-range too.
• Bass definition is slightly different between them, but both good.
• I swapped the input tubes to EL33s, and the texture of the bass became bloomier than the Mogwai, but still good. It was nice for orchestral and jazz.

Mine vs RH84 (Again, different tube sets may change the comparison):

• Bass is different on each of them. The RH84 has "fatter/bloomier" bass, whereas mine has "punchier/faster" bass. The subbass detail retrieval difference is present in this comparison too.
• Highs seem to be slightly more present/extended on my amp than the RH84.
• Overall clarity is a step above the RH84. It makes the RH84 seem a little "veiled" in comparison. The RH84 isn't veiled on it's own per se, but in direct comparison it felt that way sometimes.

Other random notes:

• I'm really enjoying the EL33s as driver tubes. They definitely give the sound more body.
• The sakuma-style with the Mazda EL33s (white base paint) in both positions was really harsh. Not a good combo. But the mullard EL33s in both positions was much better.
• Favorite combination so far is EL33s drivers and TT21 outputs.

Overall I'm very happy and impressed with how this turned out. There's still a couple minor things that need to be done, but for now I'm just enjoying the music!

Now, some pictures.

Sakuma-style with Mazda EL33s in both positions:


Mullard EL33 driving TT21:


My tester tubes when I was testing circuit changes constantly. New production 6SN7 driving 6L6GCs:

The rear:

 

[carlman14]

Favorite combo so far: Made-in-France EL33s driving GEC KT8Cs paired up with the Verite Closed. Punchy, clear, and resolving. This sounds dang good.