Tubelab SE with 45 tubes only

[operksoy]

I am planning to build a Tubelab SE amp that is fine-tuned for the 45 tubes. I have no plans to use other tubes on this one since my main goal is to get the best clarity even if it comes at low power.

For the power transformer, I am planning to use Edcor X131s. Thanks to rknize for customizing it with Edcor! With its 260-0-260v HV winding, I think it is a good choice for 45s.

I will go with Electra Print OPTs since I have read only very good things about their quality. I have a question about the OPT specs for my "45-only" TubelabSE:

Based on tubelab's website, I am planning to run the 45s biased at 30mA on 280V. To have the OPT gapping close to this, I am thinking about getting the ElectraPrint OPTs with 40mA rating, rather than the typical 50mA listed on EP website for 45 tubes. Since I am pretty new to tube amps, I am not sure if I would be better off with a 50mA OPT since most builders use the 60/65mA version for the universal Tubelab SE amp.

I would appreciate any feedback, especially if you have tried 45 tubes on the Tubelab SE.

Thanks!

[rknize]

You should know that this transformer comes in a little "hot" when used with 45s. The secondary winding was designed for 175mA, so the 260-0-260 come in higher because I forgot to take into account the lower plate current of the 45s. Also, the load on the 6.3V winding is about half as with 300Bs, so the primary and the core have lees load. The result is that you get about 265-0-265 from that winding.

This yields about 320V B+ when the amp is in "45 mode" (I'm using a choke with 82-ohm DCR). All of my 45s (about 20 or so now) work fine at this voltage, but it is technically exceeding the plate voltage spec of a 45. I believe George has done the same, though he has those NX-483s in there now.

If you are planning to use some ultra-rare NOS blue glass globe Acturus tubes, you might want to consider lowering B+ a bit. One option is to ask Edcor to tweak my design slightly and move the 260-0-260 taps to 240-0-240. That should yield 280-300V B+, depending on what choke you use.

[operksoy]

Thanks for the heads-up. I am planning to use the Triad C-14X choke with 150ohm DCR. That may help bring the B+ down a bit but probably not more than ~9v. I will check if there are other options out there.

[tubelab.com]

All of my 45s (about 20 or so now) work fine at this voltage, but it is technically exceeding the plate voltage spec of a 45. I believe George has done the same, though he has those NX-483s in there now.

I have about 15 or so 45 tubes, all very used and in varying condition from excellent to "3 for $12 on Ebay". They all work fine at 325 volts, and I think they sound better at 325 volts than they do at 275, but maybe it is just the extra 1/2 watt. I have not tried any globe 45's because all of mine were purchased for less than $20 each and you don't get globes for that price.

I was at a flea market in the middle of nowhere a few years ago when I found the chassis of an old Sparton radio lying in the dirt. It had 7 globe tubes in it with a tag that said "all tubes replaced in 11/1929." I bought it for $20. It contained a pair of NX-483 output tubes which turned out to be a 45 with a 5 volt filament. These tubes are now 80 years old and still work great, even at 320 volts.

[operksoy]

So I can use the Edcor XPWR131. That sounds great!

At 320-325v B+, I assume the 45s should be running with 25-27mAs. If that is correct, it will be safe to use the 40mA rated Electra Print OPTs (rather than 50mA) for better gapping, right?

[nicoch58]

HI not expert but lundhal suggest better use the opt near max current rated not Higth current one,normaly a good trafo have 15% safe more current then rated

[Gluca]

best thing to do is asking jack elliano himself what primary inductance his OPT will have when gapped for 40 or 50mA.

[operksoy]

After a few months of collecting all the parts and eBay bidding for a bunch of tubes, I managed to (sort of) complete my TubelabSE project. It still needs to be put into a nice enclosure before I can call it complete but it is alive and currently burning in with Miss Sarah Vaughan.

Since I am planning to have everything except the tubes, transformers and the motor-run cap under the hood, I installed the caps to the other side of the PCB.

The power transformer is an Edcor XPWR131 and the OPTs are Electra-Print. Jack was nice to provide a 3K tap on the OPTs (in addition to 5K) in case I need to use them with 300Bs in the future.

Thanks to all who have posted about their TubelabSE experiences in the past. Reading through various posts helped me a lot with this pretty smooth build.

Here are a few pictures:

[kevinkr]

Congratulations, and cool. I have always enjoyed 45 based amplifiers and am actually building a new pair of speakers to go with mine (not a tubelab) so that I can put it back in service.

[rknize]

Lookin' good!