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Dhoff 12th February 2008 01:58 AM

tubelab SE 300b power transformer
I'm trying to find a hammond 300 series power transformer for the tubelab SE w/ 300b tubes. Any similarly priced alternatives are welcome.

Requirements are
640 to 650 v center tap at 150++ ma
5v at 3 amps (I might use a NOS rect. that needs more current than the 2amps that is required on the tubelab site)
6.3v at 4amps

Hammond makes a 300series supply which would be perfect, but I can't find it anywhere. I'd like to purchase from Angela, as they've got good prices (and I bugged him w/ a few questions, so want to repay the favor). If another vendor has a perfect match for me, I'll go w/ the match.

The angela universal doesn't have the current needed for the fillaments.
I was thinking of trying a hammond 372JX
300-0-300 w/ multiple primaries.
Would 600v C.T. be enough power for a 300b?
My other thought was to try using the 110v primary w/ my power which is closer to 120v. I don't know the math associated w/ transformers, so don't know how much that would raise the voltage on the 2ndaries.

Thanks for your help. I understand the rest of what is going on in these amp circuits, but I don't have much (any) experience with transformers or building power supplies.

Tubelab_com 12th February 2008 02:32 AM

I have been using surplus transformers in my 300B amps that deliver 640 VCT. My supply of them is getting low, so I am searching for a new source. Some users built 300B amps using the Hammond 276X transformer which is rated for 640 VCT. The problem occurs when some Hammonds put out far more than they are supposed to. I recently saw a 276X put out 708 VCT with a 175 mA load. The no load voltage was 740 volts. This caused some grief, and I can no longer recommend the 276X transformer.

Since Hammond transformers have a reputation for too much voltage, I have ordered a Hammond 272JX transformer for my next 300B amp. I couldn't justify the higher cost of the 300 series. I will test it as soon as it arrives, but I just ordered it yesterday, and it comes from the other side of the country.

I can assume that you are in the US, and have 60Hz power. If this is true you can get away with using the 110 volt tap on 120 volts to get a little more output (about 10%). The rectifier tube will see over 5 volts on its filament which could shorten its lifetime a little.

Dhoff 12th February 2008 06:41 AM

Are you going to try the 272jx soon after you get it, or is that a project for another time?

Do you have any trouble w/ the 200series from Hammond? The over voltage thing is an issue, but you came up w/ a good solution to that. I guess the other negative I heard about them was that they hum.
I really don't want a transformer that makes noise. This will usually be playing at a low volume.

Tubelab_com 12th February 2008 03:20 PM


Are you going to try the 272jx soon after you get it, or is that a project for another time?
There is a built TubelabSE board and two Edcor OPT's sitting on my bench waiting for it, so it will be tested ASAP. My work schedule doesn't allow for Tubelab work every day, but it will likely get tested on the weekend following its arrival.


Do you have any trouble w/ the 200series from Hammond?
I have used a few Hammond 200 series power transformers, and I have used dozens of the Allied 6K7VG and 6K56VG transformers that are essentially 200 series Hammonds only cheaper. They get hot which is typical of the 200 series. I have not had any of them buzz when used with a well designed tube rectifier circuit.

Other users do complain of hum, so it may be a real issue in very quiet environments. Most of mine went into guitar amps. I built at least 10 TubelabSE's using the little Allied transformer though and there was no hum issue.

Tubelab_com 20th February 2008 02:55 AM

2 Attachment(s)
When I got home from work today there was a box from AES on my doorstep. How long do you think that it took me to get sound from the moment I opened the box?

After dinner and an hour of mindless reality TV with Sherri, I ripped the box open at 9 PM and had sound at 9:34. I played the amp for an hour. During this time the transformer remained cool. It is a relatively cool (about 68 Degrees F) tonight, so the line voltage is up to 124.5 volts. I measure 372 volts of B+ with the 300 B's biased to 70 mA each. I had 390 volts on the hot side of the choke. With a more normal line voltage, I would expect 355 to 365 volts.

I didn't take any other measurements, I just wanted to listen.

Dhoff 22nd February 2008 11:17 PM

Have you worked with this transformer anymore? The initial results sound promising.
When you were running it, did it sound alright or would you rather be running the tubes a little hotter?
(to be honest, this is my first tube amp, so I won't know the difference no matter which transformer I get)

Tubelab_com 23rd February 2008 03:10 AM

No, the amp hasn't been turned on since the initial test. I have been stuck at work until 7 or 8 PM this week. After I posted the initial report, I noticed that the 6.3 volt filament winding was puting out 7.2 volts. This is also a common complaint with Hammond transformers. The filament voltage is regulated in the TubelabSE, so the tubes are safe. The regulator however must eat this extra voltage causing it to generate extra heat. A larger heat sink is needed on the regulator IC.

I plan to do some more testing on this amp this weekend. I will be making a cabinet for it in wood shop class Monday night.

I think that this transformer is a good choice for 300B amps.

Evenharmonics 5th March 2008 03:22 AM

What are my choices?
I too have Hammond 272JX and a pair of 1627SEA OPTs. Plus I have a bunch of 50uF and 30uF motor run oil caps (both 370 V AC). I started to look into Tubelab SE amp and was wondering what my choices can be. I already have a pair of Sovtek 2A3 but am willing to try 45 or 300B if it means better sonics. By the way, I have 105db horn speaker actively bi-amped to cover 500Hz and up which is the reason why DC heater amp caught my attention. What would you recommend? Thanks.

Tubelab_com 6th March 2008 12:17 AM

I did some more experiments using the Hammond 272JX and a TubelabSE board. I found a bunch of 307A / VT225 tubes in my warehouse so I wired a pair in place of the 300B tubes and performed some testing last Sunday. It was unseasonably warm and my line voltage was 119 volts. B+ varied form 345 to 355 volts. Some prefer to operate 300B tubes with a higher voltage, and I usually tend toward running most tubes near their maximum voltage with a higher than "normal" load impedance. I would like to run 375 to 395 volts on 300B tubes, unfortunately there are very few transformers available that can make this voltage easilly. I have been using some surplus transformers that are 325-0-325 volts, but I am running out of them. The Hammond 276X or 273CZ (funny mounting) are marginal for current ratings if you crank up the tube current, and work but get quite hot. The 273BX puts out too much voltage, especially when the line voltage is high. The 372JX should be ideal since the "universal primary" allows for some fine tuning of the voltage, but this transformer is too expensive for my use.

The 300B tubes sounded good at 350 volts and I was planning to build a 300B amplifier using this combination.....until I heard the 307A's. I will build this amplifier, I just haven't decided whether it will use Chinese 300B's or vintage 307A's. The expensive 300B's are not in my budget.

The 350 volts that this transformer produces is too much for any "normal" 45's or 2A3's. The Hammond 1627SEA is a 2.5K ohm OPT. That is too low for a 45 tube, but is OK for a 2A3 or 300B. I am not familiar with Sovtek 2A3's, so I can't say if they can deal with 350 volts. The voltage could be lowered a bit by using a choke input, but I haven't tried this combination.

I have a friend who has 106db horn speakers that use active subs. Many of my amps have been played through his speakers. I have listened to all of his amps also. My favorite amp on his speakers is a TubelabSE using 45's (the Lexan amp). Unfortunately a 45 amp would require all new iron. He has a 2A3 amp that sounds nearly as good, but I don't know the details on that amp.

coresta 6th March 2008 01:35 PM

Hi Tubelab ! My SEP 307's are on the lab with a 6C6 in front .. i'll send pics in the next few days ;)

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