George's most recent advice is that the power transformer should be 700VCT (350-0-350). Looking at Hammond's potted offerings I see the 373XP rated at 700VCT @ 127mA:
Hammond 373XP
* Edit: Some other rectifier tubes run on 2A heater current but that would be right at the transformer max with no margin.
- 5V/2A - this would tie me to the 5AR4 rectifier tube @1.9mA heater current but that's not a big deal to me.*
- 6.3V/4A - this should be sufficient: 2xKT88 =3.2A, 1x12AT7 =0.3A, Total 3.5A Other output tubes (EL34, KT66, etc) would need even less heater current.
Hammond 373XP
* Edit: Some other rectifier tubes run on 2A heater current but that would be right at the transformer max with no margin.
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The Tubelab web-page here gives minimum current rating at 175mA and there are designers out there who would advocate going for more.
The schematic gives options for different power supply configs and depending on what you are going to use Hammond would have suggestions in their design guide Here.
Can you say any more about what you plan to do? If you can post more detail someone with experience in your chosen config will be around.
The schematic gives options for different power supply configs and depending on what you are going to use Hammond would have suggestions in their design guide Here.
Can you say any more about what you plan to do? If you can post more detail someone with experience in your chosen config will be around.
I have looked at and studied the website for this amp extensively. The section on power transformers is based on the initial recommendation of a 750 VCT with 375-0-375 @ 175mA minimum. More recently George has recommended 700 VCT with 350-0-350. Hammonds at that level have a current rating lower than 175mA, hence my question.
I'm not "designing" anything. I have populated the standard SSE board supplied by George, exactly per his assembly instructions on the site, the only change being that I left out R1 in place of an external inductor and I did not install ss recitification with D1 and D2.
It would seem that there may not be a potted Hammond PT suitable for this design at that CT voltage rating. The 373BX would work though.
I'm not "designing" anything. I have populated the standard SSE board supplied by George, exactly per his assembly instructions on the site, the only change being that I left out R1 in place of an external inductor and I did not install ss recitification with D1 and D2.
It would seem that there may not be a potted Hammond PT suitable for this design at that CT voltage rating. The 373BX would work though.
Yah sorry. Wasn't suggesting you were designing. I was thinking of some EE's who advocate using transformers rated for higher current.
(Looking at the Hammond paper I was surprised to see that for a full wave bridge rectifier into a capacitor input filter, they recommend drawing only 62% of the transformer's ac current rating. Basically echoing what the engineers were saying. )
I didn't know what current your output tubes were going to be running at . Nor how you were configuring the power supply. The data sheet suggested potential to run them quite hot, so was thinking of that too.
Anyway, you clearly know where you're at. Good luck with the build !
Thanks
(Looking at the Hammond paper I was surprised to see that for a full wave bridge rectifier into a capacitor input filter, they recommend drawing only 62% of the transformer's ac current rating. Basically echoing what the engineers were saying. )
I didn't know what current your output tubes were going to be running at . Nor how you were configuring the power supply. The data sheet suggested potential to run them quite hot, so was thinking of that too.
Anyway, you clearly know where you're at. Good luck with the build !
Thanks
No issues here 
You have hit on the problem that doesn't seem to resolve with the potted transformer I mentioned. George recommends running KT88s at about 80mA (or even a a little bit higher). I would have to use the unpotted 373BX which is 350-0-350 @201mA or the potted 374BXP, which is 375-0-375 @201mA and learn how to drop the B+ down substantially.
You have hit on the problem that doesn't seem to resolve with the potted transformer I mentioned. George recommends running KT88s at about 80mA (or even a a little bit higher). I would have to use the unpotted 373BX which is 350-0-350 @201mA or the potted 374BXP, which is 375-0-375 @201mA and learn how to drop the B+ down substantially.
How much voltage do you need to drop? What plate voltage are you aiming at? George's chart for the KT88 gives figures for a wide range, from 350V to 500V.
That I'm going to have to take some time to study and figure out. In keeping with George's design objective, I would like an operating range that is suitable for all three families of output tubes: EL34, 6L6GC and KT88, without changing components for each type.
Of course, the short, smart *ss answer is: however much drop is necessary to deliver the same plate voltage with a 700VCT transformer as would have been delivered with a 750VCT transformer
Of course, the short, smart *ss answer is: however much drop is necessary to deliver the same plate voltage with a 700VCT transformer as would have been delivered with a 750VCT transformer
I see that the transformer with the 750VCT secondary also has a 50VAC bias winding which is not used. I think it might be possible to use that to buck the transformer, but would hopefully someone else with more experience of that can confirm. Here is the principle ...
Bucking and boosting ...
Bucking and boosting ...
Take a look at the Edcor XPWR-155 to see if it fits your needs.
https://edcorusa.com/collections/tu...cts/xpwr155-700v350-0-350-200ma-6-3v-5a-5v-3a
I've used it twice in SSE builds & have been quite satisfied
https://edcorusa.com/collections/tu...cts/xpwr155-700v350-0-350-200ma-6-3v-5a-5v-3a
I've used it twice in SSE builds & have been quite satisfied
In my case it's primarily a matter of physical design, layout and personal taste. Of course, there are miles and miles of threads about the pros and cons of potted transformers. I haven't seen any "cons" that would put me off a properly designed and professionally made potted power transformer (allowance for extra heat dissipation being a major factor).
I did look at that transformer but two things:
- I began this thread asking about a potted transformer; the Edcor isn't.
- there was a comment in a previous thread about some people experiencing "buzzing" with Edcor power transformers. I don't know if that was an isolated problem or more common but it did put me off Edcor.
I'm not at all averse to a toroid for power transformer and I did look through the Toroidy site a bit but I didn't see any 350-0-350 models (or close). If I missed one (some) a link would be great. I'll take another look.
Toroidy does not offer a 700V CT power transformer. They are also limited on ones that offer a 5V winding for tube rectification.
Their full listing is here:
https://sklep.toroidy.pl/public/assets/Mains_for_tubes_ENG.pdf
Their full listing is here:
https://sklep.toroidy.pl/public/assets/Mains_for_tubes_ENG.pdf
I have ordered different custom transformers from them and they have usually been around an extra 20%. Drop them a line.
This one is close ...
TSTA 0200/003
A few more turns on the secondary, and 6.3v instead of 5V for another secondary. Bish bash bosh!
This one is close ...
TSTA 0200/003
A few more turns on the secondary, and 6.3v instead of 5V for another secondary. Bish bash bosh!
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