New output transformer speced as 450vdc max operating voltage

[pjd3]

Just noticed this.

I have two output transformers for this current build, they are the
'Musical Power Supplies - OT22PP "oversized Deluxe Reverb" with output power spec'd at 30VA.

What I didn't notice before was a spec that states "Max operating voltage - 450vdc". This is somewhat concerning because I was expecting to see a B+ of about 460vdc in this up-and-coming stereo amp. I have to assume that the 450vdc max spec is referring to the max voltage to the primary center tap however, I have never seen this spec stated on any other output transformer. It is a good-sized output transformer, every bit as big and heavy as I remember for 20 watt type amps.

Its not much extra to make a couple of B+MOSFET reducer boards to bring down the B+ a bit but, I just never expected to have to be concerned with this. Would you?

I did send an email off to the guy at Musical Power Supplies so, lets see if I get a response. I'll report back with his comment.

Thank you,
Best,
PJD3

[B Ingram]

'Musical Power Supplies - OT22PP ... a spec that states "Max operating voltage - 450vdc". This is somewhat concerning because I was expecting to see a B+ of about 460vdc in this up-and-coming stereo amp. ... I have never seen this spec stated on any other output transformer. ...

Ignore it, because it's not a useful rating.

The spec sheet also says "HiPot Test Ratings 1000Vac minimum".
The Peak Voltage = 1000Vac x 1.414 = >1.4kV ----> so how is "450vdc" a concern?


Just build your amp. If you're very nervous about the transformer, buy from a company whose product you trust (though the price could be higher).

[pjd3]

Thats why I bought from this company, because the prices are more comfortable for me at this time.

I did see the hi pot test specs, and although that is good to see, I don't find it unreasonable to wonder what the max operating voltage is all about. Why wouldn't I when its such a common practice across the board to make good note of specifications when putting together a system, especially tube amps that do have certain limits despite their headroom. If I did't want potential customers becoming concerned about buying my product, I certainly wouldn't be presenting useless specifications that held the potential to ward off customers by needlessly concerning them over something that doesn't matter.

My instincts did tell me that I'd likely be fine going forward but that doesn't stop me from being curious what that spec is about, and how it was arrived at.

Thank you,

Phil d

[Stevem]

I have never seen such a rating before, but with that being said such a rating should be stated with a max operating temperature I would think.

I can only assume that the 450 volts stated is what the enamel insulation is rated at on the primary winding wire used.

[pjd3]

Hello, and thanks for your comments.

Here's the response I received from the "guy".


"Yes.

It's OK at 460Vdc B+

I was having a similar conversation with another OT22PP customer recently. My spec should be revised to rate to 600Vdc.
Actual HiPot testing is 1000Vac (~1300 Vdc)"


So, it didn't cost much of anything to quickly inquire on this from the company owner. It snuffs out any last bit of concern on my part and he won't have any more customers wondering what that spec is all about.

Best,
PJD3

[R.G.]

Hmmm... the transformer is marked 450Vdc max, but the maker just instantly responds that it's fine at 460?

My very next utterance would have been "Can you tell me why it's marked 450V then, if it's fine at 460? Should it have been marked 460? Or 480? Will it die at 500?"

[pjd3]

Yes, in hindsight I wish I did ask why this spec was present and how it was actually determined.

I have however found other instances of inaccurate or More-than-one numerical specification for the same parameter and brought it to his attention. I believe it was the max HT current on one of his PT's, (something like both 90mA and 120mA on the same page). He thanked me for bringing it up and said he would be going back to fix it. Maybe there are a handful of specs on the site that need some attention.

I'm going to respond to his response and ask what the spec was meant to reflect, or if it was a mistake of sorts.

Thank you,

Phil D.
'

[pjd3]

OK,

Just received a very comprehensive explanation from the guy at MPS explaining the 450vdc max operating voltage spec for his OT transformer. I'll give you the first of a good handful of paragraphs that he wrote out for me. I think it does help clarity what happened with that spec.

"This value rolled forward from my initial work in 2005-2006 on my 1st couple of Guitar OT offerings.  Those were basic EL84/6V6 SE and PP output xfmrs and I was indicating these could handle a reasonable B+.  Those parts did also pass 1KVac HiPot test and 450Vdc was a very conservative value used.  My standard PT’s at the time were 380Vct/550Vct/570Vct/650Vct so that 450Vdc rating reflected this. "

So it looks like the 450vdc spec was simply a conservative value attributed to the first round of OT's that he put out many years ago when he was just lifting off with the company.

If you want to see the whole email, let me know. It gets into a more specific breakdown of tests and evolution of his product and testing. Interesting. I thought that was a pretty generous use of his time to explain all of this.

Best,
Phil D

[Helmholtz]

Specifying a max. DCV across the OT primary makes no sense at all.
There are only a few DC volts across the primary, determined by the winding DCR and the idle current.
What really matters is the max. primary AC current and the isolation voltage between primary and secondary/core.

[B Ingram]

... I don't find it unreasonable to wonder what the max operating voltage is all about. Why wouldn't I when it's such a common practice across the board to make good note of specifications when putting together a system, especially tube amps that do have certain limits despite their headroom. If I did't want potential customers becoming concerned about buying my product, I certainly wouldn't be presenting useless specifications that held the potential to ward off customers by needlessly concerning them over something that doesn't matter. ...

At the end of it all, you seem to have found ratings that aren't typically present in other-manufacturers' data, conflicting information on this company's data sheets, and a willingness/ability to instantly "change max ratings" when questioned. It seems their "data" generate questions and anxiety, when instead it should provide clarity and inspire confidence.

[pjd3]

The maker of these transformers did also mention in his last email to me that of the thousands of transformers he's shipped out, they are fine with B+ voltages up to and beyond 650vdc with little to no problems, the majority of any problems were primarily user error, such as operation with no load.

He also said I was one of two reports back to him on the 450vdc operating max, and that he will be going in to change that in any spec sheets that contain that spec. Good idea I guess.

Best,

Phil D.

[R.G.]

I'm glad the maker was so open about this. As noted, it's a surprising thing to find on an OT data sheet, both because it's an unusual thing to specify, and because the maker didn't seem concerned about it.

I can think of all kinds of business-evil possible causes for the spec to be there, but the maker being willing to just remove it makes it likely that it was just an oversight.

Of course, now the maker knows that most customers don't read the data sheets.

[pjd3]

Yes RG,

I was only one out of two people he claimed as calling out the spec. I think it's important to not inadvertently underrate product specs. I wasn't going to buy one of his PT's and started looking at Hammond and Heyboer, but then noticed another spec on a different page for the same transformer, and it was the difference between me purchasing or not purchasing it. The guy has been really stand up in terms of expediency and communication, and I'm happy to give him my business - I wouldn't want him losing business over a damn inadvertent spec. But, as you say, it looks as though people don't read specs as closely as I may be assuming.

Ha, one guy on this thread silently yelled at me for even being concerned about it - "Just build the thing!"

Best,

Phil D

[TUBEDUDE]

Good to know as I have an OT20PP for a future project. Thanks for running this down for us Phil.

[pjd3]

Hello all,

Probably not a concern either way but, thoughts welcomed.

The PT's I'm using for this stereo amp build each have both a 6.3vac/3A coil and a 6.3/5 @ 2A. Obviously, the 2A coil can be used for either signal tubes or rectifier tube heaters.

As is, the total current for heaters per amp is about 1.9A (two 6V6's plus three 12ax7's). I suspect that the PT doesn't care whether the 3A coil is used by itself to supply all of the heaters or if the two coils split the job, say use the 3A coil for the 6V6 pair and use the 2A coil for the 12ax7's. Would you be aware of any reason why one approach would be better than the other? Does the PT really care?

Thanks,
Best,
PJD3