Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: W5FH on May 11, 2023, 08:36:18 am
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Hello-
Possibly it is just my own peculiarities but I have struggled with understanding the current ratings on the HV secondaries of the Hammond 290 series power transformers. Nearly all secondaries are center tapped and used with full wave rectifiers whereby the center tap is grounded. An example is the 290-DX that shows a HV secondary 650 VCT at 230 mA. Since in nearly all cases the center tap is grounded the secondary will supply 1/2 the voltage. In my feeble mind, since this is the common usage, I felt like the secondary power rating was for only half the voltage. But a conversation today with a Hammond engineer gave me a surprise. I was told, in no uncertain terms, that in all 290 series transformers the HV secondary current rating is at the full winding voltage. In other words, the 290-DX HV secondary is rated for 650 volts at 230 mA (leaving center tap unused/floating such as with a bridge rectifier). I was originally thinking the secondary winding current rating of 230 mA was for only using a full wave rectifier with the center tap grounded (thus yielding 1/2 the voltage).
What brought this to light with me was the use of the 290-HX which is the replacement for the Marshall JCM800 100 watt series. Attached is the 290-HX datasheet. It shows a 350 VCT HV secondary rated at 420 mA. In the Marshall power supply the center tap is unused and left floating; a bridge rectifier is used. Thus the full voltage of secondary is used in this case, very much out of the norm for most designs.
One of the things that helped confuse me was the maximum primary current rating shown on data sheet. This specification is not the loaded primary current. It is the primary excitation current. I was looking at this current figure thinking it was related to maximum current allowed in the primary during use, thus yielding power capability.
I was surprised that the 290 series HV secondary current rating is for the full secondary voltage. But after learning this let me ask - - in the case of the 290-DX with the 650 VCT at 230 mA HV secondary. This represents 149.5 watts of power. Is is correct to say that if using this secondary with a full wave rectifier/center tap grounded, at 1/2 the voltage, that the current draw could be increased in order to yield the same power? I am suspicious that the current rating would probably still be near the 230 mA rating even at 1/2 the voltage due to wire size in transformer or principles of operation in the transformer.
I asked Hammond if they could clarify this in their 290-series product info description, and give VA ratings on these transformers.
Just looking for a little help clearing these clouds out of my head!
Thanks all.
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... I asked Hammond if they could clarify this in their 290-series product info description, and give VA ratings on these transformers. ...
The 290HX data sheet that you attached says "150VA" on the label in the dimensional drawing.
Though most people aren't shopping by "VA" in Hammond's "Guitar Replacement" lineup, which is why they don't make it as obvious as they do on their other power transformers. Most people just pick based on amp-model.
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Thanks for the reply Hot Blue Plates-
I did not see that "150VA" on the label, thank you for pointing that out. So, 150 VA rating equates to 150 watts, correct?
If you add up all of the secondaries full wattage ratings (using the full 350 volts at 420 mA on HV winding) it totals 195.9 watts. If you use 1/2 the HV secondary winding (175 volts at 420 mA) the transformer wattage is 122.4
This is what confuses me about the Hammond ratings and their engineer telling me the 290 series HV secondaries are rated for their full stated current at the full voltage (not 1/2 voltage on center tapped ones).
Can someone set me straight on this, thanks.
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Imagine the center tapped output winding as 2 separate windings, with one end of each connected together. If using the 290DX without the center tap, the 2 windings are in series, that will deliver 325v@230ma each. That gives you 650v@230ma (149.5va). If you use the center tap, you now have 2 windings in parallel, that will deliver 325v@230ma each. That gives you 325v@460ma (149.5va). When you use the center tap, the voltage is cut in half, but you now have 2 windings in parallel, which doubles the current capacity.
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What’s weird is that the mains input of 100W Marshalls notes that the equipment can draw 375W.
See p5 https://cdne-marshall-assets.azureedge.net/content/uploads/3cd92104-9519-4ad9-a3c4-671d2808d301.pdf?sv=2021-08-06&spr=https&se=2023-05-15T15%3A56%3A52Z&sr=b&sp=r&rscd=attachment%3Bfilename%3DM3311.397_1959HW+QSG_CATS-00262-02_web.pdf&rsct=application%2Fpdf&sig=S9rPhKB44mFJPr4iOquwpZXdpwa7ki8uVLIZNdZPcak%3D
Dunno how that lines up with its mains transformer only being rated for 150VA?
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Could that 150va rating be the no load primary draw? I really don't see any reason to list that as a specification though.
Edit: I think that 150va on the 290HX label is a typo. I have a Hammond 373HX here, which is another 100W amplifier transformer and is rated at 369va. Adding up the secondaries (282.1va of HV, 56.7va for 6.3v filaments, 30va for 5v filaments) comes to 368.8va. Seems to line up perfectly with the va rating on the label.
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Could that 150va rating be the no load primary draw? I really don't see any reason to list that as a specification though.
I don’t think so, 200mA (@ 120V) is the upper limit for acceptance (only 24W).
Edit: I think that 150va on the 290HX label is a typo. I have a Hammond 373HX here, which is another 100W amplifier transformer and is rated at 369va. Adding up the secondaries (282.1va of HV, 56.7va for 6.3v filaments, 30va for 5v filaments) comes to 368.8va. Seems to line up perfectly with the va rating on the label.
That looks to be a somewhat bigger, beefier transformer though.
The laydown version of 100W Marshal mains transformer 290HZ notes 196VA
https://www.hammfg.com/files/parts/pdf/290HZ.pdf
So yes, 150VA for the 290HX seems incorrect.
As other than the mounting, the 290HZ seems identical.
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Yes, it's definitely a beefier transformer, but my point was not to compare the 2. I was just pointing out that the 373HX label states that it's 369VA and that matches the VA of all the secondaries summed. The 290HX isn't even close. The label of the 290HZ that you linked to also matches the va of all its secondaries exactly (196va).
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Tender, you're right.
Hammond -- as much as I love their Z mounted transformers (I do !) -- do not market their trannies based on VA, as other manufacturers (mostly the toroid guys) do. It's just the nature of business practices. Hammond has their way, Edcor another, and Mercury Magnetics even less information, all different. All make great transformers, and I buy from all three companies.
We just have to figure it out. And yes, it is irritating.