Current Ratings for amp and Power Transformers

[Gtr0]

Hi Everyone (who reads this :-) )

Something, for a long time, has befuddled me to a point that I ignored it... until now. That is current ratings of power transformers and as a secondary topic, estimating amplifier current needs for power supply etc.

When trying to guesstimate current requirements of an amplifier, does one draw conclusion from zero-signal plate and screen current readings or maximum-signal? Depending on the tube, they can just about double. In the case of a pair of 6L6GCs, RC-30 quotes zero-signal at 116mA and 210mA at max-signal.

I assume that we use zero-signal since this is how we take all of our measurements etc.

I also assume zero-signal because most replacement transformers for, say a Marshall 1959, quote the secondary at 300mA (Dagnall) to 420mA (Hammond). However, zero-signal currents on plates, screens, and 3x 12ax7 (assuming .003A) would equate to some 309mA requirement.

Do I have this all right?

[Helmholtz]

Do I have this all right?

It is certainly not that simple. First of all transformer currents are RMS AC and not directly comparable to the DC currents after the rectifier. While it is clear that AC power must at least equal DC power, AC and DC voltage levels are typically different, so currents must be different as well.

It is not quite clear to me how manufacturers rate their power transformers and if all of them use the same rating method. I did some power calculations and it seems that current ratings are often based on some mean or time averaged amp power. Means that there are unknown assumptions about average power consumption/output involved.

Hammond specifies their 50W PT at 150mA@690V corresponding to a power of 103.5W and the 100W PT at 420mA@350V corresponding to 147W. While the power of the 50W PT corresponds to the calculated power demand at full output, the 100W PT value does not.

https://www.hammfg.com/files/parts/pdf/290GX.pdf
https://www.hammfg.com/files/parts/pdf/290HX.pdf




(From your post above it is not clear if you mean a 2x6L6 amp or a 4xEL34 (Marshall 1959) amp.)

[Gtr0]

(From your post above it is not clear if you mean a 2x6L6 amp or a 4xEL34 (Marshall 1959) amp.)

Yea, sorry - I was referring to EL34, but I am working on an amp that has 6L6GC ATM, so I was reading both pages in the RC-30. I pointed out 6L6GC because zero-signal to max-signal nearly doubles, zero-signal = 116mA and max-signal = 210mA, where it does not with EL34 (according to the RC-30 info), EL34, respectively, shows 120mA and 143mA.

[Helmholtz]

where it does not with EL34 (according to the RC-30 info), EL34, respectively, shows 120mA and 143mA.

According to my EL34 (Mullard) datasheet, max signal plate current (for Pout=55W and Vp=425V) output is 2x120mA = 240mA. (Zero signal plate current depends on biasing and is not relevant here.) The DC power demand for the same output power varies very little with tube type.

What is the RC-30?

[Gtr0]

Sorry, RC-30 is the 1975 RCA Receiving Tube Manual - RC-30 is the revision.

Just out of curiosity - what is your datasheet resource? I always to to the RCA tube manual. But of course if I remember correctly, Mullard defined the EL34 if not straight up invented it.
Duncan Amps has a neat resource for datasheets with a search.

[Gtr0]

While it is clear that AC power must at least equal DC power, AC and DC voltage levels are typically different, so currents must be different as well.

Hammond specifies their 50W PT at 150mA@690V corresponding to a power of 103.5W and the 100W PT at 420mA@350V corresponding to 147W. While the power of the 50W PT corresponds to the calculated power demand at full output, the 100W PT value does not.

Does this include that the 100 watt version requires the use of a "voltage doubler" to get up to +/-494 VDC - or am I blowing smoke out of my ass and totally not understanding any of this? haha. I am trying!! I swear! Your the resident Marshall genius, as far as I can tell, and I would absolutely love to understand this.

[Helmholtz]

Sorry, RC-30 is the 1975 RCA Receiving Tube Manual - RC-30 is the revision.

Just out of curiosity - what is your datasheet resource? I always to to the RCA tube manual. But of course if I remember correctly, Mullard defined the EL34 if not straight up invented it.
Duncan Amps has a neat resource for datasheets with a search.

Thanks, this book I have. The EL34 example given is for a 40W stage with cathode bias and very high (60mA) idle current. So it is not comparable to the other 55W fixed bias examples. But if you add max plate (143mA) and screen (44mA) currents and multiply with supply voltage (450V), you get 84W DC power consumption at full power. This again confirms that DC power demand with class AB is always a little higher than twice the output power, meaning a power conversion efficiency of 84/40= 48%.

[Helmholtz]

Does this include that the 100 watt version requires the use of a "voltage doubler" to get up to +/-494 VDC

Could you elaborate? Not sure if I understand.

You don't need a "voltage doubler" (I actually don't like this term being used for the Marshall type fullbridge rectifier with CT) to get 500VDC. The "voltage doubler" circuit would work just the same without the center tap (but you would need at least 600V rated filter caps). Without the CT everybody would call the circuit a standard fullbridge rectifier. The center tap only balances the voltages across the stacked filter caps, otherwise no net current flows through the center tap.

[Helmholtz]

I collect tube databooks as well as paper copies of datasheets I find on the web.

The pentode (e.g. EL34) was patented by Philips/Holland. Since 1927 Mullard (and a number of other tube manufacturers like Valvo and Mazda) belonged to Philips.

The beam power tetrode (e.g. 6L6, 6V6, KT66) was invented by EMI/UK as an alternative to power pentodes and to avoid paying licence fees to Philips. But they sold their patent to RCA who produced the first 6L6.

[Justin Thomas]

As for tube data sheets, I still think tubedata.org is thebest thing out there. Often, for common amp tubes, Frank has multiple sheets from different manufacturers, and also sheets for multiple iterations of the same tube. It's a bit cumbersome at times, but you can usually compare & see RCA & Sylvania & Tung-Sol all for the same tube, differences in how they rate, etc.

Justin

[Dave H]

Hammond specifies their 50W PT at 150mA@690V corresponding to a power of 103.5W and the 100W PT at 420mA@350V corresponding to 147W. While the power of the 50W PT corresponds to the calculated power demand at full output, the 100W PT value does not.

Perhaps the PT current ratings are something to do with the 690V PT being intended for a 2 diode full wave rectifier whereas the 350V PT is for a bridge rectifier? Hammond states in their “Design Guide for Rectifier Use” that the DC current available from a 2 diode full wave rectifier is 1 x Transformer RMS current. i.e. 150mA for the 50W PT. For a bridge rectifier they say the DC current available is 0.62 x Transformer RMS current. i.e. 260mA for the 100W PT. Neither transformer is rated to supply enough DC for continuous full power output (250mA DC for 2 EL34 at 50W)

[Helmholtz]

Perhaps the PT current ratings are something to do with the 690V PT being intended for a 2 diode full wave rectifier whereas the 350V PT is for a bridge rectifier?

Yes, there are probably different power factors to be taken into account. Will look up the Design Guide tomorrow.

Neither transformer is rated to supply enough DC for continuous full power output (250mA DC for 2 EL34 at 50W)

Completely agree to this.

[vintagekiki]

When trying to guesstimate current requirements of an amplifier, does one draw conclusion from zero-signal plate and screen current readings or maximum-signal? Depending on the tube, they can just about double. In the case of a pair of 6L6GCs, RC-30 quotes zero-signal at 116mA and 210mA at max-signal.

When comparing have you look for which working classes of service data refer to.
The information you provide (page 353) is for PP class AB1 (two tubes).
For zero signal and small signal levels, the amplifier work in class A, and when the signal increase the amplifier will work in class AB and accordingly the anode current will be proportionally higher.
When designing a power amp, all data refers to max values.


https://archive.org/details/RCA_RC-30_1975 (pages 25-38)

[pdf64]

Bear in mind that the HT current draw when overdriven may be significantly higher than at the onset of sine wave clipping.

Here’s a nice tube type info database, don’t even need to hit ‘return’ https://tubedata.altanatubes.com.br/search/

[Dave H]

Will look up the Design Guide tomorrow.

I should have posted it but I couldn't find it. Here it is.

Hammond Design Guide For Rectifier Use.pdf