Can someone explain transformer ma and amp ratings

[yalesmith]

Can someone explain power transformer ma and what is good in certain applications. I have noticed 50 and 100 watt amps that are powered by transformers with low ma ratings and then 30 watt amps with a much higher ratting. Can someone please explain.

THANKS

[John_P_WI]

Read and understand everything from VacuumVoodoo in this thread:

https://tubeamparchive.com/viewtopic.ph ... ransformer

[ToneMerc]

Yes you are correct, you will see is a Fender Bassman at 240mA and JTM45 at 150mA. To compound the issue the current ratings are not expressed in universal parameters across the different power transfomer manufacturers either.

It can be a slippery slope because some of the charm of some amplifiers circuits are the underated power supplies and filtering.

I've ordered quite a few custom power transformers from Heyboer and I spec my HT current at my required AC voltage spec.

Usually for a wound transfomer the HT current rating is based on a 75% duty rating. Here's my guide for a center tapped HT winding power transformer.


(HT winding voltage x HT current) x .75 = Supporting Watts


My personal minimum rule of thumb for the following tubes are

60-70mA - 6V6
90-100mA - 6L6/EL34
125-135mA - KT88

For example take a quad tube 6L6 tubes. I know 335V rectified will put a quad of 6l6 around 440-445V on the plates. So I want a PT that supply at least 440V to a quad of 6l6s.

(335V * 400mA or .4A) *.75 = 99 Watts

For a voltage doubling supply, I double the current rating on the winding.

Others may do it differently, but this works for me.

TM

[xtian]

TM, don't you add in preamp tubes?

[Colossal]

TM, don't you add in preamp tubes?

To account most accurately, sure you would add in preamp tube current. But since preamp tubes draw on the order of 2-4mA (so lets say 16mA for four 12AX7s at full tilt), that's still just 4% of the total for a 4x6L6 amp. TM's method will get you in the ballpark and then it's usual to upsize the PT by some percentage to ensure it has sufficient headroom.

[ToneMerc]

TM, don't you add in preamp tubes?

To account most accurately, sure you would add in preamp tube current. But since preamp tubes draw on the order of 2-4mA (so lets say 16mA for four 12AX7s at full tilt), that's still just 4% of the total for a 4x6L6 amp. TM's method will get you in the ballpark and then it's usual to upsize the PT by some percentage to ensure it has sufficient headroom.

Exactly Dave

TM

[martin manning]

Another quick-and-dirty way to get at this is to use the max plate and screen dissipation numbers for the power tubes, and add a half-watt for each 12AX7 triode, and a watt for each 12AT7 triode (the preamp stages are typically run at about half of their max dissipation). If there is a tube rectifier, it will eat a few watts, so you can account for that too. Then, divide the total by the RMS voltage at the HT secondary to get the HT current required.

Example: For a 4x 6L6 Twin Reverb, it would be 4*(30W Pa + 5W Pg2) = 140W, plus 7W for the five 12AX7 types and the one 12AT7 for a total of 147W. The Twin Reverb PT's secondary is somewhere around 330 VAC, so 147/330 = 0.445A... 450mA is the standard rating.

[Colossal]

These calcs could (should?) be made into a little Excel app for a Sticky download!

[Firestorm]

Well, as long as we're on this subject, I can think of classic amps where the speaker was underrated, the OT was underrated, certainly the filters in the PS underrated, but I can't think of any where an underrated PT did anything but cause grief. Am I wrong about this? (It did happen once before:: ).

[martin manning]

I recall some 100W Marshall amps have a hot combination of plate voltage and load impedance. The only thing that keeps them from red-plating is the PT voltage sag...

[Firestorm]

I recall some 100W Marshall amps have a hot combination of plate voltage and load impedance. The only thing that keeps them from red-plating is the PT voltage sag...

Happy accident, don't you think? You wouldn't design that way. Ken went the other way: overrated everything in the PT.

[Cliff Schecht]

Well, as long as we're on this subject, I can think of classic amps where the speaker was underrated, the OT was underrated, certainly the filters in the PS underrated, but I can't think of any where an underrated PT did anything but cause grief. Am I wrong about this? (It did happen once before:: ).

I look at old BF Fender Deluxe's and think the PT on them is, well not underrated but spec'ed to encourage some droop. I've worked on a few with original transformers and the thing that surprised me (initially) was that with a high wall voltage (i.e. above 120V), the heaters only get up to maybe 6.6V, what I consider well within the limits of what the tubes can handle. The B+ also will drop to about 430V which makes the first cap able to be a 450V variant instead of a 500V variant (I've seen many techs use 450V for this cap and I have as well without issue).

[martin manning]

I recall some 100W Marshall amps have a hot combination of plate voltage and load impedance. The only thing that keeps them from red-plating is the PT voltage sag...

Happy accident, don't you think? You wouldn't design that way. Ken went the other way: overrated everything in the PT.

You didn't say it had to be intentional... Just a case where it caused something other than grief!

[yalesmith]

Sorry for the delay as I ask a question and disappear for a week but i was traveling. Ok so a hammond 290cx (http://www.hammondmfg.com/pdf/EDB290CX.pdf) pushing two 6550 tubes in a ceriatone afd 35 (50 watt amp) is not enough sufficient power or ma. This is maybe why the amp squeals, sounds and acts funny?

Thanks

[martin manning]

I would say you need about 50% more HT current (say 320 mA) for that amp to push it. I don't know about the squealing, it's probably unrelated.
(this post has been corrected by edit - MPM)