Proper PT selection question

[surfsup]

I built a AX84 HO amp as many here know (and helped me with)...

I did the mod to add an EL34 as the SE output tube after the two 12ax7 preamp tubes.

the PT is a Hammond 269EX
269EX 43 115 60 380V C.T. @ 71ma. 6.3V @ 2.5A - X5

As per the specs, the 71ma rating should be 150% of what you expect the amp to pull, right?

EL34
So two 12ax7s at 10mA each = 20mA
one EL34 at 25W max biased to 60% = 15/230V = 65mA
20 + 65 = 85mA
85mA is greater than 71mA which exceeds the PT rating yes?

EL84
Even the EL84 at 12Watts max at 90% = 10.8/230V = 46mA
20+46=66mA

66 is barely less than 71. I thought good design warrants 150% of what your tubes will draw. So if the PT was properly selected, the PT should be rated at 130mA (for the EL34 mod)? So is the ax84 design choice to use the 269EX a good one and I am wrong in my assumption? Or should I consider a heavier transformer? Thanks.

[Cliff Schecht]

10mA per 12AX7 is a bit high IMO. I say more like 1-3mA per triode usually although it can be even less!

[ToneMerc]

66 is barely less than 71. I thought good design warrants 150% of what your tubes will draw. So if the PT was properly selected, the PT should be rated at 130mA (for the EL34 mod)? So is the ax84 design choice to use the 269EX a good one and I am wrong in my assumption? Or should I consider a heavier transformer? Thanks.

Here's my personal HT current rule of the for SE PT's

6V6= 75mA
6L6/EL34=95mA
KT66-KT88=115mA


The parallel tube PTs that I have made double this rule.


Yes, I think that PT is too small.

TM

[surfsup]

Well that's great news...

So I guess the math/assumption was correct?

Maybe I'll go with the el84 SE then, and pull out the octal socket entirely and use that socket in the champ clone I wanna build next.

With the EL84, it seems like it would be okay.

[ToneMerc]

Hammond are conservatively rated, if it states 62mA it's most likely closer to 82mA. I would use it for a single 6V6.

TM

[guitarmike2107]

Where did you get the 150% idea?

SE amps dont have the same current change from idle to full output power that push pull amps have, so you can get away with transformers that are rated at the idle current + a small percent for safety. you dont need 150% with SE amps.

If you pull more current than a transformer is rated for it will drop the voltage and likely shorten the life of the transformer. but it will work, and if its only slightly underspec it will work for a long time.

You can also choose to bias the output valve slightly colder to reduce the strain on the PT..

Also running your heaters off a seperate 6.3v trannies would reduce the load on the PT and so help it to run cooler and happier. 6.3v trannies are cheaper than a new big one.

Lots of ways to skin a cat...

[surfsup]

Hammond are conservatively rated, if it states 62mA it's most likely closer to 82mA. I would use it for a single 6V6.

That's good to know thanks.

Where did you get the 150% idea? SE amps dont have the same current change from idle to full output power that push pull amps have, so you can get away with transformers that are rated at the idle current + a small percent for safety. you dont need 150% with SE amps.

Read it on ampgarage and did not make a distinction between different requirements for PP and SE so thanks for that info...but again, good to know SE are not as demanding. I'm getting lower voltages on my amp so the current is higher than it needs to be too, so that's an additional small problem.

[surfsup]

Well if I may comment more on my own comment. The Hammond 269EX is 71mA. Using one of these power tubes:

EL34
So two 12ax7s at 5mA each = 10mA
one EL34 at 25W max biased to 60% = 15/230V = 65mA
10 + 65 = 75mA
75mA is greater than 71mA

EL84
Even the EL84 at 12Watts max at 90% = 10.8/230V = 46mA
20+46=66mA
66mA is just under 71mA

But....doesn't the OT take some mA as well? So that puts be back to the idea that the Ax84 HO kit, even with the EL84 PT has too small a PT based on the Hammond specs (forget it is underrated...)

[Alexo]

Yer thinking too much.

That PT will work great for an EL84 or a 6V6 amp, but I would not use it for an EL34 or a 6L6, etc.

End of story.

...that said, I'm pretty sure I used one of those for a push-pull 6AQ5 amp, at idle, it is pulling just over 100% of the high voltage winding and the same goes for the heater winding.

It actually works great, but that tranny does get hot!!

But I digress..

Go low-watt with this one. Have fun!

[guitarmike2107]

no its fine for an el84, the only source for power(current) in the amp is the power transformer, the output transformer does not require another power source to work.

besides the current thing, you wouldn't really want to run an EL34 at 230v B+

[surfsup]

okay I'll go back to the el84 and use the octal for my next build. Or, I have two 6v6s so maybe I'll try one of those first before soldering stuff again.

besides the current thing, you wouldn't really want to run an EL34 at 230v B+

I am trying to figure out the low voltage thing, just don't seem to have any good ideas.