Which PT to use?

[Madison]

This is for my EL34 SE project, diode rectified.

I have two PTs here to choose from.

#1
280-250-0-250-280V @100mA
6.3VCT @2A
5V @2A

or

#2
325-0-325V @70mA
6.3VCT @2A
5V @2A

My gut says use #2 and save #1 for a PP amp.

Which would produce the best tone ?
I am not so concerned with output volume in this case.

[firemedic]

I don't know about how it would affect TONE, per se, but it seems like the lower-voltage PT should go in the SE amp, & the PP should have the higher voltage one.
But either one will work PP or SE.

[stratele52]

No 2 is good for your SE project

Best tone ? the better quality one . Same quality ; the amp design, quality of each parts , choose high end filter capacitors like Sprague , F& T ,don't use asian import caps. And very important : output transformer.

[Madison]

Ok, I decided to go with #2.
Just made more sense all around.

[jojokeo]

Madison,
I'm in the middle of a changeout from using a 260v @ 100mA to a 325 @ 100mA tranny. Pretty much your same ones you have. The lower voltage one worked and the amp sounded great but the smaller tranny was getting everything sucked out of it and running hot w/ not as much headroom and power as I think it should have. I will know for sure when I get a chance a complete it hopefully this weekend.

[Leevi]

I would select the alternative #1 since it allows more current.
See the data sheet below:
(open the pdf-file and see "Class A")
http://www.r-type.org/exhib/aam0018.htm

/Leevi

[DummyLoad]

#1 @ 250Vrms = 350Vdc
#1 @ 280Vrms = 392Vdc
#2 @ 325Vrms = 455Vdc

loading will probably see a reduction of 5-10%.

EL34 rated @ 25W Pdiss max. - usually idle for A1 is 90% of max Pdiss = 22.5W

#1 @ 250Vrms bias current 22.5W/350Vdc =  64mA
#1 @ 280Vrms bias current 22.5W/392Vdc =  57mA
#2 @ 325Vrms bias current 22.5W/455Vdc =  49mA

either transformer will work - #1 may be at or slightly exceed current rating when @ full power due to g2 current.

personally, #1 @ 280Vrms is what i'd run - a good transformer load would be around 3.5k. sim shows around 350Vdc full load.

--DL

[sluckey]

Both transformers are rated 6.3VCT @2A. EL34 needs 1.5A. Does that leave you enough juice for your other tubes?

[Madison]

>>The lower voltage one worked and the amp sounded great but the smaller tranny was getting everything sucked out of it and running hot w/ not as much headroom and power as I think it should have.

Might be okay in this case.
I am looking for grind.
I'll be doing the VVR so I guess I can get it down to the level of option 2 in the long run?

>>a good transformer load would be around 3.5k. sim shows around 350Vdc full load.

I got a gigantic 5K 15 watt SE OT.
I think it'll be good?
Thanks for the maths!

>>Does that leave you enough juice for your other tubes?

I think I am okay; two additional 12ax7s.

Hoping to power up the chassis in the morning on this.
IOW (for me and my luck) expect problems!!

Thanks guys.

[jojokeo]

My orignal tranny is 260v @ 100mA and 6.3 @ 3A When biased at 25w Pdiss the voltage on the plate dropped to 250V. This was using a 150r/10w cath resistor and only a single 12ax7 also built with VVR. The tranny voltage dropped about 23% - almost about 80v & got so hot you couldn't keep your hand on it. You're going to be running an extra preamp tube also from mine - you're really going to be pushing either tranny w/ the ones you've got. I raised the cath r to 200r/10w and the Pdiss lowered to DL's calcs at 22watts and the tranny doesn't get quite as hot. You've got to keep into account the screen's current usage and the small amounts of your two 12ax7s. You won't fully have enough current for the B+ nor will you from the heater current - those tranny's are going to really cook. You should really keep an eye and hand on it if playing full power for 30 minutes or more. Lowering voltage with the VVR will allow it run cooler but also less power too of course.

As for the OT impedance, I'm using one giving 2.5k which sounds much better and made more power and headroom than the 5k tap. I've got plenty of grind too if wanted at various levels but wanted also to have it be clean in a particular bias setting in a high powered situation. Grind you will get no matter what if that's your preference especially when you lower the VVR. The lower it's adjusted, the easier it is to overdrive el34.

[Madison]

>> 150r/10w cath resistor
>>got so hot you couldn't keep your hand on it

I'll bet!

330R here.

[Leevi]

1. Measure the voltage over the resistor
2. Then count the current = measured voltage/resistor value
3. Then count the power = current * measured voltage

If the power is less than 10W => no problem

/Leevi

[jojokeo]

My comments were about the PT running hot, not the resistor. It's the way Madison posted his reply that makes it seem confusing. With that sized resistor his Pdiss is likely around only 16 watts or less - this doesn't mean his amp is putting out this power only what the amp is biased at w/ his tube and cathode resistor being used. The amp "should" be biased as close to 25w as possible (or even a tad higher) but then it's his amp to what he pleases.

[Madison]

I went ahead and swapped the PTs and put the bigger one in.
The smaller one was getting pretty warm.
It's all good so far.
Maybe some tweaking to do.
Thanks guys for all the great tips and ideas!
With any luck I'll have this thing put together in a few days.