Single ended el34 amp

[fossilshark]

I have a 5000 ohm 15 watt output transformer, some 12ax7s, and a power transformer with 430v, and 6.3v secondaries. I want to do a single ended el34 amp, are there any designs out there that use just 12ax7s for the preamp?

[sluckey]

are there any designs out there that use just 12ax7s for the preamp?

Plenty. Is your output transformer single ended? Or push/pull?

[fossilshark]

I was using it single ended with 2 6v6s in parralel before i upgraded. It does have a center tap though.

[sluckey]

Several forum members have built EL34 SE amps. Maybe they will chime in. Meanwhile, go snoop around this site...

     http://www.ax84.com/home.html

[shooter]

that use just 12ax7s for the preamp?

any fender, or other amp, where the TS sits 'tween V1a and V1b has worked good for me.  You don't need tons of signal to drive SE.

[SILVERGUN]

Here's one I built and liked...
Due to the "tweed'ish" style tone stack it has somewhat more of a flat frequency response as compared to the more traditional mid-scooped tone stacks you see in many later Fender, Marshall, etc.
This makes for a very "in your face" tone with no place to hide, but also drives the output tube very easily and gives a big/bad sound.
You could always sub in a more traditional TMB stack if you want to scoop out some mids because there is plenty of signal there for the dumping.
 
When you say that your PT has a 430V sec. winding, do you mean 215-0-215?...is it center-tapped?

[terminalgs]

It does have a center tap though.

are you talking about the output transformer? Is this an ultra-linear tap? or a true center tap?   PP and SE OT's are built differently. 

[tristanc]

I made one of the AX84 SELs using an EL34. http://www.tristancollins.me/guitar/ax84-sel-from-ampmaker-n5x-build/

I later modified it to Merlin's High Gain topology and added an effects loop. This sounds great and is really usable:
http://www.tristancollins.me/guitar/merlin-high-gain-preamp-with-el34-se-output/

Schematic here: http://www.tristancollins.me/wp-content/uploads/2016/10/SEL_MerlinHG-FX.png

[SILVERGUN]

Very nice looking work, schematics and website in general, Tristan
Thanks for sharing!
 

[fossilshark]

the power transformer is just 420v with no center tap, of course this pretty much eliminates a tube rectifier option but i want to use a solid state rectifier anyways since it doesnt matter for single ended amps

[sluckey]

the power transformer is just 420v with no center tap, of course this pretty much eliminates a tube rectifier option but i want to use a solid state rectifier anyways since it doesnt matter for single ended amps

That PT will produce 590VDC unloaded B+.   

[fossilshark]

here is my schematic and here are my questions:
what value should the 2 power supply resistors be, i know the el34 should be about 380v but im unsure as to what the 12ax7 should run at. also, what should my fuse be rated at?

[kagliostro]

Your B+ is too high, a resistor will not be efficent to reduce it to the level you need

as Sluckey told 420v AC became around 590v DC

if you want to use that transformer without a tube rectifier, you must use a choke input instead of a capacitor input

this way you'll have B+ at around 380v DC

other way will be to use Zener/s (or an amplified zener) or to use a VVR to drop the excess of B+ if you use a capacitor input

Franco

p.s.: You say the PT has 420v AC but your schematics show 400v AC .... ??? Which one is the right value ?

[fossilshark]

Its 400v i checked earlier. What value choke would i use? Ive never used one before

[kagliostro]

I think you must use a 10-15H choke able to afford the current consumption of the whole amp

I hope PRR or HotBluePlates will share their opinion

--

Here you can read something about choke input

http://el34world.com/Forum/index.php?topic=21055.msg223079#msg223079

Franco

[HotBluePlates]

Its 400v i checked earlier. What value choke would i use? Ive never used one before

400v * 1.414 = 565vdc.  25w / 565v = 44mA, and 565v/44mA = ~12.8kΩ (as a 1st-guess suitable SE load).

And you have a (likely) push-pull output transformer.  You're considering adding a heavy choke the lower the B+.

It seems to me you might be frugal, and are attempting to reuse transformers for an application to which they're not suited.  You'd probably be happier with the results with either the correct parts, or a circuit which suits these parts...

[fossilshark]

I have to stretch $20 to last me my next 3 paychecks lol. I think i might just get the proper power transformer

[terminalgs]

 

I was using it single ended with 2 6v6s in parralel before i upgraded. It does have a center tap though.

the power transformer is just 420v with no center tap,

Does your output transformer have a center tap?

[HotBluePlates]

I have to stretch $20 to last me my next 3 paychecks lol. ...

Been there, done that.

... I think i might just get the proper power transformer

What exactly are the power & output transformer you have (model #, etc)?  I ask because it's unclear whether the OT you have now is a single-ended transformer or not.

[PRR]

> i know the el34 should be about 380v

No, the EL34 can be run almost any voltage and current that suits your Output Transformer.

> What exactly are the power & output transformer you have (model #, etc)?

+1.

[fossilshark]

I got them off ebay this is the only information i have on the

[fossilshark]

It would be really great if these could work

[sluckey]

The OT is for a push/pull amp, not a single ended amp. That's what the "PP" on the label means.

[fossilshark]

I was using it as a single ended, what problems could that cause?

[HotBluePlates]

I was using it as a single ended, what problems could that cause?

You can also drive a car which has 4 flat tires, but it's not the best way to get around...

It will work, sound will pass through it.  The transformer core may saturate very early and constrict the power passed through to the speaker (though it's possible to see this as a "bonus").

Normally, single-ended output transformers have an air gap and much bigger cores than push-pull transformers rating for the same power handling.  That's because there is unbalanced d.c. (the tube's idle current) passing through a single-ended OT.  Magnetization due to that idle current eats up a portion of the core's capability to transfer power from the primary to the secondary, so the core must be larger than a push-pull OT (which is inherently balanced, assuming idle currents to each side are similar).  The air-gap also helps offset the effect.


If you had to make use of the output transformer, a single EL34 might give you as much as 12w output in a perfect world.

Power = Voltage²/Impedance, so
Voltage = √(Power * Impedance) = √(12w*5kΩ) = ~245vdc
25w / 245v = 102mA idle plate current (!)

Indeed the EL34 data sheet has a single-ended condition for 250v on the plate (and screen) and 100mA of idle current. They use a 2kΩ load, perhaps to reduce distortion a bit, which is still at 10% for a claimed 11w of output power.  There's also 15mA of screen current at idle, for a total of 115mA through the 106Ω cathode resistor (for about 12.2v of bias).  Total B+ needed before considering any other losses would be ~265vdc.

So quite a ways off from your 400vac (* 1.414 = 565vdc) power transformer; I wouldn't use them together.  400vac is really too much for most guitar amps as well.  And unfortunately, Basler's site is no help is finding out the current rating of that 400vac winding (though presumably high given the 6.5A rating of the 6.3vac winding).  I'd guess the PT was meant to support 4x EL34's (6.3vac @ 4.5A) plus plenty of current for many preamp tubes, in a high-volted 100w+ push-pull amp design (assuming it was meant for guitar/audio).

[PRR]

The PT likely supports a 560V B+. (I doubt it is any kind of guitar amp PT).

Push-pull, 5K load suggests an 80 Watt output.

The 15W OT will not be happy delivering over FIVE times rated power. (May not like 560V DC either.)

I think you got a Mack engine and a Corolla gearbox. Not a good fit.

If you accept that a P-P OT will suck in a SE amp, I would rate it about 1/4 of the P-P rating, 4 Watts. 4 Watts in 5K is (look at 6V6) about a 230V B+. You want like 170V AC winding.

[fossilshark]

im also working on a x4 6l6 100 watt amp, would The power transformer be good for that?

[sluckey]

Impossible to say with any certainty without knowing the current rating for that 400V winding.

[fossilshark]

How do i figure out how much current a plate draws? Do i calculate it based off the plate resistor?

[shooter]

x4 6l6 100 watt amp, would The power transformer be good for that?

the operating plate voltage would be a good start
example;
480vdc * 50mA ~ 24W per tube *4 ~100w + #preamp tubes @ 5mA per tube
so 50*4 + 3*5 ~215mA total.
also have to verify the filaments which look ok based on the PT label

[kagliostro]

How do i figure out how much current a plate draws?

Refer to the datasheet of the tube, there you can see the consumption in accordance with B+ and applied load



Franco

[PRR]

> How do i figure out how much current a plate draws? Do i calculate it based off the plate resistor?

Exactly. (Audio amps rarely run near spec-sheet show-off numbers.)

Assume plate sits at half the supply voltage. So half of B+, divided by Rp, is a quick-guide to stage current.

For Fender 12AX7 1.5K 100K, plate sits nearer 70% of B+. Assume each such stage is effectively a 330K resistor loading full B+.

[frankenxtein]

  I was assuming these transformers 1st secondary voltage was a  combined total.  200+200= 400 sort of thing ...... right at about $30w/shipping. It caught my eye.
        It's not near the size of my giant almost 20lb. Hammond AO32 power supply  transformer. There isn't much data on these anywhere.

Basler Tube Amp Power Transformer 120 VAC 6.3V 6.5A 350-400V 28 VCT BE32901001

(From the eBay posting) "Item is one tube amp power transformer for converting 120 volts AC into 6.3 volts AC, 400 volts AC (per label), and 28 volts, center-tapped. High voltage winding is actually around 350 volts on these as measured, though label says 400 volts. The maximum output current of this transformer is not known. This transformer is made by Basler and has part number BE32901001. This transformer measures 3.1 inches tall by 3.75 inches wide by 4 inches thick. It mounts using four fasteners spaced at 2.5x3.125 inches. 6.3 volt secondary is 6.5 amps, other currents not specified. Mounts though 2.25 x 2.5 inch hole."  Probably a dumb question but hey fossilshark .... Did you measure the voltage on that PT?

I was looking for something to power a PP 6BQ5 , and thought this PT secondary would be too light. I was looking at the 6.3vac to be able to run more than a single preamp tube with that pair of 6BQ5's (my situation) I got 1.7A spec on my 6.3 vac. The "Tube Amp Power Transformer Current Draw Calculator" has me needing 1.85A.

[PRR]

> what value should the 2 power supply resistors be

[HotBluePlates]

... I was assuming these transformers 1st secondary voltage was a  combined total.  200+200= 400 sort of thing ...... right at about $30w/shipping. It caught my eye.
...
Basler Tube Amp Power Transformer 120 VAC 6.3V 6.5A 350-400V 28 VCT ...

Notice the 28v winding indicates a center-tap ("28VCT").  The 400v winding does not; and fossilshark's photo confirms it.

Just to be safe, select one of these transformers by taking the d.c. voltage you want and dividing by 1.414.  Want 350vdc?  Get a 350v/1.414 = ~250vac secondary Could be 250VAC for a full-wave bridge, or 250-0-250v or 500V CT (same thing said 2 ways) for a full-wave rectifier.

With the perennial complaint of too much supply voltage, you're probably best off shooting low.  Almost no one seems to complain about having too-low supply voltage.

[frankenxtein]

Thank you for that info HotBluePlates

For now I'll stick to robbing PT's & OT's from old tape recorders , single ended  simple small amps. it's challenging enough for now.

Not interpreting/understanding some of these specs right , is the reason my PR40 power supply is pretty much a doorstop until I learn a bit about this stuff. 

For what it's worth the seller on eBay also sells a Kustom 72 Coupe output transformer The schematic for that one shows 4 6l6/el34.