SINGLE ENDED AMPLIFIER (3K OUTPUT)

[Jonas]

I have a single ended (25 watt) output transformer with 3K primary and 4, 8, 16 ohm secondary. What power tube(s) would be a good match for this transformer, either single or dual parallel?

Also considering mixing to 2 different output tubes at the same time, such as 2x EL84 + 1 KT66.....

[tubeswell]

EL34 types, 6L6 types

[shooter]

i've built SE amps with every combination of tube(s), once I built using the KT88 I quit playing the "what tube game" and just built different PRE's and always used the same PA, make life simple


you need more info than provided
plate volts is pretty important

[PRR]

...you need more info than provided
plate volts is pretty important

If this is a Properly Designed Product then the plate voltage is implied by the OT specs.

You paid for (and carry!!) 25 watt output. With tetr/pentodes this will be 63W plate dissipation.

The stage V/I will be about the rated impedance, 3k.

Don't make me get the calculator, but 450V 150mA is in the ballpark (a bit high but everything sags).

The 63W dissipation suggests two or three of the usual big bottles, or HK257.

IMHO: this is not an at-home amp (too loud!) and not a gigging amp (too heavy and not quite loud enough).

[shooter]

 
that's what roadies are for
the guitarists like them in small venues, played in class A (think lounge music)
they also hold there own mike'd into the PA systems when you want to let the hair grow long, bring a 2X12 cab around 98db spl n 23Wrms
 will hold it's own

[dwinstonwood]

once I built using the KT88 I quit playing the "what tube game" and just built different PRE's and always used the same PA, make life simple

shooter,
I'm really starting to like your philosophy. Playing around with preamp triode biasing&bypassing, cascodes, parallels, CF's, hi&lo filters, voltage dividers, and various tone stacks is where the fun is for me. The heck with phase inverters and negative bias supplies! I think my next amp will be a Class A/cathode biased tube on it's own turret board, and a separate preamp board that can be modded. 

[shooter]

it's not my philosophy, I stole just like the rest, being a contrarian helps 
my best liked by the guitarists has 1 input, split into 2 gain stages blended (ala plexi) with TS's into a DCCF (ala Merlin) into a driver (optional), a PA, heavy metal OT, feeding good 2X12
of about 10 very good city guitarists, NONE would believe me when I said it's 23W.
someone used it with a 1X10, put the amp knobs where he wanted n used guitar only to play some amazing clean light jazz, adjust guitar, out came some P-funk, tweak guitar, grungy pop rock all at reasonable city close houses volume
then the Marshall full stack got mentioned, I left before company showed up:)

[Jonas]

I'm not looking to maximize power, I would be fine with around 10ish watts, tone more important ... maybe the extra iron could be good for better bass response???

I have a PT with a dual secondary 210-0-210 and 310-0-310; anticipating B+ to be around 300VDC or 440VDC respectively using SS rectifier.

For preamp, I'm thinking starting with a standard blackface fender preamp like in the AB763 and tweak from there....

[AlNewman]

Where'd you get the power transformer?

[tubeswell]

I have a PT with a dual secondary 210-0-210 and 310-0-310

So two separate High Tension windings? Or one winding with 210V taps? (If the former, then a separate 210V winding would be handy for a screen supply)


What’s the heater winding rated for?

[Loomer]

i've built SE amps with every combination of tube(s), once I built using the KT88 I quit playing the "what tube game" and just built different PRE's and always used the same PA, make life simple

Debated whether to post this as didn't want to hijack, but believe a few will be wondering and ultimately, being a possible option, is relevant to the OP's request - would you mind sharing the SE KT88 PA design?

It isn't a popular SE valve, but one I've heard being mentioned as a great sounding one before...

[Jonas]

Edcor PT

yes only one HT winding with (2) taps 620V/420V (310-210-0-210-310) at 200mA center tapped, 6.3V at 6A and 6.2V at 2A.

Loomer, happy chimed in

[shooter]

https://el34world.com/Forum/index.php?topic=31066.0
reply #5 has a PP version (the best liked pre)
here's a SE using the KT88
many undocumented changes were done on both schematics but both will get you a working amp.

[labb]

The SEL amp from the old AX84 site is a pretty good SE amp with a kt88 tube

[Jonas]

Shooter thanks for sharing. Have you ever combined different output tubes at the same time? I'm not 100% sure how to approach this. If I wanted to run a pair of EL84's with a KT66, is this as simple as adding another cap off the plate feeding the output tube or would I need a splitting circuit to feed the EL84 pair and KT66 separately? 

[dwinstonwood]

I have Blencowe's (Valve Wizard) books on preamps and power supplies. Fantastic books. Essential.
I just wish he would publish one on Power Amplifiers.
I'm reading Morgan Jones chapter 6, but, it would be great to have Blencowe's illustrative, intuitive, and comprehensible language on this whole mysterious ordeal involving power tubes, B+ and OPT primaries.
Thanks.

[PRR]

> wish he would publish one on Power Amplifiers.

Have you read both/all pages on his website?  https://www.valvewizard.co.uk/

[dwinstonwood]

Thanks PRR. I completely forgot that was there. 

This helped a lot, too:
http://radioremembered.org/outimp.htm

[shooter]

I've done a 6K6 n 6V6 in a PP cathode biased
I've done ||SE 84's, quad 84's, ||6V6SE, ||6L6SE, and 3SE 6AU6
for those following along;


SE amps ain't like PP amps, they are a Class A amp, they don't like to be overdriven at the PA (with a few exceptions), they will amplify pretty much whatever you feed them, a scope is REAL handy to set up the drive signal to the PA, figure out why "it sounds like sheeet", they are NOT as versatile as a PP amp, they have no "built-in" hum cancelling that you get in PP, so clean, well thought out, layouts are important. The big $ stuff costs more.


MY personal recommendation, build a simple "fender style" 5(E)F1(2) SE amp.  it's about 4W so you can play most everywhere getting used to SE sound.  tweak the pre like a kid in the candy store, understand the tweaks.
SE is a tinkers amp, NOT "i need to gig on Friday, let me slap this together amp"

[Jonas]

Thanks for the help shooter, I'm up an running!!
I didn't follow your schematic exactly but the topology is pretty much the same - I used first half 12AY7 to volume control into second half 12AY7 into 12AU7 cathode follower. Added a second channel (5879) mixed into 2nd half of 12AY7. running KT88

What are you (and everyone else) setting your bias at? Are you keeping close to 100% MPD?

[shooter]

I've run everything from ~~~85% to 120%  usually set it to how much "good OD tone" the PA adds AND the amount of "clean-swing"
so, adjust it for your ears, and pocketbook

[Jonas]

do you have any fault protection circuitry built in for power tube failures?

[shooter]

 
a fire extinguisher is typically near by.

[Jonas]

I have a couple more questions......is there a preferred OT impedance for a single ended EL84?
Also, what is your screen voltage on the singled ended KT88?

[HotBluePlates]

...is there a preferred OT impedance for a single ended EL84? ...

The EL84 data sheet says things from 4.5kΩ up to 7kΩ.

A single-ended EL84 manages 6w output power when racing downhill, with the wind at its back.
It can have up to 300v on its plate, so 12 watts / 300v = 40mA ma idle current.
Hammond's 125BSE is rated for 5w and 45mA of idle current.
The 125BSE can be configured for 5kΩ or 10kΩ primary.
If you're idling at 100%, that's 300v and 40mA ---> 300v / 0.040 A = 7.5kΩ
   -  Since the 7.5kΩ splits the diff between the available primary impedance's, you can go either way.
   -  Going lower to 5kΩ probably avoid excess screen current, and tilts toward more 2nd harmonic distortion.
   - (The diagram is for a different tube, but all pentodes/beam tubes have similar tendencies)

[shooter]

I typically use an UL OT taped at 40%, so I just let the OT decide what voltage n current it wants

[DummyLoad]

I built one of the attached on my breadboard - loads of tone, but alas I didn't build it. It's a parallel SE pair of EL34s that wants a ideal load of 3K with the B+ indicated, makes around 20W. The OT I used is a custom Edcore part. They offer a 3K, 3.2K, and 3.75K parts as standard with the 25W line-up.

The PT you have may be a bit shy on the B+ for the plan attached, but should be close enough.

This is basically a 2204 lead preamp with a paralleled 12AX7 driver to simulate the same gain structure as a 2204 with a P-P output stage; IOW, the added gain of the paralleled driver subs for the missing LTPI gain in the SE plan.

In case you're wondering why the 3.5K OT; I had originally designed for +/- 450V B+, but after experimenting, the sweet spot seems to be with +/- 400V B+.

The variable NFB helps tame some of the that ice-pick like tone of the 2204 preamp.

--Pete
   

[Jonas]

THANKS EVERYONE!

Pete, thanks for the schematic, I will give it a try. I also plan to A/B 5654 and 5879 and try EL84 in the circuit, but please keep the suggestions coming!!!

Jonas

[sluckey]

Here's a little SE amp with a 5654 that I did a few years ago. Might give you some ideas...

     https://sluckeyamps.com/smoky/smoky.htm

[scstill]

Here's a little SE amp with a 5654 that I did a few years ago. Might give you some ideas...

A great story. I worked on an ATC system in my twilight years at Hughes (early 90's) and was a air defense radar engineer before that. Small World.

I appreciate how Smoky came about and especially love the front window and rear mirror.

I'm glad whenever I see point to point wiring (99% of mine are). Very appropriate for old amps & radios great way to preserve and honor the character of the period. Perfect for any toaster too.

On the tech side, why did you use a choke with a ss rectifier?
Learned something new with your gain control. Is it worth doing? and would it work with a triode preamp?

[sluckey]

On the tech side, why did you use a choke with a ss rectifier?
Learned something new with your gain control. Is it worth doing? and would it work with a triode preamp?

I don't think I could have crammed another tube in the toaster. Maybe, but it would have thrown the whole symmetry off. The choke came from a gutted AO-43 chassis and had been rolling around in my spare parts bins long enough. When I realized it had the exact same footprint as my OT, I just had to polish it up and use it. It rounded out the symmetry perfectly. And why not use it? The little Fender 5E1 Champ used one before Leo cheaped out, dropped the choke, and made the popular 5F1 champ.

That gain pot should work just fine with a triode too. It's really a useful control.

I like symmetry!     

[Jonas]

thanks for the schematic sluckey! I tacked in a 5654 then tried a 5879 for a quick test.... liking the 5879 better (less fuzz)

How come you never see pentodes with tone stacks? It seems simple one knob tone control or rotary cap is the norm...

[Jonas]

looking for recommendation on type of power tube cathode resistors for SE KT-88? would it be a better choice to go with a 25W resistor (with built in heatsink) mounted off-board, or is standard 10W sandbox on-board OK?

[shooter]

dealers choice
I typically start with a handful of "standard" resistors that either get tacked down, of just gator-clipped til i'm reasonably happy with "tone" and dissipation, then I use the pretty gold bolt-downs that match closest to my tweaking value

[Jonas]

on to the next modification on my SE amp...... looking for advice

I would like to add a LINE OUT jack - source derived from 8 ohm output - using a switched speaker jack and a 25W 8 ohm resistor as load when speaker is not plugged in.

When in use will the 25W 8 ohm load resistor adversely effect the tone of the LINE OUT jack or is it recommended to use an inductor style circuit for the attenuation???