30w class A amplifier

[fossilshark]

Hi all, one of my friends has an idea for a class A 30 watt amplifier for super cleans and super headroom. Initially I was hoping to find a single tube that would put out 30w in class A (it would be a monster of a bottle of course) but that doesn't seem practical.

I found this KT88 circuit and it got me thinking, would putting a couple KT88s in parallel be a bad idea?

Does anyone have any recommendations for a class A amplifier schematic that could do 30 watts?

Any advice is appreciated.

[Jalmeida]

I was looking into something along these lines and was told not to go this route. That getting 30W single ended even from a KT88 was not likely. And the cost for iron to power it wasn’t worth it. No clue what you could get from a SE KT88. Sluckey, PRR or the other more seasoned and knowledgable braintrust may be able to give you more specifics and math it out for you.

[kagliostro]

My friend is building a PSE

not a clean amp but a strong 2 x GU50 PSE amp

http://www.diyitalia.eu/viewtopic.php?f=22&t=11085&hilit=Evil+twins

---

BTW a tube like 813 is what you need but always there is the OT problem and maybe also a HV B+ level

Franco

[AmberB]

Well, you can get close to 20 watts from a single KT-88 SE amp with the right B+ voltage and the right iron.  I looked into building one, but the output transformer is rather expensive.  You might have to get a custom wound SE output xfmr for 30 watts output.  It would be big and have a low primary impedance, half the impedance of a single KT-88.

[bmccowan]

The tube HiFi builders have more experience with high power SE amps. It may be useful to fish in their pond. For guitar amps I do not see the advantage. Super Clean/Super Headroom - to me that's the Fender Twin - why go SE? Unless its just to experiment and then I totally get it.
Hammond has a 30W OT for about $150. Parallel up 3 EL-34s. Buy one of those too-big PTs that show up on Ebay and start soldering.

[AmberB]

Another option is to build a 30 watt class A push-pull amp.  You can do that with a pair of 6L6s or EL-34s with cathode bias and a bit lower plate voltage.  Also, loading the 6L6s with 6.6K load instead of 4.3K ohms will lower the output power and probably make it cleaner sounding.

[HotBluePlates]

... Initially I was hoping to find a single tube that would put out 30w in class A (it would be a monster of a bottle of course) but that doesn't seem practical.

I found this KT88 circuit and it got me thinking, would putting a couple KT88s in parallel be a bad idea? ...

Echoing bmccowan, why does it have to be single-ended?  One can do push-all class A and have a much easier time getting a usable output transformer.

For example, let's say you go single-ended with a pair of KT88s in parallel, and a reasonable supply voltage of 400v plate & screen.  Idling at 100% dissipation, this 40w tube passes 40w / 400v = 100mA per tube, or 200mA total.

You need an output transformer than can handle 200mA(!) of unbalanced DC, which will be a huge, air-gapped core.  Hammond has you covered with the 1650SEA, but it weighs 11(!) pounds.  A 100w Twin Reverb output transformer (the 1750W here) weighs only 4 pounds, so you basically need 3x the weight to get less than 1/4 the power compared to a Twin Reverb.  So the schematic won't be the problem so much as doing mechanical design to decide how to reinforce your chassis to support that 11-pound beast (plus a big power transformer).

That 1640SEA has a 1250Ω primary, so if we assume the limiting-case of the KT88s swinging from 200mA idle up to 400mA and down to 0mA (200mA peak), then power output is:

   200mA Peak x 1250Ω = 250v Peak
   200mA peak x 250v peak = 50w peak
   RMS Power = Peak Power / 2, so 50w Peak / 2 = 25w RMS ---> 1/4 of the Twin Reverb's power output

_________________________________________

To get more power output, we would need a higher primary impedance and somewhat lower idle (and peak) current.  Hammond's page for big single-ended OTs shows the 1650SEA had the highest unbalanced DC for the 30w class, reinforcing the need to move to lower idle current.

6550s and KT88s need to have high voltage on the plate to get their high power outputs.  Meanwhile, screen voltage usually stays low, making it awkward to build a power supply that doesn't match the typical use-case of the screen volts being 1/2 the plate volts (like 600v plate, 300v screen) along with Class AB operation.



All that said, "25w" is close enough to "30w" being only -0.8dB and likely not a big enough difference to notice.  However, think about the cost & weight for what is essentially a "Deluxe Reverb that's 0.6dB louder"...  I think you'll be very under-whelmed after the expense & hassle to build.

[fossilshark]

These are all excellent points, the iron does kind of make it cost prohibitive. If my friend really really wants to pursue this ill just go with a hi fi circuit with available parts and tweak the tone section for guitar.

thanks for all the feedback!

[PRR]

...Initially I was hoping to find a single tube that would put out 30w in class A (it would be a monster of a bottle of course) but that doesn't seem practical.

What is this word, "practical"? Sounds like a challenge.

[bmccowan]

That 257B looks so cool I think I'd just stick it in a socket with the heaters wired and display it in my house - make up BS to tell house guests what it does.

[fossilshark]

https://www.ebay.com/itm/185975258636

If i had a little more knowledge on how to build this into a power amp i absolutely would...

{EDIT- truncated tracking code in eBay link -PRR}

[shooter]

That 257B looks so cool

the early CAT scanners i worked on used 2 similar tubes that floated on the 140Kvdc for "fast regulation".  they "smoothed out" the ripple on the DCv.
i'd use the dead one's for birthday gifts for the nieces n nephews, ya I was that guy 


no cool factor with these, the small one spec'd at 50W_cw(class A) and the bigger one 500W_cw. we operated them class D at 25Kw in the MRI systems

[kagliostro]

https://el34world.com/Forum/index.php?topic=10262.0

Franco