200- 500 watt Tube Amp project

[Michael Koster]

Multicore amplifier

I found Hammond has 280 transformers at about 250 ea. they also have smaller units available next size down is a 120 watt size.
But the 200 + watt monster does weight 30 pounds. So probably you'd have to build 2 amps just so they could be moved.
Or utilize a relay rack, on wheels no less.

Bottleheads has a few kits but they are way out of my price range and don't really produce any meaningful power.

SS amps produce the raw power but none I have listened too sound all that great, If they did, why would I have a 50 year old amp in the listening room?

I may downsize this project once again, I guess a 120 watt design is the more practical route to take

As for a chassis, maybe I can make use of that old dinosaur HP server after all,it's weigh is 120 pounds and is 1ft high, 17 wide and 24 deep...

You're almost down to 100W, where there are plenty of choices in tubes, OPT, power transformers, etc. for really what amounts to a barely significant power difference (3db between 200W and 100W). And no need to parallel output tubes if you use 6550s or KT88s and have some flexibility in the biasing of the output stage. At that point there are OPTs for $90, tubes for $25, you can use a 480:120 control transformer, and the parts cost goes into the $250 per channel range. plus if you really want the extra 3db for say a subwoofer, just bridge 2 channels per side. You should be able to cram 4 or 6 channels into a chassis that size, powered by a 1 to 1.5KVA machine tool transformer.

Cheers,

Michael

 

[toddbailey]

Yes, sometimes, well most of the time life is full of compromises.
I think going from 30 to 120 wpc will make the project more successful.
At that rating, I could probably find something commercially available. plug in and play even...
However the thought of driving 2 to 4 kt88 in a apx 200 wpc push pull config does sound interesting with either output trans.
Once you get past the cost of the transformers, the rest is pretty affordable.


I looked at the Mac mc-3500 and all I can say are you kidding me? 10-20K each! Not in my lifetime...


btw: I'm using a pair of APT-1 amps in mono mode for the sub-woofers (about250wpc 4 ohm)

 

[toddbailey]

How about a Dynaco MK VI ? 120 wpc if I recall.....

 

[danFrank]

I looked at the Mac mc-3500 and all I can say are you kidding me? 10-20K each! Not in my lifetime...

I have a pair of output transformers from the McIntosh MC3500 / MI350 amps that I got from a guy back east that junked the amps because he only wanted the power transformers from them to build a ham rig.

I'd be willing to sell them if somebody gives me an offer I cant refuse....
Daniel

 

[Tubologic]

If 300 watts of power is enough for you you should check VTV (Vacuum Tube Valley) Issue 12 (1999) , p. 31-32 where there is a detailed description of a 300 watts triode "booster" amplifier using only commonly available parts. (stock Hammond transformers ) The circuit uses a 1650W OPT driven by a push-pull of eight SV572-160 triodes.
For higher power you'll need to use transmitting tubes (usually triodes) and play with lethal voltages in the multiple kV magnitude. Over 300 watts of power,multiple push-pull based circuits are not recommended due to tube matching and reliability issues.
Another good reference book is: "An Approach To Audio Frequency Amplifier Design" (Publ. G.E.C Ltd. England, reprinted by Audio Amateur Press,1994) covering audio amplifiers from 5 watts up to 1100 watts ! (using a single pair of giant V1505 triodes @ 2.5 kV plate voltage)
Such high power tube amplifiers can still be built but are definitely not a beginner's project due to the very high voltages used,heat dissipation issues and safety considerations. If you're unsure, better to ask the assistance of an experienced technician or HAM.

 

[richwalters]

You're almost down to 100W, where there are plenty of choices in tubes, OPT, power transformers, etc. for really what amounts to a barely significant power difference (3db between 200W and 100W). And no need to parallel output tubes if you use 6550s or KT88s and have some flexibility in the biasing of the output stage.

I'm finding forcing 100W o/p power from a pair of TungSol New Edit 6550 will shorten their lives.They aren't up to it and may last only 2000hrs. Parallel running using KT90's at lowish B+ to get the same power will last infinitum. This is my approach.

richy

 

[rmyauck]

Rich,

Could the new Tungsol KT-120's be made to work, for both great sound and long life? Parallel PP at KT-88 running conditions!

Thanks!

Randy

 

[Michael Koster]

I'm finding forcing 100W o/p power from a pair of TungSol New Edit 6550 will shorten their lives.They aren't up to it and may last only 2000hrs. Parallel running using KT90's at lowish B+ to get the same power will last infinitum. This is my approach.

richy

Tell me more; what B+, what g2 voltage, idle current? What are safe levels vs unsafe levels? Lots of ways to get to 100W and I assume you're not playing 100W continuously...

My plan is to run a pair of 6550s at 560V B+, g2 = 300V, idle current 25mA, load =5K plate-plate. I expected that to be easy on the tubes. What's wrong with these values?

Thanks,

Michael

 

[richwalters]

Rich,

Could the new Tungsol KT-120's be made to work, for both great sound and long life? Parallel PP at KT-88 running conditions!

Thanks!

Randy

I think it was Jim McShane who mentioned this in another thread who mentioned the KT120 is a different animal. I agree, it takes a serious driver to control it, a class A complementary Mos or sim. I prefer using the EH KT90with conventional Williamson config drivers; it's a nice one-up from the KT88.


r:-

 

[richwalters]

Tell me more; what B+, what g2 voltage, idle current? What are safe levels vs unsafe levels? Lots of ways to get to 100W and I assume you're not playing 100W continuously...

My plan is to run a pair of 6550s at 560V B+, g2 = 300V, idle current 25mA, load =5K plate-plate. I expected that to be easy on the tubes. What's wrong with these values?

Thanks,

Michael

I haven't tried true pentode (perhaps I should have mentioned this before)..... UL mode at high voltages is rather unkind. I don't run TungSol 6550 New Ed above 35W plate and screen together above 470V with I quies ~ 70mA. This may seem lenient but many designs of yesterday specified even higher values. The Svetlana version seems able to handle better but I don't have any in stock to try.

r:-

 

[banat]

Hi All,

I'm interested in building a dual channel tube amp that can produce up to 500 w rms per channel.

But I'm not finding any schematics for my little beast. Does any one know where I might start to make the concept a reality using parts available in today's world.

thanks

For that output power requirment `ordinary` line of out power tube have no chance for long time proper safe operation(el34,6550,gu50,kt....).Must use Transmiting power tube-glass, DH thoriared tungsten kathode,line is QB2/250(813),QB3/200,QB3/300,QB3,5/750,QB4/1100,QB5/1750 and QB5/2000. From this condition(continous service) of Out power reqirments(500w one chanel) easy finish beam Power Tetrode(pentode) 813(extremly good and long life tube) 4x QB2/250(4x 813 ) or GU13 excelent Russian equivalent-Class AB,UBA 1500v,Ug2 750v,Raa 4,65k,Ug1-85v,Ia steady-2x50ma,Ia max-2x 305ma,WinG1-0W, P out-520W,eficiency 57%.Upper anode Voltage is 2250V for continous service so with only 1500V on Anode Long time Operation is more secured.Opt windings can be isolate with paper and Silikone paste(very good up to 7,5kv B+ OPT suply).I made more powerfull Amplifier/Modulator for Rf use and must say next-there is no speaker they can not drive,behave like 3xpower SS amp,And sound is more than Ok.Good Luck

 

[richwalters]

Originally Posted by toddbailey
Hi All,

I'm interested in building a dual channel tube amp that can produce up to 500 w rms per channel.

Okay your'e back to Sq1. It's going to end up a home brew.
Have a peep at the GEC 400W for simplicity
http://www.triodeel.com/gec400w.gif
and redesign the dreadful driver stage to something more solid.
SOWTER AUDIO TRANSFORMERS does an o/p tranny.

richy

 

[Michael Koster]

I haven't tried true pentode (perhaps I should have mentioned this before)..... UL mode at high voltages is rather unkind. I don't run TungSol 6550 New Ed above 35W plate and screen together above 470V with I quies ~ 70mA. This may seem lenient but many designs of yesterday specified even higher values. The Svetlana version seems able to handle better but I don't have any in stock to try.

r:-

Hehe I've given up on UL... I think it was one of those times when history took a wrong turn, back in the glory days of hi-fi. It probably served it's purpose well but it's not IMO an easily scalable approach to higher power designs. As you demonstrate, it can be done but at a rather heroic price.

By running pure tetrode mode with local feedback, I expect to get respectable output power levels and still be easy on the tubes (15-20W plate dissipation), without going to the parallel connection. Efficiency goes down running tubes in parallel, depending on good dynamic matching, not to mention the extra heater power, etc.

Thanks!

Michael

 

[Rick Miller]

VAC/VALVE AMPLIFICATION COMPANY - We Build the World's Finest Audio Components

This company has in production an 440W monoblock all tube amp. I have heard it and it has tremendous dynamic range. Go to the current products section and you will find it along with other tube amps.

 

[toddbailey]

I'm very under-educated when it comes to designing tube amps, but have to ask.

Could one run 2 6550, or one of the kt series into a hammond 120 opt
perhaps 4 into a hammond 280 watt opt successfully?


I'd like to build something in the 100 to 200 watt range,

I don't have much info on the Hammond transformers so it's probably a dumb question

 

[Rick Miller]

I would think that two KT-88's with a 120W transformer, and four KT-88's with a 280W transformer would work. Two KT-88's will produce 100w.

 

[ChrisA]

I'm very under-educated when it comes to designing tube amps, but have to ask.

Could one run 2 6550, or one of the kt series into a hammond 120 opt
perhaps 4 into a hammond 280 watt opt successfully?


I'd like to build something in the 100 to 200 watt range,

I don't have much info on the Hammond transformers so it's probably a dumb question

Yes I think you have it right. Two of the KT88 in push-pull can produce up to 100W. Notice how I worded it "can produce up to...". I think you'd want to use four per channel to get into the 100-200W range

But this is NOT a suitable project for a first amp. The project would cost you close to $1,500 by the time you are done.

KT88's according to the data sheet like to be spaced on 4" centers, so the 8 tubes would fill an 8x16 chasis. The transformers weigh a ton. You need to build it as a pair of monoblocks as 8 of the KT88 tubes plus all the transformers and chokes is just to much for one chassis

Build a more modest amp first. Base it on 6L6GC and go for 35 to 50 WPC

 

[nigelwright7557]

I'm very under-educated when it comes to designing tube amps, but have to ask.

Could one run 2 6550, or one of the kt series into a hammond 120 opt
perhaps 4 into a hammond 280 watt opt successfully?


I'd like to build something in the 100 to 200 watt range,

I don't have much info on the Hammond transformers so it's probably a dumb question

How about a MOSFET/valve hybrid amp ?

You can have a valve front end driving a class AB MOSFET amplifier.
No large output transformers to buy.

 

[Michael Koster]

I'm very under-educated when it comes to designing tube amps, but have to ask.

Could one run 2 6550, or one of the kt series into a hammond 120 opt
perhaps 4 into a hammond 280 watt opt successfully?


I'd like to build something in the 100 to 200 watt range,

I don't have much info on the Hammond transformers so it's probably a dumb question

The 120W Hammond 1650T is 1900 ohms plate-plate and is meant for 4-6 parallel tubes of the 6550/kt88 (el34, 6v6GT, 5881...) ilk at ca. 420V B+

The 280W hammond 1650W is also 1900 ohms plate-plate and meant for 6-8 parallel 6550/kt88 tubes at higher B+ voltage, up to ~600V B+ for the full 280W output.

For a pair of tubes like 6550s, kt88s the typical plate-plate load in class AB is somewhere around 3000 to 5000 ohms plate-plate and 100W maximum. Hammond makes the 5000 ohm 1650R and Edcor offers a big selection including 3300 ohms, 4200 ohms, and 5000 ohms.

 

[Michael Koster]

... (...6v6GT...) ilk at ca. 420V B+

that should be 6L6GC