100 watt power amp thoughts

[garyboy]

Ok tax return season is here and i'm itching to build something big for myself. At first it was thinking of using 4 6550s of kt88s for a marshall build go get as close to 100 watts as i can. So i was sitting down and down a little math to kinda figure the tubes output biased at 70% just to get an ball park idea of what i can kinda expect to see. Here's what i got

6550/kt88 42w x .7 = 29.4w x 4 = 117.6w

I know that because of variations in you circuit, tubes, and power supply you won't actually be 117.6w. Then I got thinking alittle that i wanted to do something different so I thought that since kt-77s are a 25w tube and i have quite a few of them laying around. why not run six of instead of 4 6550s/kt88s. Plus i also read at the tone-lizard.com that you can put a 20mfd. 500v in parallel on pin 4 and increase your grid resistor to 5k 5w for a really cool sound. He said you'd loose a few watts but the sound was really good. I believe he called that running true pentode or something like that. Here the figures i got of the kt77s.

kt77 25w x .7 =17.5w x 6 = 105w

I think that would be perfect for something you don't see everyday. Then i can compare the sound of this amp w/ another one w/ 6550s/kt88s. Just a thought i had.

[HotBluePlates]

... was thinking of using 4 6550s of kt88s for a marshall build go get as close to 100 watts as i can. So i was sitting down and down a little math to kinda figure the tubes output biased at 70% just to get an ball park idea of what i can kinda expect to see. Here's what i got

6550/kt88 42w x .7 = 29.4w x 4 = 117.6w

I know that because of variations in you circuit, tubes, and power supply you won't actually be 117.6w. ...

Idle plate dissipation does not equal power output, especially in class AB amps.

Also, you should be able to get 200w of output from a quartet of KT88's. See the Marshall Major. The Ampeg SVT originally used a sextet of 6146's, but later switched to a sextet of 6550's for 300w of output (and often measuring more than that on the bench).

Best bet, skip the math and copy data sheet conditions, or copy a known guitar/bass amp. At least then, the heavy lifting of power output stage design is done for you.

If you really, really want to design it yourself, we'll need to start from (sort of) scratch and figure out all the details for the output stage.

Course, with KT77's, you could copy a 100w Marshall amp, use a quad and be all set.

... Plus i also read at the tone-lizard.com that you can put a 20mfd. 500v in parallel on pin 4 and increase your grid resistor to 5k 5w for a really cool sound. He said you'd loose a few watts but the sound was really good. I believe he called that running true pentode or something like that. ...

I have no experience actually using KT77's. Something I read recently that makes sense is that they are sonically between the 6L6 and EL34, having faster breakup than 6L6's and less compression than EL34's. This makes sense, because the "KT" series tubes are "kinkless tetrodes" which was the euro-name for beam power tubes (6L6's and 6V6's are beam power tubes, EL84's and EL34's are true pentodes).

I haven't read that part of the Tone Lizard site (is it back up and running now? was offline for a while), but I can only imagine he's applying the name to suggest the resulting sound is different than normal operation of beam power tubes.

So the only critical part from what you report is the use of "too-big" screen resistors. A high-value screen resistor causes the screen voltage to drop when the tube draws more screen current, which happens during peak currents. The drop in screen voltage then limits plate current somewhat, and the net effect is a compressed sound.

This is something I suggested a few years back on this site, and I know Tubenit has experimented with using 3kΩ or so screen resistors. The added compression would typically only happen at high volume settings unless you get really crazy with increasing the resistance. 2kΩ, 5kΩ, whatever gives you the sound you want without limiting power output beyond what you find acceptable will work.

[PRR]

Quote:
copy data sheet conditions
copy data sheet conditions
copy data sheet conditions
copy data sheet conditions
copy data sheet conditions

[PRR]

For 100W, I'd go 4 or 6 6L6/EL34.

The "large economy size" bottles (6550, KT88-100-120) are not always the best deal.

Do you buy beer in 100 gallon kegs? Or some handier size? Like 12oz bottles?

100W in one-pair KT88 is quite easy; but if one bottle dies you are pretty much sub-Watt for the night.

The big jugs are not available on a rainy night in a strange town; there's always a banjo shop with 6L6 and/or EL34.

A quad-rack down one tube will still "play". In less demanding work, the audience will never know.

All this was old hat to the old guys. Quad-6L6/EL34 iron is readily available in Fendery or Marshallike flavors; SVT and Sunn iron is more expensive, and custom iron more expensive yet (and experimental, unless you really know what you are doing or plagiarizing).

Also note that you CAN get an SVT's 300 Watts with ONE pair of say 4-125A. So why did Ampeg stuff a six-pack of little tubes? Cost. Availability. Safety.

FWIW, four 6L6/EL34 are about as easy to drive as two 6550/KT88. Max G1 resistor for 6L6/EL34 in fix-bias is 100K, for 6550/KT88 is 50K. So the grid resistance is no-difference with more-smaller bottles. The peak grid voltage is related to G2 voltage; while there are some extreme KT88 suggestions, mostly the G2 will run in sight of what you'd run 6L6 G2 at.

[garyboy]

Like I said i was am looking at building something that you don't see or hear about everyday for something different. It was just a thought that popped into my head. I'm just looking for my own sound and i'm not trying to reinvent the guitar amp. I have data sheets for the JJ kt77s that i have and have been looking at them and trying to figure out what irons to use. I'll also need to use a filament tranny for this since 6 kt77s will draw 8.4A heater current.  Here the data sheet for the JJ kt77s.

http://www.jj-electronic.com/pdf/KT77.pdf

[gldtp99]

Off topic a little, but my ears are still ringing------ a player asked me to build him an 800 watt tube power amp for his touring rig tonight---i explained that this is not a practical idea---he uses a 1000 watt solid state power amp with a floor POD as his preamp---he asked me, a few yrs ago, to build him a 1000 watt tube power amp when he was using a 1500 watt solid state power amp---i told him to stick with his 1000 watt ss power amp.
He then asked me what the most powerful tube amp I had on hand that he could try out---- I plugged him into the Sound City 120/Soldano SLO conversion i did a few yrs ago--- thru a single 4x12----i cranked the Vol and pre-PI MV to 9 each (the power dampening was on 10)---i did a quick EQ adjustment and had to leave the room because it was too loud for me--- but he had a big smile on his face-----i want down the hallway to another room, shut the door and BS'd with the room's occupant (another amp customer of mine)--- we could clearly hear the guy cranking the big EL34 x 6 SC 120/SLO amp--- he played for @15 minutes and came to the room we were in wearing a big grin--- I asked him if the amp had enough power for him--- he said that it did and he usually doesn't play that loud with his rig but he likes the headroom available with the huge poweramp--- he just wanted to see how the big tube amp felt cranked up--- and to see if the amp i built (rebuilt really) would take the abuse without blowing up--- he wants a clean tube poweramp of the same size to use with his POD preamp---I have one more inop/parts SC 120 in storage so we will probably make a deal on that one rebuilt in a clean format--- I'm thinking Fender AB763 circuit.
The six EL34's sounded very impressive cranked almost all the way up--- orig Partridge/Sound City transformers.
As PRR suggested, a quad or sextet of EL34 or 6L6GC tubes is a good base for a high wattage amp---transformers are available at somewhat reasonable prices--- you don't need super high B+ voltage levels to produce crushing power--- the one he was playing has B+ of 480V.
BIG MASSIVE POWER !!!!!! ..................................gldtp99

[garyboy]

That sounds pretty sick there makes my ears ring just reading that. Definately sounds like something I wanna do. I have a PT for a JCM800 2203 do you think that would have enough juice for 6 power tubes or do you think i need something alittle bigger? Its a Classic Tone tranny.

[gldtp99]

I have a PT for a JCM800 2203 do you think that would have enough juice for 6 power tubes or do you think i need something alittle bigger? Its a Classic Tone tranny.

this PT: http://www.classictone.net/40-18053.pdf---- It would be a poor choice for six EL34's-- not enough current capacity in the heater winding and the HT winding is only rated at 290mA---- this PT works very well with EL34 x 4---- Marshall's specs for 100 watt EL34 x 4 heads state that they actually put out 170 watts at full power, they are loud powerful amps.
My '78 Marshall 2203 (EL34 x 4) is an extremely loud amp--- I didn't have it at my shop when that player came over--- My 2203 might actually be louder that the SC 120/SLO----i don't play that loud--- but this guy would have been similarly impressed by the EL34 x 4 Marshall as he was by the big Sound City 120 conversion--- but the 2203 might have needed an OD pedal to get a similar tone as the SLO preamped SC 120.
Since you already have the 100 watt Marshall replacement PT---just go with EL34 x 4---if you're going to be pushing the amp at high levels frequently i'd suggest this OT for your project: http://www.classictone.net/40-18072.html--- i have one of these sitting here waiting on a future build---- the 2.2k primary impedance is easier on the EL34's when the amp is pushed to high levels---stock primary impedance is usually 1.7k on OT's like this.............................gldtp99

PS-- I initially rebuilt my SC 120/SLO with EL34 x 4---- but i later added the other two EL34's because of the mismatch in OT primary impedance--- it would eat EL34's until I staggered the speaker load (running the amp set at 8 ohms into a 16 ohm cab)---- I just put in the other two EL34's (and new sockets/wiring) so it would be running as it was designed------ the volume level isn't that much different, maybe a difference in fullness/depth more that pure volume.

[Willabe]

I just put in the other two EL34's (and new sockets/wiring) so it would be running as it was designed------ the volume level isn't that much different, maybe a difference in fullness/depth more that pure volume.

KOC has a Marshall type build in TUT3 that he calls the 5100. It is a 50w amp that uses 2 or 4 EL34's to get the sound of a 100w at 50w's. The difference being the fullness/depth or muscle of a 100w (but at 50w) because of the transconductance of 4 tube instead of 2.


                Brad