4xKT88 Design Help

[potatofarmer]

The schematic looks to me like 510V at the reservior caps, 490V on the screens and 485 on the plates. Look at the way he draws 8s, 9s, and 5s elsewhere.

Then again the ink ran so much it's almost a Rorschach test... is that a heater between the plates on the phase inverter, or is that a bunny...?

[g1]

but with my output transformer being rated at 120W, I was wondering if I had to bias colder to stay within specs of the OT.

I believe your concern here is that you may have up to 200W capability from the 4 x KT88. Reducing the bias is not a way to achieve less power output from the tubes.
You should probably either get a bigger OT that can handle the power, or just run a pair of KT88.
Bias levels are sometimes limited to protect undersized power transformers, like in the recent discussion about 6550 bias in Marshall amps that were built for EL34's.

[Wombaticus]

The schematic looks to me like 510V at the reservior caps, 490V on the screens and 485 on the plates. Look at the way he draws 8s, 9s, and 5s elsewhere.

Then again the ink ran so much it's almost a Rorschach test... is that a heater between the plates on the phase inverter, or is that a bunny...?

I'm virtually certain its 495V on the plates and 490V on the screens, which makes sense given the 100 ohm screen grid resistors. I reckon that operating point will indeed come in pretty close to 150W, so that Hammond might be a little underspeced. On the other hand their ratings tend to be quite conservative. I know quite a few guitar amps with 4xKT-88s near 200W have been done with the Hammond 1650T (including a well-known design from Kevin O'Connor of London Power fame, it's in one of his TUT books). But, again.... a guitar doesn't need to go anywhere near the 30Hz low end of the 1650T (significantly reducing the load on the OT), whereas the low B on a 5 - string bass is right about 30Hz, and will consequently require more from the OT.

[erock0138]

Sorry, just to confirm, can you see that there's nowhere between the OT CT and the tube screen grid that could have 'lost' 40V ?
Even 490V seems low; UL screen grid often idle at a higher voltage that their plate, especially beam tetrodes, which don't draw as much screen grid current as pentodes (I acknowledge that GEC describe them as beam pentodes).

Yes I do see that, thank you for your explanation.

[erock0138]

I believe your concern here is that you may have up to 200W capability from the 4 x KT88. Reducing the bias is not a way to achieve less power output from the tubes.
You should probably either get a bigger OT that can handle the power, or just run a pair of KT88.
Bias levels are sometimes limited to protect undersized power transformers, like in the recent discussion about 6550 bias in Marshall amps that were built for EL34's.

Yes, That's precisely my concern. I was trying to go for the size in between an SVT and a V4. Power wise. V4 is great for my gigs with p.a. support and in ear monitors, but not enough for my bar gigs with no p.a. support. So, I was hoping to achieve the one size fits all head.

I've searched quite a bit for 200W rated transformers for this application, but didn't have much luck. Do you know of any that would be available? I contacted heyboyer about some copies of the Hiwatt Dr205, and they run about $200 a piece, but I don't have the specific data sheets to accompany them (Though I didn't ask, they may supply me with them)

[erock0138]

I'm virtually certain its 495V on the plates and 490V on the screens, which makes sense given the 100 ohm screen grid resistors. I reckon that operating point will indeed come in pretty close to 150W, so that Hammond might be a little underspeced. On the other hand their ratings tend to be quite conservative. I know quite a few guitar amps with 4xKT-88s near 200W have been done with the Hammond 1650T (including a well-known design from Kevin O'Connor of London Power fame, it's in one of his TUT books). But, again.... a guitar doesn't need to go anywhere near the 30Hz low end of the 1650T (significantly reducing the load on the OT), whereas the low B on a 5 - string bass is right about 30Hz, and will consequently require more from the OT.

Yes, I've read a lot about TUT's projects, and how they "work" apparently, but the transformers are not spec'd well at all for all of the applications. I've only ordered the chassis at this point for this project, and am just debating if I want to change it all around and go with a more basic build. Just 4X6L6 UL. Which is what I'm more used to building. I'd prefer the KT88's for some extra wattage, but I wonder if it would be truly noticeably louder (Without the exception of more headroom, perhaps.) Or just a dual KT88.

I'd rather give up some wattage than stress the transformers, this will end up being my main gigging amp that I will depend on. Thank you again.

[pdf64]

How about running it with VHT that will limit the power to the OT, eg 600VCT should give a HT of ~420V.
According to the IDS and factoring in some losses, that shouldn't push the 120W OT too hard.
And the KT88 should have an easy life.

[Leo_Gnardo]

I'd rather give up some wattage than stress the transformers, this will end up being my main gigging amp that I will depend on. Thank you again.

The 120W Hammond OT in use in the amp I built way up top never seemed stressed, if anything it's loafing. And you can too. If anything, find a way to sturdy up the mounting feet, this one got bent from bouncing around the back of the van with the amp tipped up on its end.

Another possible choice for power transformer worth looking at: the BFT, from One Electron. Antique carries it & probably some other vendors too. You won't need a "helper" filament trans with this, there's 2 6.3V windings, 9.6A on one, 4A on the other. Plus it has "nested" hi voltage windings 770VCT 385mA plus 520VCT, should you choose to forego ultralinear & have reduced voltage on the screen grids. You can imagine what BFT stands for, this piglet is 15 pounds, similar to the Hammond you're considering. Overall you'll save weight without a separate filament trans and have a B+ less worrisome to your output tubes. No separate bias winding but you know how to get around that now.

For your purposes I doubt the transformer or tubes would be stressed. Just your back.

[erock0138]

The 120W Hammond OT in use in the amp I built way up top never seemed stressed, if anything it's loafing. And you can too. If anything, find a way to sturdy up the mounting feet, this one got bent from bouncing around the back of the van with the amp tipped up on its end.

Another possible choice for power transformer worth looking at: the BFT, from One Electron. Antique carries it & probably some other vendors too. You won't need a "helper" filament trans with this, there's 2 6.3V windings, 9.6A on one, 4A on the other. Plus it has "nested" hi voltage windings 770VCT 385mA plus 520VCT, should you choose to forego ultralinear & have reduced voltage on the screen grids. You can imagine what BFT stands for, this piglet is 15 pounds, similar to the Hammond you're considering. Overall you'll save weight without a separate filament trans and have a B+ less worrisome to your output tubes. No separate bias winding but you know how to get around that now.

For your purposes I doubt the transformer or tubes would be stressed. Just your back.

Okay! Sounds good, I checked out the Bft. I'd like to go with it and scrap the extra filament transformer, and like the voltages a little more. The data sheet looks all good, but am curious as to on antique supply's description, it claims to be used for a stereo amp, or a push pull 80W amp. I'm confused by this rating? Obviously the transformer is within spec on the data sheet, just wondering why the description was listed like that.

[Justin Thomas]

Here ya go - the original use of the BFT:
https://www.tubesandmore.com/products/P-TUBT-1

BFT runs TWO of these, one left, one right, voila, stereo!

Justin

[Leo_Gnardo]

What Justin posted ^^ in #25, originally intended to run two honkin' big single-ended hi fi amps, class A of course. I expect BFT is very conservatively rated, at onset of clip you'll probably get more like 100W-ish easy with KT88 or 6550 quad. Which is really just a pinch beyond 80V.