SSS KT88 schematic and info?

[Luthierwnc]

Hi Guys,

I've been putting pencil to paper on some new build ideas. The SSS is intriguing but I've only been able to find 6L6 schematics. Is there a KT88 version buried here at TAG somewhere? What would be particularly useful would be some detail on the driver bias values and voltages along with screen resistor values.

My current notion is something minimal using 4 preamp tubes and a pair of KT88 running with about 520 volts on the plates. Plenty of juice for what I need.

Thanks for any help, sh

[talbany]

Just follow the output section of an SVT .. 47K grid stoppers (for guitar) would be overkill.. 4.7K there should be just fine.. Add subtract as many tubes as you wish.. ..around 500v on plates of either 6550 or KT-88 is money!!
Have a Nice Day!....
Tony

[martin manning]

Hi Guys,

I've been putting pencil to paper on some new build ideas. The SSS is intriguing but I've only been able to find 6L6 schematics. Is there a KT88 version buried here at TAG somewhere? What would be particularly useful would be some detail on the driver bias values and voltages along with screen resistor values.

My current notion is something minimal using 4 preamp tubes and a pair of KT88 running with about 520 volts on the plates. Plenty of juice for what I need.

Thanks for any help, sh

Where's your reference 6L6 schematic that you want to convert to KT88. Looking at the SED/Svetlana KT88 data sheet, I think at 520V Va you will want to run at 300V Vg2, and use a 4k-5k load for two tubes.

[Luthierwnc]

Thanks gents.

Martin: the schematics I'm studying now are 001 and 002 in the files (or subject headings) here. Those are both 100-watt 6L6 versions. All the layouts I've found are too. There is much less information on the 150-watters. What I'm fishing for here is if we do have that flavor cataloged on TAG and I just can't find it.

At this stage of the doodling I'm looking at iron and want to know if I need parallel HV stages. My next move will probably be trying to decipher the voltages from the original SVT schematics and put them on the Piazza version Tony recommended -- unless I can find a readable original.

Part of my thinking here is that I've already got a 100-watt 102. I haven't worked with KT88s before (I do this for fun) and a 70-watt amp might fit nicely into the museum. I have restored 3 V4/VT22 Ampegs running plate volts in this neighborhood. Those were all 7027 or 7581 rigs but IIRC, guys did sub KT88s and 6550s in those amps when NOS tubes dried-up.

Thanks for looking and if any non-proprietary schematics are in the vault, I'd love to see them.

Cheers, sh

[martin manning]

SSS schematics show one CF driving each half of the power tubes, and IMO you don't need all the claptrap in the SVT for one pair of KT88's. I would just use a PT with a single HT winding for both the HT and the bias supply, and take the screen voltage from the middle of a stacked reservoir. A 100W PT shouldn't have any trouble with that.

[mobisimo]

Hey there,

Check out ElectronAvalanche's post about halfway down this page:
http://ampgarage.com/forum/viewtopic.ph ... c&start=30
He suggests values for an altered SVT driver section. It looks like HAD used his typical (give or take) long tail pair values for a PI instead of the whole Ampeg shootin match. NFB/presence is a little different than ODS. The bias source isn't explained as clearly, though I suppose one might go about it the same way as in #002. There are some pics on page 1, too.

Hope this helps.
Mo

[Luthierwnc]

Thanks again. If I'm reading the SVT schematic right, it is putting about 640 volts on the plates and 340 on the screens -- the latter coming from the second HV winding. The highest negative voltage going to the bias supply is -150 but that gets knocked down by half before it gets to the 12BH7.

Nosing around the V schematics, the V2 might have some insights. Much higher screen voltages but that can be corrected. The bias supply on those amps could also be modded.

For iron I'm looking at one of the Antek toroidal transformers:

http://www.antekinc.com/as-4t400-400va- ... ansformer/

This has two 0-400 VAC windings at 500 ma and a couple stout filament supplies. Cheap, quiet and a lot less weight than usual. I've had very good results using the little 0-280 50va versions for Dumbleators and other rack FX. On the PSU designer I should be seeing around 530 volts at the first node.

All this is supposition right now but I'll add to this thread when I get things on paper. Any more comments are welcome and thanks again for looking. sh

[talbany]

Actually if you are doing KT88's (only) you don't need a lower voltage screen supply as they can handle 600v on the screens no problem..

http://www.drtube.com/datasheets/kt88-sed1999.pdf

just run it like you would a 6L supply (1k) screen resistors and get your bias supply off a parasitic supply from 1 leg of the HT (like 002) of coarse you cannot swap in 6550's..
The only real issue then is the Stby switch..You have to break the screens and plates and bias supply B+..There are many other way's to do a Stby..


AFAIK there is no info on how Dumble ran his KT-88 Amps( my guess is he swapped in the 88's on a 6550 supply) One of the amps (newer) had what looked to be mercury Dynaco Iron at the OT, as well as the 12AU7/12BH7 Driver setup, it also had a lower gain type preamp. it was a 150W amp with 4 6550's . The PT in one of them put 525 on the plates, in the other it was around 495

IMO..KT88 much better choice than 6550's..6550's were designed for servo applications in WWII aircraft..88's were designed for audio.

Tony

[martin manning]

Actually if you are doing KT88's (only) you don't need a lower voltage screen supply as they can handle 600v on the screens no problem..

http://www.drtube.com/datasheets/kt88-sed1999.pdf

just run it like you would a 6L supply (1k) screen resistors and get your bias supply off a parasitic supply from 1 leg of the HT (like 002) of coarse you cannot swap in 6550's..
The only real issue then is the Stby switch..You have to break the screens and plates and bias supply B+..There are many other way's to do a Stby..


AFAIK there is no info on how Dumble ran his KT-88 Amps( my guess is he swapped in the 88's on a 6550 supply) One of the amps (newer) had what looked to be mercury Dynaco Iron at the OT, as well as the 12AU7/12BH7 Driver setup, it also had a lower gain type preamp. it was a 150W amp with 4 6550's . The PT in one of them put 525 on the plates, in the other it was around 495

IMO..KT88 much better choice than 6550's..6550's were designed for servo applications in WWII aircraft..88's were designed for audio.

Tony

Tony, with plate and screen both around 500V, a load line that won't over-dissipate the plate will be way below the knee in the I-V curve. Early Marshall's using 6550's worked "ok" at 465V, but as the plate voltage is increased further I think it would take a lot of screen voltage sag to keep the screen grids from over-dissipating at high power. KT88 screens are rated at 8W as opposed to 6550's 6W, but my choice would be to reduce the screen voltage.

I have read that 6550's were designed as military servo amplifier tubes too, but according to Eric Barbour, that isn't true. Tung-Sol designed them in 1955 as a low-cost, high-power audio tube. MOV's comparable KT88 came out in 1957. He mentions servo amplifier use in the context of computer tape drives, but this application emerged because they were cheap and readily available.

[Tubetastic]

Old Tung-Sol catalogue info/pics:

Vintage Tube Services

[talbany]

[Martian
With all due respect!!...Personally I think you are over thinking it!
I am looking at the plate curves (on KT-88 ) and am unclear about what you are talking about..Could you show me on the curve what you are thinking?..Thanks!
The main problem I have with plate curves are they are a general guideline for a specific operation, mainly Hi fi...The curves on a guitar amp will look much different depending on how you set it up! (Where is the tube biased!!,, are you taking into consideration screen current when biasing, how much current are the screens drawing at full power..etc,etc) which we may not even be operating close to in our guitar amps...We can take for example the Marshalls loaded with 6550's that ran there screen voltages close to the plates 500V ( beyond there limits) and still survived ..How many 6L's that run 470-480 with screen supplies close and do just fine (with screen dissipation @ only 6 watts)..How many Deluxes run there plates at 430V on 6V and last years..So IMO.. Why would a much more stout tube like a KT-88 (42w Plate/8w screen dissipation) have any problems running screens at 500v?...



Again with all due respect!

As far as the Tung Sol 6550..Many tube companies might say they designed a specific tube for Audio (different plate materials,construction methods) and call it an audio tube when really nothing in the actual design perimeters/characteristics have really changed..A kind of marketing ploy?..
Actually I don't know but would like to find out..I would love to read Erics paper on the matter,if someone has it please post it!!..

BTW..This article here says Tung Sol didn't come out with there (new improved) 6550 audio tube to compete with the KT-88 (in 57) until 1965..
I am confused?
Quote:
The 6550 was designed by Tung-Sol, New Jersey U.S.A. and first introduced in 1965. It was the "better" 6L6, with higher output better stability. It could work at 600VDC and a screen of 400VDC, and Plate dissipation of 35 watts. We need to say here, that the MO Valve company had already build an improved 6550, it was the KT66. In 1957 the M.O.V. of England striked back with the another improved tube, their famous K.T.88, later sold as G.E.C. KT88, The KT88 had higher ratings than the 6550. It had 800VDC plate, a screen of 600 VDC, and dissipation of 42 watts, outdoing all of the previous products.

http://www.jacmusic.com/KT88/kt88.htm#6550

Not trying at all to be a smart ass here,would just like to learn more on this subject!!..Looking forward to better understanding it!!

[Tubetastic]

I think Martin maybe referring to this VTV article

[talbany]

I think Martin maybe referring to this VTV article

Thanks..
Well there appears to be some different opinions on why the 6550 was designed for?

If Eric is correct (designed for audio)then someone needs to update the Wiki site to originally designed for audio but used in servo.. (to avoid confusion)
http://en.wikipedia.org/wiki/6550

IMO Not that important but interesting none the less..Still a nice audio tube for both!!...Looks like Tung sol came out with the 6550 in 55 but later upgraded (compete with the 88's in 65)
On to bigger things!

Love this kind of stuff

Tony

[67plexi]

Have a nice Power Amp steak bone to chew on

[martin manning]

Yes, that is the article I was referring to.

I am looking at the plate curves (on KT-88 ) and am unclear about what you are talking about..Could you show me on the curve what you are thinking?

If you pick 550V for plate voltage, and then a load line that stays within plate dissipation limits, say 4 or 5k, the knee in the 0V Vg1 curve looks well positioned at 300V Vg2. If you run the screen voltage up to 550, the whole family of grid curves moves up and to the right, and the load line will cross far below the 0V Vg1 knee. That means a loss in headroom and the screen current will begin to shoot up long before Vg1 is driven to zero. Basically this type of tube seems to be capable of delivering current far beyond its plate dissipation limits when the screen and plate voltages are similar.

The main problem I have with plate curves are they are a general guideline for a specific operation, mainly Hi fi...The curves on a guitar amp will look much different depending on how you set it up! (Where is the tube biased!!,, are you taking into consideration screen current, how much current are the screens drawing at full power 5% 8%?..etc,etc) which we may not even be operating close to (in our guitar amps)..Example:..When we bias our tubes we normally do not take into account screen current ..

The Svetlana KT88 curves are not a very complete set since they don't show screen current in pentode operation. However they convey a lot of information about the tube, regardless of where you choose to operate it. You have to scale them if you want to use a different screen voltage, but the bias point you choose has no effect, and screen current is very low at idle conditions.

We can take for example the Marshalls loaded with 6550's that ran there screen voltages close to the plates 500V ( beyond there limits) and still survived ..How many 6L's that run 470-480 with screen supplies close and do just fine (with screen dissipation @ only 6 watts)..How many Deluxes run there plates at 430V on 6V and last years..So IMO.. Why would a much more stout tube like a KT-88 (42w Plate/8w screen dissipation) have any problems running screens at 500v?...

6L6 at 480V plate and screen is a much different situation than a 6550 at the same voltages. With a 4k load the 6L6 is inside its plate dissipation limits, and the load line goes right through the knee of the 0V Vg1 curve. That tells me that the screen dissipation will not be a problem. For the 6550, there is no problem with plate dissipation, but since the load line (even 3k4) crosses well below the knee (a little more than half-way up). The screen current is going to take off long before the grid voltage goes to zero, and that will be very hard on the screens.