kt88 build

[shooter]

 I have a 640(320-0-320) supply, schematic below.  Wired the B+ and screen section then did an unloaded test, wound up with 880vdc, (640*1.414=904).  I’ve looked at schematics with voltage readings n find supplies with 300-0-300 showing 380-400ish at B+ (guessing loaded).

Can I use 640*.707=453 to estimate my “loaded” volts?

The build –  2 kt88’s (AB1), 3 12**7’s, 1 6sn7.  I’m expecting 75-90mA per kt idling n 5-10ma’s for the pre’s. 

If I was to pick up a big resistor as a PS load, would I calc R using 880, or something close to 500.  I’m using .2A for I.

Thanks

[sluckey]

Can I use 640*.707=453 to estimate my “loaded” volts?

No. It's not that simple. A PT that's rated for 200mA will have a higher output voltage than a smaller PT that's rated for 100mA if they are both connected to a 100mA load.

[shooter]

That's why im askin cuz I don't want to toast the tubes!  N the resistor would be a dummy load so I can see what the tubes would see without using them as test subjects.  The schematics are kinda a meld from a Marshall  and Sunn.  So if I just move the CT to ground that will lower B+?  Do I need half wave instead of the bridge?

Wouldn't the Dummy load simply limit current to whatever I fix R at and still give the power supply enough load to show me something close to "real world" volts?

Anyway, thanks for taking the time.

[DummyLoad]

B+ should be around 450-480V. i ran a sim with PSU designer. assumed you have a weber marshall 100W PT based on part number in schematic. sim attached. load stepped from 100mA to 200mA. the transformer source impedance is estimated based on the ratings; e.g. 640V @ 400mA. if you provide the primary and secondary DCR, you can get somewhat more accurate results, but not worth the effort.


--pete

[Ritchie200]

Please do not pay any attention to the old KT88 spec sheets that all the manufacturers continue to use to try and impress us users.  There are maybe two that will live for any time north of 600v - and not that much north.  The old Gold Lions could do it, but you better have some exceptional sockets, a VERY exceptional OP tranny, double insulation on your B+ runs, no sharp solder joints, etc...  It's just not worth it to push them that hard.

People are probably tired of me suggesting this, but one of Marshall's best sounding amps (besides the Major!) was the Park 75.  Essentially an old 50 with two KT88's in the OP for ~75 watts.  The current KT88 production crop will live a long and useful life at 400v and you will have an amp with plenty of grunt.  Plus it's well trodden ground for output architecture.

Just MHO based on a lot of expensive smoke.
Jim

[shooter]

Thanks for the SIM run, that's about what I expected as a loaded voltage.  My problem is, I have the tranny but there is no more mad money!  Soo I'm stuck with what I have.  I hope to run the kt's about 480 to 500.  They have been working over a year in a stereo at 470.  I'll r e-wire the CT, hook a big-O Resistor to the supply n see where I wind up.

Thanks all for the help.

[clyde]

I built a 2xKT88 head in 2006 with 425-0-425 PT, 1650R Hammond OPT, running north of 600v on both plates and screens with the biggest issue being tube microphonics when the client wanted it turned into a 1x12 combo.  This amp is gigged almost every weekend with guitar and steel guitar.

[PRR]

> Can I use 640*.707=453

NO!

For a *single* winding into a rectifier-capacitor filter:

Vac * 1.414 = Vdc

With a 320-0-320 you have two choices:

Ground the CT ("0"), each 320V AC side will give 452V DC. For proper transformer use you need two diodes.

Use the whole 640V AC winding, get 904 VDC. This needs four diodes (a bridge).

900V DC (even 880V) is insane for the popular tubes and for builder-user safety.

(Also the bias plan you show, I don't think will work.)

You want the TWO DIODE, Grounded CT plan. You will get around 450V DC.

At 450V, the most-obvious bottle-set is two or four 6L6GC/EL34 with Fender (or Marshall) 50W or 100W output iron. (Especially if this PT is Marshall 100W iron!) You *could* replace the 'L6/'34s with '88s, a very handsome amp, but a lot of mad-money.

[shooter]

Thanks PRR, that’s the way I went, I was originally following a marshal 200w PS!  The supply is working fine n so is the amp!!!  I call it the “ProwPlus”, The Prow is my SE quad EL-84, soon to get a 2 knob preamp section.
(my thinking on the .707 was if unloaded was 1.414, would loaded be .707)

I attached the current revision schematic.  I eliminated the 6J6.  It worked fine for modulating the trem osc. with the main signal but wasn’t enough amplitude before it all distorted.  I put the trem in at the IS tranny CT like Gibson did.  It needs more gain, maybe.  Need a real musician for that. All in all it sound good to me n I can’t hear so it must be ok!  Again a real musician will tell me that along with tone.  If it passes muster then it’s on to a cab, proper faceplate, probably FX out/in, and line out.

Credit where it’s do;
Channel 1 is a mesa rhythm section (lite)
Channel 2 is from Sluckey’s Rock-ola, with a gain stage added.
The PI is based off a Gibson Skylark along with the tremolo.
The PA section is patterned after Tubinit’s Tweed overdrive special
The PS is sorta  piece-meal.

Thanks to all for keeping me on track!

 

[PRR]

There's no direct path to "0.707*Vac".

When using the *center-tap* rectifier, sometimes the Vac is expressed as the total. 640V in this case. Then you first divide by 2 (to get each side), then multiply by 1.414 (to get peak). Which does work-out to 0.707, but just confuses what really happens.

That's no-load. Actually true with-load, but the Vac will drop under load.

How much? About the same as the water-pressure in your shower. Might be nearly full pressure. Then someone flushes a toilet, someone opens the garden hose, another toilet, and then the washer starts a rinse. With enough load, 100% pressure can be Zero % pressure.

50% pressure is pretty poor for a shower, and terrible for a transformer. It implies that half your electicity is being wasted as heat in the transformer!

10% to 20% drop of voltage under "normal" load is more likely.

I computed an ideal 452V above. If the amp is on at all it will be less. If the PT is a reasonable match to the OT, I would expect FULL power B+ to be 10% lower or a bit less, 410V to 390V.

[shooter]

Thanks PRR, that's just where she landed, 462vdc unloaded, 398vdc everybody idle'n.  Gonna try the min/max on the fluke n see if I can get a feel for the PS "swing" as I'm playin at max settings.  I can troubleshoot anything, but design is a whole other beast, with related parts!

[PRR]

> term you were looking for might have been flow.

No.

1) You don't know how dirty I get. I need to PRESSURE-wash the tractor grime off.

2) Domestic water systems usually work in the near-sonic range, where flow does not vary "much" with pressure.

[jazbo8]

The PA section is patterned after Tubinit’s Tweed overdrive special

None of the TOS versions has KT88 or perhaps I overlooked it... Anyway which OPT are you going to use?

[DummyLoad]

The PA section is patterned after Tubinit’s Tweed overdrive special

None of the TOS versions has KT88 or perhaps I overlooked it... Anyway which OPT are you going to use?

i don't think shooter is claiming that. he just stated that the PreAmp was patterned after tubenit's TOS. probably not a bad plan if the PI is tweaked to drive KT88s.


--pete

[jazbo8]

i don't think shooter is claiming that. he just stated that the PreAmp was patterned after tubenit's TOS. probably not a bad plan if the PI is tweaked to drive KT88s.--pete

I was curious about the PA, not the PreAmp... the PA is suppose to be based on the TOS, but I'm puzzled , isn't the Skylark SE? So what PI is he referring to? Below is the quote again...

Credit where it’s do;
Channel 1 is a mesa rhythm section (lite)
Channel 2 is from Sluckey’s Rock-ola, with a gain stage added.
The PI is based off a Gibson Skylark along with the tremolo.
The PA section is patterned after Tubinit’s Tweed overdrive special
The PS is sorta  piece-meal.

[shooter]

I used, as reference, the ga5 with IS PI, concept for my PI, bought one UIS-1 also the trem came from the ga5.  the IS tranny is capable of 1:1 or 1:2, currently 1:1.  I used TOS MV and biasing scheme in my PA but only had a 100k gang pot instead of the 250k so that's what's in currently.  I'm using a weber 022889 OT.
I Got the gain stages through the PI at about 26vacp-p so the kt88's are driven ok, still need to scope everything but the weathers great so.......  I'd like to experiment with "switching" the IS tranny between 1:1 and 1:2, kinda like an odd gain/overdrive feature.

[jazbo8]

I used, as reference, the ga5 with IS PI, concept for my PI...

Thanks for the clarification, now I get it, love to see a copy of the full schematic - there are a lot of interesting things going on with this amp - certainly don't see it everyday...

[PRR]

> I send my TO 35 over to you ... now my tractor's weight has doubled.

Well, the Fergie 35 is a small rig, so "double" isn't much? <G>

3,000 pounds without ballast, 30-35HP. If you have the Gas engine, I know it VERY well (Continental 134). Same size as my Honda sedan but a lot more grunt.

Ford 4500 is 66HP and (oddly) 4500 pounds. Mine has the loader and backhoe, full 2-seat cab with heater, total weight is above 9,000 pounds.

"Double" is 3,000 pounds of clay? That's a 2/3rd load in my loader. (Well, per factory docs.... my pump and seals are not at their best.)

The nearest 12" water line is 10 miles away. No, I can NOT wash your tractor-- there's not enuff water in my little well.

You can model the water problem in SPICE, though not with the standard electronic parts.

Just for fun, you can model my ditch. It crosses several rock outcrops with various wide and narrow flumes and a pipe. At high flow it bottlenecks in the pipe. At some medium flow it bottlenecks in the narrow cut at the lower rock. At very-low flow both of these are free and water barely slides over the wide cut in the upper rock.

[PRR]

Oh how cute!! Gotta be one of the smaller Fords ever built.

[shooter]

I live in farm country, have friends with tractors, me I'm just a long haired country boy with a 1.5" shallow well that flows at a rate that is calculated in hours! But, there's no hurry here, that's for the city folk.

 So if I put a 30 06 shell in my 30 30 It would go further?

Anyway, I do have a question, the kt's are each idling at 60mA 400dvc, 24watts, would that be the amps average rating, 24watts?

I did attach the schematic somewhere earlier in this post, just before the plumbing isle I think!
That's sorta why I did this build, had everything except the OT and IS tranny so, why not see

thanks guys
 

[PRR]

> would that be the amps average rating, 24watts?

No.

A push-pull amp can idle at "zero" and still make FULL output. (But it would sound rough at small outputs.)

For the full power rating we need the volts and the load impedance.

Weber 022889 is rated 2K plate to plate. That is VERY low for any two tubes; the tubes will strain badly. It is spot-on for *four* of any of the large tubes (6L6GC, EL34, KT88, 6550). With 400V-450V of B+, output is in the 80-100 Watt zone.

You actually could run two KT88 into about 2K load but as *triode*, for around 25 Watts. This would be too "polite" for rock-and-roll, also way heavy for its power.

[jazbo8]

I did attach the schematic somewhere earlier in this post, just before the plumbing isle I think!

Are you referring to this one? It contains only the output stage.
Agree with PRR, the Weber OPT is really not suitable for 2xKT88s in pentode mode, unless you double the speaker load value on the secondary, i.e., 16 Ohms on the 8 Ohm tap, or 8 Ohms on the 4 Ohm tap, etc.

[shooter]

Thanks, my original design was gonna be 4 kt88s, then I thought I'd start with 2, get it "right" then add two.  I probably didn't say but I am using 2X12.  Sorry about the schematic, I have pieces all over the computer!
Hopefully this is the correct one.

[DummyLoad]

this one is the one you need, or something like it for KT88 powered 200W guitar amp. another option is the hammond 1650WA   


--pete

[jazbo8]

Thanks, my original design was gonna be 4 kt88s, then I thought I'd start with 2, get it "right" then add two.  I probably didn't say but I am using 2X12.  Sorry about the schematic, I have pieces all over the computer!
Hopefully this is the correct one.

Great, thanks for sharing.

[shooter]

That's it, I'm moving underground, guilty of calculating in public!!  what a world!

I'm running the amp with an 8ohm speaker, (soon 2 16ohm), reflecting 4K to the tubes (normal tranny specs 2k @4ohm).  Will that effect ot power rating, specifically current?

Finally got scope on the amp, there is no perceivable PA distortion, so my guess is I'm not driving the kts hard enough, (need larger AC in?).  The bias is about 28v, input signal is between 46-55VAC-p-p.

can I do it either way, more input or lower bias?   

thanks N I promise not to calculate in public. 

[jazbo8]

I'm running the amp with an 8ohm speaker, (soon 2 16ohm), reflecting 4K to the tubes (normal tranny specs 2k @4ohm).  Will that effect ot power rating, specifically current?

4k is a good load for 2 x KT88, compared to the 2k, it will have lower output power and peak current.

[Willabe]

But there is something wrong with the bias info, you said earlier you were getting 400V @ 60mA at idle, but that can not happen with just -28V of bias, it should be ~-45V, perhaps that's why there is no PA distortion, as they are no where near fully driven.

Less -vdc bias (-28vdc) will cause the power tubes to run hotter. More -vdc bias (~-45vdc) will cause the power tubes to run colder.


                    Brad   

[shooter]

The 28 is voltage at cathode resistor, forgot to measure grid to cathode, will get that tonight.  She is running lower power than I expect, volume wise, it's about equal to my SE 6550 running about 99.5% of max plate.

thanks again

[jazbo8]

The 28 is voltage at cathode resistor, forgot to measure grid to cathode, will get that tonight.  She is running lower power than I expect, volume wise, it's about equal to my SE 6550 running about 99.5% of max plate.

thanks again

You don't need to measure the grid voltage, based on the figures provided, the estimated maximum output power is just short of 60W with the KT88's just cruising along.

[shooter]

thanks jazbo, I got a few different resistors coming, has 470 now, getting a 430 and a 390 to try n see if I can get some PA distortion, if that don't get it I'll rewire the IT for 1:2 n see.

Got the cab built, found in my barn some nasty looking white-washed wood, turned out to be burl'd maple, tiger eye, worm holed n all.  It's about 100 - 150yrs old according to  the 'ol guys that I talked with.

still need knobs, faceplate, n back plate but  it's summer!

thanks for the help

[DummyLoad]

cool looking wood & nice cabinet.


--pete

[shooter]

Thanks dummyload, I was quite surprised when I planed it down!

Finally got a real musician to play through it!  He says leave channel 1 alone, get more eq headroom (mainly more treble) outta channel 2 n bump the "loudness".  I think I've got the loudness figured out by making the kt's sweat a little harder, but Ch#2 has just a simple 1 knob tone, ala early tweed, will increasing the range of the 2 caps get me closer or should I drill more holes and put in a "stock" 3 knober?

overall he was impressed with the sound, breakup, and range of style he could get.

The trem was what I thought, not enough amplitude, so while I'm in there i'll try and get some more gain outta the tube, he seemed happy with the freq range, which to me  isn't enough, think it's like 8 - 15HZ, is that about a normal range?

thanks for getting me this far!

[sluckey]

think it's like 8 - 15HZ, is that about a normal range?

Is that a guess or did you measure the frequency? 8-15 is really fast. 4-10 would be more typical. I like to do 3-10 in two ranges for all my trem stuff.

[shooter]

I scoped it, being just electronic, not musical, I didn't have a clue.  Since it does work, just need more amplitude, once I get that I'll putz with the tank caps n try and move it down some.

Thanks sluckey

[shooter]

Did some more experimenting on my amp, was looking where, in the signal path, clipping/compression occurs.   I attached the waveforms, bottom trace is from v4b plate, the top trace v2b input, grid.  The coupling components seem to be compressing the signal, which I can accept, except it’s not symmetrical? 

Changed cathode biasing on the kt88s from 470r, about 57ma, to 212r, about 90mA with  no signal in, I calculated I.  Using 90mA*400vdc plate I get 36W, 57mA*400 was 23W.  I’m looking to get a perceivable volume increase without changing tone to much.  Just wanted to be sure I was coloring inside the lines before I jack a big signal in.

for what it's worth, the interstage tranny I'm using has about a 7volt loss with about 50volts in when wired 1:1.  I have it wired 1:2 now and it's about unity.

Thanks for the insite   

[shooter]

I settled on 220ohms for the kt’s, used the spare ½ 12ax as a cathode follower after the channel select switch, pulled the MV pot.  It’s acceptable to my untrained ears, So the next thing I’d like to try is move channel 1’s tone stack and place it after the channel select switch, making it a common tone stack for both channels then send it to the cathode follower.

 My question, the grid resistor on the cathode follower, (V3a) is a fixed 1meg.  The gain pot after the tone stack is a 1 meg pot feeding V1bs grid, can I use the pot in place of the fixed 1meg on the cathode follower? 

Instead of using the fixed 1 meg for V1a though, I was gonna measure the pot with the gain knob at 5, the point when the channel starts distorting, and use something fixed close to that value for V1a’s grid.  I was also gonna bypass channel 2’s tone stack.

Thanks for your help.

[sluckey]

My question, the grid resistor on the cathode follower, (V3a) is a fixed 1meg.  The gain pot after the tone stack is a 1 meg pot feeding V1bs grid, can I use the pot in place of the fixed 1meg on the cathode follower?

Yes, but the bottom of the pot must connect just as the bottom of the fixed 1M. IOW, don't connect the bottom of the pot to ground.

That V3a CF has a flaw that you should take care of. The CF circuit relies on bootstrapped bias. As drawn, the 100K pot or the 250K pot (depending on the position of channel selector switch) will interfere with that bootstrapped bias. The fix is to put a coupling cap between the switch common terminal and the grid of the CF.

[DummyLoad]

attached schematic with slucky's suggested correction.


--pete

[jazbo8]

The grid bias resistors are way too large for the KT88s, no? Mullard recommends 270k~470k (depending on output power).

[shooter]

Thanks guys, I will make the right mods now, I got the block diagrams down, the details I'm still learnin.  I "got" the values from copying a MV circuit, but I think I took it outta context!  Both builds b4 this with kt's I did use 470k.   Got my hands full trying to cure design or manufacturing flaws in a valvestate  so my amp will be waiting for a couple more days.  Thanks again, If Purdue was like this forum I probably woulda got my EE!
 

I didn't know the context of bootstrapping so went to the wizards site, which prompted an edit here and a question.  Can I do away with the .047uf caps from the plates of V2a and V2b making it dc coupled? If I can, will I also have to move the 2 pots to the plate circuit, in series with the plate and fixed plate resistor?

Again, thanks

[sluckey]

Can I do away with the .047uf caps from the plates of V2a and V2b making it dc coupled?

No. Those gain pots complicate things. The simplest solution is the single cap on the switch.

[shooter]

Thanks Steve, That's kinda what I thought, n that's what I'm shooting for, simple.

[shooter]

Got the valvestate off my bench, told the owner he might wanna consider selling or swapping it!  Had 15 cold solder joints 1st time in, another 10 this trip plus curled foil runs n bubbling caps! 
Anyway, got my build back on the bench, changed the tone stack to a common stack for either channel.  Added the .001uf after the stack, before the CF.  Changed the grid resistors on the kt’s to 470k from 2.2meg!  Left the 1 meg pot as a gain pot for V1b and left the CF having a fixed 1meg.  The results were great, both channels sounded crisper, fuller, less bite n mud, but it’s one of those 2 step fwd, 1 step back sorta things, Ch#1 now breaks into oscillations at about 4ish on the 1st gain knob.  Didn’t have time to scope it, maybe tomorrow.  Also to my worn ears appears I lost some gain but there should still be enough in the pre’s to get it back.  Ch2 has lost some of it’s darkness, seems to be moving closer to a cleanish tone.  I left the 1 knob tone in on ch2, I’ll let a real musician tell me if it’s worth leaving in.  I attached the current schematic below.

Thanks to all for helping improve this one, think it’s gonna be a keeper!

[DummyLoad]

split the power rails on CH1. yeah, add more parts.... :-/   

[shooter]

Well I chased the gremlin in circles till he got stuck, so I gave him a case of 40yr old Blatz, and carton of Luckys.  While he was busy I added a 68k between the input of Ch1 and the grid of V1a, took out the antique 82pf and replaced with  a new 24pf n 47pf in ||.  I took out the 1Meg gain pot and replaced it with a fixed 470k n 500k pot.  Haven’t seen the gremlin in about 2 hours of scope n guitar.  If he stays away one more day gonna see if I can eck out a couple more volts from the preamps cuz the tone stack is eating up about 7-12vac.

Would moving the tone stack behind the CF solve the impedance mismatch or will I just be moving it down the line?
DummyLoad, I left the power rail alone for now, didn’t see any oscillations riding there and I’m only about 60mV for filament hum.  I did add components though

Thanks again for your time.
Dave