Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on February 09, 2013, 04:30:41 pm
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Been asked to build a power section using KT88 for about 80 watts. In reading the data sheets seem like I can use a BF twin OT. I have been not very successful in finding any guitar amp builds using KT88. Seems like it shouldn't be much different except in looking at HiFi build schematics it looks as if the cathode resistors are separate and one of the data sheets state the necessity of this.
Questions:
All of the schematics I have seen have a much lower voltage on the screens. Why is this necessary? Also, the only amp I know of using PP KT88 is one of the Redplate amps.
Would there be much different in cathode bias as opposed to fixed? If it is fixed, it seems like all the HiFi designs use dual bias. Don't know why. Anyone?
Anyone seen a schematic for guitar using KT88 using near 600v on the plates?
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Check the Sunn schematics. They also made guitar amps ussing the same power section. They originally came with KT88s, then switch to 6550's Using a stock Sunn (Dynaco) PT (410-410), you will get about 570 volts if you use a solid state rectifier. You can make it UL or pentode, just use 1000 ohm 5 watt resistors on the screens if you go pentode mode. I have built the amps both ways and Sovtek KT88's seem very happy. You could also use the Sunn phase inverter and Power section and use fender pre-amps.
Good luck,
Ernie
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Separated bias for each tube is there to try to balance the difference in the tubes
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The KT88 datasheet report for cathode AB1 PP 300v max on G2
http://www.r-type.org/pdfs/kt88.pdf (http://www.r-type.org/pdfs/kt88.pdf)
K
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Not sure that i have heard of any KT88 guitar amps, mostly there were chosen for Bass or PA.
For example there was the English 4xKT88 Sound City LB200.
I have a schematic, but the file is way too large too post. It is on the internet.
It used a dual rail power supply and Partridge transformers. I scored one of the OTs but cannot tell you much more about them.
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You might look into the Marshal Major.
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You must confide in Doug :wink:
http://www.el34world.com/charts/Schematics/files/soundcity/soundcity200plus.pdf (http://www.el34world.com/charts/Schematics/files/soundcity/soundcity200plus.pdf)
http://www.el34world.com/charts/Schematics/files/marshall/major_1967u_lead_200w.pdf (http://www.el34world.com/charts/Schematics/files/marshall/major_1967u_lead_200w.pdf)
here a couple of Weber
https://taweber.powweb.com/store/6s100_schem.jpg (https://taweber.powweb.com/store/6s100_schem.jpg)
https://taweber.powweb.com/store/6s100_layout.jpg (https://taweber.powweb.com/store/6s100_layout.jpg)
https://taweber.powweb.com/store/6h100_schem.jpg (https://taweber.powweb.com/store/6h100_schem.jpg)
https://taweber.powweb.com/store/6h100_layout.jpg (https://taweber.powweb.com/store/6h100_layout.jpg)
K
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In reference to the screen voltage HI-Fi builders that I have seen make us mere guitar amp builders worthless hacks that have no clue about sound because we dare drive our amps into distortion intentionally.One of their many pet peeves seem to be that the screen voltage has to be x amount (400 V max prefer less) andscreen voltage must be regulated and ind. bias for each tube and a pot to even PI and so on and so forth.I dont have a link but you should check out a couple of web sites (1) Lenard Audio and (2)ESP Elliot Sound Products.Some of there ideas are interesting and maybe we are all hacks but if u are gonna do a Hi-Fi it is a very different ballgame and those guys make us look sane and reasonable when it comes to TONE!(Have u seen what a NOS 300B goes for?) Also I dont think a fender stack will work cuz u are passing a wider frequency range but I may be wrong.Def check out Lenard audio at least the valve section as he goes through a KT88 amp from front to back explaining why this or that is done and what it does and u may be able to use his design/schem or some of it.Good Luck!
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what does this person, <been asked to build KT88> want the amp to do Ed? I have seen them in some designs for bass mostly. Is this person a bass player or are they looking for something 'unusual'?
:icon_biggrin:
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Been asked to build a power section using KT88 for about 80 watts. In reading the data sheets seem like I can use a BF twin OT. I have been not very successful in finding any guitar amp builds using KT88.
As I can read he wants an 80W amp and he is talking about a guitar amp and a guitar amp OT
if otherwise he was interested in a bass amp I think he would ask for it, and probably, but not sure, for a higher output power
K
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My question is about the motive to have guitar amp using tubes MOST designs do not use for guitar? Does he want to re-invent the wheel using Ed's time? I'm just asking, not judging.
:icon_biggrin:
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Hiwatt, 400w 6xKT88.
Imagin carrying that thing around with a transformer set that handels 400w's? :laugh:
http://www.el34world.com/charts/Schematics/files/hiwatt/hiwatt_400w_sta400.pdf (http://www.el34world.com/charts/Schematics/files/hiwatt/hiwatt_400w_sta400.pdf)
Brad :icon_biggrin:
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what does this person, <been asked to build KT88> want the amp to do Ed? I have seen them in some designs for bass mostly. Is this person a bass player or are they looking for something 'unusual'?
:icon_biggrin:
He is wanting to get an amp tone close to a fender twin without all the weight he has in his twin he currently plays. He played a Redplate with 2 KT88 and 1 12" speaker. He liked it and it weighs less than half of his BF twin.
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My question is about the motive to have guitar amp using tubes MOST designs do not use for guitar? Does he want to re-invent the wheel using Ed's time? I'm just asking, not judging.
:icon_biggrin:
He is not a time waster. He is a professional player and has a need. Really he is not trying to reinvent as there is an amp he got the idea from. Redplate makes it. I have played one and it is far from a bass amp. Very spanky sounding.
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Thanks for the direction guys. This guy is just getting close to my age and is beginning to understand why I no longer use my twin for gigs. He needs a large stage volume as he is quite a player and does gigs where the house holds 5,000+ people. He is the house player at a place here in Atlanta called Wild Bills. We have been friends for years and play together often. We played a Redplate and he asked if I thought I could come up with something similar. I told him I did not know but we could give it a try. This is why I am considering this.
I have on hand everything I need to build it except an OT. May end up being a waste of time, but I am sure I will learn a few things.
http://redamps.com/blackverbduo/ (http://redamps.com/blackverbduo/)
This is a link to the amp. It is very nice and the weight is very manageable as well. It uses t KT88 for 80 watts and has a very similar tone to a twin. I guess I should have added this to begin with.
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Hiwatt, 400w 6xKT88.
Imagin carrying that thing around with a transformer set that handels 400w's? :laugh:
http://www.el34world.com/charts/Schematics/files/hiwatt/hiwatt_400w_sta400.pdf (http://www.el34world.com/charts/Schematics/files/hiwatt/hiwatt_400w_sta400.pdf)
Brad :icon_biggrin:
No I cannot, but I would like to play one.
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No I cannot, but I would like to play one.
Ed your a brave man.
Brad :laugh:
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Scott Splawn at Splawn Amplification is offering several of his amps with KT88's. I doubt they are linear output but check out a sound clip at http://www.splawnguitars.com/amps08.htm (http://www.splawnguitars.com/amps08.htm)
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He is wanting to get an amp tone close to a fender twin without all the weight he has in his twin he currently plays. He played a Redplate with 2 KT88 and 1 12" speaker. He liked it and it weighs less than half of his BF twin.
I'm guessing a lot of the weight savings will be the 1 less speaker and the smaller cabinet.
I say that because a power/output transformer set for an 80w amp is still an 80w PT/OT, regardless of the tube type used. So, he won't save much weight by having 2-fewer tubes and sockets.
Not sure that i have heard of any KT88 guitar amps, mostly there were chosen for Bass or PA.
Remember, an output stage is an output stage regardless of how it's being used.
The paying customer wants a KT88 output stage... I'd sell it to him even if were a harmonica amp, or for his theremin rig.
As I can read he wants an 80W amp and he is talking about a guitar amp and a guitar amp OT
if otherwise he was interested in a bass amp I think he would ask for it, and probably, but not sure, for a higher output power
This just means that he doesn't need the OT to reproduce sound at full power at as low a frequency as in a bass amp, so the OT could be a little lighter.
So again, let's not get distracted.
Been asked to build a power section using KT88 for about 80 watts. In reading the data sheets seem like I can use a BF twin OT. ... in looking at HiFi build schematics it looks as if the cathode resistors are separate and one of the data sheets state the necessity of this.
Re-look the data sheet Kagliostro posted.
You would like to have screen voltage similar to the plate voltage, because then you don't need a 2nd power supply for the lower screen voltage. You also need 80w.
It seems you might have looked at the Class AB1 Ultralinear condition, using cathode bias. But power output is cited as 50w, so not enough.
An easier case is the Class AB1 Ultralinear Fixed Bias condition, 460v B+, 4kΩ plate-to-plate and 70w output. That's the one to copy. You could probably use the Hammond 1650NA (4.3kΩ, 60w) or 1650KA (3.4kΩ, 50w) with satisfactory results. They're rated for full power down to 30Hz, so you need not worry about the apparently low power ratings.
I submit that The fixed/cathode bias switch of the RedPlate Amps model is probably to allow cathode bias of the 6L6's when installed for lower power. If you do this, you will need dual bias controls for the fixed bias (one control per tube) and individual cathode resistors.
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All of the schematics I have seen have a much lower voltage on the screens. Why is this necessary?
My question is about the motive to have guitar amp using tubes MOST designs do not use for guitar?
It's not in your case. If you buy a 12-cylinder Ferrari engine, would you be happy paying all that money if it were hamstrung to yield only 120HP in your car?
The KT88 were big, expensive imported tubes, capable of big power output. The data sheet claims the tubes can output 100w per pair or 200w per quartet, and with designs not typical in most guitar amps, you can certainly do that. But regarding the "typical guitar amp tubes" question, you could make a 60w guitar amp with 6x 6V6's, but that's something you won't see because it's easier to use a pair of 6L6's to get close enough.
And since 4x 6L6 (for 100w) would probably have been easier to buy in the drug store and most people don't need a 200w guitar amp, you don't see KT88's much in guitar amps. But that does not mean there is any reason you can't use a pair of KT88's for guitar.
So let's assume you were an amp maker and used KT88's to get every last bit of that 100w/pair, 200w/quartet. You would keep raising the plate voltage to increase output power, but you run into the problem of not being able to keep the screen at the same voltage as the plate. Say screen voltage stays at 450v while you raise the plate voltage. You might find you hit your 100w goal at something like 550v (for example).
It would be awkward to derive 550v and 450v supplies that were both reasonably constant, so an easier path is to make the screen voltage half the plate voltage. You could have a power transformer with a bridge rectifier and series input filter caps, with the 300v tapped from the lower filter cap and 600v from the upper filter cap. The lower screen voltage reduces plate current and offsets the increased plate voltage when the supply voltage was raised to allow the easy power supply setup.
Ultimately, the plate voltage (voltage swing) and screen voltage (current swing) are selected to attack each part of the equation for power, though economics often push them to be similar.
Anyway, copy the datasheet condition for fixed-bias and 70w output.
Would there be much different in cathode bias as opposed to fixed? If it is fixed, it seems like all the HiFi designs use dual bias. Don't know why. Anyone?
You'll need fixed bias, unless you want to start using uncomfortably-high supply voltages (which will increase costs as you need more power supply components, caps, etc). You should also see from the datasheet that for similar supply voltage you will get less output power than in fixed bias (mostly because of the change in plate-to-cathode voltage).
Genalex advised independent cathode resistors and independent bias adjusts so that each tube could be given exactly what it needed. I think they also wanted to make sure a fault in one tube didn't wipe out the whole output stage.
For you & me, this is an easy way to not need to worry about perfectly matched tubes, as the twin bias adjusts allows each tube to be dialled in to exactly the same idle current even if they're grossly unmatched. Power output will be reduced somewhat for very mismatched tubes.
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... if u are gonna do a Hi-Fi it is a very different ballgame ...
Yes, and why it is not worth splitting the same hairs they are. They have different design goals and asusmptions, so their answers will come out differently than ours.
Also I dont think a fender stack will work cuz u are passing a wider frequency range
The guy is playing a guitar, so there's no such thing as "passing a wider frequency range" unless the customer wants the amp to have a wider frequency range. Even then, a guitar puts out the sound it puts out, so there's not much reason to debate frequency unless the customer wants more treble than typical guitar speakers allow.
So a Fender tone stack (or any other type the customer likes) will work just fine; the output section has no real bearing on the preamp, except for how big a drive signal it will require from the phase inverter.
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Yea I apologize for wasting a post and time I interpreted it that the guy wanted a HI-Fi amp I did not realize it was for a guitar amp-I promise it will be the last time I post after 3am :BangHead:
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Not wasted; you had valid points. It just came from the stand-point of what you'd seen, which was hi-fi.
But there are a lot of different assumptions in hi-fi, and sometimes they boil down to the desire of the designer to use a novel or esoteric circuit element because of an assumed advantage. Anyway, unless you're in the designer's head, it may not be wise to copy idea's from them.
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How about this idea? He owns a BF TR correct? but it's too heavy to lug....sure, my back knows his pain
1. strip the chassis out of the "heavy 2-12 box" and set on bench. Take off whatever parts that add weight but you don't need them to operate the Twin as a preamp.
2. find a really solid amp design that is reliable and user friendly (Dynaco MkIII comes to mind) use KT88s in place of the 66s that they usually run. make few tweaks to adjust for diff. operating parameters.
3. put the twin 'head' in a nice wood cab with handle. If he wants a combo with 1x12 or 1x15 or ? build that being careful to use aged wood (pine) to reduce weight and enhance tonality
4. player now has a classy looking head or combo which is much lighter and easier to carry. He has his Power Amp in something to make it portable too. He has his KTs making 80+ and no hoakie circuitry to color the sound, only Mr Twin doing his blackface thingy. Carry them into the gig, plug in and play.
Note: the basic idea here is the exact rig a rather notable geetar picker used that served him very well. I speak of one Mr. Jerry Garcia. He kinda went big on his power amp with the biggest McIntosh power amp made. But, he had groupies, fans and roadies always willing to carry his equip for him (some guys are just lucky).
:icon_biggrin:
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Yea I apologize for wasting a post and time I interpreted it that the guy wanted a HI-Fi amp I did not realize it was for a guitar amp-I promise it will be the last time I post after 3am :BangHead:
It's all good. I appreciate your input. I am in the middle of reading what you suggested. Just because it is HiFi doesn't mean I cannot learn something.
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No I cannot, but I would like to play one.
Ed your a brave man.
Brad :laugh:
Not really Brad, just lost most of my hearing in the 70's using a plexi and 8, 10"s cab. I can feel 400 watts better than I can hear it.
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All of the schematics I have seen have a much lower voltage on the screens. Why is this necessary?
My question is about the motive to have guitar amp using tubes MOST designs do not use for guitar?
It's not in your case. If you buy a 12-cylinder Ferrari engine, would you be happy paying all that money if it were hamstrung to yield only 120HP in your car?
The KT88 were big, expensive imported tubes, capable of big power output. The data sheet claims the tubes can output 100w per pair or 200w per quartet, and with designs not typical in most guitar amps, you can certainly do that. But regarding the "typical guitar amp tubes" question, you could make a 60w guitar amp with 6x 6V6's, but that's something you won't see because it's easier to use a pair of 6L6's to get close enough.
And since 4x 6L6 (for 100w) would probably have been easier to buy in the drug store and most people don't need a 200w guitar amp, you don't see KT88's much in guitar amps. But that does not mean there is any reason you can't use a pair of KT88's for guitar.
So let's assume you were an amp maker and used KT88's to get every last bit of that 100w/pair, 200w/quartet. You would keep raising the plate voltage to increase output power, but you run into the problem of not being able to keep the screen at the same voltage as the plate. Say screen voltage stays at 450v while you raise the plate voltage. You might find you hit your 100w goal at something like 550v (for example).
It would be awkward to derive 550v and 450v supplies that were both reasonably constant, so an easier path is to make the screen voltage half the plate voltage. You could have a power transformer with a bridge rectifier and series input filter caps, with the 300v tapped from the lower filter cap and 600v from the upper filter cap. The lower screen voltage reduces plate current and offsets the increased plate voltage when the supply voltage was raised to allow the easy power supply setup.
Ultimately, the plate voltage (voltage swing) and screen voltage (current swing) are selected to attack each part of the equation for power, though economics often push them to be similar.
Anyway, copy the datasheet condition for fixed-bias and 70w output.
Would there be much different in cathode bias as opposed to fixed? If it is fixed, it seems like all the HiFi designs use dual bias. Don't know why. Anyone?
You'll need fixed bias, unless you want to start using uncomfortably-high supply voltages (which will increase costs as you need more power supply components, caps, etc). You should also see from the datasheet that for similar supply voltage you will get less output power than in fixed bias (mostly because of the change in plate-to-cathode voltage).
Genalex advised independent cathode resistors and independent bias adjusts so that each tube could be given exactly what it needed. I think they also wanted to make sure a fault in one tube didn't wipe out the whole output stage.
For you & me, this is an easy way to not need to worry about perfectly matched tubes, as the twin bias adjusts allows each tube to be dialled in to exactly the same idle current even if they're grossly unmatched. Power output will be reduced somewhat for very mismatched tubes.
Some of your replies are funny as hell. I will look over the data sheet again. You are correct, I was looking at the cathode because the schematic I was reviewing (HiFi) was cathode. Dual bias is not a problem and I do have the iron for the job. If I can skimp a little on the OT, I may have that as well. It is worth trying anyway as I have nothing to lose.
The weight difference is strange as I was not able to actually closely inspect the Redplate amp. I will have to to finish the project, but the owner of the music store was not there when we test drove the amp and the employee was afraid to let me get a closer look. The owner will.
There is more weight reduction than just one speaker and cab size it seems, but the iron in the amp is large. The PT I have for this weighs 9 lbs. He is not interested in all the bells and whistles on the Redplate. Like me, he turns on his amp and plays and doesn't do too much tweaking once he has it setup. Really what he wants is a AB763 with loads of headroom that he can carry up a flight of steps by himself.
I was impressed with the Redplate.
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How about this idea? He owns a BF TR correct? but it's too heavy to lug....sure, my back knows his pain
1. strip the chassis out ..
2. find a really solid amp design that is reliable and user friendly (Dynaco MkIII comes to mind) use KT88s ...
There is more weight reduction than just one speaker and cab size it seems, but the iron in the amp is large. ...
Check the RedPlate, but seriously... 80-90% of the weight of an amp is the speaker(s) and transformers. My 25w Standel weighs close to the same as a Super Reverb (maybe even more), because the big 15" speaker and enormous transformers.
And transformers to deliver enough VA for 80w (whether that's the PT or power handling of the OT) are the same size regardless of what tube type/circuit topology you use.
Unless you guys lugged around the RedPlate to know it was much lighter, you may have assumed it was lighter because of the smaller size.
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I just found a modern production amp running 88s while looking for another type of amp. This one is from Bogner and is sort of a take-off from his Uberchall if I read the website promo stuff correctly? Anyway, something for inspiration if any type of techno, schemo stuff is available for this amp?
http://www.bogneramplification.com/twin-jet (http://www.bogneramplification.com/twin-jet)
:icon_biggrin:
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How about this idea? He owns a BF TR correct? but it's too heavy to lug....sure, my back knows his pain
1. strip the chassis out ..
2. find a really solid amp design that is reliable and user friendly (Dynaco MkIII comes to mind) use KT88s ...
There is more weight reduction than just one speaker and cab size it seems, but the iron in the amp is large. ...
Check the RedPlate, but seriously... 80-90% of the weight of an amp is the speaker(s) and transformers. My 25w Standel weighs close to the same as a Super Reverb (maybe even more), because the big 15" speaker and enormous transformers.
And transformers to deliver enough VA for 80w (whether that's the PT or power handling of the OT) are the same size regardless of what tube type/circuit topology you use.
Unless you guys lugged around the RedPlate to know it was much lighter, you may have assumed it was lighter because of the smaller size.
We compared both amps. We took his to the store with us. I could easily carry the RedPlate in one hand while holding a guitar in the other. I know what you are saying about the weight tho. 2 tubes are not going to make that much difference. I am not sure why the amp is so much lighter as it does have a large ceramic high watt speaker. I spoke with the owner of the music store and she said I could remove the chassis if I were careful. I am going to do that Saturday.
There is a place where I play called Smith's Old Bar and the stage is up a flight of steps. Lot of steps. I could carry this amp one handed with no problem.
I do know what you mean tho. My favorite SR weighs in at a hefty 73 lbs. Could not believe how much changing the speakers to Webers combo of 2 Alnicos and 2 ceramics added that much weight. The RedPlate with the iron for KT88 weighs 62 lbs, but I can easily get another 10 lbs out. I will not be having to near as many parts. The RedPlate has so many bells and whistles and my friend never uses the reverb on his amp. I built him a small effects amp years ago with tube reverb, tremolo and made a single rack space for a processor, but his main setup is the twin with 4 pedals. In reality, he could get by with just a phaser. Like I mentioned, he is a great player so he doesn't need as much effect help as me.
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I just found a modern production amp running 88s while looking for another type of amp. This one is from Bogner and is sort of a take-off from his Uberchall if I read the website promo stuff correctly? Anyway, something for inspiration if any type of techno, schemo stuff is available for this amp?
http://www.bogneramplification.com/twin-jet (http://www.bogneramplification.com/twin-jet)
:icon_biggrin:
Thanks, I will see if I can get info on this.
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Sounds like you have an excellent handle on the situation.
I haven't weighed my Standel; I'd be curious to find out where it lands. Funny, I still haven't added a handle because I originally wanted a custom-made leather handle (of a style not generally available). Then I noticed that due to the cabinet dimensions of my up-sized version, it would be inconvenient/uncomfortable to pick mine up if it did have a handle. The cabinet is just a bit too tall (an original 25L15 is more compact, but I had a change I wanted made, even if it wasn't absolutely necessary).
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Sounds like you have an excellent handle on the situation.
I haven't weighed my Standel; I'd be curious to find out where it lands. Funny, I still haven't added a handle because I originally wanted a custom-made leather handle (of a style not generally available). Then I noticed that due to the cabinet dimensions of my up-sized version, it would be inconvenient/uncomfortable to pick mine up if it did have a handle. The cabinet is just a bit too tall (an original 25L15 is more compact, but I had a change I wanted made, even if it wasn't absolutely necessary).
Yep, I was just confused about the KT88 in reading the data sheet. Reason? I was reading the wrong thing.
That handle thing can be a chore sometimes. Just like a super reverb, the amp is really too tall to pick-up with the handle on top. I have to hold it with both hands in front of me. YOur Standel cab is so nice it would be hard for me to put one on it either. The original handle is cool with the chrome ends that hide the handle and that 50's Art Deco corners. It looks it should be in a diner somewhere. I have a friend who is an artist as well as a geetar player. He painted a leather handle in a wood cab I made him to match the wood grain. From about 10 feet away you cannot even see it. Very cool. BTW, I found a nice JBL for a good price.
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Waddia mean nobody was building KT88 geetar amps!
One of the nicest sounding amps Marshall built (besides the Major, of course...) was the Park 75. It was basically a 1987 with KT88s. With the about 390v on the plates, they were able to produce 75 watts without breaking a sweat - but that was not UL. Not sure what your buddy wants on the front end, but pick a tried and true UL circuit and have at it. Just watch your NFB so it doesn't sound like a Fender UL amp.....
Best toob on the planet! :icon_biggrin:
Jim
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Maybe take a look at the Dr. Z Route 66 for inspiration.
Jim
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And you don't necessarily have to have NFB from the speakers to the phase inverter. You may consider leaving room for those components if you choose to use them, but taste-test UL without it first.
Ultralinear already has a degree of negative feedback in the output stage due to the screen taps on the primary winding.
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Maybe take a look at the Dr. Z Route 66 for inspiration.
Jim
Thanks Jim, I looked at the Route 66. Found a schemo and it looks like a pretty cool idea. No NFB and V1 is a EF86. Of course the voltages are not what they need to be and it has a tube rectifier, (nothing wrong with tube rectifiers) but other than that it looks very interesting. It looks like the doctor is making a JTM 45 with a little cue from Vox. There are a few resistor values which are different than I am used to seeing. I have not played one of these before, but I'll bet it would not need a treble booster in front of it.
Attached a layout someone has done. Looks like some improvements can be made here. Anyway, now I need to do the math to get an idea of how to revise the schematic to KT88.
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Anyway, now I need to do the math to get an idea of how to revise the schematic to KT88.
No you don't.
Use a Twin Reverb PT (double-check filament current requirements, but you should be fine), and one of the Hammond OT's I recommended earlier. Copy the desired fixed bias voltage from the data sheet (assuming about 450v B+), and you're done.
You modify the bias circuit slightly if the center of its range doesn't land on the data sheet bias voltage, and give reasonable range on either side.
No NFB and V1 is a EF86.
I'm telling you, there is a pretty fair amount of feedback by virtue of the ultralinear connection.
For anything other than very high current, low plate voltage peaks, the shape of the plate curves are very much like a triode. And that yields a lower source impedance at the tube than you'd get with pentodes with no feedback, which improves speaker damping.
It's somewhat like having NFB, but without and actual loop wrapped around the OT and output tubes. That also has the effect of keep the output stage pretty stable, even when using tubes notorious for wanting to sing on their own (like KT66's).
I once tried KT66's in a 6L6 amp and had to increase the value of grid and screen stoppers to keep it from wanting to oscillate or ring.
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Yeah, too much NF and you will end up sounding like those crap UL Fenders - way too tight and lifeless. As HBP said, you will get just enough "naturally" through the UL design. I have not played or actually heard a Route 66 either. However, every review that I have read has been an A+ glowing report. All things being equal, the UL design has a pretty full bodied response, so the Vox influence should give it the shimmer a guitarist would need. The Guitar version of the Major has basically a top boost on channel one. It screams compared to the normal channel which is somewhat boomy. The Route 66 looks pretty cool! I've never seen the schemo! Thanks for doing the legwork. Darn...now I'll have to add that to my build list... :icon_biggrin:
Jim
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No NFB
I'm telling you, there is a pretty fair amount of feedback by virtue of the ultralinear connection.
For anything other than very high current, low plate voltage peaks, the shape of the plate curves are very much like a triode. And that yields a lower source impedance at the tube than you'd get with pentodes with no feedback, which improves speaker damping.
It's somewhat like having NFB, but without and actual loop wrapped around the OT and output tubes. That also has the effect of keep the output stage pretty stable, even when using tubes notorious for wanting to sing on their own (like KT66's).
KOC and quite a few of the members on the Power Scaling forum spoke very highly of what HBP is saying. There were quite a few amps built that way and the guys really like them.
Brad :icon_biggrin:
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Anyway, now I need to do the math to get an idea of how to revise the schematic to KT88.
No you don't.
Use a Twin Reverb PT (double-check filament current requirements, but you should be fine), and one of the Hammond OT's I recommended earlier. Copy the desired fixed bias voltage from the data sheet (assuming about 450v B+), and you're done.
You modify the bias circuit slightly if the center of its range doesn't land on the data sheet bias voltage, and give reasonable range on either side.
No NFB and V1 is a EF86.
I'm telling you, there is a pretty fair amount of feedback by virtue of the ultralinear connection.
For anything other than very high current, low plate voltage peaks, the shape of the plate curves are very much like a triode. And that yields a lower source impedance at the tube than you'd get with pentodes with no feedback, which improves speaker damping.
It's somewhat like having NFB, but without and actual loop wrapped around the OT and output tubes. That also has the effect of keep the output stage pretty stable, even when using tubes notorious for wanting to sing on their own (like KT66's).
I once tried KT66's in a 6L6 amp and had to increase the value of grid and screen stoppers to keep it from wanting to oscillate or ring.
I have so much used iron, I am going to attempt to use what I have already. BTW, I was not questioning the no NFB suggestion. On most of my amps I have the ability to switch it out anyway.
Question: You mentioned 3.4k for the OT. Where did you get that? The Zout suggest 6.5k ohms to 7K ohms is plate is 460. I have the perfect OT for the build if a 3.4k will be optimal. If not, I do have others. If it doesn't work out I can always get the Hammond iron. I checked last night and I have a PT which will put it into the 100 watt range, but may not be the best avenue running the tube at full tilt.
Anyway, I get to take my time on this so it should be a great learning experience for me.
This has gotten me very interested after reading the reviews on the Route 66. I guess I will call mine the Interstate 88. It is such a simple build so it must be a tone machine.
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No NFB
I'm telling you, there is a pretty fair amount of feedback by virtue of the ultralinear connection.
For anything other than very high current, low plate voltage peaks, the shape of the plate curves are very much like a triode. And that yields a lower source impedance at the tube than you'd get with pentodes with no feedback, which improves speaker damping.
It's somewhat like having NFB, but without and actual loop wrapped around the OT and output tubes. That also has the effect of keep the output stage pretty stable, even when using tubes notorious for wanting to sing on their own (like KT66's).
KOC and quite a few of the members on the Power Scaling forum spoke very highly of what HBP is saying. There were quite a few amps built that way and the guys really like them.
Brad :icon_biggrin:
Yep, I am big on messing around with NFB. I am trying to come up with a way to get a proper presence response when using the LarMar Master on Marshall style builds, but a different topic for a different day. I have always said the best thing you can do to a Super Reverb is disconnect the NFB.
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Interstate 88 - I like it!
Earlier Erney mentioned he uses Sovtek (I think?) KT88's. I know Erney has used this tube on several of his builds and I would listen to what he has to say! I have not had good luck with the Sovteks as they do not last long in my amp. I've used =C= for years and they have not let me down, and they sound good. Certainly doesn't mean that in an amp that doesn't beat the crap out of its tubes that they will do just fine! Like I mentioned earlier, with 390v on the plates, the Park 75 made 75w easy. That is loud enough anyway. If you are pushing 100w and you need to turn it up to get the grind, that might be painful. Maybe keep your plate low, your tubes will last a long time, and IMHO there is nothing like a KT88 pushed into distortion. If it comes at a manageable volume - woowhooo! Yeah a 100w Super Lead or a Major cranked on the full stack is heaven, but at what cost? Lots to consider!
Jim
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Question: You mentioned 3.4k for the OT. Where did you get that? The Zout suggest 6.5k ohms to 7K ohms ...
Pish-posh! :icon_biggrin:
Kagliostro's original post, with the Genalex KT88 data sheet. I have no idea what you're looking at on it, so I'm gonna duplicate the exact condition on it I keep referring to (top of page 4).
Push-Pull. Class AB1. Fixed Bias. Ultralinear Connection.
Va,g2 (b) 460v
Va,g2 (o) 453v
....
RL(a-a) 4kΩ
-Vg1 -59v
Pout 70W
Forget Zout; that refers to the output impedance of the plate/screen of the tube. It has relevance if you're trying to figure damping factor, or see how much output impedance is reduced by the effective feedback of the UL stage compared to pentode operation.
Example:
You use a 4kΩ:8Ω OT. Zout is indicated on data sheet as 6.5kΩ.
- Impedance ratio is 4k/8 = 500:1
- 6.5kΩ/500 = 13Ω
- Speakers seem under-damped because an effective 13Ω source seen at the secondary is driving the 8Ω load. (Not necessarily bad for guitar)
- Result: Only light additional NFB might be used to reduce the effective 13Ω source below 8Ω, which also explains why amps that have "typical guitar amp feedback" along with an UL connection seem too-stiff.
- If you connected the 8Ω load to the 4Ω tap the load would see a 6.5kΩ/1000 = 6.5Ω source impedance at the secondary, providing decent damping albeit with a reduction of power. This might sound subjectively a little tighter, but not unyieldingly stiff.
I then looked at available transformers with UL taps from Hammond, and selected 2 likely candidates while considering the fact that their rated bandwidth is much wider than you need so you can derate the transformers somewhat to reduce cost/weight while still getting 70w of output.
I assumed the 70-80w target power output was a non-negotiable. Do your on-hand OT's have UL taps? If not, you won't get all that power, without using much higher plate voltage (550-560v) and lower screen voltage (~300v) which then means your power supply becomes more complex and costly.
Does using the on-hand OT without UL taps also allow you to use an on-hand PT with 550-600v output in a bridge rectifier arrangement so you have an easy source for the 300v screen node? If not, you're trading the cost of buying a new OT for the cost of buying a new PT.
So less work, less thinking if you simply get the UL Hammond OT, unless you have one of those already.
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oops... HBP, just read your figures and looked at the data sheet. I need to pay more attention and quit blabbering on... Obviously they were running more than 390v in the Park. Still, 460v (or lower) is better than 560v. Those data sheets are talking about the cream of the crop KT88 ever manufactured. The crap we have today do not like the 600v+ applications. I have spent a lot of money proving that....
Jim
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Question: You mentioned 3.4k for the OT. Where did you get that? The Zout suggest 6.5k ohms to 7K ohms ...
Pish-posh! :icon_biggrin:
Kagliostro's original post, with the Genalex KT88 data sheet. I have no idea what you're looking at on it, so I'm gonna duplicate the exact condition on it I keep referring to (top of page 4).
Push-Pull. Class AB1. Fixed Bias. Ultralinear Connection
Va,g2 (b) 460v
Va,g2 (o) 453v
....
RL(a-a) 4kΩ
-Vg1 -59v
Pout 70W
Forget Zout; that refers to the output impedance of the plate/screen of the tube. It has relevance if you're trying to figure damping factor, or see how much output impedance is reduced by the effective feedback of the UL stage compared to pentode operation.
Example:
You use a 4kΩ:8Ω OT. Zout is indicated on data sheet as 6.5kΩ.
- Impedance ratio is 4k/8 = 500:1
- 6.5kΩ/500 = 13Ω
- Speakers seem under-damped because an effective 13Ω source seen at the secondary is driving the 8Ω load. (Not necessarily bad for guitar)
- Result: Only light additional NFB might be used to reduce the effective 13Ω source below 8Ω, which also explains why amps that have "typical guitar amp feedback" along with an UL connection seem too-stiff.
- If you connected the 8Ω load to the 4Ω tap the load would see a 6.5kΩ/1000 = 6.5Ω source impedance at the secondary, providing decent damping albeit with a reduction of power. This might sound subjectively a little tighter, but not unyieldingly stiff.
I then looked at available transformers with UL taps from Hammond, and selected 2 likely candidates while considering the fact that their rated bandwidth is much wider than you need so you can derate the transformers somewhat to reduce cost/weight while still getting 70w of output.
I assumed the 70-80w target power output was a non-negotiable. Do your on-hand OT's have UL taps? If not, you won't get all that power, without using much higher plate voltage (550-560v) and lower screen voltage (~300v) which then means your power supply becomes more complex and costly.
Does using the on-hand OT without UL taps also allow you to use an on-hand PT with 550-600v output in a bridge rectifier arrangement so you have an easy source for the 300v screen node? If not, you're trading the cost of buying a new OT for the cost of buying a new PT.
So less work, less thinking if you simply get the UL Hammond OT, unless you have one of those already.
I will get the Hammond, but I have 3 UL transformers. 2 I know will not work, but one might. It has a 43% reduction and is designed for 70 watts, but id does not weigh 8 lbs like the Hammond. It weighs 6.5. Got the chassis folded up and welded. Time to punch and put together a material list. I decided to put the Route 66 preamp with an AB763 as well. It is so simple, why not?
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I will get the Hammond, but I have 3 UL transformers. 2 I know will not work, but one might. It has a 43% reduction and is designed for 70 watts, but id does not weigh 8 lbs like the Hammond. It weighs 6.5.
It will almost certainly work. Remember, the Hammond has much wider bandwidth than typical guitar amp transformers, which is why the core is so much bigger.
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> is designed for 70 watts
At what B+ voltage?
Is that anywhere near the B+ voltage you intend to run?
Remember that if B+ is changed 1.4 times, the optimum impedance is changed *2 times*.
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All of the schematics I have seen have a much lower voltage on the screens. Why is this necessary?
So let's assume you were an amp maker and used KT88's to get every last bit of that 100w/pair, 200w/quartet. You would keep raising the plate voltage to increase output power, but you run into the problem of not being able to keep the screen at the same voltage as the plate. Say screen voltage stays at 450v while you raise the plate voltage. You might find you hit your 100w goal at something like 550v (for example).
It would be awkward to derive 550v and 450v supplies that were both reasonably constant, so an easier path is to make the screen voltage half the plate voltage. You could have a power transformer with a bridge rectifier and series input filter caps, with the 300v tapped from the lower filter cap and 600v from the upper filter cap. The lower screen voltage reduces plate current and offsets the increased plate voltage when the supply voltage was raised to allow the easy power supply setup.
Ultimately, the plate voltage (voltage swing) and screen voltage (current swing) are selected to attack each part of the equation for power, though economics often push them to be similar
HBP do you have any schematics we could look at that shows how they tapped 300v from lower caps and 600v from the upper filter caps. I have several sets of kt88 and want to use them in a guitar amp someday.
I dont really know anything about ulta linear outputs so regular push pull would be better for me, but I could read up a bit and see what would be good for a guitar amp .
Come to think of it I have a VHT amp schematic that uses 2 kt88 for 60 watts I should check the voltages and filtering scheme on that one also.
Thanks Bill
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HBP do you have any schematics we could look at that shows how they tapped 300v from lower caps and 600v from the upper filter caps.
Here's a simple way to do that...
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Here's a simple way to do that...
...and here's another:
(http://i497.photobucket.com/albums/rr333/valvetone/2013%20Projects/voldoub_zps81d0dd31.gif)
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> is designed for 70 watts
At what B+ voltage?
Is that anywhere near the B+ voltage you intend to run?
Remember that if B+ is changed 1.4 times, the optimum impedance is changed *2 times*.
Cannot answer that. It is a Saratoga SI-310-0, it was a pull from a phase linear amp from years ago. I was wrong about the weight. It may be too heavy for this project. It weighs in at 14 lbs. Pulled maybe 1981 from what I remember to be one of their models which was in a glass enclosure. I am not able to find specs on it, but I do know it was a replacement. May not work, but since I have had it for (wow) over 30 years I figure if I toast it I have not lost anything. When it gets right down to it, I will just probably spring for the Hammond as it is $135 from Digikey.
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HBP do you have any schematics we could look at that shows how they tapped 300v from lower caps and 600v from the upper filter caps. I have several sets of kt88 and want to use them in a guitar amp someday.
I dont really know anything about ulta linear outputs so regular push pull would be better for me, but I could read up a bit and see what would be good for a guitar amp .
Guys like us looking at guitar amps have a tendency to assume screen voltage has to be the same or close to plate voltage in an output stage. That's not the case, it's just easier and cheaper to do that.
The only difference between normal push-pull operation and ultralinear (as far as wiring) is that plate and screen voltage must be essentially the same, and you have screen taps on the OT that attach to the screens instead of a node of the power supply. It's generally easier to hook up the output stage UL than typical push-pull, and requires fewer components.
Depending on how clean you want your power supply to be, you might move the choke from between the plate and screen node (of a typical push-pull stage) to a point ahead of the plate node. You'd need to uprate the choke to handle all the amp's current, and you'll likely add a filter cap to ground ahead of the choke.
In other words, cap to ground -> big choke -> cap to ground, with the 2nd cap feeding the CT of the ultralinear OT. Screen taps of UL OT go to screens, which may have smallish 100Ω or so stoppers. The 2nd filter cap then would go to a probably largish resistor before a following filter cap feeding the phase inverter.
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I got to take apart the RedPlate. There is really a lot going on inside in the preamp and a couple of tricks in the power section. It does use cathode bias for 6L6 and has, you guessed it, a UL OT without a sticker. There is a stamped number on the bell housing. Looks like Heybour stuff. Anywho, it looks fairly straightforward build if you remove all the bells and whistles.
Question
What is the (b) and (o) on the data sheet as in
Va,g2 (b) 460v
Va,g2 (o) 453v
I read where (b) is anode supply voltage and (o) is zero out. Could someone elaborate.
Also I posted a gut shot of a Route 66 if anyone is interested.
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Question
What is the (b) and (o) on the data sheet as in
Va,g2 (b) 460v
Va,g2 (o) 453v
I read where (b) is anode supply voltage and (o) is zero out. Could someone elaborate.
"b" in all cases on the data sheet is the supply voltage. "o" is a condition at idle or "zero" signal output.
In your selected portion, 460v B+, and after OT d.c. drop, 453v measured at the plate and screen.
Of course, that sounds a little fishy to me, as the plate will have more winding resistance in the way, and in reality is probably a volt or 2 less than the screen.
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It does use cathode bias for 6L6 and has, you guessed it, a UL OT without a sticker.
So the UL taps are hooked up but no NFB loop?
Brad :think1:
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UL OT without a sticker
What does that mean?
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UL OT without a sticker
What does that mean?
i'd guess no part number, so no specs?
respectfully,
--pete
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Which is fair... I took the stickers off the Hammond transformers I used in my 25L15. :laugh:
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It does use cathode bias for 6L6 and has, you guessed it, a UL OT without a sticker.
So the UL taps are hooked up but no NFB loop?
Brad :think1:
That is correct.
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UL OT without a sticker
What does that mean?
Pete nailed it. No part number and no specs. I am going to order the hammond HBP (Brian) suggested. From the photo on hammonds Web site it looks the same and dimensions are the same. Specifically 1650NA. Should work well.
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Which is fair... I took the stickers off the Hammond transformers I used in my 25L15. :laugh:
I emailed Redplate and asked. That is the difference. If I asked you, you would tell me. They wrote me back offering to sell me one of their "custom" OT's. $276.48 plus shipping. It is not a Heybour. I called Alden at Heybour and he told me they do not make the transformers for Redplate. Strange, but the bell covers are not painted which is the only thing that makes me think it is not a hammond.
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They wrote me back offering to sell me one of their "custom" OT's. $276.48 plus shipping.
At that price why not get a dozen?
Brad :l2:
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.... Strange, but the bell covers are not painted which is the only thing that makes me think it is not a hammond.
Half the bell covers on the Hammonds I got had rust bubbling underneath the paint.
I had already planned to paint the endbells of my transformers, but the rust made it imperative (to me) to strip the paint, remove the rust and repaint.
Maybe Redplate had rust on their endbells and decided to use bare metal (or whatever they used).
I'll sell you my "BluePlate Special Transformers" for only $220/ea. :l2:
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You guys are having waaaaay too much fun with the reply from Redplate. Good information tho. I know if I ever see a Redplate amp that needs help, I will have to get it cheap. OTOH, they are really nice amps with some unique approaches. I just think they smelled something fishy, like I was going to build one of their amps. You all know I would never do anythihng like that. :l2:
Bet I can build it for less than $600 not counting that I have a cab and speaker and chassis already plus all the small parts. The deal is I get the BF twin and $1500, but I have to give him a 75 silver twin I blackfaced. Got that thing for $400 last week off craigslist. Dude said he already had about 20 calls on it and the first one there got it. Craigslist. He lives about a mile from my office and had cash in my pocket. He said he may have listed it too cheap. Ya think?
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maybe redplate is using edcor iron? edcor offers unpainted end-bells and custom colours... the only other winder i can think of right now is magnetic components.
--pete
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maybe redplate is using edcor iron? edcor offers unpainted end-bells and custom colours... the only other winder i can think of right now is magnetic components.
--pete
Thanks for the info. I forget about Edcor, but they do not have and specs for the UL Transformer, but will make it. Magnetic Components only makes one currently and it is not sufficent. I ordered the hammond yesterday, but now I have to put the build aside for a few weeks. I have too many amps and I have to get busy getting them to a couple of stores on consignment so I can make more and have too many again.
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Here's a simple way to do that...
...and here's another:
(http://i497.photobucket.com/albums/rr333/valvetone/2013%20Projects/voldoub_zps81d0dd31.gif)
Daryl do you happen to have one of these with a center tap off the transformer or how i would draw it onto this one sorry. Im kind of visual and yes I am trying to take the easy way out by asking.
Thank you Bill
PS. I do have the Valve Wizards new book on power supplies I just havnt read much of it yet
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> one of these with a center tap off the transformer
What Sluckey posted (Daryl just showed a different path).
(http://www.el34world.com/Forum/index.php?action=dlattach;topic=15132.0;attach=35407;image)
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> one of these with a center tap off the transformer
What Sluckey posted (Daryl just showed a different path).
(http://www.el34world.com/Forum/index.php?action=dlattach;topic=15132.0;attach=35407;image)
The reason I asked is his showed 210-0-210 was getting 600v on the top node the the output transformer I want to use shows 410-0-410 i thought his full wave rectifier was a doubler to get that kind of voltage off of it. i need the regular rectifier set up . Actually I need to get farther in to valve wizards book sitting here in front of me then i will understand these things better.
Thanks Bill
PS off to read :happy1:
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Ok finally I understand the voltages showed are figurative not actual . That confused me that will work perfectly.
Thank you
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Ok finally I understand the voltages showed are figurative not actual .
No, those are real numbers. A 212-0-212VAC PT is the same as a 424VAC with CT. And 424 x 1.414 = 600VDC unloaded.
Your 410-0-410 will yield 1160 and 580 volts! :think1:
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Yes, when I first brought up the idea of arranging the supply like this, I had the bridge rectifier in mind.
The transformer you want to use was wound for a full-wave rectifier (not bridge) configuration, and will output way too much voltage.
For what it's worth, Darryl's circuit is also a doubler, and kind of like a half-bridge.
So ultimately, you'll need a different power transformer to easily get full- and half-B+ voltage and at a reasonable voltage level.
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I can get one and use darryls circuit with no center tap 0-385 400ma get just over 500 volts on the mains. but triode store has a bunch of dynaco replacement stuff good ul transformers could just go that route
4.3k rated at 60/120 watts i dont know how they figure the 120 watts but thats what it says. Like you said push it at 470v get 70 watts from a pair of KT88 tubes.
Thanks Bill
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I can get one and use darryls circuit with no center tap 0-385 400ma get just over 500 volts on the mains.
This circuit is a voltage doubler. If your secondary AC voltage is 385, then the B+ is 385 x 1.4 x 2 ≈ 1078 VDC. :huh:
(http://i497.photobucket.com/albums/rr333/valvetone/2013%20Projects/voldoub_zps81d0dd31.gif)
You could use a bridge rectifier with your non-CT 385VAC secondary, to produce a B+ of about 539VDC, but this circuit will not provide a ½B+ voltage.
(http://www.expertsmind.com/CMSImages/668_Bridge%20rectifier1.png)
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Wouldn't these work with a PT with no CT and give B+ and half B+?
Or are they voltage doublers too?
Brad :icon_biggrin:
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Wouldn't these work with a PT with no CT and give B+ and half B+?
I 'believe' both of those are disfunctional.
There are alternatives to getting 600/300 volts without using a hard to find transformer. You could just use a 425-0-425 (or close) to give you the plate voltage of approx 600. Then use a VVR to get the 300 for the screens. Replace the 1M pot on the VVR with two 470K resistors to get a fixed output from the VVR that's 1/2 the input.
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Wouldn't these work with a PT with no CT and give B+ and half B+?
I 'believe' both of those are disfunctional.
Thanks for correcting me on those Sluckey.
Sorry for the misdirection guys.
Brad :w2:
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> Wouldn't these work
"B" certainly does nothing at all. Imagine ALTernating current through the transformer secondary. One way, one diode blocks current. The other way, the other diode blocks current. NOthing comes out.
"A" will charge-up the top cap, but there is no complete path through secondary and bottom cap. You will have useful DC on the top cap, no useful DC on the bottom cap.
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The iron from a Dynaco Mk III, but with silicon rectification, will be close to your 80 Watt goal.
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Thats what I was thinking also.