modifying kt88 pp ul amp

[iorga]

Hi guys, I m new to tubes
i ll have loads of questions, and I m still reading a lot.
But basically I plan to do a stereo tube amp in a single chassis based on the
Class A Ultra Linear (UL) 12SL7 SRPP / KT88 SIPP Tube Amp


in diyaudio projects.
with KT88 and 5751 tubes.
I have questions about the transfos, I think the 1605P from hammond should be ok for each channel but what about the output transfo? Is it some kind of 1650PP at 210 euros?!?



Now, and i know this is not the main question, anyway it s another necessary condition for me to go on with it, i plan to do a couple of mods, but i don t know if they are possible:

i ll do one chassis, for a single stereo (two of those blocks) system.
if i do 2 mono amps, and join their signals in an operational amp in addition mode, i would be able to have stereo, mono, left or right sound, am i stupid?


I plan to use an old amp chassis. Big, cute, nice pots and switches.

Can I easily connect its headphone jack to the circuitry?
How can I add inputs to the schematics?
(i don t think this is that hard)
How can I add 4 and 8 ohms for the speakers?
(i have def no clue here)

My old amp is 2x60W and has nice vu meters. Can I plug them easily to the schematics? Where?

I know I need to learn basics, but as I d like to do that greatly I must ask if these things are at one's reach. Is it just plugging?

The Power switch has a LED on it.
Is that complicated to make it work?

There s also a function "switch" (tuner, phono, tape): can I integrate it in those schematics? Would anyone have some example of such a union?

I won t need any of the other wheels, so I ll just let them there without connecting them.

If you have nice schematics it could be cool.


Many thanks and sorry for the messy character of this post!


Regards,
I.

[Herr Grau]

Originally Posted by iorga

Hi guys, I m new to tubes
i ll have loads of questions, and I m still reading a lot.
But basically I plan to do a stereo tube amp in a single chassis based on the
Class A Ultra Linear (UL) 12SL7 SRPP / KT88 SIPP Tube Amp

I think KT88 are a good choice, as are PP and UL operation. But SRPPs are known to be bitchy and I would propably not build one without the knowledge (and time) how to polish it, should it not perform to your satfisfaction.

I'm currently putting the finishing touches on my UL PP KT88 Amp and have to tell you: By now I would already do a great many things differently. If I were you, I would try building this. Interstage transformers may seem expensive, but you save coupling caps, tubes and other parts + there is less to do wrong + I think they are excellent phase splitters.

Originally Posted by iorga

I have questions about the transfos, I think the 1605P from hammond should be ok for each channel but what about the output transfo? Is it some kind of 1650PP at 210 euros?!?

Transformers in the signal path are always a bottle neck, so I think you should make shure to use good output transformers (and interstage, if you take my advice...). I don't know where you are from, so I can't give you local recommendations.

Originally Posted by iorga

i ll do one chassis, for a single stereo (two of those blocks) system.
if i do 2 mono amps, and join their signals in an operational amp in addition mode, i would be able to have stereo, mono, left or right sound, am i stupid?

If you have a chassis and it's good, then use it. Not having to do the metal work is a major advantage.

Originally Posted by iorga

Can I easily connect its headphone jack to the circuitry?
How can I add inputs to the schematics?
(i don t think this is that hard)
How can I add 4 and 8 ohms for the speakers?
(i have def no clue here)

For the headphones you just source the incoming signal directly behind the volume pot out again. High gain sources like CD Players can drive headphones themselves.
If you need 4 and 8 Ohm outputs, you have to choose an OPT with those outputs. After that it's as simple as connecting them to jacks.

Originally Posted by iorga

My old amp is 2x60W and has nice vu meters. Can I plug them easily to the schematics? Where?

Depends on which range they measure in.

Originally Posted by iorga

I know I need to learn basics, but as I d like to do that greatly I must ask if these things are at one's reach. Is it just plugging?

The Power switch has a LED on it.
Is that complicated to make it work?

Although I think everyone can build an amp if the schematic is good enough, you should ensure that your basics are straight, so you don't electricute yourself - the voltages in an tube amp are very dangerous. Don't take this lightly. High voltages can let the flexors of your fingers cramp, so that you can't let go of a once touched wire. Once a current of about 50mA runs across your heart in the relative refractory time, youre done for. So if you're not certain, don't do it. Otherwise: Go ahead and try! You might want to buy the books of Morgan Jones, those will solve most of the other questions thatll appear sooner or later in the project.

Getting LED to run is not that hard, just look under "LED" in wikipedia, the basics are all there.

Originally Posted by iorga

There s also a function "switch" (tuner, phono, tape): can I integrate it in those schematics? Would anyone have some example of such a union?

If you have a source selector, thats a good thing. Just figure out where the output is and you can use it as-is.

[iorga]

Thanks!!
I m in Barcelona, Spain
I ll hav a closer look at what you suggest;
right now i m concerned of the reusable or not chassis:
i can just unplug the function switch and provided i plug where it should be plugged, i can just use it as it is?

that s good news isn t it?


As for the security concerns, i ve read so far the 11 first pages of the sticky post and i m still reading! wow
i know it is dangerous
A friend of mine, experienced with such devices (he built a couple, that s enough for me), and both having read the warnings list, I think it could be the minimum necessary things to do...

You suggested me another scheme, why?
Why is there a concern with SRPP? Can t i just get rid of it then?

Thanks again!

[iorga]

The scheme you suggested, did you try it?

Actually, i d like
a PP UL class A with KT88 tube amp.

Then for the extras, i d like 4 inputs, 0-4-8 ohms, headphone jack, power led and vu meters usable, and the Mono left, Mono Right, Stereophonic modes available too.

(N.B.: The speaker i m planning to get are the supravox 215 RTF 64 BIC or 215 signature Bic, in TQWT boxes, respectively 96 or 93 dB.)

Cause I listened to such an amp, and enjoyed a lot. So if I keep the scheme similar (no access to the scheme, sadly), i would have same sound...

If possible, i read that nowadays we can do SE benefits in PP operation mode, without having SE disadantages. I don t know how one can perform this. I d be glad I had such schematics in my hand for the amp i d like to have.
Does anyone have such a scheme?


When you refer to transfos as the bottleneck, .. i ve seen a lot of designs identical, so... are you saying this way is not good?



btw, what OPT stands for? (operational amp?)?

[Herr Grau]

Spain is Europe, that's good. We have some very good transformer makers here.
You might want to look at AE Europe, Tribute, Lundahl (e.g. here), Amplimo and IG Präzisions-Wickeltechnik.

Regarding SRPP, I just quote from frihu.com (translation included):

SRPP is nothing for a quicky and wants to be 'raised' (means: optimized). Only very seldomly the circuit will work just like that with the given source, output stage and speaker combo. Either the amplification is too low or too high, either the output impedance is right or not, either the tube is within it's working point or not. A tini-tiny wrong turn at the input can turn the high end SRPP into a noise generator. Last but not least you're on with termic shifting. Ergo: Either youre a designated SRPP-freak with masochistic tendecys - or not.

Just 'removing' it doesn't do the job, you have to come up with something to substitute it with. And without experience in the circuit design department, that's gonna be al little tricky.
I would tell you to stick with a working schematic without bells and whistles. And since it seems to be your first project, you might want to look for simplicity. Not always is 'more complicated = more better', sometimes you find the best results in pure, simple things. The schematics always look far more easy then the wires in real life

right now i m concerned of the reusable or not chassis:
i can just unplug the function switch and provided i plug where it should be plugged, i can just use it as it is?

Sorry, I don't understand what you mean.

[iorga]

i ve been reading a bit more.
it s been SO helpful
The howtos in this web are really cool.

I hae a doubt on the layout of the 2 mono layouts in a single stereo chassis: how can i join them together in terms of power supply (2 plugs?).
Will I have the space in a 42x35x15 chassis?

What I asked was for the input selector pot: the old amp already has one.
But only 3 inputs. I plan 4 inputs. Maybe 5.
I figured out roughly where those inputs ought to be, as well as the 0 4 8 ohms speakers imped.

The thing I am concerned also is about the physical position of inputs: must be far away from power.
In a dual monoblocks config, I m afraid that distance won t be that big.

Isn t it easier to have one main power input? How can I do this?

OPT = output transformer.

The 5751 are the rectifier ones from the driver stage? Am I wrong?

[Herr Grau]

Originally Posted by iorga

i ve been reading a bit more.
it s been SO helpful
The howtos in this web are really cool.

I hae a doubt on the layout of the 2 mono layouts in a single stereo chassis: how can i join them together in terms of power supply (2 plugs?).
Will I have the space in a 42x35x15 chassis?

The chassis is big enough. But to give you hints on how to do it, I'd have to see it. If you make fotos, we can suggest where to put what and how to lay out.

What I asked was for the input selector pot: the old amp already has one.
But only 3 inputs. I plan 4 inputs. Maybe 5.
I figured out roughly where those inputs ought to be, as well as the 0 4 8 ohms speakers imped.

The thing I am concerned also is about the physical position of inputs: must be far away from power.
In a dual monoblocks config, I m afraid that distance won t be that big.

Isn t it easier to have one main power input? How can I do this?

1. Ok, I realy think that you don't have the whole monoblock/stereo thing out correctly. Monoblocks mean that you build two seperate chassis, each with their own power supply. The only difference in a stereo build is, that you build the two circuits into one chassis and possibly let them share a power supply. If you already have the chassis, take it and build stereo!

2. If the input selector works - take it. If it has to few positions - dont take it. I would rather settle for one input less than to fit a new input selector in it. It is expensive and takes time and metal work skill.

3. Yes, you should always try to keep away the signal path from the power supply, more acuratly electromagnetic fields produced by the power transformer and every wire that carries AC. This 'keeping away' can be done by isolating the two things with a sheet of iron connected to ground or by distance, since the intensity of an electromagnetic wave decreases with r².

The 5751 are the rectifier ones from the driver stage? Am I wrong?

Yes, you are wrong. The 5751 ARE the drivers. Rectification is in your mentioned project done via solid state.


If you have help building the amp, you'll propably be fine. But you might realy want to learn a little more. Try reading the pages of Boozhound and Valve Wizard. You should know how the basic stuff works and most importantly get your grounding right. Otherwise you'll end up with an expensive hum generator. I want to strongly advise you once again not to build that OddWatt Project. You are better of not using an SRPP and not trying to build something too complicated.

[iorga]

great!

TFYH!
I understood the mono blocks had 2 separate PSU and what i thought was that a stereo one is basically 2 monoblocks in the same chassis sharing the same power unit. If I ask it is because my aim is just to try to insert a Mono / stereo / mono left / mono right switch and i don t know exactly how to do this.

As for reading: you re absolutely right. Yes I do need to read more, and I m on it. And yes I want to learn more. I read those pages, they re on my computer now, it s really pedagogic. I am still at the beginning...
About the rectifier: ok. It s not properly exxplained how it rectifies, that s hy.
Sin wave -> abs(sin) is done by the rectifier. Ok; these can be tube or solide devices. Then the CRC or CLC (Pi thing) is there to make the signal as flat as possible (i don t have the terms): the camel back done by the rectifier is not good enough, we d like something DC perfect. So CLC.
Well, i still have NO schematics. The one you suggested me, did you try it? Or is it just the one you think would be perfect?


I m still studying a lot the way things work. I have to read a lot.

About the chassis: the input selector switch has 3 positions. It s supposed to work.
Is it THAT hard to put another switch? I mean: the hole is done, the knob is there, shouldn t it be fine to put another switch whose diameter is compatible with the knob?
Too hard? (Then I would follo your advice)


The pics: i have something in my mind, but that is not mine yet. I d rather keep discrete until it is, so please forgive me not putting right no the pics.


You would go for a tube rectifier?
And for the transfos? I need one output transfo (hammond or another) per channel, and I also need the "main" one, the one that gives the power. This one is not clear in its characteristics, cause i haven t found yet any schematics of a ultra linear class A stereo kt88 push pull.

Night and thanks!
I began this. When my mind is set, full and ready, I ll gather parts. Then I ll see my friend. This is going to be long...
Please be patient...

[iorga]

sorry you must become crazy:
I meant
Yes i d like to share the power supply between the 2 "blocks", but with 1/ possibility to choose the stereophonic /mono mode, and 2/ i have no idea of which transformer use here (and i knew monoblock meant 2 chassis; but i used that term even if then i was thinking on stereo= 2 mono blocks on same chassis)

Again, sorry: the tube is not the rectifier, the tube is the DRIVER.
(what does rectification then? this absolute value? it s not said in the how to page)

Night!

[Herr Grau]

Originally Posted by iorga

I understood the mono blocks had 2 separate PSU and what i thought was that a stereo one is basically 2 monoblocks in the same chassis sharing the same power unit. If I ask it is because my aim is just to try to insert a Mono / stereo / mono left / mono right switch and i don t know exactly how to do this.
[...]
I meant
Yes i d like to share the power supply between the 2 "blocks", but with 1/ possibility to choose the stereophonic /mono mode, and 2/ i have no idea of which transformer use here (and i knew monoblock meant 2 chassis; but i used that term even if then i was thinking on stereo= 2 mono blocks on same chassis)

If I understand correctly, you want to build a stereo amplifier (two channels, one power supply) with the option to make it a bridged mono amplifier. I hope that's what you mean because otherwise I have no idea what youre talking about and why.
So I drew up the basic way to do this with a 2-position 3-way switch:



In position 1 of the switch you get standard stereo operation, in position 2 only the upper input is used and the outputs are bridged together on the upper speaker.

Originally Posted by iorga

Again, sorry: the tube is not the rectifier, the tube is the DRIVER.
(what does rectification then? this absolute value? it s not said in the how to page)

About the rectifier: ok. It s not properly exxplained how it rectifies, that s hy.
Sin wave -> abs(sin) is done by the rectifier. Ok; these can be tube or solide devices. Then the CRC or CLC (Pi thing) is there to make the signal as flat as possible (i don t have the terms): the camel back done by the rectifier is not good enough, we d like something DC perfect. So CLC.

Let's take the schematic I suggested and I'll explain it along.


The upper circuit is only the amplification unit. Power is only inserted at two points: The center tap of the OPT and the primary of the interstage transformer. This inserted voltage is generated and filtered in the lower circuit, the PSU. In this case, the power transformer generates 230V which are then rectified by a bridge rectifier consisting of four diodes. What that does is basicly inverting one of the half waves into positive, so that you have a pulsed DC. To smooth that out, you have a LCL filter for the output stage and another RC-filter for the input. In the end you have a more or less smoothe DC to run your amp with.

Originally Posted by iorga

You would go for a tube rectifier?

Yes, I definitly would. This is a matter of disagreement though within the tube fan community. Personaly I like tube rectifiers and will always use them.

Originally Posted by iorga

And for the transfos? I need one output transfo (hammond or another) per channel, and I also need the "main" one, the one that gives the power.
This one is not clear in its characteristics, cause i haven t found yet any schematics of a ultra linear class A stereo kt88 push pull.

You need one main transformer if you build a community PSU (which I think you should and want to do). The quality of this is not completely unimportant, but not that critical. I would try finding a toroid winder localy. If you don't find one, the german ones surely ship to spain. What you want to look at very carefuly are the OPTs. Their quality basicly determines the quality of the amp. So I strongly disagree with Hammonds and point to the manufactorers I mentioned earlier.

Originally Posted by iorga

Well, i still have NO schematics. The one you suggested me, did you try it? Or is it just the one you think would be perfect?

No, I have not tried it. But I plan to build some varity of it. I'm currently playing with the schematic changing it to my taste. But I'll have quite some time to think about it, since I've got no money for another build at the moment. I can show you, what I've come up with so far, adapted to UL and tube rectification (and - for you very important - with a complete grounding scheme):



I can't account for the PSU to be perfect though.

Originally Posted by iorga

About the chassis: the input selector switch has 3 positions. It s supposed to work.
Is it THAT hard to put another switch? I mean: the hole is done, the knob is there, shouldn t it be fine to put another switch whose diameter is compatible with the knob?
Too hard? (Then I would follo your advice)

I can't tell you that without having seen it. Those things have a habbit of not being standard interchangeable parts. The main thing is that a good input selector costs real money. The cheap stuff will drive you crazy. Look for ELMA, they cost around 50€.

]I began this. When my mind is set, full and ready, I ll gather parts. Then I ll see my friend. This is going to be long...

That's good. Take your time, don't rush it. That helps avoiding mistakes.