• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

With a little help... KT88 PP UL Amp

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0-100mA fsd (full scale deflection)

You may want to re-think about the o/p stage protection: I often use a 10R resistor in the o/p cathode legs (ideal monitoring point) and fit a single protection fuse in primary CT to B+.The resistor in the cathode legs offers cheap AC regenerative feedback and drastically reduces IM thd. It's up to you if you want to choose this. Again user preference.
My experience of fuses is if you use them near power signal paths as per CT tranny then use a small cap after them to decouple, which extinguises the AC arcing once they fuse.

My experience is that the 10R option in the cathode legs works better for 6550 tubes than the KT88's. The output tranny being the ultimate quality component. (The new 6550's I find slightly harder sounding tube than new KT88' types...OMO that this mod responds better)
Majestic o/p trannies 18 sect designs like this option, Sowters,50/50 generally have 14 sect designs, often this mod not so responsive with their transformers.
Again other members have differing ideas and inputs.
richy
 

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I drew that up. And I implemented the Bias supply of the Eckland design.
Thing with that is that he has a 70V tap on his main transformer; if I want to stay with my filament transformer (which I come to like since it's a couple of times cheaper than an extra winding at the main trafo), I'd need a 20V:230V ratio. Closest I can get to that is 18V:230V, which would result in about 80V instead of 70V. Is that okay?
Eckland specifies FRED 1200V diods. I found somewhere that thats equal to a TO-220AC Schottky. Right?

Oh, almost forgot, the changed schematic:
An externally hosted image should be here but it was not working when we last tested it.


A quick overview over the changes (please check if there are mistakes!)
- Implemented Ecklands Bias supply
- Substituted fuses in cathode legs with 10R-resistors, put in a 315mA (does that value apply here too?) Fuse between center tap and B+, accompanied by a 1uF cap to ground
- Implemented a Bias Amperemeter (question here is whether I have to take out the resistors for measuring via switch or if I can just switch on my amperemeter parallel to the resistors)
- Removed the Preamp power supply

P.S. I still don't know whether I can use the Sowter OPT... It'd be good to know, since the british pound is getting more expensive by the day...
 
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Yeah, I wanted a tube rectifier for B+ from the very beginning (cause of the soft start and soft clipping). But since I realy dislike half wave rectifiers (and noone wanted to specify the resistors for me... :p ) I simply copied Ecklands Bias Voltage supply into the Keroes Circuit. Gonna be a couple of bucks more for the diodes, but I'll sleep better, because it looks "more perfect" to me this way. :cloud9:
 
Well, I have most of the parts and will do the casework any day now...
The progress can be followed in my blog, but it's in German.

For the english reader just the major facts:
Custom wound toriodal XFormers from Rondo-Müller Germany are already on my shelf, as are the cheapest chokes I could get (from Jan Wüsten for 15€ pp 5H 300mA). There will be no electrolytic capacitor in the amp (exept in the DA-blocker): The PSU has all film caps, motor-start MKPs for the Bias supply and "the good ones" (~5€ pp) for the main supply line.
Coupling caps are paralleled Polystyrol which I encapsulated in bee wax (pics are in this post). Resistors are all NOS Vitrohm carbon composite, exept the ones that have to be matched, which I took metal film precision resitors for. Cases are Hammond 1441-28, LS-terminals Dynavox, RCA-jacks the isolated ones from Neutrik. I'm still waiting for my OPTs from Bud Purvine (O/Netics, Washington) which should be ready in about a month + shipping time. Valves will be Siemens 5692, EH KT88 and some 5V4-type which I still have to choose.
Until now it's just an expensive bag of parts not realy worth taking pictures of....
 
...Resistors are all NOS Vitrohm carbon composite, exept the ones that have to be matched, which I took metal film precision resitors for....... I'm still waiting for my OPTs from Bud Purvine (O/Netics, Washington) which should be ready in about a month + shipping time....

Hi Grau,

nice to hear an update from you!
I am curious which Bud Purvine OPT did you go for, I read somewhere he has level 1, 2... to 5 and each higher level is a doubled price for the OPT as there's much more labour for each higher level, I wonder now what speces are your OPTs going to have and how much did you have to pay?

About the Vitrohm composite resistors are you aware of all kinds of drawback these types of resistors are dragging with or what is the reason you have chosen these over other types?

Cheers Michael
 
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I'm just curious: isn't this circuit a bit much for a 5V4 rectifier? The specs for this tube call for a maximum RMS plate voltage of 375V for a capacitor input filter, yet the transformer used has a 400-0-400 secondary. I just figure there's something out there I'm overlooking...

If you look at the CLC, the first C is small enough (10uf) that it is still considered a Choke input filter, so you resulting Voltage should be just over .9x Primary VAC, so my guess is around 375 V.

That low value first Cap sets the output voltage on this CLC. Now a high value first C will get you a full fledged Cap input filter, and resulting voltage would be more near 1.4xVAC...

PSU designer should confirm this...
 
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Actually, from PSU designer, that first cap should be more like 3uf to get around 375V...

I had to guess at Primary/Secondary DC resistance on power tranny and on choke...the real values can change the output voltage quite a bit...
 

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I am curious which Bud Purvine OPT did you go for, I read somewhere he has level 1, 2... to 5 and each higher level is a doubled price for the OPT as there's much more labour for each higher level, I wonder now what speces are your OPTs going to have and how much did you have to pay?

I told him I needed an equivalent for a TO330 and he seems to have no problem building something around the specs... The OPTs are going to be 100W-types, which would be 140$ for Level I, 260$ for Level II and 520$ for level III. I choose Level II, since it was about the money i planned on spending.
He said that he was using type I for his own amps though. I realy liked that he didn't try to talk me into anything expensive.

About the Vitrohm composite resistors are you aware of all kinds of drawback these types of resistors are dragging with or what is the reason you have chosen these over other types?

There was a simple reason why I choose these resistors. I read an interview with Howard Dumble once and he said one should stick to carbon comp resistors. So I do. I'm somewhat bound to rely on what experts say.
 
Building a Tube Amp for long term - Introduction

If you have a weak spot for good music, if you already built good speakers, like to build your own things and suffer of a bit of a gear acquisition syndrome too, sooner or later you'll get the idea to build your own amplifier. I already showed HERE and HERE how to build a very good solid state amp for quite little money. Those mono blocks found their place in the world - just not with me. You couldnt say that I'm underamplified anyway, I own a NAD C372. But at some point you gonna hear a tube amp and then the damage is done. There aint no where back. Just forward. Into the holy land.
I want to show with these articles, that it isn't that complicated to build very good hifi components yourself.
In this introduction the single steps are going to be listed in a short fashion, which are all going to be explained in greater detail in the specific article later on. It is the intention of those articles to show you what to focus on and discover possible mistakes. I am going to explain my own decisions alongways, so that beginners will have a "cooking recipe" at the same time that can be successfuly recreated without deeper research.

Step 1 - What do I need, what do I want?
First step is to determine what you want and need:
- Which tubes?
- Which modus of operation?
- Which features?
- Which adresses do I need to know?
Step 2 - Circuit
Secondly you have to find yourself an adequate circuit
Step 3 - Choosing the components
- Output transformers
- Power transformers
- Coupling capacitors
- Components of the power filter
- Resistors
- Tubes and accessories
- Wire, jacks, cases
Step 4 - Casework
Step 5 - Wireing
Step 6 - Euphoria
 
Building a Tube Amp for long term - 1.1 - Which Tubes?

What defines a tube amplifier? Of course: The tubes. That's where the fascination begins. Now - what is a electron tube?
Mainly it's a glass bulb with a vakuum in it. Other parts are: Two plates and a grid in between. If we put a voltage between the plates, an electrical field is generated. If you now place electrons between the plates (we do that with a heater), they are going to start traveling towards the positive plate, the anode. By applying a voltage to the grid in between we can controll the whole process. Luckly for us, what happens at the grid is in the end directly proportional to what happens at the anode and that's basicly why we can use a tube as an amplifying element.
First off: I am lously at explaining such things. And secondly, I myself lack the actual insight. If you want to understand the exact principles and learn how to design a simple amplifier, I can only recommend the site of Boozhound Labs, which is very insightful and easy to understand.

The first thing you propably want to do is selecting your output tube.
This is a point where it's important to know the market: If a tube is still in production is a major criteria for me. I don't like the idea of choosing a perfect tube for the job and then ending up searching garbage bins and spending hundereds of dollars on NOS. Not that NOS would be a bad thing - of course the best tubes are still old western world produced ones. But personaly I want to be independed of this supply and rather accept slight losses in quality. Plus for the popluar tube types there are much more good, readily available circuits.
The most popular output tubes are:
EL34, EL84, 6C33C, 2A3, 6L6, 6V6, KT66, KT88, 300B
The second important point: Try to listen to all the tubes intersting to you. That's not always simple, since the other tubes and the type of circuit are influencing the sound of the amp very massivly - but if you find something you like, there is no problem in copying circuit style and all the tubes. So try it. Most importantly, when the time passes, you get an idea of how certain tubes do.
El34 and EL84 are no bad types and very suitable for beginners. There are tubes, power supplies, output transformers, circuit boards and amp kits for little money (with the circuit of loetstelle.de you can realize a very nice amp for about 200$). But still, for an amp I want to keep indefinite time they didnt seem to be the right choice. 6L6 and 6V6 I know very well and I would choose them any time for a guitar amplifier. In my oppinion they aren't exactly first choice for hifi though. Much better are KT66 and KT88. The character of those two tubes is very similar, they are nice sounding and quite powerful tubes. The KT88 has just a little more power than her little sister. The only real opponent in my eyes was the 300B, which is a very powerful real triode, making it a good hifi tube and though a quite popular choice for high class amps. The 6C33C and the 2A3 I don't know and couldn't arrange to hear in a senseful frame of time.

In the end intuition is always the deciding factor: I chose KT88 for my output stage. What was in the gain stage I did not care that much about, which is propably not the best way to look at things. The 6SN7, which was finaly chosen, were already in the circuit, they are well knows around the world, solid, nice sounding and easily available. These small tubes are even more of an own sience department than the big ones and you can find almost infinite amounts of information about them on the net. I actualy don't know any realy bad ones to beware of though.
 
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Building a Tube Amp for long term - 1.2 - Modus of Operation

The title is a little distraction, since we actualy have to chose several modi.
"Single Ended or Push-Pull? That is ... another one of those questions..!" One of those which split the fan base. Single Ended means that one tube is handling the whole signal. Of course this holds more potential of acoustic quality than splitting the signal in any way. The downside is realativly little power. Some people argue that you don't need that much of power, a 5W amp is very sufficient in many cases. Hafler explained though, that you want to have massive power reserves to account for suddenly changing dynamics. I tend to believe that. Our ear works logarithmicly, which means that even slight changes in subjective volume are related to enormous difference in the initiating power source. At least that I do know from studying medicine.
I wanted a little more power. More reserves. More dynamic potential. So I choose Push-Pull. With this modus of operation two output tubes each handle half of the work. I believe that the drawbacks in quality with a good circuit are marginal.

More influence to the sound has the decision how to wire the tubes. The background theory is a little to complex. What you should know is that triode mode is the best sounding variety. With all the other modes you treat ideality agains power. I choose ultra-linear, because I found it to be a good compromise between quaility and power. A KT88 you too could wire as a pentode, which would be furthest on the side of power.

Last but not least you have to choose the bias variant. Bias means the current that determines the working point of the tube. This graphic illustrates the differences in circuit between auto- and fixed bias: Klick
"Fixed Bias" means that you set the bias current to a fix point. You could think of cathode or auto bias as the tube drawing the current it need itself, determined by the resistor and capacitor from cathode to ground. You eliminate the necessity to set the bias manualy and the need of a external bias supply. Nevertheless fixed bias is highly recommended, because you have the option of using different tubes or varying the point of operation without changing the circuit. Besides it has technical advantages, since higher tube current doesnt alter the bias current. I clearly settled for fixed bias.
 
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