• 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.

Newbie - 1st time build 50w KT88 Amp

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I have read several threads from other newbies before me over the past years to not make the mistake of asking the same question twice. However, I do apologize if these questions have been asked before. Over the past years I have been into personalized speaker building for friends, family and others with interest in getting something unique.

However lately I have been bored with the sound of my Onkyo HT-R395 (Receive/Amp) and been thinking about finally building my own tube amp. There are some concerns however regarding quality components and design which i will list further below. One other declaration is that I have some electronics building skills, as in, cross-overs for 2 and 3 ways speakers. But other then that, I am very rusty as I never practiced my electronics skills that I learned in school.

That said, I appreciate any input from you veterans regarding my 1 time build.

NOTE: Money is not really a problem, that said, I consider 2000USD the absolute maximum for this build. I found this AMP which looks nice and seems to work for my needs, but Id rather build it myself then to buy a built amp: RAPHAELITE EP65 KT88 6550 Tube Amplifier Hi End Pull Push Integratred Tube Amplifier With Fully Coupling Output Transformer

1. The outcome I look for is the highest possible quality. I will build this AMP only once, so that it lasts for the rest of my life as I have no interest whatsoever to build several over the years (I have a steady job now, but hey, that can change anytime). What components are recommended as I am looking for the KT88 tube type? (I have personally heard amps with this tube and sound amazing) And 50 watts is a must.

2. Having a list of high quality component names, where do I get started? Should I go to the local bookstore and buy a book or books about designing and building tube amps? If so, what do I read first etc?

3. Safety: I did study electricity (high voltage) in school. During that period I have experienced my share of shocks. That is no fun, and I am well aware of the dangers. However, tube amps are a different breed of electronics I normally work with (cross overs, and for my day job fiber optic fusion splicers). What should I put special attention towards when working on a tube amp without getting shocked.

Thank you.
 
$2000USD? We can definitely help you figure out an heirloom-quality amplifier to build with that kind of money for sure.

Where are you located? That will dictate the parts choices some, especially with regards to transformers and chassis. You should be putting the majority of your budget into the transformers for sure.

For capacitors, if you want longevity try to stick with film types rather than electrolytic types where possible, poypropylene-in-oil motor-run/industrial types are very good in this respect, they will last a lifetime if kept cool enough. You may need electrolytics in some places, like cathode bypass caps if going cathode bias, or in the bias supply if going fixed bias.

50W per channel with the KT88/6550 is a pretty "classic" goal, and pretty easy to accomplish well. The circuit is much more important than the brands of parts here.

Are you opposed to using sold state parts anywhere? I would go for tubes on everything except for the power supply, here is where silicon diodes will have a great impact on overall quality and complexity, as well as cost.


As far as books, there are many available that cater to varying degrees of prior experience. Many are also available in PDF form, scanned in from the heyday of electronics when everything was tube. There are a ton of places to look, but the archives/downloads section at Pete Millett's site is a good place to start- tubebooks.org - Vintage info from the age of vacuum tubes

The book I would recommend most is "designing Valve Amplifiers" by Morgan Jones-
http://a.co/d/0gvSDBW 4th edition
http://a.co/d/eyrkuQc 3rd edition

Both have good info, and are more or less the same, with a few differing projects being shown. A bit pricy, but if you hunt around you can find them used for cheap, or the occasional discounted PDF on offer as well.

There are a bunch of builds shown here on our forum too, to varying degrees of complexity and power levels. I would do a search for "KT88 push pull" and similar terms and absorb everything you can for a start.
 
$2000USD? We can definitely help you figure out an heirloom-quality amplifier to build with that kind of money for sure.
Heirloom-Quality = Lovely, I will think of a name for this amp with these words :)
Where are you located? That will dictate the parts choices some, especially with regards to transformers and chassis. You should be putting the majority of your budget into the transformers for sure.
Location: Mexico-City, which makes sourcing parts a bit more complex, but import is not an issue as I deal with that for the company I work for on a daily basis. I used to live in the Netherlands, but I left that birds nest over 15 years ago now.

As for the transformers, I expected that to be part of the quality point for a great amp.
For capacitors, if you want longevity try to stick with film types rather than electrolytic types where possible, poypropylene-in-oil motor-run/industrial types are very good in this respect, they will last a lifetime if kept cool enough. You may need electrolytics in some places, like cathode bypass caps if going cathode bias, or in the bias supply if going fixed bias.
Ok, noted
50W per channel with the KT88/6550 is a pretty "classic" goal, and pretty easy to accomplish well. The circuit is much more important than the brands of parts here.
As for brands of parts, I get to read a lot about fake tubes etc and how it influences (from some people´s POVs) the sound quality. Thats why I rather ask then assume.
Are you opposed to using sold state parts anywhere? I would go for tubes on everything except for the power supply, here is where silicon diodes will have a great impact on overall quality and complexity, as well as cost.
I am not exactly sure what you are asking here.
As far as books, there are many available that cater to varying degrees of prior experience. Many are also available in PDF form, scanned in from the heyday of electronics when everything was tube. There are a ton of places to look, but the archives/downloads section at Pete Millett's site is a good place to start- tubebooks.org - Vintage info from the age of vacuum tubes

The book I would recommend most is "designing Valve Amplifiers" by Morgan Jones-
http://a.co/d/0gvSDBW 4th edition
http://a.co/d/eyrkuQc 3rd edition

Both have good info, and are more or less the same, with a few differing projects being shown. A bit pricy, but if you hunt around you can find them used for cheap, or the occasional discounted PDF on offer as well.
I have seen that book before and will check with my local book store if they can get it as I rather read paper books then those new fancy electronic versions. I do a lot of computer work during the day for my day job, it does not necesarily enhance my vision (as I am already The Glassed and Furious).
There are a bunch of builds shown here on our forum too, to varying degrees of complexity and power levels. I would do a search for "KT88 push pull" and similar terms and absorb everything you can for a start.
I will search for those terms and read up on that.

My hope is that by december 2019 (so about a year from now) I have a working amp. That means that over the upcoming months I can read up on other build threads and gather the information I need to get this going.

One thing I didnt put in my OP is how difficult it would be to put some VU meters on it? I know it is just for show, but give the AMP something more to look at on the panel then some blank selector/volume knobs.
 
So, Since I've played with a few different designs that all shoot for "50W" class sort of power, I'll try not to overwhelm you with this stream-of-consciousness style post on my experiences getting there :)

Keep in mind that to get good performance and linearity at these power levels you're looking at ~450-475+ for the supply voltage. If going tetrode connected you will definitely want an adjustable and preferably regulated screen supply to get the best results.

The first design I would recommend (assuming solid state helper-parts are acceptable) would be the wonderful and now semi-famous 6L6GC AB2 thread- 6L6GC AB2 Amp

It's a long read, but addresses the majority of the issues with other similar power amplifiers- It has powerful drivers supplying the output tube grids for elimination of blocking distortion (a very worthwhile upgrade with typical audio grade pentodes and tetrodes!) allowing greater power without audible distortion, as well as increasing possible output power. The cascade of differential drivers also assures excellent balance, so that everything stays nice and linear. An excellent overall design, if a bit complex. The circuit as shown will work as-is with the KT88, with adjustments to bias, and proper transformer selection. Reasonably versatile design.

6l6ab2.png

Schematic found here- 6L6GC AB2 Amp

This would be a lovely choice for a lifetime build, in my opinion. I built one for a good friend a couple years ago and its in use pretty much whenever he's home.

Something like a modernized 5-20, Williamson, or similar would also work. It's hard to fault the splitter-driver-output topology when implemented correctly. Something like the Heathkit W5M maybe, but with 6SN7 instead of 12AU7. This one is cathode/mixed bias KT66, but you get the idea-

W5M.GIF

I would go for a 6SL7 voltage amplifier/concertina front end, 6Sn7 driver/differential, and then mosfet grid drivers with fixed bias. Unbypassed cathode on the first 6SL7, with some global feedback to it. With a good output transformer phase shift will not be an issue here. Just my two cents :)

Hopefully some other big names will pop in soon and give some advice too, always good to hear multiple ideas.
 
Last edited:
Heirloom-Quality = Lovely, I will think of a name for this amp with these words :)

Location: Mexico-City, which makes sourcing parts a bit more complex, but import is not an issue as I deal with that for the company I work for on a daily basis. I used to live in the Netherlands, but I left that birds nest over 15 years ago now.
Not too bad then, Antek would be my recommendation for the power transformer, and Edcor for the output transformer (although power could come from here too, and it would save on shipping) and most parts shouldn't be terribly expensive to source then.
I am not exactly sure what you are asking here.
I was meaning silicon diodes, transistors, etc. Using them for the power supply circuitry will save money and perform very well, as well as make less unnecessary heat. In some places they can help set the operating points for the tubes as well, without imparting any real sound character of their own.
One thing I didnt put in my OP is how difficult it would be to put some VU meters on it? I know it is just for show, but give the AMP something more to look at on the panel then some blank selector/volume knobs.
It wouldn't be too difficult, and there are a variety of meters that folks use. I haven't used one for years, but they are pretty common.
 
Last edited by a moderator:
As a very broad generalization, classic amplifiers using more expensive output valves are likely to be better. Amplifiers using KT66 may be better than EL34, which will be more powerful than EL84, and ECL82 or ECL86 are at the bottom of the heap. Curiously, amplifiers using KT88 maybe worse than any of these, because they may have been designed as public address amplifiers, purely for their high output power. The quality of output transformers is crucial. Poor output transformers will be small for their rating, although C-core transformers may be an exception to this rule, and are almost always a guarantee of good quality.
So now that the KT88 is critisized by this man I am wondering what would be better quality, thats lasts long and sounds superb!
 
He's mainly refferring to classic amplifiers as units, as back in the days of yore it wasn't common to need higher power amplifiers unless for larger venues or public address use, these amplifiers often had compromises in order to supply the brute-force SPL required, and the transformrs may have been limited in bandwidth, and the power supplies marginal (IE, "good enough") to fit the non- hi-fi application.

This wont apply to a competent and thoughtful modern design that uses high quality wide-bandwidth transformers. Don't worry :)
 
Ok, good to know, as I really like the size and design of the tube. As far as the parts you mentioned so far, 100-200 dollars for a AnTek transformer and 60 -100 dollars for the Edcor output transformer each. I suppose that besides the tubes that would possibly be the most expensive parts for a tube amp built, besides the housing?
 
As I read across this thread, I saw the first schematic in post #5 and instantly recognized it as one of my designs. It has come back to life here: Tubelab Universal Driver Board, 2015 version

And can be seen here: YouTube

As a very broad generalization.......Curiously, amplifiers using KT88 maybe worse than any of these

And as a more accurate statement, very bad amplifiers, and very good amplifiers can be made with most any tube. Some tubes are inherently more suited to good audio than others, and the KT88 IS one of the good ones. It all depends on the design goals of the engineer, marketing, and finance departments behind the design, as to whether maximum power or maximum sound quality is the main design criteria.
 
3. Safety: I did study electricity (high voltage) in school. During that period I have experienced my share of shocks. That is no fun, and I am well aware of the dangers. However, tube amps are a different breed of electronics I normally work with (cross overs, and for my day job fiber optic fusion splicers). What should I put special attention towards when working on a tube amp without getting shocked.

Thank you.

Reminds of my first tube pre amp. I built it up and tested it and it didn't work. So I turned it off and touched circuit and got a huge shock. My tutor told me to discharge the smoothing capacitor before touching circuit. So next time I discharged the smoothing capacitor and still got a shock ! I had forgotten to turn it off at the mains. Remarkably I have reached 61 without killing myself.

So I now always have a LED in series with a resistor off the B+. This gives visual indication that the capacitor is still charged.
 
In regards to the transformers, both power and output, is Hammond similar to the brands already mentioned? Or is it better? Pricing seems significantly higher though, 400 dollars is pretty steep.

Hammonds are nice but run pretty hot. I would derate them a good bit to keep them cool. To be honest I would probably prefer Edcor for a bigger amplifier like this, or a toroidal Antek power transformer.
 
Nasty, ive been shocked once by 360VAC (32A) working on a 3 phase engine. Long story short, it was the teachers fault and the test mechanism was changed after that. 230V 16A at least 4 times and 120v probably 2 or 3 times because bad wiring on equipment.

Becoming 38 hopefully this 25th :D

As far as the LED in series with a resistor off the B+ : can you explain more? Im currently going through learning to read the schematics of tube AMPS and it would certainly help me to prevent getting shocked by high voltages working on one.

Good to know, the reason I ask is that I found a DIY kit tube amp called MOD 101 Guitar Amp kit : MOD 101 Guitar Amp Kit | Mod Kits DIY

And they seem to supply the Hammond transformers both power and output for that kit. But since the prices are pretty high I started wondering if the price really is justified in regards to quality or not.

Other point I remember is that an old friend back in the Netherlands always had all his audio equipment cables with gold plating. I have seen that tube sockets are also available with gold plating.

Question, does that make any difference at all in regards to sound?

I forgot to mention that the guitar tube amp would not be for me but a nephew of my wife who is an avid guitar player and at age 17 running around going to gigs every couple of weeks. This would be for his birthday in may next year, and might be a solid good build to get a first tube amp done before actually building one for myself.
 

PRR

Member
Joined 2003
Paid Member
...audio equipment cables with gold plating. I have seen that tube sockets are also available with gold plating. Question, does that make any difference at all in regards to sound?...

A cheap connector, gold plated, is a cheap connector with a higher price tag.

Cheap connectors always give trouble.

And much gold-color plating is not gold. There are good gold-tone contact alloys, but also much fool's-gold.

Whenever possible (difficult today), buy very-good connectors from very-good makers. Springs that grip, now and for decades. Let the maker decide what plating is suitable for the quality he aims for.
 
The first design I would recommend (assuming solid state helper-parts are acceptable) would be the wonderful and now semi-famous 6L6GC AB2 thread- 6L6GC AB2 Amp <snip>
Lets assume for a moment I would start with the design you provided. Are the 6sn7 tubes replaceable for lets say 12AT7? Or would I have to change the design for that? Also, many people talk about bias, and on some really expensive amps like the MC275 i have seen front tube sockets supposedly to BIAS the amp. What in gods name is that? Can you point me in the right direction for some solid info?

Thanks :D
 
Nasty, ive been shocked once by 360VAC (32A) working on a 3 phase engine. Long story short, it was the teachers fault and the test mechanism was changed after that. 230V 16A at least 4 times and 120v probably 2 or 3 times because bad wiring on equipment.

I forgot to mention that the guitar tube amp would not be for me but a nephew of my wife who is an avid guitar player and at age 17 running around going to gigs every couple of weeks. This would be for his birthday in may next year, and might be a solid good build to get a first tube amp done before actually building one for myself.
I'm sorry to butt in, when you're getting really excellent advice, but thought I'd add a positive recommendation to try the kit first. Designing, laying out, be-bugging and constructing a whole amplifier, which needs to spring fully formed from the head of Zeus, is a really big first step. Not impossible, and people did it a century ago, but still a very big step. Safety issues, for example, cannot be added later - they must be there at birth. I don't want to seem even slightly negative, just very conservative.

BUT that's way too many times to have been electrically shocked. Proper procedures were not taken, and any one of these incidents could have killed you. In firearms safety we say that you're responsible for every bullet fired, that there are no "accidents", only negligences. Something along these lines is true for any potentially dangerous activity. Rant mode off.

All the best fortune in the new year,
Chris
 
I'd recommend you go have a look at - How to design valve guitar amplifiers then get a valve like an ECC81/2/3 or whatever, draw a load line, power it up and go from there. You will learn and retain far more knowledge than asking questions, though I'm not for one minute trying to discourage question asking. Also read The Radio Designers Handbook whilst mucking about on the bench with actual circuits. First thing though make a cap discharger, stay safe.

To echo Cris Hornbeck's advise, to build a decent amp from scratch is a big jump with your level of knowledge. I was about at the same level two or three years ago when I started to try and build a 100w valve amp from scratch. I've learned a lot in that time, and the amps eventually worked, but they are far from perfect or even near perfect. It's a hard road with lots of setbacks but hopefully fun too.

Andy.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.