another beginers first amp thread!

[graeme uk]

hi, first of all, hello all, i just managed to sort my registration out.

I was going to buy this kit http://www.worldaudiodesign.com/wadp...s/kit6550.html
but the company is no longer trading, apparantly.

So i looked at the diagrams, printed it all off, including the layouts and instructions.
Ive read it all thoroughly, cross checked the schematic with the layout and i think i understand it all well.
I have no idea what each component actually does (well i do a little) but to be honest, it looks quite easy.

I have no knowlege of electronics but am very practical, i can build a car, im a jeweler and carpenter and can turn my hand to most things.
I understand the dangers too!
I wont have the money for a few months so i can learn more first, but i have lots of questions! (sorry)

here we go.

1. what kind of mains trans and output trans do i need? The parts list lists 2 mains, 2 output + chrome caps and 2 mains + chrome caps.
Am i missing something? My untrained eye only see's one mains.

2. how can i add a phono stage and a headphone socket?

3. i know the values, but what make/type of resitors and caps should i use?

4. my chassis design is in 3 parts, the inputs however will be under the mains trans. Should i shield them? I was going to use a wood/lead/wood sheet. Is this needed, will it work?

5. do the output trans decide the ohmage (sp) if so, 4 or 8? Im going to build speakers after to match the amp.

Theres probably going to be more questions, but thats all that springs to mind for now.

Thanx

[Sherman]

Welcome!!!

You will find this forum to be a great source of information!

I'm not an "expert" but I have built a few tube amps so I'll toss in my 2 pence.

1- You can use a single power transformer if it meets the required specs, the toughest one being the rectified current. The PTX shown supplies 350mA which is quite a lot.

2- In the tube world a phono stage is usually a separate amplifier with its own power supply and chassis. The power needs to be ultra clean for a phono stage and it is usually easier to make it a separate unit that then connects to the regular inputs of your amp.

I can't comment on putting in a headphone jack as I haven't yet built a headphone amp.

3- When you start talking specific types of components you will get almost as many answers as there are amp builders. I would recommend using metal film resistors throughout. 5% would be fine (most tube circuits were designed when 5% was considered "precision" . If you wish you can certainly use 1% resistors.

You can use quality electrolytics in the power supply. I'm partial to motor run capacitors for the power supply but they are much bigger (though cost isn't that much more). For the coupling caps use something like Hovland or Auricap. For the bypass caps you can use high quality electrolytics.

4- Definitely shield your inputs. Use shielded wire from the inputs to the selector switch (if you have one) and from the selector switch to the volume pot and from the volume pot to the input of the first tube. Ideally you want to keep your low voltage/current sources as far from the AC stuff as possible. Any noise they pick up will be amplified.

I've built several amps using wood chassis with a metal top plate that have been very quiet, no noise or hum. Just pay attention to the layout. Keep your inputs as far from AC as you can and use shielded wire.

5- Yes, the output transformers determine your speaker impedance. Most modern output transformers give you a choice of at least 4 ohms or 8 ohms with some also offering 16 ohms on the secondary (the side that connects to your speakers, the side that connects to your output tube(s) and B+ is the primary of the OPT).

Hope this helps and I'm sure others will kick in also.

[graeme uk]

yeah, helps loads, nead to clear a few little things up though.

1 so i need a 350ma mains trans, is that enough info to source one?

2 i used a seperate phono stage with my old cyrus ss amp so understabd seperate is better, if i have to keep it seperate ill build this first to gain experience. I just wondered if i could build it in for neatness and one less mains plug.

same deal with the headphone socket.

if im going to need seperate amps for phono and heaphones, would i be best of building a pre amp, with a phono stage, and switching the output between a headphone amp and the main power amp?

3 good info on the resistors, but i dont know which caps are which. As i said, i dont know how amps work, but im still confident i can build one. I could re-build engines before i understood how they worked. Im sure ill know before i start the build though, im trying to learn as much as i can! Once i know what ones are what ill understand your advice.

4 how far away is far enough? I was going to use shielded wire anyway, i just thought it may be a good idea to put the inputs ect in a seperate lined case inside the main chassis. Worth it? (although this wont matter is i end up building a pre)

5 so the same trans can give 4 or 8 ohm? I dont need to decide before i buy the output trans?
Which is best, 4 or 8, or does it just depend what speakers you use. Is there any difference between a 4 0hm amp with 4 ohm speakers and the same amp in 8 ohm with 8 ohm speakers?

Thanx, im learning more all the time. Will get to the library soon and get some books.
Sorry my questions are so basic!



I also wanted to ask if there are any recomended types/makes of volume and selection pots.

[Eli Duttman]

Graeme,

I was wondering why you selected a "50" WPC amp as a 1st project. You mentioned a Cyrus SS amp in your last post. Are you planning to use the same speakers with the tubed amp as you use with the SS amp?

I took a look at schematic you linked to. I like the O/P stage, but I'm less than thrilled by the small signal circuitry. An asymmetrical pentode/triode paraphase phase splitter is employed. For a number of reasons, I'd prefer either a differential or "concertina" splitter.

You need a 350 mA. DC B+ supply. The RMS current rating of the rectifier winding will depend on the type of supply you construct.

An EXCELLENT UK source for transformers is Sowter. Sowter's model U069 looks like it would be right in the WAD O/P stage. If you order from Sowter, be sure your "iron" has the ultralinear screen grid taps.

[Sherman]

Don't worry about the questions. We've all been there!

1 so i need a 350ma mains trans, is that enough info to source one?

If you build according to the schematic you will need a PTX with three secondary windings, a high voltage secondary 425-0-425 volts at 350mA, a 5V secondary at 6A and a 6.3V secondary at 9A. That is a pretty big PTX.

2 ...I just wondered if i could build it in for neatness and one less mains plug.

same deal with the headphone socket.

if im going to need seperate amps for phono and heaphones, would i be best of building a pre amp, with a phono stage, and switching the output between a headphone amp and the main power amp?

It would be easier to do a preamp with a phono stage and headphone jack than to put those things in an integrated amp. You might look for a used Dynaco PAS 3 preamp. They had a phono section (and I believe a headphone jack).

3 ...i dont know which caps are which. As i said, i dont know how amps work, but im still confident i can build one...

Electrolytic caps are inexpensive but don't sound good when in the signal path. They are used in power supplies and generally have large values (40uF to 100uF or more). Caps used in the signal path typically are small-value (.22uF up to maybe 1uF).

If you are good at following directions, are patient and understand the dangers of the high voltage you should be able to build an amp! The issues come in when planning things like the layout. If you don't know 'what' and 'why' it can be tough to plan a good quiet layout.

4 how far away is far enough?...

That is a great question! Really. My answer is "I don't know" and "as far as possible". The standard advice is try to keep the AC and rectification cirtuitry as far as you can from the signal circuitry. Also if you must cross signal circuitry over AC do it at a 90 degree angle.

5 so the same trans can give 4 or 8 ohm? I dont need to decide before i buy the output trans?...

Yes, most OPTs will give you separate taps for 4 ohms and 8 ohms. Look for a transformer that gives you both. You can put three binding posts on your chassis for each channel, one 8 ohm, one 4 ohm and one common. That way you could connect either a 4 ohm speaker or an 8 ohm speaker (but not both at the same time).

You must match the OPT to the circuit. Your main considerations are the primary impedance (what your output tubes want to see) and the current capacity. This circuit is a push-pull circuit so you have to use a push-pull transformer rather than a single-ended OPT.

I haven't read all the pages so I don't know the values of the OPTs but a PP KT88 circuit probably wants a primary Z of between 4K and 6.6K and a current of maybe 200mA.

I also wanted to ask if there are any recomended types/makes of volume and selection pots. [/B]

Alps Blue Velvet is a pretty good volume pot.

I would like to recommend that you get a copy of 'Valve Amplifiers' by Morgan Jones. Another good book is 'A Beginner's Guide to Tube Audio Design' by Bruce Rosenblitz.

Hope this helps.

[graeme uk]

Thanx , things are becoming clearer.

I went for this amp, not because its 50w per channel, but because it was integrated, got good reviews, and was available in kit form with loads of back up.
The kit form part doesnt matter now as the companys stopped trading.
I do like the fact thats theres lots of information available.

Im not going to use the sme speakers, ive got a pair of mission 733 speakers. I like em, but was never quite satisfied with the cyrus straighline/733 combo. Im going to build speakers to match whatever amp i build.

It looks like ill be best of building a pre and power, andone suggest a good schematic?
I need a phono stage and a headphone socket.
As for power, i dont know really. I guess the more the merrier, just cos it will make it easier to match some speakers as there will be more choice. (i dont know if thats true)

Im confident i can make a good job of building an amp, i just needto know exactly what needs to be done as i dont understand them yet.
Ill get that book, and as many other books as i can find, and read lots. Ive also been thinking about finding a local electronics course.

As i mentioned, i have time to learn first as i cant build anything till i have the money. Im wainting for the sale of a house to finalise, then its all go!

[graeme uk]

the output transformers suggested above are 240 GBP each!
Why? I priced the mains and output trans from wad and it was about 350 GBP for all three, but they aint trading any more.
Wad sold the kit with valves for under 600 GBP
( i cant find the pound sign key )

[pedroskova]

World Audio Design looks to still be in business. At least it appears that you can order things from their web site.

[graeme uk]

yeah, but people were saying on their forum they have stopped trading and need someone to take over.
Ill try when i have the money, it would be easier to get a kit for my first build.

I was going to get a pair of quad II's and rebuild them.
Anyone know how they sound compared to modern valve amps like this wad?

[dhaen]

It does seem that change is happening to WAD. There is a new web site link after 6 Jan. Info posted on their forum.

The KiT6550 sounds quite good. I've built several of them for people, and repaired a couple more. The standard of cosmetics are good for the price. On the negative side, the supplied mains transformer does get a bit hot, and the thing makes a bit of a thump when you turn on - something that could be eased by a retro-fitted soft-start circuit.

Construction is straightforward. One tip I would give is to discard the supplied hook-up wire and buy some proper stuff. Also, to avoid crosstalk, don't run both channels through twin screened to the selector switch as they recommend, use individual cables.