I have some voltage questions!!!!

[Spookyman]

Hello and welcome to my first post!!
I have decided to design and build an amplifier. I am finding the design phase difficult because
A) I have no money and so I am using parts either "reclaimed" from other amps, or found in old boxes in the shed so I am having to shape the design around what I have available rather than designing it with the "right" parts
and
B) I have no knowledge of, or experience with electronics.

Still, building something that's directly wired to the mains supply, putting out 450 ish HT dc voltage and containing capacitors rated at 600v can't be all that tricky as an introductory project can it?

So to my voltage questions.
1) Looking at some old schematics shows the cathodes of the valves going to ground. Am I correct in assuming that this is not actually grounding to the chassis but is actually "grounded" to a low voltage supply approx 25v?
2) On the old schematics I have seen the tone controls are shown as providing a path to "ground". Is this actually grounded to chassis and not to the 25v supply?
3) Because I am using reclaimed parts my power transformer is ripped, sorry carefully removed, from a Marshall JTM 30.
It is for use in the UK and so accepts 240v ac. There are two tags offering 25v, two 6.3v, and two 325v. I am assuming that in each pair one is + voltage and the other - voltage. eg in the 325v pair one tag offers +325v with respect to ground (0v) and the other -325v. Am I correct in my assumptions?

I will be grateful for any help anyone can offer me!!

[Tubis]

1. No, if its to ground on the shematics its to ground. The tube is biased on antother pin with a negative voltage. Ground doesnt have to be chassis.
2. Same ground as tubes.
3. The two 25VAC should be added to get 50VAC that is rectified to -70VDC.
The 6.3VAC seems strange. Are you sure theres not 3.15-0-3.15. That vill read 6.3VAC between the 3.15VAC?
The 325VAC should also be 325-0-325. That will be rectified with 0 to ground and a diod from each 325VAC that will be 455VDC.

The only negative voltage supply should be the -70VDC

Finaly. Are you shure you want to do this with no electronic knowledges?

[Spookyman]

Oh I'm sure I want to do this!
I have always found that the best way to learn something is to jump in feet first and have a bash!
The voltages are what are printed on the transformer. The 6.3v seemed strange to me which is why I posted.
I have aquired an early 1960s army and navy electronics training manual from a jumble sale -it's about 3 inches thick and packed with loads of information. Starts explaining the basics including what avalve is and how it works, by the end of the manual you're constructing radio transmitters, audio amps, televisions, radar systems and it's all valve!!! So by the time I start soldering I'll have a bit more knowledge. Should be ready to start next weekend.
Thanks for the reply, it's great that there are these helpfuls communities we can turn to!!!
I'm sure that I'll be posting alot in future.
When I get home (am at work now) I'll take a multimeter to the tranny to clarify that 6.3v issue.
When you say ground doesn't ahve to be chassis, what else would you use?
Would you ground the chassis seperately? Or not at all?

[Tubis]

The circuit ground could be seperated from the chassis ground. In old designs the circuit and chassis are almost certain the same but in newer amps some components could be between the circuit ground and the chassis ground. The chassis is then directly connectet to the wall.

[Robert M. Martinelli]

"no country for young men"

Paraphrasing a famous movie : tube amp building is "no country for young men" - at least not for the unexperienced ones....

I must agree with Tubis,
building a tube amp from scratch is certainly not for beginners, it' s too difficult and risky, I strongly encourage you to start with something different, meanwhile, you can continue to participate to this forum reading the posts and asking questions, and don' t be ashamed to ask questions even they sound silly, as someone said "nemo nascitur doctus" ( no one is a natural born doctor ).

There is a lot of knowledge to be found here, and a lot of knowledgeable people willing to help and I' m sure you 'll learn a lot in a reasonably little time...

First "lesson" : the negative voltage is not appplied to the cathode, but to the control grid of a tube. Tubes work by means of a filament which stimulates thermally the cathode which emits electrons which are attracted by the greatly positive voltage on the plate. The variation of the control grid voltage causes the grid to reject electrons to a lesser amount ( when it rises from the initial ( bias ) negative value towards zero ) or a greater amount ( when it falls below the initial ( bias ) negative voltage and becomes "more negative" ) - this happens because electrons are negatively charged, so they get rejected by negative voltages - This way the flow of electrons from the cathode to the plate can be precisely controlled.

Setting the bias is one of the factors that define the operating point of a tube.

The reference point of a tube is its cathode, so you have two ways of making the control grid negatively biased : the first one is referred to as self-bias, auto-bias or cathode bias - a resistor is placed between the cathode and the 0 V, since current is flowing through it you have a voltage drop across the resistor, making the cathode positive with respect to the 0 V. The grid then becomes negative if referred to the cathode. The VOX AC30 is a good example of this technique ) - The second way to properly bias the control grid is putting the cathode to 0 V, and applying a negative value ( obtained from the power supply section ) to the control grid. On some amps this bias voltage is fixed, on other amps it is adjustable, but the way it works is still the same.You can find such a technique on Marshall tube amps.

Hope this helps

Enjoy!

Best regards

Bob

[Spookyman]

Thanks for your comments guys!
Voxrules, If I read your post correctly, if I "Cathode bias" then I will not need the - 70v and so will not need to use the 25v tags on the tranny at all?

If I elect to use them and apply the -70v to the control grid then I can connect the cathode directly to ground and so will not need a resistor between cathode and ground?

This is fun!!

[Tubis]

Do not apply all of the -70 to the grid. You should trim that with a trimpot for differnet tubes. Some want -30 some want -65. And different plate voltages makes for different control grid negative voltages as well.

[Robert M. Martinelli]

Thanks for your comments guys!
Voxrules, If I read your post correctly, if I "Cathode bias" then I will not need the - 70v and so will not need to use the 25v tags on the tranny at all?

If I elect to use them and apply the -70v to the control grid then I can connect the cathode directly to ground and so will not need a resistor between cathode and ground?

This is fun!!

Hi Spookyman,
your understanding is formally correct, but not applicable in the real world; I advice you to search some tube amp schematics ( there are tons of them on the internet ) and try to see for yourself what things look like..... AC30s have no negative voltage section inside their PS section, because they' re cathode biased - Marshalls do have it ( an adjustable one ) so if you are willing to build something resembling an AC30 you could choose cathode bias, if you' re intentioned to build a Marshall clone you can add a reverse diode and a filter cap on the 50 ( 25+25 ) VAC section of your power tranny....

the main difference is you' re dealing with EL84s in the first case ( VOX ), and a cathode bias of -10 volts is enough, so the self-bias method is OK, in the second case you' re dealing with EL34s, and the bias voltage is low as -40 volts, so you should use a big cathode resistor and this would come to the point of limiting the current, this is the reason why Marshalls and EL34s are NOT cathode biased and why they need a separate negative bias circuit. Hope this clarifies this issue.....

Best of luck!

A last word of caution - ensure yourself you have fully understood the tube amp technology before starting to tinker with them - They can kill you with the same ease of a bullet in your head - Be careful !

Hope this helps

Best regards

Bob

[Tubis]

Hm, a looked up the schematics and the PT is like you said.

[R.G.]

Oh I'm sure I want to do this!
I have always found that the best way to learn something is to jump in feet first and have a bash!

That's very laudable, and a great way to learn and have fun at the same time.

However, you would not want to, say, learn to handle poisonous snakes this way, would you?

The "slow down" advice you're getting is from people who know just how easy it is to get shocked and possibly electrocuted. Messing with the voltages inside a tube amp is a great way to get killed, and no, that is not an exaggeration. Be careful. Get someone who has some experience to help you out.

[tubeswell]

Hi Spookyman

I started out by modding an existing production amp (Peavey Classic 30) a couple of years ago, which I tinkered with alot for 18 months or so, and then did my first builds this year. I recommend that as a way to go if you are interested. You probably need to get a copy of the schematics and layouts of the amps you are going to mod if you are going to make any sense of this. One of the posters as already mentioned this.

Here is a site stacked with classic Fender amp schemes and layouts:

http://www.ampwares.com/ffg/schematics.html



I suggest you visit as many websites as you can, as there is a guzillion forums and pages around now which discuss amp modding and building.

Firstly, this site is useful for explaining Ohms Law in simple terms and has some simple tutorials:

http://www.valvette.us/dcca02.html


Next, I found this page very useful for basic info about tubes, and it contains a whole heap of good links to get you started (including RG's own site.)

http://www.diystompboxes.com/pedals/tubedummy.html


(Before you start any tinkering or dismantling) you need to learn about how to safely discharge residual charges in tube amps that have been switched off - there is some information here:

http://studentweb.eku.edu/justin_holton/caps.html


For a reservoir of basic technical information on all sorts of different types of (NOS) tubes:

http://www.r-type.org/


Even if you have been doing this stuff for a while, you can still kill yourself if you even make the slightest forgetful mistake. I have shocked myself once already (and it was mainly because I was following a repetitive sequence of checking voltages, and switching the amp I had built on and off, and connecting and disconnecting meter alligator clips and it was 10:30pm and I forgot my own sequence, and for a fraction of a second, one of my fingers brushed on of the A/C Mains fuse contacts, just as my other hand was hitting the off switch, but too late and zap - 240VAC ripped instantaneously through my hand, leaving my whole forearm tingling! I don't even want to say I'm lucky to be alive, but can I?. I had to cart myself off to the A&E dept at the local hospital there and then to have an ECG to satisfy myself I was going to pull through. Two important lessons I took from that:

1) I'll never work on amps at 10:30pm again - the slight fatigue your body starts to feel at that time of day, that affects your thinking and that you are not consciously aware of, can be fatal, especially in a repetitive sequence with (as RG puts it) 'rattlesnakes'.

2) I had gotten overconfident and was being lazy with my self-discipline - I was switching the amp off at the amp switch but not disconnecting the power cord from the wall socket! - I was about to switch the amp off and put my hand into it, but I got things in the wrong order for a fraction of a second. Can I also say: never, Never, NEVER do what I did.

Fortunately I was on an insulated floor, and I had the other clip of the meter that I was holding the +ve clip of, still grounded to the chassis and the current flowed only instanteously mainly through my hand and forearm before I switched the amp off. I was so angry with myself for being so stupid. I was attracted to valve amp building because I wanted the sounds that I couldn't afford to buy off somebody else, but it nearly killed me. This sound that is sooo attractive about valve amps invariably can lead musicians to this. So please be careful.

Unfortunately I am addicted to amp building - infact I always had a closet fascination with electronics from when I was a boy - messing around with crystal radios, model (RC) aeroplanes and building simple transistor amps and all that stuff. Maybe I'm a muso who is a wannabe tech - how sad is that?)

[Steve Conner]

Not as sad as a tech who wants to be a muso.

BTW, those pairs of terminals on the transformer do not produce two voltages with respect to ground. They're floating, which is to say that 325V or whatever appears between the two terminals, and they're not connected to anything else, just to each other via the winding.

And what's more, the voltage that appears is AC and needs rectifying. The kind of transformer you have is intended for use with bridge rectifiers, do a quick google search to see what one of those is and how to wire it up.

[cminor9]

Before I ramble on, let me cut to the chase: before you build your amp, do your homework. If you don't like homework, then do the minimum required. IMHO, the minimum is as follows:

Step 1: read up on how to work on a tube amp SAFELY. You can find that info anywhere. Basically, drain filter caps before you do anything and don't poke around in a live amp without your wits about you and a hand in your pocket. Unless you don't care much for your pulse, bilateral symmetry, and neural synapses. You know: life and limb.

Step 2: download and read this book: http://www.pacificrecone.com/JackDarrBook.html. It won't take long to read. I read it in a week, one chapter at a time. You could read it in an evening, but you have to force yourself to go slow. The point is not to read the book to say you've read it, but to really grok the material. No theory, no math. It's a book designed for technicians, not engineers.

Step 3: Start building, but realize that with a meager foundation you'll have to learn from some mistakes. You will get frustrated.

Welcome to the forum. I hope you heed the advice here and don't have to learn stuff the hard way. If you looked up "not fun" in the dictionary, you'd find a picture of yourself learning stuff the hard way.

[Spookyman]

Many thanks - I seriously did not expect so many people to be so helpful!
Especially toward someone who doesn't really know much about what they're doing (yet)!!
Still, the learning process goes on...
So, I will need a bridge rectifier. My initial reading shows they are not complicated affairs. Simply four diodes, and then a filter arrangement. I have access to a boxful of capacitors I found at the back of the garage, many of which are rated at 450, 500, 600v so should be adequate. Have checked them with resistor and meter to make sure none were holding any charge, none were. I am 99% certain that they were purchased brand new many years ago and have never been used. I do need advice as regards the diodes though. Is there any particular type that I should be looking out for that would work well with the voltage output from my transformer?
Also the resistors in the filter, do people here have any preference as regards value / tolerance / wattage for use in this application?

[Steve Conner]

Diodes: 1N4007

Capacitors: Read this before using old electrolytic capacitors. They are usually fine, but you should reform them before use to make sure. You don't want them to cause mysterious faults that you'll waste time chasing, or worse, explode and spew goop in your face.

This only applies to electrolytics: other types are OK, except paper in oil which should be discarded on sight