Basic tube amp question(s)

Rolbista · Post by **Rolbista** » 12 Jan 2012, 23:00

So, I am in the process of building my first tube amp (a Marshall 2061) and before I start soldering anything, I'd like to clear up some things. I have plenty of wire salvaged from an old computer power supply and I read somewhere on the net (unfortunately can't remember where) that it can be used in a tube amp. Is that ok? Also, what power rating should the resistors be? The big ones are described - 10 watts, 5 watts and 2 watts but the rest isnt. is 1/4 watt ok? Cause thats all i can get in my location. If i put two 5w resistors in parallel, will that give me 10 watts rating overall? Again, what voltage ratings should the caps be? Likewise, big ones are described: 450V, 63V, 10V but the rest is not. i assumed 63V is cool for the rest but im not sure. I'm using Ceriatone Lead and Bass layout. Sorry, but i usually find myself having questions and problems no one else has and end up asking directly.
PS since i'm already asking - there is 130R 5W resistor on the schematic, but i was only able to acquire 150R, is that cool, or is it too big of a difference? Cheers

kleuck · Post by **kleuck** » 12 Jan 2012, 23:21

Hello
All resistors should be at least 2 watts in a tube amp, 1/4 watts are not guaranteed for hundred of volts, and some will dissipate more power than that.
Anyway, 12AX7 grid stoppers and cathode resistor can imo be 1/4 watts (though I use only two watts resistors, even in these places)
12AX7 cathode caps can be of low value, the DC voltage is only a few volts, but AC can be larger, so i never use anything under 40 volts.
Remember : tube amps can became dangerous, so derating is not an option for components.
If you use two identical resistors in // you end indeed with a resistor of half the value, twice the power rating.

Rolbista · Post by **Rolbista** » 12 Jan 2012, 23:53

what about the wire?

juanro · Post by **juanro** » 13 Jan 2012, 00:52

Yes, salvaged PC power supply wires are ok to use. I (we) use that a lot.

For the resistors, besides what kleuck said, there's also the mechanical factor... 1/4w resistors are pretty small, flimsy and with thin leads. Tube amps usually are made either point to point, with components flying everywhere, or turret/eyelet, where the R's are stretched much more than on a PCB, so using 2W aids with the mechanical strength of the build.

Regards,
Juanro

kleuck · Post by **kleuck** » 13 Jan 2012, 11:52

juanro wrote:Yes, salvaged PC power supply wires are ok to use. I (we) use that a lot.

Yeah, they are intended for high currents (max voltage in an PC is 12 volts) and they are well isolated.

DrNomis · Post by **DrNomis** » 13 Jan 2012, 12:16

PC Power Supplies don't use a heavy Power Transformer to step down the mains voltage to that which is suitable to run a PC, they use what's called a "Switchmode Power Supply" system, basically the AC from the mains is rectified into raw DC, then this is converted to high-frequency AC, stepped down by a small transformer, and then rectified again into raw DC where it is finally filtered and regulated into steady DC, Switchmode Power Supplies are more efficient than traditional Transformer-based Power Supplies....

kleuck · Post by **kleuck** » 13 Jan 2012, 12:27

DrNomis wrote:Switchmode Power Supplies are more efficient than traditional Transformer-based Power Supplies....

Theoretically. but a lot of them are under 70% with cheap passive PFC (power factor corrector) which is not greta, a PT based- power supply can exhibit almost 80%.
When correctly built, a Switch mode PS can be between 80 and 90%.
Why i bought (years ago) a Seasonic S12, completely worth the (high) price.

Dr Tony Balls · Post by **Dr Tony Balls** » 13 Jan 2012, 15:16

Unless otherwise stated on the schematic like the 5W and 2W you mentioned, 1/2W resistors are standard in tube guitar amps and have been since the dawn of time. Original 2061s used 1/2W and so did JTM45s, 1987s, 1959s, etc. I have no idea why kleuck would declare that you should use 2W, but thats just silly. For the caps, you're 330uF cathode bypass cap should be about 50V or higher, and the other film caps for that amp should be 300V or greater.

kleuck · Post by **kleuck** » 13 Jan 2012, 15:31

Dr Tony Balls wrote:Unless otherwise stated on the schematic like the 5W and 2W you mentioned, 1/2W resistors are standard in tube guitar amps and have been since the dawn of time. Original 2061s used 1/2W and so did JTM45s, 1987s, 1959s, etc. I have no idea why kleuck would declare that you should use 2W, but thats just silly.

Just because it's in the datasheet of the resistors, line "voltage rating"

Old amps did not have any ground, so, you will build yours without too ?

For example, the 1/2 watts one i can source are not rated (or at least i don't know for sure the rating) ; 1 watts metal oxyde are rated for 350 volts (not a sufficient derating for me in most places, particularly in a 2061 where B+ is almost 400 volts...when the amp is drawing current) when 2 watts are rated for 500 volts.

Dr Tony Balls · Post by **Dr Tony Balls** » 13 Jan 2012, 16:30

almost all 1/2W film resistors are rated for 350V max working, and 700V overload. I happily use them at ~450VDC all the time without a problem (so do millions of other valve amp owners).

DrNomis · Post by **DrNomis** » 13 Jan 2012, 18:16

I use 1Watt 5% Carbon Film resistors in my tube amp head for most of the resistors, 5Watt 1k wire wound types for EL34 Power Tube Screen Grid resistors, 5Watt 1 Ohm wirewound resistors for the power tube cathode resistors, and 1/2Watt 5% Carbon Film resistors for preamp tube cathode resistors....

I think that 1Watt Carbon Film resistors are better voltage-rated than, say 1/4Watts....

kleuck · Post by **kleuck** » 13 Jan 2012, 18:36

Dr Tony Balls wrote:almost all 1/2W film resistors are rated for 350V max working, and 700V overload. I happily use them at ~450VDC all the time without a problem (so do millions of other valve amp owners).

Fine, but i won't do the same, nor recommend to anybody to do the same.

Rolbista · Post by **Rolbista** » 13 Jan 2012, 20:54

Wow, generous replies! And dang, cause 1/2 watt resistors will be hard to find in all the values I need in my neighborhood and I don't feel like ordering a few resistors via mail... Is this 1/4 watt so much of a difference? Factory schematic says the 330uF electro cap is rated for 10V... The rest is for 630V which seems silly to me since caps before them (two 47uF) are rated for 450V, so what's the point? Good to hear about PC wire, got plenty of it and many colors too!

kleuck · Post by **kleuck** » 13 Jan 2012, 21:17

Can you publish the schematic ?

Rolbista · Post by **Rolbista** » 13 Jan 2012, 21:31

yeah, can't see any disclaimer. I'd like to note that I'll be going by Ceriatone layout, and have the marshall reissue schematic just for comparison and reference, there are some differences:

kleuck · Post by **kleuck** » 13 Jan 2012, 21:58

OK, The 330µf is rated 10 volts, cause the DC voltage here is only about 1 volt (but i would rather use a 35 volts cap).
Supply caps are 500 volts, cause the voltage is about 380 volts.
Others are 630 for no special reason, except the "standard" voltage ratings, don't worry, they are not of high values, and won't be big -if you do PTP or similar, they have to be big enough though, but if you find caps rated at 500 volts, they would be fine too.
I use PIO in my axe, rated for 630 volts cause...that's what i found

diagrammatiks · Post by **diagrammatiks** » 14 Jan 2012, 05:07

power supply caps should be rated for max b+ voltage.

the 2 450v caps are in series to sum their voltages together.

I really wouldn't use a commercially rated 1/4 watt resistor in an amp.

You could get away with it in some places if you know exactly what the voltage requirements are for each stage but it's not a good idea for a first build.

It would be much easier to use 1/2 watts everywhere except for the first dropping resistor which needs to be 2 watts or more.

if you want to use something cheap but still good you can grab vishay ccf60 resistors.

the only caps that can be low voltage are the cathode bypass capacitors.

all the coupling capacitors should be at least 400v.

phatt · Post by **phatt** » 14 Jan 2012, 08:38

Re Wire rating,,
As long as you realize that while the computer wire will probably be ok for for 300 / 400 HT,,,, just be aware that when you build a really big monster that has 600/700 VDC you may want to revisit the question in more detail.

Just Be aware that the AC swing in the output transformer primary can be double the HT Voltage

Phil.

Rolbista · Post by **Rolbista** » 14 Jan 2012, 09:37

Ok, I'm gonna grab some 1/2 watters or bigger, shops in my town don't have all the values in 1/2 but i was able to locate some in 1 and 2 watts ratings. Soo are these caps for 400V expensive? will 4$ be enough for all (approximately)?

phatt · Post by **phatt** » 14 Jan 2012, 10:23

Rolbista wrote:Ok, I'm gonna grab some 1/2 watters or bigger, shops in my town don't have all the values in 1/2 but i was able to locate some in 1 and 2 watts ratings. Soo are these caps for 400V expensive? will 4$ be enough for all (approximately)?

Ok,, sounds like cost is an issue,,, in which case start scrounging.

Your computer psu quite often has 400 Volt 330uF electro's,,as do almost most anything that uses SMPs's. Hint.
Stack two together for around 150 uF (add a pair of 220k resistors for balance)

Also those thow away one shot film cameras have 300 Volt cap's inside. Be careful pulling them apart as they often stay charged.

Re resistors,
1 Watt carbons should be easy enough to find.
As mentioned already the issue is the modern 1/2 watt/ 1/4 Watt Resistor is *The wire* is way too fine to be of practical use in an eyelet board layout.
1 watters allow you to do old style layouts.

As to voltage ratings for resistors,,, Much like my previous comment.
If you intend to build really big Amps with very high supply volts you may need to do some reading.

For small Amp builds where the HT is below 400 ish VDC then it should be ok.
As for Cap's the general rule is +20%,, so if your VDC is 15 Volts and you use 16 Volt Caps then expect it to fail very SOON. Winky.

Tons of helpful stuff for you to get your head around those tricky q's here.
www.ozvalveamps.org

site dedicated to Aussie Valve Amps but it's all there if you look.
Check out the *Lamington build* if you wish to build on the cheap.

Have fun,,Phil.