magnatone ancient amp

[ilyaa]

alright seems good -

one question: the cathode resistor is unbypassed - its at about 30V. i get a great signal swing from the PI, but the power tubes are only putting 13 watts or so into 4 ohms. im probably going to replace them (they are quite old, too), but am wondering about the unbypassed cathode resistor. does that seem odd for a push-pull 50 watt-ish design?

[Roly]

am wondering about the unbypassed cathode resistor. does that seem odd for a push-pull 50 watt-ish design?

Yes, it does.  No signs there was one in there?  The few Magnatone ccts I looked at had one.  That would introduce quite a bit of local NFB to the output stage.  It is generally much more common for the cathode bias resistor to be bypassed with an electo of suitable ripple current rating.  Lash something in and give it a burl - you can always take it out again.

I'll just remark that the 40 to 50/60 watt step is generally the break point where designers switch from cathode to fixed bias, not that there is any hard and fast rule about it, but generally with all things considered...

[ilyaa]

cool!

put in a 47 uF cap and got more clean power as well as a squar-er full power overdrive (at least on the scope) - and more power overall, actually.

we'll see what happens when i put new tubes in -

[Roly]

{...the cap will explode?  }


Just off the top of my head 47uF is a bit light for a 50W output stage.  I'd expect something more like 220uF to 470uF to have sufficient ripple current rating.

[ilyaa]

really??

i calculated for a 40 Hz-ish cut-off - transconductance of 6BG6 is about 8mA/V - the cathode resistor is 250 ohms. 47 uF seems to be just right for that cut-off.

i didnt realize i was worried about ripple current there....and i thought a bigger cap would have created bias excursion problems.

am i missing something?

[Roly]

If anything a bigger cap will tend to hold the bias steadier than a small one (think: a one farad cap is like a battery).

As with the filter caps in a s.s. amp power supply we can calculate a value from the required ripple and current draw, but the available caps may not align with both the required value and ripple current rating, and it is common that we have to use a higher value to get sufficient ripple current rating.

In the case of cathode bias we want an RC time constant that is long enough to satisfy the lowest frequency we want to reproduce, say 40Hz, but again the ripple current rating of the cap that will give us that frequency will typically be too low to handle the 200-300mA expected (in a 50-60W amp).

As we often have to do, where we have different constraints we have to use the most conservative (e.g. transistor voltage, current, and power ratings).

[ilyaa]

what about this:

http://www.davidsonamp.com/sf/images/twin5c8.gif

isnt that bypass cap too small, according to what youre saying?

[Roly]

That's a very old example and when that was built you could get 25uF caps the size of a C-battery and they had ripple current ratings known as "enuf".  You will be hard pressed to find a 25uF cap these days with that sort of ripple capacity - a modern 25/25 might be 5mm dia and 10mm long.

If we dig a bit, the ripple current limit is a thermal internal heating limit.  The ripple current flows through the cap and causes heating by passing through the internal loss resistance.  With a tiny capacitor it has very little surface area to get rid of this heat and if you over do it the electrolyte will boil and the cap explode.  Older caps were lossier than modern ones, but they also had a lot more surface area to get rid of the heat.

[ilyaa]

hmmm i see i see -  good to know i was totally unaware of those constraints!

would a 220 uF 50V cap work better? its still small physically.....

where could i find the ripple current limit or calculate it for a modern cap?

[Roly]

would a 220 uF 50V cap work better? its still small physically.....

where could i find the ripple current limit or calculate it for a modern cap?

Try looking here.

[ilyaa]

hm i see very funny serves me right

hard to figure out all the parameters for a cap i already have - but i looked at all the similar ones (size/temperature/voltage rating) and my 220uF guy should be alright. the 47uF would not have been!

[Roly]

A conservative rule of thumb might be "1mA per uF".

[ilyaa]

hm now you have me all paranoid about ripple current -

would a physically large, high voltage 47uF have a high ripple tolerance generally? or does it really depend cap to cap and better to just err on the side of caution?

i attached a datasheet for one i have - a 47uF / 350V - its 16x20 mm - but there is no ripple current on the datasheet (in general ive found that parameter hard to find......)

PS its 30V over a 250 resistor at idle so 120 mA of idle current....

[Roly]

Nah, look, you just want a reasonably chunky cap.  I wouldn't go above about 50/63V, but anything physically bigger than the required micro 47uF, and maybe low ESR if it's to hand, but a physically big cap that means it will most likely be 100uF-470uF.

You don't have to be paranoid, but many people ignore it and come unstuck, so just try to keep it in mind with any electro cap, value, voltage, ERS/ripple current.

This Jaycar page gives the ripple current ratings of sundry electros;
http://www.jaycar.com.au/search?q=electrolytic%3Arelevance&page=1

e.g.

Value: 47uF
Working Voltage: 25V
Lead Spacing: 2mm
Size dia x H: 5 x 11mm
Ripple Current mA: 100
Rated Temperature: 105 Degrees C
Mounting: RB - PC Mount

105 degrees but only 100mA, however there are quite a few other options depending on the highest expected cathode voltage;

Value: 100uF
Working Voltage: 63V
Lead Spacing: 5mm
Size dia x H: 10 x 12mm
Ripple Current mA: 280
Rated Temperature: 105 Degrees C
Mounting: RB - PC Mount
-
Value: 220uF
Working Voltage: 25V
Lead Spacing: 3.5mm
Size dia x H: 8 x 12mm
Ripple Current mA: 320
Rated Temperature: 105 Degrees C
Mounting: RB - PC Mount
-
Value: 220uF
Working Voltage: 50V
Lead Spacing: 5mm
Size dia x H: 10 x 17mm
Ripple Current mA: 430
Rated Temperature: 105 Degrees C
Mounting: RB - PC Mount

...all under a dollar.

You can use this as a rule-of-thumb guide because other manufacturers will be trying to meet similar specs (fraud aside).

[ilyaa]

cool, thanks Roly -

i think this amps good to go - louder sounding and all cleaned up -