Topic: Newbie and Not-so Noob Thread: Questions (FAQ) on anything Tubes

[JojoD818]

Virtual centertapping is done by connecting 1 100R to each tap (AC) and joining the other end of the two resistors. I saw this usually on filaments.

when is it most appropriate to ground the virtual center tap?

Does the resistor value vary with the given voltagesm?

AC heaters needs to be connected to the tranny in a balanced/equal way. Unfortunately, AC heaters must have a DC path to HT ground in order to define the tube's heater to cathode voltage so it is appropriate to connect the center tap (virtual or not) to ground. One of the ways to achieve this is as you described above using equal value resistors or a humdinger (hum pot).

The most ideal way of defining the heater's DC path is to have a center-tapped heater winding which on several ocassions, 2ny recommends a bifillar winding technique for a matched voltage output. But if a center-tapped winding is not available then fixed or variable resistors can be used to define a midpoint (virtual ground).

For DC supplied heaters, it should be worth treating the heater wirings as if it were carrying AC as this would ensure that the there wouldn't be any heater induced hum. Normally, the output of a DC heater supply has no center tap and the ground of the DC heater supply is often connected directly to ground but as in AC, it would be better to derive a center tap using a pair of matched fixed resistors or using a humpot.

One excemption of connecting the center tap (virtual or not) of heaters to ground is if the tube's heater to cathode voltage has already been exceeded then you must connect this center tap (again virtual or not) to an elevated voltage reference.

Resistor values vary with increased heater supply voltage, it's value is chosen in such a way so as not to load the heater winding too much but also to draw enough current to force a voltage midpoint.

 

[ATJr.]

Virtual centertapping is done by connecting 1 100R to each tap (AC) and joining the other end of the two resistors. I saw this usually on filaments.

when is it most appropriate to ground the virtual center tap?

Does the resistor value vary with the given voltagesm?

i use 2 470 ohm resistors connected to each side of the filament winding, the midpoint is then connected to a voltage devider supplying dc voltage, i used 100k and 47k, the junction of the two resistors connect to the 2 470ohm reistors, the end of the 47k connect to ground and the end of the 100k connects to the B+.

that is why biffiliar wound filament ransformer winding will save you two resistors or a wire-wound resistor, just connect the center tap to ground or to a suitable B+ source.

[JojoD818]

btw, just want to add and clear it for eXg.... virtual grounding can be used for both AC and DC heater supplies.

hth,
JojoD

[eXg]

napakalinaw po.... salamat sa inyo!

[eXg]

how exactly are these voltages measured?  cathode pin and filament ground for the 100vdc?  what about the 450vdc? what's the appropriate application of 100vdc and 450vdc?

sample rect spec:

[wraith]

Hi,

Gurus say that it is the 2nd  order (or x order) harmonic distortion that give tubes a euphonic, irresistably sweet  sound. Will I get/experience/hear  that if I use a tube pre-amp with my SS integrated amp?

Thanks in advance.

 

Hi.

I have tried this, both on a SS Integrated amp, and a SS power amp.  I get better response on the power amp pairing... most likely because my integrated amp has no inputs that will bypass its preamp stage.

The thing with pairing a tube preamp with an SS integrated amp (without a bypass input) is that the amp will be driven by 2 inline preamps -- the tube, and the preamp section of your integrated amp.  Yes, the even-order harmonics of the tube will be noticable, but so too will the odd-order "harsh" harmonics of the ss preamp section because the tube preamp will have a tendency to overdrive the ss preamp section.

On the other hand, if your SS integrated amp has a pre-amp bypass input, you should be fine.

[JojoD818]

how exactly are these voltages measured?  cathode pin and filament ground for the 100vdc?  what about the 450vdc? what's the appropriate application of 100vdc and 450vdc?

sample rect spec:

it depends on the application of the tube.... for instance, in an SRPP topology, the tube on the top can very well be breaching it's cathode to filament limit. sometimes even phase splitters (or inverters) have high cathode resistors that elevates the cathode voltage from ground.

cheers

[markmlists]

2ny and Sonic Boom--Thanks.

My Nad has main-in/pre-out section. Salamat uli.

[ATJr.]

how exactly are these voltages measured?  cathode pin and filament ground for the 100vdc?  what about the 450vdc? what's the appropriate application of 100vdc and 450vdc?

sample rect spec:

these are values that must never be exceeded or else safety of the tube is not guaranteed. so if you have tube applications, you have to be watchfull of this. for instance, cathode followers that are dc coupled to the preceeding stage, or phase splitters.

for srpp's the top tubes should have floating(no part is connected to ground) filament supplies so as to comply with this spec.

[Jakob]

will a circuit part failure (resistor, capacitor, etc..), in any way damage the vacuum tubes (or vice-versa)?

thanks,

 

[ATJr.]

will a circuit part failure (resistor, capacitor, etc..), in any way damage the vacuum tubes (or vice-versa)?

thanks,

 

yes, same as with ss amps, compoment failures can lead to tube damage,

[eXg]

Re CHOKES:

from this link, http://republika.pl/diytriode/drugi_eng.htm, I quote:

"
I removed second choke in each PSU unit.  It is another proof, that not always something, which is theoretically perfect, sounds the best. Double chokes make sound stiff and hard, dry and shouty, very precise but mechanical. Of course, there is a problem with hum using only one choke, but can be solve easily.
"

So when do we need a second choke? Does the choke's H value affect the B+ (increase or decrease the voltage)?

[arnoldc]

re: heater-cathode voltage

the tubes will survive (not for long) if this parameter is exceeded. in my case, i have a preamp wherein the second stage is a cathode follower and the cathode voltage is 156 volts. the heater voltage is 40 volts so the cathode-heater is at 116V which exceeds the 80V limit.

since the tube i use has center tap for the filaments, i can easily lift the voltage to something to make it safe, however since my heaters are AC and lifting it with DC will cause a huge negative swing. it's ok for low-voltage filament tubes with 6.3V but not this one with 40 volts!

two solutions are possible: a) change the cathode resistor (lower) to lower the voltage difference, but increasing current draw, b) change the B+

i went for the second option.

re: circuit part failure

depending on the design, the failure of the first stage for example in a direct-coupled circuit could damage the power tubes (in a driver-DCed-power tube topology). there are variations to the direct-coupled like money, free lunch, etc. to make it safer.

tubes are robust, but should not be abused. the failure of the cathode resistor will screw up the bias of a power tube for example. it will survive but not for prolonged period.

re: multiple chokes

i use dual chokes on my type-45 amplifier (2W) and the person babysitting it now described its sound as warm sounding with balls on the bass. so it is not a generalization that dual chokes are bad (duh).  when designing a PSU you should consider the requirements of the tube rectifier- they specifiy how much inductance it can take on either capacitor loaded or choke loaded mode. also the current rating of the choke should be close to the current drain of the circuit. in my experience, too much current capacity does not contribute much for filtering. example, if the circuit will draw like 30mA (in a preamp application) the chokes should be aroung 40-45mA. i used a 100mA once and alang kwenta sa filtering.

the choke's DCR will contribute to the voltage drop.

finally, not all chokes are designed for choke loading, some are smoothing chokes. input chokes require higher than usual insulation due to the large voltage swing being an input choke. when in doubt, use the choke in a CLC fashion instead of LC.

[ATJr.]

tubes are robust, but should not be abused. the failure of the cathode resistor will screw up the bias of a power tube for example. it will survive but not for prolonged period.

yes, also an open grid leak resistor will increase plate current that leads to tube plates turning red, this is speacially severe in case of fixed bias amps. that is why a small resistor is installed at cathodes to serve as a fuse in this evetuality. cathode bias schemes are less prone as there is significant resistance in series with the plate to limit current in case grid leak resistor opens up.

[ATJr.]

So when do we need a second choke? Does the choke's H value affect the B+ (increase or decrease the voltage)?
 
 

chokes in commercial amps are being avoided because of costs, but here where cost of core and copper are low, it is almost a crime imho to avoid them, that is if you are a serious diy'er

[devo]

question lang.... I plan to install an small exhaust fan (used on PC's) beside my tube amp to slightly cool down the tubes when operational. Will this be a pro or a con as far as tube characteristics/performance are concerned. Amp is not on an enclosed cabinet......... inputs be very much appreciated, thnx.

[JojoD818]

almost all datasheets of power tubes warns that power tubes need adequate ventilation, proper spacing between tubes etc.

i think your move would be on the "pro" side. 

[ATJr.]

question lang.... I plan to install an small exhaust fan (used on PC's) beside my tube amp to slightly cool down the tubes when operational. Will this be a pro or a con as far as tube characteristics/performance are concerned. Amp is not on an enclosed cabinet......... inputs be very much appreciated, thnx.

better that the fan blows air into the tubes imho...

[arnoldc]

also depends on room temperature. if you're going to blow air at mainit din, wag na lang. palagay ka na lang ng aircon sa audio room mo ako nga i barely use my ceiling fan pero buhay pa mga amp ko

[devo]

thanks JojoD,2ny,arnoldc for all the inputs........ installed 1 each blowing the transformers and the tubes. the audio room is already airconed but since power rates are up, I thought of the fans as a cheaper alternative, besides, I only use it at night. thanks again.........

[ATJr.]

yes fans are very cheap indeed, you can get these 120mm dc fans with controllers to vary the speed to your liking.

[eXg]

which ones are good or bad caps (types not brands) for bypassing duties? 

[eXg]

can anyone make out which terminals to bypass from this pic?  or do i just bypass all of them 

[Jakob]

yes, same as with ss amps, compoment failures can lead to tube damage,

failed component -> failed tube -> will cause other tubes to fail also?

(sorry no-brainer yata tanung ko  )

what are the methods/designs employed to protect the tubes from sudden parts/component failure?

thanks again gurus!!!

 

[ATJr.]

failed component -> failed tube -> will cause other tubes to fail also?

(sorry no-brainer yata tanung ko  )

what are the methods/designs employed to protect the tubes from sudden parts/component failure?

thanks again gurus!!!

 

very good question!
if you understand how ohm's law works, analysis of the output stage becomes a no-brainer, in terms of possible failures.

most failures are in the driver and output tubes, more so in the outputs. when the grid leak resistor opens, the grid on the output tubes are biased into 0volts, therefore big current flows, your output transformer can be damaged because of heat. this is more prevalent with fixed bias output stages than with self bias, as there is not much resistance to limit the flow of current.

but tubes unlike ss devices does not fail abruptly, there will be signs, like distorted sound or output tube plates turning cherry red, so you have time to turn the amp off.

in the tube data sheets you will notice that grid leak resistor recommendations are given, they are best followed, or lower than those can be used, however, lowering the grid leak resistor value lowers the gain of the previous stage.
use of grid chokes are a good counter measure, ie. very low dc resistance compared to grid leak resistor.

[Jakob]

thanks a heap sir 2ny 

[gaol]

I just switched from SS to a tube amp.  I listen 3x a day:
1) in the morning from 6:30 - around 8:00am;
2) from 11:00am - 12:30pm
3) from 5:00pm - 10:00pm

With my SS amp, I used to leave it on 24/7 (as also recommended by the manufacturer), but with tubes I understand this is not advisable.

What would be best to prolong the life of the tubes (I understand that switching on-off is very stressful to the tubes):
a) switch it on and off for every session (on-off 3x a day)
b) switch it on in the morning, leave it on until 10pm (on-off 1x a day), and just set volume to 0 in between sessions
c) switch it on in the morning and off after the 2nd session (12:30pm); then switch it on at 5pm for the third session and off at 10pm (on-off 2x a day), volume at 0 between 1st and 2nd session.

Your inputs would be greatly appreciated.

Cheers!
GAOL

 

[ATJr.]

What would be best to prolong the life of the tubes (I understand that switching on-off is very stressful to the tubes):

you have to understand that tubes have cathodes, and as such suffers from emission depletion with time, this is unavoidable no matter how youse your amp. this is more so with power tubes! small signal tubes normally will last  you a life-time and will outlast most electrolytics in your amp. it is the power tube that is prone to exhaustion!

the better sequence would be, turn heaters on, then say 12 seconds later turn on the B+, when powering off, reverse the sequence.

[eXg]

what material/gauge make excellent ground wires?

 

[ATJr.]

ga14 solid copper wire, you can tin it with solder.