230V / 50Hx mains: heater voltage?

[mikkosjun]

Hey everybody, I'm posting first time here!

Just got my Crack up and working after finding a couple of cold joints and burned leds.

I have 240 V transformer and as title says there is this 10 V difference to our finnish mains power. After listening few hours, voltage list is mainly perfect. Only things you may notice is B5/B2/2/4 which gives 165V (manual gives this B+ voltage 170 V) and after those 22,1 kohm resistors 1/5/A1/A6/B1/B4 which gives 79V (manual 90V). Is there any reason to fiddle with power supply resistor/capacitor values to gain this extra 5V to B+ voltage? If there is, could some engineer explain and calculate corrections, I can pick up suitable components somewhere.

Heater voltage says 5,3VAC. This is 1,0V less than stated in original tube datasheets. Does this have possible effects to sound? There is nothing to for this, because power comes directly from transformer.

In the coming month I'm assembling Speedball, which I already have. At the moment mains 50 Hz buzz can be heard about at 80% maximum volume, but this is normal I think this kind of machines and I'm listening max 50 %. There is some gentle hum which is not dependant of volume level but I haven't done enough tests with placing amp away from computer equipment, monitor, cheap interconnects, different sources, so I can't comment on that yet.

My experience with headamps is very narrow, so I don't have direct comparison. With HD800 improvement is in any case really significant to opamp and capacitor-modded EMU 0404 USB! Bass is more relaxed and grooving and soundstage is more natural, I'll say easier to listen. 

Wooden base and chassis plate is finished as in picture, let's see, if this is final or am I trying to get it even better.

Thanks for Bottlehead for fun with this kit, building manual is excelent and customer service also over Atlantic perfect!

Mikko

[Paul Birkeland]

A 5-10V deviation on the B+ is no big deal, I'm not so sure about the 5.3V on the heaters, is that what you're measuring at the transformer? 

[earwaxxer]

Nice build Mikko! - welcome!

[mikkosjun]

So, voltage between transformer terminals 4 and 5 is 5,3 V and logically same readings between B7/B8 and A9/A4+A5. Any expectations?

Just noticed a mystical efect. When knocking chassis plate lightly I hear in headphones weak metallic click. Same kind of phenomenon as I would knock a turntable and hear a light "bumb" in speakers. May be my hearing is just tricking me, but it doesn't seem to come outside, because sound is turning when I turn my head, same efect with two different headphones. Do tubes have that kind of reaction to sudden fast movement? Not depending on volume level.

What do people mean with "dead silent"? Should I expect from Crack total silence with high-end headphones, not source connected, 100 % volume? Is the light mains hum in higher volume levels reason to hunt some grounding weakness? Resistance check was perfect. This hum is also more pronounced when gadget has no source connected and gets softer when I connect my usb soundcard.  I don't have another tubes to test their influence. When beginning assembly I lightly dropped 12AU7 tube to floor carpet when opening package. Any kind of music even low volumes cover these efects and sound is great. Solid state EMU card is totally silent even with full volume.

Playing some time before Speedball to get familiar with amp characteristics. 

When I have time to continue testing, I inform this thread. 

Mikko

[Laudanum]

Tapping the top plate and then hearing sound through the speakers is normal.  It's called "microphonics".   The tube is "transmitting" the vibration through to your headphones.   "Dead quiet" doesnt account for tube microphonics.  The solution is to NOT tap on the chassis or the tubes 

Other than that, yes, many (probably most) of the members here report the amp to be "dead quiet" or have a "black" background with the volume turned up and no input signal.   You can try shorting plugs on the inputs and then listening. This will isolate the amp from the source or cables.   I would start from there.   There are far, far better troubleshooters here than myself and Im sure they'll chime in to help.

[Doc B.]

Dead quiet means quiet (other than the microphonics - this is more about hum and hiss) when you have the volume set to the maximum level at which you would listen. With many headphones you would never turn the volume up all the way it would be painfully loud. And if your headphone is painfully loud with the knob turned all the way up it might be possible that you would hear some noise at that level with no music signal.

If you have a tube that is microphonic it might also add a little bit of hum.

[vetmed]

Like PB I'd be concerned about the heater voltage. Can you see the heaters glowing? In my admittedly limited experience voltage that low would cause very low or no visible glow. Do you have the meter set to AC? If so then touch the leads together and see if there is any residual reading. If so you would have to add that to the reading you obtain. If your meter is not auto-ranging make sure it is set to the appropriate range. If you are still getting low heater voltage may I suggest you try another meter? I believe that operating the heaters at such low voltages will shorten tube life. OTOH if you like what you hear with it running at such low heater voltage then replacing tubes more frequently than you would otherwise is no big deal since these tubes are relatively cheap. Good luck


Robert Lees

[Doc B.]

If it works and sounds good let the tubes run in for a while and then remeasure that heater voltage.

[mikkosjun]

Thanks for everybody about comments. I'll listen my Crack for a while and follow its habits. The multimeter was set to AC, but it's a cheap unit, so possibly not so reliable. Now I have an explanation to the microphonics phenomenon. The heaters are glowing, but without experience I can't say, are they hot enough. In fact, beforehand i was waiting for more visible glow, that is "the thing" often advertised when there is some tube gear around. 

Mikko

[mikkosjun]

I got cheap 6AS7GA to test possible differences in the amp behavior compared to original 6080. After powering up seemed, that tube didn't heat at all, no glow. Fortunately I put the plate upside down to chassis to test voltages, because after a few seconds: EXPLOSION! The tube glass broke to several pieces accompanied with loud sound and spark. Luckily I didn't see what it looked liked around the tube

I had been testing voltages earlier with original tubes, nothing to mention in values. I can't figure if I did  short something with the meter probe, heater voltage just before accident was 5,1 Vac. Nothing serious, old power tube back and music flows as before again.

Can somebody confirm, was this tube at all possible substitute for original, as I read in the forum tube rolling thread and as I read in several specsheets in the internet? Did I not notice something essential, pin order or something as severe mistake? Or was it just broken individual tube? What kind of wrong voltage can explode a tube like that?

Another question: Is it possible to test just for interest suplying heater voltage with outboard DC power supply, for example cheap wallwart switching AC/DC converter?  I've got one with conficgurable voltage 3 - 12 V, 1000 mA (6V being nearest step in selector)?  As stated in forum and tube datasheets, it should not be done with 6080, because its heater current can be 2,5A. But with 12AU7? Do I need some extra regulator?

Mikko

[Doc B.]

Was the center black pin broken off? If so, the tube was probably inserted in the socket the wrong way. If the tube was in correctly it is possible that the tube had a leak and had gas inside.

[mikkosjun]

The tube bottom and the center pin are okay, bottom of the tube is grey, some kind of ceramic? But not black. In the center pin there is that guide line and when I inserted it, it went nice and smoothly to socket. May be defect tube.