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dscottguitars 6th May 2012 04:26 AM

6L6 current-voltage problem
 
Hi, I have a bassman clone I'm building and my tubes, russian 6L6's are glowing red at 420 volts and 47mA of current. When I add more negative bias voltage to get the red out, the plate voltage goes up to about 470v with only 22mA. Clearly within the plate dissipation limits.



I did notice since I move to Minnesota this week that the wall voltage is at 119v versus 115 in my California home.

I can't figure out what's going on...

dscottguitars 6th May 2012 04:29 AM

schematic
 
1 Attachment(s)
here is a drawing...

nazaroo 6th May 2012 08:28 AM

Are they really 6L6GC? or 5881/6L6? The latter can only take 400 volts.

Are you measuring the current properly? Red-plate seems like far more current than 40 mA.

What is the voltage on the screens?
I see you are hooking the screens up to "B".
Seriously try disconnecting that and hooking them up to
a more sensible voltage, like "D" or "E". (i.e., less than 350 volts on screen, as per design-center values in typical operation).

At least see if the tubes work like that first.
If they can't work in any scenario, I hate to say it,
but maybe the tubes are already bad/cooked,
or maybe you've cooked the screens letting them go into runaway.

First measure the voltages all along your Power Supply,
then pick a low one for screens, and isolate that with another resistor in series/cap to ground to keep the current from influencing the other stages.

That is, figure out a good resistor value and hook it up to "B", with a cap to ground, dropping the screen voltage by 200 volts (use a 10 watt or higher resistor).


One other possible thing you MUST check.

Disconnect your feedback loop from the output tranny,
because you may have wired it backwards, i.e., POSITIVE FEEDBACK by mistake!!!!

hardwire a 100 ohm resistor across the output tranny (where the speakers would go),
to protect the transformer from spikes. It won't affect sound.

Don't put an input signal into the amp while the feedback loop is disconnected.

Printer2 6th May 2012 12:43 PM

When you reduce the current through the tubes you are dropping less voltage across your power supply and the output voltage goes up. It may be best if you put some series resistance between your first cap and the rectifier.

Loudthud 6th May 2012 11:10 PM

Check for an ultrasonic oscillation. Connect your DVM across the speaker terminals and set to AC volts. With no signal the DVM should measure in the 10s of millivolts or less. An oscillation will read several volts.

dscottguitars 7th May 2012 04:33 AM

From what I know-limited-the screen voltage should be close to, but lower than the plate voltage. But with the max according to the 6L6 data sheets, 450 volts on the screen, would that extra 7 to ten volts higher be a problem?


I'm sure they are 6L6GC, that's the spec I bought them, but they are with Russian print. And I don't think less than 350 volts on the screens sounds right to me.

I can add a small resistor between the first cap and rectifier, like I have on the low power side of the switch shown which is 1.5K. And, I will try testing the current in the low power mode too, to see if it's stable at around 19 to 20 watts dissipation. Any size recommendation for that resistor Printer2?

I'll also check for the ultrasonic oscillation.

As for the feedback loop, I have it wired correctly. It is a variable feedback control to go from a mere 1% to 22%. It sounds okay, just the current is low and I cannot get close to the max dissipation of 21 watts, which I like a target of about 18-19 watts dissipation.

Thanks for the suggestions...

DavesNotHere 7th May 2012 10:02 AM

Please note: Audiophile builders, go away. These things are build different then those boutique amps.


no, the little bit of voltage increase would add some headroom
I run mine 520V plate, tie a 1 k resistor from screen to plate.
I would tie the screen resistor to the B+ connection going to the center tap of the output transformer then use a switch to different taps on your power supply to the output transformer/screen supply.
On the output tubes, the cathode resistor should be low in value (if not directly tied to ground). 1 ohm, 10 ohm, and 100 ohm (with bypass cap around 220uf ) works nice, but I like a piece of wire thank you.

on bias, I would make that as a master volume control that is set up so I can go from cut off to OD (in a voltage divider network).

the more ways it can go non-linear the better.

dscottguitars 7th May 2012 03:45 PM

Dave, not sure I fully understand. the 1K resistor between the screen and plate is fine, but do you mean connecting the that resistor to the B+? And which end the plate or screen? I am assuming the plate side, but want to be sure of what you are saying. And then the switch to different taps on my power supply also confuses me.

Also, the cathode resistor shown in the bias circuit is for switchable cathode-fixed bias.

I have the values set so I can do the cutoff to OD on the bias pot, but I'm having trouble finding the exact value that does not go too far into OD and make the plates glow.

I had a pair of Baldwin 6L6's but damaged them by when I accidentally touched the bias line to the B+: BIG SPARK AND LOUD POP.

Not sure if the new russian tubes are that different of if I damaged something else.

Thanks...

dscottguitars 7th May 2012 08:14 PM

Now I am wondering if these tubes are rated as high as the 6L6GC. They have 6n3c and the number 0184 etched on the glass. I've tried to look up specs for this but don't get consistent info.

I did test the current in the low power mode with 365 volts on the plates. The current was 53mA, which calculates to ~19 watts dissipation. That is what I expect and want.

Does anyone have any info about these tubes that may explain why I cannot get the power dissipation I am targeting?

The bias voltage, when adjusted to get rid of the red glow is what I expect, about -45 volts. But the current running through them is quite low.

Also in cathode bias mode, the current is only 12mA. According to the Weber bias calculator online, they should be running at about 29mA given the parameters I have:
Plate voltage-489v
Cathode voltage-46v
Cathode resistor value-750 ohms

nazaroo 8th May 2012 07:54 PM

Quote:

Originally Posted by dscottguitars (Post 3015529)
Now I am wondering if these tubes are rated as high as the 6L6GC. They have 6n3c and the number 0184 etched on the glass. I've tried to look up specs for this but don't get consistent info.

I did test the current in the low power mode with 365 volts on the plates. The current was 53mA, which calculates to ~19 watts dissipation. That is what I expect and want.

Does anyone have any info about these tubes that may explain why I cannot get the power dissipation I am targeting?

Any 6L6 will run good at 350-400 v.
If they won't run properly at higher voltages,
they probably aren't made to...why damage them?



Quote:


The bias voltage, when adjusted to get rid of the red glow is what I expect, about -45 volts. But the current running through them is quite low.


Also in cathode bias mode, the current is only 12mA. According to the Weber bias calculator online, they should be running at about 29mA given the parameters I have:
Plate voltage-489v
Cathode voltage-46v
Cathode resistor value-750 ohms
46v / 750 r = .061 = 30 mA per tube.

If you're reading 46v, then you must be conducting 60 mA!

How are you trying to measure current?

These values look good:

I'm running my Svetlana 6L6GC at:

Plate = 520 v
Bias = - 50 v
Rk = 1500 R (1K5) per tube.

Idle current = 33 mA per tube (aiming for 40 mA idle).


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