noisy noisy noisy noisy noisy tube

[dsavitsk]

Originally posted by EC8010
However, it occurs to me that that may not be the problem. It may be that you have RF oscillation - that can cause microphony and noise. Have you got 1k carbon grid-stopper resistors on the grid pins before any other connections? Also, a 100nF between the top of the programming resistor on the anode CCS and the bottom of the cathode circuit would help.

There are 1K Riken resistors right on the pins. I don't know the difference in sound between microphonics and and RF oscillation, but I do know that if you talk into the tube it comes out the speakers.

For the 100nF, I'm not sure I understand where it would be placed. By top of the programming resistor, does this mean right where B+ first connects to the CCS? And for bottom of the cathode circuit, is that just ground?

Originally posted by stalker
Hi,

Hum is always a problem with the 6as7/6080. All the schematics i've seen used a lot uf's. Your power supply looks good but maybe needs more capacitance. Look at this schematic:

http://www.audiokit.it/ITAENG/KitEle.../interfsch.jpg

I have also found that some 6as7's in certain circuits make strange noises, like the ones you described.

Hmmm. According to the duncan amps program, ripple before the CCS is just a few micro-volts. The CCS should reduce it even more. I do have a couple of 56uF Solens I could cram in there, however. Also, would it be worth changing the heater to DC? It doesn't really sound like AC hum, but maybe that's what is inducing the tube noise?

Pete's original schematic (http://www.pmillett.com/images/lowmu_sch.PDF) used a 7236 (though he does suggest the 6as7 as ao option.) I think I can use it by changing just a couple of resistors, so I might give it a shot too.

-d

[EC8010]

Yes, and yes, to 100nF positioning. RF oscillation can sometimes show up as microphony.

[Tube_Dude]

Originally posted by Tubes4e4
I think your point is moot, since in a grounded cathode stage a 6AS7 has a gain of 2 (at most) - it doesn´t amplify noise much.

I have called attention for the fact (post# 16 ) , that probably the noise came from the CCS...but nobody care...


dsavitsk..what is the voltage between the anode of the tube and the + rail?...I suspect that the transistors are in saturation state , therefore having a very low impedance between the tube anode and power rail..

[dsavitsk]

Originally posted by Tube_Dude
I have called attention for the fact (post# 16 ) , that probably the noise came from the CCS...but nobody care...

I care, but the design is used in a lot of places, and I've not heard of anyone having stability problems with it.

Originally posted by Tube_Dude
what is the voltage between the anode of the tube and the + rail?

About 35V, which is well within limits of the circuit. All things considered, I am running this pretty conservatively.

[Tube_Dude]

Originally posted by dsavitsk


I care, but the design is used in a lot of places, and I've not heard of anyone having stability problems with it.

Can you try to put the capacitor that I told you in post # 16 and see if the noise change anything..?

[Cassiel]

I agree with Tube dude. It takes 5 minutes.

[Tube_Dude]

Originally posted by stalker
I agree with Tube dude. It takes 5 minutes.

Hi stalker

Specially because the LEDs are only biased at 2 mA , to low for a LED , I'm afraid...

[dsavitsk]

Some updates:

I increased the bias on the led's to 4mA (Doc at Bottlehead actually suggests biasing to 10% of the current over 10mA whilePJ says 20% I may increase closer to 8mA at some point to see what happens.) I also put a 1uF film cap across each one of the led's. I tried some of the noisy tubes, and they were still noisy. They may have been a little less noisy, but not hugely.

However, I happen to be fortunate to live down the street from Triode Electronics (down the street is relative here. Chicago is a big place.) I called them, and even though their website says they are closed on Saturday and they don't list the 6080 as a tube they carry, they told me they had some and I could come in to get them. They sold me a couple of Philips JAN 6080WC's. I've only tried one of them so far. At first there was a high pitched squeel -- less like oscillation, and more like ringing due to vibration. This went away after about 20 minutes. Other than that, almost all of the noise is gone. The tube does not seem to sound quite as good as the Tung-Sol. The Tung-Sol had a huge open sound (though part of that may be microphonic reverb) while this one is a little dead and muffled sounding. It is getting better -- or I am getting used to it -- with use.

So, thanks to everyone who has helped here. I think there is more to be done, but at least for now things are working reasonably well.

-d

[dsavitsk]

I seem to have been premature here in declaring everything okay ...

Most of the major noise is gone, however, I am now getting a high pitched squeel that comes and goes. It could just be that it is this tube's special noise, but I doubt that.

There are 1uF film caps across each LED in the CCS as in the schematic above. I also added 0.1uF VitQ caps to bypass the cathode bypass caps -- this improved the sound remarkably. I did measure the caps before installing them, but I am open to the possibiliy that one of them is leaky and that they are the culprite -- I don't remember the squeel before putting them in. It does not change as the volume changes.

However, I also think that I should pull out a lot of the wiring and rethink the grounding. Any thoughts on things to look at before doing this, whether a leaky cap is a possibility, and ways to improve the grounding scheme?

-d

[EC8010]

Bypassing an LED with 1uF won't do much. An ideal (for this purpose) red LED has a slope resistance of about 5R, and even a less than ideal red LED is likely to be less than 20R. 1uF will only begin to affect 20R at 8kHz, and would only affect the best red LED at 32kHz.

I think you're right, it's the grounding (isn't it always?) that's likely to be the problem. Star ground the whole thing to one point. Even if it doesn't cure the problem, it's the right way to go.