Noise - 6J5 - 71a -300B amp

[hal3101]

Hi.

I need some input from the experts! Have a stereo amp 6J5 - cap coupling - 71a interstage - 300B amp. The 6J5 is CCS loaded with a common B+ together with 71a (fixed bias) driver tube. The output tube is a 300B with a separate B+ and fixed bias. The amp is up and running but with too much noise. The problem is the 71a tube. I measure a lot of noise on the anode of the 71a tube. The noise is increasing when I move my hand next to this tube. The noise level on the output of the amp is around 20mV. Filament on the 6J5 is ac and the 71a and 300B is dc (regulated).
The interstage is a LL1660. I have tried different positions for the interstage with no luck. The noise level on the B+ is low and so is also the bias voltage noise for the 71a and the 300B.

Need some tips.



TorH

[Shoog]

It may be just possible that your anodes on the 71a are picking up interference. Try adding a 10R resistor on the pin as an anode stopper.

Shoog

[jazzbo]

What is the CCS on the 6J5? It could be oscillating....

[hal3101]

I will try to add a anode stopper resistor to check this. Thanks for the tip.

If the CCS on the input tube was oscillating I would suspect that this would be able to be measured on the grid on the 71a tube ?

TorH

[jazzbo]

Yes. If you are using the IXYS chip as the CCS, they can be prone to oscillation if far from the anode of the tube. I have one, even with a gate stopper right at the pin, that will oscillate when a meter probe goes near it...

[hal3101]

Added an anode stopper - no change. Changed from fixed bias on the driver tube to cathode bias - no change. Removed the ccs - no change. Removed the driver tube - noise on the output dropped to 1 mV. I think that the problem is airborne noise and the 71a is picking up this or maby some vierd grounding problem. Can try a different position for the driver tube but more than 5-7 cm is not possible.

TorH

[kevinkr]

Originally posted by hal3101
Added an anode stopper - no change. Changed from fixed bias on the driver tube to cathode bias - no change. Removed the ccs - no change. Removed the driver tube - noise on the output dropped to 1 mV. I think that the problem is airborne noise and the 71a is picking up this or maby some vierd grounding problem. Can try a different position for the driver tube but more than 5-7 cm is not possible.

TorH

I use 71A and 26 in a variety of applications and they are very susceptible to electro-static coupling from surrounding metal objects. Try shielding them with foil grounded to your chassis to start, if that works look for some antique tube shields and mountings. (Antique radio restoration sites often have these.) I'm currently using ugly, but effective cylindrical shields made from soldered copper foil. To be completely effective I have a little "hat" soldered to the top of the shield. The shield is grounded to the chassis.

I use a pair of of 71A driven by 5842 in my headphone amplifier and the output noise floor is below the measurement limits of both my Amber 3501A and my FFT analyzer (~100dB SFDR)

Some time ago I had a problem with a large hum increase with my 26 dht transformer coupled line stage. I had recently purchased an old Mitsubishi DA-F10 tuner which I installed on the next shelf up on my rack. It had a 2 conductor line cord and I noticed that when it was plugged in the hum level at my speakers increased about 10dB - even with the tuner rca cables disconnected from the pre! I started to get suspicious and flipped the orientation of the tuner line cord and noted that the hum level dropped significantly. I immediately installed a 3 wire line cord and this source of electro-statically coupled hum vanished.

[kevinkr]

Something else I find helpful with these tubes is to create a pseudo center tap using a pair of resistors rather than just grounding on one or the other side of the filament supply. A pair of 27 ohm resistors one from each filament pin to ground (or cathode bias network) works really well. Always seems a bit quieter to me, and seems to improve rejection of residual ripple on the filament supply. (There really shouldn't be any.. )

You can do the same thing on the 6J5 filament supply and even elevate it 20 - 30V above ground.

Also fixed bias is just fine with the 71A (171) and I personally prefer not to have the large cathode bypass electrolytic cap in the signal path. (Frankly large film, even PPIO aren't that great compared to no cap and the proper operating point.)

71A is pretty linear. Previously cited headphone amplifier also had rather low levels of thd.

Once you get this sorted out you will have a great sounding amplifier, you have managed to combine a number of my favorite tubes into your design..

[korneluk]

Kevin,

Sidebar please.

Originally posted by kevinkr
I use a pair of of 71A driven by 5842 in my headphone amplifier ...

Is that transformer coupled? If so, what transformers are you using?

Originally posted by kevinkr
...the output noise floor is below the measurement limits of both my Amber 3501A and my FFT analyzer (~100dB SFDR)

Which FFT analyzer do you use?

TIA,

-- josé k.

[kevinkr]

Originally posted by korneluk
Kevin,

Sidebar please.


Is that transformer coupled? If so, what transformers are you using?

Hi Jose, the output transformers were custom built to my specification by Jack Elliano at Electra-Print. The primary impedance is 5K and the they have two 8 ohm secondary windings which can be connected in series (how I am using them) for 32 ohm headphone drive or parallel for 8 ohm speaker use.

Drive from the 5842 is LC coupled. (100H choke load, and coupling cap.)

I think I should mention the amplifier is caged and the power supply is on a separate caged chassis.

The output of this headphone amplifier is not attenuated prior to the headphones, and it is shockingly quiet. Frankly I did not expect this level of performance. Sounds great, unfortunately I lost interest in headphones shortly after I finished it. (Didn't get the Grado headphones I wanted for Christmas and too cheap to buy them for myself.) I might use it as a tweeter amplifier at some point down the road.

Which FFT analyzer do you use?

TIA,

-- josé k.

It is software based - latest version of audiotester, along with hardware interface (I think you have the schematic already - as I posted it to another thread.) and an M-Audio USB Transit. It's suprisingly good, although not in the same league as the M-Audio PCI based Audiophile 2496 which has considerably better SFDR and supports 96K 24 bit in full duplex mode which the transit can't due to various usb bandwidth limitations. (It's usb 1.1 only and on my old slow laptop.)