Pentode Noise Tests

[DaveP]

I use a very thick busbar, a 40W iron can only just solder onto it because it conducts so well.

The earth terminal at the IEC socket goes direct to the chassis via a short wire and a nut, bolt and washers.

The power supply earths are all joined together, there should of course be complete cancellation of ac within the power supply itself.  So a bridge rectifier earth is joined to the first cap -ive and any centre taps on heaters or transformers.  This goes to the far end of the busbar.

The amp grounds are connected along the rest of the busbar keeping each tube grounds as close as possible to avoid excessive loops.  Finally, the other end of the busbar is connected to pin 1 and the chassis on the input socket.

I was nervous the first time I tried this but my V241-76 project shows that it works in practice.

Hope that is clearer.

DaveP

 

[scott2000]

I was nervous the first time I tried this but my V241-76 project shows that it works in practice.

Hope that is clearer.

DaveP

Yes...Clearer....

I'm Trying to take a break  from  rather than breaking things trying to improve stuff... lol .......

Thanks!!!

 

[abbey road d enfer]

I'm having a bit of difficulty understanding this. So, basically all the power supply caps  should be floating and their negative leads tied to chassis at the input jack?? I have an older power supply that's just grounded through the can capacitor's shield right to the chassis so, after removing this can and installing regular electrolytics individually, I'd just tie the negative leads together then off to the input jack shield to chassis???? Non isolated jacks.....

For best performance, the rule of thumb is ground follows signal, and capacitor negatives should be connected to the reference point of the stage they are decoupling. That means the capacitor that decouples Vx should have its negative tied to the cathode resistor of Vx (assuming usual common cath or cath-follower). Since currents are quite small in low-level tube stages, and due to practical reasons (such as dual canned capacitors), it's not always possible to strictyly adhere, but for high-level stages, it is important to adhere strictly. Solid State is even more demanding.

What about an AC heater center tap hum pot.....This ground to them as well?????

It should be connected to the cleanest ground possible, generally at the input.

So the power amp is grounded at the input jack

Not so; the power amp is grounded at the phase-splitter, that is grounded to the preceding stage (tone or whatever), that is in turn grounded at the input stage. That's hierarchical grounding. Exactly what is described in Merlin's paper.

  , when using screened wires, where is this location of grounding?

Very debatable, but if the shield is connected on one side only, IMO it should be at the receiving end. Should hardly matter anyway in an enclosed chassis.

 

[bluebird]

For best performance, the rule of thumb is ground follows signal, and capacitor negatives should be connected to the reference point of the stage they are decoupling. That means the capacitor that decouples Vx should have its negative tied to the cathode resistor of Vx (assuming usual common cath or cath-follower). Since currents are quite small in low-level tube stages, and due to practical reasons (such as dual canned capacitors), it's not always possible to strictyly adhere, but for high-level stages, it is important to adhere strictly.

I agree this is the most successful grounding scheme for me. Although I don't ground at the XLR input. XLR pin one is always to the chassis for me and the main filter cap/tx center tap goes right to the AC in ground at the chassis. The higher gain stages are on a buss that goes further out from the main cap ground point. Smaller filter caps for each stage ground close to the cathodes of that stage like Abbey suggests. XLR out pin 1 usually is grounded to the last amplification stage ground point.

Solid State is even more demanding.

Hmmm, I don't know, maybe technically but I've always had a way harder time getting tube circuits quiet.

 

[gridcurrent]

what Abbey said:  +1

and from RCA tube manual RC30,  page 90.

 

[Tubetec]

My grounding layout is good , problem turned out to be the small choke needs to live a few inches further away from the mains transformer, I added longer leads and at certain orientations outside the the confines of the psu enclosure its perfectly quiet . I think the only way forward is to mount the choke can on the outside of psu box.I'll have another look tomorrow and see how it goes .

 

[abbey road d enfer]

I agree this is the most successful grounding scheme for me. Although I don't ground at the XLR input. XLR pin one is always to the chassis for me and the main filter cap/tx center tap goes right to the AC in ground at the chassis. The higher gain stages are on a buss that goes further out from the main cap ground point. Smaller filter caps for each stage ground close to the cathodes of that stage like Abbey suggests. XLR out pin 1 usually is grounded to the last amplification stage ground point.

You can get away with this because of the isolation provided by the xfmr, but for unbalanced inputs, the reference bus should be grounded close to the input.

"Solid State is even more demanding."
Hmmm, I don't know, maybe technically but I've always had a way harder time getting tube circuits quiet.

Typically, currents in a SS circuit are about 10x of vacuum-state, so ground connection resistance becomes a significant factor. Even more when you compare p-2-p wiring with PCB.

 

[DaveP]

Typically, currents in a SS circuit are about 10x of vacuum-state, so ground connection resistance becomes a significant factor. Even more when you compare p-2-p wiring with PCB.

Abbey makes a very important point here.

I think it's a good idea to post the RCA grounding instructions from gridcurrent here:-

RCA Grounding Instructions
Any amplifier having two or more stages should be constructed with a straight-line layout so that maximum separation is provided between  the signal input and output circuits and terminals.
Power-supply connections, particularly those carrying ac, should be isolated as far as possible from signal connections, especially from the input connection. Signal-carrying conductors, even when shielded, should not be cabled together with power-supply conductors.
Internal wiring for ac-operated tube heaters, switches, pilot-light sockets, and other devices, should be twisted and placed flat against the chassis.
All connections to the ground side of the circuit in each unit should be made to a common bus of heavy wire. This bus should be connected to the chassis only at the point of minimum signal voltage, i.e., at the signal-input terminal of the unit.

This is exactly what I do now in my builds and it works.

DaveP

 

[scott2000]

I agree this is the most successful grounding scheme for me. Although I don't ground at the XLR input. XLR pin one is always to the chassis for me and the main filter cap/tx center tap goes right to the AC in ground at the chassis. The higher gain stages are on a buss that goes further out from the main cap ground point. Smaller filter caps for each stage ground close to the cathodes of that stage like Abbey suggests. XLR out pin 1 usually is grounded to the last amplification stage ground point.

Are you referring to the safety ground where you ground the tx center tap and main filter cap?????

So this kinda goes against the grounding at one point only?? Basically grounding at both ends of the bus bar?? Or does the input ground not connect to the bus??? Are these inputs considered isolated from the chassis??? Thus the ground added????

Any thoughts on running the output tube's cathode grounds to the first filter cap????

 

[abbey road d enfer]

Are you referring to the safety ground where you ground the tx center tap and main filter cap?????

tx center tap and main filter cap should not be grounded at safety earth.

So this kinda goes against the grounding at one point only??

No. It just goes against a wrong concept.

Basically grounding at both ends of the bus bar??

Just one end, the correct one.

Or does the input ground not connect to the bus???

Yes, connected to the bus.

Are these inputs considered isolated from the chassis???

Not necessarily.

Any thoughts on running the output tube's cathode grounds to the first filter cap????

That is perfectly correct AND compatible with all that's been written.

 

[Tubetec]

I moved the choke to the outside of the psu enclosure ,definately an improvement on the previous situation,but still some trace of hum left . I think I'll have to look at mounting the choke on the preamp chassis itself ,well away from the mains transformer.

 

[abbey road d enfer]

I moved the choke to the outside of the psu enclosure ,definately an improvement on the previous situation,but still some trace of hum left . I think I'll have to look at mounting the choke on the preamp chassis itself ,well away from the mains transformer.

What happens if you replace the choke with a simple resistor? Maybe you've reached the limits of the choke's contribution to noise, and need to find the last fraction of dB elsewhere...

 

[Tubetec]

Thanks Abbey,
I guess its worth a check at any rate ,just to see .
Line up of the smoothing components is, rectifier ,220uf x 2 in series first filter (with balance resistors),3kohm resistor,100uf,10H choke, 82uf ,1K ,56uf ,1H choke,12uf (anode connection),75kohm, 100uf to ground ,3.3kohm resistor , and 6.6uf foil (screen bypassed to cathode) .Ht at the top of the anode resistor is 320 volts ,anode is 225volts ,screen is 195volts. As I have a very large cap on the feed to the screen it takes a couple of minutes for the voltage to build up to full after switching on the Ht. Its very quiet as voltage builds ,as the screen volts rise towards 195 volts the partition noise gets progressively worse, theres around 10-15db rise in noise between say 60 volts and 195volts on the screen. From what I understand its a trade off between low noise(low screen volts) and better linearity, higher noise with higher voltage . Spec sheet gives curves for 220volts on the anode and 150 or 200 on the screen. Any further comments very welcome.

 

[DaveP]

Many vintage compressors ran their screen grids in the 40-60V region.

DaveP

 

[Tubetec]

The characther of the noise changes quite a bit from low to high voltage also , its starts off a quiet 'hush' type sound,with very little high frequencies  then as screen volts increase ,more hiss, which is much more noticable, comes in. I'll try removing the second choke all together  and see what happens and maybe use a pot to vary the screen volts and see if theres a good balance to be found .I guess at higher input levels/higher screen volts  more noise will end up masked where at lower input levels a lower screen voltage might give better results .The quest continues....

 

[Tubetec]

With controls set to flat on the equaliser and 60db of gain added to the output I get a noise level around -20db on my meter ,the C3G is supposed to be able to put out 70 volts with minimal distortion ,Am I correct in saying I have more than 115db dynamic range in this case

 

[abbey road d enfer]

Its very quiet as voltage builds ,as the screen volts rise towards 195 volts the partition noise gets progressively worse, theres around 10-15db rise in noise between say 60 volts and 195volts on the screen.

That is probably accompanied by a similar gain increase, so S/N ratio may not change as much.

 

[abbey road d enfer]

With controls set to flat on the equaliser and 60db of gain added to the output I get a noise level around -20db on my meter ,the C3G is supposed to be able to put out 70 volts with minimal distortion ,Am I correct in saying I have more than 115db dynamic range in this case

70 volts rms? That's about +39dBu. If this is confirmed, the DR is indeed 119dB. But in order to take advantage of it, you need the equipment around it to be able to manage the same levels. In most cases, max level is about +20-26dBu; that means a usable DR of about 100-106dB.

 

[Tubetec]

Thanks for your comments Abbey,

Heres some further reading on the Siemens C3G

http://www.jacmusic.com/techcorner/ARTICLES/English/Portraits/C3g/C3g-C3m-info.html

I guess as a driver valve into a hi Z maybe 70 volts is possible ,probably not into line level 20 kohm load though ,anode load is 10kohms in this case .

I'll draw out the schematic tomorrow and see if any improvements can be made .