Inductor at plate to prevent power tube oscillations is good practice ?

[kagliostro]

Using tubes like 807 - 832 - GU50 as AF power tube I'm wodering if it will be good practice to take precautions against oscillation

like they do in this big PA amp (12 x 100W EL34 tube modules paralleled) where plate is feed through an home made inductor

formed by a 10R resistor covered with 8 turns of wire





Will this be a good practice ? A simply inductor like 8 turns of wire on a (not wirewound) 10R resistor will be enough efficient ?

Thanks

Franco

[PRR]

The problem is parallel tubes. We know our circuit to 20KHz. But tubes work out to 20MHz. The two plates on each side separated by unknown wire inductance and small capacitance everywhere can get into "push pull oscillation" across the few inches of wire.

The resistor is critical, and sufficient.

I'm not sure 10 Ohms is ample. I have seen 47 Ohms.

Resistor alone would reduce audio output. But even 47r against a typical 1K/plate load is not a big loss. Enough turns of wire makes it zero for audio and 47r for >~~~1Mhz.

Yet we see many audio power amps without these resistors.

[kagliostro]

Thanks PRR

When you say The problem is parallel tubes do you refer specifically to tubes like the 829 & 832

or also to RF tubes like 807 & GU50 configured for PP ?

Franco

[sluckey]

I've seen several of the BIG amps (Sunn, Ampeg, Vox, etc.) use a resistor on the plate of output tubes, but I don't recall any that used a small inductor.

[kagliostro]

Ciao Steve

A rapid search and I've seen some Ampeg (SVT) that uses 10R 5W resistor on 6550 tubes plates

no able to find such a configuration for Sunn and Vox but I belive they can have used the same method

EDIT: it was under my eyes, Vox 100 use a 100R

Franco

[sluckey]

Sunn 1200, 2000, and Model T.

[kagliostro]

Sunn 1200 - 2000 - Model T = 47R 5W

Model T Super = 50R 5W

 

Thanks

Franco

[trobbins]

An anode stopper of that type was not an uncommon vintage combination, and well known in the amateur radio field - sometimes also seen on vintage ss amp output stages.  A modern option is to use ferrite tubes/beads sleeved on a resistor's leads, to add a few extra tens of ohms loss above a MHz.


Good practise - yes - it means you have appreciated what could happen - and Philips had that nous.

[kagliostro]

....... well known in the amateur radio field

Yes, you are right (of course), I've many radio amateurs friends and I've seen those inductors there a lot of times

what I was wondering was about the opportunity to add it on AF amp (and now I know it can give benefits)



Thanks

Franco

[trobbins]

Nice 807 build indeed !

I managed to fit a PRO2 resistor and some ferrite beads inside the ceramic top covers of an 807 quad I have on the bench.  Peace of mind 😉

[xm52]

Ampeg has been using a ferrite core on each SVT 6550 plate instead of what was, a plate resistor in early revisions. This goes back to the 90's on all their SVT models. You can see them in the image below and on the attached SLM schematic.


The ferrite bead manufacturer's part number:

All American: BF45-4001
Murata: BL01RN1A1F1J
TDK: BF45-4001

[kagliostro]

I managed to fit a PRO2 resistor and some ferrite beads inside the ceramic top covers of an 807 quad I have on the bench

You remembered me some cute 4 x 807 tube with a "tower" inductor at the middle of the cross, something like this

You can see them in the image below and on the attached SLM schematic.

Oh, I see, they seems WK200 (but they aren't), I didn't know about this particular on the SVT

Thanks for the infos

Franco

[xm52]

...they seems WK200 (but they aren't), I didn't know about this particular on the SVT

FYI, I added the ferrite bead part number in my post above. They are pretty inexpensive.

[Merlin]

Yes, you are right (of course), I've many radio amateurs friends and I've seen those inductors there a lot of times

They are used in radios to peak the frequency response, i.e. to get wider bandwidth at the expense of higher-Q. However, inductors in the anode tend to reduce stability (because of the higher Q!), not improve it. For stability in an audio amp you should add resistors to the anodes, not inductors.

[xm52]

High frequency noise and oscillation suppression in amps is a very interesting topic. Ferrite beads are useful in taming oscillations but they have to be selected properly for whatever their application is, they have resistive, inductive and reactive properties depending on the frequency. The resistive element becomes dominant at higher frequencies which helps.

When Ampeg replaced the SVT plate resistors with the ferrite beads, they also increased the screen resistor from 22 ohms to 220 ohms. At the same time they removed the diode that was in parallel with the screen resistor.