Tubes: 811A vs 572B

[WA2MGB]

My amp takes four   572B tubes.    I understand that the 811a is a drop in replacement...though more limited in performance.  Given the significant price difference, the 811A  being inexpensive compared to the 572B's... is the loss in performance substantial to warrant spending the extra $ on 572B's?

The amp is a Gonset GSB-201

Tnx

[K0BG]

Four 811s have about the same plate capacitance as two 572Bs. Yes you might (!) be able to use them in a pinch, but I wouldn't. The other way around (572s in place of 811s) seldom works. Pay the extra and enjoy the long life driven to the same level as the 811s.

Alan, KØBG

[WB2WIK]

Ummmm...I've had GSB-201s which came factory equipped with 811A's.  If yours uses 572B's it must be the later version (GSB-201 MK III?) or some previous owner retrofitted it with 572B's even though it didn't need those tubes.

I'd find out which is the case, because 572B's can be operated at much higher plate voltage than 811A's and this is not necessarily a backwards-compatible change.

WB2WIK/6

[AB5Q]

Maximum specified plate voltage for the 811A is 1500 VDC (50% Duty) and for the 572B it is 2750 VDC. Exceeding the tube's maximum voltage rating could result in a plate to grid arc and grid destruction. If your amplifier operates at a plate voltage of less than the maximum allowable for an 811A you should be fine.  It is also important to verify the maximum specified plate current and plate power dissipation for the 811A to make sure you will not exceed those ratings as well.  Tube sockets, plate connectors as well as filament requirements are the same between the 811A and 572B.  Mechanically it is a simple swap, however there are a few electrical variances that complicate things.

Here's a link to the data sheets for both tubes, take a look.

http://www.svetlana.com/docs/tubeframe.html

[KG4PFO]

In short....putting 811's in a amp designed for 572's is NOT a good idea at all......kiss your 811's goodbye.
Putting 572's in a amp designed for 811's on the other hand is acceptable in most cases.

[W8JI]

I agree with WIK.

I never saw a GSB 201 that used 572B's from the factory. Every one I saw used 811A's, although 572's were often installed later to be a HD replacement.  

What is the plate voltage? DON'T stick a meter on the anode, read the front  panel meter and tell me.

 

[K8AC]

I believe the 201 MK III came factory equipped with 572Bs.  MK II was the last with 811As.

[WA2MGB]

Mine is the Mark IV....factory equipted with 572B's....

[W8JI]

Did they raise the HV when they added 572's?

Never saw a MK III I guess!!

Or did they leave the HV about 1700V?