Bad Cat Hot Cat 100/Redplating/eating o/p tubes

[robertamp]

Hi all,am new to this,greetings from Devon in England.Problem is with a Badcat Hotcat100 head that is redplating and burning out el34,s at an alarming rate.Being class A,cathode biased ive worked out the tube power dissipation is being well exceeded for this to occur.This amp is used on Half-power setting into a 16ohm cab.
So far it has new power supply Caps including cathodes;new octal valve bases;and replaced PI coupling caps,all cathode resistors(270ohm) measure fine as do screen grid(1kohm) and grid resistors(10kohm).The voltages measured for 230vac mains in uk are as follows:

plates-pin3= 432v; screen grids-pin4= 405v; cathode resistor drop= 26v
screen grid resistor drop= 5v; o/p tx centre tap= 435v; after choke= 425v
PI HT= 405v
Are these PSU voltages too high?Plates especially?Can anyone throw some light here,anyone had Badcat100 problems like this.All help gratefully received.I'm stumped.

[Wakculloch]

Don't know this amp so was hoping someone else would give you advice- however;
Looking at the data sheet it gives a value of 470R for the cathode resistors( if they are seperate for each valve), giving a bias voltage on the cathodes of 34V.
Yours appears low.
This would cause excess current.

[robertamp]

Thanks for replying wakculloch,i thought someone else might help too but!!!!!
The cathode resistors are 270ohm 10W in this amp and using- Ic=Vc/Rc; i get cathode current as: 26v/270ohm=96mA per tube.Then using Ip=Ic-Iscreen; i get plate current as: 96mA-5mA=91mA per tube.Finally power dissipation as:Vp-Vc x Ip= 432-26x91mA= 36.94Watts!!!per tube.How can that be at idle for a tube that has maximum dissapation of 25Watts?
If the voltages are correct i can only think that there is a heat sensitive component leaking or the output Tx is faulty.Ho Hum.Thanks for your help.

[Wakculloch]

The maximum anode dissipation is 25W, but the cathode is the sum of anode and screen which bumps this up to a max of 33W. Even that is exceeded so the remedies are:-
Install valves with high enough dissipation.
Raise the cathode resistor to bias more towards class AB1.
I would be tempted to contact the manufacturer and see what they have to say. It would be interesting to hear back from them.

[Bruce / Mission Amps]

Thanks for replying wakculloch,i thought someone else might help too but!!!!!
The cathode resistors are 270ohm 10W in this amp and using- Ic=Vc/Rc; i get cathode current as: 26v/270ohm=96mA per tube.Then using Ip=Ic-Iscreen; i get plate current as: 96mA-5mA=91mA per tube.Finally power dissipation as:Vp-Vc x Ip= 432-26x91mA= 36.94Watts!!!per tube.How can that be at idle for a tube that has maximum dissapation of 25Watts?
If the voltages are correct i can only think that there is a heat sensitive component leaking or the output Tx is faulty.Ho Hum.Thanks for your help.

With a GZ34 rectifier, ... 270@10 watts per single tube with only 380vdc on the plates is what is used in the BCH30 amp.
Assuming you have four 270 ohm resistors, I more then wonder if someone goofed and used the same value Rk at your much higher B+.
Why the B+ is so much higher I don't know, well, is this one running only on solid state rectifiers?
Obviously 37 watts per tube is absurd so you better get it figured out.
At the very least you should get yourself some 360 ohm 10 watter resistors installed to safe what is left of the power tubes.

[robertamp]

Thanks wakculloch, i'm not quite sure how you are calculating the power dissipation using the voltages etc could you explain using the data i put in first post?
Hello bruce from mission amps thanks for responding.This BCHC 100 has only solid state rectification so the voltages are higher than tube.I calculated that a 470ohm 10W resistor is needed to bring the dissipation down below 25W per tube.Am i calculating this correctly for given plate,screen,cathode voltages using: Ptot= (Vplate-Vcathode) x Iplate per tube?If not any idea how i should be?

[Bruce / Mission Amps]

Thanks wakculloch, i'm not quite sure how you are calculating the power dissipation using the voltages etc could you explain using the data i put in first post?
Hello bruce from mission amps thanks for responding.This BCHC 100 has only solid state rectification so the voltages are higher than tube.I calculated that a 470ohm 10W resistor is needed to bring the dissipation down below 25W per tube.Am i calculating this correctly for given plate,screen,cathode voltages using: Ptot= (Vplate-Vcathode) x Iplate per tube?If not any idea how i should be?

Thanks, I couldn't remember if the 100 has a solid state rectifier or not... I do not have a schematic of it.
I think you should change the cathode biasing resistor too.
As far as dealing with high plate voltages... or jut high B+ in general...I'm not a big fan of over biasing the power tubes to compensate.
I don't like using too large of single cathode biasing resistor for this BUT, in this amp, I would have no trouble seeing a 10 w 470 ohm cathode biasing resistor on each cathode.
If you do switch to four 470 ohm resistors, make sure each resistor has a new/fresh cathode bypass resistor of around 22uF@100v too.
Your plate to cathode voltage ratio should change when the new resistors are installed so remeasure under load to see if all the tubes are under 25 watts
If the amp uses a FW rectifier, it would not be hard to install some zener diodes in the hi-v center tap to lower it by 30 to 40 volts.
With four power tubes, I'd suggest using a larger 25-50 watt T03 type zener diode.

[Don Moose]

I didn't see you mention the grid voltage - unless the 400-some-odd screen grid was it.

You should make sure your PI coupling caps aren't leaking DC onto the grids - that's a really effective way of redplating.

Please measure the grid-to-cathode voltage with and without the power tubes in.

[robertamp]

Yes thanks bruce i have insalled the 470ohm 10W resistors and calculated that for the given voltages,incidentally not too different from before:-

Vp= 430v ; Vc= 29.25v(V5) / 29.70v(V8) ; Vscn= 413 / 405(pin4)
Vrscn= 8v (dropped on Rscn=1K) ; Rc= 466ohm. So Ptot per valve is:
V5= 21.9W and V8= 22.25W.So theoretically they shouldnt overheat unless there is a nother reason.
Hello Don thanks for replying.The 405 is the screen grid voltage at pin4.I have measured from control grid to cathode and get:
With tubes:
V5(pin5)= -27.80v ; @cathode R= 28.30v
V8(pin5)= -28.22 ; @cathode R= 28.80v
Without tubes:
On both pins5 negligible dc voltage ie.1-4mV fluctuating,and same V's on cathodes.I have changed both sets of coupling caps on input and output of PI-: In=10n;out=10n+47n//=57n.This amp still has a tendency to blow mains fuses when turned on hours after being on in test ok!!Very odd.Let me know if those voltages seem out.Thanks Pete.