TNDJG:006:Carlsbro Sidewinder Persistent Reverb Fault

My friend has a Carlsbro Sidewinder guitar amplifier on which the reverb valve keept blowing every few months.  He replaced it twice and it went again so he asked me to have a look at the amplifier to see if there was a fault with it.

Pulling out the front panel reveals that the preamp uses four ECC83 double triodes:

One of these is for the springline reverb with one half driving the send through the small yellow transformer and the other being the return amplifier.  Banging the springline gives the normal clanking sound through the speaker which showed that the return side was working fine.  However, there was no signal on the send side, as revealed with a quick look using the oscilloscope.


A word of warning:  this amplifier has two on/off switches.  If you turn them both off at once, the high-tension is disconnected and is not discharged.  It sits at about 480 volts almost indefinitely. After my first shock I thought about fitting a 1 Mohm discharger resistor for safety but then I discovered if you switch it off on the mains switch while leaving the HT switch on then the HT is discharged while the valves still have some heat in them. So I decided to leave this aspect of the amplifier unmodified and keep the HT switch on all the time while working on the unit.

Diagnosing the problem

A quick measure of the electrode voltages on the reverb valve send side did not look healthy.  They were
  Anode:  496V and
 Cathode: 0.2V.

The grid was at 0V, the cathode bias resistor was 470 ohms and the primary of the output transformer was 560 ohms.   Replacing the valve restored operation and resulted in the anode being at  494V and the cathode at 2.1V.  This means the replacement valve was running at roughly 4mA x 500 V = 2W.

Looking more carefully at the circuit I saw that the other seven anodes in the preamp used an HT feed that came from the power amplifier.  This was at 350 volts or so.   It comes along the little cable that caries the line out from the preamp to the power amplifier; a cable that also carries a -50 volt bias rail.

On the other hand, the reverb driver was fed directly from the main HT voltage for the power amplifier, accessed via a wire link to the output side of the HT on/off switch.  I mentioned this to my father, who has designed many valve circuits and he immediately spotted a potential design fault in this amplifier: many versions of the ECC83 have a recommended maximum nominal anode voltage of 400 volts with 500 Volts and 1 Watt being the absolute maximum ratings under all circumstances.

Clearly the new valve would not last long either!

Fixing the problem

  The fix for the problem, as my father put it, was to "lose 200 Volts or so". An easy way to do that would be to wire the reverb send to the lower tension supply, but this was likely to overload that supply and it was presumably for that reason the designers at Carlsbro connected the reverb send to the main HT in the first place.

Since the anode feed came through a wire link, the obvious fix was to replace the link with a dropper resistor and to rebias the valve with a new cathode resistor.  The nominal valve plots above show that for 4mA the anode should be at 250V and the cathode at 0.5V.


The parts needed are as follows:

• A new anode resistor:  60K ohm, 1 Watt (made from 27K+33K 0.5W resistors in series.
  
• A replacement cathode resistor 165 ohms (two 330R in parallel).
• A replacement ECC83 valve (hard to come by now - Farnell, CPC and Maplin no longer stock, neither does GEE in Cambridge, so purchased for £15.00 in PMT East Road Cambridge).
• An anode supply capacitor: I used 150 uF 400V that I had to hand, but a lower capacitance would be fine and preferably a 500V capacitor should be used so that it would not be over-volted if the send valve is removed or failed again.

Here the wire link for the anode supply is removed (dotted line to right of large blue supressor capacitor (that is across the HT on/off switch poles):

Here the new anode resistor is fitted (I moved the suppressor to the left using the handy hole in the PCB for a larger physical suppressor):

Here I have started making a cradle for the new anode capacitor.  These two pots have four large lugs to the PCB and using the top of them as well I had six fixing points for making a snug capacitor bracket using tinned copper:

Here is the anode capacitor installed and you can also see the replacement cathode resistor parallel pair (between the valve and the xformer):

To wire the anode capacitor to the track between the anode resistor and the top of the xformer I drilled a small hole in the PCB (not shown) and passed the wire through the hole for easy soldering on the track side.

New operating Point

Measuring the new operating point for the valve we have
   HT Supply 494V 

   Dropped HT Supply: 265V

   Anode:  262V

   Cathode: 0.56V.

The new current is 0.56V / 165 = 3.4 mA which is about the same as before but the valve power is now 0.86 Watts which should not blow it.

Conclusion

Well it all seems to work fine at the moment.  I suppose there is the question of whether the reverb send will suffer transient clipping with its lower headroom but this really does not matter since the springline will be very good at rounding off such corners!
I hope you find this useful and if you try it yourself then do watch out for the high voltages!

DJG
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