ENGL Savage 120 mk1 troubleshooting [ENG]

[InDecember]

Hi, first off, this post is in English as I don't speak German very well, but I've seen a lot of good knowledge on this forum, so I thought I give this a shot. Should this not be allowed, please do remove this post. Feel free to respond in German if you want to.

So, I have this ENGL Savage 120 mk1 and ever since I bought new back in 2009 it is eating outputtubes for breakfast. I have to replace them every few months or so because one starts red plating, no matter how meticulous the bias is set (by me or by a tech).

In my search for a solution I came across a label on the powertransfo with the values of the sec windings printed on it. To my surprise it doesn't have the 6.3VAC (2x3.15) tap to feed the heaters, instead it has a 7.2VAC output. As far as I know ecc83's and 6550's are designed for 6.3VAC on the filaments. I measured this unloaded (no tubes) and get 7.54VAC on the heaters and loaded (tubes in place) gives me 7.01VAC. This seems way too high.

Is it normal for Engl to use these higher voltage transfo's and could this be a cause of red plating in the long term?

I did contact Engl directly, but didn't receive any reply to this day.

Cheers,
Erik

 

[RED-DC5]

Hi first of all have a warm welcome here at MB.
I own an Engl Fireball and it works great . Please
give Engl a second call because the had vacation
during the last Monthe.
regs

 

[InDecember]

Hi first of all have a warm welcome here at MB.
I own an Engl Fireball and it works great . Please
give Engl a second call because the had vacation
during the last Monthe.
regs

Thanks!

Coincidence or not, I just recieved word from Engl stating the actual output voltage is load depended and should measure 6.8VAC under normal conditions (which is within the +-10% tolerance).
They also suggest installing a high wattage resistor in the heater circuit should these values remain too high.

Still not sure this will reduce the chance of red plating, but I guess I could try.

 

[RED-DC5]

Hi, it 's me again, perhabs it would bei the best if you send the Head to Engl in Tittmoning. The Powertubes are having diffrent Grades, per example 20 for 6L6 in my amp. Otherwise you can bring the Head to an good repairman. We have also an Engl User Tread where you can have a look at the Savage 120 users. Do you live in Germany?
REGS

 

[InDecember]

I don't, I live in Belgium, where we don't have that many qualified Engl techs. I did buy it in Germany though, when I used to go there often (at Musik Schmidt, now called Session Music near Frankfurt a.M.). I'll check out the Engl user tread, thx for the suggestion!

 

[OneStone]

Hi Erik,

first of all, you do not need a certified ENGL tech since this issue is a very basic one that any electronics repair person that knows tube stuff should be able to sort out. The person does not even have to KNOW tube stuff inside out because this is basic electronics 101:

Let me explain your issue, why it happens and how to solve it:

First of all, the tubes in question have a heater voltage range in their datasheet that tells you how big the allowed deviation from the nominal heater voltage of 6.3 V is. For most non-special quality tubes like the EL34 and 6L6GC and the 12AX7 this is +-10%. The heater voltage must remain in this window for all possible operation environments imaginable for the device that includes the tubes in question.
Now that the device (here: guitar amp) does not have a regulated heater power supply and it uses a normal mains transformer, we can expect main voltage deviations to show up on the heater voltage almost 1:1. Given that the mains voltage is allowed to deviate by +-10% from its nominal value (in Europe, that is), it is clear that the heater voltage MUST be 6.3 V if the mains voltage is at its nominal voltage of 230 V.

If you measure 7 V at the tube sockets, this is problematic because on one hand, the voltage is too far away from the nominal voltage of 6.3 V and the cathode is therefore overheated and secondly, if you push the amp hard, additional heating of the cathode occurs due to the current it has to carry. This current can be more that the tube was specified to carry because guitar amp manufacturers usually do not adhere to the limits stated in the datasheet very strictly...
This leads to cathode material being emitted from the cathode and that very material ends up on your control grid, where it leads to increased idle current due to thermal emission and the high resistance guitar amplifiers usually use in their power tube grid circuits. This means redplating.

In addition to this effect that is common for all tubes, some manufacturers seem to use cathode materials that can lead to thermal runaway in a more severe way than others do and therefore, the effect may be better or worse depending on manufacturer. Plus, the heater current and therefore heater voltage (when supplied by a not perfect voltage source) varies between manufacturers.

The ideal solution would be to have a low impedance heater power supply with the correct nominal voltage. This means a good transformer with thick copper and low resistance wiring (thick and wide PCB traces or thick wires) so no voltage drop occurs between the transformer and the tube socket and the transformer does not have significant voltage drop when the heater current changes due to a different tube manufacturer or different tube type.

If the transformer reads 7.2V AC, it simply might have too little iron, too little copper and therefore might be too high source impedance to be a real voltage source and in combination with your mains voltage and/or tubes, the heater voltage ends up too high. Or there really is that much voltage drop between the transformer and the socket so this high voltage is necessary to compensate for that? Both are not good if the heater voltage at the sockets ends up to be 7 V in actual operation if your mains voltage is within +-10 % of its nominal value of 230 V.

In your case I would recommend to have somebody add a low value high current capable resistor (hint: inrush current with cold tubes!) in series with the transformer. If this solves the issue: great. If it does not, one could change the control grid resistors from (assumed) 220k down to 120k. And of course, do not bias the amp to class A but much lower, so the system is more stable (less runaway).

Personally, I never ran into that issue, but given that the thermal runaway effect is a thing especially with some tubes manufactured today (also with NOS but much less issues there), some years ago I simulated this and the effects were exactly the ones you described when the tubes were overheated: Instability, runaway, molten glass (yes!)

Regards
Stephan

 

[InDecember]

Hi there Stephan,

Thank you so much for the detailed answer. I knew I was not going to be disappointed posting in this forum!

So if I understand correctly, heater voltages should be 6.3VAC +-10% (assuming primary gets 230VAC) , if not this could be a source for red plating and can be solved by adding a low value high wattage resistor in series with the heater circuit or reduce the value of the control grid resistors.

Knowing this, the question remains why Engl uses transformers that put 7.2VAC out (at 230VAC) in the first place? Not only do I measure 7.2VAC, it is also literally labeled on the transformer itself (see picture in original post).
If I'm the only one having this issue, it would lead me to believe mine was installed with the wrong transformer? Or am I seeing that wrong?

Thanks again!

 

[OneStone]

Knowing this, the question remains why Engl uses transformers that put 7.2VAC out (at 230VAC) in the first place? Not only do I measure 7.2VAC, it is also literally labeled on the transformer itself (see picture in original post).

This is a question that you would have to ask ENGL, not us.

It really does not make a lot of sense at all, if that voltage is actually present on the tube sockets when the tubes are installed and the amplifier is in working condition...