Restoration
60ies Klemt Echolette M40
This Klemt Echolette M40 from the early 1960s had spent the last 20 years in an old shed, unnoticed. The new owner asked us: "Do you think you can get this thing flying again?". Of course we can...
History
those were the days.
The Klemt Echolette M40 and the matching Echolette LE2 speakers were THE club PA of the 1960s in Germany. Equipped with 4 inputs that could process everything from microphones to guitars to high-level keyboard instruments and a 32W ultra-linear power amplifier with 4xEL84. With this device, the band or club was prepared for anything that might come.
So, the best of the best. Those were the days! History is that the Beatles also used it for their legendary performances in Hamburg's "Star Club".
The starting point
Oh oh!
Our customer rescued exactly such an Echolette Club PA from an old shed after more than 20 years of "resting" there and would like to use it again in the same way as in the 60ies. Namely as an amplifier system for vocals and instruments with lots of vintage charm.
A few first mobile phone photos of the owner did not suggest anything good.
Power supply electrolytic capacitors that had "vomited" their electrolyte. A power switch that was defective and was simply bypassed with twisted-pair cables. A defective fuse that was not replaced but simply bridged with aluminum foil.
Our clear recommendation
Don't turn this thing on! Bring directly to the Tube WorkShop for assessment.
After the Echolette M40 had arrived at the Tube WorkShop, we were reliefed.
Yes, the mains switch broke off internally... someone had bridged the fuse... the power supply electrolytic capacitors were dead... and the device contained the dust of the last 60 years...
But otherwise the amp was in really good condition!
The housing was still OK...
There was an original tube set with 5x Telefunken ECC83, 4x Siemens EL84 and EZ81's from Valvo and Siemens...
When measuring the windings, the transformers showed no short circuits or interruptions...
The circuit was in absolute original condition and had never seen a soldering iron since it left the factory...
The last one in particular is very helpful in a restoration, since defective components can be replaced based from factory condition and there is no need to first sift through and eliminate lengthy, well-intentioned tinkering attempts by soldering iron artists from the past.
First, the innards of the amp were freed from the dust of the decades using compressed air and a brush (tip: better do this outdoors). When the cloud of dust settled in the workshop, things looked very promising.
The plan
we'll get it "back to life".
In consultation with the owner, we have determined that the Echolette should be as "vintage correct" as possible. This means that components such as capacitors and resistors are replaced by structurally identical types wherever possible and avsailable. Safety-critical components are replaced by modern, higher-quality types. We didn't want to let "NOS fever" break out here. Overall, however, the original condition of the amp should be preserved or restored as technically and optically correct as possible.
Since the amp arrived in an unsafe condition, the first restoration step on the agenda is to overhaul the power supply. Only then can the amp be put back into operation, piece by piece, and the further measures required can be assessed.
Therefor the first steps are:
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Exchange of the 2-pin mains socket for a modern type with a dedicated ground connector.
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Exchange of the power supply capacitors
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Measurement of the existing tube set for condition and function
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Replacing the power switch while keeping the original look
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Firing up the revised power supply
Power supply
Klemt, what were you thinking?
As already written, the Power supply electrolytic capacitors had already started to “vomit” electrolyte and were more than obviously exhausted. This is also normal for 60-year-old electrolytic capacitors.
However, it should also be mentioned that Klemt used electrolytic capacitors with a voltage strength of 350V in an amp that had a B+ voltage of approx. 380V after the rectifier tubes. Ok, even if you assume that the mains voltage in the 60s was slightly lower than today (and therefore the B+ voltage was also lower), this is still a more than "borderline" design.
In our amp, the power supply electrolytic capacitors were replaced with mechanically identical types from F&T that have a voltage strength of 450V. We are now in the “safe area”.
A little tech tip at this point:
After removing the old electrolytic capacitors, I had the fun of checking one of them for capacity and ESR value.
Interestingly, a capacitance >50uF could be measured and an ESR value <2Ohm.
So was the capacitor still “good” and perhaps even particularly “strong” with a capacity >50uF? Um, nope!
What is crucial about an electrolytic capacitor is that it is not “leaky” (i.e. does not allow DC voltage to pass through) and all of the existing electrolytic capacitors already had a worryingly low DC resistance in the three-digit kiloohm range. So you were well on your way to creating a short circuit in the power supply. Incidentally, an increased capacity value (as in our case) is a quite good indicator, since when measuring capacity, the value is determined via a time constant during the charging process. If the capacitor allows direct voltage to pass through, it charges more slowly and a higher capacitance value is displayed.
High capacitance value = particularly strong capacitor? No... more of a sign that he is "Leaky".
But enough of the tech babble that no one understands anyway...
Next up was the Power button restoration at. This was quite a time-consuming step.
The original power switch was still there, but was broken off before the connection terminal.
Of course, this switch is no longer available and it had to bea replacement type can be found that can be integrated into the Echolette M40. We actually used this switch [LINK]. The old switch button had to be removed and applied to the new switch using epoxy resin using a device. The Echolette then had to be dismantled on a larger scale in order to make suitable holes for the new switch on the existing bracket. At the end of the day, the Echolette M40's power switch could be restored with its original appearance from the outside.
The originalsrectifier tubesfrom Siemens/Lorenz have in our eTracer tube testing system optimal values are still achieved and can continue to be used.
Finally there was still “Better living” on the program and the tube bases of the rectifier tubes were cleaned with isopropanol and the chassis was cleaned of dirt and rust with isopropanol and then treated with WD40 to protect the surface. An old toothbrush (chassis) and interdental brushes (tube base) are your friends here!
After the 2-pin power socket was replaced with a modern cold device socket with a protective conductor (and the protective conductor was connected to the amp chassis), the Echolette M40 was able to be slowly started up again for the first time using a control transformer brought to life become. Of course, there were no other tubes in the amp apart from the rectifier tubes.
Result:
All voltages from the mains transformer (HV, heater, power amplifier bias) are present and stable. Let's move on...
Power amplifier
old but not hopeless!
First we got the existing one tube set with 4x Siemens EL84 checked on the eTracer system.
3 of the good old Siemens tubes were still in usable condition. Unfortunately, the fourth showed a very high screen grid current and must therefore be put in the "Box of Shame" as defective.
Of course, the easiest way would be to install a new, matched quad of EL84's. But somehow it would also be a pity to retire the still usable Siemens EL84's directly. They're such beautiful tubes. Luckily we still had a used Siemens/Lorenz EL84 in our pool of tubes, matching the remaining 3 original tubes very well.
The final set of old Siemens/Lorenz EL84 is certainly not optimally "matched" and has a variance of 10%. But it's good enough to stay in the amp and can be replaced with a new matched quartet at any time in the future. Rock n'Roll!
Next up was the bias circuit of the power tubes to be checked.
Except for the 50uF/25V Elko for voltage smoothing, this was still absolutely OK. The 50uF bias electrolytic capacitor showed the same symptoms as the power supply electrolytic capacitors and was no longer usable. This is one of the few places where we have moved away from the "vintage correct" approach and replaced the electrolytic capacitor with a modern, radial type with 100uF and 63V dielectric strength. This simply helps to stabilize the bias voltage better and to have enough headroom for the dielectric strength at this neuralgic point. Safety first!
Another "construction site" within old amps are usually the screen grid resistors of the power amplifier.
Especially if (as with the M40) 0.5W carbon composite resistors were used. Firstly, a current that is worth mentioning flows here, secondly, these resistors are quite sensitive to heat. The bottom line is that the screen grid resistors are mostly "toast".
Same here.
2 of the screen grid resistors were already "broken" and another one dissolved into its components when unsoldering. Maybe this was the reason why one of the power tubes was bad. In this state, the output stage is simply "defective".
All screen grid resistors have been swapped out for heat-resistant 2W metal oxide types. This is not "vintage correct", but at this point simply a significant improvement in operational reliability.
And also in the power amp we had to do some "better living" in which the chassis and the tube sockets are cleaned accordingly. See above.
It's alive!
first signs of life.
With the restored power supply and the overhauled power amp section, we were now able to insert the power amp tubes and check the bias. With the existing -17.5V negative bias, the power tubes all ran stably and within an acceptable range in terms of plate dissipation. As already mentioned, the old power tube set is not optimally matched but is still OK.
Now, for the first time, "sound" could be heard again from a connected loudspeaker in the form of a slight hum and noise. This is very good, so the amp is alive again. Unfortunately, there were also some crackling and popping noises (especially shortly after switching on) that we don't want to hear. So that's what we need to take care of next...
Phase inverter
silence please!
Ok, we already carry with us some unwanted crackle and popping noises from the first power-up
This can be due to contact problems in the tube sockets (unlikely after thorough cleaning)... problems with individual tubes (could be, we use the "old" tubes)... or "outdated" anode resistors.
The carbon composite resistors used in the M40 in particular tend to become "noisy" at the anode of tubes over time. The resistances are often still within the specified tolerance range, but they tend to generate noise. Often (not always) this can be provoked by tapping the resistors with a blunt object, which then reacts very microphonically and with a lot of noise.
This was also the case within the anode resistors in the phase inverter of our M40. It made the phase inverter anode resistors the "prime suspect" and they were swapped accordingly. Since we are not dealing with "safety-critical components" here, they have been replaced with equivalent, current carbon composite resistors.
Operation successful: After the swap, the crackle and popping noises were completely gone.
Another point was to verify that the original 0.1uF SEL MKT coupling capacitors to the power stage had not become "leaky" and thus possibly negatively influence the operating point of the power stage. Fortunately, this was not the case. The old MKT's from SEL were still in good condition and can live in the M40 in the future.
Finally, we selected the existing Telefunken ECC83 preamp tubes with our eTracer system to identify the one with the best symmetry of the two triode systems. Perfect for the Phase Inverter!
Preamp
we can see the finishing line!
The first step in the process was the measurement of the existing preamp tubes.
As almost suspected, the 60-year-old Telefunken ECC83 were still a "bank" and have shown impeccable values in our eTracer measuring system. This coincides with our previous experience in which the old preamp tubes from Telefunken and Valvo repeatedly showed an incredible longevity, very good values and little microphony. "Conspicuous" in a positive sense! These tubes are just incredibly good and robust. This is also proven by the very high used prices on eBay.
Conclusion: All of the old Telefunken ECC83 can still "live" in our Echolette M40 in the future.
Since there are no really safety-critical components in the preamp, we put the Echolette M40 into operation after checking the preamp tubes.
Result:
Yes, we have a signal coming out of the speaker output. But if you input a sinus-signal, it comes out at the loudspeaker output in a strongly alienated way and the volume potentiometers also do quite a lot crackle and swoosh sounds when being used. Neither one nor the other is cool. We don't want that!
I keep getting asked what I mean by "swoosh"...
Unfortunately I somehow don't have the correct German word for it and it's difficult to describe. It's not the "scratching" that you hear with dirty pots... it's kind of a "noise floor" that you hear when the pots are operated. A bit like flushing a toilet.
So for some reason the preamp doesn't seem to be working the way we want it to.
The prime suspects here are the coupling capacitors at the anodes of the tubes between the various amplifier stages. Here, too, the issue (as with the power supply electrolytic capacitors) is DC leakage!
The coupling capacitors have the function of blocking the high voltage of the anodes so that the next tube stage has a pure AC audio signal at the input again.
Long story short, all the old EROID anode coupling capacitors in the preamp of our Echolette M40 were "leaky" and instead of a pure AC signal, you could still measure a DC voltage between 5-30V after them. On the one hand, this ensures that the next amplifier stage is at an undefined operating point and that the volume potentiometers are also subjected to a DC voltage, which leads to the "swoosh" noises described.
All of these coupling capacitors were exchanged for NOS 0.22uF film capacitors from the 1960s that we had in stock. I don't want to claim that these sound better than current components... but they simply fit better into the overall picture of the circuit.
Another construction site were the cathode electrolytic capacitors in the preliminary stage.
Although these were not obviously "bad" yet, they were replaced to give the preamp tubes a safe working point.
The same applies to them anode resistors in the preliminary stage. Even if these weren't obviously bad yet, they still face the same fate as the noisy anode resistors in the phase inverter.
Means: Exchange them for new carbon composite types.
Even if this all sounds very simple in text form: Nothing is EASY in the Echolette M40!
The components are packed so closely that an exchange always resembles a minimally invasive operation. You need a very good soldering technique and have to bend a lot around the components to operate them into the M40 and at the same time. to keep the original appearance. But it's worth it.
Of course, the preamp section also has experienced our "better living" program with cleaning of the tube sockets and the chassis.
Power-up
happy days!
The Echolette M40 was now in excellent condition again and probably in some aspects even better than when it left the factory.
Of course, in the end, every pot got cleaned with Deoxit D5 which also provides them with a protective lubricating film and extends their life. If you take it very seriously, the volume potentiometers of the individual channels would still have been a candidate for an exchange, since they are no longer "completely off" in the 0 position. For the time being, however, we have dispensed with this because it probably does not represent a disadvantage in practical use.
Another point is the heater voltage of the power tubes. This is separated from the heating voltage of the preamp tubes, which can be adjusted to a minimum hum with a trimpot. This is not intended for the heating voltage of the output stage tubes and there is also no ground reference for the heating voltage. This was retrofitted with a "Virtual Center Tap", which reduced the basic hum of the power amplifier again. With a well-matched power amp quartet, the basic hum will be further reduced.
Conclusion
great amp for individualists and enthusiasts!
The Klemt Echolette M40 from the early 1960s is alive again and sounds great!
Both with the appropriate PA boxes and as a guitar amplifier...
The 30 tube watts should not be underestimated, but at higher volumes they also add their own "sound". All in all a great vintage PA with a lot more character than a clean, modern product.
As a technician you have to say:
There is nothing "easy" about restoring this amp, as the complete circuitry is designed to take up minimal space, making soldering a surgical operation. But when you take on this challenge, it's just incredibly fun to revive such an old amplifier (which was built to such a high level of craftsmanship).
The working time required for this remains a dilemma.
A restoration as in our case requires a working time of 9-10 hours + material costs which, with a reasonable hourly rate, quickly drifts into the "uneconomical" area. However, we also want to keep all those nice old amps for the future. This is only possible with a certain willingness to compromise and idealism on the part of the workshop (as with us in the Tube WorkShop) and the awareness and willingness of the owner to invest money to preserve a piece of history!