Changing the tubes.
Before changing the tubes
Unplug the power cord from the main current and let the tubes cool down for roughly 10 minutes. Open the back panel of your Hehku and read the tube chart on the back panel carefully.
Usage of the wrong kind of tubes leads to damaging the amplifier and immediate termination of the guarantee. Always consult a professional, if you’re even slightly hesitant about what to do.
Power amp tube EL84
Changing the tubes on Hehku is relatively easy, since the amplifier doesn’t have to be biased when changing them. This is due to the fact that the main amplifier of Hehku has been cathode-biased. It must be taken to account, that the classification of the power amp tube should be kept the same, unless you are looking for some specific effect on the sound. Standard Hehkus come with power amp tubes that are classified as 3 out of 1 to 5. The smaller the classification-number, the lower volume the distortion starts and the headroom is lower.
This tube is located as the first one behind the terminal adapter. It affects the sound of its channel greatly since it’s the first tube and hence its sound is multiplied several times before it reaches the speakers. By choosing a low gain-version of the 12AX7-type of tubes, the hum can be minimized and on the other hand, it adds certain clarity and warmth to the sound. If you choose a tube with higher gain into your Hehku, the sound will be rougher and mistier – which one you prefer is entirely up to personal taste.
The EF86-tube is a speciality in Hehkus. It is connected to the amplifier by a silicone-based attenuation adapter. This is because the tube is vulnerable to picking up the vibration of the cabinet and the speakers easily like a microphone and hence, the sound may get distorted. However, the silicone adapter we use cuts these distortions out effectively and a separate cellular plastic-insulation can be added on top of it to suppress the direct vibration from the speakers. Hands on-experience has showed that this insulation-technique is extremely effective and we are proud to recommend this tube as an option since its sound provides a whole new kind of special warmth to compliment the vintage-spirit of your Hehku.
Changing the tubes
1. Grab the base of the adapter firmly and pull the tube out of the amplifier carefully.
2. Press the protective cap down lightly. Rotate it counterclockwise and pull it out from the top of the tube.
3. Place a small chisel headed screwdriver in the same hole with the tube and carefully rotate the tube out of its base a couple of millimetres. Place the screwdriver in the similar hole on the other side and continue doing this until the tube gets loose.
4. Install the new tube. The most important part of this process is pay attention to not pressing and harming the silicone-parts.
5. Install the protective cap.
6. Install the adapter back into your amplifier by holding it from its base.
The sensitiveness of this tube can be modified to meet your desires when it comes to how much distortion you would like in your Hehku.
A balanced 12AX7-tube is often used for phase reverse, but the standard tubes work on guitar-amplifiers for this purpose just fine. However, the so-called spiral heat tubes cannot be used for this purpose, for they can’t endure the high voltage coming from the phase reverse.
Rectifier-tube can be replaced by for example, 5AY4 or GZ34-tubes. They provide slightly different tension and hence may affect the headroom of your amplifier. The differences are usually relatively small and hard to detect.
The best way of optimizing the tubes on your Hehku is by consulting the experts from Hehku Customs. We offer services for ordering, installing and maintaining the tubes for an affordable price.
We also provide excellent options for EL84 and 12AX7-tubes. EL83-tubes are now available to be used instead of EL84-tubes! EL83s are a good alternative for EL84s for multiple reasons. To name a few, military-inspected Russian EL83-tubes (6P15P-E) can be purchased for third of the price of EL84s of similar quality. Since they are manufactured to be used in the military, the life expectancy for these tubes is often four to five times longer in comparison to EL84s. And the sound? Depends on everyone’s personal taste, but several of our clients have preferred the sound of EL83-tubes to the sound of EL84’s. Legend has it, that Rory Gallagher himself would have modified his Vox to function specifically on EL83-tubes. On top of this, old EL84-tubes are starting to be quite rare and even the used highly sought after Telefunken-tubes go for around 100€ these days. However, unused EL83-nos-tubes can be found at a cheaper price point due to the lower demand.
A switch that allows your Hehku to use a Russian military-quality tube 6N2P instead of 12AX7 can be installed to your amplifier upon request. The price-quality ratio in them is roughly the same as in the EL83-tubes described earlier.