Jaap Knoppert Tube amps for guitar

Another experiment with submini tubes: 6112 as preamp tube and a 6021 in the poweramp. Preamp is a regular two stage preamp, normally biased, 22u bypass cap over the first stage and 470n over the second stage. Tone stack is a baxandall type. For the second stage I used a split plate resistor (22K/75K) to make sure there’s not too much signal going into the poweramp too early. I spent most of the time tweaking the poweramp. I started with a self split push pull. It sounded ok but then I tried a parallel SE configuration with a 6021 and that sounded better in my opinion. So that’s what it is now. For the psu I used a standard Nixie power supply unit, a dc/dc converter and that works fine. Power transformer secondary is 9 V @ 1.3 A. Followed by a bridge rectifier and a cap and than into the Nixie dc/dc converter. That results in a B+ of 175 V @ 11mA for the total amp. That’s just under the max of 12mA for the Nixie unit. No problems with heat or voltage dropping. Another bridge rectifier and a LM317 are used for the 6.3V filament.
Output: around 400mWatt, Class A.


Dimensions (inches): 9 x 5 x 2

Soundclip

Labels: submini tube amp guitar

Why hide your amp behind wood and tolex? The ZABS Suitcase has nothing to hide! My main goal for this project was to make an amp with a lot of sound possibilities. From clean to mean and suitable for different styles of music. Low volume and a unusual design and housing. Top cover is made of plexi glass. With a marker you can draw your own design or make notes for example about the setting of the amp for a song.

Zabs yellow cab! A high gain amp with three preamp stages and a single ended power amp. As you can see the amp runs on only 2 tubes: a 12AX7 and a triode/pentode ECL86. The amp is based on the Hi Octane but it has a one knob tone control which acts like a sort of mid control. This gives the amp a sound range from Marshall to a sort of Vox like sound. I tried all kind of tone controls but this is my favourite for this amp. Also because it gives a bit more signal and overdrive to the power tube.
You can use just two preamp stages or all three. The third one can be switched in for more gain.
I used a one star grounding system and the amp is very silent. At first it hummed and oscillated but I found out that two things were crucial: 1. grounding the mid pin of the noval socket of the ECL86 (stopped the oscillation) and 2. the reference at the catode of the powertube for the filament (no more hum). The Siemens ECL 86 pentode runs at 260V into a 10K primary OT. Output about 3 Watt. The 12AX7 is by Tung-Sol, I like that tube very much.

Dimensions (inches): 12.2 (w) x 5.9 (h) x 4.3 (d)

I'm working on building in a tremolo into a Fender Bassman. Like in the Fender Vibroverb amp the trem is connected to the bias resistors of the output stage. At first it didn't work at all. With help from Joe Kerr and AX84 I found out that the bias of the output tubes is crucial. The Bassman is normally biased at around -45V at idle. But than the tremolo hardly had any effect. First I changed resistor *) to 100K instead of 470K and that helped a bit but not much. Later I biased the Bassman to -53V with the intensity pot at zero (bias is around -44 with intensity at max). That is the key! Listen to the sound clip for an impression. At the end of the clip you can hear a strong 'chop chop' sound. When I use a JJ Electronics tube (not in this clip) this sound is a bit less strong.
As you can see on the picture the tremolo is now ready. I use the tube from the Bass channel but everything is still there so the Bassman can be changed back to original. No new holes were necessary. The trem is working very good now; without any 'chop' sound. Volume switch is for on/off and Treble and Bass pots are for Speed and Intensity.

Sound clip 1 (No title No song Trem test; 6minutes: from no trem to max trem)

The ZABS Champ XL has gone through some stages. This is the final result. The PSU rectifier tube is a EZ81 instead of the original 5YR3. There's a switch for normal filtering or extra filtering when using a 6L6. That tube draws more current than a 6V6 and the extra caps keep the amp quiet.
It started out as a 2 channel amp with an octal triode 6SL7 and the regular noval 12AX7. Normal Champ set up but with a Moonlight one knob tonestack. It sounded good but I was not totally happy with the tone stack and sound at higher volume. So after all I ended with the normal Champ blackface set up. Preamp stage 1, Fender tonestack, Volume, pre stage 2 and SE poweramp. Also tried it with a 6SJ7 pentode just like the Champ 5C1. That is a great sounding tube but after all I changed the amp again. So finally the preamp uses a 12AX7. And the amp now has a tremolo with a 6SL7 triode. So it has become a Champ Vibro XL!

Other possibilities:
NFB can be switched 'on', 'off' or 'on with presence' for different kind of dynamics and sounds. It’s possible to switch between catode resistors so you can use either a 6V6 or 6L6 power tube. The tonestack can be switched off for a gain boost in the preamp. Also the 'sound shape' of the tonestack can be changed with a switch giving the amp more mids in the sound.
So it’s not an exact Fender Champ and it's not something new either. Sounds ok though! The 6L6 GC and the other extras is why I added the XL to the name. Amp sounds great with Hytron Jan CHY 6V6 and a Philips 6L6WGB gives a sound somewhere between a 6V6 and 6L6. Great!

The Roommate is a small single ended practise amp. When using the internal speaker you can easily use it at midnight without any trouble. This is a cheap PA speaker, rated 75 W , frequency response until 5 kHz. With a more efficient guitar speaker plugged in, it is loud enough to really enjoy the tube sound. The Roommate consists of two subminiature tubes: a 6112 triode in cascode followed by a 5902 pentode. The amp is switchable between Clean and Crunch. In the Crunch mode the amp becomes a hybride because a 2N5457 FET is used as first stage. With the gain of the FET only being about 4 x, the cascode stage gets driven a little harder and of course the pentode can be driven harder.
Output @ maximum volume is 700 mWatt. No tone control. The Roommate only has a Volume knob.
I recently changed the cascode stage into 2 preamp gain stages. The second half of the 6112 can really be overdriven and the pentode powertube can also be overdriven quite hard. I added just a little bitof feedback to keep the sound tight without loosing 'tube rawness'. Result: with the volume at half this amp sounds already like AC/DC .. Listen to the clip.
This set up gives the amp more character than the cascode to power tube set up.

For the power supply I used a torodial transformer with 3 secondary windings (9V-15V-15V); put those in series and multiplied that to 110V for the 5902 pentode and to around 220V for the 6112 triode cascode stage. The B+ for the FET is taken from the pentode rail and brought down with a RC to around 20V.
The 6.3V for the heaters is taken from the 9V secondary; I use a LM 317 and a transistor and a cap to avoid too much inrush current.
Dimensions (inches): 8.8 (w) x 8.8 (h) x 6.5 (d)