Two Stroke

Two Stroke is a double single ended (DSE) tube amp designed by Dave Hunter. I bought Dave Hunter's book 'The Guitar Amp Handbook' in December 2005 and got excited by the DIY amp project presented by the book. Because i have been tweaking my tube amps already for some time, the idea to make my own amp have been crossing my mind a lot lately.

Two Stroke specs:

-one ECC83/12AX7 preamp tube

-3 different preamp voicing options

-volume and tone control

-two cathode biased power amp tubes (Class A), possible configurations: 6K6, 6V6, 6L6, EL34, KT66, KT77, 6550, KT88

-5Y3 tube rectifier

I am seriously considering the idea to be able to use two higher power power tubes (6L6, EL34, KT66, KT77, 6550, KT88) at the same time. The original idea in the Two Stroke was to use only one higher power power tube (max of about 10 watts). I upgraded the output transformer in my part list to 20 watts but i am still uncertain if the amp requires more modifications (the power transformer i had in my part list was already oversized in power compared to original specs. The upgraded specs are 230V//325-0-55-325V 0,180A / 6,3V 5A / 5V 3A). With 20 watts of output option it sure would be a bit more versatile (like it wasn't versatile enough with the different power tube options and all).

16.2.2006

Today my amp project finally moved forward again. The amp's cabinet and speaker arrived from Weber VST. The cabinet (solid pine) is reproduction of the old tweed Fender Deluxe amp and for the speaker i chose 12" Weber AlNiCo Silver Bell to seek out some 60's Marshall tones.

Also today i ordered almost all the rest of the components (from Uraltone). I wanted to see the cabinet first before ordering the amp chassis just to make sure it'll fit in. I hope i will get all the other parts next week. At this point i could not finance all the possible power tube configurations (so far i have already been buying parts that are not much compromise in quality particularly). For power amp i ordered pair of Shuguang KT66s. I already have JJ 6L6GCs and NOS Philips Miniwatt EL34s. For preamp i ordered Ei 12AX7EG Elite. In my preamp tube shelf i already have a JJ ECC83S and a NOS Tungsram ECC83. For the rectifier i ordered a NOS RCA 5Y3GT.

The cab's dimensions are 51 cm (W) x 41 cm (H) x 23 cm (D)

18.3.2006

Tadaa, the amp is functioning and sounds good!

My intention was to write something everyday when i was building the amp but... i was too lazy.

The building process went quite smoothly (only one struggle ;) forward every weekend that i was working on the amp. As described earlier i didn't have a kit as i bought my parts from different places. The only real modifications to the part list i made were the transformers (as OT i used Hammond 125FSE and as PT i used UralTone 230V//325-0-55-325V 0,180A / 6,3V 5A / 5V 3A transformer). I also had to accept some minor component changes as i could not find the exact values. These small differencies should not have significant effect to the tone of the amp.

I made my own turret board and drilled the chassis. Soldered the components and wires according to the schematic. It all took couple of weekends to complete.

Then... what's that buzz?! The amp was working otherwise fine, the voltages were correct so i assumed i had the components in right places of the circuit. I was thinking the problem was a grounding problem and i called to Wahvarikellari's Jukka Mönkkönen and he was suspecting a grounding problem too. He gave me a couple of ideas where to start to troubleshoot. I spend quite many hours to try different grounding solutions. Nothing seemed to help. I recently bought Gerald Weber's book 'Tube amp talk for the guitarist and tech' and i started to look for some ideas from the book. At same time i was re-reading the instructions from Dave Hunter's book just to check if i had paid attention to everything. I was later to discover i actually didn't do one important thing. I noticed Dave used Ted Weber VST power transformer in his amp and the 6,3 V heater supply was center tapped while my transformers heater supply is NOT (the heater supply is grounded with the center tap). I didn't immediately see this difference as a problem (i'm not a tech) but then i started to go through the several troubleshooting concepts suggested by the Gerald Weber's book. Soon i tried grounding the heater supply without the help of the center tap and ZAP! Heh... no more buzz!

The artificial center tap: You can make an artificial center tap by connecting a 100 ohm (½ W) resistor from one side of the heater supply to ground and then another 100 ohm (½ W) resistor from the other side of the heater supply to ground. I used 4 W types because i didn't have lower wattage ones. However i will change them lower wattage ones later as they are supposed to act as fuses if the plate shorts to heater, this way burning the resistor and saving the PT. Gerald Weber also describes more advanced modification to the heater supply by connecting the other side of the heater supply to positive bias supply (and getting rid of the AC hum too). I want to try that later.

21.3.2006

Tone update. I put in two Shuguang KT66 tubes and yes these tubes seem to work quite well in this amp (too), at least for me. After putting two big tubes the plate voltage dropped quite a bit, from 340 V to 290 V. I was thinking to lift it back up and i changed the tube rectifier to 5AR4. The voltages went back up to 360 V and i had to change the cathode resistors to 680 ohms (5W) because the KT66s were running too hot. After these changes the amp got more power but the sound wasn't so nice to my ears as it was with lower plate voltage. So i changed it back. I also realized i made a small mistake designing the tube positions in the chassis. I positioned the second power tube socket too close to the speaker magnet. I had to remove the magnet cover of the Silver Bell to make room for the other KT66 as the KT66s are quite big bottles.

23.3.2006

More upgrades to the amp. I tried again the 5AR4 as rectifier tube. It was actually very good tone also (with two KT66s). Tighter and more top end. The plate voltage went up to 380 V (it went to 360 V when i tried it before, i don't know why the difference but it's not significant). Last time i had to change the cathode resistors to 680 ohms (5W) which i think was still a bit too hot for the KT66s. I decided to make the cathode resistors switchable. I put a switch to change between 470 ohms (5W) and 820 ohms (5W) (i calculated 820 ohms would be suitable, i don't have the resistors yet so i can't test the value). I also changed the cathode resistor by-pass capacitors to 33 uF/63 V. The ones i had there were 25 V types which were not enough for higher voltages. The capacitance value also increased a bit (from 22uF) which in theory should bring some more low end but i didn't hear much difference.

I ordered a choke for the amp. The increased wattage brings up the 60 Hz hum a bit too loud (it can easily start to feedback when volume is turned up). To install the choke i need another big filter capacitor. I ordered a can capacitor (50 uF + 50 uF/500 V) to replace the single 40 uF/500 V one in the amp. There's not much room left inside the chassis so i am gonna install the can in Marshall style to the transformer side of the chassis.

31.3.2006

I decided to take away the cathode resistor switch because i noticed the switch made the tone worse. Maybe i had a bad quality switch or a switch is not a good idea in anyway. Either way i took the switch away but left the other pair of cathode resistors (820 ohms/4 W) in place in case i need some more power someday (to get more power it requires to change also the rectifier tube to 5AR4). So far the amp is plenty of loud for home use.

I changed the cathode resistor bypass capacitor value back to 22 uF but increased the voltage value to 63 V. Also changed the heater supply's 100 ohm grounding resistors to ½ W type as i earlier planned.

Okay, i installed the choke (5 H/200 mA) and changed the single 40 uF/500 V filter capacitor to can type cap which is two 50 uF/500 V caps in one can. The 60 Hz hum level dropped significantly. I don't think i have a reason to try the heater supply modification anymore.

I feel the amp is quite finished so far. But you'll never know what kind of "upgrades" i decide to make to the amp later. Oh yeah... the control panel is still missing. Anyway at this point i would like to thank Dave Hunter for his help during project. Great amp!

2.6.2006

Tone update. I bought a pair of SED 6550C tubes. I was thinking to use them both but it seems that Two Stroke can't take them both. I tried but i heard some nasty sharp distortion when i hit the strings harder. I don't know what caused it. However with just one 6550 the amp screams some cool hard rock tones. Compared to sweeter tone of Shuguang KT66, SED 6550C has harder and bolder tone. More complex when distorted but not so sweet clean as Shuguang KT66.