My latest amp build was a Ted Weber kit, the 6100, which is modeled after an early 70s Orange OR120. All of Weber's kits are modeled pretty closely to the original amp but with some changes to components values as well as modified layouts to accommodate current production chassis. Like with any of my previous builds, I decided that I was not content the stock circuit so I decided to add another gain stage onto the front of the amp and use a Marshall 2203 style Hi/Lo input jack setup where the low input jack bypasses the extra gain stage for the stock amplifier sound. The goal of adding the extra stage was to get an extremely saturated sludgy/doom tone.
Adding another gain stage to any amplifier is not as simple as it sounds. At minimum half of a triode (usually 12ax7) is needed in order to create a generic gain stage. If an amp has no free triodes available then a tube socket needs to be drilled into the chassis, heater filament wires need to wired to the new socket and new triode is required. A 12ax7 requires 300mA of heater current which usually is not much strain on a power transformer but needs to be taken into account. The amount of gain stages which use a power supply node should normally not exceed 2 so additional power stage decoupling made be needed. Placement, wiring, and layout of the new gain stages components is even more critical. A badly designed layout can be prone to noise and parasitic oscillation, a condition where there is an internal positive feedback loop. All of these problems were complicted by the fact that the chassis used for the 6100 kit is normally used for 50W amps so space is a premium.
The 6100 schematic shows two 12ax7s, the first is used for two cascaded gain stages and the 2nd is for a cathodyne phase inverter. Luckily I did not need to decouple the power supply because the the last supply node is only used by the first gain stage. Due to the space limitations I decided to raise up the main eyelet board so that a pair of terminal strips could be place under the board. There was not enough eyelets on the main board so the terminal strips were required to mount the plate resistor, coupling cap, cathode cap, and cathode resistor for the extra gain stage. The picture below shows the terminal strips and the extra stage wiring.
I designed the extra gain stage so that I would not waste the other half of the 12ax7 which would be unused if I had built a generic gain stage. 12ax7s and most other 9 pin triode tubes contain two identical halves which can both be used for a gain stage. Thus each 12ax7 can be used to create 2 gain stages. For my design I implemented a parallel input stage where the two halves of the 12ax7 are wired in parallel with the plates, grids, and cathodes tied together. The schematic for this type of stage is shown below.
In this design, the plate and cathode resistor values are halved as they are pulling twice the current. The grid resistor can be kept the same as there is negligible grid current. The gain of this design is equal to the sum of the gains of each half triode and the output impedance is halved.
In addition to the extra gain stage, I also heavily modified the suggested layout as per Ted Weber's site. In my opinion, the suggested preamp wiring and component layout would have been extremely prone to parasitic oscillation with the addition of an extra gain stage. There are several capacitors which are closely located and several long wire runes from the Presence control back to the negative feedback circuit. Along with the modified layout, I utilized extensive shielded cable long signal runs to reduce the chances of capacitive coupling and extraneous noise. Lastly I replaced the stock power switch with a two position keylock switch. In the first position, the power light is turned on and in the second position the heaters are turned on. The picture below shows the keylockswitch installed.
The completed amp took a little over 1 month to wire and power up. After powering up I found I had significant blocking distortion and high frequency parasitic oscillation when plugged into the Hi Jack with the gain 3/4 full. When plugged into the Low Jack, there was no blocking distortion or high frequency. The problem was isolated to the extra gain stage. I removed the cathode capacitor from the extra stage which lowered the gain by about 10db but effectively removed the blocking distortion and high frequency parasitic oscillation. The amp still has immense amounts of gain on tap so little was lost from the removal of the cathode capacitor. Below you will see a picture of the nearly complete wiring job. For those interested in purchasing one/having one built, email me or comment and we can chat.
The amp sounds extremely heavy and very thick. Rotating the Drive rotary knob over to the far left is pure bliss. Very bass heavy and chuggy. Here is a video I made with a camera mounted on my desk.
Adding another gain stage to any amplifier is not as simple as it sounds. At minimum half of a triode (usually 12ax7) is needed in order to create a generic gain stage. If an amp has no free triodes available then a tube socket needs to be drilled into the chassis, heater filament wires need to wired to the new socket and new triode is required. A 12ax7 requires 300mA of heater current which usually is not much strain on a power transformer but needs to be taken into account. The amount of gain stages which use a power supply node should normally not exceed 2 so additional power stage decoupling made be needed. Placement, wiring, and layout of the new gain stages components is even more critical. A badly designed layout can be prone to noise and parasitic oscillation, a condition where there is an internal positive feedback loop. All of these problems were complicted by the fact that the chassis used for the 6100 kit is normally used for 50W amps so space is a premium.
The 6100 schematic shows two 12ax7s, the first is used for two cascaded gain stages and the 2nd is for a cathodyne phase inverter. Luckily I did not need to decouple the power supply because the the last supply node is only used by the first gain stage. Due to the space limitations I decided to raise up the main eyelet board so that a pair of terminal strips could be place under the board. There was not enough eyelets on the main board so the terminal strips were required to mount the plate resistor, coupling cap, cathode cap, and cathode resistor for the extra gain stage. The picture below shows the terminal strips and the extra stage wiring.
I designed the extra gain stage so that I would not waste the other half of the 12ax7 which would be unused if I had built a generic gain stage. 12ax7s and most other 9 pin triode tubes contain two identical halves which can both be used for a gain stage. Thus each 12ax7 can be used to create 2 gain stages. For my design I implemented a parallel input stage where the two halves of the 12ax7 are wired in parallel with the plates, grids, and cathodes tied together. The schematic for this type of stage is shown below.
In this design, the plate and cathode resistor values are halved as they are pulling twice the current. The grid resistor can be kept the same as there is negligible grid current. The gain of this design is equal to the sum of the gains of each half triode and the output impedance is halved.
In addition to the extra gain stage, I also heavily modified the suggested layout as per Ted Weber's site. In my opinion, the suggested preamp wiring and component layout would have been extremely prone to parasitic oscillation with the addition of an extra gain stage. There are several capacitors which are closely located and several long wire runes from the Presence control back to the negative feedback circuit. Along with the modified layout, I utilized extensive shielded cable long signal runs to reduce the chances of capacitive coupling and extraneous noise. Lastly I replaced the stock power switch with a two position keylock switch. In the first position, the power light is turned on and in the second position the heaters are turned on. The picture below shows the keylockswitch installed.
The completed amp took a little over 1 month to wire and power up. After powering up I found I had significant blocking distortion and high frequency parasitic oscillation when plugged into the Hi Jack with the gain 3/4 full. When plugged into the Low Jack, there was no blocking distortion or high frequency. The problem was isolated to the extra gain stage. I removed the cathode capacitor from the extra stage which lowered the gain by about 10db but effectively removed the blocking distortion and high frequency parasitic oscillation. The amp still has immense amounts of gain on tap so little was lost from the removal of the cathode capacitor. Below you will see a picture of the nearly complete wiring job. For those interested in purchasing one/having one built, email me or comment and we can chat.
The amp sounds extremely heavy and very thick. Rotating the Drive rotary knob over to the far left is pure bliss. Very bass heavy and chuggy. Here is a video I made with a camera mounted on my desk.
Hey Nicholas,
ReplyDeleteAny chance you could revive the images? I'm interested in adding a gain stage to my Orange OR80 so I'd really like to know what values you used around the added tube.
Kind regards,
Harold