Overview
The “Hami18” got its start as an Hammond AO-39 organ amp. It was transformed into a Marshall 18w Lite amp head. A priority was to keep as much of the original look as possible but make a functional guitar amp “head”. The transformers and tubes and obviously chassis were retained. But an additional tube needed to be added for the phase inverter to drive the push pull 6BQ5’s (same as EL84). In addition, a speaker output jack, guitar input jack, a volume pot and a tone pot, AC connector, on/off switch and a LED pilot light were added.
Original Design
The Hammond A-100 organ included a power amplifier known as the Hammond AO-39. Its chassis is shown in the pictures below in the lower right section of the rear cabinet. The Power Amp is a perfect candidate for a guitar amplifier and that is the subject of this essay.
Original Amp
The original amp was described as a power amp. The amp is a push pull design using two 6BQ5 power tubes driven by a 12AX7 preamp tube. The phase inversion for the push pull is accomplished on a separate chassis using a transformer. AC power and speaker out are received by a 8 pin octal cable, that connects to the preceding stage likely including a preamp, but not available here. The chassis was extremely clean except for the output transformer which is not original, but replaced years ago with a very equivalent design except the mounting of the replacement transformer (the original transformer stood on end). Lots of space inside the cabinet to add the guitar preamp. As well the filter caps in the can were out of spec and likely dried and in need of replacement.
Original Hammond AO39 schematic is shown below. The original power supply is solid and provides good DC voltages so it will be retained, except the filter caps have been replaced.
Chassis Modification
Cleaned up as good as possible without repainting
The original extra speaker jack was replaced with a 9 pin tube socket for the preamp tube. As well a guitar input jack and LED pilot light are added.
The new speaker jack is located on the back panel close to the output transformer
Adding New Filter Caps
The original Mallory filter can caps had fallen out of spec and were likely noisy. So these are replaced with modern caps inside the chassis. Plenty of room around the original filter can, An additional power node was added to power the preamp tube. The original filter can is left in place on the top chassis for good aesthetics. The new cap on the upper left side (positive terminal black wire) is not a filter cap but used for power tube cathode bypass as in the original design. All new caps are held in place with double sided tape. Although once wired in place they do not need the tape.
As built Hami18 schematic below
PreAmp
The preamp tube is the 12AX7 which is a dual triode. The two triodes are wired in parallel providing as much gain in the first starge as possible. This is how the Marshall 18w Lite design provides.
Phase Inverter
The preamp tube from the Marshall 18w Lite design is to use the 12AX7 for Phase Inverter. But after being wired up it turned out that the gain for this stage was too high so a 12AU7 was used in its place.
Speaker Output
The output transformer had a strange grounded center tap on the secondary. The impedance measured at 10k:4 ohm across the coil. This is a good load for the 6BQ5 pair, which by spec wants a 9k load.
The final circuit design and layout includes some updates from the original Leslie schematic to improve tone and operation. First the screens of the power tubes were actually slightly higher voltage than the plates due to the resistive loss of the OT primary, so a dropping resistor was added off the screen power node. There were not any grid stopper resistors on the power tubes; it was decided to add 3k resistors to help tame some of the high frequencies. But these were left out of the final design as the original amp did not have these and the frequency change was imperceptible.
One addition was to increase the first power supply dropping resistor from 10 to 25ohm (25w) in order to reduce the power tube voltage closer to 400v (from 450). This allowed the original GE 5881 Tung Sol tubes that came with the unit to be used. Always “keep it as original as possible” is a good motto.
Design Cabinet
The challenge was to figure handles that would not interfere with the side mounted AC cable and switch while having a comfortable and solid handle for movement. It was decided to fabricate an arch type of handle that was high enough to provide a comfortable hand grip. A board was sized to fit the amp and the handles were mounted. The wood was stained and coated with a durable clearcoat.
