I recently completed a build based on the TAD 18-watt TMB kit. (See schematic below.)
Deviations from the schematic:
- 470R / 3W screen grid resistors (in place of 100R / 2W)
- 150R / 5W cathode bias resistor (in place of 120R / 5W)
The above changes were in response to the amp initially running the EL84s wildly hot. Not quite red plating, but sizzling. In the current state, with the above resistor values, the plate voltage on the EL84s is ~354 VDC. The bias readings were an average of 42 mA, which seems to be around 125% of their plate dissipation. The power tubes are now running okay, no visual or other indicators otherwise. But of course, they are still running hot. (That might just be how this amp is supposed to be, I don’t know. But it’s worth noting our house notoriously has high wall voltage.)
The only other deviations from the TAD kit were as follows:
- Added a 47 pF cap across the two plate load resistors for the Phase Inverter
- Used a ground buss bar on the turret board instead of two buss bars across the back of pots
- Heater CT is “elevated” to cathode bypass cap of output tubes
- Mostly using metal film resistors (1 watt) for any resistors 100k or above and not in power section, with a few carbon film for the lower resistance values
- Using shielded cable (Mogami W2333) from the two inputs to grid, as well from the Volume pot (TMB channel) to V3 (cathode follower)
In its current state, the amp really does sound great, especially when you crank it. No hum or buzz, so I think grounding seems okay. TMB channel has a LOT of gain, especially on High input.
Potential issues/concerns:
- There’s some crosstalk between the channels, but I think that’s a “thing” with these?
- There’s a hiss from the speaker (whether a guitar is plugged in or not), made louder by turning up the TMB channel pots, especially treble and volume. Both High and Low inputs.
Everything about the amp when turned up and cooking I love. However, I do also like to use the Low inputs at lower volumes because I think the clean tone is fantastic. However, the amount of hiss you can get is a bit annoying. If I turn all of the pots down on the channel I’m not using, it can help, but the TMB channel is always hissy no matter what (that seems to be the main “source” of the hiss).
I’d love to find a way to reduce this hiss if possible, without dramatically killing the soul of the amp. Thoughts?
EDIT: I guess I should note I’m currently using long plate Mullards in all three preamp slots, a pair of Amperex EL84s, and a Mullard 6CA4 rectifier.
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