These Akai M8 chassis have been stripped and rebuilt with RH84 circuits AND 12B4 preamps. For the uninitiated, the M8 reel to reel has pretty good parts--a nice PT, a choke in the PS, and OPTs that I think are as good as the much more celebrated Sony 500a transformers. I used solid state rectification, and with that, the PTs do not run hot.
While the RH84 doesn't need a pre, the 12B4 is included here so it can drive sub amps easily (or a DSP or SS woofer amps for biamped OB setups). This 12B4 implementation uses a CCS on the plate and LED bias. I think there is a little cap on the sub output that I used as part of the low pass. Or maybe that is switchable? You'll have to check.
There are parts throughout that are too fancy. You'll notice some nice caps . Film caps for coupling, including a motor run after the pre. Snubbers on the rectifiers, etc. PEC pots for the main volume. All relatively low hours.
These worked last time I used them, but they have a little extra noise (an intermittent crackle--actually very little hum, even on efficient speakers). My theory is that the layout isn't ideal (I'm sure most folks here will agree). I think they'd work better if the two bits are separated. In other words, maybe pull the 12B4 preamps out and put them in a separate chassis with a separate power transformer. Or, at least revise the grounding layout.
This is like two good projects in one, and better than starting with a stock M8. Stripping them down to rebuild took longer than I anticipated as Akai used A LOT of solder. But, I have more projects than time and space, so I think these deserve a new home. They'll come with a full complement of tubes.
I did this awhile ago, so you'll have to figure out how to hook them up and use them (which jack is which, etc). I obviously goofed with the chassis set up a bit to add fuses, switches, jacks, etc as needed. But, I think the preamp circuit is very straight forward (aside from the CCS) and the RH84 follows the schematic.
I'd save these to pilfer for parts, but taking them apart seems sad. $160 +shipping. I can negotiate a bit, especially for a budding DIYer. I don't really need more projects in trade, but I'd consider interesting DACs and streamers.
Paul
While the RH84 doesn't need a pre, the 12B4 is included here so it can drive sub amps easily (or a DSP or SS woofer amps for biamped OB setups). This 12B4 implementation uses a CCS on the plate and LED bias. I think there is a little cap on the sub output that I used as part of the low pass. Or maybe that is switchable? You'll have to check.
There are parts throughout that are too fancy. You'll notice some nice caps . Film caps for coupling, including a motor run after the pre. Snubbers on the rectifiers, etc. PEC pots for the main volume. All relatively low hours.
These worked last time I used them, but they have a little extra noise (an intermittent crackle--actually very little hum, even on efficient speakers). My theory is that the layout isn't ideal (I'm sure most folks here will agree). I think they'd work better if the two bits are separated. In other words, maybe pull the 12B4 preamps out and put them in a separate chassis with a separate power transformer. Or, at least revise the grounding layout.
This is like two good projects in one, and better than starting with a stock M8. Stripping them down to rebuild took longer than I anticipated as Akai used A LOT of solder. But, I have more projects than time and space, so I think these deserve a new home. They'll come with a full complement of tubes.
I did this awhile ago, so you'll have to figure out how to hook them up and use them (which jack is which, etc). I obviously goofed with the chassis set up a bit to add fuses, switches, jacks, etc as needed. But, I think the preamp circuit is very straight forward (aside from the CCS) and the RH84 follows the schematic.
I'd save these to pilfer for parts, but taking them apart seems sad. $160 +shipping. I can negotiate a bit, especially for a budding DIYer. I don't really need more projects in trade, but I'd consider interesting DACs and streamers.
Paul
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