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Hi all, well I'm on to my next project. This time I have picked up a National RS-772 R2R machine. It is stereo, has two inbuilt speakers and when I plug in my guitar it works. But it's rather quiet and the controls are fiddly and it only works in one channel (as input is only to one channel). Inside, apart from all the mechanicals for the tape there are pairs of 16A8, EF86 and 12AX7. The EF86 are heavily shielded and I suspect they are for the recording section (the cabling is all bunched together very tightly making it very difficult to trace).
My idea is to remove all the mechanicals and make a push pull combo. I quite like the idea of having an EF86 preamp stage (just because it's different), and I think that the 16A8's may need to go because they're a bit low on oomph. So, I could make a pre-amp that's EF86-EF86-12AX7a-12AX7b-tone stack-12AX7a-12AX7(PI) and then a power stage of 2*6BW6 or 2*6P14P. Or I could split the input signal and have two SE channels (a bit pointless, I think). Or loads of other combinations, including alternating between 12AX7 and EF86 stages.
Questions! Is there such a thing as too much pre-amp gain if one is not a screaming metal-head?
If I want to use the 16V heater tap for the replacement power valves, how do I work out the appropriate value of resistor so that if I series the two 6.3v heaters I drop the remaining 3.4V to ground? Is it a case of knowing that each heater takes, say .3A, that the the current across the resistor is thus .6A and therefore requires 3.4/.6=5.6R @ .6^2*5.6=2.016W (5W to be safe)?
Would it be feasible to have switchable preamp stages? I'm thinking perhaps have an EF86 preamp and a 12AX7 preamp, both going into the tone stack and hence the recovery stage, PI and power stage. If I pursue this path, would it be better to keep the inputs separate? Could be cool for two guitars together.
Without a stereo output guitar I doubt there's any point in keeping this thing as stereo - does anyone disagree?
Another way could be to save the EF86 for another project and use three halves of 12AX7 for preamp duties, half for PI and have two pairs of 6BW6 or 6P14P for output. The electric motor is powered off one of the primary taps (115V) so by disconnecting the motor would I have plenty of leftover power for the extra output valves?
Any other thoughts?
Thanks all.
My idea is to remove all the mechanicals and make a push pull combo. I quite like the idea of having an EF86 preamp stage (just because it's different), and I think that the 16A8's may need to go because they're a bit low on oomph. So, I could make a pre-amp that's EF86-EF86-12AX7a-12AX7b-tone stack-12AX7a-12AX7(PI) and then a power stage of 2*6BW6 or 2*6P14P. Or I could split the input signal and have two SE channels (a bit pointless, I think). Or loads of other combinations, including alternating between 12AX7 and EF86 stages.
Questions! Is there such a thing as too much pre-amp gain if one is not a screaming metal-head?
If I want to use the 16V heater tap for the replacement power valves, how do I work out the appropriate value of resistor so that if I series the two 6.3v heaters I drop the remaining 3.4V to ground? Is it a case of knowing that each heater takes, say .3A, that the the current across the resistor is thus .6A and therefore requires 3.4/.6=5.6R @ .6^2*5.6=2.016W (5W to be safe)?
Would it be feasible to have switchable preamp stages? I'm thinking perhaps have an EF86 preamp and a 12AX7 preamp, both going into the tone stack and hence the recovery stage, PI and power stage. If I pursue this path, would it be better to keep the inputs separate? Could be cool for two guitars together.
Without a stereo output guitar I doubt there's any point in keeping this thing as stereo - does anyone disagree?
Another way could be to save the EF86 for another project and use three halves of 12AX7 for preamp duties, half for PI and have two pairs of 6BW6 or 6P14P for output. The electric motor is powered off one of the primary taps (115V) so by disconnecting the motor would I have plenty of leftover power for the extra output valves?
Any other thoughts?
Thanks all.