Hey everyone! I've been using this site as a resource for a while and recently rebuilt a Grommes LJ7 Mono amp that I got cheap from craigslist. This inspired me to acquire an Akai 1700 Reel-to-Reel player/recorder for its internal tube amp. This was also a very cheap craigslist find as the tape mechanism wasn't working. I know it won't be the most high-end amp but I'm trying to learn more about audio circuits and it was just so cheap to pick up.
I have been studying through the schematic in the user manual (the craigslist seller emailed it to me, so finding that was easy). The amp contains a single 12AU7 and a single 6BM8 tube per channel. The 12AU7 is acting as a mic pre-amp and the 6BM8 as the output tube. I have been able to hook my turntable outputs directly into the playback/record head jacks and the amp puts out plenty of sound when hooked up to a pair of little fostex 4" full ranges.
I would love some advice on converting the mic preamp stage to a phono input by adding an RIAA circuit. I have been reading mixed reviews about a 12AU7 preamp circuit but I would like to be able to keep original tubes in here as they are all working great. It would also be nice to strip out anything related to the record/erase functions and to swap the giant multi-layered play/record switch for a couple more sensible toggles perhaps. I am hoping to end up with a phono input and an aux input that would bypass the phono stage.
In terms of chassis layout and overall look I am liking the current vibe. The power transformer has an interesting look with lots of dangerous exposed connections as this amp previously lived inside the reel-to-reel enclosure. I will likely remove the input voltage selector and hardwire it for US voltage. I will probably relocate the fuse under the chassis, and then build a cover for the power transformer. There are some 1/4" input jacks on the front for mic inputs which I will either use as locations for toggle switches or just leave for looks with no connections hooked up. There are plenty of jacks on the back to re-purpose or replace as phono input and aux input. The original power/tone knobs are great looking and I kept the faceplate from the original reel-to-reel to cut down and put back on with some adjustments.
Any help is appreciated. This will be the first tube project where I am attempting to change anything about the circuit rather than just replacing old components to get the amp back to working.
I have been studying through the schematic in the user manual (the craigslist seller emailed it to me, so finding that was easy). The amp contains a single 12AU7 and a single 6BM8 tube per channel. The 12AU7 is acting as a mic pre-amp and the 6BM8 as the output tube. I have been able to hook my turntable outputs directly into the playback/record head jacks and the amp puts out plenty of sound when hooked up to a pair of little fostex 4" full ranges.
I would love some advice on converting the mic preamp stage to a phono input by adding an RIAA circuit. I have been reading mixed reviews about a 12AU7 preamp circuit but I would like to be able to keep original tubes in here as they are all working great. It would also be nice to strip out anything related to the record/erase functions and to swap the giant multi-layered play/record switch for a couple more sensible toggles perhaps. I am hoping to end up with a phono input and an aux input that would bypass the phono stage.
In terms of chassis layout and overall look I am liking the current vibe. The power transformer has an interesting look with lots of dangerous exposed connections as this amp previously lived inside the reel-to-reel enclosure. I will likely remove the input voltage selector and hardwire it for US voltage. I will probably relocate the fuse under the chassis, and then build a cover for the power transformer. There are some 1/4" input jacks on the front for mic inputs which I will either use as locations for toggle switches or just leave for looks with no connections hooked up. There are plenty of jacks on the back to re-purpose or replace as phono input and aux input. The original power/tone knobs are great looking and I kept the faceplate from the original reel-to-reel to cut down and put back on with some adjustments.
Any help is appreciated. This will be the first tube project where I am attempting to change anything about the circuit rather than just replacing old components to get the amp back to working.
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According to the schematic, that's not a 12AU7, it's a 12AT7. Like a magnetic phono cart., the signal from a tape playback head is low level and requires equalization. The curve for tape is NAB, as opposed to the RIAA curve for records.
Reworking the EQ network might get the job done for records. Somebody more expert than I will have to help you do that. No matter what, you need to construct a DC heater supply for that low level circuitry and the OEM power trafo is not up to the task.
If you're willing to switch to 12AX7s, the uploaded setup will work well for you and it can drive an external device, with aplomb.
Reworking the EQ network might get the job done for records. Somebody more expert than I will have to help you do that. No matter what, you need to construct a DC heater supply for that low level circuitry and the OEM power trafo is not up to the task.
If you're willing to switch to 12AX7s, the uploaded setup will work well for you and it can drive an external device, with aplomb.
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Eli- Right you are about the tubes. They are indeed 12AT7s. I have 12AU7s in one of my amps and 12AX7s in the other. I totally had those switched in my head.
Can you explain what it is about the OEM power transformer that won't work? It has been driving these same tubes already. Sorry this may be obvious but I am a bit of a noob.
Can you explain what it is about the OEM power transformer that won't work? It has been driving these same tubes already. Sorry this may be obvious but I am a bit of a noob.
Thanks for the input Ketje!
I do have a more readable version of the schematic but it was too big to attach to this thread. Is that capacitor all I would need to add to get close to an RIAA equalization?
I do have a more readable version of the schematic but it was too big to attach to this thread. Is that capacitor all I would need to add to get close to an RIAA equalization?
After you remove all that extra stuff, insert a passive RIAA network after the first stage's output
coupling capacitor, in between the first two stages. Make sure there is a 1M resistor to ground before R1.
Choose around 220k for R1 in the calculator here: mh-audio.nl - Phono
Leave the power supply as-is to start. Unless there's too much hum for you, it will work.
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Save it as a greyscale gif with a low-bit count… i shrunk your original posted schema from ~200k to <70k… or send it to me if you don’t have the tools.
dave
This is much better to read
The first stage is an HF-oscillator at record time and the pre-amp for play-back.
Little change to make it a better one with more reliable Zout (~33k//100k).
There is allready a low frequency correction for play-back.Changed the capacitor for RIAA and one extra for high side correction.
Input 47k for MM and the 100pF can probably stay there.
It's a minimal solution but after all the hole thing is not hi-fi, so why bother 😀
Mona
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Thanks for the input! I have ordered up some parts for this and another project so hopefully I'll be able to post some progress on this thing soon.
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