Micro-Documentary of Roberts 720 analog Vacuum Tube Reel-to-Reel tape recorder (like Akai M-7).

This micro-documentary shows the Roberts 720, an analog reel-to-reel tape recorder that has vacuum tube electronics. It is similar to the Akai M-7, but it lacks the Akai's cross-field bias system. Compared to the Akai M-8 that I previously owned, the Roberts 720 is noticeably quieter, exhibits a greater sense of detail, especially in the midrange; and has lower overall THD, especially at 1-7/8 ips.

The Roberts 720's level meters are superior to the Akai and have ballistics similar to Ampex professional tape decks like the 602. Therefore, recording levels can be set more precisely. The head block is better shielded, and the wiring harness layout is cleaner and better routed.

The Roberts 720's EF86/6267 input pentode is fitted with a dampener, whereas the Akai has no provision to guard its input pentode against microphonics. This may contribute to the Roberts' superior detail.

The Roberts 720 speakers are side-firing and give some sense of spatial spread, also providing better bass extension without losing highs, which are linear to 12 Kcps.

Oddly, the Roberts' motor runs all the time the deck is plugged into the mains, even if the electronics are switched off. A separate head ouptut is provided for each channel, allowing the tape drive to be used with outboard playback electronics, which can be a great advantage for archival playback of low-speed tapes.

Overall, the Roberts' electronics are simpler than the Akai M-8's. This may explain its greater sense of detail overall.

The Roberts 720 can be successfully rebiased (with a simple change of one resistor) to take full advantage of modern tape formulations, whereas the Akai M-8 cannot provide enough bias current for modern tape formulations. Apparently, the Akai crossfield bias system limits the bias flux that can be generated on tape, limiting the deck to use of older "low-noise" tape formulas, like Scotch 111.

Category:

Music

Tags:

• analog
• tape recorder
• reel-to-reel
• r2r
• Roberts 720
• Akai M-7
• M7
• vacuum tube
• electronics
• guitar amplifier
• audiophile
• sound-with-sound
• EF86
• 6267
• pentode
• stereo
• monophonic
• speakers

[Thermionicist]

satcom231, Congratulations on getting your M6 running! You're lucky to have the 15 ips sleeve! I lost mine when I moved. The motor run capacitor I used for the Roberts 720/Akai M7 was a 2.2 uF 350 WVDC type plus two 0.47 uF 350 WVDC for the speed select capacitors.

[Thermionicist]

Roberts model numbers are confusing. The 720 is similar to the Akai M-7, but it doesn't have the M-7's Crossfield heads. The 720A = an Akai M-8, and it does have the Crossfield Heads. Generally, using vintage tape, a Roberts 720 is 5 dB noisier & has slightly less high-end compared to the 720A, Some say the 720 has smoother midrange than the 720A. However, a 720 can be easily modified to take full advantage of modern tape but the 720A can't. A modified 720 can outperform a 720A. I do these mods

[Thermionicist]

Thanks for the nice comment! Based on your description my guess is that your recorder has one of the following problems: 1) The heads need to be demagnetized. 2) The bias oscillator circuit is defective. 3) The bias is incorrectly calibrated. 4) The heads are excessively worn. 5) You're trying to record onto chrome tape and the deck can record only on normal bias tape. So, I don't think changing a resistor will help but you could have a defective resistor. Let me know if I can help further.

[satcom231]

I got The M6 and cleaned it up. All tubes lit up. Ran it without a tape. But after about 10 minutes the motor slowed down to a stop. It was hot to the touch. Turned it off and have to put a new run motor cap on it. I'm going to use a 2uf 350v cap that is used on a fan motor. Everything else looks good. I also have the 15ips sleeve. What kind of motor run cap did you use?

[caseyhconnor]

Hi - what's the difference between the 720 and 720A? Can't seem to dig it up... Thanks!

[satcom231]

I have an M10. Refurbished it to the extent of all switches cleaned and lubed as well as all mechanicals. It does need new caps for the motor though. It plays and sounds wonderful. I am getting an M6 or M7 that comes with the external speakers. Can't wait to dig into that one.

[donnscottshetler]

I have the earlier model of this deck I believe it does sound on sound but uses push buttons and is stereo with a single vu meter and a switch as you mentioned it is great for warming tracks thanx for such great footage

[monomachine]

i just bought one of these off ebay. i hope itturns out to be whatim thinking, and wont need much fixing.

[CassetteMaster]

Very nice deck! And god sound quality. Can even a cheap DC biased cassette machine my improved with sound by changing a certain resistor? Because I have a "shoebox" cassette recorder with VERY distorted highs in its recording.