Roberts 770X R2R Mono Blocks

Rex Everything

Rex Everything

are you Shpongled?
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I have two new DIY tube projects coming up and was encouraged by Jaymanaa to place the posts in this section with updates for all to see. Jay has volunteered to be my mentor on this but all input will be appreciated.

These two mono blocks came from a Roberts 770X R2R that was not in great shape. The amps work fine and actually sound pretty good when a guitar was run through the unit. Anyway it looks like these will need some time and modding to get them to audio specs.

The left unit is running one each of the following tubes

6267
12AD7
6X4 rectifier
6BQ5/EL84
6AR5

The right has the same tubes minus the 6AR5

Here are pics of the right amp:

r2r1r.jpg


r2r2r.jpg


r2r3r.jpg


and the left:

r2r1L.jpg


r2r2L.jpg


bottom of right:
r2r3L.jpg


r2r4L.jpg


Does anyone know if these meters can be used to monitor power output?

All thoughts and help will be appreciated. Here's to hoping this will be a fun and fruitful project.
 
Those look very promising, especially the SE EL84 with what looks to be a pretty hefty output TX. I'll certainly help where I can, but hopefully some of the guys who "really" know what they're doing will chime in and we will both learn some stuff. :yes: I bet the VUs could be used for power out and it will just be a matter of figuring out what to feed them and how (resistor values and such). Have you been able to come up with a schematic, and if not, could you post the capacitor values? That way I can start scrounging around here for tubes and caps, since I reckon you will bring them when you come up early for the KC Fest. :thmbsp: I've got a pair of Ampex R2R monos that I've been saving for a similar project, so hopefully I'll get some ideas from your project. Like I said, I think the first step would be to get a thorough parts list going including things like wood, hardware, paint etc.....
 
jaymanaa said:
Those look very promising, especially the SE EL84 with what looks to be a pretty hefty output TX. I'll certainly help where I can, but hopefully some of the guys who "really" know what they're doing will chime in and we will both learn some stuff. :yes: I bet the VUs could be used for power out and it will just be a matter of figuring out what to feed them and how (resistor values and such). Have you been able to come up with a schematic, and if not, could you post the capacitor values? That way I can start scrounging around here for tubes and caps, since I reckon you will bring them when you come up early for the KC Fest. :thmbsp: I've got a pair of Ampex R2R monos that I've been saving for a similar project, so hopefully I'll get some ideas from your project. Like I said, I think the first step would be to get a thorough parts list going including things like wood, hardware, paint etc.....
Click to expand...

I'll PM ya or call ya on the parts list as I'm sure I'll need some directions. I'm gonna look for a schematic. From looking at other posts the 720 and the Akai M-8 are more or less the same units. I wonder if this 770 is the same as the 720 as far as the amps go as I know the Akai m-8 schems are out there.

I'm guessing most of the stuff seen in these amps are not necessarily needed as they are functions for the R2R. Hopefully a lot of it can be removed to give the amp a cleaner appearance and make it easier to work on/mod.
 
Rex,

The M-8 and the 770X are identical. I have the same set of amps, waiting on me to have time to play with them.

Here's a link to a site that not only has the schematics, but a schematic on how to easily mod them into stand-alone amps:

http://www.beyondsanityproductions.com/

Scroll all the way down to the bottom. Lotsa info, including the original M-8 service manual.

Have fun!

Don
BT
 
No problem. By the way, each amp is 6W ouput.

The best thing is, most of the mods are simply clipping things out, and soldering in jumpers to take the place of things removed.

Don
BT
 
jaymanaa said:
Great info :yes: I downloaded it and saved it if you want to work on the amps Fri. and Sat. :smoke:
Click to expand...

That would be very cool :thmbsp:

I emailed the guy that did the write up and asked about an audio amp only conversion. Still waiting to hear back from him.
 
Ive been messing with a pair of m-8 SETs for a while now. They are very detailed but a little lean and bass shy run stock. It was recomended to me to replace the cathode bypass cap on the 12ad7 (25uf 25v)(c12 on schematic) with a larger value of 100uf or 330uf. I tried both and like the 100uf better cause it retains most of the high end but smooths out the vocals alittle (which can be a little harsh stock). The 330 uf rolls off too much high for me personally. This change was recommended by Rod Childers (the guy that makes preamps and guitar amps out of them. Oversizing the main power cap to 75/75 or 100/100 from 25/25 helps the bass alittle too. Mouser electronics has nice sprauge caps cheap to do all of them.
There is a post somewhere here that walks you through the first steps of conversion to stand alone amps. You have to put a 375j tantalum cap from the vol. pot to the first post on the tape speed/ tone pot (or something like that) to get a true full and level range.
The coupling caps (c9 and c11) could be increased to .22 or .47 from .001 but I havent tried that yet as Im quite happy with the sound presently.
After you gut the white buttons out (upper right holes, take a dremmel and inlarge the hole alittle bit to fit in a fuse holder. Seperate power cords are easy enough. If you have the switchable power model you might have to move the power wire on the PT to the 117v from the 100v. Mine was like that originally.
Radio shack has nice gold plated male 1/4" to female rca plug ends that you can hook your nice rca cables right to and then to the cd player.
If you get any distortion or gain trouble, like I was having, it may be in the reed switch on the pre-out input hole. I took mine out altogether to slim things down and it fixed my problem.

Let me know if you come up with any interesting mods or suggestions for the M-8. I really love these little amps for some reason.
 
Thanks for the great post ashevillebob.

The mono blocks are over at Jays and it may be sometime before he has time to get to them. I have another Akia M-8 here that I bought recently. It is in very nice condition and I'm having a bit of a hard time bringing myself to pull it apart.

If you could help me out with a link to the thread that tell's the mods for these I'd appreciate it.

I'm pretty much a novice with this stuff but I am an experienced solderer and with the proper direction think these will be a fun project.
 
"Akai M-8 R2R SE pentode amplifiers" is the name of the post to get you started. I'm link simple (computer challenged). Ctrl C then V is my one and only trick.
While gutting the amps out there was a metal bracket that I gutted out and it made a great power cord retainer bracket for both amps. I put a "L" bend in it and drilled a hole to attach it. Ill post photos if I can figure out how.
 
Rex E, I've attached a few photos I think? As you can see, Ive switched just about every cap just for S##ts and Giggles. The 375 jule cap can be seen in the last shot. The tape speed switch becomes a variable ajustment to produce a little hump on both ends of the hz scale it seems. The tone is disabled. I used orange drops in stock value for the coupling caps(the two on either side of last shot, but not middle one). Id love to know if anyone recommends larger values for them. Ive stocked up on three more of these R2Rs, because I kept running into them at Hamfests. Oh, thanks Squidward!
 
Thanks Bob and Squidward. Loooks like the M-8 may become a parts unit for somone.

I've passed on a few other but think I'll grab them from now on.

Off to read squidwards thread.
 
Very cool threads! I have some of these units, and always meant to make them into nice little power amps -- but it always looked like a total rebuild, using not much more than the orig OPTs. You got me looking at them again.

I would think the bass response would be good. Anyone measure these OPTs?
 
I just went and looked at the unit I have here and found that it is an Akai M-7 and not an M-8.

Is there a lot of difference in these two units as far as the amps are concerned?
 
The bass response really isnt that bad, but my living room is quite tinny with glass sliders and a large rock fireplace. I usally have a quilt with several layers of batting hanging infront of the fireplace from hooks screwed in under the mantle, but the wife took over for the Holidays and kicked out my speaker farm and the quilt. Im using KG4s at present and they are a little tinny next to the bare stone. Maybe I need to bass it up with a differernt speaker from the farm in the basement. ( They grow like mushrooms down there in the dark)
 
Rex Everything, The M-7 and M-8 are essentially the same. Different brand caps is all I can see. The M-7 seems to have alot more ceramic caps, which probably have held there tolerance better than the electrolytics in the M-8.( I have a Terecorder(m-7) and a M-8) I like the metal knobs on the M-8.
I piggybacked some Sprague Vitamin Qs at .22uf 600v in parallel with the Orange Drop coupling caps and I really like the effect. Yes, the VQs are 10 times bigger than spec, but they sound amazing to me. I guess any oil paper over around .068 (three times bigger) would have the same effect and save space. Ill listen to them this way for a few days before I solder them in perminately. I have some poly syrine caps that are close to original value (.02) and I might switch them out for the orange drops if the VQs stay in. The poly syrine might help put back alittle of the symbol crash thats not quite there with the orange drops.
 
Last edited:
BrassTeacher said:
Rex,

Here's a link to a site that not only has the schematics, but a schematic on how to easily mod them into stand-alone amps:

http://www.beyondsanityproductions.com/
Click to expand...

Back from the dead........

080509colombiatombsbigxp7.jpg


I was at SA today dropping off a pile of speakers and could not help perusing. Behind a golf bag I found a Roberts 990 in mint condition. As I was looking at it over the PA they said Thursday was family day and everything 1/2-off. $4 later & I strolled out with it.

On point, the above site & link is great! The 990 appears to be identical inside to the 770 and I think I am now going to put them in the keep pile.
 
Any harm in upping all the 25uf@25v caps in these amps to 100uf@100v? I've got a bunch of the 100 but no 25@25's....
 
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