janikphoto
Lunatic Member
Thanks! That was the thread I was remembering. It was nice to re-read it, now that I've started messing with these again.
Yet Another Roberts 770 mono block conversion
- Thread starter Omegaman
- Start date
Thanks! That was the thread I was remembering. It was nice to re-read it, now that I've started messing with these again.
BrokenHill
Well-Known Member
That's a good thing to know, thanks for sharing that. I've got about 6 pairs of these darn things, and a couple complete units. I hope they sound as good as everyone says.Did they have a special tap on the OPTs for high Z headphones like certain Sonys did (600 ohm)?
I did see something about the headphone tap on the OPTs somewhere on the internet but I can't remember where, I was going to look into this more but
then I started reading about the Hi Rez portable music players that are coming out and I decided to get one when the prices come down a little and just
use that for my headphone listening.
Cold weather has me staying indoors more and working on some other projects that I have been wanting to do, I just modified a SE Zenith 4K22 console
tube amp for a stand-alone amp, I used a schematic that got from the DIY forum web site.
The little Zenith sounds very nice and I am getting some really nice stereo imaging from it although the Roberts 770x amps have tighter sounding bass
maybe that's because each amp has it's own power supply.:scratch2:
Anyway I think the Akai M8 and Roberts 770x is worth the time to modify for some sweet little mono tube amps.
janikphoto
Lunatic Member
So, technical question... On the phono/line input, there appears to be a 1m ohm resistor which leads into the mic input with its 10k ohm resistor. That goes I to the switch on top, which is known as the record switch, and then I'd guess it goes off to tubes and transformers and speakers and such. Well, that 1m ohm resistor... could I change that out to raise the input level? Say, like a 500k ohm resistor? I want to raise the input done, but the mic input is way too high to use. I'd like something in between.
BrokenHill
Well-Known Member
It has been many years since I did the mods to my pair of Roberts 770x and right now they are both mounted in a wood frame that is hard for me to see
inside, I will need to un-mount one of the amps to see inside.
When I did these amps I rooted everything I did not see a purpose for out of these amps and just use the bare minimum components to run them, I even
disconnected the meters and lights because they did not do that much anyway, I think they were for line level in recording.
Looking at the front face of the amps I have a Phono playback input jack at the upper left side that was all removed, down below on the left side I have a
1/4 inch Line jack and just below that there is a 1/4 inch Mic jack, I used a RCA jack that is mounted very close to the same place as you did yours, I then
wired it to the center 1/4 inch Line jack terminal inside the amp, looking inside I can see that there is a 1 meg wire wound resistor that is connected to the
same place also is a 10k resistor that goes to ground, the 1 meg resistor is connected inline from the RCA in jack and goes straight to pin 2 of the 12AD7
tube, at pin 2 there is also a 500K resistor that goes from pin 2 to the center pin of the 12AD7 and that is grounded at that center pin.
Also while I was in there looking around I did look at the OPT and did not see any tap for headphones I only saw a 0 tap and a 8 ohm tap and it is clearly
marked.
These amps only put out 6 watts each so they do like efficient speakers, I did get a lot better bass out of them by using a SAE XXX pre-amp to feed them,
I then turned up the volume knob on the mono tube amps to about 3/4 turn up and just left them at that and then use the volume control on the SAE pre
to control my volume on both amps, these amps drive my 94db 15 inch woofers very nicely and pump out great bass.
inside, I will need to un-mount one of the amps to see inside.
When I did these amps I rooted everything I did not see a purpose for out of these amps and just use the bare minimum components to run them, I even
disconnected the meters and lights because they did not do that much anyway, I think they were for line level in recording.
Looking at the front face of the amps I have a Phono playback input jack at the upper left side that was all removed, down below on the left side I have a
1/4 inch Line jack and just below that there is a 1/4 inch Mic jack, I used a RCA jack that is mounted very close to the same place as you did yours, I then
wired it to the center 1/4 inch Line jack terminal inside the amp, looking inside I can see that there is a 1 meg wire wound resistor that is connected to the
same place also is a 10k resistor that goes to ground, the 1 meg resistor is connected inline from the RCA in jack and goes straight to pin 2 of the 12AD7
tube, at pin 2 there is also a 500K resistor that goes from pin 2 to the center pin of the 12AD7 and that is grounded at that center pin.
Also while I was in there looking around I did look at the OPT and did not see any tap for headphones I only saw a 0 tap and a 8 ohm tap and it is clearly
marked.
These amps only put out 6 watts each so they do like efficient speakers, I did get a lot better bass out of them by using a SAE XXX pre-amp to feed them,
I then turned up the volume knob on the mono tube amps to about 3/4 turn up and just left them at that and then use the volume control on the SAE pre
to control my volume on both amps, these amps drive my 94db 15 inch woofers very nicely and pump out great bass.
janikphoto
Lunatic Member
So, it's a 1 meg resistor, which is what? In hertz, it would mean hundreds or thousands?!? I'm confused. I retry much thought the m stood for 1 million. Gahhhh... I'm gonna do some research.
janikphoto
Lunatic Member
Ok, I measured the 500k and the 1m, and the 500k reads as 0.5m, so it's half of the 1m, like I thought. Will halving this resistor allow double the signal? Also, did you remove the switch on top? The one you're supposed to lock in place... the record switch? It looks like I could just solder together the right wires, as if the switch were "on".
BrokenHill
Well-Known Member
I'm not sure about the resistors and if it would double your signal, the way it's wired in mine seems to work good so I just described how I have set up in
my amps, I may have tried wiring it in different ways but ended up behind the line jack with the 1 meg, 500K and the 10K resistors in thier places.
Oh, 1meg resistor would be 1 million ohms on the ohm meter.
I did remove the switch on top and if I remember correctly I did exactly what your thinking. I activated it in the locked position and figured out what circuit
it completed and then removed the switch and soldered those wires together.
I think you will get it all figured out soon.
my amps, I may have tried wiring it in different ways but ended up behind the line jack with the 1 meg, 500K and the 10K resistors in thier places.
Oh, 1meg resistor would be 1 million ohms on the ohm meter.
I did remove the switch on top and if I remember correctly I did exactly what your thinking. I activated it in the locked position and figured out what circuit
it completed and then removed the switch and soldered those wires together.
I think you will get it all figured out soon.
janikphoto
Lunatic Member
Here are two videos, one using the mic input, and the second uses the rca I added using the phono/radio input wires. The RCA goes into a 1m ohm resistor. That leads directly into the mic input with a 10k ohm resistor. That runs into the switch and I haven't traced it past that.
http://youtu.be/v5kW1e5rI2c
Keeping the volume the same on the Carver pre, I plug into the rca's and turn the amps' volumes up to 7 or 8, and it's still much quieter than the mic inputs on 1. I would expect the volumes to be the same once I moved the knob from 1 to about 7, seeing as how the mic input needs a lot more gain. But, the other input seems too weak if it's way too quiet at 7. I need something in between these two, and that's why I was wondering if I need to change the 1m resistor to gain some...
http://youtu.be/MQ7kyqEs1vU
http://youtu.be/v5kW1e5rI2c
Keeping the volume the same on the Carver pre, I plug into the rca's and turn the amps' volumes up to 7 or 8, and it's still much quieter than the mic inputs on 1. I would expect the volumes to be the same once I moved the knob from 1 to about 7, seeing as how the mic input needs a lot more gain. But, the other input seems too weak if it's way too quiet at 7. I need something in between these two, and that's why I was wondering if I need to change the 1m resistor to gain some...
http://youtu.be/MQ7kyqEs1vU
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On the switch in record mode, I just super glued it into place and then cut the top of the plastic off to make the unit fit into the case
I also left the inputs as found and wired a RCA from LINE/PHONO to the back
The next pair I do I will save the 110vac female connector and use it to provide power for the second amp
Also I will set up the headphone connection
I had a difficult time getting the pilot light to light in the correct position. It would light for the mono but not for the stereo setting so I had to mess around and find the right connection for full on
Don't forget to install fuses. I removed the 100vac/117vac selector switch and put the fuse there
Best of luck to you
I also left the inputs as found and wired a RCA from LINE/PHONO to the back
The next pair I do I will save the 110vac female connector and use it to provide power for the second amp
Also I will set up the headphone connection
I had a difficult time getting the pilot light to light in the correct position. It would light for the mono but not for the stereo setting so I had to mess around and find the right connection for full on
Don't forget to install fuses. I removed the 100vac/117vac selector switch and put the fuse there
Best of luck to you
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janikphoto
Lunatic Member
I think I'm going more of Brokenhill's route... removing most of the unnecessary stuff inside, and I'll put some bolts in place of the 1/4" inputs on front. Good info on the bulb. I see the 6.3v taps under the transformer, so we'll see what they do for me. And, yeah, I'll definitely add a fuse. I don't have one yet, but I need one in there.
BrokenHill
Well-Known Member
Looking at the videos I see they have slightly different versions of the Roberts tube amps, below is a pic of the jacks
on the face of mine.
Also looking at the stock schematic I see how they used the record switch in the signal path to the jacks, since mine
has been removed I had to look a the schematic to jog my memory, removing the switch does make all of this much
more simple.
Here is part of a schematic of another modified Akai m7 amp, as you can see this guy left the 500K resistor in place
and just uses a .3 cap in the signal path eliminating the 1M and 10k resistors, the m7 may have not even used those
exact resistors, I'm not sure about that.
My SAE XXX pre-amp has a lot of gain so it does fine with the 1M resistor in the signal path with the 10K to ground.
I would think you could experiment with different resistor values to get the desired results to match your needs, if you
do change the 1M resistor I think a carbon film resistor would be my choice because they are easy to get and would
probably not alter the sound to much and you can get them at any Radio Shack.
on the face of mine.
Also looking at the stock schematic I see how they used the record switch in the signal path to the jacks, since mine
has been removed I had to look a the schematic to jog my memory, removing the switch does make all of this much
more simple.
Here is part of a schematic of another modified Akai m7 amp, as you can see this guy left the 500K resistor in place
and just uses a .3 cap in the signal path eliminating the 1M and 10k resistors, the m7 may have not even used those
exact resistors, I'm not sure about that.
My SAE XXX pre-amp has a lot of gain so it does fine with the 1M resistor in the signal path with the 10K to ground.
I would think you could experiment with different resistor values to get the desired results to match your needs, if you
do change the 1M resistor I think a carbon film resistor would be my choice because they are easy to get and would
probably not alter the sound to much and you can get them at any Radio Shack.
Attachments
janikphoto
Lunatic Member
I'm reading this on the other thread, and thinking of trying a 12ax7 in the amps to see what happens...
"The last arena of tweaking (which is mistakenly often approached first)
is tube replacement. If your units still have the original NEC, or possibly
Matsushita tube compliment, and they are undamaged, they should serve
nicely. These were Japanese clones (very well made) of Amperex designs,
rugged, reliable and usually well matched at the factory. Remember the
common 12AX7 will work great in these units in place of the harder to find
12AD7 and if chosen, nothing sounds better than Mullards in this stage.
Although factory rated at 6 watts per channel (8 Ohms), by low distortion,
full bandwidth, HiFi standards, I call all of these series a more realistic
4 watt output."
I'll also study this schematic and see what I can do with it. Thanks!
"The last arena of tweaking (which is mistakenly often approached first)
is tube replacement. If your units still have the original NEC, or possibly
Matsushita tube compliment, and they are undamaged, they should serve
nicely. These were Japanese clones (very well made) of Amperex designs,
rugged, reliable and usually well matched at the factory. Remember the
common 12AX7 will work great in these units in place of the harder to find
12AD7 and if chosen, nothing sounds better than Mullards in this stage.
Although factory rated at 6 watts per channel (8 Ohms), by low distortion,
full bandwidth, HiFi standards, I call all of these series a more realistic
4 watt output."
I'll also study this schematic and see what I can do with it. Thanks!
BrokenHill
Well-Known Member
I have tried different 12AX7 tubes and they all sounded close in these amps, the 12AD7 tube is supposed
to be a low noise tube so I just put them back in, I noticed no difference in sound level at all between the
different brands I tried.
These amps are very quiet and that's one of the high points about them.
to be a low noise tube so I just put them back in, I noticed no difference in sound level at all between the
different brands I tried.
These amps are very quiet and that's one of the high points about them.
jaymanaa
RIP 1961-2018
Did you leave the EF86?
BrokenHill
Well-Known Member
I removed that tube because I just don't use it, that probably changed some of circuit voltages a little, if you want to know
what the voltages are at the tubes just let me know and I can pull one of the amps out and take some measurement at
any of the tube pins you would like to know about.
janikphoto
Lunatic Member
I only have three tubes in mine right now. I pulled anything that could be pulled and still let it play. The other thread suggested to pull the unused tubes, so I did.
BrokenHill
Well-Known Member
I was going over tracker-x modification instructions again and he explains everything very nicely, looking at the
number 6 and 7 in the list of instructions he explains the importance of the tone control mod and how it affects
the bass and headroom of these amps.
When I did the work to my amps I followed the tracker-x modification very closely and even used silver mica
and the orange drop caps where he instructed to use them, also somewhere on the internet I read that the amps
really don't put out enough power to use the VU meters effectively for output and that they actually degraded the
sound slightly for HI FI use,
I can't remember were I saw that and I really don't know how true that is about the VU meters but I disconnected
them anyways I don't want them in the signal path.
I thought tracker-x was very accurate on his opinions about these amps when modified like he did and I find them
to sound just like he describes after implementing all the mods exactly as he explains.
Speaker Synergy is a must with these modified amps.
number 6 and 7 in the list of instructions he explains the importance of the tone control mod and how it affects
the bass and headroom of these amps.
When I did the work to my amps I followed the tracker-x modification very closely and even used silver mica
and the orange drop caps where he instructed to use them, also somewhere on the internet I read that the amps
really don't put out enough power to use the VU meters effectively for output and that they actually degraded the
sound slightly for HI FI use,
I can't remember were I saw that and I really don't know how true that is about the VU meters but I disconnected
them anyways I don't want them in the signal path.
I thought tracker-x was very accurate on his opinions about these amps when modified like he did and I find them
to sound just like he describes after implementing all the mods exactly as he explains.
Speaker Synergy is a must with these modified amps.
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