Roberts Model 721 and 990 Tube Amps: Can I run These in parallel?

[Bigwoo]

I had scored the 990 monoblocks last year and was planning on running that pair for my main tube system (W/ a nice 1970 Kenwood tuner) but now that I scored the 721 unit I'm wondering if it is possible to run 1 monoblock from the 721 and 1 from the 990 for the L channel and the same for the R channel?

Or do I have to keep the same model amps together and run 2 completely separate systems?

 

[perchdog]

are you in detroit area ?? i think it was a 721 i left behind at an estate sale sat.

 

[Bigwoo]

Nope, sorry.. Nowhere near MI. Hoping someone will know how to put these to the best use.... I see that one is 80w and one is 100w and I've heard that 2 amps being run in parallel must have the same watts/should be the exact same model or one will overpower/burn up the other. I suppose that I could run the pair from the 990 to one channel and the pair from the 721 to the other speaker, but I don't know if there would be a noticeable difference in sound. (?)

 

[perchdog]

ummm,
that gas to be current draw... i left the 721 but took the 770, i think output is more like 5 watts. it is a single ended el84 amp

 

[Tube Radio]

Yes the amp will be no greater than 5 watts output. The Roberts 770/AKAI M7 amps must be run in the cabinet they came out of unless you want to modify the power connections. A word of warning. If you do decide to restore these amps be very careful as to what physical size the replacement audio coupling caps are. Use caps that are too large physically and you WILL get feedback. Also there is a cap that has a wire wrapped around it. That wire must be wrapped around the new cap as well. IMHO these amps require a lot to get them right and are only 5 watts output which is why I sold the two I had. Now if you were to gut the amps and used just the 6BQ5 tube and a single triode then you may have something worthy of daily use. Not saying they are not good as is, but IMHO it is just much easier doing it that way. Now if you could find some good efficient open back speakers that have decent sized cabinets (would hold at least one 12" speaker and about a 4" tweeter) you could mount each amp in one speaker near the top front. That would look real good, but you would need to pot an input jack on the back of the amp. Now if the power transformer puts out at least 300-0-300 Vac you can use a SEP 6BQ5 circuit from a Magnavox amp schematic and get excellent results.

 

[perchdog]

Hi,
I am not sure what I am going to do with them yet. I grabbed the rtr because it was there at a decent price. I did look at a few threads the other night and thought they may be decent to pair with my Aristrocrates but the sad fact is it will be quite sometime and many projects completed untill I can get near them.
For now I wiill just leave them as-is and possibly pass them along. But then again you really never do know :scratch2:

Thanks for the advice though

 

[sloober]

They will make decent little monoblocks. All the ones I have seen, can easily be removed from the cabinet ( I have 6 of these, just got 2 more today) The power transformers are on each unit, just need to rig up a fuse and power cord. I have converted 2 into guitar amps, 2 for music listening, and more stuff on the way. They do sound pretty good when recapped. About 5 watts max.
John

 

[perchdog]

Sloober,
I realize this but just have bigger fish to fry at this time... I read a few threads, that and what I seen when I opened the cab looked to me like some real posibilities. Just too much in front of it right now which I cannot seem to get to...

 

[Tube Radio]

I understand where you're coming from.

 

[Tube Radio]

You could always leave the amps and reel to reel as is and restore them then connect the speakers to them. That way you will have a way to play 1/2 track stereo reel to reel tapes unless you already have a reel to reel that plays 1/2 track tapes.

 

[Tube Radio]

Do you still have the tape transport that went with the Roberts 770/Akai M7 amps. If so PM me.