Roberts 1630 6BM8 based, stereo practice /recording amp

[Toxophilite]

I'm picking this up this eve. Like most Roberts reel to reel stuff it's pretty modular and contains a compact stereo amp with 2 12AT7s and 2 6BM8s. Might be a good platform for a stereo guitar amp. If I wanted to try fender AB763 style EQs would I need both halves of the 12AT7 or could I put it in between the 12At7 and the triode of the 6BM8, freeing up a triode for fun things like tremelo, or both together..possibly reverb. Maybe use 12AX7s in place of the 12AT7s? Just tossing around ideas.
I have the service manual from the 1710 which I have been informed is identical. The schematic is in two pieces which i couldn't combine. There's some overlap.

[sluckey]

I stitched it together. Not perfect but should be usable.

[Toxophilite]

Thanks! My internet computer doesn't have the fancy layout and design programs that my not connected to the internet music computer has on it While I can make them talk using a thumb drive, I had had enough trying on this computer already.
thanks again.


If used as a guitar am would this be better as a 'nuke and pave' ? as my brother says...
At some point it had some 'new' tubes Sylvanias for the 6BM8s and a raytheon 12AX7 in one of the 12AT7 spots. Kind of hoped it had 2 12AT7. But still.
bringing it up on the variac.
Cute little amp.

[tubenit]

I drew this up recently.  Maybe some idea in the schematic that you can use.  You could just eliminate the reverb and/or tremolo if you wanted

For more 6BM8 ideas look here:
http://el34world.com/Forum/index.php?topic=25046.0
http://el34world.com/Forum/index.php?topic=17863.0
http://el34world.com/Forum/index.php?topic=12526.0


With respect, Tubenit

[Toxophilite]

Thanks, definitley some good ideas.


Looking at a vibro champ it seems like I could possibly manage with an Ab763 style EQ using half a 12AX7 and the triode from the 6BM8 before the pentode power tube. If I did a two triode reverb and a one triode tremelo, maybe with the addition of a 6AV6 or another dual triode I would have enough stages to make a versatile little practice amp.

Any opinions on whether it's doable?


It would be take everything out and rebuild anew inside the stripped chassis using the transformers and existing tube sockets.
probably using left over Hammond cone boards or terminal strips.

The Gibson GA 1Rt has some single ended 6BM8 ideas too but no EQ

[sluckey]

Have you actually pulled the amp chassis from the R2R? I've only messed with one R2R. I used a Roberts 1057 as a donor to build a toaster oven. The chassis was just a bunch of bent plates bolted together that actually formed part of the structure for the tape transport. It looked nothing like a guitar amp chassis. I salvaged the transformers and tubes and threw everything else in the trash.

Let's see some pics after you sever it from the R2R.

[Toxophilite]

Well they're all different. I've built several excellent amps/preamps out of modular roberts preamps from old 770s and Akai M-7s and the like.
 This one is also completely modular requiring no needless brutality. The amps simply unbolts and unplugs . THat is why I purchased this model. The real mess is inside the amp chassis because of oscillators and a large multi switch etc., that's why it would be a nuke and pave.

The removed amp looks like this. Mine has retaining clips on all the tubes which would facilitate mounting in a variety of configurations.
Thanks for the input, but I have that aspect figured out and am looking for some help/input on designing the basic circuit now.
Stereo, single ended maybe the addition of a tube and reverb and tremelo

[shooter]

Your Gibson choice seems near perfect
as to

but no EQ

buy a guitar with a tone knob 
you can "add" the left channel as clean, the right as verbalo, blend to taste

[sluckey]

Oh yeah. That already resembles a guitar amp.

This was mine. Not so good. Heck it wouldn't even set flat!

[Toxophilite]

Your Gibson choice seems near perfect
as to

but no EQ

buy a guitar with a tone knob 
you can "add" the left channel as clean, the right as verbalo, blend to taste

Funny! but I definitely like have a 3 band EQ. I like to scoop the mids. I play Gretsch's for the most part so no tone controls and on my guitars that have them, I don't use them anyway.I will say I had one of those new champion 600s for short bit (should've kept it but I gave it to my brother for his lower volume rocking.) and the way it's EQ was structured actually sounded quite good as is.

[shooter]

fwiw I like to structure the tone as a fixed part of the amp.  I get the musicians twitch when there's 1 or no tone knobs, but once they get over the shock they typically find every tone n sound they want before the amp and the amp just adds the secret sauce at the end. most players I know have 1 - 3 tone knobs on the guitar and "tuned" by their guitar tech to the players style 

[Toxophilite]

There's a lot of different ways to do it out there. I've been playing the same sort of guitars live for about 25 years now and I've always used fender amps and I always get the same sort of sounds that I'm quite happy with out of them and I use the same sort of settings. I rarely tweak a lot live. Mostly volume. 

[Toxophilite]

I've disassembled the R2R (Thanks S.Luckey) chassis into it's separate components


Has a pair of low powered 5x7 speakers . Their steel 'baffles' hold the amp to the bottom and the tape deck to the top.
It all just unbolts/unscrews. Doesn't have a conventional fuse holder as the inside of theh amp was acessable via a grill on the bottom of the cabinet allowing fuse replacing.
Amp doesn't have the retaining clips on right now as I was cleaning it a bit, Very hifi/guitar amp looking as is though it could use some additions to the chassis.


Also S.Luckey here's a couple of Roberts amps/pres I'm halfway through converting. I used to do these regularly following step by step instructions but I've been halfway through this pair for about 4 months now...It doesn't take very long actually
Yours looks and awkward. These units just unplug and unscrew. They do need plugs and fuses added


They each have their own power supply and single ended 6BQ5 power amp. a 12ax7 (often 12AD7) and a EF86.
I typically build a guitar pre mini amp with the 12ax7 and the 6BQ5 and then one line level or transformer balanced mike pre with the Ef86. Preamp out for the guitar amp 2 band EQ and a NFB adjusting pot. They have a load resistor on the speaker output. I've used them for recording guitar for clean and dirty sounds and for goofing around at home. I've used the line channel for preamping bass and other instruments and I've used ones with a mike pre as...mike pres for recording.
Cool units, Nice transformers and a choke! Cool look.

[Toxophilite]

Seems both sides work. I was trying it for an hour or so just now quietly. Maybe the right side is a little cleaner.
Overall it seemed to have a lot more headroom potential

I have to go in through the tape head inputs as I think everything else goes to tape. I was using the dry and echo from my space echo
With a 6x9 speaker I had it sounded quite full on the dry (left channel), I was using an EQ pedal to scoop it out some.
The echo came through clear on the wet (right channel).


I had some old Reel to reel tapes I was listening to , good stuff actually and humourously one that was the Queens birthday in 1957 with a cannon salute and military band. The pinch roller is  raunchy though and has a huge capstan dent in it.
I have very little use for a reel to reel though.

[shooter]

this caught my eye since PRR was talking about voltages back when... it another thread
the back plate does say 120vac

[Toxophilite]

this caught my eye since PRR was talking about voltages back when... it another thread
the back plate does say 120vac

Whew! Thanks goodness I don't plan to motorize my amp.

[Toxophilite]

I was looking at the Gibson GA 1RVT as I realized the  earlier GA-1RT while it is an amp is actually  meant  to be a sub reverb amp driven by a 'main' amp.


It has a one tube reverb using a 7199 (which there are cheaper equivalents of) and a 6eu7  preamp(which is electronically very similar to a 12AX7 but with a different pinout.


Nice to put a vibro champ front end on a Gibson GA-1RVT PA

[shooter]

6eu7

fwiw those tubes seem to be pretty finicky microphonic.  I repaired a few G-amps and got the ok from owners to change out to AU AX.  One owner wanted authentic, think he bought 4 and one worked well enough to leave the shop, with warning 

[Toxophilite]

Yeah I would use whatever is cheap, plentiful, reliable and does the job.


Could one just use a vibro champ circuit but with 6BM8s? kind of like how the AB763s arethe same amp but with different power tubes and power supplies.


I was also looking at the ECL86s I have ..which have half a 12AX7 as the triode and a bit more poop. Of course the pinout is a little different requiring some switching around

[Toxophilite]

Okay starting yanking things out and then done good wired it !
(Just kidding)
After I yanked out the oscillator and the multi switch, I figured out what everything was basically and then
roughly wired it up (with marrettes and tape though) just to try it  using only the 2nd half of the dual triode, through the EQ and volume and then into the  triode of the 6BM8 and so on. I only added a 33k resistor on the guitar input as there was only a cap. Left everything else alone


Before I had tried it going into the tape head inputs AS IS


Works way better  now, It was distorting before and now it's cleaned right up and sounds clearer. Voltages are up too all around.


Just wanted to try that out to explore the possibilities and familiarize myself, it will NOT be left wired in this atrocious temporary fashion...

[Toxophilite]

In checking things out I noticed a fair amount of differences between the Roberts 1630 and the 1710 schematic I have (Though I was told they were the same) so I made my own schematic as things stand now with the multi-switch and the oscillator removed and the amp wired up for guitar testing using just one triode of the 12ax7 preamp tubes (I switched to 12ax7 for now).
Note: the 1st half of each preamp 12ax7 is currently NOT being used, thus not really hooked up


There were a couple of coax left unaccounted for which don't seem to be entirely necessary:
-one coming from the power tube which could be for recording maybe as teh 1710 schematic seems to send it right to front of the schematic and the recording head.
-and another coming off the bottom of the tone pot which looks like it maybe goes to the headphones out?




I think I will go with a vibrochamp front end.
Wondering about a few things:

- lack of cathode resistor/cap on 2nd 12ax7 triode

For tremelo:

-vibrochamp 2 triode tremelo wiggling the preamp
or
-Gibson GA-1RVT 1 triode tremelo wiggling the power tube 'bias'
or
-S.Luckey's vactrol tremor-lator (not sure if it's appropriate for single ended small wattage amps)


-Also ponder either the addition of a 12D7 for reverb circuit or perhaps just the addition of a 6AU6 and use that and one of the extra triodes if using a 1 triode tremelo


Probably I will switch to SS rectification once I get the power supply figured or reconfigured
Main can cap  40 + 40 / 350V with 400V surge
That would be the only potential problem area with increased voltage from SS rectifier that I can see

[shooter]

SS rectification

that will color your tone, maybe.
if she don't hum now, might leave sleepin gremlins sleep 

[Toxophilite]

Thanks, the amp , as is, doesn't have any special tone and I usually play completely clean so having a bit of extra B+ is usually a good thing. Also less heat, safety ect. All my guitar amps are SS rectifiedm as I'm not looking for power supply sag and compression.
I would also probably see about new Electrolytics too ,and then move the whole Power supply into the corner to free up room in the chassis.


Any takers on the choice of trem, reverb, and cathodes questions?


The gibson 1RVT also has a one tube reverb using a 7199 I think. There's an inexpensive Russian equivilent too. I've never heard that reverb though.

[Toxophilite]

Hmm
Well Toxophilite, I'm not sure what some of those things do as well as I simply don't have the knowledge. I wish I could help because you're clearly having some quandries.


What? You've tried SS rectification and it gives you about 25v at the first filter cap?
Of course you subbed in some higher voltage caps for the first two filters before trying right?
Oh good well then a little extra voltage should help.


Well about the tremelo quandries, hmm, as your a fender man and you plan to use this regularly in your apartment and for recording and acoustic rehearsals then I suggest using the fender vibro champ and while your at it you might as well use the vibro champ trem too even if it does use an extra triode, think about the reverb as an option for later and maybe leave some space.


What's that?? you were looking at magnatone 210s too..hmm maybe some hard to find parts and also you don't know if you even like magnatones, so scarp that idea also the limited EQ.


Now about those bits your not sure of , I say clean them out and hope for the best...


Thanks me!!

[sluckey]

Sounds like y'all got it all sorted out.   

[Toxophilite]

Well as it is oft said, (and something I often say at dinner.).. "The lord helps those who help themselves" and it seems like that's what is intended here.
I was wondering what I said...however with forums and the internet one can never tell how others interpret what you say. For eg: I had someone send me a very nasty insulting PM a year or two back because I responded to a query on baffles with something other than the methods used by fender. It was like I was insulting someones family, religion or politics. Bizarre.


I did notice last night in desperately emailing my electrical engineer brother-in law for help that my schematic had some errors.

-I was curious about using the vactrol trem circuit in this amp


-I was also wondering about my plan to use a vibrochamp front end and the existing PA and preamp tube prior.


I was going to change the 250k plate resistors on the 6BM8 triodes to 100K and maybe add a cathode bypass cap.


I will also be adjusting the power dropping supply resistors in an attempt to achieve some more preamp voltage


Here is the 'fixed' schematic
The SS rectifier voltages are printed smaller above the tube rectifier voltages

[shooter]

insulting someones family, religion or politics. Bizarre.

It goes back as far as I can tell to the Babylonians and their Engineering projects 

[Toxophilite]

I have a question if I may


I'm reconfiguring and have drawn up a schematic to show what I am aiming for, for now
My question is regarding the tremelo insertion, the cathode, and the negative feedback look of the 6BM8 triode of the right channel. (see circle with ?? on the schematic)
The original Roberts 1630 has NFB attaching right at the cathode while the vibro champ has it attaching after the cathode resistor and bypass cap (I added a bypass cap to the original in my schematic)


Do I need to emulate the vibro champ triode cathode and NFB circuit to achieve the tremelo??


I was asking about tremelos earlier as this was one of my concerns


I was also querying then if i could use a half triode tremelo like the tremor-lator or the Poweramp bias wiggling Gibsons GA-1RVT

[sluckey]

I would build the left channel exactly like a vibro champ. Tremolo and NFB just like the VC. Forget about what roberts did on this channel.

As for the right channel, keep the power amp basically the same as the Roberts, including NFB.

BTW, that right channel preamp needs a coupling cap between the plate and the James tonestack.

[shooter]

THANKS!
I was gonna ask the same question, still have a minor one, (for Steve:);
did you ever measure the AC outta the tremolator, my quandary, will it be so "hot" wiggling a cathode I need to choke it enough that I don't gain it's benefits 

ok, you can have your thread back 

[Toxophilite]

Thanks very much. I'll do that.

And thanks for the heads up about the coupling cap


I used the James stack because I had it already made, on a dual concentric potentiometer (I believe it was one I tried in the Ampeg stereo twin I had for a while). I had adjusted it slightly so when it was cranked it had a very similar EQ curve to a AB763 tone stack but with about 10db less insertion loss.
As faceplate space was at a premium it seemed like a good idea. Also it gives me the potential for  two slightly different sounding channels.


If I wanted to mix the left channel into the right channel power amp, could I do it as fender does it with a couple of 220K resistors and a mixing pot prior (say 1 meg or 500k)

[sluckey]

(for Steve:);
did you ever measure the AC outta the tremolator, my quandary, will it be so "hot" wiggling a cathode I need to choke it enough that I don't gain it's benefits 

I have 110VRMS @5Hz on the plate of the Trem-o-Nator oscillator. But you don't need this circuit to wiggle the cathode bias of a small tube. They are very easy to wiggle.

     http://sluckeyamps.com/bandmaster/bandmaster.pdf

[Toxophilite]

A couple more questions if I may:


-Would it be beneficial to up the plate voltage of both the 6BM8 triodes by using 100K plate resistors?
That's what I was planning initially


- also mixing the left channel pre into the right power amp (this is for my particular guitar setup , I did this on my stereo deluxe too.). Fender style with 2  220k resistors and a potentiometer prior?

[shooter]

1st, thanks Steve, kinda what I thought
next

prior?

I got lost at prior, personally I don't see an advantage to mix on a stereo, blend possibly 
Guess I would put it at the MV, I use 470k's in my plexi style, but 220K - 470K seems to be the typical range

If you change plate R, doublecheck, or you might know, if this changes the tubes bias point 

[Toxophilite]

Thanks.
This is what I was thinking for mix and left channel done up as vibrochamp (see schematic)


Shooter,


I use a Roland RE-150 space echo. It has separate echo and dry outputs. I have dry in one amp and echo in the other. However I like there to be still some 'stereo' signal when the echo is turned off so I typically have some signal from the dry amp fed into the wet channel, maybe about 60% of the dry signal in the wet channel. This what I am trying to achieve here


Steve,


Should I be having a 220K to ground on the grid of the 6BM8 pentode like the vibro champ or leave the 500k that is there already? I guess what I'm asking if the 220K is specific to the 6V6 and the 500k to the 6Bm8?

[shooter]

sounds like you have a component Ampeg SET 

[Toxophilite]

sounds like you have a component Ampeg SET 

I used to have a stereo echo twin, briefly, It had many cool features that I got some great ideas from, but it's power to weight ratio was not my thing,,similar to a Gemini I had too . Cabinets made of lead.

[shooter]

yep, fixed 1, that was enough, you need the blue(?) pill to make you small enough to get around that chassis 

[Toxophilite]

Both channels up and running and sounding good, very quiet so far(noise wise) Cool!


Next build the tremelo circuit and devise someway of mixing the left channel into the right.




Any suggestions on how to mix the left channel preamp into the right channel power amp , though maybe at the earlier triode stage. ??


- I'd like to be able to adjust the blend
- Tremelo only on the left channel

[sluckey]

Any suggestions on how to mix the left channel preamp into the right channel power amp , though maybe at the earlier triode stage. ??

Be aware that any passive mixer will also put the right channel preamp into the left power amp. There will be a lot of loss in your latest schematic.

May I suggest just put a 220K mixing resistor on the wiper of each volume control. Then put a switch between the grids of the following triodes. Use the volume controls to blend.

If you are still looking for a one triode tremolo I have a couple you may consider. They both vary the bias of the push/pull power tubes but should work just fine for a single end power amp. You should also look at shooter's 1482esq thread.

     http://sluckeyamps.com/lil_maggie/Magnatone_M2.pdf   (see page 2)
     http://sluckeyamps.com/RCA/Ampeg_J12B.pdf    (see page 1 and 2)
 

[shooter]

since I bounced between proper use of a SE OT vs improper, I was gonna use your GA-1 trem at the PA tubes, just like Gibson did, but I'm going improper use so Steve's trem

[Toxophilite]

I was asking about the Gibson GA-1RVT 1 triode tremelo a few posts back.


It does a wiggle on the 6BM8 pentode .


Would that allow me to mix the left channel into the right channel right after the 6BM8 triode ?

I will put the gibson schematic up again

I will ask again about the vactrol trem , Could one insert  it in the same way gibson is using the trem in the GA=1RVT?

[shooter]

Could one insert 

when I did my thought experiment I convinced myself "yes, with some extra tweaking n unforeseen results" 

[sluckey]

I will ask again about the vactrol trem , Could one insert  it in the same way gibson is using the trem in the GA=1RVT?

Maybe. It was not designed with that in mind and AFAIK no one has tried it. You could be the first. You're in new territory now, so don't be afraid to be a pioneering leader. Experiment.   

But why bother. You have now seen at least three circuits the use a single triode and wiggle the output tube grid/s. One circuit even has a LED on the cathode of the oscillator tube so it would be dead simple to replace the LED with a VTL5C1. Something else to think about... you were not happy with the tube driven VTL5C1 before. I would just go with one of the proven circuits. The Maggie M2 uses 6GW8s which are very similar to 6BM8s.

[Toxophilite]

you were not happy with the tube driven VTL5C1 before. I would just go with one of the proven circuits. The Maggie M2 uses 6GW8s which are very similar to 6BM8s.

I was never 'not happy' with the vactrol circuit, I was just wanting more tremelo than it was providing with the vactrols I had ordered.
 Once I tweaked the insertion point ala 2deafs suggestion for a 'cold' vactrol it worked very well with lots of tremelo. I posted a thank you for that.


I had been asking about the different tremelo circuits as it was something that was going to inform my choices for my preamp circuit, NFB etc etc. Also I was thinking about how many triode stages I had available and the possibility of adding reverb which would necessitate adding a triode or a dual triode.
Oh and I just noticed that the Gibson circuit has no depth control and the magnatone circuit uses varisitors. hmm.

{EDIT - untangled quote - PRR}

[Toxophilite]

If I was to use the Ampeg circuit or another PA bias wiggle circuit, could I just do what Gibson did with the PA insert  on the GA 1RVT ?


Inserting between two 220K resistors, one to ground and one one the pentode grid

[Toxophilite]

I built the vibrochamp tremelo into the amp, It's pretty good, sounds like tremelo, has the usual quirks
of bias vary tremelo. I followed the existing aesthetic of the amp in point to point wild scramble. My tremelo layout was much neater till I realized I hd the two cathodes reversed..also I should've moved the terminal strip to the side to leave some possible room for a reverb circuit...which as I am now using a dual triode trem will now require an entire other dual triode..or more.
So far so good,


Still have to tweak the power supply (take note of the horrors I have going on!!), install a 3 prong plug, footswitch jack, bright switch etc.

[Toxophilite]

Hey can one do the LED thing on the cathode of vibrochamp tremelo, it seems to be pretty well the same circuit. Humorously the cathode resistor combo I used on the tremelo in this amp was one I took out of another amp and replaced with an LED

[sluckey]

Hey can one do the LED thing on the cathode of vibrochamp tremelo

yes

[sluckey]

Here's a very simple mod you can try that will quickly and easily let you compare the VC trem to the bias vary trem. I think it would be worth the time to temporarily add the addition cap and INT pot (I'd just let the pot dangle). Just set the INT pot you don't want to hear to zero and play with the other INT pot. If you like the bias vary trem well enough then you can free up a triode.