Roberts 1630 6BM8 based, stereo practice /recording amp

[Toxophilite]

Thanks that was neat to try. It would seem to be that PA wiggling tremelo was stronger than the VC style, but that mostly translated into more whop whop. Basically you got similar usable quantities out of it, Likely with the PA bias vary you could get a bit more depth at slower speeds. Mind you I currently have a 22K pot in my VC tremelo circuit as I forgot to buy a 25k and I had that on hand.

[sluckey]

If you want to pursue the bias vary trem, try this simple filter to help smooth and set the max intensity. Fender uses such a filter in all their bias vary trem circuits.

Another trick to help reduce/eliminate "whoop whoop" is to put a diode across the intensity pot, cathode to ground, anode to top side of pot.

[Toxophilite]

I'm going to the diode idea.


Hey I have a layout question. Looking at my chassis and contemplating a reverb.
I know the reverb circuits tend to be sensitive so I was wondering where to put a tube. First instinct is smack dab between the other preamp tubes, but maybe closer to the front of the amp would be better noise-wise. Does it make a difference?

[sluckey]

There are two sets of double RCA jacks on top of the chassis. I'd look into using one double jack to connect the tank and removing the other. Try to put the RT in the space of the removed double jack. Maybe put the extra tube on the other side of of the double jack. Be flexible. Components under the chassis may not allow this.

The recovery tube is the sensitive circuit that will be fussy about where it's located. The driver, not so much.

[Toxophilite]

Hey thanks!


I was going to try another one tube 12DW7 reverb as I have a 12DW7 lying about so both the driver and recovery tube in the same bottle, I could orientate the recovery side of the tube away from the power supply and power amp end.


I was thinking along the same lines  as you regarding the RCA jacks but I was going to put a tube socket where one was .
It's actually easier to do it your way .They RCAs screw in and out easily.


-Were you thinking the RCA closet to the inputs (top left corner) for the tank , or the other one?


There really is a fair bit of space available despite it being a very small amp 12" x 6", sone of the terminal strips can moved or rotated,


There's the spot I mostly drilled out which was the old multi switch hole. I  was thinking of the putting the tube there but then I thought about how close it was to the PA and plugged it with a  pair of fender washers.


If I tidied up the power supply it could take up way less  space right in the corner. So far it's working and very very quiet. One of the quietest amps I've worked on and there are wires going everywhere. I suspect , however that the addition of a reverb circuit would help introduce some noise....they always seem to be noisy, or at least mine are
I will include a better picture of the chassis.

[Toxophilite]

I have a question regarding the heaters and tube sockets. The amp, originally being a sort of hifi has separate heaters and a hum balancing pot for each channel, Likely not necessary in a guitar amp but a cool feature. (my 1980 super has hum balancing)


When I rewired the preamp I used the one tube for both preamps, thus negating the usefulness of the hum balance/separate heaters.
Frankly I forgot about the heaters and just thought it was better to use the socket closer to the input side of the amp for the 1st stage of preamps and the one closer to the power supply for the tremelo circuit.




Is this okay or should I swap it around with a preamp and half the trem circuit in each 12ax7 and be able to utilize the hum control?

[sluckey]

Since you have two separate filament windings, you need two hum pots. Of course you can replace the pots with two sets of 100Ω artificial center taps if desired, but I would keep the pots.

The original had two tubes on one winding and two tubes PLUS a pilot lamp on the other winding. I would put the additional tube on the winding without the pilot lamp just to keep the current about the same in each filament winding.

I would not swap the tubes around. Keep the preamps in one bottle and the trem in the other bottle.

BTW, which tremolo circuit did you decide to use?
 

[Toxophilite]

Thanks.
I went with the vibrochamp as that was the original suggestion and maybe I wasn't feeling adventurous enough.
It works. I tried it as PA bias wiggle too which worked as well.


My interest in different tremelos was primarily in one triode versions (princeton, and your tremor-lator) to free up a triode, maybe to use as a mixing stage for a reverb circuit.


I should really buy a  'breadboard' so I can experiment with different circuits without it being a harum scarum soldering and desoldering, component destroying nightmare!


I just actually learned what a breadboard was a couple days back. Seems pretty damn handy.

[shooter]

bias vary trem

I'm not to experiment stage yet so I'll let Mr T burn up things n take notes 

will the G1 "bias" effect current flow through the PA tube enough to "worry" about or does Rk "fix" the current to max and trem just brings it down from there? 
thx
dave

EDIT    page #s matter 

[Toxophilite]

Because I didn't want to commit myself (my girlfriend complains about this too) I did 2deafs variation on a fender reverb inserted into a vibro champ..I kind of breadboarded it but it's really more like a cheeseboard. Tube socket, terminal strip and transformer mounted in an old defunct shure M67 chassis with wires everywhere!


It actually seemed to work very well. Even with a plain old 12au7 (instead of the 12adw7) it got every day usable amounts of reverb, with a 12ad7 is was well into surf territory.
I'l include the schematic 2deaf posted in my other posting that I had before this stereo amp showed up.
Maybe I'll actually build it into the amp now


S.Luckey, I was asking before but do you think it would be best to have the reverb tube socket on the input side of the existing RCA jacks or visa versa ?

[Toxophilite]

To make my question clearer, I put some circles and squares on a picture of the chassis.


Circles are 3 potential reverb tube socket locations
Squares are 2 potential reverb transformer mounting locations (outside the chassis though) One set of RCA jacks will be kept and used of course.

[glass54]

Silly Question (maybe I missed something  )
Shouldn't the 1M resistors on L and R input Jacks be going from Pin 2 (grid A) and Pin 7 (grid B) of V1 to Ground. and of coarse the Input Jacks firmly fitted to the chassis for good grounding/shielding. See AB763 Jack inputs as an example. 
Kind regards,
Mirek

[Toxophilite]

Thanks, you're right! My schematic has an oops, my amp doesn't though.I will fix my schematic. 


I will now repost my actual question in hopes that someone will answer it.

Regarding reverb tube socket placement:

-rectangles are transformer options
-circles are reverb socket options

[sluckey]

Where will the pot go?

[Toxophilite]

I have an idea for the pot, thanks.



-What I'm undecided about is the transformer and single 12DW7 socket placement

You were suggesting placing the transformer where one of the dual RCA jacks was, using the other RCAs for the tank, and putting the socket on the other side.

However you didn't specify which RCA or which side, so I've been trying to clarify ( a few times now in the last bunch of posts) before I drilled anymore large round holes in my chassis.

I can drill holes no problem, it's the un-drilling I'm not so good at.

[Toxophilite]

Any takers on this one tube reverb, tube socket and transformer placement question?

(I have a spot on the front panel for the reverb pot already)


 I was hoping to use the amp at a rehearsal tomorrow, I've already tested the reverb circuit and I'm keen to install it this eve.


There's already 2 sets of RCA jacks one of which will be used and the other removed to accommodate the reverb transformer


-Circles are tube socket options
-rectangles are the transformer options

[Toxophilite]

Well not getting much feedback on my simple question and I want to get the ^%#$*%^#$(# amp finished.
I would ask my brother in law but he's not familiar with guitar amps and things like 'reverb'


I looked all over the internet but it was mostly fender amp stuff and at best vague, and mine is laid out NOTHING like a conventional fender layout.


So I guess I'll go with this because I figure whilst it's good to keep the reverb away from the PS and the PA it is also good to keep higher voltages away from the sensitive inputs. I will orientate the12DW7 so the recovery stage is facing the front of the amp
On my other $#^%$# diagram this would be T2 and S3

[DummyLoad]

swap reverb x-frmr and tube location?


--pete

[DummyLoad]

.   

[Willabe]

Well not getting much feedback on my simple question

Probably means that your guess is as good as any body else's here.

[Toxophilite]

Reverb installed ala 2deafs suggestion and it seems to work very well. After drilling a hole in the spot I suggested I quickly changed my mind and put it smack dab between the other two and slightly forward which made all the connections nice and short. Extra hole but that's why the Powers That Be invented fender washers.
Same harum scarum point to point wiring and still very quiet. even messier as i accidentally wired the 12DW7 backwards... YES!


Gain is of course reduced though there is enough for my purposes. and I have sent a little more B+ downstream which was necessary anyway for the reverb circuit


Still have to install footswitch jacks for the tremelo at least and maybe the reverb too.

[Toxophilite]

On my amp I have a fender TMB control on the one channel and a James stack on the other.Typically I run the fender stack on my amps at around T-4-4.5  M- 3   B-5. This produces a bright mid scooped sound with full bass.

Using Duncans tone stack calculator I discovered I can get the same EQ curve with the James stack on T=9.5 B- 9
and with about 8db less attenuation.

I'm contemplating putting the James stack on both channels which would free up a spot for the reverb pot.
However I'm not entirely sure if I could use the same reverb insertion.


It would also be nice to have a way to mix the left channel into the right.
It wouldn't be so bad if it went both ways, like a passive mixer

[Toxophilite]

Oh heck! I should've tried the ampeg stereo echo twin one triode speaker driven echo.just need a recovery stage.


I cheese-boarded it in and it sounds quite nice, a little more like a plate or hall and HUGE when cranked.


Seems to make a lot of sense in a small stereo amp because you basically have a reverb driver set up
Especially a low wattage one that's pretty well already right as a reverb driver.
So you don't add a transformer and you just need one triode for a recovery stage and boom into the other amps PI, or in my case the triode of the 6BM8.


If could sort a one tube trem (the super echo twin also has a one triode bias wiggle for it's Power tubes) one could dispense with the extra 12ax7 altogether..maybe I should drill another hole in the chassis for old times sake!

[sluckey]

If could sort a one tube trem (the super echo twin also has a one triode bias wiggle for it's Power tubes)

You already tried that but decided to go with the VC trem instead.

[Toxophilite]

If could sort a one tube trem (the super echo twin also has a one triode bias wiggle for it's Power tubes)

You already tried that but decided to go with the VC trem instead.

Not really , I tried, as per your suggestion, (thanks as always!) inserting the VC bias trem at the PA rather than the triode stage prior.

[sluckey]

If could sort a one tube trem (the super echo twin also has a one triode bias wiggle for it's Power tubes)

You already tried that but decided to go with the VC trem instead.

Not really , I tried, as per your suggestion, (thanks as always!) inserting the VC bias trem at the PA rather than the triode stage prior.

Well, you didn't follow my instructions! The tremolo oscillator triode connected directly to the PA grids via an INT pot, exactly like the echo twin. Only one triode was used to wiggle the output tube bias.

[Toxophilite]

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.

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.

What did I miss? These were your instructions and it seemed to me to be using the existing VC tremelo but trying different insertion points. still the entire triode used. When you were talking about 'freeing up a triode' did you mean using the princeton style single triode bias vary or the vactrol style? THere were no specifics. It's hard to guess intention in print.


I did try this test  as suggested and found both options to work but I had the VC tremelo all wired in already as per initial suggestion

[Toxophilite]

So, Is it bad to have a single triode tremelo and a reverb recovery stage in the same 12ax7 bottle?

[sluckey]

Should be OK. Fender 5F11 (probably others) have the trem osc triode in the same bottle as the input preamp triode.

[Toxophilite]

Well i tried that again to make sure and I realized what you were doing. I did do it just as suggested originally I just didn't realize that the point I was 'taking' the tremelo from bypassed a triode. I guess the thing with really small amps is that so often one runs them close to their max anyway that using a bias vary on the PA sends it into distortion quite easily.

If could sort a one tube trem (the super echo twin also has a one triode bias wiggle for it's Power tubes)

You already tried that but decided to go with the VC trem instead.

Not really , I tried, as per your suggestion, (thanks as always!) inserting the VC bias trem at the PA rather than the triode stage prior.

Well, you didn't follow my instructions! The tremolo oscillator triode connected directly to the PA grids via an INT pot, exactly like the echo twin. Only one triode was used to wiggle the output tube bias.

[Toxophilite]

Pretty well decided on the VC tremelo


Weighing the pros and cons of fender style reverb vs Ampeg echo twin style

- fender style reverb

pros - nice fender sound I'm  used to
cons- needs fancy 12adw7 tube and I only seem to have one (pulled from my princeton build with one tube reverb)
       -Not 'stereo' though this could go either way(pro or con) and maybe could be remedied by inserting the return into the second channel?
         - some signal loss

-Ampeg echo twin style


pros -nice full hall sound
       - stereo (this also goes both ways the stereo sound is nice but so is the mixed effect)
       - less components, use any 12ax7 and no reverb transformer necessary. making use of the existing OT and PA to       drive tank


Cons - Nice full hall sound (rather BIG on the low end, less splashy and the 'stereo effect separates the effect) but I found I can attenuate the low end by putting a cap on the return line ala fender, I could put a switch on the cap for variety.
       


Any opinions?

[Toxophilite]

-Cork sniffing reverbs with my evil cheeseboard
-Then some roughed up incarnations using the original Roberts R2R cabinet 13.25' by 13.5"
Ugly but decent sounding 6X9 as the speaker for the dry channel
Two original 5x7" speakers for the second (echo) channel
Their side mounting really gives the echo a room filling sound.


I've since reorientated the spring reverb tank, upside down.
Originally with it upside down on the top I had a bit of trouble with the 'tray' clanking on the tank so I moved the suspension springs up a hole, which worked.
Original back faceplate remounted.
I think I'm going to use the unused RCA preamp inputs as foot switch jacks for the trem and reverb and make a foot switch just for this amp.
 Need to put grill cloth on the baffle, fix a couple small things


I have two questions


-I still would like to mix the channels
Normally I would have 100% dry sound in one channel
and in the other channel would be echo combined with about 60% dry sound (60% of the volume of the dry channel)
This way I retain a full sound when the echo is turned off


- There's a .0025 cap (2 .005 actually in series to make a smaller amount) between teh pentode control(?) grid and the plate.


Is there as sort of a high end noise suppression? Are they necessary for a guitar amp?
It does seem a little dark

[sluckey]

There's a .0025 cap (2 .005 actually in series to make a smaller amount) between teh pentode control(?) grid and the plate.

They are actually between the plate and screen grid. Not needed for guitar. I would remove them.

[Toxophilite]

Hey, thanks for taking the time!
Do you know what they were there for?

[sluckey]

Don't think of the cap as being between plate and screen. The screen is at AC ground through the node B filter cap. So, the cap is really between plate and ground. The cap passes hi freq to ground and this will make the amp sound darker.

That's what the cap does. I have no idea why Akai did this.

[Toxophilite]

Cool and thanks again. I was thinking it might have to do with the top end. Maybe it was to deal with tape hiss?

[PRR]

> spring reverb tank, upside down. Originally with it upside down on the top I had a bit of trouble with the 'tray' clanking on the tank so I moved the suspension springs up a hole, which worked.

AHHH! Hammond lists different tanks for different mountings. I see you are right: the difference is how the side-springs are rigged to get best clearance for different directions of gravity. And we DIY guys can change that. (A mass-producer would want them all pre-set for design direction to avoid labor.)

> There's a .0025 cap.. between teh pentode control(?) grid and the plate.

As Sluckey says, really between screen B+ and plate. So right across the OT primary. A naked pentode into a speaker makes a rising treble. In an AM radio this brings up the static. Even in a "good" system it can be annoying. The cap rolls-off highs. This cross-OT cap is a VERY common connection in low-price tube amps. Other than guitar amps where some top-lift may be desired. Depends some on what speaker you pick.

[Toxophilite]

I hope I'm not sounding too much like a broken record but I would like, if possible to be able to mix some (say 50-60% of the left channel into the right channel.


Would it be possible to do it right before the pentode stage of the 6BM8 ?


Here is my current schematic:

[tubenit]

i accidentally wired the 12DW7 backwards... YES!

Reverse 12DW7   JJ ECC823
http://www.audiotubesupply.com/jj-ecc823reverse-12dw7

[sluckey]

This simple mixer will put about half the left channel into the right channel. Unfortunately it also puts about half the right channel into the left channel. All simple passive mixers will do the same.

Replace the resistor with a cathode follower if you don't want right channel getting into the left channel.

[Toxophilite]

Thanks
I guess then if i want to do it how I imagine I need a mixer stage



If I switch to the ampeg reverb I would end up with a free 12ax7 triode.

i accidentally wired the 12DW7 backwards... YES!

Reverse 12DW7   JJ ECC823
http://www.audiotubesupply.com/jj-ecc823reverse-12dw7

I already switched it back but that would've been a good solution if there had been some available locally (out of stock right now)


Is there an issue with putting a regular 12ax7 in the reverb spot. I tried it briefly and it seemed to work fine. Fire? Flood? famine? Amp implosion?

[Toxophilite]

I was originally thinking of this like I did on my other stereo amp, However that was between the 'reverb mixer' stage and the PI. It seems there might be something amiss with the grid resistors to ground (500K) and the 500k pot sending signal to ground.


Do I need a triode stage for this?
, and if I had one available how would I implement it?

[2deaf]

Is there an issue with putting a regular 12ax7 in the reverb spot. I tried it briefly and it seemed to work fine. Fire? Flood? famine? Amp implosion?

Definitely fire, flood, and famine and with the way things been going, I wouldn't rule out amp implosion.

12AX7 will work.  Lots of people use 12AX7 drivers.  Poor form, but lots of people do it.

12AX7 just doesn't swing a lot of current like 12AT7 and 12AU7 do.
It doesn't take very much grid signal to overdrive a 12AX7 as compared to a 12AT7 or 12AU7.  In this application the gain stage before the driver can easily overdrive a 12AX7, but a 12AU7 will give it a pretty good run for the money.
The 1.5K cathode resistor is a good choice for a 12AU7 at that voltage, but a poor choice for a 12AX7.

But, yeah, a 12AX7 will work.

[shooter]

It doesn't take very much grid signal to overdrive a 12AX7

Wow, learnt me something! thank you, Was thinking I needed an AX to allow MORE swing 

back to regular programming 

[Toxophilite]

Is there an issue with putting a regular 12ax7 in the reverb spot. I tried it briefly and it seemed to work fine. Fire? Flood? famine? Amp implosion?

Definitely fire, flood, and famine and with the way things been going, I wouldn't rule out amp implosion.

12AX7 will work.  Lots of people use 12AX7 drivers.  Poor form, but lots of people do it.

12AX7 just doesn't swing a lot of current like 12AT7 and 12AU7 do.
It doesn't take very much grid signal to overdrive a 12AX7 as compared to a 12AT7 or 12AU7.  In this application the gain stage before the driver can easily overdrive a 12AX7, but a 12AU7 will give it a pretty good run for the money.
The 1.5K cathode resistor is a good choice for a 12AU7 at that voltage, but a poor choice for a 12AX7.

But, yeah, a 12AX7 will work.

I just tried it briefly out of curiousity as I couldn't  find my other 12DW7...I'm having this horrible feeling I accidentally trod on it earlier this year. I was trying a tube that I thought might be it, that had it's script rubbed off, I hoped it might be an indicator if it worked. It did and then I wondered and tried a regular 12AX7. I might switch to the ampeg circuit or buy a JJ 12dw7.

[sluckey]

Tubenit has used a 12AX7 in his one tube reverb circuit for about 10 years. It works but will not do Dick Dale.

[tubenit]

I probably have built between 7-10 amps with a one tube reverb using a 12AX7.  Only recently did I try a 12DW7 which is what I am using now in the amp that has one tube reverb. 

Tubenit

[Toxophilite]

Most things sound better with a JBL
Picked up a  little  D 216 and it sounds very good. A perfect speaker for a compact clean amp, nice and full, not too bright either.
Pretty efficient as well. Small amp has ballooned up to 21.6lbs but I think worth it. The 'stereo' effect from the delay coming out the 2 side mounted 5x 7's is pretty great. Still have to finish the faceplate. My camera is $%#&%^$^%ed!!


Still thinking about the Ampeg 'stereo' reverb and another gain stage for mixing.

[Toxophilite]

Hello.
I have a specific question about mixing my channels together.


There have been some nice suggestions regarding the amps configuration as is and the limitations inherent.


However I can reconfigure and adjust if needed.


So what do I need to properly mix the left channel into the right one without involving simultaneously mixing the right onto the left.


Do I need to use another triode stage?


If so I can make one available.


If I had an extra stage available could someone possibly show me how this is done please?

[tubenit]

So what do I need to properly mix the left channel into the right one without involving simultaneously mixing the right onto the left.

Can you explain that question please?? 

With respect, Tubenit