Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Toxophilite on September 19, 2019, 09:13:10 pm
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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.
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I stitched it together. Not perfect but should be usable.
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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.
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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
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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
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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.
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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
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Your Gibson choice seems near perfect
as to
but no EQ
buy a guitar with a tone knob :icon_biggrin:
you can "add" the left channel as clean, the right as verbalo, blend to taste
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Oh yeah. That already resembles a guitar amp.
This was mine. Not so good. Heck it wouldn't even set flat!
(http://sluckeyamps.com/smoky/rr3.jpg)
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Your Gibson choice seems near perfect
as to
but no EQ
buy a guitar with a tone knob :icon_biggrin:
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.
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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 :dontknow:
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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. :icon_biggrin:
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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.
(https://i.imgur.com/MzRBaZp.jpg)
(https://i.imgur.com/sdSKBbw.jpg)
(https://i.imgur.com/pUa7hAV.jpg)
(https://i.imgur.com/B0WGvLH.jpg)
(https://i.imgur.com/p6ogkKn.jpg)
(https://i.imgur.com/RJRRfj4.jpg)
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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.
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this caught my eye since PRR was talking about voltages back when... it another thread
the back plate does say 120vac
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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.
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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
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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 :laugh:
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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
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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...
(https://i.imgur.com/B3ZVct4.jpg)
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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
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SS rectification
that will color your tone, maybe.
if she don't hum now, might leave sleepin gremlins sleep :dontknow:
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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.
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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!!
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Sounds like y'all got it all sorted out. :icon_biggrin:
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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
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insulting someones family, religion or politics. Bizarre.
:icon_biggrin:
It goes back as far as I can tell to the Babylonians and their Engineering projects :laugh:
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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
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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.
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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 :dontknow:
ok, you can have your thread back :laugh:
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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)
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(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 :dontknow:
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
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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?
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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 :dontknow:
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
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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?
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sounds like you have a component Ampeg SET :icon_biggrin:
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sounds like you have a component Ampeg SET :icon_biggrin:
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.
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yep, fixed 1, that was enough, you need the blue(?) pill to make you small enough to get around that chassis :think1:
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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
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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)
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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
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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?
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Could one insert
when I did my thought experiment I convinced myself "yes, with some extra tweaking n unforeseen results" :laugh:
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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. :icon_biggrin:
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.
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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}
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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
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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.
(https://i.imgur.com/XhlWgWk.jpg)
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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
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Hey can one do the LED thing on the cathode of vibrochamp tremelo
yes
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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.
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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.
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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.
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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?
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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.
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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.
(https://i.imgur.com/wyiYWR5.jpg)
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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?
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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?
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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.
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bias vary trem
I'm not to experiment stage yet so I'll let Mr T burn up things n take notes :icon_biggrin:
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? :dontknow:
thx
dave
EDIT :think1: page #s matter :think1:
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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 ?
(https://i.imgur.com/hv2ZQyA.jpg)
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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.
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Silly Question (maybe I missed something :w2:)
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
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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
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Where will the pot go?
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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.
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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
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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
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swap reverb x-frmr and tube location?
--pete
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. :dontknow:
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Well not getting much feedback on my simple question
Probably means that your guess is as good as any body else's here.
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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.
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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
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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!
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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.
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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.
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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.
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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
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So, Is it bad to have a single triode tremelo and a reverb recovery stage in the same 12ax7 bottle?
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Should be OK. Fender 5F11 (probably others) have the trem osc triode in the same bottle as the input preamp triode.
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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.
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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?
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-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
(https://i.imgur.com/SaVLSmh.jpg) (https://i.imgur.com/7AuhOHg.jpg)
(https://i.imgur.com/iiSuMee.jpg) (https://i.imgur.com/B4R9E2d.jpg) (https://i.imgur.com/EN0JxmO.jpg)
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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.
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Hey, thanks for taking the time!
Do you know what they were there for?
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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.
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Cool and thanks again. I was thinking it might have to do with the top end. Maybe it was to deal with tape hiss?
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> 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.
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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:
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i accidentally wired the 12DW7 backwards... YES!
Reverse 12DW7 JJ ECC823
http://www.audiotubesupply.com/jj-ecc823reverse-12dw7
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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.
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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 (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?
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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?
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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.
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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 :think1:
back to regular programming :laugh:
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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.
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Tubenit has used a 12AX7 in his one tube reverb circuit for about 10 years. It works but will not do Dick Dale.
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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
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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.
(https://i.imgur.com/3edKeSu.jpg)
(https://i.imgur.com/pBupvQp.jpg)
(https://i.imgur.com/yLyIMg7.jpg)
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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?
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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?? :dontknow:
With respect, Tubenit
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I want to mix the guitar sound from the left channel of the amplifier in this build into the right channel of the same amplifier.
I believe with passive mixers I end up having the mix be both ways.
Ideally I do not want that. I just want to mix some of the left channel into the right channel power amp.
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Good to have a healthy obsession that keeps one off the streets.
As it appealed to my streamlined approach to this amp and I thought something different with a strong 'stereo effect' would be cool I installed the Ampeg echo twin style verb. It doesn't need a transformer, only one triode, small component count, and 'stereo'
The left channel OT powering the tank and sending it back into the right channel at the 6BM8 triode. Works great as tested before. VERY FULL sounding, which is cool but sometimes overpowering on the bottom end at higher levels of verb.
Switching the cap on the plate of the recovery stage 12AX7 triode from .0047 to .002 or even .001 gives one more of a brighter fender sounding verb and still plenty of it.
Would it be too nuts to have a switch so one can switch between the two sounds of reverb?
Sort of an Ampeg/Fender reverb switch.
I was really tempted to build both circuits in to the amp as there is room and have two different verbs for kicks..but I worry it might be a little noisy, especially feeding one into the other. I was goofing around with it briefly and it sounded pretty cool.
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Would it be too nuts to have a switch so one can switch between the two sounds of reverb?
It's getting harder and harder for me to draw a line between too nuts and acceptable nuts, but switching the capacitance of the coupling capacitor has been done innumerable times. More than one way to do it, also. You could switch the capacitance of the cathode bypass capacitor, instead, to achieve a similar goal. You might want to lower the capacitance of the bypass capacitor in any event. A lower value capacitor will cut the reverb faster when the footswitch is used.
Loading the reverb tank output with a 22K resistor rolls off the upper highs. You might try a 100K resistor or larger to see if you notice any difference.
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Acceptable nuts, this is where a squirrel comes in handy as they can tell if a nut is unacceptable without having to crack it open.
Thanks a lot for all the ideas and options. I will enjoy trying those out
Part of what I'm hearing is the short 3 spring generic tank I'm using due to use due to the size of my 'cabinet'.
I might build a purpose built cabinet eventually.
The Ampeg circuit has more top end sizzle and splash with a long fender style tank.
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Reverb sorted! Thanks!
I have a question regarding the preamp outs!
They're hooked up to the plates of the 6BM8 triode with a .02 cap. They seem pretty hot and the right channel distorts
Is this a less than ideal preamp out for going into a board ? is there a way to make it happier?
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> the preamp outs!
Signal from drivers to power tubes is over 10V.
You want less than 1V!
No doubt it distorts hotly. You are overloading what you are going into.
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Thanks
any idears?
I was contemplating switching the 'preamp outs' to 70s fender style line outs coming off the speaker jacks.
2.2k in line(series), with a 270 ohm to ground
Unless somewhat has a better solution to use with the preamp outs?
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any idears
since you are designing more like audiophile than guitarile you need to limit the input signal to the desired output, can be done anywhere before the input jack, otherwise you'll have to do voltage dividers and scope the path. OR dump gain stages for CF's might get you there
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Hmm
I am building a guitar amp. But one intended mostly for clean playing, like the way most of the classic amps were designed.
My 1980 silverfaced super (last of the silverfaces) has an effective line out off the speaker jack that I've incorporated into every amp build I've done.
I find it very handy late at night when I have a song idea I need to work on but can't actually mike my amp. I can get quite a nice sound going straight in that way. Going straight in with stereo reverb and echo etc. is even better
I was curious if anyone had an idea about an effective way to achieve the same end using the preamp outs but tweaked?
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Just put a 10:1 voltage divider on the preamp out jacks. I'd start with a 100K and 10K. Maybe even make the 10K a pot.
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> any idears?
What does each amplifier stage do?
Make signal bigger!
If signal out of stage 13 is too big for your needs, move the tap back to stage 12.
--- also --- tapping signal out of the INside of a NFB amplifier is "bad" because the internal signal is trying to correct the distortion and response of the amplifier. Not a ton of NFB here on a 12AX7+"6AQ5" with 2700:47 NFB, but still not the best idea.
BTW, that signal at the 47 is more guitar-level and VERY low impedance. Taking headphones here would screw-up the amp, and the DC here may upset external loads, but 10uFd+470r from that point to a jack may make a fine output.
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Interesting and thanks.
Perhaps I should've been clearer.
Anything that will allow the effects to come through too. In this instance I believe it would have to be at or after the triode stage of the 6BMB. Tremelo is inserted at the cathode of the left 6BM8 triode. Reverb comes back on the grid of the right 6BM8 triode
Also I would like to be taking a similar output from both channels
I know the late 70s fenders are often scorned but the line out on the speaker output worked pretty well considering.
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I know the late 70s fenders are often scorned but the line out on the speaker output worked pretty well considering.
Well, just do it. Or you could try this...
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Cool I will try a few of these options
Thanks
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Tried 470 to line out and 47 to ground on the pentode grid (517k total to ground). Sounds good but very quiet, Needs almost all my preamp gain to get a strong signal to the computer via the board. For fun I tried swapping the resistors to ground, 56k, 68k, 100k. Signal got progressively a bit louder, probably completely workable as it is digital and it doesn't need to be a really loud regardless.
Also tried inserting a 100k in the original preamp out arrangement with 10k to ground.
Louder signal, works well. At least as well as the other fender arrangement I mentioned.
Is this 110k running in parallel to the 470k to ground on the grid of the 6BM8 pentode?
( I had to replace the original 500k with a 470k resistor as the leg broke at some point. Will fix the schematic)
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To confirm the implementation of this:
Hooking up to the junction of the 47 ohm resistor and the 1.5k resistor on the left channel 6BM8 triode?
The 10uf cap + 470 ohm resistor arrangement, in series?
See schematic screenshot in red 'PRR-amp output'
The only 10 uf caps I have handy are polarized electrolytics
( I mistakenly tried it with a .1 cap in series with a 470 resistor tied to the junction of the 47 and the 1.5k resistor and got a strong and very crisp signal)
> any idears?
What does each amplifier stage do?
Make signal bigger!
If signal out of stage 13 is too big for your needs, move the tap back to stage 12.
--- also --- tapping signal out of the INside of a NFB amplifier is "bad" because the internal signal is trying to correct the distortion and response of the amplifier. Not a ton of NFB here on a 12AX7+"6AQ5" with 2700:47 NFB, but still not the best idea.
BTW, that signal at the 47 is more guitar-level and VERY low impedance. Taking headphones here would screw-up the amp, and the DC here may upset external loads, but 10uFd+470r from that point to a jack may make a fine output.
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This provides satisfying mixing of the left channel into the right channel without visa versa. It does seem to emasculate the right channel some requiring it to be turned up a fair bit more to achieve the same result from the right channel input.
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This seems to work better for mixing so far, Anybody see any potential problems with the left to right channel mixing circuit? IS 220K optional for mixing for some reason?
I noticed there were a lot of mistakes in my schematic. Kind of embarrassing. :embarrassed: One was that somewhat due to cutting and pasting.
I forgot to show that the right channel 6BM8 triode gets it's B+ from node B via a 250k resistor. Which is the original way Roberts had the power configured. For some reason I changed the left channel 6BM8 triode to the C node with a 100k resistor. It seemed to make sense as I was thinking of that channel as a vibrochamp.
I took 500k pot out of the mixing circuit as it seems to be best maxed and seems to sound the same either way.
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I've been playing this amp around the house for a while and I like it. However it never seems to have the same sparkle and prettiness my deluxe build has. I thought it might the little 8' JBLS but I tried one with my deluxe and it sounded great.
The amp seems, in general, to lack top end.
is this something inherrant about the vibrochamp circuit or just my strange configuration of it?
Is it the reverb ciruit or lack thereof. I have the Ampeg echo twin 'stereo' transformer driven reverb.
Any suggestion?
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The amp seems, in general, to lack top end.
might have tried these, but If not;
disconnect the nfb and see if it moves in the right direction
boost the PA bias (If possible) to close to 12W/tube , right direction?
boost the 25uf bypass to 47uf, right direction?
at the end of day you can squeeze just so much from the little ones :icon_biggrin:
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Thanks. I had some success with a 4.7K resistor in the feedback circuit in lieu of the stock 2.7k required by a vibrochamp.
Now to further reveal my vast ignorance.
I'm curious if negative feedback looks are designed around the output tap of the OT?
For example if one were using an OT with an 8ohm output tap with a vibrochamp circuit would one use the same negative feedback loop circuit? (same value components etc.)
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This is helpful:
https://www.aikenamps.com/index.php/designing-for-global-negative-feedback
Scroll down to:
Using a different impedance tap on the output transformer
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Thanks. I found that link while I was trying to read up on the idea and couldn't open it. Now I can thanks and it has provided some insight though for some reason I can't quite wrap my head around it yet.
I will keep trying and hopefully it will penetrate my thick skull. I do want to understand.
Out of curiousity would someone mind explaining how (and if) a negative feedback circuit in a vibro champ build would be different if one was using an 8 ohm instead of a 4 ohm OT
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doesn't matter that it's a vibroXXXXXXXX or a voxxxxxx
the section SG pointed to shows the square n root stuff, so you take the "original" values fender used, plug it into the equation using the new tap.
me, I scourge through schematics til I find "it", then tweak it to taste.
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Out of curiousity would someone mind explaining how (and if) a negative feedback circuit in a vibro champ build would be different if one was using an 8 ohm instead of a 4 ohm OT
With all else being equal the 8ohm OT will provide more feedback voltage than the 4ohm'er.
As tweakers, we have been known to replace a feedback resistor with a pot to experiment.
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Hi Toxophilite,
I am new in this forum and got to this thread while looking for opinions on vibro champ vs Gibson GA-5rt circuit. Coincidentally the reason I am curious about those two circuits is because I was lucky enough to get my hands into a handmade (heathkit style) stereo amp that has 2 6BM8 tubes. The amp already sounds great as it is but I want to spicy it up a bit and I had very similar ideas to the ones you had. I converted an old radio that used a 35L6GT power tube into a Fender Bronco/Vibro Champ a while ago and I loved its sound. I later sold that amp. So now I wanted to use parts from this stereo amp I got and build another one. I was thinking about building one channel with the vibro champ circuit and the other with the gibson ga-5rt but I never heard one of those gibson amps before. So... not sure if that's what I want. :) I was wondering if you have any sound clips to share about your "frankenstein" amp experience :) Thanks, Gui.