First circuit diagram I've ever drawn...will it work?!

[Jennings]

Hi Folks!

I've just completed my first ever circuit diagram.  Never done one before so this little project has basically taught me how to draw, what the symbols mean, and how this all relates to what I see in the chassis in front of me.  Please excuse the rather shoddy quality of the layout in my drawing...it is a first attempt ever for me, and it's been a process of trial and error with various software and printout of diagrams

Will this idea work do people think?  What are the potential issues, if any?  Have I got my circuit wrong or made mistakes in any way?  I'm trying to use the output stage of my Marshall Artiste (basically a 50w Marshall Bass output section) but couple it to a JTM50 preamp stage and a Vox AC30 preamp stage.  Do people think that either of these would not marry well to the output I'm working with?  Are there better options?

Basically I'd like a twin channel amp, one with a fairly classic marshall crunch when cranked (thus the JTM50 idea), and the other give a bit of a different flavour.  I love AC30s, so figured that using the AC30 pre would be interesting, if not totally Vox like with the Marshall output.

[Jennings]

OK...I spotted the first problem!  I'd accidently forgotten to adjust a couple of V3b differences between the Marshall and Vox preamp sections.  Corrected now in the circuit attached here.

[Willabe]

Please excuse the rather shoddy quality of the layout in my drawing...it is a first attempt ever for me, and it's been a process of trial and error with various software and printout of diagrams

Not at all shoddy. Great job.

If there's any mistakes the guys here will find them.


              Brad     

[Jennings]

Cheers Brad...I actually quite chuffed how quickly things on the chassis and layout diagrams quickly started making sense on the schematic.  Still, it's early days for me and my skills!  Haha!!!  Anyhow, I just spotted I'd not linked the AC30 channel to the choke, so I've updated the diagram

[PRR]

Two circuit typos.

Eye-strain!! Don't JPEG line drawings!! GIF does not render skin-tones well but won't fuzz-up line drawings and is universally supported. PNG can be fuzz-free and is nearly universal now.

[Willabe]

The filter cap off the choke is backwards. 

Edit; Doh, suppressor grid was right, screen grids  were not tied to power supply rail. Sorry about that. You should also add the appropriate value screen R's.

               Brad          

[Jennings]

Thanks for the quick replies guys...really useful info for a novice like me!  I've updated my drawing accordingly...and this time saved as a GIF file too!  I much prefer my fuzz in pedals rather than drawings!   

I'll make sure I have the parts I need this weekend and then get building!

[Willabe]

Hi Jennings,

Please see edit in reply #5.


            Brad   

[stratele52]

Big job this sketch ,
 I see the output tubes bias circuit is connected AFTER the stanby switch. This will damage your output tubes.
Pout this wire  BEFORE the standby switch.

[Willabe]

I see the output tubes bias circuit is connected AFTER the stanby switch. This will damage your output tubes.
Put this wire  BEFORE the standby switch.

Like this,

[Jennings]

Good spots guys!  Thanks...I'll do that!  Keep 'em coming if anyone spots any more faults...I'm hoping that with an amp this complex (build wise, for me!) to get as much right from the start!  Wouldn't be able to do it without you guys!

[J Rindt]

Big job this sketch ,
 I see the output tubes bias circuit is connected AFTER the stanby switch. This will damage your output tubes.
Pout this wire  BEFORE the standby switch.

Pardon my novice question.....
If the SB Switch is before the bias circuit, is that a problem.?
Is it possible the Power Tubes can red plate (or draw A LOT of current)  before the negative bias has time to develop.?
Can it happen that fast.?
Thank You

[Willabe]

If the SB Switch is before the bias circuit, is that a problem.?
Is it possible the Power Tubes can red plate (or draw A LOT of current)  before the negative bias has time to develop.?
Can it happen that fast.?

Yes!

You see what is happening if the B+ is applied __Before__ the bias is ready to _Tame_ the OP tubes output. They will run wild (at first) until the grid bias is stable.  

Why?

Because it takes a certain amount of time for the -bias filter caps to come up to their full charge/voltage, the time period is determined by the  "time constant" of the circuit, caused by the C's and R's in the -dc bias sup. circuit, that's to be applied the grid's of the power tubes. The -dc bias filter caps have to charge up to their -dcv before the B+ is applied to the plates of the power tubes.

They might not red plate but, they will be stressed, which can easily and should be prevented by moving a single wire connection in this case.  

But you already figured it out.   

              Brad    

[stratele52]

+1 Willabe

Jennings , I did not look at your schematic more, this does'nt mean everything is fine or .....wrong

[Jennings]

Well I've learnt something there about power tube stress that I hadn't considered before...cheers!  I'm guessing my circuit might still have a few bugs that I might not know about yet, but hopefully we've got the main ones out the way.  One thing I'm also wondering is whether one channel might be louder than the other?!

[sluckey]

One thing I'm also wondering is whether one channel might be louder than the other?!

That's possible, but since the topology is nearly identical, I'd expect the difference in volume to be minimal. But so what? You have a volume pot in each channel.   

[Jennings]

Good point!  In fact probably no bad thing really, thinking about it...I can always use one as a boost by A/B-ing them.  OK...I'll start the builing this week then.  I've got the cab in a good state ready for the chassis, and cleared out the bits of the Artiste circuit that I don't need already.

[Jennings]

OK...parts on order etc...one quick question...are all my electrolytics all orientated the correct way round now in the schematic?  Just to double check!

Cheers,

Jennings

[sluckey]

are all my electrolytics all orientated the correct way round now in the schematic?

Not yet. You still need to make the corrections PRR and Willabe suggested.

[Jennings]

Ah yes...forgot I hadn't uploaded the changes I made based on what they'd said already...how about this:

[sluckey]

Still not right. See reply #5.

[Jennings]

Crikey, you're right...I saw that but clearly didn't save the change...amended now.  Cheers for your invaluable help...think I need to stop staring at the screen awhile!

[Willabe]

You still have the suppressor grid (grid 3) hooked up to the B+ rail and the screens (grid 2) are not hooked up to the B+ supply rail.

Grid 2 goes to B+ supply rail, grid 3 goes to ground with the cathode. Grid 3 might be internally connected to the cathode on some tubes.

             Brad    

[Jennings]

Sorry Willabe...I'd missed what you'd said before about that while I was worring about the cap orientation!  OK...I've red-lined my drawing appropriately (SmartDraw free trial ended!).

[Jennings]

Update:

I've built the circuit as a mark 1 attempt...few issues to sort out currently.

1)  Low output on the Vox channel, and a sort of crackle like a cheesy fuzz pedal on the notes when played.  I think this is party due to a possibly iffy V1 socket.  Also I might try lowering or removing the 220k resistor connecting the Vox preamp to the PI to address the volume balance with the JTM channel.

2)  JTM50 channel sounds ace...but the tone controls don't seem to work properly, and don't do too much to the tone or volume of the amp even when completely off!!!  Clearly a wiring issue there!

3)  On both channels, when you turn the preamp vol to between about 9 and 10 the amp sort of shuts down and just squeels.  Not sure what's causing this?!

Other than those issues we're up and running.  By the feel of playing it in the current state the Vox channel actually does give a fairly good AC30 ish flavour, and the JTM side really gives a sweet plexi type tone without the harsher treble of the metal face 70s Super Lead.  Sounds nicer than my JTM45 clone!  Once I get the faults ironed out and back into the 2x12" combo cab I think this is going to be a fantastic amp.

[Jennings]

I've fixed the JTM side tone stack...I'd not noticed a jumper bypassing the 56k resistor which I'd accidently not snipped from the original amp.  Fine now.

regarding problem 3 above...as I move the vol in and out of the zone which cuts the amps sound and makes the squeel, I notice the power tubes 'flash' slightly.  Something certainly not right there.  I'll get some voltage readings when I have a mo...off out to the cinema tonight with the missis.  To make up for paying the amp more attention than her!

[sluckey]

Disconnect the NFB loop from the PT secondary. Does that help the squeal? If so, reverse the OT primary plate leads and reconnect the NFB loop.

[PRR]

> Disconnect the NFB loop from the...

OT secondary.

[PRR]

> turn the preamp vol to between about 9 and 10 the amp sort of shuts down and just squeels.

Layout.

Bring it up right at the edge of squeal. Move a dry lead pencil point around inside. If you find a place where squeal changes, that's where some output is sneaking-back into some input. Move wires to reduce sneakage.

[Jennings]

Thanks sluckey and PRR...by examining the NFB circuit as you guys suggested I found that the NFB resistor was duff...replaced with a good one and that's helped a bit.  I've played with the layout, and that's further reduced some of the issues with creepage from one part of the circuit into the next.  And by doing all that I've noticed that the two preamp stages seemed to be coupled to each other such that when plugged into the JTM side, if you adjust the Vox side's treble or bass control then it affects the sound still!  Should I separate the two preamps with a switch, or can I use some other way of preventing the two preamps impacting each other in this way?

[sluckey]

Should I separate the two preamps with a switch, or can I use some other way of preventing the two preamps impacting each other in this way?

On the top preamp,,, put a 220K mix/isolation resistor between the treble wiper and the .022uF PI input coupling cap. This will provide some isolation between the preamps. You already have a 220K on the lower preamp.

[Jennings]

Thanks for the tip...added the resistor.  Trouble is I now have total motorboating, no sound at all apart from that on the whole amp!  I made a couple of other tweaks too, so I think it may be to do with that and maybe my current grounding set up.  I'm going to go back to basics tongiht and tidy the whole thing up...then see where we are after that.

[sluckey]

It's a good idea to change only one thing at a time when tweaking. The motorboating has to be related to one of those tweaks. Adding that isolation resistor would not have done that.

[Jennings]

Wise words...that'll teach me in rushing ahead.  I made a change to grouding points (including the grounds for the EL34 tubes) at the same time and suspect it could be one of those which has a short back into the circuit somewhere.  I also think my 32uF filter cap on the AC30 side has a dodgy leg, so I'll try a replacement in its place.

Additionally, it seems normal practice to put both inputs into either tride of one ECC83.  Usually the inputs are physically close together, but as mine are a fair way apart I actually have them on separate valves.  This means they are both on one triode of a valve which has the next gain stage on the other triode.  The JTM channel input is on the same valve as the AC30 channel's last valve stage.  Could this be one of my sources of the signal bleed/interference problem that I haven't totally solved yet?

I'm going to tidy the layout up some with the gounds and other bits being near each other.  Tidying wires hasn't worked completely, so I'm looking at component location too.  while I'm there I was wondering whether it was worth changing which triodes I had doing what job.

[Jennings]

Very kindly indeed Ray has helped me by drawing up a much better flowing representation of my cut-n-shut Vox/Marshall circuit on JSchem...I've attached it here in editable form.  Feel free to make any improvement suggestions folks...Now's a perfect time as I've got to revist what I've built so far anyway!

[Jennings]

Thought I'd give a little update with where I got to! 

I never did get the "AC30" preamp to work connected up...even when I took it apart and re-built it with new components.  The only components I didn't change were the pre-amp valve sockets and input sockets.  I just couldn't see where it was going wrong, so currently I have taken that channel out and just left the knobs/pots and jack sockets in place to fill the front panel.  I've just wired it up as a two input JTM50 preamp into the original Bass 50 output stage, and reunited the amp with the combo cab.  Had to have a new baffle fabricated, as the old one was destroyed by a previous owner.  The amp sounds great!  Just wish I could have made use of that spare one and a half ECC83 sitting unused currently and two inputs.  I expect I'll have to take a breather and come back to it later!