Click on the image to see a larger version of the layout. The associated schematic can be found here. Note that the layout does not include C7, C13, C14, C18, R24, R25, R26 or R30 as these will be mounted elsewhere in the chassis.
Progress has been slow up to this point but all the effort that was put into the design is starting to pay off and the amp is coming together quickly. I have made one minor change to the plan: I was going to put the flying-leads onto the valve sockets to keep the turret-board tidy until it was put into the chassis but I realised that would be rather impractical so the flying leads are now soldered onto the turrets as you can see. It makes the board a little more difficult to work with but it would be very difficult to solder flying-leads onto turrets in the chassis. Much easier this way and I discovered a little dodge to keep the flying-leads out of the way.
I have a ValvePower guitar combo that uses perforated turret board so I had a close look at that and noticed that the flying-leads were pushed through the holes to keep them out of the way of the rest of the circuit. A neat idea that I adopted immediately – you can see the effect in the picture.
I haven’t designed this into the circuit but I need a way to cut the low bass. This requirement has come up a few times in conversations with bass players and it makes sense. In a domestic environment rooms are small and the wavelengths of bass notes are relatively large. At home bass-player will have to reduce the amplitude of low bass to avoid problems. By how much is the big question; more research is required.
Christmas has come and gone and I’ve finally had the chance to finalize the layout for the Northcourt turret-board. It’s pretty small because it has to be to fit in the case so the components are pretty tightly packed in. Some of the components will be mounted off-board to save a little space: C7, C18 and R30 will be mounted on the tone controls and R24 and R25 will be mounted on the valve sockets.
You’ll have noticed that the turret-board has lots of holes in it. This allows the turrets – which are screw-mounted – to be easily re-positioned. A useful approach when you’re as fond of making mistakes as I am. The turrets and board came from www.ampmaker.com. The turrets aren’t cheap but the approach does simplify things.
The next step is to wire up the chassis with flying-leads to attach to the turret-board. Hopefully, I will get the time to do this soon.
I have been off travelling for a few weeks so I haven’t had much time to work on Northcourt Mk2 but I did decide to replace the old flimsy plastic valve sockets with new ceramic sockets so that’s what I did today. When I decided to rip out the old sockets, I didn’t think that the diameter of the new sockets is 4mm larger than the holes cut for the old sockets (22mm rather than 18mm). Oops!
I first thought of using a reamer to enlarge the holes but I couldn’t find many reamers large enough and most were too expensive for my tastes. I was also a bit worried about how long it would take to ream out that large a hole by hand. The solution that I eventually settled on was a stepped drill bit. It was cheap (less than £10) and large enough (just) to cut a 22mm hole.
I tried to use a battery powered drill with speed-control but that did not have enough torque so kept stalling. In the end I used my old Bosch but it was hard to control and the enlarged holes are not particularly neat as you can see from the picture. Nevertheless, the job was done quickly and, when the valve sockets are in place, the ragged edges will not show. I’m not sure if I will use this approach again but it produced an acceptable result.
During an exchange on tone controls on the AX84 forum I started to wonder why guitars and bass tube amps have such similar tone-stacks. I reasoned that, if the frequency responses of the instruments are different, the frequencies controlled by the tone-stacks should be different.
The middle frequency is the frequency that is left unchanged when you alter the treble and bass controls and is usually around 1kHz. But why?
From what I can see, the reason why it is around 1kHz is lost in the mists of time but it has been suggested that it is the middle of our hearing range. Listing the octaves in Hertz from the lower to upper thresholds of hearing we have: 20, 40, 80, 160, 320, 640, 1280, 2560, 5120, 10240, 20480. So 640Hz is the middle value which is pretty close to 1kHz (although 1280Hz is closer) so perhaps the suggestion is true. You can go to the Eliot Sound website for a little more information about this but the important question is how does this apply to guitar and bass amps?
I have used the Tone Stack Calculator from Duncans Amps to create PDF files that show the ranges of the standard tone-stacks from Fender, Marshall and Vox. You can see that, although the middle frequencies move around a little, they all agree that it sits in the 500Hz to 1kHz range. It is worth noting that 1kHz is the fundamental frequency that you get when you play the top string of a guitar at the 19th/20th fret so pretty much everything above 1kHz is an harmonic. So the treble control on a guitar amp pretty much controls the amplitude of the harmonics while the bass control controls the amplitude of the fundamentals by raising the output amplitude of the bottom string relative to the top string (the bottom E-string on a guitar is 82Hz while the top E-string is 330Hz).
This makes some sense for a guitar but a four-string bass guitar generates 311Hz at the 20 fret of its top string. The bottom string is 41Hz when played open and the top string 98Hz. Shouldn’t this be reflected in the tone-stack? The Fender Bassman 5F6-A circuit was copied by Jim Marshall in the early 1960s and, from that point on, there appears to have been little distinction made between guitars and bass guitars until semi-conductors made more sophisticated tone controls affordable.
it may be that properties of the human ear make changes to the tone controls unnecessary. The frequency response of the human ear can be seen on this interesting diagram. Our ears are most sensitive at around 2kHz and our hearing tails off below 200Hz. Some would regard 200Hz as the upper end of “bass” and too much bass between 100Hz and 200Hz can sound “boomy” so it would seem to be sensible to be able to compensate for our poorer hearing and the possibility that the bass might sound boomy by being able to control these frequencies. That’s almost what the guitar tone-stack does but it starts around an octave higher.
This is interesting stuff and will be the subject of further investigation. I’ll have to leave space on my Northcourt Mk2 layout to try out different tone controls.
I decided that almost everything had to go so I spent some hours last night removing components from the Northcourt chassis. I had intended to keep the valve sockets but, in trying to unwrap the component leads, it became pretty clear that the sockets were flimsy and it wasn’t going to be worth the effort. The rivets will be drilled out and the sockets will be removed too. That will really only leave the paper-insulated transformers and the chassis itself which is pretty much what I expected. The amp was around fifty years old and the capacitors and resistors were showing their age.
The amps I have built up to this point (see the side-bar) have placed all the valves in a row which has allowed me to use tag-strip – between sockets and in front of the tone controls – to mount the components. It results in a messy layout but it does allow the circuit to be changed fairly easily. That was how I was hoping to lay out the Northcourt but I can’t see how it would work. The valves/tubes are laid out in or square rather than a row and are widely spaced which would require long leads between sockets (as per the original layout).
Putting tag strip in the middle of the square doesn’t work because of the can capacitor. Placing a tag-strip between the ECC83s (12AX7s) and the tone controls would work well but that’s a relatively small part of the circuit; certainly not enough to justify the approach. Clearly it is necessary to use a tag-board or turret-board. I would prefer to leave the valve/tube sockets visible for rework but there is only 40mm between the ECC83 (12AX7) and EL84 (6BQ5) sockets which does not provide enough space for larger components (e.g. smoothing capacitors). The only sensible approach seems to be to use a wider turret-board and allow it to cover some of the sockets (perhaps only one). Flying leads will have to be soldered onto the socket(s) before the turret-board is put into place.
I’m going to use the perforated turret-board and screw-in turrets provided by www.ampmaker.com to allow the circuit to be changed relatively easily. The-screw-in turrets are expensive – especially when compared with tag-strip – but it’s the only sensible approach. Thinking about expense; the amp cost me £85 and the capacitors from AudioCap cost £16. Now that the initial circuit and layout have been sketched out I have been able to buy the remaining bits and pieces which has added another £50 to the cost giving a total of £151 which which seems like a pretty good deal for a hand-wired bass amp. Now the parts have been ordered so the next post should show some real progress.
After reading Merlin Blencowe’s books (you can find out more about them in an earlier post), I have been inspired to change the circuits of both Little One and Northcourt Mk II. The changes aren’t major but I’m finding Merlin’s books incredibly useful so, if you have any interest in building guitar valve/tube amplifiers buy them before they go out of print (as they surely will).
The NOS Svetlana EF86s arrived from the Ukraine and I love them. I only ordered two but when I heard them I was compelled buy another pair. With one of these in Little One it doesn’t seem to matter what valve/tube you use for the phase inverter or the power stage; it always sounds good. I will produce some recordings ASAP.
I was looking for a can capacitor for Northcourt Mk II and found the AudioCap website. A good source for specialist audio capacitors. The capacitors for the Northcourt rebuild were ordered from there and arrived within a couple of days. Other components will be ordered from AmpMaker but the next step is to plan the layout. More on that later.
This follows on from my previous post about a small bass amp. In the time since I made that post I’ve been considering my options with the help of the guys over at basschat.co.uk. I had it in my head that bass amps would require some sophisticated EQ circuitry to be usable but it turns out that the big issue – for a valve/tube amp – is making sure that your output transformer is large enough. People who like valve amplifiers seem to be happy with simple tone controls.
I have taken the time to draw out the schematic of my Northcourt Fifteen and it it turns out that it is different from the schematic I found on the web. The differences are primarily in the power supply and in the tone-stack which are two areas that will need to be redesigned to make sure the amp produces a good, solid bass tone. I used the Duncan Amps Tone Stack Calculator to model the tone controls on my Northcourt amp as it is today and I have uploaded the plot as a PDF file. There is good control of the bass but almost no control of the treble output. The values of most of the components in the tone control will have to change.
The power supply seems to have unnecessarily large reservoir caps for the two ECC83/12AX7 valves/tubes. The first ECC83 has two 50uF capacitors; one for each side! Normally, when the two halves of a preamp valve are working out of phase they are fed from the same reservoir cap because one valve is pulling more current while the other is pulling less so the action of one tends to compensate for the action of the other. That is why the phase-inverter (the second ECC83) can fed from a single reservoir cap but, even then, it appears to be larger than in needs to be.
I have sketched out my first thoughts on a revised schematic and you can see it here but I’m not going to build anything until I have had time to read more of Merlin Blencowe’s books. Feel free to give me any feedback in the meantime.
While browsing the Valve Wizard website I discovered that Merlin Blencowe had written and self-published two books called “Designing Tube Preamps for Guitar and Bass” and “Designing Power Supplies for Tube Amplifiers”. I have long been a fan of Merlin’s clear and informative style of presentation so I decided to buy both and I’m glad I did.
From my experience it is unusual to find as much useful information in a modern book on Valve/Tube Electronics. I have only had time to read sections of the book about Preamps so far but Kevin O’Connor’s “The Ultimate Tone” is the only other book I have read which has provided so much food for thought. Merlin’s books explain the fundamentals of circuit design so do not make for easy reading but Merlin takes a practical approach which makes the subject easier to digest.
I bought both books from Amazon in the UK. Both books are still available despite Merlin’s warning that the volume on Power Supplies is now out of print. At just over £40 including postage for both books, I think they offer exceptional value for money.
Today’s Class D amps and improved speaker technology make bass rigs ever smaller, lighter and cheaper which is a good thing for the gigging bass players among us. I don’t think there can be any argument about this but I’m not a gigging bass player – I’m a guitar player who plays the bass for fun – so I’m more interested in tone, size and power output. I want my bass rig to sound good at a sensible sound-level and to be small enough to fit into a home environment. I have an old Park (by Marshall) practice amp but the sound quality is pretty dreadful and dialing in a decent tone is almost impossible. If I play the bass through a simple headphone amp, leaving the tone-controls flat, the sound quality is hugely improved but I don’t like using headphones. So I need a new rig and that’s it in the picture; an old valve amp sitting atop an Ashdown Mi10. I love the size of the mi10 cabinet; the rig doesn’t even reach my knee. It probably isn’t a very efficient speaker but, in this application, it doesn’t really need to be.
Unfortunately it doesn’t work yet and it probably won’t work for some time. The amplifier is a “Northcourt Fifteen” that dates from the 1960s and needs some TLC and a bit of a redesign to tailor it for a bass guitar. I can’t find much information about these amplifiers anywhere but I did find this snippet on an eBay listing:
A lesser-known part of the story of British amplifier manufacture in the sixties, Northcourt specialised in affordable “cage” amplifiers for guitar and bass. They were closely related to R.S.C and Linear, who sold similar amplifiers to be used for music playback and P.A applications. Like their ‘Hi-Fi’ cousins, Northcourt designed their amplifiers to be used in “ultra-linear” mode, which resulted in very clear, transparent amplifiers with a low degree of distortion. Due to their linear nature, Northcourt amps never really reached the output levels suggested by their names. The Northcourt 15 was realistically somewhere between 10 and 12 watts before clipping.
If anyone has any information I would be very grateful.
Looking at a schematic I found on the web confirms the ultra-linear design. This approach is more common in hi-fi amps than in guitar amps but I think it would be very suitable for a bass and 10 watts of output will be quite enough for my needs. The output transformer has a 3Ω tap and the Ashdown Mi10 is a 4Ω speaker so there are no problems there. The tone-stack is closer to a Baxandall tone-stack than the usual guitar tone-stack but it’s not one thing or the other. The input is also peculiar, having a volume control for each input placed before the first valve. My immediate thoughts are to use the two inputs for active and passive pickups and use one volume control to govern pre-amp gain and the other as a master volume to control the input to the power stage.
The reservoir capacitors seem rather large for the pre-amp valves where only a milliamp or so is pulled so I am not sure whether I should try reconditioning the can capacitor or replace it with more sensible values. Looking at the inside of the amp reveals a lot of old carbon resistors and some ceramic capacitors that should probably be replaced. The paper-insulated transformers appear to be intact. It may be that the output transformer is a little small for bass but I don’t have a five-string and the feedback loop should compensate to some extent.
The next step will be to sketch out some ideas for the new schematic. I won’t get it right first time – I never do – so please feel free to share any thoughts on you have on this project.