Amplifier Musings

Nomad

Over the years, I've had an interest in making my own amp. I have some previous experience - made the Maplin Millenium 4-20 stereo hi-fi amp from a kit, and built a single-ended EL84 guitar amp using tagstrip and point to point wiring. Also messed around with valve preamp circuits for a bit using a plank of wood, some nails, and tinned copper wire to link the bits together. While I'm certainly no expert, I have a reasonable understanding of valve circuits (and the associated risks).

I'm not sure if this will ever come to anything, but I feel it would be good to open it for discussion, if only to improve understanding for myself and maybe others. Of course, better understanding may well make everything look more feasible and lead to making a real amplifier.

Here are my initial thoughts on the amplifier's features...

Combo, either 1x12 or 1x15.
Prefer a ready-made cab and chassis.
Power about 30-40W.
Must be super clean.
No PCBs - all turret board. tagstrip, point-to-point.
No silicon in the signal path.
Valve-driven FX loop.
EQ: Bass, mid, treble, presence.
Overdrive channel, perhaps with its own EQ.
Possible input for a Hammond organ (if so, must have overdrive).
Possible reverb, valve-driven if so.
Option to use open-back or closed-back.

It must stay clean at volumes louder than I can get away with in the living room. I can get away with my Laney Cub 10 pushed into power amp overdrive, and the new amp must stay clean at louder volume levels than that. I realise 30-40W is probably too much for even my tolerant neighbours, and I have no particular plans to take this out of the house, but my reasoning is that, for the amount of effort that will go in, I'd rather have something that could be used - clean - for drummer-volume jams and maybe gigs.

A special Hammond input may not be needed if I can work out a better way to take an output from the organ, or treat the existing output, such that the amp doesn't get damaged (I believe the existing switch is connected to the speaker output in the Hammond - A100 has built in speakers).

I'm not especially interested in trying to design a circuit from scratch. There are plenty of existing designs, and I'm more inclined to mix and match from those.

I'm interested in using 6L6 output valves, for no particular reason other than they seem to be popular for Fenders, which have a strong reputation for staying clean.

Are there standard 6L6 output transformers that are designed to deliver the sort of power I'm looking for?

Any suggestions for 6L6 power amp circuits to consider?

Although the main requirement is that the amp be super clean at louder volumes, it would be good to have the option to reduce the output volume and induce power amp overdrive. Assuming I can do this by reducing the HT, can I safely use the same output transformer? If I want to have variable/switchable max power, should I be looking at cathode bias?

Phase splitters - what are the pros and cons of the various types? (In other words, why use two stages of a preamp valve, when you can use one?)

Any suggestions for a schematic drawing program that is free or cheap? (Doesn't need to do circuit simulation.)

monquixote

You could get a kit from here: http://www.ampmaker.com/store/home.php

Nomad

Been to the AmpMaker site a few times, but none of their designs is really what I'm looking for. Possible source for components, though.

Found this open source schematic creation program...

http://sourceforge.net/projects/tinycad/

Seems pretty good so far, although valves are limited to a single triode, tetrode and pentode (no half of a double triode). Haven't seen a jack socket symbol yet. However, the user manual says there is a means to edit symbols, so I'll look into that.

ecc83

Hi Nomad.

Get and devour Merlin Blencowe's Tube Pre Amplifiers for Guitar and Bass. There you will find all you need to decide on PI type and how to keep things clean (or indeed, make them dirty!) .

These are just MY random thoughts....

6L6? Fine bottle but if you want clean you want a valve that is easy to drive,EL34 IMO.

Might be expensive but look into using an "ultra linear" output transformer.

"No Silicon in signal path". Will you stand for a solid state rectifier? This will allow bigger caps and give a "stiffer" power supply (sag means less power = UNclean!)

"All valve FX loop". Merlin gives several circuits but it always seems daft to me when guitar FX range from very decent (cough!) solid state designs to really crap old Germanium tat!  A top range chip such as the NE5532 will add buggerall to the sound.

And why a circuit drawing software? For a one off, pencil and paper.

Good luck.

Dave.

NervousJohn

The main reason for using schematic capture packages is to generate net lists for PCB layouts - it allows automatic checking and you get a layout that corresponds to the schematic.

You can also generate SPICE net lists for a simulator, but you tend to need workarounds to make the models work.

For an amp on tagboard I'd be inclined not to bother with either and look at the analysis someone else has done.

http://www.ampbooks.com/home/classic-circuits/fender-champ-5e1-preamp/

ecc83

"For an amp on tagboard I'd be inclined not to bother with either and look at the analysis someone else has done."

Indeed and you can pretty much organize the tagboard as the circuit. Unless you want to cram things into a tiny chassis or go for massive gains, valve amps are not prone to instability.

Dave.

Nomad

Dave,

I'll check out the book. In what way are EL34s easier to drive than 6L6s? (ie, what do you mean by easier to drive?)

Found this circuit last night (about half way down)...

http://ejjamps.com/articles/6V6SE-6L6GC-amplifier-circuits.html

I suspect it's intended as a hi-fi amp (referred to as a monoblock). It has extra taps on the transformer that go to the screens. Same with the Maplin stereo kit, which I think was what made that ultra-linear (was a good 10 years ago when I built the latter, so memory is shaky). Last night, I was wondering about using something like this but, in my possible ignorance, thought about doing something without the ultra-linear aspect. I ended up wondering if I should be more pragmatic and copy something guitar-based that is already known to stay clean.

Silicon rectifier is what I intend. I once had a valve rectifier amp, and found the sag interesting, but not really what I prefer. Point taken about what is in effect pedals (I'm not above reconsidering the all-valve idea).

All,

There are other reasons for using schematic drawing software. It's easy to read, easy to edit the design, you can move the bits around when you reach the edge of the paper, and it's less hassle to publish online to get suggestions from others. No PCB manufacture intended, but I dare say a human-readable netlist could be useful for checking the physical circuit once it's built, if only to make sure that all bits are connected properly. My limited experience of spice tools has left me feeling that they're more hassle than I want to get into - again, there is an argument for being pragmatic and using circuit blocks that are known to work.

It might be worth describing my experience of a clean setup, since that's my real-world reference...

Years ago, I used an old 50W Selmer Treble & Bass into a sealed back 4x12 size cab that had baffle cut-outs for 2x12. I used a fairly high output guitar (Duncan SH-5 and Hot Rails) plugged into the bass channel. The speakers were two Celestion G12H greenbacks. Tone settings were usually bass 7, treble & presence 6.

The sound was very clean and easily kept up with a drummer. When playing chaka-chaka funky stuff, the attack was excellent and never sagged. Low notes were solid with no flub or wooliness. If anything, the overall tone was a bit harsh, but, as I've learned recently, this could be due to the magnets in the pickups (both ceramic). I also felt that it was great for chords, but not so good for lead - not enough mid, even though my tone settings weren't especially scooped.

To an extent, I'm looking to get something similar in terms of clarity, attack and bass, but perhaps a bit more musical sounding (and accept that different pickups might be all that's needed for that). The option to dial in more mid frequencies seems like a sensible idea.

Misterg

+1 on Merlin's book.

The Selmer T'n'B schematic is here:

http://www.vintagehofner.co.uk/selmer/schematic/leaf.html

It uses a couple of EL34s on ~450V (Although, frankly, the type of valve has little to do with how 'easy' it is to keep clean - it's more about designing the circuit to suit the valve in question, rather than adopting a 'lego' approach to join different chunks together without necessarily understanding the ins and outs - in particular preamp voltages. All IMHO).

For what you want, I would suggest persevering with *LTSpice* - it has a graphical interface and produces passable schematics. There is also an (excellent IMHO) valve library for it by Steph(?) Bench. Zip file & instructions here:

http://www.intactaudio.com/forum/viewtopic.php?t=523

If you can master LTspice (and it's a lot easier than some of the others), you can model the circuits that interest you and see how they respond, then try some modifications and see what effect they have. Although it is satisfying to model a complete amplifier, you can learn a lot just by modelling one or two stages, and it is much quicker. When you've digested it all, then you can design your 'dream' amp using the bits that you've learnt and tweak it in software before building it for real. You can then fine-tune it by ear, knowing that your design is fundamentally sound.

£0.02

ecc83

The EL34 is easier to drive than the 6L6 because it has about twice the "mutual conductance" (gm) at about 10mA/V . This means in simple terms, that change of 1 volt on the control grid will give rise to a change in anode current of around 10mA whereas for the L6 the change would only be around 5mA . The result therefore is that you need only about 1/2 the drive voltage from the PI for the same power output  and less drive volts means less distortion.

Tapped OPT? Just an idea. It is a way to get lower distortion without overall NFB but I found a map for the Selmer T&B MK1 and that uses EL34s in a very common circuit with an ECC83 and no UL traff but overall feedback. Many ways to skin the cat! ...These guys do both "budget" guitar OPTs and hi fi jobbies. Not a lot in it for price.

http://livinginthepast-audioweb.co.uk/index.php?p=xfrmrpp

Drawing software? Been doing it with p&p so long I can't spare enough of my life to learn it on a bloody PC! I also have a scanner/copier which helps.

Dave.

ICBM

ecc83 said:

The EL34 is easier to drive than the 6L6 because it has about twice the "mutual conductance" (gm) at about 10mA/V . This means in simple terms, that change of 1 volt on the control grid will give rise to a change in anode current of around 10mA whereas for the L6 the change would only be around 5mA . The result therefore is that you need only about 1/2 the drive voltage from the PI for the same power output  and less drive volts means less distortion.

While that's absolutely true, it's interesting that the reverse "sounds" to be the case! - 6L6 amps generally sound cleaner than EL34 ones - and even in the same amp where you can run both types. 6L6s have a clearer, smoother sound and EL34s a more complex, 'hairier' sound.

jpfamps

I would probably go for a 6L6 design too.

re power amp, there is no point reinventing the wheel, and as there are plenty of off-the-shelf repro transformers, you can borrow the power amp design from an existing amp, which if you are inclined to go with 6L6s would be a Fender, so I would probably go for a BF Bassman output stage (which of course also determines the PI).

If you are not bothered about a valve rec then Tube Town in Germany do some nice toroidal mains transformers at a good price, and for output transformers you have a choice of Mojotone (from Allparts UK), or Hammond. 

If you want maximum cleans I would go with a BF Bassman style transformer as this has a larger core and is interleaved, so will have less insertion loss, and more bandwidth.

You do need to pay attention though to what taps are available on the transformer secondary as many repro Fender transformers only have one secondary tap like the originals (which may work for you). Most manufacturers all offer a version with more taps.

I wouldn't go down the ultra linear route to be honest. We've experimented with it and whilst the clean sound is nice, once you start to clip the power amp it doesn't sound great. Also neither 6L6s nor EL34s were really designed to run ultralinear (although that doesn't stop people trying.

guitarcookie1

Nomad said:

Been to the AmpMaker site a few times, but none of their designs is really what I'm looking for. Possible source for components, though.

Found this open source schematic creation program...

http://sourceforge.net/projects/tinycad/

Seems pretty good so far, although valves are limited to a single triode, tetrode and pentode (no half of a double triode). Haven't seen a jack socket symbol yet. However, the user manual says there is a means to edit symbols, so I'll look into that.

I have the Ampmaker Double Six and unless you switch out the EQ it stays clean right around the dial...

ecc83

Ok, now the heavyweights have weighed in I shall bow out and 6L6 seems to be that way to go. (can I mention KT88?)

" Also neither 6L6s nor EL34s were really designed to run ultralinear"

Don't know about the 6L6 F but surely this was a key design point of the Mullard 5-20?

Note, I am talking "hi-fi" now I appreciate that guitar amps are a special breed.

Dave.

jpfamps

ecc83 said:

Ok, now the heavyweights have weighed in I shall bow out and 6L6 seems to be that way to go. (can I mention KT88?)

" Also neither 6L6s nor EL34s were really designed to run ultralinear"

Don't know about the 6L6 F but surely this was a key design point of the Mullard 5-20?

Note, I am talking "hi-fi" now I appreciate that guitar amps are a special breed.

Dave.

You can run both 6L6s and EL34s UL, however they weren't designed for this mode of operation (unlike say the KT77 or KT88 which were).

Fender did of course employ UL on their higher powered amps in the late 70s with over 500 VDC on plate and screens, which is not great with current production valves.

EL34s have a higher screen grid rating. The Selmer Zodiac 30 uses a pair of EL34s in UL configuration; I guess this was a hi-fi influenced design.

Hi-fi tends to be class A operation and hence lower HT and screen voltages, and aren't usually driven hard into clipping...

Remember the screen voltage in UL goes up and down with the anode voltage.

Another disadvantage of UL  is that more care is needed to reduce hum in the output stage, either by filtering the HT with higher cap values, or balancing the current draw of the power valves (or both), as the screen supply doesn't have another LC or RC filter to reduce ripple on the screens. (In pentode mode it's the ripple on the screen supply that causes hum

If you want really clean, either KT88s or 6550s (which were used in the Leslie cabinet amps) is a good option. I've used KT88s on several amps, and even with modest HT voltages (480 or so) 70W is achievable, and you are using the valves very conservatively, so should be very reliable. However, it will require a little research to find off-the-shelf transformers for these valves, whereas if you are copying a Fender output stage it's easier to identify the correct parts.

hywelg

Sounds to me like you are describing a Dumble HRM clone, with 4 6v6's or 4 6L6's with half power option. Plenty of schematics for an added valve FX loop. Clean all the way up (not on OD side though obviously, though it can be set to 'pushed' clean) provided you dont goose the input volume or bypass the stack.

Nomad

Thanks, all, for the inputs so far. I would like to throw a curve ball into the discussion...

Stereo.

After many years with mono pedals, I have been gradually replacing things and have ended up with some stereo pedals (MXR Tremolo, TC Nova Delay and HoF reverb, Boss stereo volume pedal). Yesterday, I ended up ordering two of the Harley Benton G112 cabs loaded with V30s. The two cabs make more sense for me than a single 2x12. I can stack them together, or use one on its own, and connect (with appropriate wiring) to my Laney Cub 10. Or I can put them on either side of the room in a stereo setup. There is also the possibility of replacing the screwed-on open back panels/baffle extensions with single 18mm ply boards to make the closed back. Although this would be primarily for guitar, part of the reasoning behind getting stereo pedals was for use with keyboard as well (particularly a Rhodes sound using a MIDI controller board and hardware sound module.

Either way, the addition of the 1x12 cabs mean that maybe I'm looing at making an amp head rather than a combo.

Assuming the Maplin Millennium 4-20 still works (hasn't been used in years), I dare say I'll be trying that initially to see how things go in stereo. If I like the idea of stereo, it would seem that I have a couple of options: build a stereo amp using lower powered output valves (eg, 6V6s), or cannibalise the 4-20 for its output transformers and valves.

The 4-20 is a bit heath-robinson - it has a single PSU for both channels, which is known to be marginal for running both at max power, and is built into three ally boxes held together with little joining plates. The output transformers are quite small (not much bigger than what might be used for 6V6s). It's unlikely to ever be used for hi-fi and is gathering dust, although I feel it would be a shame to pull it to bits if the cost of a couple of transformers and 4 smaller valves isn't too much. A new power supply would be needed in any case, I'd still want to redo the circuitry on turret board, I suspect some sort of stereo preamp with EQ would be added, and it would need a new chassis and put into an enclosure.

If I was to reuse the 4-20 OPTs and valves, can I (should I?) disconnect the UL tappings and use resistors (or whatever) to the screen grids? Or would I be better considering something new, about 15-20W per channel, using 6V6s? How should I approach the preamp? Dual inputs with ganged pots and a pair of EQ circuits? If I wanted to use it as a mono amp, can I have a pair of standby switches and use one to kill one channel?

Nomad

Cabs arrived during the week, and I've been trying them with the Millennium 4-20 hi-fi amp. I like the stereo guitar thing. Never really done that before, other than a couple of short trials through the hi-fi, and never really got going because there's always paranoia about damaging the speakers. No worries with the robust guitar speakers (Harley Benton G112 cabs with V30s).

This amp and speaker combination has bags of volume - even playing fairly gently with clean sounds, it can go louder than I dare use at home. The original Millennium 4-20 mains transformer was under-specified for a stereo setup (fine as a monoblock), and they were known for failing. Mine failed some years ago, and I'm pretty sure that I bought the uprated transformer that came out later. This also has a slightly higher HT, and the amp can apparently max out at 27Wpc with this. Max power is delivered with a 220mV input. I did try a couple of heavy strums with the guitar clean and all volumes at max, and my impression was that there was some harshness - not sure if the amp was being pushed, or whether my ears weren't responding favourably. It was fairly early on, and the volume was bordering on discomfort. At anything less than this, it's completely clean.

One thing I've found is that it needs EQ for guitar - there's a lack of top end and general muddiness without it. My GEQ pedal has been on almost permanently with this setup, whereas it's hardly ever on with the Laney cub 10.

Hmmm....