Finished building my first amp!

thermionic

I've had the ampmaker site bookmarked for a long, long time, but I finally decided to give one a go. The old SE-5a has been improved with a couple of front panel switches so I ordered a kit a few weeks ago:

https://i.imgur.com/DuxetYa.jpg

It's a lot of work, I spent at least two wet and windy weekends on it. The instructions are fantastic, especially bits that help avoid you getting into over-fiddly situations where you can't get a soldering iron near where you need it because there's previous wires in the way.

https://i.imgur.com/uSbU0Ur.jpg

https://i.imgur.com/MhOHIfy.jpg

https://i.imgur.com/MM6Siqf.jpg

The instructions come with a list of comprehensive tests split into several stages. I followed them carefully because I've never worked with high voltages before. There was one voltage that was 100V lower than it should be but it didn't take me long to spot the mistake (I used to be an electronic engineer) - a 1M resistor when it should have been 100k. I had damaged a couple of resistors (twisted the legs off) and bought spares from Maplins, so luckily I had a spare 100k to put in the right place.

https://i.imgur.com/DDAMn2Z.jpg

Thermionic emission!

https://i.imgur.com/T8KsCm9.jpg

The finished article. Currently with an EL34, hoping for a proper Marshall sound. I have an EL84 to try as well, courtesy of Karltone.

https://i.imgur.com/MLIx8rn.jpg

I have a cab on order and I'm looking for a used Greenback for it, but I couldn't resist using the 2x10"s of my AC15 for a quick run-through. It sounds pretty good, the VVR voltage scaling works well, turn up the gain and the master, bypass the eq for a really smooth power valve distortion.

ThePrettyDamned

I tell you what, that lead dress doesn't look like it's your first! I assume it's probably nice and quiet

Glad to hear it sounds nice, too.  How much power do you get with the el34?

Nice work

thermionic

It is pretty quiet, yes. I'm reasonably happy with the wiring even if I didn't get it quite as tidy as I'd hoped. For the heater wires I put the ends in the chuck of a drill and hooked the other end to somewhere convenient (the brake lever of a nearby bike in this case). Perfectly twisted wires in 3 seconds!

According to my calculations, the plate dissipation is 9.0W with an EL84 and 6.9W with the EL34. The output is supposed to be 5W (I'll have to read up a bit more on valve amps to see how plate dissipation relates to output)

ecc83

Yes, that is bloody good for a first attempt!

Tip on the heater wires. Hook the wire to something fixed, like a bench fixed to a wall! Then, as the wires tighten up "go with them a bit" then stop the drill and let it relax. Takes a bit of practice but you can get a really tight twist that does not unwind.

And, my personal bête noir! Bare mains connections! I am surprised the kit did not contain boots and sleeving for deading conns!

But very well done.

The actual power into a load is not really related to the valve dissipation. You need a big, 100W, load resistor of the correct value and a source of steady 1kHz sine tone (computer) . Connect an AC meter across the load and P=V squared over load R....VERY rough and ready without a scope but it will tell you if the power out is in the ball park. (10W for EL34. 5W for EL84).

Dave

thermionic

Bare mains connections certainly had me on my toes when I was testing it. Very careful to remember to disconnect it from the mains every time I needed to attach a multimeter probe. I was was very aware of the DC on the filter caps, but it's easy to forget that there's 240V AC lurking there before you've switched it on at the front panel. 

That's how I did the heater wires Dave, although I could have probably have given it a few more turns before releasing the trigger of the drill and got it a little tighter. I probably left them a little too long in general, knowing the consequences of trimming a wire too short would mean desoldering at the other end...

ESchap

Great stuff @thermionic ; Damned addictive hobby!  what's next?

TheOtherDennis

Top work that man. Have a wow.

martinw

thermionic said:

Bare mains connections certainly had me on my toes when I was testing it. Very careful to remember to disconnect it from the mains every time I needed to attach a multimeter probe. I was was very aware of the DC on the filter caps, but it's easy to forget that there's 240V AC lurking there before you've switched it on at the front panel. 

Insulating mains connections only makes sense in equipment where all the 'lethal' voltage terminations can be covered.

The point being that in most modern equipment (from hifi to industrial stuff) once you're past the power supply there are only 'low' voltages present on the system (5V, 12V 15V etc). It's easy and logical therefore to insulate any exposed mains or other 'high' voltage terminals, and render the equipment as close to 100% safe as possible. This enables reasonably safe Live working.

Additionally, most modern equipment can be regarded as essentially 'off' when unplugged.

Neither of these are the case in a valve amp. It can never be regarded as 'off' due to the filter caps, and there are so many dangerous contact points in the amp that the only safe way to proceed is to regard ALL terminals as live ALL the time. NEVER touch anything EVER inside the amp unless you know you have unplugged the amp, drained the caps and checked with a meter that there is no voltage, AC or DC, anywhere.

IMO it is bad practise to insulate some of the lethal voltage terminations in an amp, worse than insulating none of them. It's about the mindset of the technician/operator and the level of care exercised, and might lead to a belief that Live working is in some way safe.

ICBM

I agree with martinw.

I do tend to sleeve mains connections if I'm working on an amp which already has them done and something there needs replacing - I just think it's good practice to keep it consistent (the alternative would be to remove them all completely) - but I don't on anything which doesn't have them in the first place.

I also don't like to twist wire pairs like that - because you are twisting them, they have a tendency to unwind. The best way to do it is to secure them at one end, then *pass* the wires round each other - like plaiting a girl's hair, if you have daughters - without twisting them. You can get them just as tight that way if you put a bit of tension on, and they never unwind because that would actually require the wire to twist.

Nice job, thermionic!

equalsql

Nice looking job you've done there. I built my first valve amp too last year and the sense of satisfaction when it was completed was enormous. I really enjoyed the build... and I learnt a lot too... which I plan to apply to my next build.
Hope you had as much fun as I did.

More pics when it's in the cab...and some sound samples/video would be nice too

usedtobe

Nice job!

hywelg

ecc83 said:

Yes, that is bloody good for a first attempt!

Tip on the heater wires. Hook the wire to something fixed, like a bench fixed to a wall! Then, as the wires tighten up "go with them a bit" then stop the drill and let it relax.

Then give them a waft over with a hot air gun just to warm the plastic, its stops it 'relaxing' when you take the tension off.

chillidoggy

Now I'm really getting interested! Excellent job!

jpfamps

martinw said:

thermionic said:

Bare mains connections certainly had me on my toes when I was testing it. Very careful to remember to disconnect it from the mains every time I needed to attach a multimeter probe. I was was very aware of the DC on the filter caps, but it's easy to forget that there's 240V AC lurking there before you've switched it on at the front panel. 

Insulating mains connections only makes sense in equipment where all the 'lethal' voltage terminations can be covered.

The point being that in most modern equipment (from hifi to industrial stuff) once you're past the power supply there are only 'low' voltages present on the system (5V, 12V 15V etc). It's easy and logical therefore to insulate any exposed mains or other 'high' voltage terminals, and render the equipment as close to 100% safe as possible. This enables reasonably safe Live working.

Additionally, most modern equipment can be regarded as essentially 'off' when unplugged.

Neither of these are the case in a valve amp. It can never be regarded as 'off' due to the filter caps, and there are so many dangerous contact points in the amp that the only safe way to proceed is to regard ALL terminals as live ALL the time. NEVER touch anything EVER inside the amp unless you know you have unplugged the amp, drained the caps and checked with a meter that there is no voltage, AC or DC, anywhere.

IMO it is bad practise to insulate some of the lethal voltage terminations in an amp, worse than insulating none of them. It's about the mindset of the technician/operator and the level of care exercised, and might lead to a belief that Live working is in some way safe.

I completely agree with this, working on a valve amp whilst it's live is never safe.

What does amaze me though is the number of amps that don't have bleeder resistors on the filter caps (including Blackstar amps).

ICBM

jpfamps said:

What does amaze me though is the number of amps that don't have bleeder resistors on the filter caps (including Blackstar amps).

A bleed resistor adds about 0.1p cost to the amp and has no benefit whatever from the manufacturer's point of view .

thermionic

There are bleed resistors on this - it's quite informative to watch the voltage drop while you still have the multimeter connected.

One thing that puzzled me though - the instructions stated unplugging from the mains after testing each voltage. Good practice, as the AC is still live as mentioned above. However, I thought the bleed resistors needed the ground connection through the mains plug (when switched off) to discharge the caps. I tried it both ways and the voltage dropped at more or less the same rate. If I've pulled the plug out, where's the path to ground?

ICBM

The ground the voltage drains to is the amp's own internal ground (chassis usually), it doesn't need an external connection.

thermionic

I realise the resistors are connected to a ground wire that is soldered to a tag bolted to the chassis, but where does the charge go from there? 

The IEC socket earth is also soldered to a tag bolted to the chassis. I thought the bleed resistors needed this connection to earth (i.e. the earth) to discharge. If there's no connection from the chassis to a "bigger" earth there's nowhere for the charge to go apart from distributed around the metal of the chassis.

viz

Wow.

Misterg

thermionic said:

where does the charge go from there? 

The charge just needs to get from one end of the capacitor to the other - think of the capacitor as a little battery that has been charged up when the amp was plugged in - the bleeder resistor will 'run it down'.

Nice work, btw!

thermionic

Ah right, of course - current dissipated as heat, I²R.

Pretty sure I read somewhere about leaving the mains lead plugged in but switched off to discharge filter caps, I should check the books I have...