Tube amp build (s)

[JohnH]

And who thought a studio on the second floor without a lift was a good idea????

Well, to be fair it is an artist's studio/gallery, so they probably weren't expecting people to drag amps and cabs to their exhibitions, ha ha ha

You could always swap out the heavy V30s (4.7kg each) for some neodymium magnet speakers. I like the Celestion Neo Creambacks. Weighs almost nothing in comparison (1.9kg each). Make two 1x 12” cabs and put the V30s in these as they’d be far more manageable. That would make the cab 5.6kg lighter, a considerable weight saving. Though if the cab chassis is really heavy, then you probably wouldn't notice much difference. Just a thought as you’ll probably be buying one new speaker anyway.

Good thinking Simon, I'll check them out

[Simon Barden]

Other neo speakers are available from Celestion and other makes like Eminence. They aren't cheap because neodymium is more expensive than a standard ceramic ferrite magnet (though you need less of it). On a par with the cheaper alnico speakers.

The Neo Creamback has a nice sound, less mid-forward than the V30s. Less efficient at 97dB vs 100dB for 1W input, but that does mean you can turn the amp volume up just that little bit more. Still go nice and loud though if necessary.

[JohnH]

I've been a bit slack at posting progress pics, but I've managed to get quite a lot done. The amp is pretty much all wired up - I'm just waiting on the faceplates to arrive so I can connect the IEC socket.

I've glued the headshell together, but haven't routed the roundover on the outside edges, or attached the internal bracing. I haven't got the tolex or other hardware for the shell yet either, so I can't do much more there.

Once all that's done I just need to buy a variac and build a current limiter and I can begin startup procedures!

An update on the cost tally:
$120 for the chassis
$41 for wire and resistors
$80 for capacitors
$47 for turret board, stand offs, assorted hardware, and an iec socket
$163 for tubes
$30 for timber for the head

[Andy40]

Hey John, Thanks for the updates. I think I ended up costing about $900 - $1000 in the end with all the bits including speaker, transformer, cabinet, tolex, grill cloth and the cabinet hardware. I wasn't trying to spare expenses though.

Definitely give some thought into replacing the ceramic 5W 270 Ohm resistor with a 10Wone. It reduces the heat a lot.

I also put in a bleed resistor to make it much safer to work on the amp so I don't have to keep bleeding the caps every time I need to work on it.

[JohnH]

Thanks for the tips Andy!

I did actually put in a 10W resistor, based on the same advice I'd read in another forum.

I think I forgot to mention too, while I was ordering parts I decided to swap from the mojotone layout to the Rob Robinette one:

[Simon Barden]

I certainly think the Robinette one is a lot safer electrically, with the live going through the fuse and then the switch. The Mojotone one has the live fused and the neutral switched, which works but is effing unsafe in my eyes! It may be original, but original doesn't always mean good!

The Mojotone also has a pointless ground switch shown, which is there only for appearances sake as it doesn't actually work, with a capacitor used for looks only. Your chassis probably has a hole cut for it though. Some people use the switch to add in a negative feedback option. https://robrobinette.com/5e3_Modific...ative_Feedback

[JohnH]

Yeah, I did a lot of reading which led me to the same conclusion. I'll have to get some pics this weekend, but I've got the live and neutral both switched, and obviously deleted the unnecessary ground switch and capacitor.

I realised after it arrived that the chassis has an IEC hole large enough for a socket with inbuilt fuse, so I think I will use one of those. It removes some extra wires and frees up another hole in the front of the chassis.

I'm keen to try the negative feedback option, but wanted to build it stock first so I can concentrate on making it work before I start changing anything. The chassis I have has the ground switch hole, but it also has a few extra holes at the rear, so there's lots of options for adding things later if needed

[JohnH]

Finally got the faceplate and wired in the iec socket.

I went through the rob robinette start up procedure (although to be fair I was a bit lazy and didn't build a light bulb limiter).

Voltages seemed ok so I plugged in a guitar. It sounds great but started red plating, so stopped and did voltages again and it seems I accidentally skipped V2 pin 7, which should be 17v but gives me 3v

Bit too tired to figure it out tonight. Will post pics and voltages later for anyone who has any ideas

[JohnH]

I can't edit that last post for some reason, but I should add heaters measured 6.1 across the pins, not referenced to ground.

I did have a bit of a weird thing measuring the heaters though. I measured the high voltage secondaries with black probe on the chassis and red probe on the pin (and got a stable reading of 350) but when I did the same for the heaters it just kept rising way past the 3v I expected (I took the probe off when it hit 45v because it freaked me out). Measuring across the pins gave me 6.1 which seems reasonable but I didn't think I'd get a runaway reading with the black probe to ground?

Is that just me missing something obvious?

[JohnH]

Oh, now it looks like my voltages have disappeared lol. I'll try again tomorrow, ha ha ha