B15-inspired Amp Head Build

[JapanAxe]

Test fit of eyelet boards (one partly populated) and most front and rear panel components.

[JapanAxe]

Yesterday was all about metalwork. Most of the chassis hole are now drilled and/or punched. The PT leads will pass through the rubber grommets immediately beneath where it is mounted, but I need to finalise the points at which the OT leads will enter the chassis as they need to be away from the preamp area.

[Richard R]

I've been following this with interest. Electronics was never my strong suit,  though I still have my battered copy of Horowitz and Hill from Uni days. 

[Hellzero]

Very neat work !

[JapanAxe]

11 minutes ago, Hellzero said:

Very neat work !

Thank you! This time I used a 30mm panel cutter for the valve socket holes instead of a step drill.

[Mikey R]

Mert told me you were building an amp from scratch! I'll follow this build with interest!

[bertbass]

Following with interest as well.

[JapanAxe]

I finally drilled the small guide holes that I will eventually widen for the self-tapping screws which will secure the cover to the chassis. I have ordered what I hope will be suitable screws. I also drilled a hole for the safety earth bolt and exposed bare metal around this and the input socket hole using a Dremel-type tool.

All the panel hardware is now firmly mounted except for the input socket. It's always satisfying when the valve sockets are in. Next up, a load of soldering.

[Jabba_the_gut]

5 hours ago, JapanAxe said:

I finally drilled the small guide holes that I will eventually widen for the self-tapping screws which will secure the cover to the chassis. I have ordered what I hope will be suitable screws. I also drilled a hole for the safety earth bolt and exposed bare metal around this and the input socket hole using a Dremel-type tool.

All the panel hardware is now firmly mounted except for the input socket. It's always satisfying when the valve sockets are in. Next up, a load of soldering.

Really liking this build - it is looking superb.

On 14/06/2021 at 22:07, Richard R said:

I've been following this with interest. Electronics was never my strong suit,  though I still have my battered copy of Horowitz and Hill from Uni days. 

Crikey - had forgotten about that book from my uni days too!

[Merton]

5 hours ago, Jabba_the_gut said:

Really liking this build - it is looking superb.

Crikey - had forgotten about that book from my uni days too!

@Mikey R and I probably still have ours lurking too 🤓 I was always stinky poo at electronics though which is a shame because I’d love to be able to do something like this

[JapanAxe]

You may have noticed there are no valve retainers fitted. I did buy ‘bear trap’ retainers but decided not to fit them - the valves will stand above the chassis and the head is for home use only, plus it is a pain to remove valves from bear trap retainers.

[JapanAxe]

I re-checked my layout this afternoon and found a couple of silly mistakes (plus another after I took the photo). After that I had a bit of a solder session and finished populating both eyelet boards, as well as fitting most of the flying leads, except the connections to the ground bus, which will be made with the boards in place. It's always worth checking component values on a meter before installing them - I found a stray 1ohm resistor in my 220k bag!

[bertbass]

Nice!

[JapanAxe]

I've now installed the input jack, screened cables, and tone stack board.

[Jus Lukin]

-

Edited March 14, 2022 by Jus Lukin

[JapanAxe]

7 minutes ago, Jus Lukin said:

Looking great so far!

This is the fun part!

[JapanAxe]

I have now fitted both transformers and made the connections for the mains and HT. The OT primaries are above the chassis at present as they will pass through at a different point.

[Bassybert]

Very nice indeed, enjoying seeing the progress of this build.

[JapanAxe]

14 hours ago, Bassybert said:

Very nice indeed, enjoying seeing the progress of this build.

I'm always a bit fearful that, when I do my pre-checks and plug an instrument in, I'll be greeted by a tsunami of humming and buzzing.

[Beer of the Bass]

2 hours ago, JapanAxe said:

I'm always a bit fearful that, when I do my pre-checks and plug an instrument in, I'll be greeted by a tsunami of humming and buzzing.

Getting the output transformer secondary leads reversed is a fun one - you end up with positive feedback around the power amp (instead of negative), and a loud howl that made my dog hide under the sofa the time it happened to me.

[JapanAxe]

17 minutes ago, Beer of the Bass said:

Getting the output transformer secondary leads reversed is a fun one - you end up with positive feedback around the power amp (instead of negative), and a loud howl that made my dog hide under the sofa the time it happened to me.

Yep, done that one - only scared myself though!

[JapanAxe]

Today I made more connections from the transformers to the panel-mounted hardware, added the remaining resistors to the valve sockets, and installed the eyelet board. I ran a copper bus through all the speaker ground connections to make it harder for them to work loose. I have to remember to run another cable from this to the main ground bus, otherwise there will be no ground reference for the NFB circuit.

My box of bits is now looking somewhat empty - valves, knobs, feet, fuses, pilot bulb, and a few screws.

[JapanAxe]

As of last night, all the connections are now made apart form the 6.3V heater supply. The stray white wire is the heater centre tap.

[JapanAxe]

All the heaters are wired up now. Those pink and purple wires sit well above the audio connections to the valve sockets. I have elevated the heater centre tap by connecting it to the cathodes of the output valves - as I understand it (see Blencowe 3.17), this creates a DC leakage current that is saturated, and consequently unaffected by any AC component from the heaters.

All the other bits are now fitted. Time to fire up and plug in - NOT. I'll put this aside and come back to it tomorrow, when I'll check all my connections against the schematic and layout, both visually and with a meter, before running through a staged start-up sequence.

I may change my mind about the knobs - I've already done so several times!

[jimbobothy]

… very cool, well done 😎