Rebuilding a Broken Crack

[TypodCrow]

TLDR;
I recently bought a pre-loved Crack that was unbelievably mishandled by USPS. While moving through the motions of insurance refund from the seller/USPS, I thought I'd take a crack (pun intended) at rebuilding it. This thread will be my outlet for questions/notes I have for the process.

Original condition when sold to me:
The last set of repairs done on this unit were outlined in this thread: https://bottlehead.com/smf/index.php?topic=11140.15
Pictures available here: https://imgur.com/a/5I3K7xF
I'm unsure whether the steps outlined in those posts were followed, however, it seems the point is moot since the amp was quite dishelved when it arrived. I will be doing a rebuild, instead of a repair.

Mods on the amp as recieved:

• Speedball installed ('nough said)
• Alps blue pot installed
• Hotodded (ERSE MPX 100uF 250V) + bypassed (ERSE MET 1uF J 250V) output paps
• Choke (Triad C-7X) replacing the first resistor in the C-R-C-R-C filter
• PSU caps bypassed (ERSE MKT 2.2uF J 250V)

Rebuild steps:

• [Done] Deconsruct the amp and clean up the extra solder.
• [Done] Buy replacement tubes.
• [Done] Rebuild the wood chassis; will refinish it later when the weather is warm outside.
• [Done] Replace broken parts
• [Done] Source Maxhawk's tube switching boards. Details can be found by reading ths thread: https://bottlehead.com/smf/index.php?topic=5989.0
• [Done] Rebuild back to as-sold state
• [In Progress] Install togglable output transformers (Hammond 119DA) for lower impedance loads. (Yes, I know other amps exist. This is a personal choice I'd like to stick to for now.)
• [In Progress] Add Maxhawk's tube switching board. (Because I have an addictive personality. Pun intended.)
• [Dropped] Maybe add some vumeters because I have them sitting around and they're pretty.
• [In Progress] Make it all fit inside the stock chassis. Personal goal.

Broken parts to replace:
These will be sourced from https://www.digikey.ca, since I am Canadian and need to purchase the output transformer from there anyways.
Big thanks to the Doc, and Paul, and Derek for helping choose suitable replacements. I'd be lost without you guys.

Tubes: Found them from a friend.

Hookup Wire: The ever helpful Maxhawk has agreed to include what I'll need with the tube switched board shipment.

250V 1A Fast Acting GMA fuse.
270ohm 5W resistor in the power filter (it is cracked in two).
Speedball's N2222A is blown (confirmed by checking terminal impedance when disconnected).
Thermal pads for the Speedball TIP50's. Mica, as suggested below.
Hookup wire for signal path. I know I'm ideally looking for 20 AWG PTFE Solid Core. Settling for PVC taking ths suggestion in comments.
I'm taking the 600V suggestion, instead fo the 300V so that I can use the remains for my Stax Amp build one day.
https://www.digikey.ca/short/pqrm95

Thanks for reading folks, and in advance for your help.
Nelly

[Paul Birkeland]

Stranded hookup wire is a good way to take a broken kit and make it worse.  Solid core is the only way to go.

If the 270 ohm resistor that crosses over the power transformer is the one that's broken, then you're better off replacing it with a 10W part so it will reach across all the way.  I don't think the Yageo has leads that are long enough.

[TypodCrow]

Solid core is the only way to go.

If so, how badly will I be hurting the sound by using PVC instead of PTFE? I'm losing on the order of 200 bucks to get a single color or PTFE 20/24 AWG for 100' that I don't need and don't have anyone to resell to.

If the 270 ohm resistor that crosses over the power transformer is the one that's broken...

When I was taking it apart, the choke was installed across the transformer. I'll recheck my hand drawn schematic and compare it to the manual to see if I misquoted it being the first 270ohm in the filter. Either way, I'll be putting it back together in the same fashion (choke across transformer).

Lastly, what are the minimum specs for the thermal pads on those TIP50's? That seems to be one of the last things I can't decide on for part ordering.

[Paul Birkeland]

You can use whichever thermal pad you'd like.  We (usually) supply mica ones. 

Stranded wire doesn't hurt the sound, it's just impossibly hard to work with and the strands tend to go places where you don't want them.  The wire also doesn't stay where you put it. 

[TypodCrow]

Stranded wire doesn't hurt the sound...

Awesome thanks; I'll do some more due diligence tomorrow. If I can't find any decently priced solid core, I'll try to find a soldering pot on campus (University) to help with the clean insertion of the leads into the terminals.

[Paul Birkeland]

While the solder pot can help, what you're left with is a stripped lead that's tough to bend around terminals. 

When you're shopping for solid core wire, be sure if it's copper that it's plated with something and not just bare copper with PVC over it.  You want a barrier against oxidation on the leads that you strip.

[Deluk]

I don't know how the Bottlehead spares department works but wouldn't it be easier to just get the wire and components, those that were in the original spec of course, straight from them? Start from scratch. Build the new Crack and then add the SB after, as is the normal way to go. Mods later as and when.

[oguinn]

I picked up CAT5 and shielded twisted wire from Bottlehead before. Plus a bunch of other replacements for things I botched while building my kits.

[TypodCrow]

I don't know how the Bottlehead spares department works but wouldn't it be easier to just get the wire and components, those that were in the original spec of course, straight from them? Start from scratch. Build the new Crack and then add the SB after, as is the normal way to go. Mods later as and when.

Unfortunately, shipping to Canada gets expensive. The solid core wire is the only thing I haven't yet found a true replacement for.

[oguinn]

I'd imagine a run of solid core plus shipping from BH (if they'd do it, I don't work there and don't speak for them, obviously) is probably less than what you'd mentioned in the opening post about losing a couple hundred on a huge spool.

[2wo]

Parts connection is in Canada, they have wire...John

https://www.partsconnexion.com

[Deke609]

I'm in Toronto, and I use the following from Digikey.ca.

Fatter PVC insulation (I believe this is the same brand/model of wire that BH ships with kits): https://www.digikey.ca/product-detail/en/general-cable-carol-brand/C2118A.12.03/C2118R-100-ND/16236

Thinner PVC insulation: https://www.digikey.ca/product-detail/en/alpha-wire/422001-RD005/422001RD005-ND/4928094

They both cost about $32 CDN for 100' plus I think about $8 shipping.  Both are tinned.

To save money, you could order just one color and wrap the ends in differently colored shrink tube or electrical tape to identify red, black, white, etc.

[Paul Birkeland]

The 300V rated wire is just fine for a Crack.

[borism]

I have bought small quantities of silver plated teflon wire on ebay a number of times. One seller (navships) was very reasonable and reliable.

[Deluk]

"Unfortunately, shipping to Canada gets expensive." At least get a quote from Bottlehead, keep it in the family as it were.
The parts you appear to need, including 2-3 metres of wire weigh ounces so I doubt if postage would be very much. A light jiffy bag is all that's needed. A CD is probably heavier. A 100m reel of wire, that's a different matter.