Rewiring my ST-70 with VTA board: change to 20awg wire or stick to 22awg?

Happy new year, tube-lovers!! A quick question for you wise ones if you please......I'm currently rebuilding an ST-70 (as per my rebuild thread) and I'd purchased some 20-gauge wire to complete the job. However on re-reading the VTA board instructions, the advice is to stick to 22-gauge. No problem.

My question is though: is there anywhere in the amp where using the 20-gauge is worthwhile or desirable? Or shall I return it to the store? I'll be getting some 22awg anyway for all the wiring to the PCB, as per instructions.

Thanks!

Regardless of diameter, my vote is for Teflon insulated wire as it withstands much higher heat.  You can solder very close to insulation without melting it and contaminating your joint or having excess bare wire exposed.  Especially nice if you are dealing with shielded wire.

This guy on eBay sells all sorts of gauges and variations of stranded, solid and shielded cable in multiple colors in 10 foot segments.  Mostly seems to have Mil Spec surplus wire.  

http://stores.ebay.com/Johns-Silver-Plated-PTFE-Wire-Shop?_rdc=1

Thanks for the link! His stuff looks well-priced, I'll give it a whirl.

Only caveat with teflon wire is you should plan to strip the wires a bit further back than you would with any other wire. Touch the gun to the jacket, and you've the potential for contamination in the joint.

vtshopdog wrote:Regardless of diameter, my vote is for Teflon insulated wire as it withstands much higher heat.  You can solder very close to insulation without melting it and contaminating your joint or having excess bare wire exposed.  Especially nice if you are dealing with shielded wire.

This guy on eBay sells all sorts of gauges and variations of stranded, solid and shielded cable in multiple colors in 10 foot segments.  Mostly seems to have Mil Spec surplus wire.  

http://stores.ebay.com/Johns-Silver-Plated-PTFE-Wire-Shop?_rdc=1

I looked it up and found what I had posted was not within the rules so I deleted it. My apologies!

sKiZo wrote:Only caveat with teflon wire is you should plan to strip the wires a bit further back than you would with any other wire. Touch the gun to the jacket, and you've the potential for contamination in the joint.

Teflon very inert (does not generally combine with other compounds) and has melt point around 600 F - would ordinary lower melt point poly jacket wire have an advantage if touched by hot iron?

Depends ... I keep my iron set around 720F ... that's more than enough to cause the teflon to decompose and contaminate the solder, resulting in possible voids and blistering.

It's an easy work around though. I strip long as I said, then bend the wire to provide a stop yet keep the jacket away from any hot metal, solder, then trim the extra length tight to the solder once it cools.

Or just use standard solid core wire. No issues as long as it's 600v rated, as most hookup wire is. Also easier to work, especially if you're twisting wire, as teflon will fight you some and want to go pretty much where it wants to.

I think I've got a bit stuck on this. In the end I used 22awg solid core wire and just finished hooking all the filament power wires to the board. But just now I found a reference that says it should be 20awg (from the tubes4hifi VTA-70 amp build manual). I wonder whether I should rip out what I've done. Better do it now than risk something melting or problems later. Bah. Lowest point of the project.  

On the other hand the only thing I've used the 22awg for so far is the filament power supply to the PCB and that's a low voltage anyway, right? Apologies for asking such basic questions, this is a big learning curve for me!

The filament may be low voltage but current is a factor.

Just think how much better your build will be after redoing......

Wire size ratings are by current, not voltage. A maximum rating is obtained by how much current it can sustain before melting (basically). In a tube amp, this is somewhat complicated because the wire lives in a hot space and we expect it to do its job a long, long time.
I would never undersize heater wires for the power tubes, and neither would I buck established practice- which would be to use a twisted pair of 18awg in this spot. Honestly, I think you should re-do the heater wires.
Voltage rating is different, it mostly refers to the the wire's insulating capacity.

all wire is rated by current and voltage, the wire size determines the current ability, the insulation determines the voltage rating.
Common 22g solid wire is rated at 5 amps, stranded wire is rated at 2 amps. Most common wire is rated at 300vdc, and 600v insulation is an option.
Personally I use 22g wire to wire everything in my amps and preamps. Only the power wires would ever use more than 1 amp.
So I'll use 20g wire for the AC wiring to the power transformers. KT88 filaments use 1.5 amps of current, so use 20g wire there.
20g solid wire is rated at 6 amps and stranded wire at 3.5 amps.
This stuff isn't common knowledge, but it only took a 10 second google search to find the info.

Thanks guys - I had to buy a new power transformer so all that wiring is correct: but I'm going to re-do the wiring I put in for the filament power supplies so that it's 20awg throughout.

I'm still creeping ever closer to the finish though!

Has anyone had any problem with using the 300v vs 600v rating for hook up wire used in a ST 70 or St 120? There are some pretty high powered volts running around in these amps.

GreggW wrote:Has anyone had any problem with using the 300v vs 600v rating for hook up wire used in a ST 70 or St 120? There are some pretty high powered volts running around in these amps.

Volt rating refers to the insulation jacket.  Assuming the inner wire is identical you will do no harm having higher rated wire insulation than needed other than slightly higher cost or difficulty sourcing.  OD probably be thicker too, which might be an issue in some situations.

300 v through a 600 v wire no problem, but reverse scenario under right conditions may burn through insulation and eventually arc to ground.  My take is to always meet or exceed specification simply as a hedge against my own ignorance or the less than best practices observed in my basement workshop.

We are DIY'ers building one off projects, not corporations pumping out thousands of units - to me a one time extra $12 for wire to reduce the chance I get electrocuted flipping my power switch is money well spent.

I bought some hook up wire from the Shack that I'm now certain is 300v rated. I put a few pieces in my ST120 and so far, no problem. That said, it's time to spend a few more bucks and get some higher voltage rated wire....