From Scratch BoM questions

[SteveH]

All,
So I have this factory wired ST70 sitting here.....And I don't think I really want to change it (as I think I should do OK with it on ebay Yellow_Light_Colorz_PDT_05 )
My question is this - below is a list of the items I purchased to replace on the origional ST70, along with a couple of items I know I will require. What else am I missing ? I am using Shannon's board, along with an SDS Cap board (so no need for any of the bias pieces). I know there are some bias resistors on the tubes, as well as some others around. Any help would be GREATLY appreciated !

I have or know I need:
1 - Chassis from Triode
1- PA060S
2 - A470
1 - C354
1 - DIY Tube Driver board and components
1 - SDS Cap Board and components
5 - Octal Sockets
2 - Isolated RCA Jacks
1 - Power Cord
1 - On/off switch
1 - Fuse Holder
2 - 10k Bias Wipers
6 - Binding Posts

Misc screws/bolts for the chassis and tube sockets
Various Teflon Wire

So - I believe all that I need now are:
2 - 15.6 ohm bias resistors (I think I still need these....)
4 - 1k resistors for the grid stoppers

Does that sound about right ?

Thanks again !
Steve

[erichayes]

Change 2ea 15.6Ω to 4ea 10Ω 2 watt. That should do it.

[Shannon Parks]

Go ahead and start bolting it together!

[SteveH]

Eric,
Can you elaborate a bit on that ? I assume that changing the resistors down to 10 ohm will make my voltage calculations easier; but where do the other two need to go ? On my amp currently there are just the two 15.6 ohm resistors on the rear tubes.

Thanks again,
Steve

[Ty_Bower]

The original ST70 design had a single, shared 15.6 ohm resistor on the cathodes of each pair of output tubes. The weird value was supposed to allow the owner to use a new alkaline battery as a reference (to 'calibrate' their volt meter). The cathodes shared a common resistor to keep the part count low (cost savings), but required matched output tubes. If the outputs weren't matched, one tube might draw more current than the other.

Shannon's board has provisions for individually biasing each output tube in the pair. So, you need a separate cathode resistor for each output tube (total of 4) instead of a resistor for each pair (total of 2).

You've noted the 15.6 ohm resistor is connected "only" to the rear tube (on pin 8, the cathode). If you look closely, there should be a wire tying pin 8 on both tubes of the pair together.

[SteveH]

Ty,
Yep; you're right. Thanks for the heads up! How's life treating ya anyway ? We never did catch up earlier in the year - let me know if you want to stop by.

Thanks again,
Steve

[TomMcNally]

The weird value was supposed to allow the owner to use a new alkaline battery as a reference (to 'calibrate' their volt meter).

Ty !

Alkaline batteries hadn't yet been invented when the ST-70
came out. They say "D type flashlight dry cell" ...
those silver Everyreadys with the cat on em ?
Can you still buy those !

:)

... tom

[SteveH]

I remember my grandmother would always have a stash of 'em; along with the free flashlights that came with the boxes (days before blister packs!)

[EWBrown]

Those were the old style carbon-zinc batteries. Averaged 1.56VDC for a new "fresh" one. Henve, two tubes consuming 50 mA each would generate 1.56VDC across the 15.6 ohms resistor. Similarly, the MK___ used a 11.2 ohm resistor,, so that the two KT88s / 6550s combined would consume 140 mA 0r 70 mA each, and give a 1.56VDC reading

FWIW there were some neat tricks to "recharge" these carbon zincs, like injecting saline solution, very gentle re-charging, etc. Some worked reasonably well , some didn't.


Best ST-70 approach is to use individual 10 ohm, 1W or 2W resistors, 1% tolerance. This simplifies the current measurement, 50 mA cathode current will read as 500 mV, etc. Current in mA will be the DC reading in millivolts divided by 10.

The actual wattage dissipated through these 10 ohms is very small, 500 mV X 50 mA = 25 milliwatts each.

HTH

/ed B in NH

[TomMcNally]

The actual wattage dissipated through these 10 ohms is very small, 500 mV X 50 mA = 25 milliwatts each.

They make nice fuses too. My big Velleman K-4000 with PPP EL-34's
used to blow up cathode resistors regularly when it had "Tesla"
tubes (now JJ) Since I modded it and put Svetlana's in it, no problem.

[SteveH]

Interesting stuff. So then should I go for .5w units ?

Thanks,
Steve

[TomMcNally]

I'd go with 10 ohm 1 watt "flameproof" because if a tube
goes, you don't want that resistor stench ! Flameproof
just open up odorlessly (hopefully)

[SteveH]

Thanks Tom...And I imagine that 1/2w should be enough for the grid stoppers eh ?

[EWBrown]

Grid stoppers can be 1/2 W or 1/4W for the G1 grids, there is virtually no current being drawn through them.
I typically use 1/4 watt carbon comp, anything between 220 ohms and 1K.
(Got lots of vintage 1/4 and 1/2 W cc resistors in the junkbox, accumulated over the years (and decades). For grid stopper duty, the tolerance isn't critica,l 5% or 10% is fine.

For the screen G2 grids, 1W or 2W resistors would be safer, as the screen grids do draw some amount of current.

/ed B in NH

[SteveH]

Gents,
Last night I went through my crusty ST70 and got it back into working order. I replaced all of the sockets, the can cap, and bias circuit (actually just bypassed the Se Rectifier and replaced everything else). It really sounds better than what I would have thought - having the original driver board in there and all...
So now it is time to get moving on the 'new' one....What does everyone use for binding posts on Triode's chassis ? I imagine they need to be fully insulated; but I really like the all metal binding posts that I was able to use on my ST35...Any suggestions ?
I am still tossed about using those 6.3v dampers on the 5v windings...I may just run two cool 5v FW rectifiers in parallel as Ed recommended. Maybe some Arcturus CX380's or something Yellow_Light_Colorz_PDT_05

Better yet, dual 274A's
http://cgi.ebay.com/Western-Electric-274-A-274A-Rectifier-Vacuum-Tube_W0QQitemZ310007374784QQihZ021QQcategoryZ67816QQssPageNameZWDVWQQrdZ1QQcmdZViewItem