Stancor A-3800: what tubes?

[MHProd]

The OT is 5K primary Z. Looking at building a plexi style circuit with around 420V on the plates.

The OT is rated @ 80ma, would it be ok to use a pair of KT66 or EL34?

Maybe a quad of 6V6?

[didit]

Hello --

Stancor had spec'ed 6L6 along with a few assorted triode tubes.

Stancor OPT 3800.jpg

Be mindful of the 80ma max DC current and keep overall output power target sensible, and all should be fine.

Best .. Ian

[MHProd]

I know about the 6L6, but was wondering about KT66 or El34...

I heard about guys building a Liverpool around it, which has a quad of el84s.

If I lower the B+ some, would a quad of 6V6 be ok?

[pompeiisneaks]

EL34's want 100mA current per tube, that datasheet he showed said 80mA current. I think KT66s want 80mA per as well. This is the point didit was making about being careful on the output power.

~Phil

[Phil_S]

Treat the primary as 10K. That makes the secondary 8Ω, 16Ω, 32Ω. You can use a pair of 6V6 like this. I don't think 80mA will support a quad of any typical common guitar amp tubes. If you are adventurous, you might try a quad of EL95, but what's the point? You'd get more watts from a pair of 6V6.

[didit]

I know about the 6L6, but was wondering about KT66 or El34...

Much is similar across KT66, EL34 & 6L6, etc... An AB PP amp for one can be adapted with minor tweaks for another. Might not be pleasing as the original however. Need to do a bit of engineering to find suitable current & voltage to stay within operating limits, or perhaps find someone skilled to help. Could be an opportunity to develop better understanding of loadlines.

If I lower the B+ some, would a quad of 6V6 be ok?

Lower B+ less on 6V6s, and dependent on bias point less plate current. Somewhere I've an A-3800, and destined plans on building an amp akin to a Rocket but with 4x6AQ5. A back-of-envelope sketch with ~260V on plates and ~24W estimated output, and some arithmetic showing less 80mA DC through each side of the primary. Storage a mess, or I'd pull it out and share. If memory serves, worked backward from 6L6 at ~350V delivering 30W, which seems a sensible reverse engineer of Stancor's spec.

EL34's want 100mA current per tube, that datasheet he showed said 80mA current. I think KT66s want 80mA per as well. This is the point didit was making about being careful on the output power.

Minor tweak .. specification states EL34 tolerate 100mA maximum at the cathode and should work up to that, depending significantly on supply voltage and maximum total power dissipation. An EL34 will readily bias up and function quite nicely with quite a bit less. As will any of the loosely similar class including KT66, 6550, KT77, etc... As an example the "build" PDF here at TAG has recommended bias for Trainwreck Express of ~40-45mA per EL34. A little too hot for an A-3800.

Best .. Ian

[MHProd]

Didit, what tweak would you have in mind?

For a pair of 34s or 66s, if I keep the B+ under 400V, would that be OK?

I have a JTM45 clone here which has around 440V on the plates. Should I check the cathode current to see where it's at... as a reference point?

[SoulFetish]

Hello --

Stancor had spec'ed 6L6 along with a few assorted triode tubes. Stancor OPT 3800.jpg

Be mindful of the 80ma max DC current and keep overall output power target sensible, and all should be fine.

Best .. Ian

The 80mA max DC rating is for each phase(each half) of the primary winding. So, if you are designing for 2 tubes (ie, 2 x EL34s), you just need to ensure that they don't exceed a bias current of 80mA per tube. Or, 160mA total for both.

[didit]

Hello --

Stancor had spec'ed 6L6 along with a few assorted triode tubes. Stancor OPT 3800.jpg

Be mindful of the 80ma max DC current and keep overall output power target sensible, and all should be fine.

Best .. Ian

The 80mA max DC rating is for each phase(each half) of the primary winding. So, if you are designing for 2 tubes (ie, 2 x EL34s), you just need to ensure that they don't exceed a bias current of 80mA per tube. Or, 160mA total for both.

Correct, except would advise regarding it as max current spec & not the lower idle bias.

[SoulFetish]

Hello --

Stancor had spec'ed 6L6 along with a few assorted triode tubes. Stancor OPT 3800.jpg

Be mindful of the 80ma max DC current and keep overall output power target sensible, and all should be fine.

Best .. Ian

The 80mA max DC rating is for each phase(each half) of the primary winding. So, if you are designing for 2 tubes (ie, 2 x EL34s), you just need to ensure that they don't exceed a bias current of 80mA per tube. Or, 160mA total for both.

Correct, except would advise regarding it as max current spec & not the lower idle bias.

If it were, I don’t think they would have specified it as max DC current. For instance, DC is really only present at idle. Unless one side does not receive any signal. In this case perhaps the 80mA max factors in offset to keep the transformer out of saturation.
Interesting that I don’t see a max RMS current rating though.