Powering up a new amp build

[rdjones]

OK, hooked-up power from Variac to amp turned on power switch while in standby raised voltage to 120v... nothing. No measurable voltage on heaters, nothing lights up, no power to tubes. I'm guessing there's a problem with the PT?

Odds are on it being a couple wires transplanted on the primary side.

Assuming your transformer matches the diagram the two primary windings will be in parallel for 120V (series for 240V).
If one of the pairs (brown and blue for example) are swapped they will be "out of phase" and not produce any secondary voltage.

I can't tell how PT primary is wired from the photos, are there 4 wires for the primary ?

RedDog Steve

ps - James Gang fan, Kent State class of '74

[funkgang49]

Yes, there are 4 wires. The White Primary lead is connected to AC Power receptacle. Black Primary lead is connected to one side of Power switch. Other side of Power switch - Black lead is twisted around White Primary and then connected to top of 2amp Slo-Blo fuse receptacle. Black lead goes from side of 2amp fuse recept. and connected to AC Power receptacle. Blue & Brown leads are twisted, cut and shrink tubed (not connected).

Odds are on it being a couple wires transplanted on the primary side.

Assuming your transformer matches the diagram the two primary windings will be in parallel for 120V (series for 240V).
If one of the pairs (brown and blue for example) are swapped they will be "out of phase" and not produce any secondary voltage.

I can't tell how PT primary is wired from the photos, are there 4 wires for the primary ?

RedDog Steve

[martin manning]

You want to tie black and brown together, and blue and white together, and then treat them like you have one pair of primary leads.

[Firestorm]

What Martin said.

But on a previous note, is this SE? One output tube? Or push-pull? You said Princeton so I got confused. If it's SE, I think the impedances are out of whack. Champs ran 6V6s at 8K-10K. Just checking.

[funkgang49]

But on a previous note, is this SE? One output tube? Or push-pull? You said Princeton so I got confused. If it's SE, I think the impedances are out of whack. Champs ran 6V6s at 8K-10K. Just checking.

Yes, this is SE. Tweed Princeton (5F2A) sort of. I will be running 6L6, 5881, & EL34 mostly. I can drop down on impedance selector to bring up impedance to 6K for 6V6.

[rdjones]

Yes, there are 4 wires. The White Primary lead is connected to AC Power receptacle. Black Primary lead is connected to one side of Power switch. Other side of Power switch - Black lead is twisted around White Primary and then connected to top of 2amp Slo-Blo fuse receptacle. Black lead goes from side of 2amp fuse recept. and connected to AC Power receptacle. Blue & Brown leads are twisted, cut and shrink tubed (not connected).

As noted on the spec sheet both windings are part of the primary and all 4 wires should be used to connect the two in parallel for 120V source.
However - even with just half the primary being used you should still have some voltage on the secondary at reduced current capability and regulation.
There may be something else preventing a complete circuit on the primary.

If you want to follow modern safety guidelines (highly recommended) I suggest using these connections:
"0 V" (low side) black and brown transformer primary wires together connected directly to the power cord receptacle neutral (low side) terminal marked "N".
"120V" (high side) primary white and blue wires together connected to one side of the mains power switch.
Other side of power switch to the ring of the power fuse.
Tip of the fuse holder to the high side "Line" or "L" terminal of the power cord receptacle.

You can check for the presence of secondary voltages more easily from the outside of the chassis by testing at the rectifier socket with the tube pulled.
5VAC at pins 2 and 8, ~630VAC at pins 4 and 6.
Bring up the AC to a lower value on the variac and test for reduced voltages to avoid overvolting the caps at the end of the dropping string if there's no tubes installed.

Hope this helps.

Steve Pompura
RedDog Thermionics
Nashville, TN

[funkgang49]

You can check for the presence of secondary voltages more easily from the outside of the chassis by testing at the rectifier socket with the tube pulled.
5VAC at pins 2 and 8, ~630VAC at pins 4 and 6.

Taking measurements from within the chassis - Hot or red lead of DMM (set to VAC) clipped to Pin#2 of Rectifier socket, black lead clipped to Pin#8 or to chassis ground?

[Structo]

Yes, you only ground the black probe to the chassis for DC. (usually)

[funkgang49]

As it stands now, I have both Yellow Secondary leads connected to Pins#2&8 and both Red Secondary leads connected to Pins#4&6 of the Rectifier tube then I've run a red wire from Rectifier socket pin#8 to tip of fuse(1/2 A Fast-Blo) then from ring to one side of Standby switch. The other side of Standby switch then connects to (+) side of first Filter cap. Both Green Filament Heater wires are connected to the jewel light then twisted black & yellow wires connect to the Power tube and then on to the (2) Preamp tubes. Both the Green/White 6.3V Filament wire and Red/White B+ wire (not Red/Yellow as listed on spec sheet) are connected to chassis ground at PT mounting bolt. Would this be correct?

[rdjones]

You can check for the presence of secondary voltages more easily from the outside of the chassis by testing at the rectifier socket with the tube pulled.
5VAC at pins 2 and 8, ~630VAC at pins 4 and 6.

Taking measurements from within the chassis - Hot or red lead of DMM (set to VAC) clipped to Pin#2 of Rectifier socket, black lead clipped to Pin#8 or to chassis ground?

The 5VAC for the rectifier filaments is floating with respect to ground, so only connect to the pins 2 and 8, not chassis.

The only reason I suggested to make the measurements form the outside of the tube socket is for safety, the inside connections are more cramped with more chance of slipping a probe tip and making a short. I use some heat shrink around the probe to insulate most except the very tip to avoid shorting inside a close quarter chassis.

rd

[rdjones]

As it stands now, I have both Yellow Secondary leads connected to Pins#2&8 and both Red Secondary leads connected to Pins#4&6 of the Rectifier tube then I've run a red wire from Rectifier socket pin#8 to tip of fuse(1/2 A Fast-Blo) then from ring to one side of Standby switch. The other side of Standby switch then connects to (+) side of first Filter cap. Both Green Filament Heater wires are connected to the jewel light then twisted black & yellow wires connect to the Power tube and then on to the (2) Preamp tubes. Both the Green/White 6.3V Filament wire and Red/White B+ wire (not Red/Yellow as listed on spec sheet) are connected to chassis ground at PT mounting bolt. Would this be correct?

Yes, this sounds correct.

rd

[funkgang49]

Thanks Steve. I made changes to wiring per your suggestions and I'm ready to power-up and take measurements. Unfortunately I've gotta take care of other business tonight so I'll have to wait til tomorrow evening.

[funkgang49]

OK then. Everything wired up correctly. DMM hooked up to pins 2&8 and set to AC. Power cord plugged into my Variac. I slowly raise up the voltage and ...Voila! Nothing! No readings, nada, zippo. WTF! Alright so where do I stop getting AC? Let me check the power cord - nope, she's good. Is my Variac shot? Nope, she's good. Is it the power receptacle?! YESSS! WTF! I couldn't have destroyed it when I soldered the leads to it could I?! Can I really be that big of a hack?! Don't anyone answer that! So I go dig out another one (I bought several a few years ago) fresh, still in its package. I open it up and a little white plastic thingy falls out of the open end of the receptacle - huh? What's this for? Looks like it should hold one end of a... yes, that's right, FUSE!! DOOH, DOOH, and another Double DOOOOH!
Everything is up and running now. I've spent the last 2 days making adjustments - tweaking the tone of both preamp tubes, upping the voltage to the power tube and first preamp tube. Adding in the NFB resistor (it's quieter now and has more head room). The amp sounds wonderful no matter what power tube I throw in there. I've tried NOS 6V6GT, NOS 6L6GC, NOS 5881, Svet. EL34, JJ 6L6. I even threw in a JJ EL84 with a Yellow Jacket adapter - SWEET!
I just have one niggling issue: the 6SL7 has considerably less output than the EF86 (which sounds incredible by-the-way). I'm currently running the 6SL7 at 310v before the 100K power resistor, 187v measured at the pin, 1.2K/10uf at the cathode, 33K at the grid (hooked directly to pin with sheilded cable going back to the input jack), 1Meg to ground. Any and all suggestions as to how to get more output from this tube?