Terry Stinger Superlite IIb 2.7 PCB - Anyone familiar?

[Zarrir]

Hi, i need help with a 18 watter using the above PCB. I am using the latest schematic (v4 see attachment).
It is a bridge rectified board. The schematic shows the Power supply section using a MPS NON-CENTER TAPPED transformer.
I intend to use a Hammond 290PAZ power transformer on this amp and the heater wiring is CENTER tapped.
Can i simply connect the center tap wire to star ground?  Or should i remove the two 100 ohm power supply section resistors?
Thank you!

[kagliostro]

you can remove the resistors and connect CT to ground or leave it there and don't connect the CT to ground

Which is better ? ....... Give it a try (is so simple)

Franco

[Zarrir]

Thank you Franco. How about the four diodes? On another PCB from Terry (Baby Will), somebody mentioned that keeping all four diodes there and connecting the transformer heater CT to ground fried the transformer. In that case, it was recommended to remove D3 and D4 as well as the above resistors.   The boards look very similar so this is my fear. In case i decide to leave everything exactly like the schematics above, is it ok to just tape out the heater center tap wire?

[sluckey]

I think there is some confusion about bridge rectifiers, center taps, and heater center taps. The schematic you posted is not a bridge rectifier. Some of the Baby Will schematics show a bridge rectifier. You can get into trouble if you attempt to connect the HT center tap to ground as shown in the schematic you posted ***AND*** use a bridge rectifier with the negative terminal also connected to ground.

But, in no case will connecting the center tap of the heater winding to ground harm a PT. The heater centertap is totally unrelated to the HT center tap.

[Zarrir]

Thank you Sluckey! Do you happen to know the name of this particular SS rectifier arrangement? Maybe a two fase diode rectifier?
So can i safely ground all secondary center taps (high voltage and heater) in the above case? Should i keep or remove those two 100 ohm resistors?

[kagliostro]

Here you can find a bit of info

http://www.electronicsandcommunications.com/2013/04/full-wave-rectifiers-theory-and-circuit.html

the arrangement with 4 diodes (two by two in series) is a Full Wave rectifier

where the ability of the diodes to afford higher voltage is obtained connecting it in series

as an example if you connect two 500V diodes in series you can use it in 1000V circuits

---

If you connect a transformer with a CT to a Bridge Rectifier you'll burn the Bridge if the CT is connected to ground an if it is not connected you'll have the B+ doubled

Franco

[Zarrir]

I think these names contribute for the confusion. Because the Bridge Rectifier is ALSO a Full Wave rectifier.  Well, i think i will be on the safe side by grounding all secondaries and powering up this baby.  My only doubt remains about the two power supply resistors.

[PRR]

> these names contribute for the confusion

Agree.

[sluckey]

Do you happen to know the name of this particular SS rectifier arrangement?

The school I attended and the theory books I used referred to the rectifier circuit in your schematic as a "FULL WAVE CONVENTIONAL RECTIFIER". That name also applies to most of the dual diode with HT center tap rectifier circuits.

Do a google search on the phrase "FULL WAVE CONVENTIONAL RECTIFIER". You'll find lot's of easy to understand reading.

[Zarrir]

I have just powered it up for the first time using a light bulb dimmer.  Both power tubes lit up, nothing from the preamps.  Standby to the ON position, the light bulb went on dimly.  All knobs were at noon. Guitar sound playing fine but kind of low volume for 5 or 6 seconds. And then slowly faded to zero. No bad smelling. Too bad a sign or should i continue to check the voltages? Any idea on where to start debugging?  Sorry for newbieness.

[kagliostro]

What have you done about the resistors ?


K

[Zarrir]

I left them on board, Franco. Bellow is a general picture and also a close up of the power wiring on the board.  The big white sag resistor is connecting the PCB Standby exit point to the Standby Switch.  Will move it to a better place soon. Should i remove this sag resistor and R1/R2 as well and try again? Also, i double checked all component values and noticed that i misplaced a 56k resistor on R16 position. It was supposed to be 47k. Sorry to bother you guys.
As soon as i switch the Standby On and Off again, the volume goes back on and lasts another 6 or 7 seconds just to fade out to zero again.
Done some measurements: Between the two green heater input wires = 4.5 VAC . Between each heater input and ground = 2.1VAC
                                            Between each HV input and ground = 240 VAC .  Output from Diodes to Ground = 215 DC  All on Standby

[Zarrir]

UPDATE:  I have lifted the heater CT from the ground and the sound does not fade out anymore. Amp seems a little low on volume only. Voltages are the same as above, very concerned about the 4.5VAC on the heaters.

UPDATE 2: Went ahead and unplugged the dim light bulb advisor. Uhuuuuuu, this thing rocks like shit now. Voltage on the heaters a little low though (5.5VAC).  280VAC on HV.  Amp seems a little microphonic. What if i reconnect the heater CT to ground and remove the two 100 ohm resistors from the power supply together with the Sag resistor?  Please shred some light on a newbie! Thrilled to hear it rocking now!

[sluckey]

What if i reconnect the heater CT to ground and remove the two 100 ohm resistors from the power supply together with the Sag resistor?

NO! Look at the schematic. Those two 100Ω resistors do not connect to ground. They connect to the cathodes of the output tubes. We call that elevating the heaters. Just tape off the heater CT and rock on.

BTW, the PT wires in your pics do not match the colors for a Hammond 290PAZ. Why???

[Zarrir]

Thanks again Sluckey!  Actually it is a Brazilian knockoff of the Hammond. They simply replace the green/yellow heater CT wire with blue  AND the red/yellow HV CT with black. Because they do not carry those bi-coloured wires  .
Would you keep the sag resistor?   What do you think about the 5.5VAC between heaters? Too low? Thanks for your help! It sounds great but too chimy on my Strat. Will try a Les Paul later. Maybe because i only had Sovteks and Tungsols here in stock.

[sluckey]

What do you think about the 5.5VAC between heaters? Too low?

Well, that is low. Most people complain the heaters are too high. What is your wall voltage? And what are the specs on that 'knockoff' PT?

Just short out your sag resistor and let your ears decide if you want it. But you said it rocks like shit. I'd be inclined to button it up.

[Zarrir]

Wall voltage 120V .  The specs are exactly the same as the 290PAZ. The Company that makes them down here is very capable, 45 years in business. Good quality iron. Despite not having bi-coloured wiring.... Should i bother to replace the currently installed R16 (56k) with the correct value (47k)?
Been studying a lot, in a few days will not ask silly questions anymore. I hope....

[sluckey]

I'd put a 47K in there but I doubt it will make a big difference.