A quick PT question

[gsr]

Hello all,

I have a donor pt that is labeled 350-0-350 @ 70ma. How does that stack up for a 5e3 build? Is 70ma enough? Thanks

[Chuck H]

The voltage would be roughly correct (a little high) but 70mA just won't cut it... Sorry. Not sure of a good use for that PT. With high volts for a guitar preamp and not enough current capacity for a full amp nothing comes to mind.

Chuck

[txstrat]

How about a 5E3 with a single ended power stage? Your PT shouldn't have a problem with that.
I once built a champ II circuit and lacking a PP OT I did just that.
In the pic I just added a 5F1 power stage into the 5E3 schem. (keep an eye on the values of the filter caps, since the B+ would be higher than 450V)
This worked out fine for me, but it might be tweakable with different resistor values on the plate and cathode of the former PI triode. May the experts chime in?
edit: the 6V6 should be a JJ 6V6s due to the voltages - or change it to a 6L6GC

[d95err]

How about a 5E3 with a single ended power stage? Your PT shouldn't have a problem with that.
I once built a champ II circuit and lacking a PP OT I did just that.
In the pic I just added a 5F1 power stage into the 5E3 schem. (keep an eye on the values of the filter caps, since the B+ would be higher than 450V)
This worked out fine for me, but it might be tweakable with different resistor values on the plate and cathode of the former PI triode. May the experts chime in?
edit: the 6V6 should be a JJ 6V6s due to the voltages - or change it to a 6L6GC

The negative feedback on the design shown in the schematic is not working. The feedback is shunted to ground by the cathode capacitor.

[txstrat]

The negative feedback on the design shown in the schematic is not working. The feedback is shunted to ground by the cathode capacitor.

Thanks.
I don't know if it's necessary anyway, since I believe part of the 5E3 sound is the lacking of NFB.
Changed the pic with optional NFB.

[gsr]

The strange thing is, the donor amp was made in hong kong, with english (woden) transformers and GE tubes. The tube compliment is 5751 v1, 12ax7 v2, 2x 6v6, 5y3 rectifier. I haven't fully traced the schematic and the amp wasn't working when I got it. Someone had clipped out the pio coupling caps. There are 5; all .01 350 volt. I'm still learning, but at first glance it looked like a 5e3 type circuit, yet I wondered about the 70mA being too low.

[gsr]

How about a 5E3 with a single ended power stage? Your PT shouldn't have a problem with that.
I once built a champ II circuit and lacking a PP OT I did just that.
In the pic I just added a 5F1 power stage into the 5E3 schem. (keep an eye on the values of the filter caps, since the B+ would be higher than 450V)
This worked out fine for me, but it might be tweakable with different resistor values on the plate and cathode of the former PI triode. May the experts chime in?
edit: the 6V6 should be a JJ 6V6s due to the voltages - or change it to a 6L6GC

I have built a 5e3 already but not a champ. Is this pt good enough for a 5F1 regular build? Or do you think the 5e3 pre would be cooler?

[txstrat]

I've had a look on Hammonds website (Hammond Mfg. - REPLACEMENT & UPGRADES - Tube Guitar Amplifier - Power Transformers).
The PT 290AX claims to be a replacement for the Tweed Deluxe. The spec sheet says 650v@81ma. When you click on the image of the transformer and look at the sticker you can clearly see 70ma. Maybe an older version. But anyway it WAS a replacement for Tweed Dulexes @70ma.

[txstrat]

I have built a 5e3 already but not a champ. Is this pt good enough for a 5F1 regular build? Or do you think the 5e3 pre would be cooler?

If you have the 5E3 already why not go for something different?
A 5F1 is a great amp. Not too loud, but that could be cool for home use or recording.
How about the heater current of your transformer? A 5F1 needs less than 1 ampere @6.3 volts.

[gsr]

6.3 is 3A. 5v is also 3A. If I did the champ, could I still use a 6v6. I have Tung sols and GEs. I don't have any 6l6 on hand.

[txstrat]

With the PT having 350 volts secondary you'll get 493 vdc rectified.
Even considering a voltage drop of the rectifier tube that would be easily over 450 volts.
You need filter caps and a power tube, that can stand the voltage.
Don't know if Tung Sols or GEs 6V6 do. JJs go up to 500vdc.

[gsr]

I added a photo of the pt (sorry it is backwards). On the primary side, there are 2 black, 2 yellow and 2 red wires. This is different than my first build and I want to make sure I get it right. I know to cap off the yellows. I then connect the 2 reds together and connect them to the amp side of the switch. The 2 black wires connect together with the incoming house mains neutral wire. Is this correct? Also, since the pt was designed for 100 and 110v, will modern mains voltage increase the secondary even more than 350-0-350? Thanks for the help

[gsr]

With the PT having 350 volts secondary you'll get 493 vdc rectified.
Even considering a voltage drop of the rectifier tube that would be easily over 450 volts.
You need filter caps and a power tube, that can stand the voltage.
Don't know if Tung Sols or GEs 6V6 do. JJs go up to 500vdc.

I have 500 volt filter caps. I recall reading somewhere about increasing the 250 ohm cathode resistor on the power tubes. I have a 300 ohm 10 watt. Will this bring down the voltage a little, and is this a good idea?

[unparalleled]

The voltage would be roughly correct (a little high) but 70mA just won't cut it... Sorry. Not sure of a good use for that PT. With high volts for a guitar preamp and not enough current capacity for a full amp nothing comes to mind.

Chuck

I have been experimenting with 12BH7 as PP output tubes. 5-7 W.
A 5E3 Pre with one or two of these could produce a useful little amp?

[gsr]

I have been experimenting with 12BH7 as PP output tubes. 5-7 W.
A 5E3 Pre with one or two of these could produce a useful little amp?

12BH7 appear to be mini dual triodes. I'm still pretty new at this. They would work as power tubes? Could you shed some light as to their application? Thanks