Tube Amp Archive

Fender Blues Jr. PT

https://www.tubesandmore.com/products/t ... unior-15-w

If I'm not mistaken, it says B+ 330VDC @ 68mA

Never mind, just saying.

Stevem wrote: ↑Sat Sep 09, 2023 6:36 pm It’s a great price for that PT because it only seems to provide.55A on the B+ secondary side..../cut

Again, that is 550mA to me. The 6X4 rectifier tube can only safely deliver 77mA steady current. Where is the rest coming from?
The schematic says it all and datasheets.

It’s a great price for that PT because it only seems to provide.55A on the B+ secondary side. That’s really only enough for something like a 5 watt champ if I am Not mistaken . That is 550mA. How much does he need? It's a 100VA PT. Edit: from the schematic, I calculated the amp needs about 91mA B+,...

Would you consider a toroid transformer? If so, then this would be a good match and the price $42 is right. I know, it doesn't have an "oldie" look. Steel cages are also available.

https://www.antekinc.com/content/AS-1T250.pdf

That is odd. Exactly 2:1. Colors messed up? What is the voltage Brn-Blue?

Try to power the OT secondary winding from half of the filament transformer (3.15V). Measure exact voltages on primary/secondary. Knowing the winding ratio, one may evaluate the condition of the OT primary winding. Connect some load, like 2 x 47k on the primary. Edit: Martin already preceded me on t...

I think my next step is to buy another OT and give it a try. If that doesn't work then simplifying the power supply may be worthwhile, though pushing the screen voltage that high concerns me! Don't buy an OT yet. Unless you shorted the primary to GND for two hours, I don't think the OT is blown. Te...

I could manage to get 2.25W out of this amp in LTspice simulation, with some circuit tweaking. Input signal to phase inverter 1V p-p. Pentode grid sine clipping at 1.8V: 1.5Wsim_grids.png AC voltage swing at the plates: 1.5Wsim2.png Output sine on the 8R load resistor: 1.5Wsim.png RMS Power calculat...

Forrest Cook built a single ended mini amp with a similar pentode and got only 100mW out of it. I don't have any experience with these tiny TV pentodes. http://www.solorb.com/elect/musiccirc/6U8AmpV3/ Point of concern is your dirty & clipping input signal. Suggest removing all pre-amp tubes and conc...

14.5 mA each tube? That is class A bias, 3.9W. Suggest to change to class AB bias (50-70% class A) that would be about 7.25 - 10.15 mA each tube, to keep it within limits.

Since you have access to an oscilloscope, use an 8R load resistor and measure the signal there. Calculate the RMS output power, then you know for sure.
In the meantime, jump temporarily (short) the D1 Zener diode and see if that makes any difference.

Your amp circuit is similar to this Nakid Micro Amp but with an unconventional power supply. https://robrobinette.com/How_the_Saldano_SLO-100_Works.htm#SLO-Nakid_Micro_EF80_Pentode Personally, I would not use a dual pentode in one bulb. These are not matched and when one goes bad then both need be b...

Looks like a Russian 6P3S (6L6 derivate)