Recent content by jbrew73

Which brand kt120 are you using? (Edit: I may have answered that myself. I see only Tung Sol kt120 listed online.)

I tend to agree with you on that. I thought our wall voltage was historically around 123vac. I will leave it on the 125 for now move on to the 2nd amp and some other projects. I’ll keep an eye on the wall voltages for a few months and make a final decision on which tap to use. I would hate...

I know that my B+ voltage is a little low due to my 118-121vac wall voltage being wired to the 125vac power transformer tap. I moved it to the 115vac tap and my B+ went up from 430vdc under load to 467vdc under load. At those voltages my output power went from 66 to 77 watts during testing. I...

Used Sovtek 6550 of unknown origin are currently in the amp. I have similar results with used vintage Tung Sol 6550, used Winged C 6550 and new KT88 tubes. I even popped in a few pairs of kt66 because my limited bias range also works with them. The kt66 would make 45 watts. All other tubes...

The schematic says approximately 70 watts before clipping occurs so I think 68 is probably fine. With no nfb the ef86 and phase inverter are clean when the output stage clips. However at the point of output tube clipping the driver tube starts to clip and I think the power tubes are drawing...

How would I go about testing for this situation? My ef86 stage needs an input of .044vrms to reach full power in the output stage when open loop and .394vrms when in closed loop.

Thanks for the excellent detailed response. It helps tremendously. I understand the first 4 paragraphs pretty well but the last 3 will take a little time for me to fully grasp.

Thanks to everyone for the help. I will do some more testing if for no other reason than to learn from the experience.

The ef86 is dc coupled to the phase inverter. If I pull the phase inverter tube or lift the nfb with the phase inverter in place the clean signal gain increases exponentially higher. The ef86 stage works great without a nfb signal. I believe that the nfb is causing what I am seeing. I just...

That sounds very close to what I’m seeing. I’ll look more closely next time I work on it.

The 5 ohm resistors are matched very closely and I have confirmed the balance by measuring the current across the output transformer primary windings. With the feedback disconnected the output clips symmetrically. Both signals start to square off evenly. With feedback I think the signal...

The voltages are within a few percent of the schematic.

I have a pair of Allen 75 amps that I am rebuilding. So far I have replaced the electrolytics and film caps. I have also rearranged some components to get the speaker outputs moved to the back, added an iec connector to the back , added power switch and pilot light to the front. No circuit...