output transformer suggestions????????

[yunger]

well, i had a fender pro jr that failed so i built a whole new circuit to co-exist with the existing chassis and transformers. i noticed that the lower frequencies sounded hollow or thin. i just assumed that was the nature of a 10" speaker until i plugged a blackface deluxe reverb i was working on into that same weber 10" speaker. the speaker is capable after all of producing satisfactory bass.
i therefore am blaming the cheap, stock, pro jr ot. i am no transformer expert so i welcome any recommendations any of you may have. i've been considering the hammond 1600 series but don't know if the hifi response would be a bad thing or not.
i also thought about going the "matchless" route with a 4k primary or at least an 8k w/ multi-tap secondary.
feel free to call me on ignorance or any foolishness on my part. i've also attached the schematic to my amp in case the problem lies within the design.
thanks.

[Enzo]

You made a guess that the speaker was the problem, but found out that was not correct.

You have made a guess that the transformer is the problem. What did you base that conclusion on? You built your own circuit, how do we know the circuit itself doesn;t have the problem?

Regardless of whatever failure the original amp suffered, before the failure, was it thin and hollow sounding then? If not, the transformer has not changed, only the circuit around it.

[Enzo]

Neither one of your preamp stages have bypassed cathodes, that will reduce bottom end response as well as gain. Have you considered raising the value of those .01 coupling caps? Temporarily tack another .01 in parallel with each for a quick find-out.

Are those 18v grid voltages real at the phase splitter? Try measuring from grid to cathode directly.

[yunger]

Neither one of your preamp stages have bypassed cathodes, that will reduce bottom end response as well as gain. Have you considered raising the value of those .01 coupling caps? Temporarily tack another .01 in parallel with each for a quick find-out.

Are those 18v grid voltages real at the phase splitter? Try measuring from grid to cathode directly.

enzo, thanks for having a look.
i did try doubling the coupling caps as well as bypassing cathodes early in the tinkering phase of the build. as for the low end, things just seemed to get muddy. since the power amp is a conventional design, and the speaker was ruled out, i just assumed the pj tranny was a problem. i take from your comments that i'm barking up the wrong tree again.
honestly, i don't remember the low end charicteristics of the amp before. all i remember was ear-piercing highs and overbearing mids.

the 18vdc grid voltages measured the same when grounding the chassis. when measuring with one test lead on grid/one on cathode i got something like .76v.

[Enzo]

I don;t know whether you are wrong about the transformer or not. I just think it is not smart to assume that without doing a little checking to find out.

I have a 50 watt Fender Bassman OT nearby, and it is my universal test OT. I don;t care what the amp is, if I suspect a bad OT, I clip this thing in place of the suspect. It doesn;t matter if the impedances are wrong or anything else. It will be close enough to tell me that the other transformer was not working or was OK after all. SO you can experiment, even with "wrong" transformers to find this out.

OK< you got 18v on the grid when measured to chassis. But when you measured from cathode you got .76 volt. Your cathode is sitting at 49v. SO either your grids are sitting at 18v or they are at 48.24v. Can't be both.

I point this out because your meter is a great tool, but only as long as you know its limitations. It is not a remote observer. When you connect it to your circuit, it becomes part of that circuit. Normally, it has a high enough impedance that it doesn;t affect things much - it doesn;t "load" the circuit. But in circuits where impedances are already high, then the meter impedance is close enough to the other stuff to alter the reading.

Even a great meter with a 10Meg impedance can screw with you. Imagine you have a 10meg meter impedance. And a 9v battery. Meter reads the battery as 9v. Now add a 10k resistor in series with that battery. Your meter still reads about 9v. But what if we took a 10meg resistor? Clip your meter to the negative post of the battery. CLip a 10meg resistor to the + post. Now measure between the megative and the free end of that resistor. What will you measure?


ABout 4.5 volts. Your meter impedance and that resistor made a voltage divider.


If your PI grids were really at 18v and the cathode at 49, that would be a bias for the grid of -31 volts!!! Tough to get tone through that. By measuring this grid to ground, your meter is facing the resistors in the circuit, plus any grid leak voltage the tube might accumulate.

Since there should be no grid current, by measuring from cathode to grid, you get a much more accurate reading.


BAck to topic. SOmetimes a thin sound is not the result of not enough bottom, it can be just too much top.