isolation transformer install

[g1]

plate = 135v, screen = 127v and cathode = 9.5v. Since these now all look good, would this point to a look back at the OT?

Check the peak signal voltage at the grid first.

[Randall]

OK. Here is what I see for voltages driving the input with 45mV as measured with a non true RMS Fluke:

preamp

grid - 0.125v p-p
plate - 11v p-p

output

Grid - 11v p-p
Plate - 100v p-p

3.9 ohm load - 1.5v p-p

[Randall]

I sure could use some help with why this amp is producing such low output, cause I'm pretty in the weeds here. I'm back to suspecting the OT again, I wish I had one hanging around to try.

[g1]

Have you tried the neon bulb tester on it? If you haven't built one it will come in handy for transformer testing:
https://www.premierguitar.com/articl...ormer-tester-1

[Leo_Gnardo]

That ^^ and/or try a substitute OT. Most hot chassis amps only turn out a watt or two anyway. A Champ OT, maybe even a reverb drive transformer clipped in as a substitute may pass a better wattage figure, then you know what you have to do. Also do you your power test at say 500 Hz on up as low frequencies drop precipitously with tiny iron.

[Randall]

When I try the neon bulb test it does flash a bit, but not as much as a DRRI PT I have laying around. I suspect more iron means more flux and more neon power?

Shame but I do not have another single ended transformer to clip in.

I tested power at 1 KHz.

[eschertron]

Plate - 100v p-p

3.9 ohm load - 1.5v p-p

suggests a primary impedance in the 17k Ohm range. Is this reasonable? Could the load actually be a 2R speaker?

[Leo_Gnardo]

suggests a primary impedance in the 17k Ohm range. Is this reasonable? Could the load actually be a 2R speaker?

Hmm, don't rule it out. I've seen 2 ohm speakers in some 50's Premier amps. When it comes to funky old amps, anything goes.

[Helmholtz]

When I try the neon bulb test it does flash a bit, but not as much as a DRRI PT I have laying around. I suspect more iron means more flux and more neon power?

Shame but I do not have another single ended transformer to clip in.

I tested power at 1 KHz.

Could you verify the 180 Ohm DCR of the OT primary, as shown in the schematic? Is the 1.5Vpp across the 3.9R load just below clipping?

[Randall]

"Could you verify the 180 Ohm DCR of the OT primary, as shown in the schematic? Is the 1.5Vpp across the 3.9R load just below clipping?"

The OT primary measures 265 ohms, the secondary is 0.3 ohms. The top of the wave is squashed a little with reference to the above measurement.

OK, I actually do have a single ended transformer in my shop, it is the reverb drive tranny from a Vibrolux Reverb. I subbed it and did indeed get a higher wattage measurement, but still it is not right. With the reverb tranny I get 0.5 watts, with the "original" I only get about 0.2 watts.

[Leo_Gnardo]

OK, I actually do have a single ended transformer in my shop, it is the reverb drive tranny from a Vibrolux Reverb. I subbed it and did indeed get a higher wattage measurement, but still it is not right. With the reverb tranny I get 0.5 watts, with the "original" I only get about 0.2 watts.

That shows you're heading in the right direction. You could try a number of different load R's, see which one gives you max power, then that's your target speaker nominal ohms rating.

What a lot of trouble to go to for such a peewee amp. But it is somebody's pride and joy. Chalk up the hassle as a "learning experience."

[Enzo]

Have we looked at the signal at the power tube grid? I mean is the tube being driven fully or is the signal there already weak?

[Randall]

No, it's not the original speaker, it is in fact 8 ohm. But I am not testing with the speaker, I am testing with a 3.9R 5w resistor.

"Have we looked at the signal at the power tube grid? I mean is the tube being driven fully or is the signal there already weak?"

Yes, as previously stated:

OK. Here is what I see for voltages driving the input with 45mV as measured with a non true RMS Fluke:

preamp

grid - 0.125v p-p
plate - 11v p-p

output

Grid - 11v p-p
Plate - 100v p-p

3.9 ohm load - 1.5v p-p

[J M Fahey]

You are NOT driving power tube to full power.
You pulled "drive it with 45mV at the input" out of the blue.
Rated power is measured at output reaching clipping, all you got so far is "The top of the wave is squashed a little with reference to the above measurement." which in the Guitar World means "still clean"
Increase drive signal to 100mV ... or even 150mV if necessary, so *bottom* peak visibly clips, let devil take care of the top one, and remeasure.
Please post a screen picture here.

Single ended amps, and doubly so *cheesy* ones, hardly clip symmetrical.

That said, you have 138V +V so maximum plate can swing down is to some 40 or 50V above ground, (tube saturation voltage) and high twice that, so maximum *possible* swing is some 200Vpp or so.

About twice what you are measuring so *potentially* you could have 4X the power you are measuring now.
In practice I doubt you can get more than 1W RMS.

Which is ample power for an AC/DC table top radio, your amp is not different from that.

Way back then it was more than enough for a Kansas or Oklahoma picker, relaxing at home after a hard day´s labour, or a kid trying to learn Rock licks he heard on the radio.

[Randall]

Juan, thank you for joining in, I always learn something from you.

Here is the waveform driven at input with 1KHz at 120mV (measured with Fluke non true RMS), output is measured across a 3.9 ohm 5 watt resistor with mystery OT. Scope is set at 0.5v/div for approx. 2.5v p-p. My calculations put that at .20 watts. 2.5 v p-p / 2 = 1.25 v peak, 1.25v * .707 = 0.88 v rms, (0.88 * 0.88)/ 3.9 = 0.20 watts.