Fender 65 Princeton Reverb --Reverb Circuit Send failure

[nevetslab]

Strange. An inductance of 28.1H should have an impedance of 176.5k @1kHz, ignoring the DCR. Did you measure impedance with the tank connected?
But still, the 125A20B has a step down ratio of 50:1. This makes the 8 Ohm reverb transducer impedance reflect as 20k at the primary - a perfect match for the ca. 20k source impedance of the driver circuit.

Or did you measure serial AC resistance (Rs) and not Z?

Ah...that was AC resistance. It was without the xfmr being connected. The Inductance reading was nearly the same both at 120Hz and 1khz, while the serial AC resistance changes substantially (3.03k @ 120Hz, 6.04k @ 1kHz). The amp has already been picked up, so I can't go back in to see what I've done wrong using the GenRad 1568 Digi-Bridge. I'll have to wait for another Fender Amp to come thru with the same Reverb Driver Xfmr to look. No doubt did something stupid. I also forgot to check the DCR, as well as plate voltage on the driver tube. Quick fix, Reverb working again, I didn't look back to check on those things.

Note: Typo......it's a GenRad 1658 Digi-Bridge, NOT a 1568.

[SoulFetish]

"GenRad 1568 Digi-Bridge"?

dude, you have better toys than batman

[Helmholtz]

If it's a GR1658 it doesn't measure impedance.

[nevetslab]

"GenRad 1568 Digi-Bridge"?

dude, you have better toys than batman

That was a typo.....it's the GenRad 1658 Digi-Bridge. I did once own a General Radio 1568 1% Wave Analyzer that I had bought for $25 in beautiful condition at a swap meet. Sold that years later, having better instruments to use for that function.

[nevetslab]

If it's a GR1658 it doesn't measure impedance.

You're absolutely right. One would have to calculate that from the measured Rs and the inductance Ls......Z = Rs + jwLs. The Operator's manual gives all the formulas for the various relationships section 3.4.2.

[Helmholtz]

You're absolutely right. One would have to calculate that from the measured Rs and the inductance Ls......Z = Rs + jwLs. The Operator's manual gives all the formulas for the various relationships section 3.4.2.

Yeah, just as with most LCR meters. BTW, the Z formula fails if the component exhibits a self-resonance in the vicinity of the measuring frequency. Generally LCR meters have problems if the component/circuit has inductive as well as capacitive properties.

[nevetslab]

Back to the original problem with the amp, which turned out to be this shorted 37099 Driver Xfmr Primary.........I've never encountered one of these failing that way. For a very brief time when I had the chassis on the bench, tracking the signal thru the tube stages and onto the Reverb Driver stage, I WAS seeing signal of proper magnitude on the driver tube plates, then, the next time I checked, the xfmr failed, and I had NO signal on the plates. During both measurements, I did NOT have the reverb tank connected to the xfmr secondary. Would that have been final cause of the xfmr to fail? Though, when the amp came in, I still had NO Reverb, with a good tank and good return circuit.

[Helmholtz]

During both measurements, I did NOT have the reverb tank connected to the xfmr secondary. Would that have been final cause of the xfmr to fail?

There is at least the theoretical possibility of a parasitic oscillation at the self-resonance of the primary causing excessive primary voltages without a load (as with other tube "power" amplifiers).
But I think it is common practice to leave the tank disconnected while servicing, so a damage to the reverb transformer doing so doesn't seem very likely. Maybe the newer transformers are more critical, though.

[nevetslab]

There is at least the theoretical possibility of a parasitic oscilllation at the self-resonance of the primary causing excessive primary voltages without a load (as with other tube "power" amplifiers).
But I think it is common practice to leave the tank disconnected while servicing, so a damage to the reverb transformer doing so doesn't seem very likely. Maybe the newer transformers are more critical, though.

You're right. I've never given the absence of reverb tank loading on the Driver Xfmr a second thought, when I have to pull the chassis from the cabinet, while the tank is well secured inside the pouch, mounted to the cabinet floor (Fender and similar amps) or up on the inside front panel on Marshall amps.

[TimmyP1955]

I've worked on tons of Fender RIs that had open tank inputs. Never any damage to the reverb transformer (well, maybe one - I replaced a bad transformer over 10 years ago - but who knows if it was owing to a no-load situation).