Vibro-Champ AA764

[lowell]

Initially came in with low output... but also a petrified 2-prong cord. Just replaced the power cord and now the hairs on my forearm no longer sizzle when resting my arm on the chassis taking voltage readings.

This is an odd one... The B+ is quite high as is the filament voltage. I have 7vac on the filaments. Someone thought it was a wise idea to cut off the center tap and run the filaments with one end of the secondary grounded. A single ended amp would be the last amp I would do that to if I was ever gonna do it... which I wouldn't. SE amps already have their share of hum.

So the B+ is 415vdc!! the secondaries are at 365vac. This is about 50v higher than the schematic indicates.

Yes all tubes are in and working except the vibrato cause the circuit isn't working. But that's not enough of a load reduction to cause this.

If the can cap was leaky or there was excessive ripple the voltage would be low.

So being that the filaments are also high... could we say we have a bad PT?

http://ampwares.com/schematics/champ_vibro_aa764.pdf

[Enzo]

What is your mains?

Transformers really don't have a failure mode where the voltage changes.

[lowell]

Wow it's rather high here at my house... 124vac. 6% higher than 117vac which I guess is what the schematic assumes. However... shouldnt the secondary be 6% higher too? 415vac vs 355vac is 17%.

[Enzo]

Is it a 125P1B? Is it a real 5Y3?

The schematic doesn't show a center tap on the heaters. It is normal for one side of the heaters be grounded.

Schematic says 315vAC from the transformer. I get 445vDC from that, minus the 5Y3, and I am not thinking 90v drop on a 5Y3 sounds normal. Are you starting with 315-320vAC?

I don't put much stock in schematic voltages.

[lowell]

Thanks... it's odd they chose that heater arrangement. There is a grn/ylw wire on PT that was cut off.

PT has 022772 and 8316718 stamped on it. That is indeed a 125P1B.

I have 365vac on each end of secondary to chassis. The HT secondaries are center tapped. JJ 5Y3 and 6V6 installed.

[lowell]

Sorry I had taken those measurements with the original vintage tubes in it. Not JJ. Also... after replacing the power cord the amp sounds "normal." The volume/output issue is gone. Doesn't make a whole lotta sense... but I can't complain cause it sounds good. So I guess it's ok... still odd voltages though eh?