New power transformer issues

[randomair]

I have a Sound City L100W Mark III that I just replaced the power transformer on. The issue I am having is low output, the output is extremely low for 100 watt amp. When I turn it to 9 its about as loud as a small practice amp. I went through the schematic and checked doing the "pop" test on all of the plates and grids and everything seems relatively normal. I checked all screen grid resistors and bypass caps, they are all ok. The resistors in the filter cap section were funky, one read 660k, another 350k another 300k so i replaced them all to normalize that section.

I began to get suspicious when V1 plate was only reading 190 volts, schematic states 95v on 1 and 6. I checked the rail and I am getting 260vdc where I should be getting 335vdc! Coming from the + side of the rectifier I am reading 330vdc where I should be reading 410vdc. I checked the output of the power transformer at the secondary by first checking at the inputs of the rectifier, getting 236 volts AC. I pulled the secondary mains from the rectifier and sure enough, I am only getting 236 volts AC coming from the secondary mains with no load. The schematic states 350 volts AC. Am I right in presuming my transformer is underrated for this amp?


Below is the schematic for L/B100W. As the schematic states my voltages are way off, too low.

http://www.soundcitysite.com/sc_webpages/B100_schem.jpg

Thanks!

Edit: amp has been re-tubed with known good tubes by the way.

[Enzo]

Is the mains voltage set right? Are you sure you wired the various primary taps correctly?

Leave the amp in standby but power on. Schematic says 350VAC for your high voltage winding - is yours close to that? Is your 6vAC pretty close?

[randomair]

My heaters are measuring 4.6AC and the wiring diagram for the power transformer I got from Heyboer. Everything should be wired properly. I am just getting 100 VAC less on the secondary mains. The voltage is set properly.

My 115 tap is measuring 150VAC, my 225v is measuring 68VAC and my 245 is measuring 150VAC. What the heck is going on here???

[Enzo]

So both your secondaries are 25% low - there almost has to be something wrong on the mains side. Is the transformer heating a lot?

Wait, I am assuming something. You are not installong a factory transformer, you are installing a replacement brand. Is the replacement a wire-for-wire drop-in? Or is it an equivalent but with different taps? In other words, the original had four series taps on the same winding, while these days you are more likely to see split primaries. Tell us more.

[randomair]

Your assumption is correct, this is a newer replacement from Heyboer in Michigan. I sent them the original Partridge and instead of rewinding they sent me a new transformer. he wiring is not a match drop in. I had to get the wiring diagram from them, the PT is not heating. By the way, your insight is priceless right now.

[Enzo]

So post or link the wiring guide.

Is it split primary or multitap?

[randomair]

There is no link to the wiring diagram, it was given to me over the phone. I guess my main question here is why my 117vac line is running 150vac? and why the amp now has a 10amp fuse in replace of the 3amp that was originally listed on the schematic on the primary after the switch and before the taps. Some one has etched on the chassis "10 amp fuse" in replace of the 3 amp. The only thing I can imagine is it was modified at some point, but why is the question?

[randomair]

Well, in my quest to bring this amp back to life, I have noticed some modifications to V2. Someone has jumpered pin3 and pin8 together and removed the two 1k preset trimmers and 100mf caps. In replace of the two trimmers they have added just a 2.2k resistor. I tried pulling the 2.2k resistor and using my resistance sub box tried a 1k resistor to see if this would make a difference in volume, it does. By doing this I get a decrease in volume and awful distortion. ALso, seeing -10 v on the wiper one bias pot no matter the setting of the pot, the other bias pot is capable of changing the voltage to proper -35. Going to check the and see if the pot is faulty and check the 100mf cap to ground.

[bluesfreak]

According to my plan both cathodes of V2 do ground via a 1k Pot and a 100µF Cap (see attached schematic) and indeed this could be a root cause for your problem. Also ensure that if your bias pot is dead the tubes did not suffer...you might have burnt already one side...

[Steve Conner]

My heaters are measuring 4.6AC
...
The voltage is set properly.

One of these two statements is false. (or your voltmeter is broken)

[randomair]

One of these two statements is false. (or your voltmeter is broken)

Its possible I have "amp repair fog" in my head. Ill check it again.

I am going to go ahead and change V2 back to stock and swap the bias pot.. What is a safe voltage rating for those V2 caps? I see 0.6v test points on the schematic.

[Alex R]

Most bypass caps are 25v. Small cheap and safe, though you could go even lower.

47uF would sound just the same.

[randomair]

Cool thanks. I have plenty o' those. I am really hoping this will solve my issues but yes I am concerned about the output tubes, stress and such. The pace will be glacial.

[Alex R]

That 2.2K should work kind of ok. Try a bypass cap across it.

[randomair]

I did, it didnt help me. Unfortunately my problems may be larger than they seem? ***As a side note I have individually checked EVERY resistor in this circuit, point by point out of circuit.