Troubleshooting Help, New 183, Low Voltages

[jlatrace]

Well I finished my 183 earlier in the month. I have spent a week of my spare time trying to find my mistake without success. My eyes and mind are tired and I am asking for some help. So I'm posting some pictures hoping someone else can see what I keep missing.

The problem is that the amp has uniformly low voltage as follows (unloaded readings - no tubes):
B+1 = 389 vdc
B+2 = 389 vdc
B+3 = 378 vdc
B+4 = 298 vdc
B+5 = 298
Heaters = 2.9 vac
Relay power = 5 vdc (this is correct, I am using 5 vac regulator and relays)

I have already verified that it is NOT the PT by disconnecting the PT and checking the voltages. The transformer is a mercury magnetics tranny I purchased from another builder. Also I have switched the OT secondaries to make sure it had nothing to do with oscillation (although I really didn't think that was the problem).

Here are some pictures.

[jlatrace]

More pics

[ToneMerc]

Well I finished my 183 earlier in the month. I have spent a week of my spare time trying to find my mistake without success. My eyes and mind are tired and I am asking for some help. So I'm posting some pictures hoping someone else can see what I keep missing.

The problem is that the amp has uniformly low voltage as follows (unloaded readings - no tubes):
B+1 = 389 vdc
B+2 = 389 vdc
B+3 = 378 vdc
B+4 = 298 vdc
B+5 = 298
Heaters = 2.9 vac
Relay power = 5 vdc (this is correct, I am using 5 vac regulator and relays)

I have already verified that it is NOT the PT by disconnecting the PT and checking the voltages. The transformer is a mercury magnetics tranny I purchased from another builder.

.



What are the voltages at each PT secondary leg with only the primary connected? I wouldn't rule out the PT because your relay supply that has it's own dedicated PT, appears to be fine. What primary voltage is the
PT primary wired for and what is your AC main voltage, I'm assuming it's 120?

TM

[jlatrace]

TM,
Thanks for the quick reply. Here are the B+ readings (unloaded) with only the primary connected (secondaries are disconnected & capped with wire nuts):
B+1 = 388
B+2 = 388
B+3 = 377
B+4 = 297
B+ 5 = 297

I am montoring my variac with a DMM and it was reading 120 vac when I tokk these and the other voltage measurements.

While I'll be out the bucks paid, I hope it is the PT.
Les

[ToneMerc]

TM,
Thanks for the quick reply. Here are the B+ readings (unloaded) with only the primary connected (secondaries are disconnected & capped with wire nuts):
B+1 = 388
B+2 = 388
B+3 = 377
B+4 = 297
B+ 5 = 297

I am montoring my variac with a DMM and it was reading 120 vac when I tokk these and the other voltage measurements.

While I'll be out the bucks paid, I hope it is the PT.

I already know the DC rectified voltages are low. I want to know the AC voltages at the output of the PT secondaries.

Also, if the secondaries are capped off with wire nuts and only the primaries are connected, it' s impossible to have B+ at the nodes. You didn't answer my question off how the primary is wired, but I'm assuming it only two leads, 120v only.

Let's cut this in half........at the blue bias wire where it terminates to the bias board, what's the AC voltage?

At each HT secondary same thing.

Better yet, disconnect the B+ feed to the main power board, you should have around 465-475V DC at the HV board eyelet. if you don't have voltage that high, verify the AC at the secondaries.

TM

[CHIP]

Which Mercury Magnetics PT?

[jlatrace]

Thanks for the replies. Here is what I have now. I will get more information later today (including disconnecting the B+ from the main power baord and taking readings.

PT is a ToneClone FBFTP rated for 335 vac on the secondaries. When I measured the secondaries before installing the PT they measured 306 vac, 30 vac too low, but it should still give me about 430 vdc rectified.

All of these measurements are taken at 120 vac line voltage.

[jlatrace]

TM,
Sorry, I was watching the football game and trying to respond. The unrectified B+ is 278 vac and the unrectified bias is 35vac. I am using Jason's board set. I'll get the B+ vdc reading with the main power board disconnected later today and I'll get new AC readings too.
Les

[odourboy]

The problem is that the amp has uniformly low voltage as follows (unloaded readings - no tubes):
B+1 = 389 vdc
B+2 = 389 vdc
B+3 = 378 vdc
B+4 = 298 vdc
B+5 = 298
Heaters = 2.9 vac

Not exactly uniformly ... the heater is cut in half compare to the others, which are only reduced. Does this heater have a center tap? Maybe you've grounded a hot lead on the heater taps, which is pulling everything else down?

[ToneMerc]

Thanks for the replies. Here is what I have now. I will get more information later today (including disconnecting the B+ from the main power baord and taking readings.

PT is a ToneClone FBFTP rated for 335 vac on the secondaries. When I measured the secondaries before installing the PT they measured 306 vac, 30 vac too low, but it should still give me about 430 vdc rectified.

All of these measurements are taken at 120 vac line voltage.

It's your PT, with only around 306VAC you are only going to see around 407-410VDC rectified and loaded, slightly less if it's biased hot. Also, there's something going on with the filament winding.

With what you have given me you should have 432VDC unloaded, right around 408 loaded and if 306VAC is only providing 389VDC rectified, with nothing connected, it's a paperweight.

It almost seems like you have one of those funky Twin II power transformers.

TM

[CHIP]

I have that transformer and it is different from yours. Mine doesn't have copper foil wrapped around the coil. It has brown paper. Mine has a center tapped filament, (yellow and green wire).My bias supply wire is blue and red, I don't see that on yours.

[jlatrace]

It has a filament center tap, it's not being used and it is capped with a wire nut. The heater reading is for 1/2, so double it (5.8 vac) to compare to 6.2 vac.

I'm thinking it is a paper weight too. I have a MC twin PT that I will put in. First I'm going to check the filaments and the B+ reading off the rectifier board (unhooked ) as suggested by TM.

Thanks to everybody; the folks on this site are very generous and I very much appreciate your help and advice.

[ToneMerc]

It has a filament center tap, it's not being used and it is capped with a wire nut. The heater reading is for 1/2, so double it (5.8 vac) to compare to 6.2 vac.

I'm thinking it is a paper weight too. I have a MC twin PT that I will put in. First I'm going to check the filaments and the B+ reading off the rectifier board (unhooked ) as suggested by TM.

Thanks to everybody; the folks on this site are very generous and I very much appreciate your help and advice.

You could check DC at the HV board output, but the fact that your PT HT leg voltage is either 278 or 306VAC verses 335VAC with only the primary input connected will result in low rectified DC voltage.


Glad to help

TM

[jlatrace]

TM
I understand, its a bummer. No more ebay "savings" for me!
Les

[Structo]

Not sure I understand why you didn't ground the CT of the heater winding?
Or use an artificial CT with a couple 100R resistors to ground.

Damn these amps get very crowded around the power tubes.