Bad PT? resolved!

[T Wilcox]

Hello all

Well I finished 2 builds this weekend, a TW express and a HRM w/EL-34's.
The TW turned out great with all voltages in spec. The HRM on the other hand had some issues. Powered it up and biased my tubes and everything seemed well. Even played it for about 10 minutes and man it sounded great. So I thought hey before I get too far I better check my voltages and make sure everything is good. This is when I found these readings.

V1- PIN1 = 215 PIN3 = 1.9 PIN6 = 221 PIN8 = 1.8

V2- PIN1 = 243 PIN3 = 2.1 PIN6 = 252.8 PIN8 = 2.0

V3- PIN1 = 347.8 PIN3 = 66.3 PIN6 = 334.8 PIN8 = 66.3

V4- PIN3 = 511 PIN4 = 507 PIN5 = -45.3 PIN6 = 511

V5- PIN3 = 511 PIN4 = 506.1 PIN5 = -45.3 PIN6 = 510.5

BIASED AT APPROX 35MV

So quite obvious that everything is way high.

I pulled my HT wires and metered off the board and on one leg am reading 304VAC and the other leg am reading 410vac. This is a Magnetic components PT PN# 40-18073 which should be 340-0-340.

http://www.classictone.net/40-18073.pdf

I am still learning my way through alot of this stuff and would like to get any expert opinions on what could cause this or if there is anything else I could check before sending this back to Triode to test and replace which I am sure will take atleast a couple weeks.

Has anyone here ever gotten a bad PT out of the box?
Is there anything I need to check in my amps wiring that could have caused this?
BTW in the 10 min's of playing this thing sounded awesome, no redplating ,and no blown fuses, but with the voltages so high Im sure something would fail eventually just not sure what , the tubes? the PT?

Thanks in advance for any comments

Todd

[David Root]

That is odd. Which wire is being used as the HT centertap? Red/Yellow or Red/Green? Was it disconnected when you made the HT VAC measurements or was it still connected to the chassis ground?

[T Wilcox]

Hey David
I am using the red/green as the 0v on the bias ckt.
Correct me if Im wrong but isnt the center tap the SS orange wire

The only wires disconnected when measuring were the 2 reds!

Thanks

Todd

[David Root]

Er..no it isn't. Red/Yellow is the HT centertap. Orange is the electrostatic shield between primary and secondary windings and it goes to chassis ground like the red/yellow should.

Red/Green wire goes to the bias supply circuit. Sounds like you were using that as the HT centertap and grounding it?

However if both the orange and red/yellow are connected to chassis ground and red/green goes to bias supply, and you still get the wacky AC VAC, something is wrong.

[T Wilcox]

So in other words if I just put the red/yellow to ground and the red/green on the bias I should be good?

I will try that when I get home today.

I did just guess between the 2 wires since nowhere in the spec sheet does it say which is which. Where did you find this info? Like me the hard way?

Thanks again

Todd

[David Root]

Yes, that's right. The diagram is rather misleading. Red/green is the 50V bias supply. By connecting that to ground you unbalance the two sides of the HT. Red/yellow is the HT centertap.

The way the wording is written I could see you might think that red/green is the HT CT, but it's not.

[Firestorm]

Red/yel to ground. Red/green - 50V bias tap. Reds - 340V winding ends.

[T Wilcox]

Thanks guys

When I had the choice of using either a green stripe or yellow stripe i automatically thought green/ground yellow/hot. Man MC makes good products but they're documentation blows.

Todd

[talbany]

Todd
If your not sure you can always ohm it out

Tony

[T Wilcox]

Thanks Tony

Yeah I just made an uneducated guess on that one. Luckily no sparks this time
What should I read if I ohm it out?

Todd

[Structo]

The orange wire is the shield, it goes to ground.

Look at the text below the transformer symbol on the documentation.

It says that the Red/Green is the bias tap.

[T Wilcox]

Hey Tom

The orange was always grounded! It was whether the red/green or the red/yellow was to be grounded which stumped me. i had a 50% chance and guessed wrong. I do believe one of your previous posts explained the orange a while back which I somehow remembered.

But in that same spec sheet do you see anywhere where it says which of the 2 bias taps is 0v?

Thanks

[Structo]

Yes, the text below the symbol says:

RED-RED/YEL-RED = 340V-0-340V (680V CT)

RED/YEL-RED/GRN = 50V BIAS

Also, if you look carefully at the transformer symbol the RED/YEL wire #5 is shown dead center on the symbol. So it is the center tap.
While you usually can't trust those all the time, in this case it is accurate.
Strange they reverse the colors in the text to YEL/RED.

So then the RED/GRN is above that so that is the bias tap.

Don't worry that was a very easy mistake to make.

You did power up with a current limiter didn't you?

Hope it didn't harm anything.

[T Wilcox]

No I didn't use current limiter. ( yeah I know your thinkin Jackass )
This is what I get.
Anyways weird thing was before checking voltages the amp worked with it wired that way. Actually sounded damn good!. Hope with it wired correctly it will sound the same.
I will swap the wires this afternoon after work and report back tomorrrow.

BTW I see what your looking at on the spec sheet now, but man they sure have a crappy way of detailing rather important info.

Anyways hopefully all checks out well

Thanks again

Todd

[talbany]

Todd
, HT wires are usually both soild colors (most transformers are red) but I have seen other colors.. If you stick your ohm meter on them you will usually get a low ohm reading like 8 ohms or so..If your searching for the CT of that winding look for the wire that reads 1/2 that value 4 ohms.. 8-4-8 you found the CT..This also tells you if it is a balanced ..You can do the same with OPT

Personally I don't think you did any harm to the PT..Don't sweat it

Good Luck!!

Tony