Please help with my first build 5F1 with no sound

[icy_wind500]

This was an earlier pic, I think he's reflowed since then....

http://a6.sphotos.ak.fbcdn.net/hphot...79728901_n.jpg

It's poss there's a wire been trapped and shorting? but it would have shown up by now, all I can think is there's a component fault somewhere.

yes I've reflown all the joints at the back and insulated with a piece of cardboard from the box the kit came in

[tubeswell]

Checking the PT is simple. If the PT is making ~6.3 VAC or so between each end of the heater winding, and ~650VAC or so between each end of the high tension winding, and 5VAC or so between each end of the rectifier winding, it'll be fine.

To check the OT, you need to unhook the OT primary and secondary windings completely from the rest of the circuit (with the amp off and unplugged), and hook each side of the secondary directly up to one side each of PT's 5VAC winding, and then (making sure the now-unconnected OT primary wires are not touching anything else) switch the amp back on and carefully measure the VAC across the OT primary. Make sure the OT secondary and the PT primary are not contacting/shorting to any part of the chassis or the amp's circuit when you do this. Its best to use insulated gator clips to hook the VAC meter up to the OT primary, and its best to use mains-rated insulated screw connectors to hook the Ot secondary to the PT 5V winding. The VAC between each end of the OT primary winding should be 160VAC or so (assuming it is an 8k:8R OT) with about 5VAC applied to the secondary winding. (Make sure your meter is on the 'hundreds of volts AC' setting to measure this)

The point of taking the board out is to check the wiring on the back of it throughly, and also to make sure the back of the board is properly insulated/separated from the chassis.

[icy_wind500]

Checking the PT is simple. If the PT is making 6.3 VAC between each end of the heater winding, and 650VAC between each end of the high tension winding, and 5VAC between each end of the rectifier winding, it'll be fine.

To check the OT, you need to unhook the OT primary and secondary windings completely from the rest of the circuit (with the amp off and unplugged), and hook each side of the secondary directly up to one side each of PT's 5VAC winding, and then (making sure the now-unconnected OT primary wires are not touching anything else) switch the amp back on and carefully measure the VAC across the OT primary. Make sure the OT secondary and the PT primary are not contacting/shorting to any part of the chassis of the amps circuit when you do this. Its best to use insulated gator clips to hook teh VAC meter up to the OT primary, and its best to use mains-rated insulated screw connectors to hook the Ot secondary to eth PT 5V winding. The VAC on the primary should be 160VAC or so (assuming it is an 8k:8R OT)

Checking PT, between heaters (green) is 6.83V AC, (yellow) is 5.07V, (red and white) is 579V

[tubeswell]

Checking PT, between heaters (green) is 6.83V AC, (yellow) is 5.07V, (red and white) is 579V

I thought on the other forum that you said the high tension winding measured 670? Was that another PT? ~580VAC centre tapped is 290-0-290VAC, which will get you about 320VDC B+ with a 5Y5GT rectifier.

650VAC (which is 325-0-325VAC with the center tap) will get you around 360VDC B+ with a 5Y3GT rectifier.

[icy_wind500]

I thought on teh other forum that you said the high tension winding measured 670? Was that another PT?

That multimeter was bad so all the voltages were wrong

[telenut62]

It's the lower 550V secondary on the Classictone PT

[tubeswell]

Is there a higher secondary on that PT?

[telenut62]

........630V

[icy_wind500]

the red one I think

[tubeswell]

I'd use the 630 winding

(Its getting late here guys and I've got work in the morning - I'll catch up with you tomorrow)

Pete

[icy_wind500]

ok changing now

[tubeswell]

But also might pay to check the OT and the back of the main board like I said. (And take care with your working hours)

[telenut62]

B+ will be over 400V...this'll be interesting

[icy_wind500]

green ( 6.84V)
yellow (5.02V)
red (655V)

[icy_wind500]

But also might pay to check the OT and the back of the main board like I said. (And take care with your working hours)

So now that the PT looks right, I guess the only problem is under the board or OT.....
otherwise I have a $300 paper weight ....

[telenut62]

Always a process of elimination, try the OT test when your fresh and clear of mind.

[icy_wind500]

should I try OT first or pull the board first

[telenut62]

You've checked the board? Try the OT...but be very careful.

[icy_wind500]

I'm not sure what color is what .... I have red blue which are primary and black green which is secondary right ?

[telenut62]

This is your OT?...Transformer, Output, 8 W, Single Ended | Antique Electronic Supply LLC

[icy_wind500]

yes !

[telenut62]

Ah the link didn't come up....go to post #86 at TDPRI there is a link to a schematic to confirm the primary/secondaries

[icy_wind500]

Ah the link didn't come up....go to post #86 at TDPRI there is a link to a schematic to confirm the primary/secondaries

ok I think we found the problem....


Its only 1.2V across the primary....

[telenut62]

Well well....it's a 5K OT? but I think it should read higher than that, get on to the supplier show them what you've found and get a refund or a replacement.

[icy_wind500]

I've just emailed Dave !

Oh what a relieve

[icy_wind500]

Well well....it's a 5K OT? but I think it should read higher than that, get on to the supplier show them what you've found and get a refund or a replacement.

By the way.... would the OT take out any other parts along with it since its not working?

[loudthud]

ok I think we found the problem....


Its only 1.2V across the primary....

It looks kike the meter is set to DC volts. That won't work if you are trying to measure AC volts.

[Steve Conner]

Yeah, he's looking at the power tube idle current times the DCR of the primary. Ought to be a bit more than 1.2V I should think.

Not a complete diagnosis though: we don't know whether the OT primary is shorted, or the power tube is failing to pass current. What voltage were you getting across the power tube cathode resistor?

[tubeswell]

Yeah, he's looking at the power tube idle current times the DCR of the primary. Ought to be a bit more than 1.2V I should think.

Not a complete diagnosis though: we don't know whether the OT primary is shorted, or the power tube is failing to pass current. What voltage were you getting across the power tube cathode resistor?

He was getting about 7 Volts (across 470R) with a B+ around 360 IRRC

[tubeswell]

B+ will be over 400V...this'll be interesting

If the HT winding is 655 end to end, it'll be 327.5-0-327.5 taking into account the centre tap, which will deliver about 360 B+ with a 5Y3GT in this amp.

[telenut62]

All good in theory Tubeswell, but his AC supply is prob closer to 250V....as mine is, and that throws the B+ out quite a bit. As he's using a NOS 6V6 prob best to keep it close to the 350V. Come to think of it, would a bad 6V6 be a poss cause of his probs. I don't know if he's swapped the tubes?

[telenut62]

Ah yes it's set on the V- setting, missed that last night...was passed my bedtime...oh dear!

[loudthud]

The main problem with the amp seems to revolve around the low screen voltage. That lowers the voltage to the preamp. I had a tube once that had a broken connection to the plate internally. It drew lots of screen current, there was still good plate voltage at the socket, but no plate current and low cathode current. No plate current, no sound from the OT. Is there a DC voltage drop across the OT that would indicate plate current is flowing?

Just thinking out loud. My .022

[tubeswell]

He said the meter was on the AC setting (over on the telecaster forum) FWIW

[tubeswell]

If he's getting 327-0-327 on the secondary with the wall voltage that he's got, that will nevertheless amount to about 360VDC B+ with a 5Y3GT for this amp. (But if he's got a sovtek/russian 5Y3, it'll be higher)