#124 started

[amplifiednation]

So it was the Heaters?

Forget the overdrive for now. Get the amp working first. If the overdrive isn't engaged there's only two preamp tubes.

Can you post more pics? You might have a missed connection in the tone stack. Also you need to confirm you wired the overdrive relay to pass signal when not engaged. One of the connections needs to be jumpered. Check continuity.

[larsvictor]

Good morning Marcus,

I´ve been looking at the pictures you posted on 4th of january. Regarding the heater wiring again: I cannot find the two ~100R Resistors from the 6.3 VAC winding to ground. If you omitted those resistors, that would be the most likely source of hum.

Are the relays switching? Do you hear the "click"? If the relay is enabled you should measure 0 VDC at the anode side of the diode, if it´s disabled there should be 5 VDC at the anode.

Posting a list of voltages would be a good start.

Lars

[norburybrook]

Morning all, hope you've all got your popcorn ready for today's episode


Right B+ voltages

B+1 429vDC
B+2 428
B+3 405
B+4 273
B+5 266

Valves

V1 plates 177 and 173 , cathodes 1.35

V2 one side is 166 and 1.5 the other is 270 and 2.1


V3 one side is (pin 1) 271 but the other side I didn't get a reading because as soon as I attach the probe the amp motorboats really loudly regardless of amp volume!!!!!!

also the hum disappeared when I probed pin 2 or 7


V 4 and V5

pins 3 428
pins 4 -50
pins 5 427
pins 6 429


does this shed any further light as to what's not right?

obviously one side of v2 is squirrely and so is the phase inverter and the voltages are all about 20v too low.

@lars, the heaters have a center tap so no need to add the resistors.


Marcus

[larsvictor]

also the hum disappeared when I probed pin 2 or 7

which valve?

Voltages on V2b look strange indeed. With 2.1VDC on the cathode there obviously is current running through the cathode resistor, but there´s almost no voltage drop across the plate resistor. Did you measure the plate resistor? Could it be damaged ?

Lars

[norburybrook]

Lars, V3 for the hum.

just measured the V2 plate resistor and it's showing 100R!!

I'll swap it out now and see what happens.

OK, took out that plate resistor and it was measuring 100r instead of 100k Must have a been a factory error when packing.

I SUPPOSE THATS WHY YOU SHOULD MEASURE EVERYTHING FIRST BEFORE SOLDERING IN PLACE.

lesson learned.

that's sorted out the V2 voltage issues.


So back to the phase inverter.

this is where the hum is coming from.


I've got 245 and 275v on pins 1 and 6 and touching these with the meter probe to ground reduced the hum.

pins 2 and 8 are 55 and then cathodes 35

touching pins 2 or 7 also reduces the hum instantly.


I've swapped the valve for a known good one and it's the same.


marcus

[larsvictor]

Glad to hear you fixed the V2 problem.

What happens to the hum when you turn the master volume all the way down ?
If it dissapears it's more likely a problem in the peamp section. If not, I'd agree that V3 is the source.

Lars

[norburybrook]

the hum does vary with the master volume and presence pots. It's still loud though whichever way it is.

The motor boating sound is back again when I probe pin 1 on V3

Marcus

[larsvictor]

Do I get this right ? Pins 3 and 8 are 55VDC and the bootstrap node ( the center of the 4 resistors) is 35VDC? That is a voltage drop of 20V across a 820R resistor. That is a current of 24mA which is way too high, isn't it?

Lars

[martin manning]

I think there is there is a bad solder joint or something miswired in the PI. The 30V difference between pins 1 and 6 is too much. Check the all resistor values and connections carefully.

[norburybrook]

no Lars,

Pins 3 and 8 are 53VDC at the moment and the center of the 4 resistors is 50VDC


thanks again for your help here.

marcus

[norburybrook]

I've been staring at this all day now.

OK on V3,

I've checked every resistor value that has a connection to the valve.

they're all as they should be.

I've checked continuity between the valve pins and their connections on the board.

they too check out.


I've reflowed all connection on the board and on the pins of V3


I'm at a loss.



Marcus

[larsvictor]

Ah, good! Martin has his popcorn ready. Help is coming.

Ok. That makes more sense with 53 and 50vdc.

[erwin_ve]

Marcus: where is the PI section grounded? At the Return jack? Is there continuity?

[norburybrook]

it's grounded at the negative end of the 1.0uf then along to the return jack.


there's good continuity between the return jack and the board connection and also any ground on the chassis..

M

[martin manning]

Measuring at the center of the four resistors is the right way to get grid voltage. You have ~2 mA flowing in the tail now, which seems reasonable. Are your plate voltages still far apart? Is the PI trimmer centered? Still have the hum?