5f4 tweed super build?

[Chuck H]

The voltages on THAT tube are really odd. Both triodes are wrong. I'm looking at the layout and trying to figure what would cause this.

Just for the heck of it try a different tube in that socket.

There are a couple of 56k and 1.5k resistors all clumped together in that area of the layout. If a different tube doesn't fix it, look a fourth, fifth and sixth time at your wiring in this area.

[chuckb]

Hey ChuckH, I'll take a look at these resistors and wiring again. i did by the way, change out the tube and still nothing different. I'll keep you posted. I'm thinking I may need to pull the board and check to see if there might be a problem to the wiring underneath. I was hoping I didn't have to do that again, but at least it will give me a peace of mind knowing everything is ok there. i guess i'm second guessing myself here, but i won't know untill I take a look. Pulling the board will be my last resort though.

[frus]

The relationship between pins 7 and 8 should be a couple of volts. They're too far off from each other. Not sure why.

It's impossible to measure the grid voltage (pin 7) in this setup with an ordinary DVM. It's bootstrapped so it has a big input impedance, and DVM internal resistance is messing up the reading. So it can be just about anything, depending on the make etc. I had the same question, and someone on Weber forum cleared it up. Pins 6 and 8 are about right, so I'd say that section is OK

[Chuck H]

Could be. I've seen the same thing measuring LTP PI's too. Right now we're primarily focusing on the cathode voltage for the driver (not a high impedance) whick measures over three times as it should.

[frus]

I'm thinking I may need to pull the board

Yeah, I'm thinking the same
btw, I'm NEVER going to put the wires on the underside of the board EVER again in ANY amp I make
It's really a PITA, especially if you want to do some mods

[frus]

Could be. I've seen the same thing measuring LTP PI's too. Right now we're primarily focusing on the cathode voltage for the driver (not a high impedance) whick measures over three times as it should.

yes, I'm inclined to think there is a miswire somewhere on the board

[chuckb]

Hey guys, thanks for the replys. I'm about to pull the board again and re-check the wiring underneath. I know it's a PITA and I'm really not wanting to do it, but at this point I think it may possibly be a mistake in the wiring I have there. I'll keep you guys posted. I'm really looking forward to getting this amp up and running.

Chuck.

[MWJB]

"3rd 12ax7 p1= 306v
p2= 0v
p3= 3.676v
p6= 315v
p7= 0v
p8= 3.637
Tube # 3 has a jumper on pins 3 & 8."

Assuming what you call "3rd 12AX7" is the tube nearest the input jacks (V1 in the signal chain, or the input stage), there is something amiss here. It looks like your plates (pins 1 & 6) are sitting at B+ rail voltages (they should be about 100v lower). Your 12AX7 is drawing over 20mA per side...it should be drawing more like 1mA per side. What is the voltage drop accross the resistor connected to pins 1 & 6? Check that you have not got the wrong resistor values here, they should be 100K (100,000ohms).

2nd (middle/cathode follower) 12AX7 actually looks right. Note the voltages in the 5F4 schematic are apropriate for a 12AY7, not a 12AX7 (even though the 12AX7 is designated). Actual plate & cathode voltages should be >50% higher than the schematic. Voltages at pins 1, 7 & 8 should be near enough the same.

1st 12AX7 (V3/Phase inverter in this amp) has a similar issue to V1 ("3rd 12AX7"), little voltage drop from the B+ rail but loads of current being dissipated through the tube (was 4mA, now 2.66mA). I suspect an incorrect plate resistor value at pin 1 (again, should be 100,000ohms/100K).

[chuckb]

Hey MWJB,
Thanks for your reply. The Resistor across p1 & 6 of v#3(or as it is correctly referred to as v1 ) is 1w/100k
The plate resistor is 1w/100k at p1 of the PI. I did a voltage check on p1,6,&7 of the PI and the readings are as follows;
P1=210.5v
P6=329v
p7= 210v.

charles.

[MWJB]

Chuck, I am now more confiused than before...

V1 - at the inputs, farthest from the power tubes, should have the same voltage at pin 1 as at pin 6. Just over 200vdc (roughly 300vdc at the junction of the 100K plate resistors in an inverted "V" shape). Confirm.

V3 - The phase inverter, next to the power tubes, should have a little more than 210v at pin 1 (though not necessarily much more), 329v at pin 6 sounds about right, but there is no resistor from pin 1 to pin 6 on either of these tubes (V1 or V3). The PI (V3) has a 100K at pin 1 and a 56K at pin 6.

V2 Does have a 100K from pin 1 to pin 6 & 7? This tube should have the same voltage at pins 1 & 7, so this looks like the tube you are discissing?

Count the tubes (V1, V2, V3 etc) from the inputs towards the power tubes rather than from R to left...because some amps are oriented differently & we always need to know that the signal from your guitar goes into v1 from the input jacks.

Post some pics of the circuit board & tube socket wiring. And/or print off a 5F4 layout, scrub out the voltages and enter exactly what you have on your amp & copy/link it to the thread.

[chuckb]

Hey mwjb, sorry for the confusion. Ok, going the correct direction, V1 input,V2 cathode follower,V3 PI. These are the voltage readings I just took.

V1-p1=204v
p6=212v
@ the juction of the two 100k resistors(inverted v)=309v

V2- p1=198.6v
p6=308v
p7=198.5v
p8=199.7v

V3 (PI) p1=357v
p3=6.8v
p6=295v
p7=24.25v
p8=67.4v

Again sorry for the confusion.

Chuck.

[chuckb]

Hey MWJB, here's the layout I'm using.

[MWJB]

I know what the layout looks like, the idea was that you put your voltages on it ;-)

V3 (PI) is still a problem. You only seem to be dropping ~1v accross the pin 1 plate resistor, therefore you do not have a 100K resistor between it and the power supply. The voltage here should be around 245vdc.

[chuckb]

Hey MWJB, V3 (PI) p1 has a 100k resistor. Like on the layout I have a lead from p1 to a 100k which is attached(inverted V) with a 56K resistor. could it be possible that the 100k resistor is faulty? I measured the value of this 100k resistor and it reads as it should 100k. I had no way of posting the voltages I measured to the layout.

chuck.

[chuckb]

I changed out the 100k resistor and replaced it with another 3w/100k. voltage taken on p1 is 314v.

Chuck.