5F6A won't break up

[MWJB]

Hi Vince, no, that's clear thanks. Just seems low on the negative dc that's all?

6.1VAC at the heaters seems a little low too.

Got any close up pics of the circuit?

[vince76]

here are the pics

v.

[Diablo]

Check to see if your input jack wiring is the same as this layout from Ted Weber - this layout is correct.

https://taweber.powweb.com/store/5f6a_layout.jpg

[MWJB]

You need to tidy up that wiring, you have several excess meters of wire in there. Plate wires can be long and pressed down to the chasis flooor. Grid wires must be short and not run parallel/close to plate & heater wires.

Explain your grounding scheme? Preamp grounds (inc PI & preamp filter cap) get grounded to input jack end of the amp. Output grounds, main & screen filter caps get grounded to PT bolt. Seperate wires for each ground path, don't link them on the board.

What's the big DPDT between the power tubes?

Where are your screen grid resistors (470ohm @ 1-5W at pin 4 of the 6L6) and grid stoppers (1500ohm at pin 5)?

What is the value of the preamp dropping resistor (next to preamp filter cap)?

You say you want it to be comparable to a RI Ltd, but you have not copied the RI circuit (see Fender.com, support, amplifier schematics). You have the Marshall style tone stack, the 56K slope resistor gives a brighter, middier bite...could be percieved as cleaner. The RI has a 0.1uf bass cap, you have 0.22uf. The RI also has 6.8K PI tail (& a different, quieter, presence control), your 10K tail will give less gain.

It may be an illusion but it looks like your 100ohm virtual centre tap resistors connect to your bias pot?

[MWJB]

"The RI has a 0.1uf bass cap, you have 0.22uf" sorry should be 0.022uf.

[vince76]

Wow thank you MWJB...I have a lot to learn
I will shorten wires and re-ground (I have to better study grounding. I have some links).

The switch is an half power switch

the screen grid resistors are not well visible but they are on pin 4 and 6 of 6l6

I'm not using grid stoppers... should I? (happy to learn more about)

the preamp dropping resitor (I hope I understand from the schem) is 820ohm

I will take a look at that schem...I tought mine (5F6A) was the same schem of fender 59 LTD.

The virtual CT resistor are grounded at PT bolt

What I discovered after checking wiring 100+1 times is that I was wrong with input jack wiring (I never used this tipe before... think marshall stile)... If Loudthud and Diablo had not told me to check again I think I will never discovered... I hope that rewiring correctly let the preamp voltages be more close to spec..

[MWJB]

"I'm not using grid stoppers... should I? (happy to learn more about)" If your layout is good then you might get away without them...it is safer to install 1.5 to 2.2K at the build stage. Look at AB763 socket wiring.

"the preamp dropping resitor (I hope I understand from the schem) is 820ohm" No, the cathode resistors at V1 pins 3 & 8 and at V2, pin 3 are 820ohms. The preamp dropping resistor (10K in the original schem) is the vertical resistor on the board, connecting to the preamp filter cap and V2, pin 6.

"I will take a look at that schem...I tought mine (5F6A) was the same schem of fender 59 LTD." There are about a dozen different, legitimate, permutations of the 5F6A circuit. The RI Ltd is closest to the latest 5F6A circuit, the main differences being in filtering (totem pole filters at main & screen supply - a good idea - and 40uf for the preamp filter, 20uf should be fine here).

[vince76]

ok it is 10Kohm @ 2W

sorry, V

[vince76]

another strange thing

don't know if could interest to someone, anyway...
I was rewiring the input jack and decided to take a rough measurements of the resistors on board... while measuring the presence res, as I touch one lead with the MM the lead came out from the resistor...
I t was broken but invisible.

V

[Diablo]

don't know if could interest to someone, anyway...
I was rewiring the input jack and decided to take a rough measurements of the resistors on board... while measuring the presence res, as I touch one lead with the MM the lead came out from the resistor...
I t was broken but invisible.

V

Presence resistor? Do you mean the negative feedback resistor that connects the primary OT coil to the presence pot? I bet when you replace this resistor and fix up the input jack wiring, the amp will return to normal.

[vince76]

thank you Diablo.
yes the NFB resistor, excuses for my non technical language.
I hope it will return to normal. the only thing that annoy me is that find a carbon comp resistors here is almost impossible... so, for the moment I will stick in whathever i find.

V

[vince76]

Repalced the resistor...now sounds good to my ear.
voltages remains the same I posted before. I don't know If I have to search for something else wrong or those values are acceptable.
Thank you to all helped me.

vincenzo

[Diablo]

Vince:
Did you rewire the input jacks too? I'd still be concerned with your voltages on V1, V2, and V3. I can't make any suggestions what else might be wrong because I just don't have the expertise.

[MWJB]

I'd try a few different brands of tube in V2 & V3 (V1 looked OK), see how that affects the relationship between V2 pin 6, pin 1 & pin 7 (pin 6 looked OK, just 1 & 7 were a little low, but not necessarily indicative of a problem).

[vince76]

diablo:
yes, I rewired the input jacks too, now I think they're ok.
MWJB:
OK I will try. I noticed that comes less light from V2 filament respect to the other two tubes (excuse for the explanation, but I think you can understand) maybe it's the tube.

vincenzo.