5F6A won't break up

[vince76]

Hy guys,
I recently built a 5F6A clone from a ceriatone kit. Apart from a hum problem solved virtually center tapping the 6.3V tap of PT It was a successful building.
I had
12AY7 in V1
12AX7 in V2
12AX7 in V3
6L6GC in the power section and
GZ34 as rectifier tube

I noticed that it won't break up even at very high vol so I put a 12AX7 in V1 position. The situation is better now to my ear, but it never really break up...
I'm wondering if there's someting I can do to make the sound closer to a Bassman 59 ltd fender reissue.

thanks, vincenzo

[loudthud]

On any new build there is the possibility of a wiring error or bad component. Voltage measurements usually point to where the problem is. One thing I would check is the heater voltage because of the unknown combination of your line voltage and the transformer's primary rating. Please post some voltages.

[mikeboone]

You may want to experiment with other rectifiers. A 5AR4 is pretty stiff and not going to offer much sag. Higher voltage is going to contribute to clean headroom.

I put a 5Y3 in my AC30 CC and it really gave it a more vintage feel.

[vince76]

thank you guys for your reply
loudthud: I have cheked wirings. thy're ok. maybe a bad component... I checked the voltage too, but since I replaced the preamp tube I will check them again and post soon... In any way all voltages were a 15-20% higher than specs. Maybe the 5Y3 as suggested by mikeboone is a good solution?

thank you again... my amp simply wouldn't be sounding without this forum

vincenzo

[Diablo]

Is this amp built with the standard 5F6A circuit and component values?
Post up your voltages so we can see if they're typical.
I think a 5Y3 rectifer is too low a power rating for the 5F6A. Here's some info on rectifier ratings which shows the voltage drop and amperage ratings:


5Y3 -60 volts @ 125 mA
5U4GB -50 volts @ 275 mA
5U4 -44 volts @ 225 mA
5V4 -25 volts @ 175 mA
5AR4 -17 volts @ 225 mA

I'm using a 5U4GB rectifier in my 5F6A, and the plate voltage is 462 volts. The amp uses Philips 5881 tubes and a 12AY7, and it breaks up nicely when I crank up the volume. A hot pickup (humbucker) gives more breakup than a Strat (single-coil).

[vince76]

thank you very much diablo.
Very useful information for me...
the amp was built following the original schematic and using the right values for components. there is only the variable bias mod.
I will post the actual voltages tomorrow

these were measured some time ago:

B+ measured at pin 8 of GZ34 (I hope is this the point where to measure)
465vdc

v5(6l6gc)
pin 3: 463
pin 4: 460
pin 5: -41
pin 6: 463
v4(6l6gc)
pin 3: 463
pin 4: 463
pin 5: -41
pin 6: 463
v3
pin1:208
pin3:64
pin6:200
pin 8:62
v2
pin1:165
pin3:1.22
pin6:357
pin8:177
v1
pin1:223
pin3:2
pin6:230
pin8:2


preamp tubes: two 12AX7, one 12AY7 but I can't remember where the 12AY7 was at the moment I took measurements

thank you,
vincenzo

[Diablo]

OK Vince. Here's your voltages, my voltages and the Fender schematic. I have a 12AY7 in V1, 12AX7 in V2 and V3, NOS 5881 in V4 and V5, and NOS 5U4GB in V6. Your power tube voltages look fine. V1, V2 and V3 volts don't look right to me.


Vince76 Diablo Schematic
v5(6l6gc)
pin 3: 463 463 430
pin 4: 460 463
pin 5: -41 -50 -48
pin 6: 463 463
v4(6l6gc)
pin 3: 463 463 430
pin 4: 463 462
pin 5: -41 -50 -48
pin 6: 463 462
v3
pin1: 208 272 230
pin3: 64 46 34
pin6: 200 275 236
pin 8: 62 46 34
v2
pin1:165 210 180
pin3: 1.22 1.16 1.2
pin6: 357 350 325
pin8: 177 211 180
v1
pin1: 223 177 150
pin3: 2 2.88 2.5
pin6: 230 174 150
pin8: 2 2.88 2.5

[vince76]

wrong preamp voltages

Thank you diablo,
now I have confirmed suspicion that something may be wrong in my amp... but... anyone have an idea about what is wrong?

v.

[Diablo]

I would check the value of all the resistors to make sure they agree with the schematic, and look for wiring that doesn't agree with the schematic. I would start looking at valve pins that have the wrong voltage, and check the next resistor in the circuit that supplies that voltage. Keep working your way back in that circuit toward the supply filters.

[loudthud]

Vince76, I searched your previous threads and found a picture of your amp. It looks nice.

On V3 pins 3 and 8 are connected together. They should read the same voltage! Check this.

There is a 4.7K resistor between the screen supply and the phase inverter supply. What is the voltage on each side of it and what is the voltage on the 10uF(?) cap that is the filter for the preamp?

What is the voltage on pin 8 of V3 and V4?

What is the AC voltage from pin 4 or 5 to pin 9 of V1?

Some resistance checks (power off and caps discharged) 5 or 10% tolerance accepted.

With no plugs in the input jacks, pins 2 and 7 of V1 should measure 34K to ground. With a cable connected between the high and low inputs of the respective channel, the same pins should measure 1.034 megohms.

Pin 2 of V2 should measure 135K with both volume controls at minimum and 635K with both volumes at maximum. Pin 6 of V2 should measure 100K to ground. Your meter may react strangely with the capacitors in the tone stack and not give you a stable reading.

With the Bass Treble and Mid controls at maximum the center of the Treble control should measure 1.275 meg (+/-20%) to ground.

Measure from pin 5 of V4 and V5 to the bias supply. It should be 220K.

[vince76]

Thank you guys I'm surprised by your attention
loudthud:thank you very much for these simple instruction.
This is what I get after I put back the 12AY7 in V1:

In V3 pins 3 and 8 are connected together. They should read the same voltage! Check this.

OK I have 49.9 on both pins

There is a 4.7K resistor between the screen supply and the phase inverter supply. What is the voltage on each side of it and what is the voltage on the 10uF(?) cap that is the filter for the preamp?

468V and 417V at 4.7K resistor
at the 10uF I have 351V

What is the voltage on pin 8 of V3 and V4?

I have 49.9 on pin 8 of V3
V4 is the power tube where you wrong on this?

What is the AC voltage from pin 4 or 5 to pin 9 of V1?

6.1V AC

Some resistance checks (power off and caps discharged) 5 or 10% tolerance accepted.

With no plugs in the input jacks, pins 2 and 7 of V1 should measure 34K to ground. With a cable connected between the high and low inputs of the respective channel, the same pins should measure 1.034 megohms.

34K on pins 2 and 7 of V1 OK
BUT
When I put a jumper cable between ch 1 & ch 2 of normal channel i have
1.086M on pin2 and 34K on pin7
vice versa on bright channel

I think I misunderstood wiring on the input jacks but I still can't understand what...

Pin 2 of V2 should measure 135K with both volume controls at minimum and 635K with both volumes at maximum.

OK

Pin 6 of V2 should measure 100K to ground. Your meter may react strangely with the capacitors in the tone stack and not give you a stable reading.

As you say my meter won't let me measure at all (it gives negative reading)

With the Bass Treble and Mid controls at maximum the center of the Treble control should measure 1.275 meg (+/-20%) to ground.

OK

Measure from pin 5 of V4 and V5 to the bias supply. It should be 220K.

OK

I want to thank you again for your time

vincenzo

[MWJB]

-41 at pin 5 of the power tubes suggests to me your tubes are way underbiased (too much current)?

Plate voltages at V2 & V3 look a little low, definitely a 12AX7 in there? Double check values of cathode & tail resistors at pins 3 & 8 if so, report back. Whats the voltage at either side of the 470ohm resistor at pin 3&8 of V3?

Single coils may need a 12AX7 in V1 to get break up at less than flat out.

[vince76]

hy MWJB and than you too
I read many of your posts

-41 at pin 5 of the power tubes suggests to me your tubes are way underbiased (too much current)?

I biased at 36mA

Plate voltages at V2 & V3 look a little low, definitely a 12AX7 in there? Double check values of cathode & tail resistors at pins 3 & 8 if so, report back. Whats the voltage at either side of the 470ohm resistor at pin 3&8 of V3?

Ihave 12AY7 in V1 and 12AX7 in V2 and V3 while measuring
the resistor value are ok
voltage of the 470ohm res are 48.9V and 47.3V

Single coils may need a 12AX7 in V1 to get break up at less than flat out.

I had 12AX7 before measuring, I just put in the 12AY7 to measure correctly.

Vincenzo

[MWJB]

How did you determine the bias?

[vince76]

MJW,
I have a 1ohm resistor tied from pin 1-8 of power tubes to ground and measure the Mv at the two leads of res.
hope I was clear in explanation (my englis sometime is awful)

v.