New 5e3 : inadequate voltages ...

[Strat76]

Hi everybody,
First I would like to introduce myself. I am Fabrice from France and I built recently my first 5e3.
It's a kit from Triodes electronic. I followed this lay out from Triodes.

5E3DeluxeLayout-Triode240V.pdf

I tried to do my best, checked out several times the connections, the resistor values, etc ...
Here is a pic of the amp ready to be tested.



Yesterday it was the test day. First I fired on the amp with a dim bulb tester and everything went well. After, it was time to check the voltages.

- Without the rectifier (5Y3) : ok except that I have 3V between each filament wire and the ground. But I believe that it's normal because I must have 6V between the two wires. Please tell me if I am right.
- With the rectifier, the behavior of the amp is ok. I have all the pops that I need when I check the tubes pin. But my voltage are not what they are supposed to be.

Here is my voltage checking :

12AY7
P(1) : 199,8 (instead of 120)
P(3) : 1,31 (instead of 2)
P(6) : 194 (instead of 120)
P(8) : The same that P(3)

12AX7
P(1) : 186 (i. of 160)
P(2) : 1,34 (ok)
P(6) : 270 (i. of 200)
P(8) : 5,13 (i. of 60)

6V6a
P(3) : 353 (ok)
P(4) : 334 (ok)
P(8) : 20,9 (ok)

6V6b
almost the same.

I checked every connection, every resistor, and everything seems to be ok. The only thing I can't check is the caps.

The amps behavior is ok but as I crank the volume pot I heard a very loud hum.
For now I removed the turret board and check everything again.

I must confess that I am a bit lost and I need some help. It looks like I have an issue around the PI, but I don't really know where it can be.

Can you first confirm that my voltages are not right, and tell me if you have an idea of the issue's source.
It will be very kind.

Thanks in advance.

[jmaf]

Hi Fabrice, welcome to the forums.

Unless I'm missing something, your voltages are all ok.

About the hum: is there zero hum when the volume is zero?

[Strat76]

Hello Jmaf, many thanks for your quick reply.
I believed that 199 V instead of 120 was far too high. In the same way I read on the Tubedepot manual that it is usual to find 60v at the pin 8 of V2. So I was a bit surprised.

About the hum, it's only when I crank the volume pot. When the volume is down, the amp is very quiet. Only the 50hz hum in the transformer.

But for now I am checking all connections they are good except for one input jack. This jack fail to make the connection with the ground when there is nothing inside. So, I cross the fingers and hope that is only that.

Can you confirm that the voltage are correct ?
I am surprised that the tubedepot manual says that the correct voltage at P(8) must be +/- 60V ... If somebody owns a good voltage chart, I take it for sure

[MWJB]

Do you actually have a 12AY7 in V1, make sure that you don't have a 12AX7 in there by mistake.

What is the voltage at the 3rd (preamp) filter cap "+"?

Don't keep pulling the board, be methodical eliminate things one by one in an orderly manner.

[MWJB]

You have shielded the grid wire to V2 pin 2...but not either of the grid wires feeding V1 pin 2 or 7? These wires are far too long & lie right on top of the plate wires. Clip them out from behind the board, re-route each of the straight out from the junction of the 68K input resistors, directly down to the relevant tube pin, keeping in front of the board & away from plate & heater wires.

[Strat76]

I DO HAVE a 12ax7 in V1. I didn't have a 12ay7 yesterday. Happily this morning I received 2X12ay7. I didn't think the voltages differences could come from that. Thanks for this idea.

Anyway I am ending to check everything. All the connections are good.
I can't say anything about the caps unfortunately.

[Strat76]

No, you're right, I didn't use shielded wire foe Pin 2 and 7 of V1. I followed the advices of Bruce on this forum.
I will do what you say ... I am beginning to rewire the board. So it's the right time
Thanks a lot for these advices ...

[guitarmike2107]

Hi Fabrice, Welcome to these forums.

Your P8 Voltage on V2 does look strange. this is the Phase inverter, sounds to me like you have grounded the 1.5k resistor under the board rather than grounded the 56k resistor.
Can you check the 56k resistors and the 1.5k resistor connected in series to V2 p8, also the turret that these two resistors connect with the 1Meg resistor must not be connected to earth.

Mike

[MWJB]

In a typical Fender preamp triode (1.5Kohms at each triode, or 820ohms at 2 shared triodes, 100Kohms plate resistors) a 12AX7 will show on its plate ~68% of the voltage applied to the 100K plate resistor at the B+ rail. A 12AY7 will show ~49%.

E.g. If you have 199vdc at V1 pins 1 & 6 with a 12AX7, you should have roughly 293vdc at the B+ rail feeding the V1 plate resistors. A 12AY7 shows less voltage because it draws more plate current than a 12AX7. in the same circuit I would expect to see in the region of 143vdc.

Tubes of the same type differ nominally so don't start stripping stuff down if voltages are, say, +/- 10vdc from those predicted.

[loudthud]

I am surprised that the tubedepot manual says that the correct voltage at P(8) must be +/- 60V ... If somebody owns a good voltage chart, I take it for sure

+60V is correct for V2 pin 8. It will vary from tube to tube. The voltage at V2 pin 7 (measured to ground) is unpredictable because the impedance of your meter changes the operating point of the tube in that circuit.


There are three large filter caps. What are the voltages on these when all the tubes are plugged in?

With the power off, amp un-plugged from the mains voltage and caps discharged, measure the resistance of V1 pins 3 and 8 to ground.

[Strat76]

Hi Mike, thanks for these kind words.
Yes I checked several times and checked again when I read you message. The 56K is well connected to the ground and is connected with the 1,5k in series to V2-8.
On the another way the 1Meg is connected to these two resistors and goes to V2-7 ....
In fact, it was one of the things I was thinking first.

[MWJB]

V1 pins 3 & 8 are fine...1.3vdc/820ohms gives about 0.8mA per triode, ball park for a 12AX7 in a low voltage 5E3.

Strat76 - again we can use this figure to compare plate voltages as 0.8mA* 100K (plate resistor) = 80vdc.
199vdc + 80vc = 279vdc at the B+ rail feeding the plate resistors...close enough to the predicted 293vdc to suggest normal operation.

[Strat76]

Well :
- C13 (first cap on the shematic - the closest to the standby switch) : 362 V
- C12 : 334 V
- C11 : 279 V

It sounded good to me ...

[MWJB]

Looks good...6V6 screens are drawing a little under 2mA each, as are the pairs of 12AX7 triodes
362-334 = 28
28/4700ohms = 5.9mA :-

334-279 = 54vdc.
54/22000ohms = 2.45mA
less 1.59mA from V1 leaves just under half a mA each for the 2 V2 triodes. 56000 x 0.43 mA = 24vdc
280-24 = 256vd...14v off V2 pin 1 voltage!

In short, your hum is unlikely to relate directly to plate voltages...double check that V2 pin 8 cathode voltage. V2 voltages will be higher than the schem, again because the 12AX7 in V1 is pushing them up.

[jmaf]

If the hum gets quieter when volume is down, I would bet on a ground loop. I don't see a central star ground on there, I think he grounded at the chassis at every point?

The main reservoir is correctly tied to the cathode resistor of the 6V6's so that would be a great candidate for star ground epicenter. Additional photos would help.

PS. Great build by the way, Fabrice. Definitely does not look like a first.