5E3 Build Odd Voltages Question

[jim911]

Hello,

I am posting for the first time regarding a 5e3 build I just completed. Picked up someones abandend project off Ebay with Mercury Magnetics Power Trans. Axiom FTDP, unknown output trans. Part Number TF-110, Ceriatone board.

Have fired it up with some older tubes and it sound great and have the following voltages I am wondering if they are correct:

5Y3 is older GE pin 2 - 7.9v, pin 8 - 8.5v, pin 4 - 373v, pin 6 - 373v

6v6 valve 4 pin 2 - 6v, pin 3 - 388v, pin 4 - 353v, pin 7 - .6v, pin 8 - 22v

6v6 valve 3 pin 2 - 6v, pin 3 - 389v, pin 4 - 355, pin 7 - .5v, pin 8 - 22v

12ax7 valve 2 pin 1 - 188v, pin 3 - 1.3v, pin 4/5 - 6v pin 6 - 212v, pin 8 - 48v, pin 9 - .6v

12ax7 (no 12ay7) valve 1 pin 1 - 168v, pin 3 - 1.4v, pin 4/5 - .5v pin 6 - 168v, pin 8 - 1.4v, pin 9 - 6.0v

Could someone please give me an idea if these are in the acceptable range for 5E3 voltages. Thanks

[tubeswell]

5Y3 is older GE pin 2 - 7.9v, pin 8 - 8.5v, pin 4 - 373v, pin 6 - 373v

Hi Jim
Welcome to the forum

You need to measure VAC on these rectifier pins (not VDC) The Power transformer coverts wall voltage AC into a range of secondary AC voltages for the rectifier and the heater filament circuit. The AC voltages will give a better idea of how its working. (I'm making an assumption here that you measured in DC, because the rectifier filament voltages look wrong)

6v6 valve 4 pin 2 - 6v, pin 3 - 388v, pin 4 - 353v, pin 7 - .6v, pin 8 - 22v

6v6 valve 3 pin 2 - 6v, pin 3 - 389v, pin 4 - 355, pin 7 - .5v, pin 8 - 22v

The plate voltage on each tube looks to be too high compared to the cathode voltage on each tube. Assuming you have a 250R cathode resistor, you would have 44mA per tube, which is about 42mA on each plate, which is too high for a 6V6. You want about 36mA max on each plate for this amp - otherwise the tubes will have a short lifespan (What size is your cathode resistor BTW?)

You either need to change to a slightly larger cathode resistor (next size up - try 330R if you have 270R etc), or get the voltage on the power transformer's HT winding down with a couple of 15V 5W zener diodes in series between the HT centre tap and the ground, both reverse-biased (with their anode/banded ends pointing to the ground) - this should lower the plate voltages to around 340V (which should also sound good BTW).

12ax7 valve 2 pin 1 - 188v, pin 3 - 1.3v, pin 4/5 - 6v pin 6 - 212v, pin 8 - 48v, pin 9 - .6v

12ax7 (no 12ay7) valve 1 pin 1 - 168v, pin 3 - 1.4v, pin 4/5 - .5v pin 6 - 168v, pin 8 - 1.4v, pin 9 - 6.0v

These look normal for 12AX7s. If you had a 12AY7 in V1, the plate voltages there would drop and you would get a cleaner fatter tone at low volumes. (A 12AY7 has a higher current draw than a 12AX7)

[jim911]

Tubeswell,
Thanks for the welcome and your response. This is a great forum with great information for a relative newcomer to building with tubes.

On the 5y3 rectifier with my meter set on AC and the amplifier power on with the meters common lead grounded to the chassis I use the 200 VAC setting and get a reading of 7.7 volts ac and on pin 8 I get 8.4 volts AC. If I measure between pin 2 and 8 using the 200 volt AC range I get 5.7 volts AC.

Setting the meter to the 600 volt AC range on both pins 4 and 6 I get 371 volts.

In reference to the power tubes I am indeed using a 250R 5watt cathode resistor. Unfortunately I don't have a 330R 5watt or other close to try. I did however have two 18 volt 5 watt zener diodes that I soldered in series with the HT center tap to ground which in the case of the Mercury Magnetics is ared/yellow wire that I had grounded to one of the transformer bolts. This lowered my plate voltages to 359v dc but within less than a minute the two zeners came unsoldered, so it must have been generating some seriuos heat which I am sure isn't good. Maybe I didn't do this correctly?

Also wondering why my heater voltages differ with both pin 2's being close to 6 volts AC, while the pin 7's are 1.3vac and 1.6vac respectively. Is this normal?

I will be getting a 12AY7 as I would like to keep this as close to stock at least to start as possible.

Thanks again for the suggestions and hopefully this additional information may shed a little more light on where my problem is.

[tubeswell]

On the 5y3 rectifier with my meter set on AC and the amplifier power on with the meters common lead grounded to the chassis I use the 200 VAC setting and ... If I measure between pin 2 and 8 using the 200 volt AC range I get 5.7 volts AC.

Umm... the 7ish and 8ish volt readings are strange, but since you are measuring between the two ends of the 5V winding and getting 5.7, I guess that's close enough

Setting the meter to the 600 volt AC range on both pins 4 and 6 I get 371 volts.

Okay, did you measure between pins 4 and 6?, or did you measure pin 4 to ground and pin 6 to ground? (Sorry I should have made that clear first)

If the voltage is between from each pin to ground, then you should get a B+ of about 420VDC on pin 8 if the rectifier is working.

In reference to the power tubes I am indeed using a 250R 5watt cathode resistor. Unfortunately I don't have a 330R 5watt or other close to try. I did however have two 18 volt 5 watt zener diodes that I soldered in series with the HT center tap to ground which in the case of the Mercury Magnetics is ared/yellow wire that I had grounded to one of the transformer bolts. This lowered my plate voltages to 359v dc but within less than a minute the two zeners came unsoldered, so it must have been generating some seriuos heat which I am sure isn't good. Maybe I didn't do this correctly?

if you dropped 36 volts and with the current draw going through that amp of about 70-90 mA at idle (for a couple of 6V6 tubes and a couple of 12AX7 pre-amp tubes), that will be expected to be between 2.5W-3W dissipation. Are you sure they were 5W zeners? (FWIW When I put zeners in like that, I mount them on a tag strip and keep a bit of wire on each end of the zener to dissipate a bit of extra heat. Also, if you use 3 x 12V, or 4 x 9V, you will get the same 36V drop but with better dissipation capability.) If they were 5W zeners, then something is making the amp draw too much current (- possibly something shorting?). What is the VDC at pin 8?

Also wondering why my heater voltages differ with both pin 2's being close to 6 volts AC, while the pin 7's are 1.3vac and 1.6vac respectively. Is this normal?

How have you got the heaters wired? You should get ~6.3VAC across the filament on each tube. You need to measure from pin 2 to pin 7 on each output tube, and measure between pins 4-5 and 9 on each preamp tube. (Note for parallel wiring, pins 4 and 5 need to be soldered together and connected to one side of the heater winding and pin 9 goes to the other side). Has your heater winding got a centre tap? (and if so, is it grounded?), or is it one where you are using a 100R resistor from each side of the winding to ground?, or have you wired it old-school Fender-style with one side of each filament wired to ground and the filaments getting 6.3VAC from one side of the heater winding only, with the other side of the winding grounded?

[jim911]

Umm... the 7ish and 8ish volt readings are strange, but since you are measuring between the two ends of the 5V winding and getting 5.7, I guess that's close enough

Yes I thought the 7.7v and 8.4v readings when measuring from pins 2 and 8 to ground were odd as well.

Okay, did you measure between pins 4 and 6?, or did you measure pin 4 to ground and pin 6 to ground? (Sorry I should have made that clear first)

I measured pins 4 and 6 to ground and got the 371v on each. I have and aligator clip on my meters common lead that I clip on to the chassis for ground to allow me to only use one hand while using the positive lead for checking voltages. Pins 2 and 6 have the two red wires from the Mercury Magnetics FTDP transformer terminated on them. Mercury's drawing shows the two red leads rated at 380V with the red/yellow wire shown in the drawing in the middle of the two red wires which I assummed was the center tap. This red/yellow (center tap?) wire is tied to a lug on one of the transformer bolts for ground.

If the voltage is between from each pin to ground, then you should get a B+ of about 420VDC on pin 8 if the rectifier is working.


if you dropped 36 volts and with the current draw going through that amp of about 70-90 mA at idle (for a couple of 6V6 tubes and a couple of 12AX7 pre-amp tubes), that will be expected to be between 2.5W-3W dissipation. Are you sure they were 5W zeners? (FWIW When I put zeners in like that, I mount them on a tag strip and keep a bit of wire on each end of the zener to dissipate a bit of extra heat. Also, if you use 3 x 12V, or 4 x 9V, you will get the same 36V drop but with better dissipation capability.) If they were 5W zeners, then something is making the amp draw too much current (- possibly something shorting?). What is the VDC at pin 8?

The zener diodes were brand new from Mouser still in the package which was labelled 18v 5w. I will try to mount them on a tag strip tomorrow and see if that will work any better.

How have you got the heaters wired? You should get ~6.3VAC across the filament on each tube. You need to measure from pin 2 to pin 7 on each output tube, and measure between pins 4-5 and 9 on each preamp tube. (Note for parallel wiring, pins 4 and 5 need to be soldered together and connected to one side of the heater winding and pin 9 goes to the other side). Has your heater winding got a centre tap? (and if so, is it grounded?), or is it one where you are using a 100R resistor from each side of the winding to ground?, or have you wired it old-school Fender-style with one side of each filament wired to ground and the filaments getting 6.3VAC from one side of the heater winding only, with the other side of the winding grounded?

I was measuring each of the heaters to ground. When I measure the heaters as you describe above all measure between 6.3 to 6.5VAC

Thank you for this additional information and it looks like I can at least check the heaters off the list.