Help Ampeg B-15NC B+ voltages low when pluging 6L6 Power Tubes

[Dom65]

Hi everyone, I hope someone can help me with a problem I have been chasing for a while...

I am trying to fix a low B+ problem that I get only when I put one (or both) of the 6L6 power tubes in an Ampeg B-15NC. So basically, when I remove all the tubes everything is on specs (or close) in the B+ voltages, from the pin8 of rectifier 5AR4 to the plate of the first preamp tube everything is ok. Same result with all the tubes In, minus the 2 6L6. As soon as I plug a 6L6 everything goes down too much. Right at the pin 8 of rectifier I see 320VDC instead of the 440VDC I should have according to schematics. All heaters are also ok. The amp works but it doesn’t have the volume it should have (I think). The OT resistance is reading perfect on secondary and close to specs on primary. The 2 halves are equal with the centre tap too. Power tubes are cathode bias. Any idea anybody of what could be the problem?

 

[Wasnex]

How much current are the tubes pulling?

Measure the resistance of the cathode resistor. Measure the voltage drop across the cathode resistor. Calculate the tube current.

Thought Exercise. Let's say you adjust the bias so the tubes cut off. The plate voltage will rise. As you adjust the bias to increase tube current, the plate voltage will drop.

The 64 B15NC schematic I found shows a 250 ohm cathode resistor shared by both tubes. The voltage across the resistor is 36V.

Current I = 36/250 = 144mA total.

144mA/2 = 72mA per tube.

Plate voltage on the schematic is 430V

Effective plate voltage (measured cathode to plate) = 430 - 36 = 394V

Dissipation W = 0.072 x 394 = 28.368W

6L6GC is a 30W tube.

% dissipation = 28.368/30 x 100 = 94.56%

 

[beans-on-toast]

Use a 6L6GC and not a 6L6, I’m sure that you meant GC.

The 250R 5W resistor often goes out of spec. As mentioned check the 36 vdc bias.

The voltages normally goes down when tubes are in place. The reference voltages are at idle. The full schematic is availableat Vintage Blue

 

[beans-on-toast]

Also remove connection K and see what happens. It goes to the centre wiper of the hum pot.

 

[boroman]

Hi everyone, I hope someone can help me with a problem I have been chasing for a while...

I am trying to fix a low B+ problem that I get only when I put one (or both) of the 6L6 power tubes in an Ampeg B-15NC. So basically, when I remove all the tubes everything is on specs (or close) in the B+ voltages, from the pin8 of rectifier 5AR4 to the plate of the first preamp tube everything is ok. Same result with all the tubes In, minus the 2 6L6. As soon as I plug a 6L6 everything goes down too much. Right at the pin 8 of rectifier I see 320VDC instead of the 440VDC I should have according to schematics. All heaters are also ok. The amp works but it doesn’t have the volume it should have (I think). The OT resistance is reading perfect on secondary and close to specs on primary. The 2 halves are equal with the centre tap too. Power tubes are cathode bias. Any idea anybody of what could be the problem?

If the 5W resistor is out of specs (towards more resistance), voltage will drop, but havent seen THAT muchlike 100+V. Usually my B15Ns were close to 450-520V. Are you sure your power transformer is original?

 

[Dom65]

How much current are the tubes pulling?

Measure the resistance of the cathode resistor. Measure the voltage drop across the cathode resistor. Calculate the tube current.

Thought Exercise. Let's say you adjust the bias so the tubes cut off. The plate voltage will rise. As you adjust the bias to increase tube current, the plate voltage will drop.

The 64 B15NC schematic I found shows a 250 ohm cathode resistor shared by both tubes. The voltage across the resistor is 36V.

Current I = 36/250 = 144mA total.

144mA/2 = 72mA per tube.

Plate voltage on the schematic is 430V

Effective plate voltage (measured cathode to plate) = 430 - 36 = 394V

Dissipation W = 0.072 x 394 = 28.368W

6L6GC is a 30W tube.

% dissipation = 28.368/30 x 100 = 94.56%

Hi Wasnex,

Thanks for your reply,

Rc reads 260.2 ohm

Voltage across is 25.35VDC

I = 97.4 mA

97.4mA/2 = 48.7mA per tube.

Va plate V4 (1rst 6L6) = 346VDC

Va plate V5 (2nd 6L6) = 348VDC

So lets say 346 - 25.35 = 320.65VDC

Dissipation W = 0.0487 x 320,65 = 15.62W. It seems very low to me.

If I remove the 6L6s (V4 and V5), I Have 536V on plate V4 and 533V of V5.

Also, with all the tubes in, when I take readings on plates of all the tube (including the rectifier 5AR4 on pin 8),
the voltages rise up and peak rapidly, then go down in about 30s then go up again for over 2 minutes before they stabilize.
Is that normal?
Once stabilized, the first 3 tubes (the 6SL7) are pretty close to specs, but obviously not the 6L6s.

 

[Dom65]

If the 5W resistor is out of specs (towards more resistance), voltage will drop, but havent seen THAT muchlike 100+V. Usually my B15Ns were close to 450-520V. Are you sure your power transformer is original?

Hi boroman,

5W? Do you mean the 250ohm 10W (cathode bias of 6L6s?).
If so, It read 260.2 Ohm. I'm pretty sure it is. Apart from a 2 prong to 3 prong conversion (and maybe the speakers output jacks), everything seems original.
Ah, also, the filter can. I'm not sure what happened there. The values are 40/40/40/40MFD instead of 40/40/20/20MFD.
It seems that someone installed in some kind of foam to float it?
See photos.

 

[Dom65]

Use a 6L6GC and not a 6L6, I’m sure that you meant GC.

The 250R 5W resistor often goes out of spec. As mentioned check the 36 vdc bias.

The voltages normally goes down when tubes are in place. The reference voltages are at idle. The full schematic is availableat Vintage Blue

View attachment 7088176

Hi beans-on-toast,

Tubes are vintage GC. The one that doesn't show the name is an old RCA made in USA.
The cathode bias resistor of the 6L6s reads 260.2V and the V drop is 25.35V.
The voltages peak then drop for about 30s and rise up to stabilize after a couple of minutes.

 

[agedhorse]

What’s all over the bottom of the can cap, either it’s a nasty failure or a horribly messy repair attempt.

That bias level isn’t all that low, especially if you consider high line conditions.

 

[Dom65]

What’s all over the bottom of the can cap, either it’s a nasty failure or a horribly messy repair attempt.

That bias level isn’t all that low, especially if you consider high line conditions.

Hi agedhorse,
Well It looks to me as someone was trying to float (disconnect) the can from chassis.
Very messy indeed.
Also the death cap was still connected even after the 2 to 3 prong conversion.
See picture of when I opened it.

 

[beans-on-toast]

Hi beans-on-toast,

Tubes are vintage GC. The one that doesn't show the name is an old RCA made in USA.
The cathode bias resistor of the 6L6s reads 260.2V and the V drop is 25.35V.
The voltages peak then drop for about 30s and rise up to stabilize after a couple of minutes.

The cap can looks expired. It needs to be isolated from the chassis. These’s an insulator that should be used. Which one depends on the capacitors discharged can.

https://www.talkbass.com/wiki/technical-amplifier/#protecting-the-rectifier-tube-from-flashover-8203

https://www.talkbass.com/wiki/technical-amplifier/#protecting-the-rectifier-tube-from-flashover-8203



When the amp is powered up with the power supply capacitors discharged, there’s an over voltage while the capacitors charge. It takes a short time for the bias to stabilize. The cathode capacitor needs to charge.

Are you using the standby switch for the heaters to come up to temperature?

A NOS 5AR4 performs better than some new production ones in terms of turn on time and voltage drop.

Is the 5AR4 rectifier arcing internally (flashover) when the amp is turned on and the electrolytic caps fully discharged? I recommend that 1N4007 diodes are added to extend the peak inverse voltage as described here.

Invalid Link Removed

 

[Wasnex]

Hi Wasnex,

Thanks for your reply,

Rc reads 260.2 ohm

Voltage across is 25.35VDC

I = 97.4 mA

97.4mA/2 = 48.7mA per tube.

Va plate V4 (1rst 6L6) = 346VDC

Va plate V5 (2nd 6L6) = 348VDC

So lets say 346 - 25.35 = 320.65VDC

Dissipation W = 0.0487 x 320,65 = 15.62W. It seems very low to me.

If I remove the 6L6s (V4 and V5), I Have 536V on plate V4 and 533V of V5.

Also, with all the tubes in, when I take readings on plates of all the tube (including the rectifier 5AR4 on pin 8),
the voltages rise up and peak rapidly, then go down in about 30s then go up again for over 2 minutes before they stabilize.
Is that normal?
Once stabilized, the first 3 tubes (the 6SL7) are pretty close to specs, but obviously not the 6L6s.

It does seem like the voltage is dropping an excessive amount considering the tubes are pulling significantly less current than expected. I would suspect excessive sag across one of the following:
1. OT primary.
2. Secondary winding of the PT.
3. Rectifier tube.


What is the DC voltage at the OT centertap or pin 8 of the rectifier? Should be 440V with the tubes operating. You indicate 320V in the OP but in post #6 you indicate 346-348V on the plates

Do you have a means to safely measure the AC across the PT HT taps? For 440VDC I believe it should measure about 645VAC. AFAIK 5AR4 should drop about 10V. 645/1.43 - 10 = ~441VDC. If the AC voltage is close to spec, I would suspect the rectifier to be the problem

Do you have a spare rectifier tube?

 

[Dom65]

The cap can looks expired. It needs to be isolated from the chassis. These’s an insulator that should be used. Which one depends on the capacitors discharged can.

https://www.talkbass.com/wiki/technical-amplifier/#protecting-the-rectifier-tube-from-flashover-8203

https://www.talkbass.com/wiki/technical-amplifier/#protecting-the-rectifier-tube-from-flashover-8203



When the amp is powered up with the power supply capacitors discharged, there’s an over voltage while the capacitors charge. It takes a short time for the bias to stabilize. The cathode capacitor needs to charge.

Are you using the standby switch for the heaters to come up to temperature?

A NOS 5AR4 performs better than some new production ones in terms of turn on time and voltage drop.

Is the 5AR4 rectifier arcing internally (flashover) when the amp is turned on and the electrolytic caps fully discharged? I recommend that 1N4007 diodes are added to extend the peak inverse voltage as described here.

Invalid Link Removed

I'm not not using the standby switch (I keep it on). I use the power switch.
No arcing on 5AR4. I tried with another rectifier tube same thing but with a few Volts more.

 

[Wasnex]

If I remove the 6L6s (V4 and V5), I Have 536V on plate V4 and 533V of V5.

This seems odd. With no tubes installed there should be no current flow through the PT secondary. So both tube sockets should measure the same plate voltage.

 

[beans-on-toast]

I'm not not using the standby switch (I keep it on). I use the power switch.
No arcing on 5AR4. I tried with another rectifier tube same thing but with a few Volts more.

Not using the stanby switch is fine but the heaters need time to come up to temperature. In part, why you have the 30 second delay. If the cap can is leaking, it will affect the charging time and performance.

 

[Dom65]

It does seem like the voltage is dropping an excessive amount considering the tubes are pulling significantly less current than expected. I would suspect excessive sag across one of the following:
1. OT primary.
2. Secondary winding of the PT.
3. Rectifier tube.


What is the DC voltage at the OT centertap or pin 8 of the rectifier? Should be 440V with the tubes operating. You indicate 320V in the OP but in post #6 you indicate 346-348V on the plates

Do you have a means to safely measure the AC across the PT HT taps? For 440VDC I believe it should measure about 645VAC. AFAIK 5AR4 should drop about 10V. 645/1.43 - 10 = ~441VDC. If the AC voltage is close to spec, I would suspect the rectifier to be the problem

Do you have a spare rectifier tube?

Pin 8 rectifier is 349V.
I did try a different rectifier. No difference.
The 320V had in my first post was from an older reading where I was afraid of letting the amp on too much time since it was taking so long to rise up.
Now I can see that it takes about 2 minutes to stabilize. So now I get 349V
In OT primaries, measuring at pin4 and 6 of rectifier I get 347VAC on pin4 and 346VAC on pin 6.

 

[agedhorse]

I would really want to know what’s up with that can cap before doing anything else.

Troubleshooting rule #1 is to investigate and correct the damage from previous repair attempts and modifications.

 

[Wasnex]

In OT primaries, measuring at pin4 and 6 of rectifier I get 347VAC on pin4 and 346VAC on pin 6.

AFAIK that should rectify to about 474V

346x2 = 692V/1.43 - 10 = 473.9V

Is that with the 6L6GCs in circuit and 349VDC at pin 8 of the rectifier?


This is from a 4AR4 datasheet.

If I am interpreting this right, line 3 of the trace should show the approximate output voltage (X axis) VS the current draw (Y axis). Line 3 = 350V per plate and you measured 346V and 347V.

You indicated the output tubes are pulling 97.4mA. If you follow line 3 over to 97.4mA, it looks like the rectifier output should be about 440V, which matches the schematic. This does not account for the preamp and screen grid current draw. You can calculate the screen and preamp current by measuring the resistance and voltage drop across the resistor in the Pi filter.

You indicated the output of the rectifier is 349V. If the rectifier is good and each plate is getting ~350V, then at 250mA current draw the output of the rectifier should still be about 390V.

So AFAIK either the total current draw is over 250mA or the rectifier is bad.

 

[Wasnex]

So basically, when I remove all the tubes everything is on specs (or close) in the B+ voltages

What is B+ without tubes? AFAIK the 440V is what you should see with the amp idling normally. With tubes pulled, the voltage should be notably higher. In some amps, B+ voltage with all tubes removed rises above the rating of the filter caps.


Could definitely be a leaky filter cap.

 

[Dom65]

AFAIK that should rectify to about 474V

346x2 = 692V/1.43 - 10 = 473.9V

Is that with the 6L6GCs in circuit and 349VDC at pin 8 of the rectifier?


This is from a 4AR4 datasheet.

View attachment 7088985

If I am interpreting this right, line 3 of the trace should show the approximate output voltage (X axis) VS the current draw (Y axis). Line 3 = 350V per plate and you measured 346V and 347V.

You indicated the output tubes are pulling 97.4mA. If you follow line 3 over to 97.4mA, it looks like the rectifier output should be about 440V, which matches the schematic. This does not account for the preamp and screen grid current draw. You can calculate the screen and preamp current by measuring the resistance and voltage drop across the resistor in the Pi filter.

You indicated the output of the rectifier is 349V. If the rectifier is good and each plate is getting ~350V, then at 250mA current draw the output of the rectifier should still be about 390V.

So AFAIK either the total current draw is over 250mA or the rectifier is bad.

Yes the reading of 349VDC at pin 8 of rectifier is with all the tubes in.
The rectifier is a 5AR4 not 4AR4.
I did test the amp with an other functional rectifier.