FIXED! 1959 Bassman 5f6a Low Output Issue. Stays Clean. No distortion

[Pinball]

Putting back together a vintage 1959 Bassman. Have no idea how long it's been down. Replaced all electrolytic caps, Checked all tubes, sprayed and cleaned all sockets, pots. Changed power cord to 3 prong.

Powers up and is pulling right under 1 amp.

Regardless of which channel I use, low output and stays clean.

As I begin to turn the volume up it makes a slight mid range "thump" around 2-3. and output doesn't really increase much at all up to 10.

All the tone knobs react as they should.

New speakers and checked all of that.

All my voltages measure close to what is stated in the schematic.

Any ideas?

[stratomaster]

Put a small signal into the input and trace it's AC voltage at the grids and plates of the signal path.  Ideally use a scope for this, but just Vac function on your meter will be sufficient.  This can help identity when the signal drops off.

Also be mindful of the cathode follower. Some modern production tubes have a hard time in this spot. Elevating the heater CT and adding Merlin's diode network can help make life easier on that triode. 

[uki]

Pictures of the circuit may help trouble shooting,

Recheck all connections.

[Pinball]

Schematic added. Looks like the signal drops a lot around the tone circuit. I’ve got 1 ohm coming from the OT feedback.

I do have some differences at the plate of the long tail Phase. One is 290VDC and the other is 201VDC. Grids for the Phase inverter are around 15VDC.

[Latole]

Did you put a new and know good tubes ? Tubes testers may not be reliable.

Read values on each pot . A too low value will send too munch signal to ground.

[pdf64]

Schematic added. Looks like the signal drops a lot around the tone circuit. I’ve got 1 ohm coming from the OT feedback.

The OT secondary is a fairly short length of fairly thick wire, so its resistance should be really low.

I do have some differences at the plate of the long tail Phase. One is 290VDC and the other is 201VDC. Grids for the Phase inverter are around 15VDC.

Are the anode load resistors 82k 100k measured resistance values reasonably close to their nominal values?
If yes, maybe try a different 12AX7 in V3.

[Pinball]

Schematic added. Looks like the signal drops a lot around the tone circuit. I’ve got 1 ohm coming from the OT feedback.

The OT secondary is a fairly short length of fairly thick wire, so its resistance should be really low.

I do have some differences at the plate of the long tail Phase. One is 290VDC and the other is 201VDC. Grids for the Phase inverter are around 15VDC.

Are the anode load resistors 82k 100k measured resistance values reasonably close to their nominal values?
If yes, maybe try a different 12AX7 in V3.

Load resistors measure very close to nominal values.


Also I tried known good 12ax7 in all three spots. No difference.

[Pinball]

So I put in a .05vpp AC sine wave in the input and turned the vol to max. Below are the VPP I measurements.

Plate signal of 1st preamp is 16.8 vpp
Plate of 2nd stage is 54vpp
Cathode follower is 56vpp
Input of phase inverter at 22.4vpp
Output of phase inverter is 236vpp (both sides)
Output of power amp a lot higher

I never see a clipped signal. Everything I can see is a near perfect sine wave.


Shouldn’t I see some clipping in the preamp? What am I missing?

As I said before I get a strange thump when starting to turn up the volume. It increases volume until around 5 and then decreases a little. It is the same regardless which channel I try. That makes me think it’s something past the first stage.


I think it’s either in the 2nd stage or the phase inverter. I am concerned with such a large signal drop after the tone stack. Is this normal for this tone stack?

[SEL49]

.05Vpp is probably not enough to overdrive anything. Turn your input signal up to .2Vpp.

[Latole]

150 mv minimum

[tubeswell]

I do have some differences at the plate of the long tail Phase. One is 290VDC and the other is 201VDC.

What control grid voltage on the 6L6s?

[Pinball]

.05Vpp is probably not enough to overdrive anything. Turn your input signal up to .2Vpp.

I turned it up to .2. Still with the volume wide open I’m not getting much clipping. Maybe the wave form gets more pointy. No compression.

[shooter]

after your answer to reply #10;
post a screenshot of your scope across the speaker wires
swinging 239VAC at 201VDC has to clip unless there's something off in your setup

[Pinball]

after your answer to reply #10;
post a screenshot of your scope across the speaker wires
swinging 239VAC at 201VDC has to clip unless there's something off in your setup

Ok you are right.

So with a .2VAC sine wave in and the amp vol max, I get 189vdc and 808VACpp, 245VDC and 748VACpp.

It is clipping at that point.

[Pinball]

I do have some differences at the plate of the long tail Phase. One is 290VDC and the other is 201VDC.

What control grid voltage on the 6L6s?

-58vdc and -39.22vdc That is measured at max vol and .2vac sine wav input.

[Pinball]

https://www.dropbox.com/scl/fi/cfnk5uvhf0qopr4rkls6m/IMG_3375.MOV?rlkey=1wv77uiicdvr8fn2uz377vgwh&st=14dxv2ff&dl=0


This is the thump I spoke of.

[glass54]

Hi Pinball

-58vdc and -39.22vdc That is measured at max vol and .2vac sine wave input.

That's not nice  (in fact really off!!) The values should be much closer. Recheck with O/P tubes out (please and let us know.
Do you have a 1R Cathode resistor (on O/P tubes) that you could check bias current.
By the way, I prefer to measure Quiescent (static DC conditions) with NO input signal.
Referring to your thump. I don't believe that is associated with Phase Inverter!
In both cases, it appears you may have some DC leakage problems?? Or possible VHF oscillation in Preamp as well as the DC issues in O/P stage?
Regards
Mirek

[Pinball]

Hi Pinball

-58vdc and -39.22vdc That is measured at max vol and .2vac sine wave input.

That's not nice  (in fact really off!!) The values should be much closer. Recheck with O/P tubes out (please and let us know.
Do you have a 1R Cathode resistor (on O/P tubes) that you could check bias current.
By the way, I prefer to measure Quiescent (static DC conditions) with NO input signal.
Referring to your thump. I don't believe that is associated with Phase Inverter!
In both cases, it appears you may have some DC leakage problems?? Or possible VHF oscillation in Preamp as well as the DC issues in O/P stage?
Regards
Mirek

Ok these measurements come with no input voltage/signal and amp at idle with volume at 0.

V4 6L6  385 plate voltage, 34mA current, -38VDC at grid, Without tube, -35.6VDC
V5 6L6  383 plate voltage, 47mA current, -37VDC at grid, Without tube, -34VDC

Also I have -49.2VDC coming from SEL rectifier.

Power off measured the following Resistors

The two 1M between grid and cathode of v3 12ax7 measured 1M and .93M
The 82k and 100k at plate of v3 12ax7 measure 99k and 130k
470 at cathode measured 520
10k in series with 470 measured 6.74k

The two 220k at 6L6 grids measure 550k and 670k

[Pinball]

Also as amp warms up at power on,I get a sizzle sound from speakers for about 3 seconds and then it goes away. It is repeatable.

All electrolytic caps have been replaced by me.

One 470 1W on V5 was replaced. It was found cracked wide open/damaged prior to start of circuit troubleshooting. Both 470 1W measure very close to spec now.

Also found the amp with a 20A fuse. Replaced it with a correct 3A fuse.

4 new 8 ohm speakers installed. Originals were shot and falling apart.


All of these findings were done prior to my first power up. This amp was in pieces and unknown history.

[Latole]

Ok these measurements come with no input voltage/signal and amp at idle with volume at 0.

V4 6L6  385 plate voltage, 34A current, -38VDC at grid, Without tube, -35.6VDC
V5 6L6  383 plate voltage, 47A current, -37VDC at grid, Without tube, -34VDC

34 A and 47 A   !!!!
You mean milli-amp (mA )  not ampere ( A ) . 

[Pinball]

Ok these measurements come with no input voltage/signal and amp at idle with volume at 0.

V4 6L6  385 plate voltage, 34A current, -38VDC at grid, Without tube, -35.6VDC
V5 6L6  383 plate voltage, 47A current, -37VDC at grid, Without tube, -34VDC

34 A and 47 A   !!!!
You mean milli-amp (mA )  not ampere ( A ) . 

fixed. Late night....

[stratomaster]

Power off measured the following Resistors

The two 1M between grid and cathode of v3 12ax7 measured 1M and .93M
The 82k and 100k at plate of v3 12ax7 measure 99k and 130k
470 at cathode measured 520
10k in series with 470 measured 6.74k

The two 220k at 6L6 grids measure 550k and 670k

At minimum replace the PI plates and the 220k bias leaks then reevaluate.  These 4 out of spec resistors likely account for the mismatch you're seeing with signal applied.

[glass54]

+1 strato
By the way Pinball, if a component is questionable regarding value, it's a good idea to change/upgrade to known values. See extract below from Leo's layout for this amp.
I believe you will now get places ie conquer the problems 
Regards
Mirek

[Pinball]

Those values were close once I pulled them out but went a head and swapped with closer ones. No difference in the amp after replacing.

[Willabe]

Those values were close once I pulled them out but went a head and swapped with closer ones. No difference in the amp after replacing.

This is not clear. You need to be more detailed.

"Those values were close once I pulled them out." What values? Close to what?

What did you pull out? "Swapped with closer ones." What does that mean?

"No difference in the amp after replacing." Difference in what?

We can't help you if your not clear in what you post. We can not read your mind.

[pdf64]

...
All my voltages measure close to what is stated in the schematic.

Any ideas?

The results of a full resistor and voltage survey may be beneficial.

[Pinball]

Those values were close once I pulled them out but went a head and swapped with closer ones. No difference in the amp after replacing.

This is not clear. You need to be more detailed.

"Those values were close once I pulled them out." What values? Close to what?

What did you pull out? "Swapped with closer ones." What does that mean?

"No difference in the amp after replacing." Difference in what?

We can't help you if your not clear in what you post. We can not read your mind.

Sorry I meant to quote “Reply #21”  Basically I stated the resistors at plate of PI and the resistors at 6L6’s grid were off. It was requested that I swap them out. Once I pulled them out and remeasured out of circuit, they were closer to original value. I went ahead and replaced them with new resistors. Once the 4 resistors were replaced, the issues with the amp did not change. Still low output and the “thump” sound when turning the volume up between 2&3. With or without an input signal.

If you look at my previous replies you will see all of the information.

Another thing to note, the current draw on the amp increases during the “thump” but goes back down after around 4 on the volume. You can see all the speaker cones move with the thump sound.

[stratomaster]

It would be interesting to have you go back and measure the in circuit resistances of the replaced resistors and the grid voltages of the power tubes at idle and the cathode current under load. That will tell you for sure if behavior improved with the replacements and you just have another issue on top of this that is dominating.

[pdf64]

... the resistors at plate of PI and the resistors at 6L6’s grid were off. It was requested that I swap them out. Once I pulled them out and remeasured out of circuit, they were closer to original value. I went ahead and replaced them with new resistors. Once the 4 resistors were replaced, the issues with the amp did not change.

Yes, as mentioned, what do the replacement resistors measure in circuit? There should be no legitimate parallel paths for those resistors.

Still low output and the “thump” sound when turning the volume up between 2&3. With or without an input signal.

If you look at my previous replies you will see all of the information.

Another thing to note, the current draw on the amp increases during the “thump” but goes back down after around 4 on the volume. You can see all the speaker cones move with the thump sound.

How are you assessing the increased current draw?

It seems to me that the key task here is to identify which stages of the amp are drawing more current, and why.

Ultrasonic oscillation, conductive board, bad coupling / decoupling caps are some possible root causes.
Voltage surveys at very low, medium, and high volume settings may be helpful.

[Pinball]

... the resistors at plate of PI and the resistors at 6L6’s grid were off. It was requested that I swap them out. Once I pulled them out and remeasured out of circuit, they were closer to original value. I went ahead and replaced them with new resistors. Once the 4 resistors were replaced, the issues with the amp did not change.

Yes, as mentioned, what do the replacement resistors measure in circuit? There should be no legitimate parallel paths for those resistors.

The 6L6 grid resistors measure 233k and 229k
The PI plate resistors measure 110.8k and 89.6k

Still low output and the “thump” sound when turning the volume up between 2&3. With or without an input signal.

If you look at my previous replies you will see all of the information.

Another thing to note, the current draw on the amp increases during the “thump” but goes back down after around 4 on the volume. You can see all the speaker cones move with the thump sound.

How are you assessing the increased current draw?

It seems to me that the key task here is to identify which stages of the amp are drawing more current, and why.

Ultrasonic oscillation, conductive board, bad coupling / decoupling caps are some possible root causes.
Voltage surveys at very low, medium, and high volume settings may be helpful.

I measure it by the Variac with amp meter.

At idle, I’m getting 383V and 37mA , 383V and 49mA on the 6L6’s. -47.6V between the two 220k resistors at the grid of 6L6’s.

Plate voltages of 1st stage are 143V and 148V at idle.
Plate voltage of 2nd stage are 158V at idle. 159.2 at cathode follower at idle.
Plate voltage PI are 293.5 and 287.6 at idle.

[Willabe]

How are you assessing the increased current draw?

I measure it by the Variac with amp meter.

What do you mean? ^^^^^^^^^^^^^

This is not clear.

[Latole]

How are you assessing the increased current draw?

I measure it by the Variac with amp meter.

What do you mean? ^^^^^^^^^^^^^

This is not clear.

Right , it is not clear.

Variac at 120 volts output or ...... ? Why variac ?
Amps meter between 6l6's plate and center tap of OT ?

[Pinball]

How are you assessing the increased current draw?

I measure it by the Variac with amp meter.

What do you mean? ^^^^^^^^^^^^^

This is not clear.

My Variac has a meter that reads amperes. When I turn the volume and hear the “thump” The meter jumps up over 1A. Normal stays around .8 amperes.

[Pinball]

How are you assessing the increased current draw?

I measure it by the Variac with amp meter.

What do you mean? ^^^^^^^^^^^^^

This is not clear.

Right , it is not clear.

Variac at 120 volts output or ...... ? Why variac ?
Amps meter between 6l6's plate and center tap of OT ?

Why not a Variac? The Variac is at 117VAC. My Variac also has an isolated transformer for protection.

The amps as a whole. The total draw that the amp is pulling. It jumps as I hear the “thump”.

[Willabe]

Why not a Variac?

Because the reading your getting is for the whole amp, not just a single power tube.

The reading your getting doesn't mean anything that you need.

[Pinball]

Why not a Variac?

Because the reading your getting is for the whole amp, not just a single power tube.

The reading your getting doesn't mean anything that you need.

If it helps, each Power tube current jumps up when you get the “thump” sound. It happens so quick, i can’t catch a good reading.

I have a Eurotube Bias probe which will give me the plate voltage and current of each 6L6.

I guess I misunderstood why he asked about a Variac. I always use a Variac when troubleshooting or using vintage amps.

[pdf64]

...
I have a Eurotube Bias probe which will give me the plate voltage and current of each 6L6.
...

Those readings, in the normal idle mode, and in the thump mode, would be good to have.

The 6L6 anode voltages provided so far seem too low.
What HT supply V DC, at the rectifier pin8 / standby switch (amp idling) do you measure?
https://el34world.com/charts/Schematics/files/Fender/Fender_bassman_5f6a_schem.pdf

[Latole]

The 6L6 anode voltages provided so far seem too low.
What HT supply V DC, at the rectifier pin8 / standby switch (amp idling) do you measure?

 right . Schematic show 430 Volts

[Pinball]

Update:

So the amp had multiple issues. They all were caused by high DC voltage passing through the coupling capacitors. All those yellow Astrons that gets praise, were bad! The two on the 1st gain stage were each causing the “thump” sound on the volume knobs. The rest were also high on DC voltage. The two at the plates of the PI were causing the large differences on the plate voltage of PI. This was causing BiAS issues with the 6L6’s. I ended up replacing every Astron except the one tied to the Presensce knob. Once this was done, the amp is functioning properly. Very quiet.

Thanks for everybody’s help! I have learned that you have to pull one leg of those caps up to properly measure for DC voltage. I was shocked to see that much DC voltage passing through these. Some were over 160VDC!

[Willabe]

I'd get rid of the SEL rectifier that's in the -bias circuit.