Marshall VS100R hum problem

[avoverdu]

OK one more test. Disconnect your DC supply and measure the DC resistance between pin 9 of the tube socket and ground.

Ok, I didn't get that. Last test I did with the other tube was using the transformer. Can you give me more detail?

[nickb]

Ok, I didn't get that. Last test I did with the other tube was using the transformer. Can you give me more detail?

Remove the tube. Locate pin 9 on the socket and put one probe in to make contact with pin. Put the other probe on a ground on the preamp board and measure the DC resistance. It should be very low.


I just thought of something else to check once it's all hooked up again. Measure the DCV on pin 3 and pin 8 to ground of thetube with it in place.

These cathode follower stages have a pretty high heater to cathode voltage applied. Often too much for some tubes. This design is bad as there is no standby switch IMHO so when cold the full plate voltage is applied to the grid and that exceeds the tube limits. Sometimee I fit an LED or neon between the grid and cathode of the follower stage. But we're straying ...

[avoverdu]

Remove the tube. Locate pin 9 on the socket and put one probe in to make contact with pin. Put the other probe on a ground on the preamp board and meauer the Dc resistance it shuldbe very low.


I just thought of something else to check once it's all hooked up again. Measure the DCV on pin 3 and pin 8 to ground of thetube with it in place.

These cathode follower stages have a pretty high heater to cathode voltage applied. Often too much for some tubes. This design is bad as there is no standby switch IMHO so when cold the full plate voltage is applied to the grid and that exceeds the tube limits. Sometimee I fit an LED or neon between the grid and cathode of the follower stage. But we're straying ...

Ok, tube is out, amp is off: Resistance between pin 9 and a nearby ground= 0.3 Ohm, from pin 9 to the main ground connection to chassis= 10.1 Ohm

Now I put the tube in place, turned on the amp and read: DCV from pin 3 to ground = 126v (it starts low but increases as the hum does)
DCV from pin 8 to ground = 0.912v (and i hear some radio interference)

Let me know

[nickb]

Ok, tube is out, amp is off: Resistance between pin 9 and a nearby ground= 0.3 Ohm, from pin 9 to the main ground connection to chassis= 10.1 Ohm

Now I put the tube in place, turned on the amp and read: DCV from pin 3 to ground = 126v (it starts low but increases as the hum does)
DCV from pin 8 to ground = 0.912v (and i hear some radio interference)

Let me know

I was thinking that perhaps the heater circuit is floating due to a bad pin 9 connection or the cathode voltage was way too high but everythng checks out as it should. There are thousaunds of these amps out there that work happily without resorting to a DC supply. Somehow, heater current is getting into the ground. We know that from an earlier test where we shorted out the signal path with the tube in place and still got the hum.

I'm stumped.

[g1]

Maybe some H-K leakage measurements on the board?
The comments about H-K leakage by nick correspond to problems with using the LPS type tubes in cathode followers, which were noted in the measurements with the LPS tube installed.

[g1]

Also, if JM's suggestion of converting to DC heater implies he thinks the noise level is normal for this model, I'm good with that too.

[nickb]

Also, if JM's suggestion of converting to DC heater implies he thinks the noise level is normal for this model, I'm good with that too.

250mV rms is a lot of hum. I don't recall these being especially bad in that respect. Am I mistaken?

For certain HK leakage is a component but how is it getting into the ground? Unless of course the test in post 19 is wrong. I wonder if there has been a misunderstanding? I took it that with R48 and R77 shorted and with the tube in the hum was there. On a re-read maybe that is not the what was meant. Perhaps the OP could clarify?

A cheap alternative to a DC supply would be to elevate the heater supply up to 100V or so with a divider* off the plate supply and isolating pin 9 from ground.

*PS
i.e. isolate pin 9 from ground by cutting track. Add 100K 0.5W from ground to pin, add 390K 1W from pin 9 to to pin 1, add 2.2uF 100V ( bigger uF and bigger volts would be OK). Check voltage across 100K after rework. Aim for 75V to 100V.

[J M Fahey]

Also, if JM's suggestion of converting to DC heater implies he thinks the noise level is normal for this model, I'm good with that too.

No, I donīt think that such high hum is normal, by any means.

But I also know there is a large batch of 12AX7 out there with higher than normal cathode to filament loss/coupling which lets hum jump where it should not.

Iīm quite certain Marshall did not meet that problem while designing and making this amplifier definitely not with the tubes they were using at the time, and one does not solve nonexistent problems.

But since this amp and tube combination hums a lot, it does not hurt at all to improve filament supply, specially because itīs easy and inexpensive.

What if you get a similar problem in the future?
Then and only IF so, repeat the solution.

[avoverdu]

I was thinking that perhaps the heater circuit is floating due to a bad pin 9 connection or the cathode voltage was way too high but everythng checks out as it should. There are thousaunds of these amps out there that work happily without resorting to a DC supply. Somehow, heater current is getting into the ground. We know that from an earlier test where we shorted out the signal path with the tube in place and still got the hum.

I'm stumped.

Well, heater current goes to ground through the tube filaments center tap. Is there any chance that any of the ICs are getting the hum in the signal?

[avoverdu]

250mV rms is a lot of hum. I don't recall these being especially bad in that respect. Am I mistaken?

For certain HK leakage is a component but how is it getting into the ground? Unless of course the test in post 19 is wrong. I wonder if there has been a misunderstanding? I took it that with R48 and R77 shorted and with the tube in the hum was there. On a re-read maybe that is not the what was meant. Perhaps the OP could clarify?

A cheap alternative to a DC supply would be to elevate the heater supply up to 100V or so with a divider* off the plate supply and isolating pin 9 from ground.

*PS
i.e. isolate pin 9 from ground by cutting track. Add 100K 0.5W from ground to pin, add 390K 1W from pin 9 to to pin 1, add 2.2uF 100V ( bigger uF and bigger volts would be OK). Check voltage across 100K after rework. Aim for 75V to 100V.

I lost track. What you suggest I can do it but it seems complicated; do you want me to run some tests again??? If we want to be sure just tell me what to do and I will

[avoverdu]

No, I donīt think that such high hum is normal, by any means.

But I also know there is a large batch of 12AX7 out there with higher than normal cathode to filament loss/coupling which lets hum jump where it should not.

Iīm quite certain Marshall did not meet that problem while designing and making this amplifier definitely not with the tubes they were using at bthe time, and one does not solve nonexistent problems.

But since this amp and tube combination hums a lot, it does not hurt at all to improve filament supply, specially because itīs easy and inexpensive.

Whay if you get a similar problem in the future?
Then and only IF so, repeat the solution.

What components should I buy to convert the filament supply (around 14vAC) to an equivalent DC heater supply?

[nickb]

Well, heater current goes to ground through the tube filaments center tap. Is there any chance that any of the ICs are getting the hum in the signal?

No heater current flows in the center tap. It starts at the transformer, through the filaments and back to the transformer. The center tap serves only to define the average potential of the heater. For current to flow in the center tap there would have to be another connection between the transformer heater winding and ground.

Please read post 52 and post 19. Is it true that when R48 and R77 were shorted with the tube in place there was still hum?

[The Dude]

I'm not sure it would be enough, but have you tried a spiral filament tube?

[g1]

That was the LPS tube he got the much worse reading with. (post #44)
In my experience they have issues with heater to cathode voltage which is why I thought it gave the worse reading.

I'm not keen on the 14V heaters, be they AC or DC.

[The Dude]

Thanks g1. I missed that. Do you know why they have issues with H to K voltages? I've not encountered that problem.