LANEY VC30 annoying noise

[GainFreak]

Hi,

I'm retubing a Laney VC30 and noticed that it has that annoying hiss which actually is more like shhhhhh.... It's there no matter which channel and even when all pots are at zero.
After I replaced the power tubes I tried to trace the noise but it looks like the only thing that eliminated it completely is when I shorted PI's input to ground. Is this amp just noisy or I'm missing something?

[GainFreak]

The noise issue was fixed by changing the first tube and even more by replacing the PI tube. It's second triode stage is actually constantly into the sound path and it looks like that noise was generated by it. When drive channel is on and volume pot is at zero no noise leaks into the power amp and it's quiet now except for very tiny amount of hum.
Now couple of more strange things with this amp:
1/ The PT is rated for 230V operation but even with less voltage the heaters are at around 7,2V which is above tubes' maximum ratings. The schematic shows 7V heaters secondary which is also above ratings.
2/ Looking at the schematic the relay switching between clean and drive should be at 18V provided the actual one (Omron G5V-2-H) is rated at 12V. According to its data sheet maximum operating voltage is 120% at 70 deg C. Currently I'm measuring 20V there! Maybe adding a 7812 will fix that.
3/ Also that 2A fuse at the heaters winding CT seems like pointless to me considering that tubes heaters draw at least double that and more. If it blows the heater will be still on but only floating.

There still some residual hum which comes from the very first triode stage and since voltages measure ok I have some doubts that that hum might be coming from the not very well filtered relay supply node.
I don't feel comfortable running an amp with 7,2V on the heaters so I'm thinking of dropping it using resistors.
Also noticed that power tubes' screen resistors are tied to pin number 1 which at least one brand tube is having problem with.
Any comments will be appreciated.


Laney VC30 service manual.pdf

[g1]

So what is your line voltage coming out of the wall?
Your schematic is not working, you need to zip it and attach as a zip file.
The schematic I have shows it as having a 240V option on the 230V model, have you tried that?

[Jazz P Bass]

The schematic shows 3.6 Vac on each heater winding leg.
That's 7.2 Vac.

I would not be concerned about it.

These amps have been running since 1995 like this.

[g1]

Ok, so this is the older version with the fused CT for the heaters. That fuse I would guess is protecting the heater winding from heater to cathode or heater to plate shorts.
It is the newer version that has the 240V tap, and also the newer version has gone to 6.3V for heaters.
Unfortunately, in my experience and with some limited experimentation, increased heater voltage increases noise.

What brand of tubes are having issues with pin1? It is listed as I/C (internal connection) so they really should not be using it as a tie point.

[Jazz P Bass]

Here is the slightly newer schematic with the 'optional' regulated 5Vdc supply for V1.

[GainFreak]

So what is your line voltage coming out of the wall?

226-227V.

It is the newer version that has the 240V tap...

This one has only 230V winding.

These amps have been running since 1995 like this.

Maybe they are but it's still a design flaw.

Anyway I added 2x0.1 Ohm resistors in series with that winding and now the heaters are at ~6,4V.
As I suspected the relay was inserting some hum. Adding a 2200uF cap in parallel with the existing cap audibly reduced that hum.
I also added a 7812 before the relay. There's still a very tiny amount of hum coming from the first stage but I don't feel like investigating any further.

[GainFreak]

Here is the slightly newer schematic with the 'optional' regulated 5Vdc supply for V1.

Again heaters - 7,2V, first tube 5V? WTF??

[g1]

Not that it matters much, but on that later schematic that Jazz posted, there is some note on the schematic about "increased heater V" on the 6073 PT (which is the 230V version). I'll guess they were having drop out issues where the line voltage was less than 220, and jacked up the heater voltage to compensate.
After 2008, the 6073 PT had a 230 or 240V tap, and heaters were spec'd at 6.3V

[GainFreak]

I'll guess they were having drop out issues where the line voltage was less than 220, and jacked up the heater voltage to compensate.

If with 226V in the heater winding is at 7,2V even 200V will produce more than 6V.

[g1]

The voltage drop as the primary voltage decreases will not always be so linear. You would have to simulate a brown out with a variac, and the original transformer version before they increased the heater voltage. All I can guess is that the original version had a problem, but this is pure speculation.
But I agree, trying to fix such a problem by over-volting the heaters under normal conditions is very poor designing.