5F6A low volume

[Chuck H]

Have you taken Helmholtz advice in posts 144 and 154 where he mentions cleaning and possibly tightening the tube sockets? If not then it's the very next thing you should do.

[Helmholtz]

I thought the V3 socket was replaced (#170)? Can you post a picture?

[vinceg]

Yes, I replaced all three preamp socket.

This morning I turn on amp and I can confirm that volume is low compared to before.

I measured voltage at pin 5 of power tubes and it is -44VDC. Is this too cold bias? Voltage across 1ohm resistors at pin 8 of power tubes is 22mV

Also, I measured heater voltage and it is very very low, only 5VAC. I measured voltage at wall and it is 210VAC, then 12.5% less than nominal primary winding of my Hammond PT, that should give 6,7VAC heater voltage unload.

Could one of these be the cause of low volume?

Anyway, the heater voltage is a problem in my opinion.

What to do?

[g1]

Can you put that other tube you had for V3 back in? Does the volume come back up?
Your line voltage regulation from the power company is horrible. You need to check it every time you are doing any work on the amp.
If your AC line is dropping as low as 210VAC, then you may have to do something about the heater voltage.
When I said 5.7V heaters was ok, I was thinking that it would not go lower.

[Chuck H]

I'm starting the think the inverse bucking transformer idea is a good one. The only rub would be that the amp may experience voltages that are too high whenever it's plugged in elsewhere.

[Chuck H]

Hey, wait a minute... My math alarm went off at some point. If 220V into a 240V primary is giving 5.7V then 210V should still be about 5.5V. Not 5V. Something else is going on.

[Chuck H]

vinceg,

Can we please see a photo of the how the transformers are mounted? Something looks odd in your interior gut shots and I need to see how the transformers are mounted to understand it.

[vinceg]

Can you put that other tube you had for V3 back in? Does the volume come back up?
Your line voltage regulation from the power company is horrible. You need to check it every time you are doing any work on the amp.
If your AC line is dropping as low as 210VAC, then you may have to do something about the heater voltage.
When I said 5.7V heaters was ok, I was thinking that it would not go lower.

No, other tube is bad, it broke and has air in. I can try another new tube.

Hey, wait a minute... My math alarm went off at some point. If 220V into a 240V primary is giving 5.7V then 210V should still be about 5.5V. Not 5V. Something else is going on.

The other day I had a 218V wall voltage and all out voltage of PT unload were about 8% less that nominal. Heater voltage were 6.2 unload. With tubes in it were about 5.7.
yesterday, wall voltage were 210V, then about 12.5% less and heater voltage were 5V with tubes in. Is normal a drop voltage with tubes in, right?

vinceg,

Can we please see a photo of the how the transformers are mounted? Something looks odd in your interior gut shots and I need to see how the transformers are mounted to understand it.

Ok, later I put pictures

[vinceg]

Now, wall voltage is 220V, heater voltage at PT unload is 6.2V and load 5.6V

I did some tests. I measured voltage at V3 pins and I discovered that there is a 28V at pin 8 cathode.

I changed V3 tube and now there is a 50 Hz background hum non affected by controls but volume is increased and cathode voltage is 35V.

Then, with one tube, hum but volume right, with other tube no hum but volume lower

Wall voltage is decreased to 210 and heater voltage with amp turn on and tubes in is 5V

[Chuck H]

I think that low volume tube must be bad (good call g1).

In my experience Hammond transformer heater voltages are a little high rather than low. Did you measure unloaded voltage with all tubes out or just power tubes out? Your loaded voltage with wall at 220V should probably be about 6V or a little more. That power transformer has a 4A (amps) filament winding and your amp should be drawing 2.85A from it (with the pilot lamp). I'm not ready to chalk up your low filament voltage entirely to the low mains voltage yet. I think something else may be contributing to the problem.

I wasn't clear about what I wanted in a photo I guess. I want to see how your transformers (both of them) are mounted on the under side of the chassis.

[vinceg]

I think that low volume tube must be bad (good call g1).

In my experience Hammond transformer heater voltages are a little high rather than low. Did you measure unloaded voltage with all tubes out or just power tubes out? Your loaded voltage with wall at 220V should probably be about 6V or a little more. That power transformer has a 4A (amps) filament winding and your amp should be drawing 2.85A from it (with the pilot lamp). I'm not ready to chalk up your low filament voltage entirely to the low mains voltage yet. I think something else may be contributing to the problem.

I wasn't clear about what I wanted in a photo I guess. I want to see how your transformers (both of them) are mounted on the under side of the chassis.

Yes, my PT has a 6.7V unloaded heater voltage with the correct 240V primary voltage.
This morning I measured unloaded heater voltage directly at PT wire by open fuse holder and this was 6.2V
This is a picture of trasformers

[vinceg]

I want to give you an update. I tried to remove V1 and hum is still there, I remove V2 and hum is stille there slightly quieter, I remove V3 and hum is disappeared. Then I swap V2 and V3 without changes, so I tried other new tubes on V3 and hum is disappeared but volume is decreased about 50% less. Then, I tried all my 12ax7 in V3 and no changes. I decided to reading voltages:


B+ 410VDC


V1
pin1 210-220
pin3 1.8
pin6 215-225
pin8 1.8


V2
pin1 188-190
pin3 1.1
pin6 328
pin8 188


V3
pin1 271
pin3 18
pin6 375
pin8 18


V4
pin4 420
pin5 408
pin6 -45
pin7 408


V5
pin4 408
pin5 408
pin6 -45
pin7 405


In the same moment:


wall voltage 213VAC
heaters unloaded 5.95
heaters loaded 4.6


I noticed that fuse holder that contain 4A fuse for heaters wires were hot, I do not know if it is normal

[Chuck H]

Would the V3 cathode voltage of 18V be taken with the tube that made the amp lower in volume?

Fuse wires shouldn't be hot. There must be resistance very near the area getting hot. There should not be resistance in the wire or the fuses to make them hot.

I don't see anything wrong with your transformer layout. I cant see all the transformer wires though. There are some "lead dress" (wire routing) issues in your build though. But we should fix the low voltage and hum problems first.

Take the preamp tube that has been causing low volume whenever it's used and throw it away. It's not good and using it confuses the tests.

[vinceg]

Yes, the V3 cathode voltage of 18V is with the tube that made the amp lower in volume and quiet but the same tube before works fine and amp had hum.
I do not know how to do

[Chuck H]

Yes, the V3 cathode voltage of 18V is with the tube that made the amp lower in volume and quiet but the same tube before works fine and amp had hum.
I do not know how to do

That tube is not good. It only makes the hum less because it makes the gain less. Don't use that tube anymore.

You should have about 5.6V (loaded) on your heaters at 213 wall volts. I think something might be wrong with your heater wiring or a tube filament. This could also be causing the hum.

Try removing the pilot lamp and re test the loaded heater voltage.