Heater voltage too low

Status
Not open for further replies.
Jump to Latest
V

vinceg

Member
Joined 2019
#1
Hi, I have a 5F6A clone whit a heater voltage too low. PT is a Hammond 290DEX wich has a primary winding 240VAC and heater 6.7VAC nominal voltage unloaded.

In my house wall voltage is a range of 205-220 VAC and I measured 6.2 VAC heater voltage at PT wires. When I connect heater wires to tube sockets I measured 5 VAC heater voltage. I think this volage drop is too much considering that all tubes absorb about 2.7A and PT secondary heater has 4A
 
#10
Galu said:
Running at a reduced primary voltage reduces the power in the secondary. This not only reduces the secondary voltage, but also the secondary current.

Whether the reduced power you are getting in the heater secondary is consistent with a drop in primary voltage of 20V remains the question!

https://www.hammfg.com/files/parts/pdf/290DEX.pdf
Click to expand...


Ok, this is. I calculated same percentage value between reduced primary voltage and unloaded secondary voltage.
But how much should the heater voltage drop be? (unloaded vs loaded)
 
#13
vinceg said:
Sorry, I'm afraid I did not understand.
Click to expand...
You only have about 5V to the heaters, the 5V6GT is a 6V6 but operates on 5V. Does not help for the preamp tubes but putting the 5V and the 6V together and changing it to dc will give you the voltage for the preamp tubes. In the end you get to use NOS tubes at a fraction of the price of ones that the amp would use normally. But you can not just go to the music store to pick them up.

Speaking of the 5V winding and boosting the voltage using it, this might be your easiest solution although not with the 5V winding. Using a 220V to 15-20V transformer and putting the 15-20V winding in series with the Hammond primary winding to give the primary the proper voltage.
 
#14
When running a 240V transformer off 220V you can expect a drop in secondary voltage of around 8%.

This is consistent with your drop in no load heater voltage from 6.75V to 6.2V.

The drop to 5.0V under load remains unexplained.
 
#15
Can you help me with this annoying hum YouTube

Printer2 said:
Speaking of the 5V winding and boosting the voltage using it, this might be your easiest solution although not with the 5V winding. Using a 220V to 15-20V transformer and putting the 15-20V winding in series with the Hammond primary winding to give the primary the proper voltage.
Click to expand...
Do you mean use two 5V6GT in place of my 6L6 and power its heaters with 5V. After put 5V and 6V together in series and get 11V, convert it to DC and power preamp heaters?

Galu said:
When running a 240V transformer off 220V you can expect a drop in secondary voltage of around 8%.

This is consistent with your drop in no load heater voltage from 6.75V to 6.2V.

The drop to 5.0V under load remains unexplained.
Click to expand...
Exact! Obviously, since my wall voltage is unstable, when transformer run at 205V a drop is around 14% and no load heater voltage is 5.8V

Since all tubes plus pilot lamp absorb around 2.8A, how much should be drop voltage at heaters? With primary drop of 8% I get around 5.9V loaded heaters, with primary drop of 14% I get around 5V loaded heaters. Is it a reasonable drop voltage?
 
#16
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
 

Attachments

  • 20190113_093438.jpg
    20190113_093438.jpg
    207.3 KB · Views: 130
  • 20190113_093450.jpg
    20190113_093450.jpg
    579.8 KB · Views: 114
#19
vinceg said:
Can you help me with this annoying hum YouTube

Do you mean use two 5V6GT in place of my 6L6 and power its heaters with 5V. After put 5V and 6V together in series and get 11V, convert it to DC and power preamp heaters?

Exact! Obviously, since my wall voltage is unstable, when transformer run at 205V a drop is around 14% and no load heater voltage is 5.8V
Click to expand...

I guess your house supply voltage drops low between 6-10 pm. The voltage can go high as 250 volts or more at midnight. This is common when the distribution transformer is far away or the utility department want to make use of full capacity of supply transformer. I suggest not to do any modification to your amp. Regards.
 
#20
vinceg said:
Ok, this is. I calculated same percentage value between reduced primary voltage and unloaded secondary voltage.
But how much should the heater voltage drop be? (unloaded vs loaded)
Click to expand...

The heater voltage should be right if the mains is right and the transformer is rated correctly.
In your case the mains is upto 10% down so that means the heater voltage will be 5.5VAC.
Temp solution would be to use auto transformer to lift voltage back up.
Or use DC heater volts and regulate it.
Or buy separate transformer with 6v3 plus +10% with current rated well within what heaters will take.
 
Status
Not open for further replies.
Top Bottom