I have recently built a st70 clone. I am using a tomiko pt-st-70 111 pwr transformer and two 1650na hammond output transformers. I purchased a choke from triode and the cap board also. I am using the pc3 with rewired tube sockets to utilize phillips 6u8a drivers. The output tubes are electro harmonix and the rectifer is jj gz34. The amp sounds great. This is my first tube amp so I am very happy with the outcome. My question/problem is the pwr transformer seems to run hot. After about 3-4 hours of use at moderate to low volume the transformer can be touched but just barely. Is this normal of is there a problem. I am using psb t1 speakers and a parasound p3 pre
2 posters
tomiko transformer
bdl marine- Posts : 12
Join date : 2012-05-24
hawaii.ken- Posts : 157
Join date : 2012-01-31
What current do you set for your output tubes?
bdl marine- Posts : 12
Join date : 2012-05-24
fixed at 265ma
bdl marine- Posts : 12
Join date : 2012-05-24
120V input
Heavy duty output for:
365--60--0--365V (265mA)
0-5V (4A)
3.2--0--3.2V (3A)
3.2--0--3.2V (3A)
hawaii.ken- Posts : 157
Join date : 2012-01-31
You need to measure the current for each output tube. Most ST-70 clones have a 10-ohm cathode resistor connecting the output tube's cathode to ground so you can measure the voltage across the resistor to get the current being drawn by the tube.
I think 55-57ma for each tube is the recommended level (.55v-.57v accross the 10-ohm resistor).
It would be helpful if you would post a schematic for your amp.
I think 55-57ma for each tube is the recommended level (.55v-.57v accross the 10-ohm resistor).
It would be helpful if you would post a schematic for your amp.
bdl marine- Posts : 12
Join date : 2012-05-24
I will do that tonight. I see where you are going with this. Thanks and I will provide that info later
bdl marine- Posts : 12
Join date : 2012-05-24
There is only one resistor per channel. Pins 1 and 8 to pins 1 and 8 through resistor to ground. This is part of the bias circuit I believe.I have 1 volt per side across the resistor as per the bias set-up instructions.
hawaii.ken- Posts : 157
Join date : 2012-01-31
bdl marine wrote:There is only one resistor per channel. Pins 1 and 8 to pins 1 and 8 through resistor to ground. This is part of the bias circuit I believe.I have 1 volt per side across the resistor as per the bias set-up instructions.
What value resistor?
Again, a schematic for your amp would be helpful.
bdl marine- Posts : 12
Join date : 2012-05-24
10 ohm 1w
bdl marine- Posts : 12
Join date : 2012-05-24
schematic is the same as original but with a triode cap board
hawaii.ken- Posts : 157
Join date : 2012-01-31
A 10 ohm resistor would give you 50ma per tube which is in the ballpark.
You can try reducing the bias to .90volt (.45ma per tube) to see if it helps with the heat problem.
You can try reducing the bias to .90volt (.45ma per tube) to see if it helps with the heat problem.