Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Guy77 on February 19, 2022, 03:02:10 pm
-
Hi everyone, its been a while since I have been here and have forgotten what a huge wealth of knowledge there is on this forum thanks to the many people that lend there experiences and knowledge.
I will be working with a Hammond 290HX power transformer on a 100 watt build in the next few weeks. I have used many different models from Hammond over the years but not this one. Right away I noticed that the Bias wire is actually 2 wires, a green and a white.
https://www.a1parts.ca/transformers/290/290HX.pdf
I assume I am sending the Green wire (COM) to ground and connecting the White to my Bias circuit ( Bassman style bias circuit)?
I will also be using a Full Wave Bridge Rectifier with a Capacitor input as shown in the drawing below.
https://www.hammfg.com/electronics/transformers/rectifier
The High Voltage secondary winding has a center tap and so I believe if I send that to ground I should not place the Neg side of the diodes to ground either?
Thanks again
Cheers
Guy
-
I assume I am sending the Green wire (COM) to ground and connecting the White to my Bias circuit ( Bassman style bias circuit)?
That's right.
I will also be using a Full Wave Bridge Rectifier with a Capacitor input as shown in the drawing below.
The High Voltage secondary winding has a center tap and so I believe if I send that to ground I should not place the Neg side of the diodes to ground either?
NO!!! Just tape off the center tap. ***OR***, if you will be stacking two caps in series for the reservoir cap, connect the center tap to the junction of those two caps.
Show us the actual schematic you are using for more detailed responses.
-
Why would you use a full wave bridge rectifier if the high voltage winding has a center tap? Unless you're looking for the full voltage across the entire winding...
-
Why would you use a full wave bridge rectifier if the high voltage winding has a center tap? Unless you're looking for the full voltage across the entire winding...
Because it’s a 350V (rather than 700V ish) winding.
The CT can be used to save on balancing resistors for a series connected reservoir cap pair https://el34world.com/charts/Schematics/files/Marshall/Marshall_jmp_superlead_100w_1959.pdf
-
I assume I am sending the Green wire (COM) to ground and connecting the White to my Bias circuit ( Bassman style bias circuit)?
That's right.
I will also be using a Full Wave Bridge Rectifier with a Capacitor input as shown in the drawing below.
The High Voltage secondary winding has a center tap and so I believe if I send that to ground I should not place the Neg side of the diodes to ground either?
NO!!! Just tape off the center tap. ***OR***, if you will be stacking two caps in series for the reservoir cap, connect the center tap to the junction of those two caps.
Show us the actual schematic you are using for more detailed responses.
Hi Sluckey, thanks for the info.
I attached a hand drawn schematic showing how I was going to wire the 290HX.
I am using 2 caps in series like you mentioned.
So your saying I should not ground the CT on the 290HX but just ground the Negative side of the Bridge rectifier?
Thanks
-
I attached a hand drawn schematic showing how I was going to wire the 290HX.
That drawing will only give you 175 X 1.414 = 247VDC. Not what you want.
So your saying I should not ground the CT on the 290HX but just ground the Negative side of the Bridge rectifier?
Yes. And connect the center tap to the junction of the two series connected reservoir caps. Look at the super lead schematic link that pdf64 posted.
-
I attached a hand drawn schematic showing how I was going to wire the 290HX.
That drawing will only give you 175 X 1.414 = 247VDC. Not what you want.
So your saying I should not ground the CT on the 290HX but just ground the Negative side of the Bridge rectifier?
Yes. And connect the center tap to the junction of the two series connected reservoir caps. Look at the super lead schematic link that pdf64 posted.
Thanks Sluckey and pdf64. I have attached an updated drawing with the changes mentioned .
The only item I need to solve is what changes do I need to make to get my B+ higher than 247 VDC with this transformer? The specs say this transformer has a 350VAC secondary winding and has 420ma.
I also attached the specs for a similar transformer that Classic Tone used to make which also had a 350VAC winding and their rectifier setup is similar to mine and i believe its getting in the 400v dc range with their rectifier setup. So I believe I am good?
Thanks
Guy
-
Interesting, I've never seen a power supply set up like that before. It kind of reminds me of a voltage doubler except that it doesn't double the voltage.
-
Thanks Sluckey and pdf64. I have attached an updated drawing with the changes mentioned .
The only item I need to solve is what changes do I need to make to get my B+ higher than 247 VDC with this transformer? The specs say this transformer has a 350VAC secondary winding and has 420ma.
You don't need to do anything else. Your last drawing will give you 350 x 1.414 = 495VDC (unloaded)
-
Interesting, I've never seen a power supply set up like that before. It kind of reminds me of a voltage doubler except that it doesn't double the voltage.
Look at the schematic pdf64 linked above. There are plenty more circuits like this too.
For any transformer secondary that has a center tap, a full wave bridge (FWB) will produce twice the B+ that a conventional two diode rectifier will. The FWB always uses the entire winding whereas the two diode conventional rectifier only uses half the winding at a time. For one half cycle the top half of the winding is used, then the next half cycle the lower half of the winding is used. Half of the winding is always doing nothing.
-
If you're using a bridge rectifier across the entire high voltage winding, is it even necessary to use the center tap if the winding has one? What I've seen in the past is that the full wave bridge is used with a high voltage winding that does not have a center tap.
-
The CT is not essential to the 1959.
The 1/2 voltage output it creates can be used for a 1/4 power option https://el34world.com/charts/Schematics/files/Fender/Fender_75.pdf
-
One thing is not too clear in the 1959 schematics PDF linked to.
Is the CT connected to the two .22uF caps before the bridge? I can't see a connection dot, nor is there a hump in the wire indicating no connection.
Also, those caps are not drawn as being polarized.
So, does a PT wired to a FWB rectifier run hotter since both windings are always working? I guess it saves money by using a 350V PT with a FWB instead of a 700V PT with a FW rectifier and a grounded CT?
Thanks.
-
One thing is not too clear in the 1959 schematics PDF linked to.
Is the CT connected to the two .22uF caps before the bridge? I can't see a connection dot, nor is there a hump in the wire indicating no connection. Also, those caps are not drawn as being polarized.
Not clear to me either but I suspect those caps are connected to the CT. No need to be polarized because they are connected across AC voltage. Besides, have you ever seen a .22µF polarized cap?
So, does a PT wired to a FWB rectifier run hotter since both windings are always working? I guess it saves money by using a 350V PT with a FWB instead of a 700V PT with a FW rectifier and a grounded CT?
The load connected to the B+ determines how hot a PT will run. I've never considered the type of rectifier being used as a factor, but I see your point. Just guessing that the temp would be about the same.
-
It’s difficult to make a fair comparison, but all else being equal, a FWB rectifier seems to draws a higher AC from its PT winding than does a 2 phase rectifier.
Difficult because the 2 phase winding carries the DC as well as AC current.
-
Thanks Sluckey and pdf64. I have attached an updated drawing with the changes mentioned .
The only item I need to solve is what changes do I need to make to get my B+ higher than 247 VDC with this transformer? The specs say this transformer has a 350VAC secondary winding and has 420ma.
You don't need to do anything else. Your last drawing will give you 350 x 1.414 = 495VDC (unloaded)
Thanks Sluckey and PDF64 for the drawings!
Have a great weekend guys.
-
One thing is not too clear in the 1959 schematics PDF linked to.
Is the CT connected to the two .22uF caps before the bridge? I can't see a connection dot, nor is there a hump in the wire indicating no connection.
Also, those caps are not drawn as being polarized.
So, does a PT wired to a FWB rectifier run hotter since both windings are always working? I guess it saves money by using a 350V PT with a FWB instead of a 700V PT with a FW rectifier and a grounded CT?
Thanks.
Hi, yes those .22uf caps do appear to connect to the center tap as Sluckey mentioned. This drawing here illustrates it as well and yes they are non polarized caps.
Its a little involved but you can see the red wire coming from the top of the large black .22uf caps and then going to one of the large cylinder caps and then to a 2nd cylinder cap and then that cap connects to the CT on the Power transformers Secondary winding.
Cheers
Guy
-
...No need to be polarized because they are connected across AC voltage. Besides, have you ever seen a .22µF polarized cap?
Thanks for the clarification! And, no, come to think of it I've never seen an axial type 0.22uF electrolytic with a 450-500V rating.
-
Thanks Guy77. And, the 1992 schematic has the connection dot. So, it was just left off of the 1959 drawing.
-
Hi everyone. I just wanted to follow up on this.
I completed the 100 watt Overdrive Special I built with the Hammond 290CX transformer and it went very well!
I completed a 12 hour burn in test as well.
I attached the schematic I used to build the rectifier and standby switches. I used no .2uf capacitors much the same way that Mojotone does in there 100 watt British kit. There was also no unwanted noise when flipping the power or standby switches.
This transformer actually runs cooler than the Hammond 290FX I have used on similar builds when its cranked.
With a 120v measured wall voltage I got 462v on the plates of each of my 4 JJ6L6GC tubes. I had the tubes running at 36 MA each, about 55% each.
Cheers
Guy