I recently built a 50 watt amp with an EL34 power section using a Stancor PC8411 power transformer, very close relative to the model you've got. Judging by the photo you posted the differences between PC8411 and PC8412 are
8411
-375-0-375 HV @ 150 mA
-5V @ 3A
-6.3V @ 4.5A
8412
-400-0-400 HV @ 200 mA
-5V @ 3A
-6.3V @ 5A
My 8411 looks exactly like your photo, grey bell covers and yellow sticker w/ratings and color identification of leads.
I found and used an older Stancor 79B50 output transformer that is roughly the same size of the 8411 PT. The sheer mass of this OT probably has something to do with it, but the amp sounds phenomenal. It is insanely loud and holds together quite tightly. I run a 5R4 rectifier tube in it to keep the HV down a little. During planning/building I was worried that the extreme voltage drop of the 5R4 would result in a big 'sag' which would make the amp suitable for blues tones but not really good for snappy, hard rock rhythm playing. Wrong! It is super-tight sounding, though not sterile.
With a 5R4 rectifier you might be able to get away with using EL34s. I think with those higher ratings though you could build an amp with a pair of KT88 power tubes and have it work great. I'm a little jealous actually! Check out this site for an approximate calculator of current draw & voltages: http://www.dreamtone.org/Calculate_Current_Form.htm
I used this as a reference when building my amp and found that the numbers were a little conservative (meaning error in your favor). I'd also place a small bet on the possibility of being able to push the PT at the very least to it's max ratings, maybe a squeege more, without worry of stress or meltdown.
Stancor transformer
Moderators: pompeiisneaks, Colossal
Re: Stancor transformer
Here are some photos of said build
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Re: Stancor transformer
Nice, and good link.
You know, really, stop fighting the 450V battle. Build the SR with a pair of EL34, which can take the voltage. You'll end up with something of a Marshally Fender. I bet it will sound nice.
You know, really, stop fighting the 450V battle. Build the SR with a pair of EL34, which can take the voltage. You'll end up with something of a Marshally Fender. I bet it will sound nice.
Re: Stancor transformer
You guys are amazing. I'm sold. By the way, I obtained a pic of the transformer in its original amp. I believe this is the way it was connected.
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Re: Stancor transformer
You've masked a few things I want to see. I am not sure where the alignment notch is on the octal sockets and I'm not sure what I see makes total sense.
Here's what I'm getting from that picture.
You've captured a pair of 100 ohm resistors that are tied to pin 7 on the rectifier socket; this pin not used by the rectifier tube. It is a common practice to capture unused pins as tie points -- it's convenient. I believe these are connected to pin 4 of the power tube sockets, likely the screen supply (not sure of the power tube in use; can't see the 6.3V filament supply which would help verify.) I expect the black wire on pin 7 of the rectifier goes to the multi-section can cap nearby as part of the screen supply.
You've labeled the can cap as a solid state rectifier.
The yellow wires on pins 2 and 8 of the rectifier are the 5V supply. I'm not aware of any 6.3V rectifier that would be appropriate with that PT. The 6.3V supply is most likely to be green and I'll vote for the green twisted pair that's next to the rectifier socket and not connected to it.
On the power tubes, I'm thinking pins 2 and 7 are the 6.3V heaters (filaments). There is a pair of 56 ohm resistors that form the faux center tap for the heaters and connected to pin 8, the power tube cathode. This elevates the heater supply as a hum reduction measure.
Unfortunately, there isn't much of a lesson here as to the power supply ladder for the donor amp. I don't think that's important. What we know is that you'll need to select a power tube that can operate with plate voltage above 500 or go for choke input (LC not CLC like that pic you posted) which will drop considerable voltage, putting you out of reasonable range for the SR circuit you seem interested in building.
I think you've either got to pick a circuit and get the right transformer, or pick a circuit that works for the PT you've got. I can tell you, if it was me, I like doing the latter. I don't think there's anything wrong with letting the things you have dictate the build. The fun is that you will get a surprise at the end. There is every reason to expect a decent result.
Here's what I'm getting from that picture.
You've captured a pair of 100 ohm resistors that are tied to pin 7 on the rectifier socket; this pin not used by the rectifier tube. It is a common practice to capture unused pins as tie points -- it's convenient. I believe these are connected to pin 4 of the power tube sockets, likely the screen supply (not sure of the power tube in use; can't see the 6.3V filament supply which would help verify.) I expect the black wire on pin 7 of the rectifier goes to the multi-section can cap nearby as part of the screen supply.
You've labeled the can cap as a solid state rectifier.
The yellow wires on pins 2 and 8 of the rectifier are the 5V supply. I'm not aware of any 6.3V rectifier that would be appropriate with that PT. The 6.3V supply is most likely to be green and I'll vote for the green twisted pair that's next to the rectifier socket and not connected to it.
On the power tubes, I'm thinking pins 2 and 7 are the 6.3V heaters (filaments). There is a pair of 56 ohm resistors that form the faux center tap for the heaters and connected to pin 8, the power tube cathode. This elevates the heater supply as a hum reduction measure.
Unfortunately, there isn't much of a lesson here as to the power supply ladder for the donor amp. I don't think that's important. What we know is that you'll need to select a power tube that can operate with plate voltage above 500 or go for choke input (LC not CLC like that pic you posted) which will drop considerable voltage, putting you out of reasonable range for the SR circuit you seem interested in building.
I think you've either got to pick a circuit and get the right transformer, or pick a circuit that works for the PT you've got. I can tell you, if it was me, I like doing the latter. I don't think there's anything wrong with letting the things you have dictate the build. The fun is that you will get a surprise at the end. There is every reason to expect a decent result.
unedited pic
Apologies. Here it is.
Yeah, it's going to be a great project, getting this thing going.
Yeah, it's going to be a great project, getting this thing going.
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Re: Stancor transformer
It would be nice to know what power tubes were originally in use with that PT, but there is serious weirdness in that picture. I had not expected the alignment notch to be at 12:00 on the left and 6:00 on the right. Here's what I can make of it...someone else may be able to pick a tube:
Wired the same on both:
2, 7 heaters
3 plate (probably); red on left, yellow on right [note 1]
5 grid (probably)
8 cathode
[Note 1: the different wire colors bother me. I'd feel much better about identifying plate if both were red.]
On the left:
1, 6 not connected
4 screen (probably)
On the right:
4, 6 not connected
1 screen? doesn't make sense.
I expect the donor amp was not wired properly and sounded like poo. The socket on the left fits the 7AC base diagram. This includes many 6L6 variants include KT??. The socket on the right, no clue.
Until, now, I've been operating under the assumption the PT is an orphan of unknown use. I don't think I asked or saw, with respect to the obvious, do we know the donor amp and can we find a schematic? That would tell us a great deal.
Wired the same on both:
2, 7 heaters
3 plate (probably); red on left, yellow on right [note 1]
5 grid (probably)
8 cathode
[Note 1: the different wire colors bother me. I'd feel much better about identifying plate if both were red.]
On the left:
1, 6 not connected
4 screen (probably)
On the right:
4, 6 not connected
1 screen? doesn't make sense.
I expect the donor amp was not wired properly and sounded like poo. The socket on the left fits the 7AC base diagram. This includes many 6L6 variants include KT??. The socket on the right, no clue.
Until, now, I've been operating under the assumption the PT is an orphan of unknown use. I don't think I asked or saw, with respect to the obvious, do we know the donor amp and can we find a schematic? That would tell us a great deal.
Last edited by Phil_S on Sun Jan 18, 2015 4:14 pm, edited 1 time in total.
Re: Stancor transformer
Just as many do now, I think they used the wires from trimming the PT for socket wiring.
The red with yellow stripe for example.
The red with yellow stripe for example.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: Stancor transformer
Both trannies came from an Oliver g150r with 7027a power tubes and a GZ34 rectifier. I believe the 7027a
Re: Stancor transformer
That certainly explains the odd wiring. 1 or 4 is the screen, 5 or 6 is the grid.Eric_BK wrote:Both trannies came from an Oliver g150r with 7027a power tubes and a GZ34 rectifier. I believe the 7027a
GZ34 will produce B+ approximately 1.2x input voltage. RC30 says for 7027a, max plate = 600, max screen = 500. The original is entirely possible with that PT. You can find 7027A new production for reasonable price, but you are not locked in. There are a number of tubes that will operate at high voltages. I just think that SR you want to build won't work with 6L6's and this PT.
Re: Stancor transformer
Very true. I was slowly coming to that realization about the SR. I was reading that the KT66 can handle a max of 550v for both plate and screen, which could work with this OT. I just have to find a board to go with it. Definitely up for the challenge!
Re: Stancor transformer
Boards are here, well worth paying the asking price: http://www.turretboards.com/TURRET_EYEL ... REVERB.htm
You want G10, FR10 or similar material. The old fiberboard is subject to absorbing moisture and becoming conductive, not a good thing. Some people prefer eyelets, but I think the majority prefer turrets. Both styles work fine.
I like the look of the red swirl material, but found the light color is more practical. I can take a fine point Sharpie to it and draw the components on the board, which makes assembly and maintenance easier.
You want G10, FR10 or similar material. The old fiberboard is subject to absorbing moisture and becoming conductive, not a good thing. Some people prefer eyelets, but I think the majority prefer turrets. Both styles work fine.
I like the look of the red swirl material, but found the light color is more practical. I can take a fine point Sharpie to it and draw the components on the board, which makes assembly and maintenance easier.
Re: Stancor transformer
Ok. Definitely getting the board and will be using kt66 tubes. I think I may go with a 5U4GB or the GZ34 for the rectifier tube. Thoughts? I'm figuring the KT66 tubes will handle the power based on the specs - 500 - 550v on both the plate and screen. Have to check the heater v.
Re: Stancor transformer
Once you've built it you can install different 5V rectifier tubes and take plate voltage readings on your empty power tube sockets. The readings will be higher without power tubes installed but it should give you a base reference of what the difference will be between rectifier types. When you do take the step of installing your power tubes, try starting with a 5U4 or a variant that will give a similar voltage drop, then factor in the difference to calculate if you'll be safe using a GZ34.
As stated earlier, the PT I used had a 375-0-375 HV secondary which will give me around 500V easily with a 5U4 when wall voltage reads at 120V. Just be wicked f**kin careful when taking these readings and be sure to drain your filter caps after every single time you turn the amp on to test things out. Make yourself a bleeder resistor/alligator clip arrangement and let it become your best pal. When you get to this stage of testing the amp you will be pretty excited and enthusiastic but you mustn't forget to take the time to drain those filter caps. This is what each and every one of those DANGER warnings on the back of amps and on people's websites is in reference to. Of course you may already be well aware of and prepared for this
As stated earlier, the PT I used had a 375-0-375 HV secondary which will give me around 500V easily with a 5U4 when wall voltage reads at 120V. Just be wicked f**kin careful when taking these readings and be sure to drain your filter caps after every single time you turn the amp on to test things out. Make yourself a bleeder resistor/alligator clip arrangement and let it become your best pal. When you get to this stage of testing the amp you will be pretty excited and enthusiastic but you mustn't forget to take the time to drain those filter caps. This is what each and every one of those DANGER warnings on the back of amps and on people's websites is in reference to. Of course you may already be well aware of and prepared for this
Re: Stancor transformer
and don't shy away from trying KT88s!