Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TheKT88KilledJFK on February 04, 2018, 02:55:42 pm
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I normally work with power transformers rated for use in mesa amps, so I expect +-350V(700V p-p) transformers and assumed all power transformers were rated in a +- manner. However, I've been messing around a lot with marshall designs but still using the 700V p-p but with full wave rectifiers that bring that down into 460VDC or below. I recently got my hands on some marshal power Tx and decided to put it in the same application where a plate voltage of a mere 245VDC presented itself in contradiction to my expectations. Looking at what a full wave rectifier should do, I assume this tranny is actually 350Vp-p and not 350-0-350 like the mesa. Does this seem right? I think going to raise the center tap with the filter section like the manufacturer suggests BUT use a bridge rectifier section.
Here is the specs for the Tx in question:
http://www.classictone.net/40-18024.pdf
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I "read" your link as you assume, 350-0-350, not sure why you don't have 700vac unloaded, so I would ask the tranny folks.
I typically buy PT without CT, and they are typically stated 0-360
here's a link to a Marshall 100w
http://el34world.com/charts/Schematics/files/marshall/Marshall_100W.pdf
does you meter have the capability of measuring 1,000VAC? lots don't like >600VAC
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To get the right voltage you have to use bridge rectifier with that transformer. CT is there so you can balance the voltage between two caps. You can do same thing with balancing resistors.
And like the datasheet says if you're only using one cap then you don't connect the ct to anything.
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I "read" your link as you assume, 350-0-350, not sure why you don't have 700vac unloaded, so I would ask the tranny folks.
I typically buy PT without CT, and they are typically stated 0-360
here's a link to a Marshall 100w
http://el34world.com/charts/Schematics/files/marshall/Marshall_100W.pdf
does you meter have the capability of measuring 1,000VAC? lots don't like >600VAC
The preamp tube plates are only getting 175 volts so I am pretty sure it's right. I would have just tested the TX unloaded but the leads are ran in a hidden manner. I'll have to test it now.
To get the right voltage you have to use bridge rectifier with that transformer. CT is there so you can balance the voltage between two caps. You can do same thing with balancing resistors.
And like the datasheet says if you're only using one cap then you don't connect the ct to anything.
That is the way that some 2203 schematics do it but they have a bridge rectifier. I think I am going to try that configuration next then attach the CT to the center of the first filter section on that voltage divider net.
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That transformer requires a bridge to get the B+ needed for a 100 watt Marshall. 350V CT means just that. Stated another way, 175-0-175. Using a conventional two diode full wave rectifier will only give 1.414 x 175 = 247VDC.
Repeat... That transformer requires a bridge to get the B+ needed for a 100 watt Marshall.
Maybe you bought the wrong transformer?
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That transformer requires a bridge to get the B+ needed for a 100 watt Marshall. 350V CT means just that. Stated another way, 175-0-175. Using a conventional two diode full wave rectifier will only give 1.414 x 175 = 247VDC.
Repeat... That transformer requires a bridge to get the B+ needed for a 100 watt Marshall.
Maybe you bought the wrong transformer?
Bought the right Tx, just used the wrong type of rectifier scheme. Your statement confirms exactly what I measured in circuit. I'm going to use a bridge rectifier with the center tap elevated at the caps like some of the marshalls.