Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mazzgreg on August 24, 2019, 04:48:27 am
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Hi. I was doing some research on solid state rectifier and found that the recommended circuit for transformers with center taps is a rectifier with diodes in series rather than 4 diodes in a standard bridge rectifier configuration. Does anyone know if there is a reason not to use a bridge rectifier with a center tapped transformer?
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In principle is there no reason to use a bridge rectifier. In the case of a centre tapped transformer you must disconnect the - of bridge and isolate it. Because if the - point touches the ground, the transformer and/or the bridge will burn out.
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if you ground the CT with a full wave bridge (four diode bridge) connected, your transformer & diodes will leak lots of magic smoke. if you use the CT properly with a full wave bridge and stacked filters, then you will have a full wave CT doubler with a voltage output twice that of what you were probably wanting.
transformers with a HV secondary that are designed for tube amps that have a CT are designed to operate in a full wave configuration (series diodes) with the CT grounded. there are other ways to deal with the CT, such as choke loading it, but that's another topic.
--pete
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Thanks for all the helpful info. I really appreciate it. I am working on a point to point amp. Circuit is pretty much a 5f10 Harvard, but my pt has no 5v winding
I will try the series diode rectifier when I get home next Wed.
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You could use an EZ81 if you want a tube rectifier. Diodes will work fine too, but your B+ will be higher than it would be with a tube.
I like the 5F10. I have an original.
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Hi. I was doing some research on solid state rectifier and found that the recommended circuit for transformers with center taps is a rectifier with diodes in series rather than 4 diodes in a standard bridge rectifier configuration. Does anyone know if there is a reason not to use a bridge rectifier with a center tapped transformer?
The vast majority of guitar amps use either a bridge rectifier or a grounded-CT rectifier (what you are calling a rectifier with diodes in series. The grounded-CT version doesn't necessarily have diodes in series, but that's another matter). Guitar amp transformers are designed for either one style of rectifier or the other, but not for both styles. Guitar amps with tube rectifiers use transformers intended for a grounded-CT arrangement.
Plenty of transformers meant for bridge rectification have center taps. The typical use of this CT in guitar amps is to balance the voltages across stacked capacitors in the first filter stage.
If you tell the folks here what particular transformer you have, they can tell you what kind of rectifier to use.
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Unfortunately, I don't have the info on the pt. I think that I got it out of an old tube record player by Silvertone. I believe it originally had a 6 volt tube rectifier, but can't be 100 percent certain. I know it was definitely a 6v6 push pull amp.
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Maybe the attached diagram will explain some of he ways you can rectify a transformer with a CT..
You sometimes see the last one in guitar amps, but as 2deaf said, the 1/2 voltage node (the CT) is used only to balance stacked capacitors. Both nodes can be used though. For example, the high voltage might be used for the plates of the output tubes and the half voltage for the screens.
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The diagrams are definitely helpful, as are all the posts. Much thanks!
I am pretty certain that I am needing the full wave and not the bridge.
To be completely honest, thecrwadon for all this is that i made a full bridge rectifier with 4 diodes and grounded the center tap along with the B-. Then, the pt got hot.
So, I disconnected the B- from ground and started checking voltages and my B+ was way high. So. Down the internet rabbit hole I went.
Found a lot of stuff about rectifiers and trannies and power supplies, but no one went so far as to explain what the different combos did.
This forum is great!
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> I don't have the info on the pt. I think that I got it out of an old tube record player
When you scrounge parts from old gear, take notes (or save) how it was used originally (which worked from the factory).
There's several ways to combine transformer, diodes, and caps. You can wind up with 200V 100mA, 400V 50mA, or 800V 25mA. Get it wrong and it will be very unhappy. Like a 120V lamp on a 240V outlet.
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HI,
i have a guitar tube amp, made in Italy around 1970s. it uses 2x EL504 & 1x 12AX7 in the power section. the preamp part is also all tube including the tremolo and reverb circuit using 12AX7's and 12AU7s. it was from a company called Sound. the model is not marked on the amp but it seems to be a Supreme 90. there seems to be several iterations of this amp under different brand names. i found some schematics online for a 220v version. i have a 120v version. they seem to match up but there are some differences in the PSU, such as i have one extra 82uF filter cap. i have brought the amp to modern safety standards such as connecting the mains hot to fuse first, then to switch and then to transformer primary. the neutral straight to transformer. i have also removed the death cap. i want to add the safety ground to chassis but from the schematic i can see that chassis was floating and it is not connected to ground. reading this topic here made me stop before going ahead because this amp uses a full wave rectifier. didn't want to add the safety ground to chassis because i don't want to blow the transformer. since that will tie the negative of the bridge rectifier to safety ground. can any one shed some light on how i would go about doing this? i've attached the schematics. thx.
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... i want to add the safety ground to chassis but from the schematic i can see that chassis was floating and it is not connected to ground. ...
The symbol circled in Red below is a "Chassis Ground" and indicates your amp's power supply was connected to the chassis (so it is not "floating").
You can tie your power cord Safety Ground to the chassis without issue, based on this schematic.
(https://i.imgur.com/AR3zU1h.jpeg)
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... i want to add the safety ground to chassis but from the schematic i can see that chassis was floating and it is not connected to ground. ...
The symbol circled in Red below is a "Chassis Ground" and indicates your amp's power supply was connected to the chassis (so it is not "floating").
You can tie your power cord Safety Ground to the chassis without issue, based on this schematic.
(https://i.imgur.com/AR3zU1h.jpeg)
Hi, hmmm. ok . i checked continuity and there wasn't any between chassis and any earth grounds on the transformer.
Thx
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Hi, hmmm. ok . i checked continuity and there wasn't any between chassis and any earth grounds on the transformer.
There's no connection to ground at the transformer.
The connection to ground is at one of 2 bridge rectifier outputs (the other output goes to the Standby switch).
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The purple highlighted lines of ghe layout are circuit common, they align with the triangular earth symbol on the schematic.
The schematic shows a connection between that and a 'planet earth' symbol at the top.
I take that to indicate a connection between circuit common and the chassis.
So I think there should be very low resistance (<1R) to the chassis from circuit common.
So if there isn't, maybe it's failed or previously been removed?