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Old 28th May 2011, 09:38 PM   #1
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Default Diodes to tube

Hi everyone, I'm a newbie here @ DIY so please bear with me.

My question is this: I have recently built the 16LS kit from S-5 Electronics and I would like to switch the bridge rectifier chip for the B+ with a rectifier tube (5u4GB maybe?). Has anyone done this or does anyone have a good schematic to do this? Could I use a separate trans. for the 5U4's heater?
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Old 28th May 2011, 10:53 PM   #2
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Well if your amp does use a bridge rectifier then you would need a full wave rectifier tube like the 5u4 plus another set of diodes to complete the bridge. It could be a hybrid bridge of diodes and a tube or even four separate rectifier tubes, like tv damper diodes.

If you could provide a link to the schematic it would make things much easier.
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Old 28th May 2011, 11:12 PM   #3
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The tube solid state hybrid is sometimes referred to as a Graetz bridge but this actually refers to the four diode topology in general. In this case two of the diodes are solid state and the tube rectifier encompasses the other two..

One potential concern is that tube rectifiers have substantial forward drop compared to soiled state devices resulting in rather lower B+ which in the case of a 5U4 might be as much as 15 - 20V lower (or more) depending on load current. My estimate of drop is based on what I suspect is a fairly modest current draw of a 100mA or so. The 5AR4 might do a little better than this, but at modest currents possibly not.

I am not familiar with the design of this particular amp, but if it uses a CRC based pi-filter the resistor could be replaced with a choke of substantially lower dcr which would help to make up some of the loss.

The other option might just be to get a suitable power transformer, and then still after that a better set of OPTs. You can see where this is heading..

Note that regardless of what you do you will need a filament transformer either 5V or 6.3V depending on the rectifier chosen. I doubt the current filament winding has sufficient capacity (probably an oddball voltage as well) for something like a 6CA4/EZ81 which otherwise could be a good choice.
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Last edited by kevinkr; 28th May 2011 at 11:17 PM.
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Old 28th May 2011, 11:14 PM   #4
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You get a higher voltage drop across tube rectifiers so you have to increase your B+ to make up for it.
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Old 29th May 2011, 12:18 AM   #5
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Quote:
Originally Posted by nigelwright7557 View Post
You get a higher voltage drop across tube rectifiers so ....
This answer is correct with regards to tube rectifier producing lower B+ as others have noted already. Whether you have to make up for it is another question altogether. In many cases the amplifier will work fine if it had more than enough power to satisfy your needs before the mod was made. You can just give it a try, and you may find the 10 to 20+ volt B+ loss may be almost unnoticeable.
To answer your other question, can I use a seperate filament transformer for the 5U4 rectifier, certainly can.

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Old 29th May 2011, 01:37 AM   #6
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Quote:
Originally Posted by oldschooltube View Post
Hi everyone, I'm a newbie here @ DIY so please bear with me.

My question is this: I have recently built the 16LS kit from S-5 Electronics and I would like to switch the bridge rectifier chip for the B+ with a rectifier tube (5u4GB maybe?). Has anyone done this or does anyone have a good schematic to do this? Could I use a separate trans. for the 5U4's heater?
The other posters have made good points. You will need to buy a filament trafo of some kind, to employ vacuum rectification. Getting rid of the inexpensive (read noisy) SS bridge rectifier makes sense, but staying all SS is (IMO) probably best. 4X 600 PIV Schottky diodes (Mouser part # 726-IDT02S60C) will be just as quiet as vacuum rectification, without the forward drop penalty already mentioned. Heck, even 4X inexpensive UF4007s will be a distinct improvement.

If you are determined to use vacuum rectification, a hybrid bridge will allow the power trafo in situ to continue in service. In vacuum rectifiers, damper diodes have the lowest forward drop. A hybrid bridge setup made from a pair of 600 PIV Schottkys and a 6BY5 dual damper will not be too far off from what's in the unit now.

The suggestion to use a CLC filter, instead of a CRC filter, is excellent. Resistance can be employed in series with the inductor to bring the B+ rail voltage in the same as OEM.

BTW, vacuum diodes can't tolerate large value caps. in the 1st filter position, like SS can.
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Old 29th May 2011, 02:13 PM   #7
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Default Re: 16LS Schematic

Thanks Astouffer. Here is the link to the schematic as I built it. BTW, this amp draws about 220ma of current on the B+ measured between the bridge rect. and the 1st filter cap. Current doesn't seem to fluctuate much between idle and full power (5ma max).

K-16ls + ton pcb (just finished pics)
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Old 29th May 2011, 08:35 PM   #8
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Quote:
Originally Posted by oldschooltube View Post
Thanks Astouffer. Here is the link to the schematic as I built it. BTW, this amp draws about 220ma of current on the B+ measured between the bridge rect. and the 1st filter cap. Current doesn't seem to fluctuate much between idle and full power (5ma max).

K-16ls + ton pcb (just finished pics)
Actually a better place to determine the amplifier dc current draw is to measure the voltage drop across that 470 ohm resistor in the power supply. (I=E/R) The output of the bridge is ripply dc with a large capacitor charging current and depending on the design of your meter you may or may not get an accurate reading of the amplifier current at this point.

Should the current really prove to be in the vicinity of 220mA the voltage drop across the resistor will be nearly 100V which is more than sufficient margin to allow the use of any tube rectifier you want in conjunction with a smaller resistor or choke. Note that for a 5U4 the input capacitance should probably be kept to 30uF or so, a 5AR4 40uF.. The 6BY5 is one of my favorite damper tube rectifiers but is best used in applications where the dc load current is not too much over 100mA total - so may not be a candidate here.
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Last edited by kevinkr; 29th May 2011 at 08:38 PM.
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Old 29th May 2011, 10:02 PM   #9
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Default Hybrid Bridge

I like Eli's post about using 2 schottkys and 1 6by5 for this. Could anyone throw out a schematic for review as I have never built a hybrid bridge with diodes and a tube before.(I just want as much info as I can get before I commit.) If for nothing else this amp uses 2x 5670 tubes that sit in front and 8x 6005 power tubes in the middle of the case and you can't tell me that a nice BIG functional bottle sitting in the back of this tread plated beast wouldn't look really cool.2011-03-15_19-02-30_659.jpg

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Old 29th May 2011, 10:07 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by oldschooltube View Post
I like Eli's post about using 2 schottkys and 1 6by5 for this. Could anyone throw out a schematic for review as I have never built a hybrid bridge with diodes and a tube before.(I just want as much info as I can get before I commit.) If for nothing else this amp uses 2x 5670 tubes that sit in front and 8x 6005 power tubes in the middle of the case and you can't tell me that a nice BIG functional bottle sitting in the back of this tread plated beast wouldn't look really cool.Attachment 224952

"Before you can truly appreciate success you must first experience failure."
Please reread post #8 to understand why the 6BY5 might not in this instance be the right choice for a rectifier tube..
(Noting that I have used this tube for a long time and like it a lot.)
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