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Old 29th May 2011, 10:45 PM   #11
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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?
My advice to you would be to change the transformer. Get one that is slightly bigger physically than the one in the kit. There are plenty of makes that are quite reasonable with their prices and one or two may actually be quite helpfull if you tell them want you want to do.
Primaries kept the same. Center tapped 6.3volts kept the same. High voltage winding to be changed to two windings of the same voltage each (maybe higher than original) but half the current of the original which will give you a centre tapped secondary to feed to your chosen valve rectifier. Add a second heater winding, the volts and current to suit the heater on your valve rectifier. This is something ANY good transformer manufacturer can do. You may even find something suitable "off the shelf".
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Old 31st May 2011, 01:20 PM   #12
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Call me crazy but how about this: connect the 2 separate cathodes of a 6by5 together to make a common cathode (basically making an indirectly heated 5u4gb without as much forward voltage drop) and then running 2 in parallel for more current handling capacity to make half of a graetz bridge. The extra filament current is not an issue as I have several filament trafos.
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Old 31st May 2011, 05:12 PM   #13
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by oldschooltube View Post
Call me crazy but how about this: connect the 2 separate cathodes of a 6by5 together to make a common cathode (basically making an indirectly heated 5u4gb without as much forward voltage drop) and then running 2 in parallel for more current handling capacity to make half of a graetz bridge. The extra filament current is not an issue as I have several filament trafos.
This should work, but I would probably place a resistor in series with each plate to assure that the load current is shared reasonably well. Something on the order of 47 ohms / 5w ought to suffice.
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Old 31st May 2011, 05:55 PM   #14
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As far as I can see, the only real advantages of a tube rectifier are:

1. Slow Start
2. ZERO Reverse Voltage Breakdown
3. Authenticity.

There are definite but arguable advantages about SLOW START HT. It is conceivable that COLD cathodes can be stripped if they are not fully warmed up when the B+ is applied to the valves. This is easilly counteracted by fitting a delay in the circuitry that supplies B+.

ZERO Reverse Current, although not completely absent from SS power supplies is not normally an issue. Your HT bridge will be doing its job.

What can never be replicated by SS is true valve authenticity.

It is awkward to replace SS with valves directly. As the previous posts direct, you need separate filament supplies and a higher VAC to achieve the same results.

You can CHEAT - SACRILEDGE I KNOW.

The SS rectifiers plus a slow start HT circuit will do the same job as the tube rectifiers. If you don't want the additional complexity of extra HT transformers and rectifier heaters supplies think about the following CHEAT - and it looks authentic.

Keep the existing SS FWB but add a slow start relay.

Fit a pair of (DEAD) rectifier valves to the chassis for appearance only.
The dead rectifier valves can be illuminated with their own heaters or with LEDs.

SACRILEDGE - MAY I BE CONDEMNED. If you are after authenticity then the problem is ten fold. If you are after appearance - SIMPLE. SS rectifiers are also much more reliable.

Last edited by Andy5112405; 31st May 2011 at 05:58 PM.
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Old 31st May 2011, 08:04 PM   #15
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A major advantage of a slow start is if you are using a choke input supply and you don't want the voltage rising to very high levels before the amplifier valves start to conduct
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Old 1st June 2011, 01:43 AM   #16
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Quote:
Originally Posted by oldschooltube View Post
Call me crazy but how about this: connect the 2 separate cathodes of a 6by5 together to make a common cathode (basically making an indirectly heated 5u4gb without as much forward voltage drop) and then running 2 in parallel for more current handling capacity to make half of a graetz bridge. The extra filament current is not an issue as I have several filament trafos.

The SS bridge rectifier in situ contains 4 diodes. To make a hybrid bridge, connect the 2 vacuum cathodes together and a plate to each end of the power trafo rectifier winding. Connect the anodes of a pair of SS diodes to ground and a SS cathode to each end of the rectifier winding. You take the "raw" B+ from the junction of the vacuum cathodes.

Vacuum rectifiers can't tolerate large values in the I/P cap. position, along with exhibiting a higher forward voltage drop. Therefore, the entire PSU needs to be reworked. The schematic for the amp shows a cheap simple cap. I/P filter RC decoupled from the small signal circuitry. I hope you have a HEFTY 6.3 VAC filament trafo on hand, as it will have to energize both the signal tube heaters and the 6BY5 heater, The 6.3 VAC winding of the OEM power trafo will be used to boost the voltage the rectifier winding delivers. There are 2 ways to connect that 6.3 VAC winding in series with the primary of the power trafo. One arrangement will raise the rectifier winding's voltage, while the 2nd lowers it. Obviously, you use the arrangement that raises the voltage. The "raw" B+ from the hybrid bridge rectifier feeds a 22 μF. cap., which feeds a 5 H. filter choke, which feeds the existing PSU filter.

BTW, While I appreciate Kevin's concerns about the limitations of the 6BY5, there's little to worry about (IMO). A careful look at the data sheet shows each plate is capable of carrying a continuous 175 mA. current. For a damper type, the 6BY5 has a (sic) low heater to cathode potential limit of 450 V., but it's more than adequate for this particular job.
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Old 1st June 2011, 01:58 AM   #17
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Thumbs up Cheat?

Andy! Cheat?!? Tsk Tsk! Cheaters never win and winners never cheat. Right now the thermostat in tube heck just got cranked up an extra ten deg. Just for you!!!! I'm just funnin' with ya. I don't mean it...the thermostat is only going up 5 degrees. I did actually consider this but it is waaay to easy and if it is easy when you succeed it just does not taste as sweet. Besides,there is only one thing cooler than tubes...FUNCTIONAL tubes. If you're gonna build a tube amp may as well use all tubes.
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Old 26th July 2011, 10:22 PM   #18
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Default Update

Just an update, I finished the tube rectifier mod on my 16ls kit and it rocks! I ended up using 2 6by5ga tubes in parallel and 2 diodes. After plugging in the tubes I held a nervous finger on the power switch and then said "The hell with it, that's what circuit breakers and fuses are for!" I closed the switch and was amazed by how quiet this amp has become. Befor the mod I could hear some switching noise and a faint hum but they are all gone now. I was able to keep the stock power tranny in service to provide the B+ and heater voltage to the 5670's and 6005's. I added a separate filament tranny to light the 6by5ga's. The increased voltage drop from the tube rectifiers is not at all noticeable, in fact I think this amp has a better and smoother sound quality with a little less B+. I kept the stock crc setup mostly in place but I yanked the 220uf first cap and replaced it with some vintage Aerovox dry electrolytics totalling 30uf and I put 3K ohms in parallel with the 470 ohm resistor to bring the resistance down a bit to compensate for some of the volt drop. I have never heard of anyone doing tube rectification in one of these kits but I highly recommend it. Aside from the obvoius sonic benefits, the two big bottles in the back really look cool!
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Old 28th January 2013, 07:50 PM   #19
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I did not want to start a new thread just for this, so,

I have a PT with two 300V secondaries and want to use two 6BY5GA in parallel.

Can I connect them like this, or is it better or just as good to parallel the two 300V secondaries and then parallel the 6BY5GAs and use only 2 diodes?

The series limiting resistors are not shown on the drawing.

Thanks.

Scott
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Old 28th January 2013, 08:55 PM   #20
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Nice job. Definitely a one-of-a-kind work of art! Is that Diamond plate hard to work with? I'm thinking about using it for my next project. Is it hard to cut?
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