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Old 24th March 2014, 02:03 PM   #1
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Default IT coupled 6BG6GA

I built this circuit using the "super" 6BG6GAs. Works and sounds great although I have to modify the fixed bias circuit to increase the voltage. I'm only able to get -30V now.

I read in another thread that when simulating the 6BY5G in PSUD2, to use the 5V4 model. I did that although there is not really a way that I know of to simulate the hybrid bridge configuration that I used. I simulated using full wave with a center tapped xfmr. The simulation showed 450V for the power tube B+ at -45V fixed bias for about 50mA/tube.

The B+ ended up being 380V, and I'm able to bias the tubes to 60mA. I'm sure the voltage will come up some when I get the fixed bias voltage up, and can bias the tubes a little colder, but probably not to 450V. I could also change the 62R series resistors to 50R safely which may help a little.

Has anyone successfully simulated a 6BY5 hybrid bridge in PSUD2?
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Old 25th March 2014, 02:55 AM   #2
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^^

I don't use that PSUD thingie to design power supplies. I use a scientific calculator and the plate characteristics. Over at Frank's, the spec sheet for this type is very sparse, and doesn't include any plate characteristics. Given that you're using a plate bias of 50mA, I wouldn't have considered the 6BY5 in the first place. The Isurge= 525mA is pretty anemic, and places a severe limit on the size of the reservoir capacitor. That will make for lots of ripple, and cut the output voltage way down. If I felt I had to hollow state the PS, I'd rather go with a 5U4GB or a 5BC3 (like a 5U4, but in the large, nine pin format) that have a stiffer Isurge= 1.0A/plate.

Still, given the difference between what was predicted and what you measure (70V) that looks to be in line with the expected forward loss across a hollow state diode.
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Old 25th March 2014, 03:26 AM   #3
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Thanks for the reply.

Separate 6BY5s are used for each channel. Only 2 power tubes + the driver for each channel. 6BY5 is good for 175mA and the circuit is drawing 116mA/6BY5. No 5V winding on my PT. I suppose I could convert to a SS FWB instead.

Vacuum tube rectifiers don't all have the same voltage drop, but I could not find a definitive answer on the 6BY5.

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Old 25th March 2014, 04:10 AM   #4
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Do you mind expanding on this topic?

...and places a severe limit on the size of the reservoir capacitor. That will make for lots of ripple,...

Thanks.
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Old 25th March 2014, 08:39 AM   #5
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Hi!

Quote:
Originally Posted by scott17 View Post
Vacuum tube rectifiers don't all have the same voltage drop, but I could not find a definitive answer on the 6BY5.
The Tung-Sol data sheet quotes 32V at 175mA. In your application you maybe have 20V drop. A 5U4 would have a much larger drop compared to the 6BY5

Thomas
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Old 25th March 2014, 10:56 PM   #6
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Hello Thomas,

Thanks. I notice that the 6BY5 hybrid supply schematics on your blog do not show resistors in series with the anodes. Are you using these but they are not shown?

Scott
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Old 26th March 2014, 11:20 PM   #7
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I really am a newbie when it comes to fixed bias and interstage xfmr phase splitters, but I have to say it has opened up a whole new world for me.

After listening to this amp for a while, the tightness and richness of the sound is absolutely amazing.
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Old 27th March 2014, 08:35 AM   #8
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Hi Scott,

I use choke input supplies which do not impose severe peak currents to the rectifier.
It is important to keep the currents within the rated limits. With moderately sized input caps the winding resistance of the power transformers is usually enough.

Best regards

Thomas
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Old 2nd April 2014, 12:01 AM   #9
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Default bias supply mod

I used a 5W 8-115VAC xfmr wired in reverse to the heater winding and changed to a FWB to feed the bias supply. This brought my raw bias voltage up to -80V, allowing greater bias range, and the output tubes are now biased at 49mA.

The B+ came up to 425V. I thought my SE KT88 sounded great, but this is the tightest sounding amp I have ever built. The interstage xfmrs are quite pricey, but well worth it.

Scott
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Old 2nd April 2014, 07:24 AM   #10
rmyauck is offline rmyauck  Canada
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Great job Scott!

12-15W/CH?
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