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Old 25th November 2013, 12:29 AM   #1
AlanJ is offline AlanJ  New Zealand
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Default How can I drop B+ Voltage

I'm new here and I hope I have posted this in the right place. I have built 300B amps from the JE labs schematics - the deluxe version.(easy to find on the web) It all works but my B+ is to high by about 25v --the power from my transformer is to high. So all my voltages are slightly high. Someone told me to put resisters before the first choke, but they were not sure. Where is the best place to add resisters to drop it the voltage?. My 5v and 6.3 v are fine-- I have plenty of 50w resisters to play with. I am using Shuguang 300BS-B and they are rated at 480 max plate voltage. Any advice is greatly appreciated. the schematic is here. I am not quite sure how to put it in the post so I have just added a web page link http://jelabsarch.blogspot.com/2012/...nd-deluxe.html

Last edited by AlanJ; 25th November 2013 at 05:02 AM. Reason: add more info
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Old 25th November 2013, 01:05 AM   #2
DUG is offline DUG  Canada
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Resistor drop will vary with current.

Power zener diode will work...calculate power dissipation and heat sink accordingly...with adequate High Voltage Isolation.

"Measure twice cut once."

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Old 25th November 2013, 04:15 AM   #3
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I don't know the circuitry. If a 5AR4 vacuum rectifier is being used to rectify the B+, switching to a 5R4 will lower the rail voltage by quite a bit. The 5R4 can't work into as large a 1st cap. as the 5AR4 can. Play safe and use 15 μF. as the value for the 1st position. Use a CL90 inrush current limiting thermistor to slow B+ rise.

BTW, the forward drop in a 5R4 is greater than that in a 5U4 too.
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Old 25th November 2013, 04:21 AM   #4
kevinkr is offline kevinkr  United States
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Provided that neither the rated plate voltage or dissipation of the 300B are exceeded you may not need to do anything.

A schematic of the amplifier or at least information on the type of rectification, and whether the output stage uses cathode bias or fixed bias along with plate voltage and current would be helpful in answering this question.
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Old 25th November 2013, 04:38 AM   #5
AlanJ is offline AlanJ  New Zealand
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good point-- I hope this added the schematics
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File Type: jpg JELsc-300BDX.jpg (27.7 KB, 281 views)
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Old 25th November 2013, 06:52 AM   #6
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On 300B You have 355 v, that is OK.
But problem. IMO, is that there are direct coupled triodes in preampl.
You can adjust modes of first and second triodes, that cathode of second triode will have for a 8 v more than plate of first triode. Try use 62 or 72 kohm instead 68 kohm, and 22 or 27 kohm instead 24 kohm to receive 8 v difference.
First plate must be lower to 8 v than cathode of second triode.
Regards!
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Old 25th November 2013, 07:20 AM   #7
AlanJ is offline AlanJ  New Zealand
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I will look into that, My main concern is my transformer. I want to drop the power after the GZ37 as I am reading about 20v to high. So instead of 436v I get 456v ect. I had the transformers wound years ago and obviously they didn't quite get them right.
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Old 25th November 2013, 08:15 AM   #8
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Hi,
do what azazello sayd. Some Volts more or less doesnt mean anything to a tubeamp.
Have fun,
Hilmar

Edit:
Another Thing you can do is: Swap the first cap in your powersup to the second cap (47 +47) and play at the position of the first cap with small capacitances maybe less than 1. You will get less tension the smaller the first cap is. Try to simulate this before with PSUD from Duncan-Amps.

Last edited by Hilmar53; 25th November 2013 at 08:25 AM.
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Old 25th November 2013, 08:20 AM   #9
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by AlanJ View Post
So instead of 436v I get 456v ect. I had the transformers wound years ago and obviously they didn't quite get them right.
That's only 4.6% high. Sounds pretty good to me!
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Old 25th November 2013, 08:42 AM   #10
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I dont know which is your final schem, but based on your first post, you can cut the line of 390 volts ..Place an ammeter in series.( careful with the probe polarity) Power-up and register the current. Devide your excess voltage (25v) with the registered current and you can find the value of resistor in ohms..add this resistor value with your existing 4.7k..Hope this help
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