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Old 10th April 2011, 03:48 AM   #1
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Default Home Brew Ampeg B25B


I am a newbie considering building a home brew Ampeg B25B bass amp. I would like to follow the circuit diagram below where possible:


I know a LITTLE about electronics and I am trying to read as much as possible before building, which I won't even contemplate doing until I am certain every thing is theoretically correct and completely safe - I have my dad (an electrician) to advise on the safety side though no amp tech to ask any detailed questions of which is what brings me here.

I intend to use a Lundahl LL9202 for the output transformer and an Antek AS-4T500 for the mains transformer. The specs are below:

Antek - AS-4T500

My problem is the Antek AS-4T500 only has 2 6.3v secondaries so I can't use the GZ34 and will have use a diode bridge - as the amp is intended to be used as a bass amp this doesn't bother me that much. I would like to follow the RCA specs for class AB1 and try to get around 70-75 volts by putting 540v on the plate and 400v on the screen as per below:


If any one could help me to modify the B25B circuit diagram to replace the GZ34 valve rectifier with a SS diode bridge and up the output to circa 75 watts aper the RCA spec I would greatly appreciate it!!!

All comments welcomed.


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Old 10th April 2011, 04:19 AM   #2
eyoung is offline eyoung  United States
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Location: Sunny SC,USA 15 min south of Charlotte NC
You can replace the tube rectifier with 2 discrete diodes(ignore heater V), I have done this on an old Gibson amp the only complication is that your B+ voltage is higher than spec however this should not be a problem . recheck the o/p bias afterward . That being said GZ34/5ar4/5u4 should not be that hard to find, tube rectifier acts as a soft start as the rectifier tube has to warm up to allow power to everything else. 500v is a little high for a 7027 , 400v is more like it I like the sound of 425-450 EL34 is a little more sturdy tube for higher plate V. 6L6/6V6(my fave)/6CA7 and they like 470 ohm grid resistor . KT66/6066/EL34 likes 1K grid resistor. hint being Fender like sound or marshall like sound. Groove Tube has a book with insight and Schematics for every popular amp ever built with a bias current spec for all o/p tubes at every voltage(second hint high bias low tube life great sound catastrophic failure when tubes die)

Regards, Elwood
"Lead me not into temptation...I can find it myself."

Last edited by eyoung; 10th April 2011 at 04:34 AM.
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Old 13th April 2011, 06:50 PM   #3
amptech is offline amptech  Scotland
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Location: Glasgow
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try this page...valve wizard...how to design valve amplifiers...

he will 'show steps from psu build to pre/power builds... it is best to read has much as poss before taking on the task as getting things wrong will be costly..
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