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

Hi,

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

http://www.unofficialampeg.com/schematics/b-25.gif

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:

http://www.lundahl.se/pdfs/datash/1620_3_7_9202.pdf
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:

http://www.drtube.com/datasheets/7027a-rca1959.pdf

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.

Regards

Jamie
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Old 10th April 2011, 04:19 AM   #2
eyoung is offline eyoung  United States
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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
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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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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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