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Old 10th July 2013, 02:02 AM   #1
JamesG is offline JamesG  United States
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Default New design with 1KV power transformer

After a long pause, I have decided to build a new 50W guitar amp using some existing parts I have. The power transformer I have appears to be 500-0-500 (1 kilovolt!). I intend to use two 6550s in PP class AB1. Obviously, I dont intend to run these any higher than 600V B+. So I am not sure of the approach to take for the power supply design. One possibility is to use a regulated supply using a pass tube or two and simply dial in the voltage I need. I have built several such supplies based on designs originating from Stephie Bench's old web site, so I know how to do that. The problem is that, to bring the voltage down 3-400V with a current draw of 200mA means a monster amount of dissipation in the pass tube(s). Any suggestions on how to deal with this would be welcome. I could just use another transformer, but I want to see if I can solve this problem first.
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Old 10th July 2013, 03:15 AM   #2
JMFahey is offline JMFahey  Argentina
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You can't use that transformer for any reasonable 50W amp.
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Old 10th July 2013, 04:35 AM   #3
jcx is online now jcx  United States
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if it has "universal" pri you could wire it for 240, run it from 120
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Old 12th July 2013, 01:24 AM   #4
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Best to get a proper power transformer.

600v is more than the typical 6550 can handle, certainly the new production tubes. Even the older tubes will be sweating a lot.

Consider a pair of 807s or 1625s with that plate voltage - but then you will have trouble finding proper output transformers.

No free lunch.

_-_-bear -- Btw, I don't actually know anything, FYI -- every once in a while I say something that makes sense... ]
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Old 12th July 2013, 03:08 AM   #5
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pair of KT120 maybe?
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Old 12th July 2013, 10:06 AM   #6
AndrewT is offline AndrewT  Scotland
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Choke input supply instead of capacitor input supply drops the PSU voltage very significantly. i.e DCV = ~0.9*Vac (540Vdc) vs DCV = ~1.4*Vac (850Vdc).
Both are very high.

Rewinding or a different transformer seems to be more useful.
regards Andrew T.
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Old 12th July 2013, 10:08 AM   #7
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a pair of 4D32 in push pull will suit that traffo...
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Old 12th July 2013, 10:29 AM   #8
DF96 is offline DF96  England
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Run it as a CT secondary to a choke input PSU. Should give you around 450V or a bit less under load. Don't mess with 1kV supplies unless you really know what you are doing.
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Old 12th July 2013, 10:54 AM   #9
Elvee is offline Elvee  Belgium
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If you use a dual half-wave rectifier, you'll be able to significantly drop the voltage without excessive losses, and you will get the same benefits as a full wave choke input supply: low internal resistance and good noise rejection.

The main drawback is the necessity to use two separate chokes (and two additional diodes) but that is not an excessive price to be paid in regard of the advantages
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Old 12th July 2013, 07:47 PM   #10
DF96 is offline DF96  England
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I have not seen that circuit before. Am I right that it gives half the DC voltage of the conventional full-wave choke input PSU?
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