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previous: Darren Well the "sort-of-pretend" 50W mars... -- 3/4/1998 8:39 PM

Re: Damn, not enough B+

3/5/1998 1:16 PM
Stephen	Re: Damn, not enough B+ Darren, I don't have time to work out the on-load voltages for this - it's probably quicker to try it and measure....I know it's not the proper kind of PS regulation for a guitar amp, but I think you could do a Choke-input voltage doubler PS, which should give you a useable B+ for a 50W amp. Depends on how big your present choke is. You'll have the 6L6 plate currents going through the choke as well, so it will need 150ma=plus continuous rating. And you'll may need a bleeder resistor from B+ to 0 V, if the amp's idle current isn't enough to pull the choke-input PS down into its (nearly) constant voltage region. This minimum current value depends on the inductance of the choke - although a 'swinging' choke may make things a bit simpler. All the tech for this is in RDH4. There are basic choke-input graphs in many tube data books, but not usally for swinging chokes, or voltage doublers. You'll have to try what you've got and see what it measures. You could probably use a regular small-current (screen and preamp) choke to see if you can get a suitable B+ - just don't try running the amp at more than 1/4 power. If this works, you might need to buy a bigger choke, but these can sometimes be found at hamfests and surplus stores. ( Do you have hamfests in Oz? If there is one in NZ, I have never managed to trace it. I am very envious of these guys in the US who can just pick up an old transmitter chassis or Tectronics scope for a few dollars) Even though this is a choke-input PS, it would be wise to hang a small cap (say 0.5uF/1kV) from the _input_ side of the choke, to ground...or put a r-c snubber across the choke...or both, to minimise transient voltage spikes across the choke. There is also a kind of voltage doubler where the output voltage can be adjusted a bit up and down by varying the size of one electrolytic cap. It has the snag that both + and - of this cap are at a fairly high voltage above chassis, so you would have to put the cap in a reliable insulating sleeve, and where fingers can't touch the outer can. I'll dig out the diagram and post it if you want. These are just _some_ possibilities. I'm sure the other guys will have some other ideas as well. - Stephen

3/5/1998 1:16 PM

Stephen

Re: Damn, not enough B+

Darren,
I don't have time to work out the on-load voltages for this - it's probably quicker to try it and measure....I know it's not the proper kind of PS regulation for a guitar amp, but I think you could do a Choke-input voltage doubler PS, which should give you a useable B+ for a 50W amp. Depends on how big your present choke is. You'll have the
6L6 plate currents going through the choke as well, so it will need 150ma=plus continuous rating. And you'll may need a bleeder resistor from B+ to 0 V, if the amp's idle current isn't enough to pull the choke-input PS down into its (nearly) constant voltage region. This minimum current value depends on the inductance of the choke - although a 'swinging' choke may make things a bit simpler. All the tech for this is in RDH4. There are basic choke-input graphs in many tube data books, but not usally for swinging chokes, or voltage doublers. You'll have to try what you've got and see what it measures. You could probably use a regular small-current (screen and preamp) choke to see if you can get a suitable B+ - just don't try running the amp at more than 1/4 power.
If this works, you might need to buy a bigger choke, but these can sometimes be found at hamfests and surplus stores.
( Do you have hamfests in Oz? If there is one in NZ, I have never managed to trace it. I am very envious of these guys in the US who can just pick up an old transmitter chassis or Tectronics scope
for a few dollars)
Even though this is a choke-input PS, it would be wise to hang a small cap (say 0.5uF/1kV) from the
_input_ side of the choke, to ground...or put a r-c snubber across the choke...or both, to minimise transient voltage spikes across the choke.
There is also a kind of voltage doubler where the output voltage can be adjusted a bit up and down by varying the size of one electrolytic cap.
It has the snag that both + and - of this cap are at a fairly high voltage above chassis, so you would have to put the cap in a reliable insulating sleeve, and where fingers can't touch the outer can. I'll dig out the diagram and post it if you want.
These are just _some_ possibilities. I'm sure the other guys will have some other ideas as well. - Stephen

Replies:

Darren Stephen, Thankyou for an obv... -- 3/5/1998 7:57 PM