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Old 14th December 2010, 09:57 PM   #1
JWK is offline JWK  United States
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Default Power Transformer - Can you use separate tap for screens?

I'm thinking on how I can make the most improvement with the easiest method for my Eico ST-70. The schematic shows 440v to the anodes with a 1800ohm dropping resistor to the screens (no choke)that gives 390v. The amp uses a 5AR4 rectifier.

I was wondering how I could make significant improvements without changing design much. I like this amp and am planning to replace a lot of components and tubes.

I have a thought for the power supply that I would like to throw out here. Weber has a power transformer for a Fender Twin rated for 400mA. It has a 320-0-320 and also tapped for 300-0-300. Is it possible to use a SS rectification on the 320-0-320 for the plates and then use the 5AR4 rectification on the 300-0-300 for the screens and preamp section? This would give me voltage very, very close to original specs and do it with a transformer that is made for my home's voltage (which is usually right around 125AC). I could use a CLC after the tube and have the choke be more around 100 ohms (maybe less, haven't checked) instead of 1800 ohm resistor. Much better filtering and much better impedance, correct?

Then again maybe you just can't do this with a power transformer. Thanks for any help.
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Old 14th December 2010, 10:14 PM   #2
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Just use large H value choke on screen supply.
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Old 15th December 2010, 12:08 AM   #3
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Then again maybe you just can't do this with a power transformer. Thanks for any help
You can do this, just remember that the total current drain off of both taps must be less than the rating for that specified for one tap. I have done this, and I have also run choke input filter using a 5AR4 and cap input using SS diodes off of the same taps to get 380 volts and 480 volts from an Allied 6K7VG.
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Old 15th December 2010, 12:26 AM   #4
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Interesting info. I have often wondered about this myself. Thought it might be a wat to get more stable screen voltage without a lot of futzing around with regulators.
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Old 15th December 2010, 11:59 AM   #5
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Just the possibility of running excessive screen current until the main HT comes up.
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Old 15th December 2010, 12:09 PM   #6
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Be careful about the protection! If the anodes (plates) run from a different circuit, please take account of what happens if the fuse blows in the anode circuit (main B+).

If the screens are left powered when the plates are unpowered, huge current will flow into the screens, and destroy your endpentodes - unless precautions are taken, eg interlocks, or current limit in the screen supply, or low value fuses.

However, if you work it all out, it is a good solution. I have done this to 1960s LANEY amps, where the B+ is 650V for EL34s. No way am I applying THAT to modern production pentodes. Luckily the Laneys have well-rated Partridge chokes, so I just add another pair of diodes (FW connexion, 2kV PIV diodes), an make a choke-input section for the screens, which pops out at ~400V. Perfect! high voltage on the anodes (for best sound), and better regulation for the screens. An individual F (flink/fast) fuse for each screen is the safest bet - its value chosen to limit the screen dissipation within the ratings, using the voltage you end up with.
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Old 15th December 2010, 01:23 PM   #7
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Quote:
Just the possibility of running excessive screen current until the main HT comes up.
The thread starter wanted to use SS diodes for plates and tube rectifier for plates, plate voltage will be first but current will be near zero until screens come up.

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If the screens are left powered when the plates are unpowered, huge current will flow into the screens, and destroy your endpentodes
This is a valid concern if there is a fuse in the plate circuit which is not too common. If you want to fuse the output circuits, place the fuse in the cathode. Be sure that the cathode bypass cap (if used) does not go around the fuse, or it will explode when the fuse blows. Other valid placements are in the B+ or the CT of the HV secondary on the power transformer.
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Old 15th December 2010, 04:23 PM   #8
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Quote:
Originally Posted by JWK View Post
The schematic shows 440v to the anodes with a 1800ohm dropping resistor to the screens (no choke)that gives 390v.
Click the image to open in full size.

Quote:
Originally Posted by JWK View Post
I was wondering how I could make significant improvements without changing design much. I like this amp and am planning to replace a lot of components and tubes.

I have a thought for the power supply that I would like to throw out here. Weber has a power transformer for a Fender Twin rated for 400mA. It has a 320-0-320 and also tapped for 300-0-300. Is it possible to use a SS rectification on the 320-0-320 for the plates and then use the 5AR4 rectification on the 300-0-300 for the screens and preamp section? This would give me voltage very, very close to original specs and do it with a transformer that is made for my home's voltage (which is usually right around 125AC). I could use a CLC after the tube and have the choke be more around 100 ohms (maybe less, haven't checked) instead of 1800 ohm resistor. Much better filtering and much better impedance, correct?

Then again maybe you just can't do this with a power transformer. Thanks for any help.
If you've already decided to replace a lot of components and tubes, then the solution is a good simpler than what you propose: active screen voltage regulation. You have a 50Vdc difference between supplies, and so any solid state regulator is going to work just fine here. You won't even need to make more holes in the chassis, or find some extra heater power.

Even if you're one of those "sand-o-phobes", doanworryboudit. SS active regulation will sound better than that simple series dropping resistor. That just says: "Cheap", and is highly detrimental to sonic performance.
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Old 15th December 2010, 04:58 PM   #9
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Quote:
Originally Posted by tubelab.com View Post
This is a valid concern if there is a fuse in the plate circuit which is not too common. If you want to fuse the output circuits, place the fuse in the cathode. Be sure that the cathode bypass cap (if used) does not go around the fuse, or it will explode when the fuse blows. Other valid placements are in the B+ or the CT of the HV secondary on the power transformer.
Hey George, I know you like fireworks, but you don't advocate building a B+ without overcurrent protection, surely. Not with SS rectifiers, anyway.

I looked at a Trace Elliott 200W Bass amp that blew a KT88 recently. This amp has separate supplies for the screen and anode, and (presumably because of the risk to the screens) the B+ fuse is rated at T2A, and does not blow, usually. This amp blew the mains fuse first, and by then, the primary winding of the OT was fried. This was a high cost repair!

Fuse in the cathode circuit might have helped here, but really you need them in each valve's cathode, or else the heater insulation sees the whole B+ on a fuse blow. The good valves get stressed along with the bad where a common fuse is used. Beware also that a cathode fuse may permit some B+ out of G1 during an internal short, too, so precautions against the grid leak resistor or driver MOSFET setting fire to the amp should be taken.

For a 50V dropper, I think Miles's suggestion is good. I've used these reliably too, on guitar amps. Fairchild FQPF3N80C etc, wired as gyrator/cap multiplier is fine, and good sounding.

This one is in a 100W 4x EL34:
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Old 15th December 2010, 09:15 PM   #10
JWK is offline JWK  United States
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Originally Posted by toprepairman View Post
Just the possibility of running excessive screen current until the main HT comes up.
How do you get excessive screen current while the 5AR4 is warming up? I would think current would not flow much if any before the B+ comes up on the screens.

edit: Oh, I see George already addressed this. Sorry. I'm very much in the beginner's stage.

Last edited by JWK; 15th December 2010 at 09:21 PM.
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