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Old 13th October 2010, 03:39 PM   #11
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Quote:
If you build a 5AR4 rectified cap input supply out of a 600VCT power transformer (2x300) and then draw 240mA from it, you will end up with a B+ somewhere around 360VDC.
I am using a Hammond 372JX feeding a Simple P-P board. (5AR4 rectifier 2 X 12AT7 and 4 X EL84). My B+ is 357 volts. Good quality EL84's (I use JJ's) don't have a problem with this voltage on plates and screens but the cheap stuff sold on Ebay glows brightly! I am using cathode bias (12 volts) so the tubes see only 340 volts or so.

Hammond power transformers always give you a few extra volts so the 300-0-300 volt transformers are really about 320-0-320.
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Old 13th October 2010, 03:54 PM   #12
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Originally Posted by malcolmfraser View Post
On the bias calcualtions isn't the max cathode bias for a EL84 65mA?
On the very first page of the GE datasheet for 6BQ5, they do indeed specify the Maximum Rating for DC Cathode Current as 65 mA. Just a few lines up above they also clearly indicate the maximum plate dissipation = 12 watts and continuous screen dissipation = 2 watts.

If you have 360 volts across the tube, you need to keep the idle current under 39 mA to stay within the 14 watt overall dissipation limit (360V x 0.039A = 14 watts).
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Old 13th October 2010, 03:59 PM   #13
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Hi!
Schematic is correct and will work without problems.
Do not know where you put voltage 470V ????....
Power transformer has a secondary and double alternation 2X300V~ rectificator tube.Tensione after rectificator-tube, continuous (DC) by filtering capacitors that if recovery is not solid-state (U ~ x1,41 = Udc), due to high internal resistance of the tube rectifier point B1 tension will be U ~ x1,3 approximately 390V at no-load consumption.
The task is likely to decrease voltage 360-350V (U B1). So the assembly will work very well.
As a recommendation, after you measure all voltages without the vacuum tube to be put on pedestals, it is desirable position to be half of Tr1 and Tr2's cursor to be to the end anode diode D1, which is the maximum negative voltage.
Regulating Tr2,the voltage measured on R30 or R31 corresponding to a current through the final tubes should not exceed the data from the catalog.
You must consider that there are many two tubes in parallel, so we measured the current split in half.
I hope I did understand.
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Old 13th October 2010, 04:07 PM   #14
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Default great advice, thanks

Wow love this Forum, thanks guys for all the great advice. So, final PSU design would look like:

The PSU spec is 360 volts for B+,
Seperate fixed bias supply that is adjustable (LM337),
A nice wad of 6.3 VDC for the heaters.
Throw in a softstart
And finally add a standby switch that drop the heater supply from 6.3 volts to 4 VDC (ala Morgan Jones's comprehensive PS) and we are good to go.

overall design concept attached, will now go away and draw up the PSU Schema, more to follow, very soon.
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File Type: jpg amp design idea.jpg (50.1 KB, 187 views)
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Old 13th October 2010, 04:56 PM   #15
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Quote:
Originally Posted by malcolmfraser View Post
Wow love this Forum, thanks guys for all the great advice. So, final PSU design would look like:

The PSU spec is 360 volts for B+,
Seperate fixed bias supply that is adjustable (LM337),
A nice wad of 6.3 VDC for the heaters.
Throw in a softstart
And finally add a standby switch that drop the heater supply from 6.3 volts to 4 VDC (ala Morgan Jones's comprehensive PS) and we are good to go.

overall design concept attached, will now go away and draw up the PSU Schema, more to follow, very soon.


Hi!
My guess is they do not justify feeding tubes with indirect heating in DC, were invented precisely this indirect heating tubes to be fed in AC (!!!)
DC heating is justified in only indirect heating tubes used in tube preamps for high-sensitivity microphone or gauges.
A neat and well planned assembly point will operate with zero noise due to AC powered filaments.
It's just my opinion!

Success in building the proposed project!
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Old 13th October 2010, 06:49 PM   #16
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Originally Posted by LAZAROIU View Post
Schematic is correct and will work without problems.

...the voltage measured on R30 or R31 corresponding to a current through the final tubes should not exceed the data from the catalog. You must consider that there are many two tubes in parallel, so we measured the current split in half.

The only component I see between ground and V3 cathode is the 10 ohm resistor R31. Similarly, the component from ground to V4 cathode is 10 ohm R30. I do acknowledge that V3 and V5 are tied together, so whatever current is measured through R31 is for both tubes. Likewise, V4 and V6 are tied together so they share the current through R30.

V=I*R. If you have 1130mV across 10 ohms, current must be 113mA. Assuming they share equally, the current across each tube would be 56.5mA.

P=I*V. With 360 volts across the tube you have 0.0565 amps times 360 volts which equals 20.34 watts. By all rights that will be a well done tube in a very short order. Is there something I am missing?

Quote:
R30,R31 = 10R/1W

Calibration Procedure:
6. Finally measure voltage between common ground and cathodes of V3 and V4 pentodes. Should be 1130 mV +/- 20mV. Repeat steps 3-5 if needed.
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Old 13th October 2010, 07:27 PM   #17
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The EL84's are rated at 12 watts plate dissipation. This circuit is pushing the tubes to near their maximum. I'd back-off on the B+ voltage a bit.
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Old 13th October 2010, 07:41 PM   #18
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Quote:
Originally Posted by Ty_Bower View Post
The only component I see between ground and V3 cathode is the 10 ohm resistor R31. Similarly, the component from ground to V4 cathode is 10 ohm R30. I do acknowledge that V3 and V5 are tied together, so whatever current is measured through R31 is for both tubes. Likewise, V4 and V6 are tied together so they share the current through R30.

V=I*R. If you have 1130mV across 10 ohms, current must be 113mA. Assuming they share equally, the current across each tube would be 56.5mA.

P=I*V. With 360 volts across the tube you have 0.0565 amps times 360 volts which equals 20.34 watts. By all rights that will be a well done tube in a very short order. Is there something I am missing?
Right!
V3 and V5 were parallel to the ground through R31 katod.
V4 and V6 were parallel to the ground through R30 katod.
You thought very well, but do not know where you pulled that current, because, as is done in the divider R37 and R38, R38 have a voltage on 100V ~, so rectifier diode D1 (half-wave) and the C10 will have a filtration a voltage above 60Vdc negativity, so we can choose any anodic current through the four EL84 tubes.
As you think is very fair, the anode power dissipation of a single lamp is too high.
Voltage setting is very broad negativity, so we can choose very accurately.
I said above that do not exceed the EL84 anode power dissipation given in the catalog.
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Old 13th October 2010, 09:17 PM   #19
rmyauck is offline rmyauck  Canada
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Hi,

I noticed first cap after rectifier is 100uF. Will kill older rect. and new ones right away . Should be 30uF for long life.

RCA SP-20 amp may be worth looking at as feedback setup is said to be better by others on this form and others. Should be able to use EL-84 tubes in it. It was recommended to swap 12AU7 for 6SN7 & 6AU6 for 6EJ7/EF184 for improved sonics. May need a preamp though! Has been said to sound great!

Good luck

Last edited by rmyauck; 13th October 2010 at 09:22 PM.
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Old 13th October 2010, 09:39 PM   #20
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Originally Posted by malcolmfraser View Post
Hi all, newbie here, with properly a silly question...

Looking at building the attached design, but with solid state PSU. If i have the calculations right B+ is at 470V, which is way above the 300 from the datasheet.

Is this right?, and just a function of the -20V fixed bias, quad EL84s and UL configuration?

Please help, confused ;-)
Hi!
I think it is interesting and schematic, which are much more time ... but it's in German!

http://www.jogis-roehrenbude.de/Vers...L84-30Watt.htm

Very good observation: Data Catalog for GZ34 say 60uF filtering capacitor is maximum at 300Vac.
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Last edited by LAZAROIU; 13th October 2010 at 09:57 PM.
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