Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: scstill on December 14, 2022, 12:37:23 pm
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Could the orientation of a choke be affected by the PT?
In the diagram below you can see how the choke and PT are oriented (both in upper right). Lower left is OT
I think their RF "planes" are different, but I'm sure how the PT is wound since it is covered
I think it is safe but wanted a second or third opinion.
Also, This choke was out of a car radio and was connected to the HV CT which I beleive would be a B- voltage used to screens of the RF and IF radio tubes. Was told that this choke might not be designed to see the B+ voltage.
Planning to have this choke after the reservoir Cap, but could it be placed in front of the 1st cap? Maybe for filtering the B+ to the Output tube plate? Currently the Output tube plate voltage is in front of the choke (as the radio was designed)
Is there a way to measure the Henries of the choke?
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Choke may be fine where it is but I'd rotate it 90°. I would also rather see it near rectifier tube.
I suggest wiring the choke and an extra cap as shown in my drawing. This will greatly reduce hum in the output tube.
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Here is a 90deg rotate near rectifier. assuming this is what was meant
This choke was originally installed above the radio PT in this same location
It has a convenient tab to bend for mounting to amp PT
Is there any trick (besides smoke) to determine the ratings of a choke? Voltage, current and inductance?
Rectifier output about 320vdc @ <60ma
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Measuring power choke inductances - Electrical Engineering News and Products (eeworldonline.com) (https://www.eeworldonline.com/measuring-power-choke-inductances-faq/)
:icon_biggrin:
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Choke may be fine where it is but I'd rotate it 90°. I would also rather see it near rectifier tube.
I suggest wiring the choke and an extra cap as shown in my drawing. This will greatly reduce hum in the output tube.
What if I got rid of the Cap in front of the choke?
That way I should be able to increase the value of the cap after the choke
The issue is that the 84 has a max first cap spec of 4uf, which seems pretty small.
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What if I got rid of the Cap in front of the choke?
Then you will have a choke input filter and your B+ will drop significantly.
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When you have limitation on the value of the first filter cap due to the rectifier spec remember that the limitation stops at the first cap, the second cap (choke or resistor between them) can be of a larger value without affect the rectifier spec
Franco
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I have also heard (maybe here) that if you add a small resistor in front of 1st cap you can increase cap value.
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The issue is that the 84 has a max first cap spec of 4uf, which seems pretty small.
Where did you see that info?
http://oldradio.qrz.ru/tubes/foreign/01/84.gif
http://frank.yueksel.org/sheets/127/8/84.pdf
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https://frank.pocnet.net/sheets/021/8/84.pdf
and that is what the Philco 926 radio used (Schematic attached) Part #64
The last figure in http://frank.yueksel.org/sheets/127/8/84.pdf also shows condenser input C=4uf for full wave. Are we misinterpreting this?
Was the discussion in another forum thread as well....
https://www.antiqueradios.com/forums/viewtopic.php?f=19&t=337749
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by '55 the tech was much better :laugh:
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When that radio rolled off the production line a 4µF or 8µF cap was very typical. There were no 20µF or 50µF caps available, at least at consumer level pricing.
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Thanks,
even though I have some 4uf's laying around,
I think I will go with a 16 or 22 as a first cap then.
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Measuring mains frequency power supply chokes that handle DC current can be not too difficult. Link below to one method plus some background. The link provided by shooter is not appropriate imho, as it is aimed at switchmode power supply choke testing.
https://dalmura.com.au/static/Choke%20measurement.pdf (https://dalmura.com.au/static/Choke%20measurement.pdf)
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most of my links are an example of how to surf the web for information, the show a man to fish thing :icon_biggrin:
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Thanks for great advice.
I have already mounted the choke
I fabricated a bracket out of a mending plate and patina-ed it with flat black
to me it Looks pretty original and sits on top the 120vac transformer in its original radio position
Hopefully whatever it's henry value will be good for this amp.
The antique radio guys were worried that since it was mounted on the B- HV center tap that it might not be insulated for the new higher B+ voltage. Guess we'll see at startup.
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Here is the choke location on the speaker/tube side of the build
Its a single ended, switched two channel, using the original radio tubes
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Here is circuit side of the build showing the Power Supply layout with 22uf
The choke is the white and black wires coming from the tube side.
Still trying to decide plate resistors.
Thinking of using 100k for all of them.
Tubes are 78, 75, 6A7;
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Missing grid-leak resistor.
Missing screen bypass cap.
Plate-mixing the two stages before the power tube will overload real easy, perhaps too easy.
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Missing grid-leak resistor.
Missing screen bypass cap.
Plate-mixing the two stages before the power tube will overload real easy, perhaps too easy.
Thanks,
I added the grid leak resistors at 1M for now.
I added screen bypass at 40nf for now.
I also added grid stops. But need some advice...
When the tube sheet is not clear about internal cap and gain are there any rule of thumbs?
In this case for the #78 I used 3.8pf and assumed 1.5pf for "pins" from ValveWizard. http://www.valvewizard.co.uk/gridstopper.html Then not having a real gain I assumed 100 (like the 12ax7; BTW why does Valve wizard use 60 for ECC83 calc, should be 100 right?). this gives a C = (5.4pf + 5.4pf*(100+1) = 600pf. For a cutoff of 20khz I calc Rgs = 1/(2*pi*20k*600pf) = 13k. If this is correct can it be used for #78 as triode and pentode?? BTW2 - thinking of replacing the remote cutoff 78 with a sharp cutoff 77...
To avoid combining two stages and overwhelming the power tube would a solution be to add a voltage divider? As shown with the two 100ks? Seems that this might affect the single channel input gain. I looked high and low (reason for delay response) and could not find a design where two channels fed a SE tube. I have found and built designs where two pre-amp tubes were combined and feeding a Phase Inverter with volume/tone control after the mix. I suppose I could just avoid combining the two but I wanted to replicate the effect of patching two channels
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Don't over-think it. You constrained yourself with tube and chassis choice. Try it, try something else. IME, one driver can about drive one power tube, so making it drag the output of another driver is liable to underwhelm (not overwhelm) the power tube. Maybe especially the '41/6K6 moreso than the 6V6.
But hey, this was not going raise the roof any place larger than a chicken shed. Even making a fraction of its potential it may be a great at-home or quiet-cafe amp.