Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: llama on June 11, 2013, 09:53:08 am
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I've gone over it again and again. But keep getting the same result.
I'm using a re-purposed PT from and old signal generator. The original schematic shows it as 1 265-0-265 VAC CT. I measure closer to 380Vac, due to the higher line voltage verses the olden days (115Vac).
So far I'm OK. But when I add a simple full wave rec (dual IN4007) the resulting DC voltage is around 350vDC. I was expecting over 425VDC. :BangHead:
i'm using the old chassis as well, and left the primary wiring intact, except for adding a three-prong wall plug and added the ground wire to the chassis ground.
Can someone shed some light on this for my dim view.
The funny thing is that I'm looking for 350VDC so if this is ok I'm happy to move forward on my 20 watt 6V6 PP amp
Attached is a schematic image.
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Hey Buddy,,,
You've gotta put a cap after your 2 diodes to see the actual rectified voltage
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The original schematic shows it as 1 265-0-265 VAC CT. I measure closer to 380Vac, due to the higher line voltage verses the olden days (115Vac).
That's a 43% increase! Line voltage has only increased about 10%.
But when I add a simple full wave rec (dual IN4007) the resulting DC voltage is around 350vDC.
SG nailed it. Put a filter cap across that 4.5MΩ resistor.
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I knew it was something simple... :dontknow:
Thanks guys... I'll try that tonight.
As far as 380 vs 265Vac - could that be an issue?
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As far as 380 vs 265Vac - could that be an issue?
Could be. But maybe that 265-0-265 was specified or measured when the PT was loaded. If you also have filament windings on that PT, what voltage do you measure on them?
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Could be. But maybe that 265-0-265 was specified or measured when the PT was loaded. If you also have filament windings on that PT, what voltage do you measure on them?
Hi Steve. :worthy1:
I'll check them out tonight, and get back thanks again!
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> and old signal generator.
> move forward on my 20 watt 6V6 PP amp
That must be a mighty beefy signal generator to be a base for a 20W amp. Is it really as big as the iron for a DeLuxe?
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But maybe that 265-0-265 was specified or measured when the PT was loaded. If you also have filament windings on that PT, what voltage do you measure on them?
OK I added a high voltage cap to ground as well, and there was 465VDC as expected. The heaters were 7.0Vac and 5.6 Vac. The Secondaries now read 374Vdc with a 40k resistor and the cap as load.
I introduced a 10H choke between the ss rec and the load. The voltage at the load dropped to 390vDC. With A 5Y3 REC i suppose it will put me in range of my goal.
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> and old signal generator.
> move forward on my 20 watt 6V6 PP amp
That must be a mighty beefy signal generator to be a base for a 20W amp. Is it really as big as the iron for a DeLuxe?
Hi PRR, The Sig Gen circuit had two 6v6's and few other octal base tubes and a GZ34 rec. Based on the heater currents of the orig circuit vs the AX84 October circuit (new build), I should be OK with the PT.
as far as Current rating on the HT secondary, I don't know, but it's slightly larger that the ClassicTone 20watt deluxe PT here : http://www.classictone.net/40-18021.pdf (http://www.classictone.net/40-18021.pdf)
I hope I covered my bases.
On a different subject, can i use two 6L6 tubes with the same circuit (just because), or is that asking for trouble? at around 360V will they pull too much?
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On a different subject, can i use two 6L6 tubes with the same circuit (just because), or is that asking for trouble? at around 360V will they pull too much?
It will add an additional 900 ma of heater current to your 6.3VAC heater winding. That would be my primary concern.
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On a different subject, can i use two 6L6 tubes with the same circuit (just because), or is that asking for trouble? at around 360V will they pull too much?
It will add an additional 900 ma of heater current to your 6.3VAC heater winding. That would be my primary concern.
An excellent reason. Thank you!
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Hi PRR, The Sig Gen circuit had two 6v6's and few other octal base tubes and a GZ34 rec. Based on the heater currents of the orig circuit vs the AX84 October circuit (new build), I should be OK with the PT.
as far as Current rating on the HT secondary, I don't know, but it's slightly larger that the ClassicTone 20watt deluxe PT here : http://www.classictone.net/40-18021.pdf (http://www.classictone.net/40-18021.pdf)
I hope I covered my bases.
On a different subject, can i use two 6L6 tubes with the same circuit (just because), or is that asking for trouble? at around 360V will they pull too much?
What was the brand and model of the signal generator?
I would not expect that a company building those would build in excess current capacity, so using 6L6's seem like pushing things. However, depending on the specifics of the original circuit, there may be sufficient current reserve; however to figure that I'd need to look at the original schematic, which is why I asked about the model of the original generator.
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Sorry Everyone...I mistyped the Transformer secondary voltage several times through out this thread. The actual secondary is 365V-0-365V...
My head knew it but my fingers were doing their own thing. So my measured voltage is close to the specified.
HotBluePlates told me that PT should live well as a Tweed Deluxe PT - Thanks HBP!!!! Since the build will be a 20Watt 6V6 amp. I plan on moving forward.
I better get back to re-planning my chassis layout...
Thank you to all of you who have read and followed my ramblings and mistakes...
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So you're tracking, the reason you had such a low measured output voltage from the rectifier with no filter cap is this:
- With no cap, there is nothing to be charged up to a steady d.c. voltage.
- The output of the rectifier without a cap is a series of half-wave positive pulses, in your case going from 0 to 380v*1.414 = 537v peak
- Because the voltage is continually changing (but never reverses polarity), your meter has a hard time tracking it and detects the average voltage. This should be (2*peak voltage)/Π, or about peak voltage * 0.637.
- Actual measured voltage may differ a little bit, depending on meter accuracy and how clean the wall voltage sine wave is. You measured ~350v, which is 350/537= 65% of peak voltage, pretty dang close to theoretical average voltage output.
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Well, the Summer is over and I'm pouring more energy into this build again. After some more experimentation, and a few minor shocks (along my fingers of the same hand), I finally bought a Hammond 270Fx PT. It's a higher wattage brother of the one called for in the Schem (270-0-270). I also bought a 6k6 Primary OT (20w Hammond).
Schem can be seen here- http://www.ax84.com/static/october/AX84_October_Club_090519.pdf (http://www.ax84.com/static/october/AX84_October_Club_090519.pdf)
Now that I have enough current capacity on the PT(do I?), what tubes can i try in the amp if it is fixed biased(separate pots for each tube)/cathode biased)? The Hammond site lists a classic OT with 6k6 primary as being able to use 6v6, 6l6, 5881, 803, EL34 tubes. http://www.hammondmfg.com/1608.htm (http://www.hammondmfg.com/1608.htm)
Is this correct in my case? I'm not looking for more output but different textures.
I humbly ask for your expertise.
-Scot
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6k6 load resistance works well with fixed bias 6V6s where the plates are between about 390V - 430V, but it will still work with lower voltages
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... Schem can be seen here ...
Now that I have enough current capacity on the PT(do I?), what tubes can i try in the amp if it is fixed biased(separate pots for each tube)/cathode biased)? ...
When wired for 6.3vac operation, your 12A_7 tubes draw 300mA; you have 3 of them, so 0.9A for the small tubes.
Your Hammond 270FX (http://www.hammondmfg.com/pdf/EDB270FX.pdf) has a 5A 6.3v winding, so you still have 4.1A left for the output tubes. You could plug in anything from 6V6's to 6550's and be fine as far as lighting the tubes up.