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In Reply to: RE: Is this feasible? Rectifying 6.3v for heaters posted by Audio Pete on December 14, 2017 at 07:40:44
Hi Pete,
Thanks.
The Citation 4 uses 6 12AX7s, each of which has the heater sections in series for 12.6 volts. They are arranged with 2 tubes in series for 25.2 volts and three groups of two in parallel for 25.6 volts @ ~ 450 ma.
I'll try to scan the OEM circuit, as well as my "balancing" circuit.
If you are interested, I'll also scan the circuit to power the LED pilot light and the B+ regulator as well.
Phil
Follow Ups:
Please send it all! It's making more sense now although it's an uncommon approach. - Pete
Hi Pete,
Here is the schematic of my updated B+ supply in my HK Citation 4.
The rectifier is now a full wave silicon bridge which feeds the pass transistor which is an N-channel power MOSFET. The heart of the regulator is the TI TL431 shunt regulator which contains the reference. The regulator is coupled to the pass transistor via an optocoupler (4N27). The regulator chip and optocoupler are powered from the heater supply.
The filter network of the OEM PS is retained and voltage from the new regulated power supply is adjusted to match the value on the schematic of the OEM supply. With a regulated supply the OEM filter is over kill but I wanted to preserve the essence of the Citation 4.
This regulated power supply is not from some frivolous desire to do it because I could. This preamp fed my Citation 5 for my main speakers but also my subwoofer (Electro Voice 30W) via a MOSFET amp which I built. Every time the fridge came on or off my subwoofer system went nuts. The regulator solved the fridge / subwoofer issue.
The remaining mod is that the ON/OFF button is now illuminated by an embedded green LED. When I was using the grunt three terminal regulator for the heater supply I lit the LED in a manner similar to the OEM by putting it and it's ballasting resistor across the regulator. That worked but the LED would vary in brightness greatly with line voltage. I was not going to have that so it now is powered from the heater winding of the power transformer ballasted with capacitors and full wave rectified. I will add that to the heater supply schemo and scan the updated version.
Phil
Phil -= What does, "Every
time the fridge came on or off my subwoofer system went nuts. " mean?
Nuts is a vague term. - Pete
As promised here is the revised schemo for the Citation 4 regulated heater supply including the circuit for the LED which is now embedded in the plastic ON/OFF push button.
The LED is ballasted by the two 3 microfarad caps feeding the bridge. In addition to the caps being non-dissipative, the load on the transformer comes ~ 90 degrees out of phase with the heater load so it does not load the transformer when it is the supplying the peak current to the heaters.
I know this is a Tube DIY forum, but for completeness I'll look for the schemo of the MOSFET subwoofer amp which drove the need to regulate the supplies in the Citation 4.
Phil
I like the way you derived the AC for the LED supply. No resistor in series, does the circuit produce just 2v? I just had to do the same thing to derive a Bias supply for an experimental amp (which didn't work out due to low B+). Couldn't you have used just one 3u coupling cap?
Hi Pete,The capacitors provide the reactance to "ballast" the current through the LED (in this case ~ 6mA). The voltage drop across the LED is ~ 2 volts at that current. Yes 1 cap could have been used but for the same current it would be 1.5 microfarad. I chose to use two as the voltage one side of the transformer is going positive as the other side is going negative. This is an effort to keep AC on the leads to the LED at a minimum.
I have attached a picture of the schematic of the subwoofer amp which forced this regulation of the Citation 4 supplies as protection against the fridge. Sorry the schemo is beyond rough. It was the schemo I drew as I was designing the amp and got scribbled on during the refining of the circuit. I really need to draw a clean copy which is more easily understandable. I hadn't looked at it for about a decade and I had to struggle and I designed it.
The output transformer is a repurposed TV power transformer. I believe it came from a late 40s Zenith TV. I did not scrap the TV but found the transformer surplus. The push-pull primary is what formerly fed the 5U4 rectifiers. The source winding is made of a center-tapped 12.6 volt winding to which a 5 volt winding is added on each side. That output circuit is inspired (stolen) from Mac tube amps. The speaker winding is what was the 120 volt primary.
I don't know how to attach more than one picture so I'll put the protection circuit in another post.
Phil
Edits: 12/21/17
Hi Pete,
This is the protection circuit. Basically it is a low pass filter, full wave rectifier, and comparator. With excessive low frequency content the relay opens removing the input to the amp. It did solve the fuse blowing issue but it was still annoying. After regulating the Citation 4 supplies the protection rarely got invoked.
I suppose a sensible person would have simply moved the 3.2 Hz zero at the input of the subwoofer amp to ~ 15 Hz, but a lunatic with a 30" woofer would not hear of that.
Phil
Hi Pete,This is a bit of a long story, but here goes.
The Citation 4, Citation 5, and the Electro Voice 30W subwoofer were part of the audio system in my old house and are to be re-united it the system in my stereo room under construction in my present house. The 30W was initially driven by a solid-state amp of conventional (for solid state amp) design. The amp screwed up and put DC on the voice coil of the 30 W. NOT GOOD! At that time fortunately Electro Voice still supported the speaker and did recone it. It was not cheap as it cost me $500 for the job. Ouch! The stereo store (Century Music In San Jose CA) where I bought the 30 W handled the crating and shipping for the 30W. The sales man at the time said "you know, when this speaker was designed amps had tubes". He said he also had a 30W and he was also bi-amping it but he was using an old Mac tube tube amp for it while driving his main speakers with a sand amp. I thought that what he was suggesting was backwards from what most folks do, but it made me think that driving a spendy speaker like the 30W with a non transformer coupled amp is probably not prudent.
A vacuum tube amp which would be flat down to below 15 Hz would be spendy. Since I didn't care about response over several hundred Hz I designed a transformer coupled push-pull amp around a big TV power transformer. This transformer was from a old energy decadent TV and was @ 500 VA. That means it has lots of iron and can go easily down to 10 Hz at @ 40 Watts. That may not seem like much but the 30W is ~ 105 dB / Watt so not a lot of power is needed. The amp uses CMOS power transistors in the push-pull output stage.
All was fine with this amp until the fridge did its thing. All that iron in the transformer is sufficient for 10 Hz but not for 2-3 Hz. When the fridge did its thing The core saturated and popped the B+ fuse. I added a protection circuit which would shut down the amp when there was much low frequency content at its input. That stopped the fuse blowing but the sub woofer amp clicking on and off when the fridge did its thing was annoying.
I was not about to part with the Citation 4 so I regulated the B+ supply. With the new Citation 4 power supply, The fridge and my 30W could co-exist.
I still have not updated the heater supply schematic to show the new pilot light circuit. Hopefully I'll get on that tonight.
Phil
Edits: 12/21/17 12/21/17
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