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In Reply to: RE: Is this feasible? Rectifying 6.3v for heaters posted by coffee-phil on December 13, 2017 at 21:41:54
Phil - This is excellent work you have done here. I am impressed. I think I understand that using a FWB for Vin and a half wave rectifier for the Vcontrol has that pin higher in voltage by one diode drop.
I don't get "three 12AX7s in series". Is that three dual heaters? And wouldn't that add up to 36v? Please send the next diagram, I am learning so much here. This is audio archeology, the original designers are pretty much dead now, so we have to carry it on.
Follow Ups:
Hi Pete,
Above is the OEM power supply of the Citation 4. Note that the junction of V1 and V4 is grounded. After adding my heater ballance circuit I did likewise to the other heater pairs so now all the heaters have virtually the same voltage.
By the way the rectifier feeding the Vcontrol pin is also a full wave bridge. The bottom two diodes are shared with the full wave bridge feeding the input of the regulator. The peak voltage at each pin is very near identical however the ripple at the Vcontrol is very low as the current is simply the overhead of the chip which is mainly the base drive of the pass transistor at the output. At the bottom of the ripple at the IN pin the Voltage at the control pin is still high enough to keep the chip active.
I will scan the heater ballance circuit and will post it as well.
Phil
Phil - I am surprised they used a half-wave for the main HV supply. Never saw that before. Price savings?
Hi Pete,
That was a selenium rectifier as was the heater rectifier. Selenium rectifiers have a reverse breakdown voltage of several tens of volts so high voltage rectifiers took a big stack. For a full wave bridge it took four stacks. That got sizey and expensive. For the low current required they got by. In my update I went to a silicon bridge. The schematic for that will follow. Soon hopefully.
Oddly my Citation 5 Power Amp which was contemporary with the Citation 4 used two silicon diodes in a full wave doubler configuration for B+ supply. That was about the time silicon diodes were becoming available so they were a bit spendy.
Phil
Hi Pete,
Above is the heater ballance circuit. Is it necessary? No, the OEM circuit worked fairly well actually. Is it silly? Probably so, but I wanted it and I could do it so I did.
The - input of the op-amp is referenced to ground. The average of the + and - supply is applied to the + input of the op-amp. If they differ either the 2N2219A or the 2N2905A as required will draw current such that the + and - voltages are symmetrical around ground.
I'm sleepy now but tomorrow I'll scan the updated B+ supply and post it.
I'll also add to the heater supply schemo to show how I did the new LED light for the off/on push button.
Phil
Clever circuit. A 741 huh! Old School! Out of 709s? lol
Unnecessary, but certainly accurate. I like the way HK used the pilot lamp as part of the filter.
If a 741 will do what you need, it is a "no drama" op-amp. Cheap as well.
Phil
Phil, I was just joking here, sometimes you don't need anything fancy to do a simple job. A 741 is fine except at high gains where they tend to be noisy.-Pete
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
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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