Calling all vacuum tube pros:  A power supply question - Page 1
Forum section: electronics / projects

author: cvanc
author_rank: Frequent Contributor
timestamp: December 11, 2018, 04:01:43 pm
content: Hi all-Please take a look at the attached schematic. I give you (drum roll please) the power supply of the legendary McIntosh C22 preamplifier.This is a highly regarded classic piece from The Golden Era. One of these in good condition can fetch thousands of dollars. Except... I don't have one of those. I have one of the other ones. It's in really sad shape right now.I don't work on tube stuff every day and might be missing something basic here, but can someone explain the nuance of this power supply? I have a couple of specific questions but would love to hear any comments.The 6 filaments are all 12AX7s. The DC filament supply ties to the midpoint of 2 of the 12AX7s. These 2 tubes serve as the cathode followers at the output of the line stage. In other words they are the output tubes of the entire preamp.So... What's up with the plate supply connecting down to the filament supply? Is there some kind of plate/filament/ground offset relationship that is considered best practice going on here? I'd love to get clarity on that.I might be forced to build an entirely new (and perhaps external?) power supply to make this unit right. I want to understand what tricks are being played and make sure to emulate anything that needs to still exist for best performance. And by 'performance' I mean low noise and low hum.OK gang, let me have itThanks.

author: 001
author_rank: Super Contributor
timestamp: December 11, 2018, 04:15:10 pm
content: it is oldtimer trick to eliminate hum from heatersIt simply bias parasitic diode behind heater and catode

author: Benta
author_rank: Super Contributor
timestamp: December 11, 2018, 04:35:23 pm
content: Sneaky circuitWhat you have is a capacitive voltage doubler with D1, D2, C29 and C30, the last is split into several sections. D2 sets the DC reference level.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 11, 2018, 04:46:42 pm
content: No, it's not for heater bias, it's all common ground. That is a common addition though.Appears they just wanted to save ~18V by stacking the HV on top of it.Also, it's 18V filament supply, oddly enough, but that's fine.I'm not convinced that doubler is drawn correctly. It seems like it's missing a ground return.Tim

author: cvanc
author_rank: Frequent Contributor
timestamp: December 11, 2018, 08:18:26 pm
content: Quote from: T3sl4co1l on December 11, 2018, 04:46:42 pmI've heard of this and I wondered but do not know the rules of thumb that apply. Is the general idea to make the filament(s) more positive than the cathode(s)? Is this done for noise reasons, or something else?Quote from: T3sl4co1l on December 11, 2018, 04:46:42 pmI kept coming back to that! It's like the transformer order was already out the door when they decided they wanted a bit more B+. I also thought of more far out stuff, like maybe the ripple signals from the 2 supplies nulled each other out if blended together "just so".Quote from: T3sl4co1l on December 11, 2018, 04:46:42 pm12AX7 tubes have a center-tapped 12.6V filament. The drawing is a bit obscure if you don't know that, but it shows the 18V supply connecting to the CT of two of the tubes, and the overall arrangement of all the filaments is as 4 18V branches each of which goes to common.Quote from: T3sl4co1l on December 11, 2018, 04:46:42 pmI kept coming back to THAT as well. The voltage doubler part (drawn unlike any doubler I've ever seen before, not that I've seen that many) clearly AC couples to common via the filament filter caps, but it doesn't DC couple to common anywhere. That has to be a drawing error, right?Thanks everbody.

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 11, 2018, 08:39:41 pm
content: Your suspicion appears correct. In a classic doubler configuration, a ground symbol should have been attached to the junction of C30A,B,C.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 11, 2018, 10:49:21 pm
content: Quote from: cvanc on December 11, 2018, 08:18:26 pmYes-- consider that the heater is hot, much hotter than the cathode. It isn't coated with emissive material, but it still manages some emission. Biasing the heaters up just reduces this source of possible noise current.Since the heaters are filtered DC, it's not much of a noise source here, but it does help with AC heating.QuoteI mean because 6.3 or 12.6 is almost always used. I suppose they wanted to save a bit on heater current, which is fair given the capacitors needed otherwise, and probably it let them use 1A diodes or something.Tim

author: Globe Collector
author_rank: Contributor
timestamp: December 12, 2018, 01:39:07 am
content: Yes, there is definitely a mistake on the negative side of the doubler and those three capacitors, C30A, B and C go to ground OR the positive corner of the valve heater supply bridge. I'd be tipping the chassis over and checking that for real, I would not take the circuit's word for it, partucularly condidering I will probably need to modify its design but preserve its function.I assume the transformer died a horrid death because one of the doubler capacitors dried out and put some sort of nasty DC component of current through the secondary and saturated the core??If the transformer in this were "beyond recovery" I'd rip it out, along with the doubler. I would preserve the HT RC filters in the upper right hand corner though. These days why would you use a doubler anyway, it produces double the ripple of a full-wave bridge and needs really big filter capacitors. These days I'm sure a higher voltage secondary transformer is no issue like it possibly was back then prompting the use of the doubler in the first instance.I would also preserve the 18v, (actually more like 20v) heater supply and arrangement and just drive the bridge from a new and possibly separate transformer. There is a little heater-cathode insulation stuff going on here, particularly with the two valves in the centre with their pins 9 going back to the 18v, these have the highest heater voltage above ground and, as you said, they are the output cathode-follower buffers which would be expected to have a fairly high cathode voltage above ground....particularly if they are DC coupled to the previous stage...so the valves with the greatest quiescent DC cathode voltages above deck are also given the highest heater voltage above deck to minimize electrolysis of the zirconium dioxide insulation over the tungsten heater wires where it touches the inside of the nickel cathode tube.I have a book somewhere, written by Philips in the mid '50's about their "SQ" series industrial valves, valves like the "E80F" an industrialized-ruggedized version of the EF80 or 6BX6, it has a whole chapter devoted to cathode heater voltage differences. I have a vague recollection that the cathode can be up to 150v or so above the heater, but only 15v or sobelowthe heater because the ZrO2 sandwiched between tungsten and nickel acts as a sort of dry electrolyte electrolytic cell, so with one polarity, reduction of the ZrO2 to zirconium metal will be reduced, but with the other polarity it will be increased due to the dissimilar metals of tungsten and nickel on either side. In this regard peak AC voltages on the heater need to be taken into account, particularly in transformerless designs with high voltage heater valves all strung in series.Here the design is very conservative by using DC on the heaters and keeping the h-k differences as low as possible.So, preserve the heater supply as is. Build a new HT supply with a bridge rectifier and largish filter capacitor....you can then choose a transformer with sufficient secondary voltage that does not require stacking on top of the heater supply. Calculate the required voltage the HT supply has to reach by calculating the drop down to 310v across the first resister, R71 in the filter. If you don't know the current draw through this resistor "guesstimate" it from the (mains consumption minus valve heaters, not output to speakers) power rating of the amplifier and plug this into "P = V.I" So if it is 15w, then use 15 = 310 x I so I is about 50mA, then you can calculate the drop across that first resistor. V = I.R , V = 0.05.1500, or 75v! So your HT supply would need to be 385v and able to deliver 50mA if the guess of 15w is correct.I recall that the 12Ax7 series of dual triodes are 300mA heater current, with the exception of the 12BH7 which is 600mA I think. So the heaters are 12 x 0.3W for each dual triode valve, or 21.6W fir all six valves.

author: David Hess
author_rank: Super Contributor
timestamp: December 12, 2018, 04:24:34 am
content: Biasing the heater to 50 to 100 volts above the cathode also increases tube reliability.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 12, 2018, 12:45:05 pm
content: It's basically a lot of "woo" crap designed to bamboozle gullible "guitar amp" nutters.Back in the day, when there were thousands, nay, millions of pieces of tube equipment in service, suchstuff was not used.Heater supplies were just plain ac --- no DC bridges or hinky connections back to the HT line to be seen!The only things that had HT on their heaters were directly heated rectifiers, & that's because the heater & cathode were the same part.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2018, 04:47:15 pm
content: Quote from: vk6zgo on December 12, 2018, 12:45:05 pmWell, for what market sector?The cheap consumer crap is what's abundant, what everyone remembers; rarely if ever seen, are the pro audio systems of the day. (I haven't seen any myself, but I don't know if you have.)Even that's inconclusive, because they would've opted for PQ types: industrial, long-life, low noise (by design and testing). Making do with junk is probably somewhere between contemporary pro-sumer sector, and modern anything.Not to say there isn't "woo" here. Oh boy is there ever "woo" here!QuoteWell, various other special cases -- low Vhk cascodes and followers (6DJ8?), high voltage parts (indirectly heated rects were on separate circuits, as often as not, I want to say..??; damper diodes are rated for high peak voltage in one direction only; etc.). But by and large yep, 6.3V was usually grounded, and maybe you had a "HUM BAL" trimmer (either end to heater power; wiper to GND) that did SFA anyway.Tim

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: December 12, 2018, 04:51:10 pm
content: Quote from: vk6zgo on December 12, 2018, 12:45:05 pmBullshit. There was a cathode to heater max voltage rating and Tektronix certainly *did* connect some tube's heater's to a higher voltage.Unless you consider physics "woo".

author: cvanc
author_rank: Frequent Contributor
timestamp: December 13, 2018, 01:18:10 am
content: Here's a little update. Please see attached image.The voltage doubler *does* have a DC path to ground, at the junction of C30B/C/D as some of you have mentioned earlier. The drawing is confusing but it actually SHOWS this ground down at the filter caps for the filament supply. The final cap in the filament circuit is C30A, which is, of course, the 4th cap of the C30 quad cap, and of course all 4 caps are in one of those old style twistlock metal cans. So one mystery solved.I still wonder why the B+ doubler connects down to the filament voltage and basically 'sits on top of it'.Thanks for all the thoughts and suggestions, it's always great to kick stuff around with you folks here.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 13, 2018, 05:15:16 pm
content: I noticed that earlier, that C30 has its odd section out there, but didn't put it together that they've drawn it wrong but it's connected correctly anyway -- good old fashioned multi-part metal can caps!Tim

author: SeanB
author_rank: Super Contributor
timestamp: December 13, 2018, 06:49:46 pm
content: They wanted a little more HT supply out of the standard HT transformer, so simply biased the HT rail with the filament supply to get this. Probably a transformer that was both in stock, common to many models and thus low cost to them, but just not enough voltage to give the rated power. Without the added voltage most will have a B+ of around 275V unloaded, and I would guess the filament supply is a 24VAC winding, so they got around 35V of boost, giving the 310V HT rail they wanted after the RC smoothing, with only the use of a standard to them transformer, that gave 115VAC and 24VAC as secondary voltages.Cheaper to have the extra capacitors and diodes than to pay for the extra copper to wind a 250VAC winding onto a custom transformer as well. Using a RC filter also saved a very expensive LC filter on the HT rail, despite the resistor dissipating a lot of power, and the associated voltage loss, more than made up by the extra 35V supply to otherwise waste. LC filter would have under 5v loss with decent hum reduction, but the size and extra cost, over the 1N4007 diode and the extra 40uf 400V capacitor.

author: cvanc
author_rank: Frequent Contributor
timestamp: December 31, 2018, 04:42:23 pm
content: To give a little closure here, this repair has been set aside while the search for a suitable replacement power transformer continues. (My theory on why the transformer failed? The original selenium rectifiers were still in there and I bet they finally shorted out)If any of you want to suggest vendors for WELL SHIELDED transformers suitable for a tube preamp, I am all ears. I am having zero luck finding one.Also, here's a different power supply from the same era that gives a good example to the "bias your filaments at some DC voltage" concept. It's another McIntosh, an all-tube FM tuner, model MR-71. Note one string of filaments is biased to sit on top of a +95V rail. Interesting.If it matters, the tubes in this DC-biased string are the oscillator V3, L&R audio out V11/V12, and MPX V14/V15. I wonder why these tubes were 'deserving' of this bias and not any of the others?

author: schmitt trigger
author_rank: Super Contributor
timestamp: December 31, 2018, 05:07:06 pm
content: A common reason is to prevent exceeding the cathode- heater maximum voltage rating.There may be other reasons.

author: floobydust
author_rank: Super Contributor
timestamp: January 01, 2019, 07:19:21 pm
content: Indirectly-heated filaments are DC biased above the heater in order to lower noise.Normally it's reverse in an amplifier circuit - the cathode is at a positive potential compared to the filament and some electrons flow from filament to cathode. Reverse-biasing blocks the charge there, I forgot the physics of it all. It's also a diode so distortion can show up on high-impedance cathode circuits.Capacitive voltage dividers have a short lifetime, the ripple current (peaks) are very high and the first capacitor (i.e. C308 quickly fails, a 40uF cap does not take much ripple current.I would get rid of it, if rewinding the transformer. You need double the secondary turns but thinner wire, not sure how much room there is on the original core. I also think the transformer needs to be double-insulated as this is not earth-grounded.For custom audio transformers I have usedSowter Transformersin the UK. They have pro vintage offerings for Neve, Fairchild, Altec, Pultec, RCA, Ampex etc. They offer no-cost custom transformer design too. Very good quality but a trip across the Atlantic for British steel.Edcor USAsells laminations, bobbins, end bells and parts to build one and are low cost.For shielding, it's just the (steel) end-bells here? I'd have to see a pic of the transformer. Shielding is more commonly done by the tube shields.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 01, 2019, 10:36:30 pm
content: Quote from: floobydust on January 01, 2019, 07:19:21 pmUse a motor run cap as a replacement - those can take a lot more current.

author: duak
author_rank: Super Contributor
timestamp: January 02, 2019, 04:24:18 am
content: cvanc, the tubes chosen for biased hum reduction are the most sensitive links in the audio frequency chain. Any hum injected there will be amplified the most. Ah, but why the oscillator tube too? Any low frequency AC injected into the oscillator will modulate its output in both AM and FM modes. Since this is a superhet, the input signal is mixed with the oscillator output producing a difference signal that is amplified and then detected by the FM demodulator, Any modulation of the oscillator's frequency will be extracted and then amplified and will appear on the output as hum.