Theoretical Question

First, some disclaimers:

a) I have a multiple lifetime of pre-blight 5AR4, GZ37 and 5U4 tubes such that my needs are met through the grandchildren - significant as they will inherit.
b) I am generally not a subscriber to the various forms of audio-phoolery such that I would think one form of DC to be any better than another given adequate filtration.
c) I am seeing all sorts of "stuff" happen as reported in this forum based on arcing rectifiers - which, in turn, take other tubes (expensive) with them when they die.
d) Even post-blight rectifiers are expensive.
e) Even post-blight power tubes are expensive - with special reference to the KT88-&-higher family.

So, Whyizzet, given all the above, that these late-model, newly-designed amplifiers are not supplied with a TDR and solid-state rectification from the git-go? It would seem to be a natural evolutionary step towards greater reliability and resilience.

I don't buy the complexity argument: After all, there are those that install auto-bias.
I don't buy the cost argument: The cost of the choke, the reduction of the 5V winding, the cost of the rectifier tube itself would go a very long way towards counterbalancing any such costs.
I don't buy the space argument.
I don't buy the inrush voltage argument: A tube rectifier, especially the 5AR4 family does not pass DC until about 75% of operating voltage anyway, it just takes a little longer. And there is nothing in the TDR circuit that a Thyristor could not address.

Yeah, this is a bit snarky, as I am running tubes (but pre-blight) in my Dynaco products (FM3,ST70) - but for the PAS pre-amps - I am using a solid-state retro-board in them with excellent results. And the ST & SCA 35s have solid-state already.

Thougths?
Comments?

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Exactly!!!

That's why, after lurking here for several years, reading/learning all I could -- especially on the VTA ST-120 which sort of has a 'unique' spin on the rectifier -- I bought my VTA ST-120 with the TDR (time delay relay board), am using a solid-state rectifier, and using the 5V winding (meant for a tube rectifier heater) for a bucking winding, since my line voltage is always 122-124 vac.

IMHO, the VTA ST-120 should never be used or sold with the 5AR4 and its variants -- it should come with the TDR, sand diodes, etc.

IF and ONLY IF the buyer demands a tube rectifier, then a GZ37 or whatever that coke bottle is called should be sold.  Otherwise you get your Copper Caps and TDR's - no questions asked!

For the VTA ST-70, yes you can have a 5AR4 rectifier, but the kit should include the yellow sheet diode mods.

Those that do one or the other above for the respective amplifiers do not have troubles......

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I use the Weber WS-1 with thermistor option.

I haven't heard about a "limiter" in there???  I haven't taken it apart or seen a schematic, but I suppose they just put some sort of thermistor in series with the B+ output pin, and make sort of a slow ramp up -- of course this is after the TDR clicks in.  I don't think the thermistor does much, or is even needed, but I figured it couldn't hurt and only added $2 or $3 to that over-priced WS-1.  I should have just soldered in some diodes underneath, or built my own plug-in unit, but oh well.........

Not to pile on but could the PT's be wound for 120-125VAC?
I'm sure this has all been covered in other threads but isn't 120v standard in the US? If so, for how long?

I'm using a variac to bring my 119-124v down to 117v. I'll soon be running a WS-1t with the auto-bias board.

Since I built my ST120 in 3/2018 I've been running the same TungSol 5AR4, TS 6550's. No issues with the 12AU7's either. I did the yellow sheet diode mod during my build.

Maybe I've been lucky- Knock clear oak veneered Klipsch MDF..?

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Seat me in the pre-blight Mullard GZ-33 and GZ-37 side of the aisle, please.
They are elegance defined. They do their job well and will do it for a lifetime.
Not to mention aesthetics. Both are b-a-a-a-d looking tubes. I put socket extensions under mine, which not only make them look as tall as Michael Jordan, but also keeps heat off both the tube and the chassis, which IMHO is a Very Good Thing.
Like PeterW, I have enough stash to supply my grandchildren through social security. Not that I expect even this one pair to fail, but a meteor could strike that side of the house. (The Mullards are kept in my gun safe.)

For my 10 cents worth.....

The ST70 was designed many years ago to a strict budget given what was available at the time. The ST120 to me is an evolution keeping the spirit of the original design. That is probaby why the repo chassis still have the octal sockets on the front.

However I have read many threads and the 5AR4 appears to be the weak link in the otherwise excellent design. So why not replace with an SS rectifier? This does bring the HT up a little and of course increase the dissipation of the output tubes if you keep the same bias current flowing. P=VI...

If I were to build another STxx then I would probably not even fit the octal socket for the rectifier, instead it could possibly be used for another quad cap to keep athestics balanced. SS rectification and the vacant 5v filament used as a buck winding on the suppy input. It should have sufficient isolation in the power transformer given that would normally sit at full HT anyway.

I would also take the decision not to fit the Triode mode switches. I have listened to mine and to me it's like buying a V8, removing 2 spark plugs and stuffing a sock up the intake.... Reduced complexity and no switches to break down. Remember they have to withstand HT voltage at standby and possibly 2 x HT peak voltages to their cases and between contact sets at full volume. (take into account UL tap ratio)

Any sonic differences changing from Tube to SS rectification I'd put down to a slightly higher and stiffer supply rail.

Steve

I now have a WZ68 copper cap and a TDR in my ST120 which was originally built using a tube retifier. The Elekit TU 8200 SEUL 6L6GC amp I built has SS rectification. The Yaqin MS30-L PP EL34 amp I have has SS rectification. Only my Magnavox 9404 EL84 retains a rectifier, a 5AR4, as recommended by Dave Gillespie. I have no problems with SS rectification and I don't think I can hear any "sonic signature" I can attribute to the presence or absence of a tube rectifier.

Technical reasons, ha, none.
It's the "Manual Transmission Effect".
You either got one, or you're one of "those" people. (sniff)

And how come nobody ever uses the cool smiley things?

 

I look more like the one on the right

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Speaking of the Weber WS-1, or the WZ68 and heat.....

Last night I took my ST-120 out of service to permanently install the binding posts that came with it (ground and 2 taps, standard on VTA ST-70/120 now days).  When I first started using it, I didn't know which tape my Quad ESL (57's) would use -- 8 or 16?  And my Maggie MMG's definitely use the 4 ohm taps, so I wanted all available.  Anyway, the Quads sound best to me on the 8 ohm tap, so I finally got around to installing those binding posts with 4 and 8 ohm taps.

Anyway, when i turned the amp over to work on it, lo and behold the Weber WS-1 copper cap just fell off!  This no doubt because the glue cooked, being in close proximity to the unbelievably hot power transformer and a KT-120.

I heard that Weber improved their glue, but apparently not -- I didn't even pry on it -- 'just fell off.

So, now I know what's inside of it:  I have the thermistor model, and there is only 4 diodes and a thermistor.  If you order without the thermistor, you only get 4 diodes I'm sure.  By 'limiter' they must mean IF you order with thermistor, cause if you don't, they ain't limiting anything.....

Anyway, I didn't get the diode part numbers yet (but they look capable of 3 to 5 amps), but the thermistor comes after the rectifier and is in series with the B+.  BTW, it has written on it "NTC 100D-11.  I think 100 means 100 ohms at 25C or something like that.  I have no idea what the 11 means.

In practice, this thermistor in the B+ makes sense -- however, what about bias?  if this thing takes an hour or 2 to settle down (it takes awhile for the copper housing to heat up), won't the bias drift during that time?  I know the bias is from a separate supply, but does a wandering B+ (slowly rising until everything is REALLY cooking) affect the bias?

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As I understand it, the Bias Supply is separate from B+ in the OEM stuff - this is not the case in the 120?

A quick search nets this:

www.bpimpexindia.com/images/products/pdf/100D-11.pdf


It looks like 100 ohms with no load at room temp, then drops to just a couple of ohms when hot.  Probably just to cut the in-rush current rather than drop voltage because the voltage drop at the normal operating point would be a volt or two at best.


Tony

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PeterCapo wrote:

Peter W. wrote:As I understand it, the Bias Supply is separate from B+ in the OEM stuff - this is not the case in the 120?

I would think it is separate.

Yes, so would I, making it independent of B+ voltage and the B+ rectifier for the most part.

Peter Capo, Kentley brought up the non-audio disclaimer on the Webers a number of years back, and none of us ever could find out why. My theory was it was the sort of thing lawyers dream up to keep people from using a product for its intended purposes. (If one ever read all the legal cautions that attend a powered lawnmower he'd pull up all his sod and plant ivy or cannabis.) IIRC, it was generally decided that the continuous-duty demands of a home audio system were more strenuous than those intermittent ones required for a head amp at a rock concert.
At bottom of a Weber autopsy is of course heat. They get every bit as component-damaging hot as a GZ-33 but in a Weber, the heat has nowhere to dissipate. I tried Webers in my M-125s and they lived for 6 months at best. Troy Madden came up with an elegant solution: remove the copper cap from its base, stick it over a dowel of about the same diameter, and drill a half-dozen holes near the top of the cap to let the air circulate and the heat dissipate, and reinstall the cap with some heat-resistant glue. (Remember, there's no vacuum to protect but you want to hide all that voltage from prying fingers and biasing screwdrivers best you can.) Troy's solution should yield a Weber that theoretically will last as long as a vacuum tube rectifier.
Those with the most grief likely start with a cheap "post-blight" 5AR4 from China or Russia out of which a day's service is the norm. However, and I'm surprised Jim McShane hasn't weighed in here, there are some very good 5AR4s coming out of Russia under the New Sensor Gold Lion livery. They appear to be very sturdy and were I to go to a non-GZ type bottle, I would go with them.
Just because I prefer big tubes for the rectification job doesn't mean you should. The GZs nowadays run >$75 for NOS, more than twice what a Weber gets.
It seems to me that s/s vs. tube rectification comes down to a Ford/Chevy argument. Just remember that the power supply is the absolute heart of any amplifier, and if it's got a murmur you're heading for trouble.

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PeterCapo wrote:AFAIK, Jim likes the new Tung Sol and GL rectifier tubes, yes.  I seem to recall the gist is that they are more rugged than other current production 5AR4.  Based upon this, I would expect they'd be fine in any Stereo 70 variant in proper working order.  But, even if they are as robust as old stock 5AR4, it still seems like too much to ask to put them in the ST-120 or M125.

I remember that old thread and something about drilling holes in the copper cap to ventilate it.  That's fine if someone is so inclined - and knows about it.  Not sure it is a practical option for everyone, unless perhaps the copper cap comes with the amplifier kit with detailed instructions on modifying it/drilling it.  Or, if the ST-120 and M125 come with instructions to go buy and then modify a copper cap.  Or, have a sticky in the forum describing the procedure.  If not, this technique could get lost in the shuffle, easily enough.  In any case, IMO anyone buying an ST-120 or M125 should somehow be made aware.

I'd still favor investigating the possibility of adding another PCB, having the needed parts, where the rectifier tube would go, above and/or below chassis.  Expresspcb is not difficult to learn.  Here it is for anyone who might like to give it a go: https://www.expresspcb.com/

Peter, another tube option is to drill an extra hole and run your tube rectifiers in parallel.
FWIW, I run a single GZ-33 in my ST-120 and a single GZ-37 in my ST-70. I like the parallel concept but wonder if that might be a bit stressful to the filament winding in the tranny.

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"AFAIK, Jim likes the new Tung Sol and GL rectifier tubes, yes.  I seem to recall the gist is that they are more rugged than other current production 5AR4.  Based upon this, I would expect they'd be fine in any Stereo 70 variant in proper working order.  But, even if they are as robust as old stock 5AR4, it still seems like too much to ask to put them in the ST-120 or M125."

FWIW, when I ordered my ST120 kit, It came with the China made 5AR4 but, I ordered a TungSol after reading about all of this. Bob said the Chinese one held up but I liked the look of the TungSol. Upscale Audio was concerned enough to call me about my order. They said the VTA's were known to eat 5AR4's. I told them I had Bob's blessing. I also put it in a review on their site to try and help all those concerned. The Shugang has never been in the amp, BTW

I've ran Mullard GZ-37 in my stock M125s for over 4 years without a hint of a problem. My wall voltage runs at 122.6 and I got it cranked down to 118.