AC30 Schematic -- I Can't Believe my Eyes... 160ma ?!?!

[bob p]

Pete and Steve recognize the things that I've been thinking about. Sag and the quality of regulation are part of the Black Magic component that I was hoping someone would mention. We're talking about a PT that's stressed beyond it's limits and probably won't regulate very well. But the flip side of the coin is that the amp is biased into a pretty hot AB1 to begin with. The hotter you bias the more insignificant the current delta becomes, so the voltage delta probably remains pretty small when the supply gets loaded.*

I honestly don't have a good feel for the relative contribution of these opposing factors. I'm hoping to learn something here.

* This was been a pretty common trick for cost containment in the tube era. David Hafler designed the Dynaco Stereo 70 with a lame-ass power supply that sagged like there was no tomorrow. But nobody noticed it when the amp was biased into Class A.

[Ted]

The original AC30 was also famous for eating tubes. Maybe you could extend tube life and get away with the 175mA PT if you modified it to run at a lower idle current.

.

I have done just that on a few occasions when I've had AC30s with 360 or more on the B+ (usually because they had non- standard PTs) which ran frighteningly hot or ate valves. Changing the cathode resistor for 68R cools things down a little. And guess what? Nobody has ever commented that the amp's sound had changed

[pdf64]

'But the AC30 is biased very hot, almost in Class A, so the current draw doesn't vary that much with the audio signal, hence sag is less of an issue.'
There's no denying that they are biased hot, but that scheme illustrates that the current rises 25% from static to max clean (ie cathode at 10V static, 12.5V max output).
My findings are that the cathode voltage may rise to nearly 20V when cranked, so supply regulation may become a significant factor under such conditions of use.
Pete

[olddawg]

I was always told that sag had more to do with rectifier tubes and their idiosyncrasies rather than transformers. I thought SS rectified amps generally have little sag unless it is designed into the power supply after the rectifier circuit via a sag resistor. It seems to me a transformer that was sagging because of an excessive current load would not last long at a all in practical use. I do know that manufactures tended to use readily available, off the shelf, components in those days. As has been said. The listed ratings for the original transformer my have been conservative. Most stated specs back in the old days were under rated by 20% in most cases. At least that's what I was taught back in the day. If it worked it worked.

[Ted]

I was always told that sag had more to do with rectifier tubes and their idiosyncrasies rather than transformers. SS rectified amps generally have little sag unless it is designed into the power supply. It seems to me a transformer that was sagging because of an excessive current load would not last long at a all.

Transformers are specified with sag - if the spec quotes "X% Regulation" it means that the transformer voltage sags by X% between no load and its full rated load. Whether they can take a load over the rated current is entirely down to how hot they get. It's not an exact science - it certainly wasn't in 1960.

I'll tell you how I think it happened. It's 1960. The R&D department at Vox are being asked for bigger louder amps. They knock up an AC30, using the smallest, cheapest, most marginal power transformer they can get away with. They're thinking "we need to make as much money as we can out of this rock'n'roll thing before it dies out in a year or two and we have to go back to making 5W amps for gentleman big band guitarists".

Two years later. Rock hasn't died. "We can't make enough of these AC30s. But 10% of them burn out their mains transformers. However 90% of them don't. And only half of the ones that do fail within the warranty period. And anyway, nobody can afford Fenders because of the import duty. I hear there's a bloke called Jim Marshall out in West London building amps now. He'll never amount to anything. Don't change a thing"

[Dave H]

I don’t think the AC15/30 black magic is in the power supply.

I made my ‘AC15’ with a spare transformer I’d had wound for an AC30 type amp. Its 275V 300mA RMS for a bridge rectifier. The ‘AC15’ is closer to an ‘AC20’ with this transformer running the EL84s at 14.5W (it has a fan). It also has a switched 220ohm ’sag’ resistor which reduces the B+ from nearly 380V to a more normal 330V. I’ve played many gigs with this amplifier over the years switching between 15W and 20W output and I can hardly tell the difference. The feel and sound are very similar. I can just about tell that the 20W setting is ever so slightly brighter but I’m not sure I’d be able to tell consistently in a blind test. Both setting sound great.

[pdf64]

See http://www.mif.pg.gda.pl/homepages/f...010/g/GZ34.pdf
Tube rectifiers may specify a level of 'protecting resistance' in series with the supply to them, in order to limit surge current (when in the capacitor input arrangement).
My understanding is that protecting resistance was usually implemented by the resistance of the B+ windings, which forces there to be fairly poor regulation, loss and heat generated in the PT.
Pete

[Enzo]

Saq/regulation need not be confined to one source. The rectifier tubes contribute to sag, yes, but so does the winding resistance. If your high voltage winding has 100 ohms of resistance, that 100 ohms can be considered in series with the winding.

That is why your little 9v wall wart transformer putsw out 14v when unloaded. Transformer voltage is specified at full current.

[Steve Conner]

I think Pete is right, the HT windings were deliberately made high impedance to protect tube rectifiers. I have an old Sansui PT that was designed for use with a GZ34. I changed the rectifier to solid state, and it gives a B+ of 550V at idle, sagging to 400V at full output. That's a lot of sag, but it hasn't burnt up yet!

[Ted]

I have a mid 60s AC30 on the bench. Yellow Mullard preamps and Sovtek EL84s.

At Idle: Vk 9.5V, B+ 320V
At Onset of Clipping: Vk 11.6V, B+ 313V
Driven Hard into clipping: Vk 13.7V B+ 297V

[Diablo]

I checked the PT specs on the TDS transformer for the Matchless DC30, and it lists 570 VCT @ .16 A.
This transformer weighs 10.5 lbs and has a lamination stack that's 1.75 x 4.375 x 5.25 inches.
Is the AC30 PT a similar size?
The transformer size/weight should give a clue as to its VA capacity.

[pdf64]

I'll check mine.
Ted, thanks for those measurements; while you've got that AC30 handy, would you mind checking the OT voltage ratios? On mine (Woden) the primary works out to somewhat over the scheme's 4k.
Pete

[Ted]

Sorry guys its gone now. But those transformer dimensions sound about right.

[bob p]

I think Pete is right, the HT windings were deliberately made high impedance to protect tube rectifiers.

Wow. Intentionally adding impedance to the power supply rail. I must be a total idiot. I always try to decrease the power supply impedance.

So how do they deliberately add Z to the windings? Smaller gauge wire? There's a two-edged sword.

[bob p]

I have a mid 60s AC30 on the bench. Yellow Mullard preamps and Sovtek EL84s.

At Idle: Vk 9.5V, B+ 320V
At Onset of Clipping: Vk 11.6V, B+ 313V
Driven Hard into clipping: Vk 13.7V B+ 297V

Thanks for posting this.

I'm thinking that knowing your bias conditions would also be very helpful. Some people have mentioned climbs to +20V under full load. That difference has to be an artifact of difference in bias conditions.