Crown SXA restoration worries

[Steve Conner]

Hi all

I just bought the remains of a Crown SXA from a friend who gave up trying to restore it. It's a rather ugly 5-rackspace stereo tube amp, and I also have the Sams Photofact, dated 1960. I'll post a scan here if anyone wants to see it, but it's in pretty poor condition, and I'm not sure it would even scan.

So far I've figured out that it's basically a Williamson amp. It was designed for EL37s, but I'm planning to run it on EL34s, since I already have a dozen of them kicking around looking for something to do. If I had a dozen EL37s, I could sell them on Ebay and pay off my mortgage.

What I'm worried about is the power supply. It most definitely isn't the export model, if there ever was one. There are two identical power transformers, and the Photofact shows the primaries paralleled, and the HT secondaries also paralleled with ballast resistors.

Now the problem is that UK mains is 240V and 50Hz. I figured out that I can series the primaries, as long as I parallel the secondaries to keep everything balanced. But that means each primary gets 120V at 50Hz. I got out the variac and tried one of the transformers at 120V, and it hummed rather loudly and draw nearly half an amp of magnetizing current: it looks like they were never designed with 50Hz in mind. Both transformers did the same, and they were dead quiet at 100V, so I don't think they're faulty.

Now, when I think of those extra losses, and also consider that EL34s draw more heater current than EL37s, and that I'd rather use more filter capacitance and bias the tubes hotter than the 34mA/tube recommended on the Photofact, it's not looking good for the PTs!

My plan is to use a small bucking transformer, like RG's Vintage Voltage adaptor, to get the line down to 220V or even 210 or less, and just see how hot I can get the bias.

I'm kind of confused, though. I thought I was buying a hot cathode-biased amp like a Dynaco ST70, that I could use in my stereo system, but it's actually fixed bias and more like a stereo guitar power amp. At least it's UL, though.

[Steve Conner]

Some more progress on this project...

I tried wiring up the two PTs with dummy loads to see how they'd perform. I used four EL34s to load the heater windings, and lashed up the rectifier and filter caps, using two 240V, 40W lamps in series to load the B+. I figured that would be about the same current as four hot biased EL34s.

I experimented for a bit and ended up using a 240V to 15V transformer to buck 15V off the line, a la RG's Vintage Voltage adaptor. That gave me 6.3V on the heaters, and 420-something B+.

The transformers never got more than warm to the touch, even after leaving it on all day, but they did hum audibly. To get them silent I had to buck 30V, and then the heaters were down to 6.0V. I actually wouldn't mind the lower B+ of this arrangement, but the low heater volts worry me. Does anyone have any thoughts on whether 6.0V is OK?

[Robert M. Martinelli]

Hi Steve!

Is this the "Crown restoration project" you were talking about time ago?

As to the heaters' voltage, I've read the "old books" and I think 6.0 VAC is still within the limits for the valves to properly operate.....but....just a thought.....

Why don' t you drop in a rectifier bridge, a filter cap and a LM317 and supply DC to the heaters ( at least in the preamp section )? This would completely eliminate your hum issues, and a single 317 can take as much as 1,5 amps ( or 5 x ECC83 if you like ), with 6.0VAC out of the PT, the voltage at the 317's input would be around 8.5 V, and, given the typical minimum voltage drop of an 317 ( 1,2 V ) you could easily adjust it to 6.3 VDC, dissipating only 2,2VDC*1,5A=3,3W across the 317, so only a small heat-sink would be needed.

All of the above of course does not apply if you' re willing to keep the amp as original as possible, should this be the case I'd bet the 6.0 VAC you currently get out of the PT will do just fine.

Oh, Talking about restoring old amps, days ago I started a thread about an old Italian amp, but I haven't been answered so far....if you could waste a few minutes of your time and take a look it would be very kind of you; your suggestions are, as usual, more than welcome!

Take care
Bob

[Steve Conner]

Hi Bob, thanks for the reply

Yes, I bought this old amp from a fellow hobbyist over in Sweden who bought it from an American Ebay seller. It was completely in pieces.

I'll have a look for your thread, though I always have trouble finding stuff because of the sheer number of subforums nowadays

I'm not bothered about keeping it original (I'm already scrapping the ugly front panel, adding balanced XLR inputs, an IEC mains inlet, and changing the 6SN7 driver tubes to 12AT7s :-O) so the DC heaters might be a good plan.

The 8.5V assumes an ideal rectifier bridge with no voltage drop, though, so in real life you would get about 1.4V drop, maybe a bit less with Schottky diodes. So maybe it would work fine with no voltage regulator at all...

The other worry, though, is that the rectifier/smoothing cap will make the RMS current in the heater windings a good deal higher, and it's already over the rating if I want to use EL34s. However, each PT also has an unused 5V heater winding, so I could use these in series to make 12.6V DC heaters for the preamp tubes, if they need it.

[Robert M. Martinelli]

so the DC heaters might be a good plan.

The 8.5V assumes an ideal rectifier bridge with no voltage drop, though, so in real life you would get about 1.4V drop, maybe a bit less with Schottky diodes. So maybe it would work fine with no voltage regulator at all...

The other worry, though, is that the rectifier/smoothing cap will make the RMS current in the heater windings a good deal higher, and it's already over the rating if I want to use EL34s. However, each PT also has an unused 5V heater winding, so I could use these in series to make 12.6V DC heaters for the preamp tubes, if they need it.

Hi Steve,
Hmmmm......well, what about simply using the 5V windings to get your 6.x VDC for the heaters? If you use diodes with a low Rd, then you can limit the voltage drop and get there with no voltage stabilizer....if your 5VAC windings have a good current capability ( and I think so, because they were usually intended for rectifiers' heaters, which draw a lot of current ) simply rectifying, filtering and considering a 1 V drop would give around 6.1 VDC, If you' re lucky, and your 5 VAC windings are some tenth of a Volt over the nominal voltage, you could hit 6.3 VDC dead-on.

As to my "strange" thread, it' s under the "mods and tweeks" subforum, and the title is " old Italian amp with ELL80/6HU8". The layout is unusual, so taking a look could be amusing.

Take care

Bob

[Steve Conner]

Well, I had a week off work, and in true geek fashion, I spent most of it messing around with the SXA I now have the left channel working, and I'm pleased to report it sounds better than I expected.

The conversion to EL34s was easy, just a matter of adding 1k screen resistors. My output transformers were 6.6k with 43% taps, so I thought I'd better use the resistors, because every EL34 circuit I've seen with that O.T. spec has them. (The Dynaco ST70 doesn't, but it's 4.3k.)

I beefed up the power supply quite a lot. The original SXA had a C-R-C filter with tiny capacitors: I used 200uF and replaced the "R" with a choke. The rectifier is solid-state, so I'm not worried about burning it out. I generally used bigger capacitors across the board, since modern ones are so much smaller in size.

Apart from all of this, and replacing the 6SN7 driver tube with a 12AU7 (I said 12AT7 above, but I was wrong) I just used the stock SXA circuit. I replaced some of the coupling caps with bigger ones, and had to check the LF stability margin all over again. It was practically zero and I had to mess around with the capacitor values again to get it back.

When I finally closed the feedback loop and turned it on, I was quite pleasantly surprised. At a few watts of output the response is flat from below 20Hz to above 20kHz, and doesn't have any peaking at the extremes. I guess those Crown O.T.s must be pretty good!

On a speaker, it sounded pretty decent too. It seems to have plenty of bass (Bob Marley tested) and while it does hum very slightly, you can't hear it over the acoustic hum from the transformers. I'm going to look at an elevated heater supply, it might help, and also would take the stress off the heater-cathode insulation in the second stage. If the acoustic hum bothers me, I'll probably just buck another 15V off the supply.

I'll post some pictures and more results when I get the other channel working...

[Robert M. Martinelli]

Hi Steve!
Congratulations for the results of your restoration! ( and for your "off work" week, holidays are a rare thing nowadays ).

As to the hum, are you going to live with it or are you planning to do something about it ? I think a DC heater supply ( at least for the preamp section ) is still worth trying....

Best regards

Bob

[Steve Conner]

Hi Bob,

Tell me about it I still have 20-something days of my annual leave left that I never managed to use this year.

I'll look at DC heaters and/or elevated heaters to see if they improve the hum, and if so I'll probably implement them. The hum level is acceptable now, but it may be worth it if it makes the amp less picky about tubes.

[Steve Conner]

It's done! And yes, it sounds pretty much like it looks.

Putting 40V DC on the heaters cured all the hum issues I was having.

[Robert M. Martinelli]

Whoooopeeeee!!

does it really sounds as it looks? Because it looks marvelous!!!

40 VDC on the heaters? I suppose you took the third picture ( the one with the heaters glowing in the dark ) before doing that

Cheers

Bob

[Steve Conner]

Nope, the heaters still have 6.3V AC across them, but the entire heater circuit is elevated at +40v with respect to ground. (By lifting the heater winding CT from ground and connecting it to a low current 40v supply, from a spare bias winding.)

I'm not too sure how it works, but it certainly reduced hum a lot, and it helps take stress off the heater-cathode insulation in the concertina PI, too.

I had to swap out a few 12AT7s until I found a pair that didn't hum or make popcorn noises. One was really noisy to start with but quietened down.

The popcorn thing had me puzzled, because the noise stayed with the socket during tube swaps. I thought it was dirt, and was going crazy cleaning the tag boards with alcohol and cotton buds.

But it had no effect, and I now think the noise came from heater-cathode leakage that took a while to settle down. The sockets are wired as mirror images, so when I swapped tubes, the triodes swapped functions, and the one that had previously been the first stage amp got exposed to high heater-cathode voltage.

And yes, to my ears at least, it sounds as good as it looks

[Steve Conner]

Update: I was talking rubbish, the problem was a leaky tagboard. When I turned it on yesterday evening, it was crackling and popping like crazy. I tracked the noise down to the plate of the first stage, and lifting this node off the tagboard made the noise go away altogether.

Damn Ebay tagboards Maybe I should have scrubbed them with alcohol before use.