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    Building, troubleshooting and testing of these amplifiers should only be
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Heathkit S-33 Information needed.

Hey everyone,
Just got given a Heathkit S-33 Valve amplifier and would like to know if anyone knows ANYTHING about this amplifier?
All i know at the moment is what my uncle told me on the phone, havent actually got the amp in front of me yet. next few days it will be here though.
Aparrently it is stereo and has 2x EL84, 2x ECF80 and 1x UU12 valves in it which leads me to believe it is some kind of SE EL84 amplifier. It needs rebuilding so any schematics or information in general would be greatly apreciated.
Many thanks,
Owen
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
Photo stolen from an eBay auction that ends in 6 minutes:
[IMGHTTPDEAD]http://headfonz.rutgers.edu/HeathS33.jpg[/IMGHTTPDEAD]

There should be nothing tricky here. Volume/balance controls, voltage-amp, passive tone controls, voltage-amp, self-biased SEP output, hollow-state rectifier probably with cap-input filter.

There are not enough tubes for a mag-phono input, so "GRAM" is probably a Crystal Phono input. This may just be a high-impedance Line input, or it may have a little EQ: treatment varied widely. The "RADIO" will be a straight line input: hack the "GRAM" input the same, and then use them as Tuner and CD or PhonoPreamp. (An external switchbox can extend you to more than two inputs.)

Very 50s color scheme and style. I would say 1956, but stereo was uncommon. Probably '57-'58 using 55-56 paints and molds.

Rust never sleeps. Tarnish will be a curse.

It is likely that all the power supply caps are decayed and should be replaced; they may burst at power-up. SEP amps run hot, and I would expect some drifted resistors. It is unlikely that both channels would be sick in the same way, so you can use one channel as a guide to the other. When in doubt, use tube-book value and plans: Heath rarely strayed far from basics, not in the low-price line.

If this is truly a HeathKIT, inspect EVERY solder joint several times in bright light. This may have been the first project the kit-builder ever attempted. Sometimes you can see the learning-curve: joints made late in the manual better than the one on the first page.

I have never seen one, so it may be moderately rare. It is however obviously a low-price product.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> When in doubt, use tube-book value and plans...

Ooops, I mis-remembered what an ECF80 was. It is a triode-pentode that was promoted for TV Tuner converter duty. The book-values soak the tubes to get good performance at 200MHz... not really what we would do for audio.

The triode side looks enough like a 12AU7 or 6SN7 that you can surely use those types as a guide to biasing. Pentodes are always tougher.

The EL34 stage should be utterly normal. About 4 Watts as Pentode, almost 2 Watts if you triode-strap it.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> the 3Ohm terminals? also it uses EL84's not EL34,

We are both doing the same thing: confusing our "3"s with our "8"s.

I'm sure the markings on the back show 8 and 16 ohms. 4 ohms on the "8" tap isn't right, but it will work, just lower power and maybe higher THD.

I know the difference between EL84 and EL34 when I see them. One of them is much bigger. You got the small one, so it must be around 4 Watts. I am sorry I typed "34" when I meant "84".
 
Amplifier arrived today, I checked it over and it all seems in order, brought it up slowly and it worked (sort off) one of the switches on the front are dodgy and all the pot's crackle. The speaker terminals are 3Ohm and 15Ohm.
There are capacitors inside that i want to replace connected from the power tube's to ground in paralell with a resistor. they are 25uF 25Volt Capacitors, Would it be ok to replace them with 22uf 100Volt capacitors (i have some nice NOS 22uF ones lying around i can use) if not i'll just order some new ones in.
The power tube and rectifier tube sockets are a little loose aswell so i will be replaceing them with some nice gold plated ones i have in my parts bin.
The ECF80's (driver tubes?) are on their own little PCB with some capacitors and resistors, the rest of the amplifier is hardwired, I will try and get some pictures of it's guits soon for you all.
Thanks,
Owen
 
Owen,

The RC combos you describe are the bias networks. The value of the cap. impacts on bass performance. Save the 22 muF. parts for another project. Buy a pair of 33 muF. parts for the EL84 cathode resistor bypasses.

Do you plan on a stock rebuild or are you thinking of going hog wild? For a stock rebuild, replace all Carbon composition resistors, as they have drifted in value and gone noisy over time. Replace waxed paper coupling caps. with 716P series "Orange Drops". Mica and/or ceramic caps. usually can be left alone.

6BL8 is the RETMA designation for the ECF80. A link to a data sheet follows.
ECF80/6BL8 Data Sheet
 
i swapped the capacitors for a pair of 22 muF ones last night and the amplifier worked wonderfullyafterwards, i will try a pair of 33muF capacitors tonight (should have some lying around) and see what it's like then. I'm going somewhere in between with the rebuild, i will be upgradeing most of the conponents as i change them. i'm going to replace all the resistors and capacitors. but first things first, i will take some pictures and change all the valve sockets, they are all a little loose and i have a load of nice ceramic ones with gold plated contacts, and i'll order some new speaker posts and phono sockets for it, the ones that are on it are a little worse for wear. I was surprised by how few conponents the amp actually has inside. It sounds awsome though. :)
Owen

Oh yeh, i couldnt see any waxed paper cap's in there they all look like electrolytics except for a few little mica cap's (i think).
 
Owen,

Mica caps. look like miniature dominoes. Ceramic caps. look like disks. Waxed paper caps. look like cylinders. Mica and/or ceramic caps. should be replaced IF they are being used for coupling purposes.

Using 22 muF. parts as cathode resistor bypasses shifts the turnover frequency of the pole up, slightly. OTOH, using 33 muF. parts shifts the pole's turnover freq. down a bit.
 
Owen,

Mica caps. look like miniature dominoes. Ceramic caps. look like disks. Waxed paper caps. look like cylinders. Mica and/or ceramic caps. should be replaced IF they are being used for coupling purposes.

Using 22 muF. parts as cathode resistor bypasses shifts the turnover frequency of the pole up, slightly. OTOH, using 33 muF. parts shifts the pole's turnover freq. down a bit.
 
piccies as promised :)
top view:
[IMGDEAD]http://www.thefilehut.com/userfiles/doompixie/IMAG0001.JPG[/IMGDEAD]

Bottom View of the PCB:
[IMGDEAD]http://www.thefilehut.com/userfiles/doompixie/IMAG0003.JPG[/IMGDEAD]

Bottom View of the rest:
[IMGDEAD]http://www.thefilehut.com/userfiles/doompixie/IMAG0004.JPG[/IMGDEAD]


You can see the capacitors that i think look dodgey on the bottom of the PCB, you can actually see the corner is missing from oen of them where the lead ataches.
the silver capacitors are the 22muF ones i put in to replace the 25muF ones that were there.
I'm going to remove all the resistors one at a time and test them and any that are suspect will be replaced with metal film type resistors, Would 1/2Watt metal film be good enough replacement or would i need more than 1/2Watt for some of the resistors and if so which ones?

Many thanks,
Owen
 
PRR said:
Very 50s color scheme and style. I would say 1956, but stereo was uncommon. Probably '57-'58 using 55-56 paints and molds.

Greetings from UK - Nostalgia time for me! I remember building a Heathkit model very similar to the one pictured. I believe it would have been in the early / mid 1960's in the heady days of Hi-Fi building and annual Hi-Fi shows at the Russel Hotel, in London. Happy days.
There used to be a Heathkit Daystrom outlet at 233 Tottenham Court Road, London, one of the many Hi-Fi (?) stores I used to frequent in my mis-spent youth. Difficult to recall sound quality of the Heathkit, but thinking about it now, it must have been fairly basic - BUT it was the fun of building it and in the process learning something about how amplifiers worked. I also built Lowther Acoustic Horn enclosures at the same time, first wooden corner units and later concrete 7 footers in the recesses either side of the chimney breast. The Heathkit certainly drove them loud!

I no longer have that piece of equipment and having just have just checked my filing drawers sadly neither do I have the construction manuals. Good luck with your renovations.
Cambshire Gordon
PS - I did find my later-build construction manuals for the Heathkit transistor stereo amplifier model AA-22U (60 pages) and their transistor FM tuner models TFMT-1 and TFMT1S or M (44 pages). These both date from about 1968. I also have specifications and schematics only for the AR-19 AM/FM Tuner Receiver and the AR-2000 Tuner Receiver - also of the same period. If anyone comes across one of these models, I could scan and email the schematics and parts lists if needed. CG.
 
Owen,

I think the resistors on the PCB are 1/2 W. Metal film parts in the grid leak positions are OK. Grid stoppers (if any) MUST be Carbon comp. All other Carbon comp. parts should be replaced with Riken or Kiwame "audiophile grade" Carbon. Metal film resistors "everywhere" will alter the amp's voice, probably for the worse. If the fancy Carbon resistors are too costly for your purse, replace Carbon comp. with Carbon comp. Other than grid leak, the only place to use metal film is in a NFB network, where precision does matter.

It's hard to tell, but it seems you do have paper caps. on the PCB.

The dodgey caps. on the PCB might be ceramic. Mica parts have axial leads. Can you determine the value of the caps.? "Orange Drops" as replacements just might be a good idea.
 
hi, i checked the values of most of the resistors and most were spot on a few were reading a little high but still within the tollerance of the resistor so i think i'll leave them alone for the moment.
the dodgy capacitors are 200pf and we measured the voltage accross them and it was 49V. i have some 50V 200pF ceramic cap's here but i didnt chanmge them because i'd like to get 100V rated ones, i'll look into the orange drop capacitors.
thanks,
Owen
 
Hey,
I've replaced all the components that looked suspect or didnt measure correct values, now i have a question, i used the amp earlier today for about an hour and afterwards the output transformers and power transformer were hot to the touch, do you think the transformers are knackered or is it just because the valves are so close to the transformers they are heating the transformers up?. theres about 2 to 3mm between both the rectifier and one of the power valves and the power transformer and about 5 - 7mm between output valves and output transformers. it looks to me like the valves are heating up the transformers as when i disconect the secondaries from the power transformer and power up it dosnt heat up atall, i checked the circuit and it all seems good, it isnt drawing excessive amounts of current from the transformer either and the voltages are all good from what i can tell. just thought i better ask. mabey i should strip the amp and make a new chassis for it so i can put more space between the valves and the transformers or will it be fine like that?
Owen
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> speaker terminals are 3Ohm and 15Ohm.

I'll be darned. What an odd choice. Fortunately this amp won't be very fussy about "mismatching".

Uh, unless my calculator is wrong (it often is), connecting between "3" and "16" gives a nominal 9.4 ohm connection, "perfect" for 8 ohm speakers. That is, as long as you do not use a speaker switch or other toy that assumes the speakers are common ground.

> an hour and afterwards the output transformers and power transformer were hot to the touch

Of course they are hot. Ripping electrons loose is hard work, also this type amp eats full power all the time and any audio output is just a side-effect.

You should be able to hold your finger on a transformer for many seconds. And most such amps DO run that hot. Hotter than that is cause for concern.

As long as you are worried: check the voltage on the output tubes' cathode resistors against the specs. Cathode resistor V/R will tell you the current in the tube. That current times the main B+ voltage is roughly the plate dissipation. Don't be shocked if it is right-at the tube's rating: they had to take it to the limit to get a respectable power. If it seems over the limit, you should do the more exact calculation, deducting screen current (estimate the ratio of plate/screen current from datasheet tables) and figuring the actual plate-cathode drop in the tube.

Philips 1964 gives EL84 Pd as 12 watts "Design Center". This was the nominal limit for production designs, with a lot of allowance for powerline and component variation. If you find the actual operating point in your amp in your home is within 20% over (~14 watts), you are fine.

You have a mains voltage switch. Is this adjusted for your local voltage? In the US, since this was built, wall power has crept from ~115V to ~120v. I gather that rationalizing UK/EUR voltages is also giving local power somewhat off from the 1950s. If you know your actual wall voltage, put the tap on that setting. If you have reason to doubt it is correct, the heater voltage should be awful close to 6.3V. The heaters will stand a wide variation, but if it is off by much then the plate voltage is also wrong. Since this design beats the plates to within a watt of their life, err slightly low instead of slightly high.

The last of the Bogen commercial tube amps would run SO hot in full-power (motor-drive) oeration that you could NOT hold a finger on the transformers, they used special insulation compounds to hold things together. I would not expect THAT much hot in a low-price Heath, but it is certainly reasonable that it will run hot, enough to hot-up your finger, but probably not enough to burn skin.


Eli has a good point, but I would not fret new 22uFd caps for old 25uFd caps in this case. The old caps had very wide tolerances. Heath usually used caps that were ample, a 13% change would be no big deal. Also on a ~5 watt amp, deep subsonics are unlikely to be audible, you may as well lose them before they saturate the output transformer. 33uFd or even 220uFd might be interesting to try, but with a newly salvaged amp on the bench and a bin of 22uFd, I sure would tack in the 22uFd.

> would i need more than 1/2Watt for some of the resistors

Go by size. 1/2W was cheapest in those days so anything from 0.0W to 0.4W they used 1/2W. You will find some larger resistors in the power output and supply circuits, compare to the size of modern 1W and 2W resistors. A modern resistor may be smaller than an old one of the same rating, but when in doubt always buy the biggest power you can afford and that will fit. Cost of few-Watt resistors is a non-issue in DIY, you may as well use 2W for any 1(?)Watt you find.

> Would 1/2Watt metal film be good enough

IMHO: Too good. Lily-gilding. Use $0.10 Carbon Film. I doubt it has UHF grid stoppers, and if it has grid-stoppers at all I suspect that carbon-film will be perfectly fine.

If you go for Golden-Ear resistors, you probably need to replace EVERYTHING to get the full impact. Coupling caps, pots, tubes, output transformers. It is a top-grade budget amp with plenty of music in it: fix what is broke, use "better" parts if Heath would have done it in 1957 (carbon film is today's generic carbon-comp), but don't go crazy. You can always go crazy later, when you decide that this is an amp that you want to live with forever.

Coupling caps can be Orange Drops: it suits the period, and Heath would have used them if this were not a budget amp, which had to compete with Eico, Knight, and Lafayette 5-watt amps and undercut the 10-watt amps.

Ceramic caps over 1,000pFd (mmF) are dubious audio paths. Use film caps. 1,000pFd and lower leaded (not SMT) ceramics are very-good. (They use different dielectric for the high-pFd caps.) But if you have the checkbook out, you might find Silver-Mica caps for the 50pFd-1,000pFd range. Lily-gilding, but very appropriate for the period, and proably not a lot of cash for one amp.

> Mica parts have axial leads

I think I have seen mica in just about every shape. I agree that the damaged cap on the bottom may be ceramic. At 200pFd a ceramic is probably harmless if sealed, but the chipped edge is Not Good and it should be replaced.
 
i have the amp running, i replaced those suspect cap's with some ceramic ones i found and replaced other caps and resistors that wernt measuring good on my test gear, I have used the amp a bit now and it sounds really nice. i have still got to change the pots and switches on the front pannel but im concidering haveing one of the inputs on the rear connected directly bypasing all the tone controls and volume controls and stuff so it can connect straight to my preamp and then i wouldnt really need to change the pots and stuff. but first things first, i need to change the connectors on the back as the ones that are there arent in too great shape. I'm not really worried about the transformers getting hot, i think its just me being parranoid. :)
cheers,
owen

edit: forgot to mention, i ripped the drivers out of some old mission 77's that had blown tweeters and fitted some 4 Ohm 2 way infinity kappa din car speakers instead and connected to the 3Ohm terminal they sound totally awsome. infact they sound better than any of my other speakers!