Heathkit S-33 Information needed.

[DoomPixie]

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]

Photo stolen from an eBay auction that ends in 6 minutes:


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]

> 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.

[DoomPixie]

Hi, thanks for your repplies, i'll change the power supply cap's and check it all over when it arrives. do you reacon it will be okay to run it with 4Ohm speakers and if so would i connect up to the 3Ohm terminals? :/

also it uses EL84's not EL34, does this make a difference to the power output?

[PRR]

> 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".

[DoomPixie]

just checking, I had a read of the auction description and im sure it said there were 3Ohm and 15Ohm speaker connections, but mabey that was a mistype aswell or part oft he numbers are worn off or something. will have to wait and see when mine gets here.
thanks,
Owen

[DoomPixie]

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

[Eli Duttman]

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

[DoomPixie]

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).

[Eli Duttman]

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.