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Old 23rd March 2005, 03:02 PM   #1
Giaime is offline Giaime  Italy
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Default Perser SA20 Restoration

Hello there! I finally have some spare time to restore this old italian stereo integrated amplfier. It's a PP of EL84, with an ECC82 driving them, a GZ34 rectifier and 2 ECC83 for the preamp.
I think it dates in the early '70s. It was of a friend of mine that hadn't a use for this, so he gave it to me. He plugged it in (I would never had done this: imagine the old rectifier that shorts and you blow the PT). It has the cathode bias and it looks like a class A, with 10W per channel (since I know another one that owns a Perser, and it is marked SA60 but it is a 30W per channel, so mine should be 10W for channel, and obtaining that from a EL84 PP would be class A or very hot class AB, or just a normal AB rated at conservative ratings).
I have a few questions:

- Power supply capacitors: I know I have to change them because they're old and not been used for a long time, but I'd like to keep the appaerance of them original. They're in metal cans: so can I open them, throw away all that's inside and put new capacitors inside them (because new capacitors are smaller than old ones)? They're 2X50+50uF plus a single 50uF, all 350V.

- Tubes: Do I have to replace them just because they're old? They're Philips miniwatt for the rectifier and 2 of the EL84, and Siemens for all the others. Nice NOS ones, aren't them? But an EL84 has a burn mark inside it, like a little explosion on the inside glass, just out of that holes in the plate. It seems to me that it's screen has burned, since I found a burned screen resistor under one of the EL84 sockets, don't know if that was the one of the burned valve. I don't have equipment to test them, so I think I have to replace the EL84 and the rectifier just for safety; I would like to keep the pre tubes instead: the ECC82 driver has wonderful chromed plates...

- Connectors: on the rear of the unit there is a mess: Speakon speaker connectors (! I hate them), DIN inputs, old-time 2 prong mains plug. There is also a 9-pin socket (identical to EL84 ones) that has many wires attached to it, some goes to the mains in some to the PT. I think it is a kind of mains voltage selector... It does miss the fuseholder since there's a circular hole near the mains plig...I would like to get rid of all these things, and put RCA connectors, IEC mains in with ground, and today-standards speaker connectors. I have found an IEC plug in a PC pwr supply: it has 2 small toroidal inductors in series with the mains. They are for filtering interferences: can I keep them? And also can I delete the two 0.01uf caps from the original mains plug to the chassis? What's their purpose?

- Do I have to replace other things? The components seem old but don't look bad. I know I have to replace the cathode bias capacitors, since a leak in them mess up the bias. And what about that 220ohm screen resistors on EL84? Do today's production EL84 can live with them or it is the case to increase them to maybe 1k?

- How, when this all finished, can I test it? Do I have to use a variac?

- It is a bit too low in power for normal sensitivity speakers like my AR18s?

What do you think? If I will remember something else I will post...
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Old 24th March 2005, 08:04 AM   #2
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I (not too long back) acquired a Technics all tube AM/ FM stereo receiver from I think 1966.

When time permits, I must lift the lid and see what is inside.
I believe we havenít had much here on valve amplifier restoration, yet . .
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Old 24th March 2005, 08:29 AM   #3
Giaime is offline Giaime  Italy
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Quote:
Originally posted by rick57
I (not too long back) acquired a Technics all tube AM/ FM stereo receiver from I think 1966.

When time permits, I must lift the lid and see what is inside.
I believe we havenít had much here on valve amplifier restoration, yet . .
Wow! Wonderful piece! You are also lucky because you have something to date your piece of equipment. Mine hasn't a date code, and also its brand (Perser) disappeared long time ago. It was a small italian brand sited near where I live. Only a person that I meet in an italian forum knows that brand. I found that he have the bigger brother of mine, a 30W per channel that uses some obscure pentodes (he says they are the same of many famous american juke boxes, and they're very rare).

I hope that this thread will help in your restoration!
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Old 24th March 2005, 10:42 AM   #4
Giaime is offline Giaime  Italy
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By the way, what kind of fuse should I use? 1.5A is enought?
I have a F1.6A fuse, that means that is fast-blow? Do I need a slo-blo?
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Old 24th March 2005, 11:38 AM   #5
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And why my mains line has a 0.047uF cap IN SERIES with it?
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Old 24th March 2005, 11:50 AM   #6
SY is offline SY  United States
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Yes, replace any electrolytics. If your caps were the twist-lock sort, there are replacements available. I wouldn't bother with hollowing out the old cans- the new ones will fit fine and you might even be able to get more capacitance in the same size package (though don't go overboard on the first cap after the rectifier!). If there are any electrolytics in the signal circuitry, replace them, too.

Replace the tubes. You want to get the thing working, tubes are pretty cheap, so toss the old EL84s, keep the other old tubes as emergency spares.

I seriously doubt that the mains has a series cap. A shunt, maybe. Double-check your circuit tracing. Make sure that you use a three-wire mains cord with the ground lead firmly connected to the chassis. Safety!

Clean the potentiometers and switches thoroughly. They're probably the weak spot, so be prepared to replace controls that are intermittent or scratchy. Do replace what connectors you can.

If the coupling caps are the paper sort or look even slghtly dodgy, replace them. A leaky coupling cap can cause lots of smoke to come out.

A Variac can save you a lot of grief. Highly recommended. If you can't swing one, you can rig up a poor-man's variac by putting a light bulb (say, 60-100W) in series with the mains feed. At power on, if it glows brightly, you've got a short somewhere. If it glows dimly or not at all, you're probably safe to connect directly to the mains.

Yes, you want a slow-blow fuse. 1.5A or thereabouts sounds right.
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Old 24th March 2005, 12:46 PM   #7
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Quote:
Originally posted by SY
Yes, replace any electrolytics. If your caps were the twist-lock sort, there are replacements available. I wouldn't bother with hollowing out the old cans- the new ones will fit fine and you might even be able to get more capacitance in the same size package (though don't go overboard on the first cap after the rectifier!). If there are any electrolytics in the signal circuitry, replace them, too.
Yes, I'll do but I don't want to spend money on expensive replacement caps: I'll stick normal caps inside them. Just for cheap...

Quote:
Originally posted by SY
Replace the tubes. You want to get the thing working, tubes are pretty cheap, so toss the old EL84s, keep the other old tubes as emergency spares.
I'll do... I bet that at the next flee market in Padua I'll find NOS ones... really.

Quote:
Originally posted by SY
I seriously doubt that the mains has a series cap. A shunt, maybe. Double-check your circuit tracing. Make sure that you use a three-wire mains cord with the ground lead firmly connected to the chassis. Safety!
No No I'm sure. I'll attach a schematic... I obviusly will delete all that stuff and put a double-pole switch, just after the fuse, and ground.

Thank you!
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Old 25th March 2005, 12:02 PM   #8
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Mmmmm... I have desoldered the filter caps, and I noticed there's no negative lugs on them. Are they supposed to be connected to ground throught the chassis? Do I have to keep this or I must connect to the chassis with a wire? Is there any improvement in increasing the capacity (obviusly not the first capacitor since more will blow the rectifier)? I'll attach a schematic...
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Old 25th March 2005, 01:02 PM   #9
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Could you take a pic of this amp ? I have never seen a tubeamp named Perser.
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Old 25th March 2005, 02:07 PM   #10
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I'll try to post pictures soon. Anyway, let's have some news from the front:

- I finished wiring the new IEC mains plug with safety ground, the fuse, the input RCAs (instead of old DINs) and the speaker outs. For those, I didn't know the polarity, since the 2 wires are the same color. I did this: since in many amps one side of the OT secondary is grounded, the side that has the lowest resistance from ground is actually the speaker negative. The other one is positive. Is that correct? I got something like 0.3ohms on one side and 1ohm on the other side...

- Pwr supply caps: I removed them and opened the metal cans. One of them (to my surprise not the one who supply heavy currents in the EL84 plates) looks like it has overheaten, and some red gook has invaded the can... I just feel too afraid of dismateling them, I thought there was the possibility of them containing something toxic, and I put them in a sealed bag waiting for proprer disposal. I recycled the plastic insulators and I decided to connect the grounds of the new caps with a wire bolted on a chassis screw, like a safety ground one. The original caps made ground contact throught their metal can, directly to the chassis. This is destroying the star ground system! I bet that at Perser they know what they did...

- I removed the cathode bypass caps at the EL84 cathodes (one for each couple of EL84, 100uF 25V) and those on the driver tube cathodes (25uF 30V). I think to replace them, since a leak on them would cause the EL84 or the ECC82 to be unable to obtain proper bias.

- I will replace the 120ohm 8W (strange unusual power rating) that goes between the GZ34 cathode and the caps, it looks overheated. And also there is a 220ohm screen resistor to the EL84 that looks like it has exploded (really!), will change that too. Btw 220ohm would be too low for modern EL84s? Do I have to raise it to maybe 470ohm or 1k? I don't know the B+ voltage (that would give us some clues) but the pwr supply caps are rated at 350V... If it's really class A I would bet for around 230/250V at the caps, since it is the maximum for EL84 to be biased in class A. Any higher would automatically be class AB to avoid exceeding plates max Pd. I bet for about 310/320V...
I hope for class AB, so my tubes will last longer and my speakers would have something to work with... they're old-time AR18s... normal sensitivity.

I'll give news when I'll buy the parts I need...
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