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Old 27th January 2005, 07:45 AM   #1
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Default Mono block restoration

Hi, I have just purchased a pair of Mullard 20 mono blocks that at the very minimum need a chassis repaint and new speaker output posts fitted. As I have never soldered,should I attempt to remove transformers etc and note where wires are from etc, or is this a job left for someone technically minded. I don't mind getting my hands dirty doing this, just don't want to screw up when it comes to resoldering.
And would your standard Hi Fi repairman be able to do this?

any help appreciated

Chris in Brisbane Australia
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Old 27th January 2005, 08:23 AM   #2
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Hi Chris,

I don't know what a Mullard 20 is, but if it is a Mullard 5-20 these are quite well known, and were built as mono blocks. A schematic has been posted on the forum a little while ago here, and a picture was posted as well. It is possible that yours mightn't look like that due to a chassis layout revision or something, I don't know - not very well versed in vintage stuff.

Generally the things will need replacement are power supply electrolytics, and occasionally leaky coupling caps or old resistors that have drifted out of tolerance. If your valves still work, don't let anyone swindle you by offering new production ones for the old ones (especially if they're Mullards). If it turns out that they're bad, all of the valves used in that amp are still in current production (EF86, ECC83, EL34, GZ34). Valves can last for decades, I know of a Leak amp which still works with its original KT66s - I am sure there are many others.

But beware! There is 465VDC in that chassis that can make you very dead!

I don't know if the local hi-fi repairman could handle something like that. I suspect many might not have ever worked on valve equipment before. You'll basically just have to ask them.
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Old 27th January 2005, 02:21 PM   #3
Sherman is offline Sherman  United States
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Default BE VERY CAREFUL!

As Jason said the voltages inside can kill you. If you have never worked on tube equipment before it is a good idea to get someone to help you out.

* Read the safety rules in this forum.
* Re-read the safety rules in this forum and print them out.
* Don't work inside while it is powered up.
* Make sure the caps are discharged before touching anything inside even after the power is turned off. Caps can hold a charge for hours or even days.
* When taking measurements inside the chassis keep one hand behind your back and make sure your meter probe is in good condition with no insulation breaks.

Having said all that, rebuilding a tube amp can be a great introduction to the hobby and can give excellent results, especially when you start with good equipment as you are doing.

Be careful, be safe and have fun!
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Old 27th January 2005, 03:34 PM   #4
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Hi, I'm very much a beginner as far a construction is concerned, but I've just rebuilt a pair of vintage Pye HF25 amps that have a kinda similar circuit.
My experiences were:

- have great respect for the voltages!

- take pictures/notes before you start and save everything you remove.

- The only thing that really matters is the output transformers. You can change/get/substitute just about anything else.

- As these things have been around for maybe 50 years, don't trust that someone else hasn't screwed them up. Especially check the output transformer taps, they've often been altered to try to get a different output impedence (If they have, c9/r13 will probably be different, too)

- The wiring itself I found to be really sound - the insulation was still intact and safe, so do the minimum to get up and running at first.

- Expect every capacitor to be leaking - even the ones that look perfect, except maybe the little flat mica ones. Use decent quality modern caps to get it going. If you want to, you can upgrade one-by-one later.

- It's worth unsoldering one end of every resistor to measure it as they can drift, but many may be OK. There's usually 20% tolerance on most anyway.

- Enlist some expert help from someone with a 'scope who knows how to diagnose problems on this kind of stuff. One of my amps behaved beautifully after just a parts replacement and simple setup, but on the other what appeared to be a bit of low-level hum showed up on the 'scope as oscillation. Took much longer to track down than the whole rebuild and was beyond my ability!

- if you need valves, avoid second hand output valves. Until the amps are running perfectly, budget Russian replacements get you up and running.

- When it's running really nicely you can look at maybe high-quality parts in the signal path (maybe not, depends what you believe!) and hiding the modern repair parts (like caps) inside the old ones if you want a cosmetic restoration. But then, once they're running, you'll be enjoying them so much you won't want to take them apart again!

regards, jeff
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Old 28th January 2005, 04:45 AM   #5
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Thanks for the advice, I respect electricity too much to play around with things I know nothing about. As it happens, only the chassis needs a repaint, and speaker binding posts added.

Guess I will find a Tech to do all the soldering stuff, and yes it appears that my mono blocks are very similar to the 5-20 pictured, but with different transformer. Its hard to tell, I am still awaiting arrival of them.

Thanks guys
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Old 29th January 2005, 01:24 PM   #6
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G'Day Chris,

I have just restored two of these amplifiers and then I built another two, all for use in a Home Theatre set-up, *as well* as for stereo sound reproduction.

Pay careful attention to the insulation of the wiring, I re-wired my original two amplifiers completely because the insulation had started to become brittle over the years. I replaced all the coupling capacitors and also the electrolytics in the cathode circuits. But I have retained the original Filter Electrolytics (they are beauties, I wasn't going to throw them away!).

The valves are all original Mullard and still well up to spec. Once I stripped the metal chasssis down, I had it powder coated and covered with a clear lacquer. The effects are stunning!

If the amplifiers have not been in use for a while, either power them up (connected to a speaker) using a variac, or otherwise wire a normal 240v/100w domestic globe in series with the AC mains input for a couple of hours to allow the electrolytic capacitors to "reform".

Refer to the following link on the Mullard 5-20, you will find lots of information there as well as pictures of amplifiers restored by other people.

Mullard 5-20

Good luck, they are excellent-sounding amplifiers.

-Eric
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Old 29th January 2005, 01:27 PM   #7
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Quote:
Originally posted by Family_Dog
If the amplifiers have not been in use for a while, either power them up (connected to a speaker)
Preferably a cheap one just in case something goes awry
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