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Old 11th October 2012, 05:28 PM   #1
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Default Heathkit WM-5

Im new and need a little help with a couple of heathkit WM5 monoblocks.

I plan on asking a few questions after some more searching, and posting a pic or two.

But for now I just wanted to know if there are any key things I should look up that are specific to these amps.

I have alot of experience with guitar tube amps, but not HiFi.

Thanks

Mark.
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Old 11th October 2012, 11:42 PM   #2
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Before you fire 'em up, check both output transformers for continuity. The earlier Peerless 16309 models are often damaged by excess current. The excess current is often a result of leaky 1.0uF coupling caps to the output tube grids. (This is way too large a value anyway, but that's another story.)

If they haven't been run for a few years, you'll need to reform the power supply electrolytic capacitors. (or replace them - your call.)

Somewhere down the road you may want to consider mods designed to keep the turn-on voltage down - it soars like there's no tomorrow for about 15 seconds, stressing the capacitors.

Let us know what their current condition is, and folks will have more to say.

All good fortune,
Chris

Last edited by Chris Hornbeck; 12th October 2012 at 12:09 AM.
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Old 12th October 2012, 02:44 AM   #3
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I took the bottom off of one to see just how far he went with them and was suprised at how much he actually did to them. Here are some pics. I dont even know if this is the one that smoked, but it has one tube socket that is in question. There is a jumper lead that has bad and burnt insulation on it. Its the bottom pic. Tomorrow I am gonna try to get a few pics in without taking up too much room.

The bottom one has the tube socket with the burnt jumper.

Click the image to open in full size.
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Old 28th November 2012, 02:04 AM   #4
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Hi Chris

Quote:
Originally Posted by Chris Hornbeck View Post
Before you fire 'em up, check both output transformers for continuity. The earlier Peerless 16309 models are often damaged by excess current. The excess current is often a result of leaky 1.0uF coupling caps to the output tube grids. (This is way too large a value anyway, but that's another story.)
Should 0.47uf be better?

Quote:
Originally Posted by Chris Hornbeck View Post
Somewhere down the road you may want to consider mods designed to keep the turn-on voltage down - it soars like there's no tomorrow for about 15 seconds, stressing the capacitors.
Can you teach me more about this i.e. how to re-design to keep the turn-on voltage down?

Thanks a lot and regards
Andersen
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Old 28th November 2012, 04:14 AM   #5
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Quote:
Originally Posted by Andersen View Post
Should 0.47uf be better?

Can you teach me more about this i.e. how to re-design to keep the turn-on voltage down?
You'd ideally like to stagger the RC coupling time constants by a factor of maybe 4 or 5, and to have the second one, at the output valves' grids, "dominate" the overall frequency response. An RC product of about a quarter second (or a little less) is a common choice, giving a good balance between frequency response and overload recovery time. For the Heathkit W-5M's 100K Ohm grid resistors I'd be tempted to choose coupling caps of 0.1 to 0.22 uF.

Soaring B+ voltages are always a problem for capacitors, but are exaggerated in the W-5M because of the 5R4 rectifier's unfortunate combination of very fast warmup and moderately high internal losses. Its low-ish perveance means that the power supply has to be designed with extra transformer voltage to accomodate, just exaggerating the unloaded voltage issues. But that's the breaks.

In the early 1960's Heathkit tried supplying what were then called "varistors", but were really a heated bi-metal switch, in the transformer primary. This almost worked, but the capacitors were newer then (weren't we all?).

I've had some luck adding a positive tempco resistor in series with the rectifier's filament. Modern AC line voltages are so much higher than the transformer's original design that there's plenty of voltage left. You might also experiment with heater/cathode rectifiers like GZ34 or GZ37, but you'll have to deal with the higher B+ they'll give. I've had good luck converting to a choke input, but needed to get the voltage soaring issue dealt with first.

All good fortune,
Chris
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Old 29th November 2012, 02:33 AM   #6
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Thank you very much for your response, Chris. Is there any reason for Heathkit to use 1uf coupling caps in that position while using 0.1uf coupling caps in earlier stages?

Currently, I am using 5U4Gs instead of 5R4s. Should I add a 1K resistor right after the 5U4G and before the choke in order to reduce B+ a bit?

Thanks and regards
Andersen
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Old 29th November 2012, 05:04 AM   #7
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Quote:
Originally Posted by Andersen View Post
Is there any reason for Heathkit to use 1uf coupling caps in that position while using 0.1uf coupling caps in earlier stages?

Currently, I am using 5U4Gs instead of 5R4s. Should I add a 1K resistor right after the 5U4G and before the choke in order to reduce B+ a bit?
There are several things about the W-5M design that we can criticize, with the ease of 20/20 hindsight. DTN Williamson himself, who made the original design, made some of the same imperfect choices in his first amplifier article (in Wireless World, very late 1940's), then corrected them in follow-ups to the original article. It's his very clear thinking that I'm parroting in my recommendations here.

A 1K Ohm resistor is very large - at 120mA that's a drop of 120 volts and considerable heat dissipation. If you don't mind a few dB less maximum output, you could try choke input filtering. Easiest thing in the world to just temporarily remove the input cap's and try it. You'll get a B+ somewhere in the range of +375 VDC and the power transformer will thank you for lightening its load. Just a thought.

All good fortune,
Chris
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Old 29th November 2012, 05:39 AM   #8
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Quote:
Originally Posted by Chris Hornbeck View Post
If they haven't been run for a few years, you'll need to reform the power supply electrolytic capacitors. (or replace them - your call.)
Please help me to understand why it is a good idea to "reform" an old, dried out electrolytic capacitor. This is most certainly bad advice and not worth the time or effort considering the replacement cost of the capacitor(s).

Last edited by scott17; 29th November 2012 at 05:44 AM.
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Old 29th November 2012, 07:06 AM   #9
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Many thanks, Chris.
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