Simms Watts Mk2 100watt head

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Simms Watts Mk2 100watt head

Postby ross_worth » Sat Jan 05, 2008 10:38 am

Hi I have an old simms watt 100 watt guitar head, not the pa version. I have just got my hands on a bias probe and have had a quick look at the amp.

I am getting a reading of 74mV from the Groove Tubes probe which seemed really high and that is as low as the bias pots will let me go. I am using EL34s but I am not sure if the amp has been modded to run something else.

Also the amp only has one of the three preamp valves inplace, should theese all be ecc83s and is this likely to affect my readings for the power amp section?

I am new to servicing valve amps but I have sorted my 50 Watt plexi and this is my next challenge. Many thanks in advance for your replies.

Cheers,

Ross
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Postby erichayes » Sat Jan 05, 2008 10:25 pm

Hi Ross,

I don't know if this http://www.schematicheaven.com/newamps/ ... _pa100.pdf
is your amp, but let's assume it is (or is close).

I'd first check the voltage at the cap of the bias filter network. If there isn't at least -60 volts there, I'd suspect the cap, or, if it's selenium, the rectifier itself. Given the amp's age, I think I'd replace both.

FWIW, this isn't my paradigm of a bias supply, but they might have been going for a grunge sound. Those output tubes are totally dependent on the health of that cap.
Eric in the Jefferson State
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Postby ross_worth » Sun Jan 06, 2008 7:02 am

Thanks for the reply, I will check the voltage at the cap later today.

It is a different schematic to my amp but I think the power amp is nearly identical.

The ony thing that occured to me was that the amp has maybe been rebiased for 6L6's would that make sense? I'll also check the plate voltage today aswell.

Thanks,

Ross
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Postby ross_worth » Sun Jan 06, 2008 11:58 am

OK just had a look at the amp and it is now giving some sensible readings on the bias settings with no changes!

I think I should replace the capacitor anyway it is 47uF at 160V so what are my options for replacement?

Can I use a 47uF @ 500v cap or does it not work like that! Also what are the best to use from experience.

Thanks,

Ross
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Postby erichayes » Sun Jan 06, 2008 5:26 pm

Of all the common output tubes having 7AC/7S/8ET basing, the 6L6 has the lowest transconductance (read: sensitivity). This means that, for a given output power, the 6L6 will require more drive voltage. Consequently, more bias voltage is needed to keep the tube from going into meltdown.

What all this boils down to is that an amp with adjustable bias and designed for 6L6s will work with EL34s (assuming the power transformer can handle the extra 600 mA/tube heater current) simply by lowering the bias voltage. The reverse, however, is not necessarily true; the values of the voltage divider resistors in the bias network generally have to be changed to get the higher bias voltage the 6L6s require.

If you want to keep the sound of the amp as close to original as possible, stick with the 47µF @ 160V cap. Mouser and AES both carry them. The higher voltage cap might not reach full capacitance at that low a voltage. If you want a slightly cleaner sound, go to a 100µF @ 160V cap. Sprague "Atom" and Illinois Caps are safe bets.
Eric in the Jefferson State
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Postby ross_worth » Mon Jan 07, 2008 1:01 pm

Thanks for that, I'll try and find a uk stockist for those caps, I'll give you guys an update when I've carried out the work!

Thanks,

Ross
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Postby erichayes » Mon Jan 07, 2008 9:56 pm

Farnell is probably your best bet. http://www.farnell.com

BTW, for some reason, guitar amps made on your side of the pond, regardless of manufacturer, use power transformers that are run maxed out to the limit. They don't take kindly to mods that involve increases in heater or B+ current in the output circuitry.
Eric in the Jefferson State
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