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Old 7th January 2010, 11:33 AM   #1
urmoo is offline urmoo  Estonia
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Default Q about STB switch

Probably totally rookie question, but still- if the FRED's are not installed, is it safe to use STB switch?
One of my FREDs finished its life after few hours of working (no STB switch installed!) and also blew away R1. I uninstalled FREDs and since then I use only 5AR4. Recently I got few Russian 5Ц3С, which should fit perfectly, just one problem- they are directly heated lamps and therefore STB switch is necessary...
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Old 7th January 2010, 06:18 PM   #2
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George (Tubelab) recommends against installing a standby switch on his website (top of the page) as some transformers can create a 2.5kV spike when the standby switch is used, which tends to destroy FRED's quite violently. Also, the FRED's that were on the parts list are not usually used anymore, as QC seems to have gone downhill and they have a tendency to fail at power down.

My Simple SE, which has no standby switch, uses 5U4GB rectifiers and works perfectly, so I would be inclined out of my experience to say do not use a standby switch.
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Old 7th January 2010, 06:45 PM   #3
urmoo is offline urmoo  Estonia
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Thank you for your answer! I understand, that STB switch can burn FREDs down, but as I dont have FREDs installed- is it safe to use STB switch? Could 2.5kV spike harm other parts of amp?

Why it is important for me- 5AR4 is undirectly heated rectifier and heating up takes some time, which allows KT66 to heat up before high voltage hits them. 5Ц3С is directly heated rectifier and therefore HV will hit KT66 while they are cold. As result the lifetime of tubes will decrease significally.

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Old 7th January 2010, 08:07 PM   #4
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I've used the center tap standby switch on one of my other amps (but not my Simple SE). I've never had any trouble with it whatsoever. That particular amp even happens to have solid state power supply rectifiers (1N4007 silicon diodes). The SS rectifiers have a similar issue as the directly heated tube rectifiers: B+ comes up very early.

I would urge you to consider a few small precautions, should you decide to try it. I installed a 1 meg ohm resistor across the contacts of my standby switch. This ensures the center tap of the power transformer is never 100% isolated from ground. Some of those who have studied the problems concerning the CT standby switch have theorized that a PT which is left floating might be to blame for some of the troubles. Putting a 1 meg ohm resistor across the standby switch ensures it always has a ground reference. You might get a couple volts of B+ out of the power supply, even with the standby switch open, but it is not enough to make any difference to cold power tubes. The resistor can be tiny - I used a 1/4 watt part.

The other recommendation I would make is to ensure you never open the standby switch while the main power switch is closed! There can be substantial inductance in the PT secondary, and trying to open the standby while the PT is energized will likely result in high voltages and possible arcing across the standby switch. Always open the main power switch first, then open the standby after the amp is powered down. Some people have even gone as far as to construct their standby out of relays and timers for fully automatic operation.

Of course, just because it has worked for me doesn't mean it won't blow up in your amplifier. Try it at your own risk, legal disclaimer goes here.
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Old 7th January 2010, 09:48 PM   #5
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Quote:
I would make is to ensure you never open the standby switch while the main power switch is closed!
One user reported that his 5AR4 sparked out when the STB switch was opened. There were no diodes in his amp.


Installing the STB switch in the HV center tap used to be a common practice. Look at many old guitar amp schematics. Any time you interrupt the flow of current through an inductance there will be a voltage spike. This is how the ignition coil in your car works. In a perfect transformer the collapsing magnetic field would transfer its energy to the other windings that have a load attached to them (the heater windings). Transformers are far from perfect and many of the currently produced transformers are a bit worse than the old ones, and so are the currently produced rectifier tubes. I have measured a 2500 volt spike on an Allied 6K7VG power transformer (made by Hammond). That is enough to blow up FREDS and spark out a rectifier tube. Not all transformers are as bad as this, but there are probably some out there that are worse.

As TY said turning the main power off before turning the standby switch off will solve the problem, but one mistake can zap a tube. If you want to try the switch, some people have reported success by putting a resistor and a capacitor in parallel across the switch. The resistor should be 470K to 1 Meg 1 watt or so, but the cap needs to be .01 uF at at least 1KV which can be rather hard to find. I have not tried this though.

There is some debate as to whether the early DC will shorten the life of your KT88's. There have been millions of TV's, radios, and HiFi sets produced over the years with 5U4's for rectifiers. No one has complained that they killed tubes. I just parted out an old Baldwin organ amp built in 1962. 7 of the 8 6BQ5's were original and they still work. All powered by two 5U4's.
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Old 7th January 2010, 10:48 PM   #6
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OK, thank you for your answers! As Russian rectifier cost me next to nothing (3$), its OK to blow one of them during some testings.
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