Standby Switch WARNING

a DIY, modified Mullard 5-20 monoblock design

Standby Switch WARNING

Postby Shannon Parks » Sat Nov 27, 2004 1:36 pm

I had a customer send me one of his new Ike monoblocks after it was consistantly blowing fuses on power up. I sent him several suggestions, which he patiently implemented to no avail, so it seemed like a good amp to debug on my bench.

It ends up being related directly to the HV center-tap implemented standby switch. After placing caps across it and using IRCLs to tame things, I figured the current was under control, so I needed to look at the secondary taps more closely. Then I about blew up my DMM a few times trying to take measurements, getting 2kV DC in the secondary from either leg to ground (with no tubes and center tap lifted, ie standby off).

At the least, this can cause a eventually blown diode which then causes a blown fuse. While having some load in there, like tube filaments, may help tame the high voltage peak, I'm not willing to risk my DMM or anyone's health on it. I'm going to discourage the use of standby switches here on out.

Those with standby switches, leave them 'on' all the time. Better yet, disconnect the center tap from the switch and connect it directly to the PCB. With the center tap grounded, I have seen no problems whatsoever. The delayed turn on benefit is small, in my opinion, to the damage that can be done otherwise.

I've researched this problem and can't find anything. Using diodes in series (two UF4007's for 2kV rating) and a high voltage cap strapped across the primary/secondary or to ground might alleviate some of this problem. But I'll see if anyone has some experience in this area and wants to share. My personal opinion has probably been made, choosing to err with caution. I tested a PA774 and saw the same thing, so this is not Hammond or diytube specific.

If you have blown fuses, be sure to check your diodes first thing. With the amp off and bled, check with your DMM on the diode setting. Touch the black probe to the band (cathode) and the red to the anode. You should see a voltage drop of about 500mV. If you swap the black and red probes, the diode should appear as an open. Otherwise, you've got a blown diode. Swap them both out and disconnect your standby switch while you are at it.

Tube gurus, opinions welcome. Let's keep this a fun, safe hobby with gear that lasts a lifetime.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3764
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington

Delaying B+

Postby Gary Kaufman » Tue Nov 30, 2004 10:33 am

Shannon -

There are only two good ways to delay B+ that I've found to be reliable. Much of my experience comes from Mercury Vapor Rectifiers, where delaying the B+ is essential.

The first way is to use a separate plate transformer, so that the primary of the plate transformer can be switched on after the filaments come up to temperature. There is even a cute trick using two switches - so that regardless of which switch you throw first the filament always come on first and off second.

The second way is to use a indirectly heated tube rectifier, such as the 5AR4/GZ34 or a damper diode.

Delaying the B+ in the EIclone is probably not necessary - as the risk of "cathode stripping" really is overstated. Many top notch amplifiers (such as the Mcintosh Mc75/275) use sold state rectifiers without a delay of any sort and have very long tube life.
Gary Kaufman
 

Postby mesherm » Tue Nov 30, 2004 5:36 pm

I design industrial control systems, so on my ST-70 clone I used a DPDT industrial adjustable time delay relay. I ran each of the two diode rectified transformer legs to a separate pole of the DP relay through a 10k 5 watt resistor (on each pole) then on to the filter network. I set the timer to 30-45 seconds. After the timing period the relay shorts across each resistor applying full B+. I selected the 10K to give around 90 volts or so of B+ maximum while the tubes warm up. The timing relay used an octal socket so I have it sitting where the rectifier tube was before. The relay cost me about $29 from Grainger and is adjustable from .1 sec to 100 minutes. So far it has worked like a charm. I also put a surge protector on the PT primary.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
User avatar
mesherm
KT88
 
Posts: 1232
Joined: Fri Aug 27, 2004 10:33 pm
Location: Alvin Texas

Re: Delaying B+

Postby Shannon Parks » Wed Dec 01, 2004 5:11 pm

Gary Kaufman wrote:Shannon -

There are only two good ways to delay B+ that I've found to be reliable. Much of my experience comes from Mercury Vapor Rectifiers, where delaying the B+ is essential.

...

Delaying the B+ in the EIclone is probably not necessary - as the risk of "cathode stripping" really is overstated. Many top notch amplifiers (such as the Mcintosh Mc75/275) use sold state rectifiers without a delay of any sort and have very long tube life.


Thanks Gary - this all makes sense.

Shannon
User avatar
Shannon Parks
Site Admin
 
Posts: 3764
Joined: Tue Mar 18, 2003 5:40 pm
Location: Poulsbo, Washington


Return to eiclone

Who is online

Users browsing this forum: No registered users and 5 guests

cron