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In Reply to: RE: Time Delay or not? posted by pictureguy on September 22, 2021 at 18:36:54
"First? If I didn't want the TD relay, could I simply install a switch so that after the heaters came up, I could manually turn ON the HV?"
A separate transformer for HV and filaments, each with its own switch on the primary side is ideal and simple to implement.
Been running such an arrangement on a tube circuit with SS bridge rectifier and directly coupled stages for years with no problems.
DT667
Follow Ups:
I agree about using a filament transformer, and it would allow the use of a damn clever idea, which I think (but don't quote me) came from the RCA Manual. That's this switch wiring configuration (shown above) which, no matter which switch is thrown first, always allows the filaments to see power first. Simple, but brilliant.
As I mentioned in my reply, you'd have to find space for the extra transformer, and as you can see from the ST-120 photo below, there isn't any space. He could use a larger chassis, but then buying the kit isn't practical. And from reading the original poster's other posts, I believe a kit is what he's considering.
Good lord, it's my diagram. I found the circuit in an old Glass Audio years ago. I bet whoever first dreamed up this circuit was extremely proud of themself.
I still use the dual switch arrangement for most of my amps.
ray
As they should have been!
I looked through my Glass Audio magazines and couldn't find the article. I did find the article from a 1994 Sound Practices. It was from a direct coupled 6922 - 2A3 amp by Ciro Marzio and Cristiano Jelazi.
Either I don't have that issue, or I forgot I saw it in Sound Practices.
I first saw this switch arrangement in Sound Practices in the '90s. I use it on everything now.
You can throw either switch and wait until the heaters / filaments warm up them throw the other for the B+. Same for shutdown. It doesn't matter which of the two switches are thrown first.
At the time I thought this was like magic. Easier to implement and more reliable than a relay.
I also use this switch arrangement, but with a twist. I connect a suitably sized resistor between the lower pole of switch 2 and the upper pole of switch 1. If you throw switch 2 first this prevents a current surge to reach the filaments of your precious DHT:s. When you throw switch 1 the resistor is taken out of the circuit. You have to remember to always throw switch 2 first, burt that shouldn't be much of a problem.
If you use a resistor as described, you're not wiring it as per the schematic. The beauty of this arrangement is that the filament transformer always goes on first regardless of which switch is thrown. Throw 1 first and the filament transformer is on. Throw 2 first and the filament transformer is on. Wait 10 or 15 seconds and throw the other switch for B+.
If you have a resistor between the lower pole of switch 2 and the upper pole of switch 1, it's across the line voltage input. It would be in-circuit always.
I think I am wiring the switches as per the schematic. The only difference is that I replace a wire with a resistor. When I throw switch 1 the filament transformer is fed directly from switch 1. There will be very little, if any, current across the resistor between the lower pole of switch 2 and the upper pole of switch 1 when switch 1 is engaged. As long as you remember to throw switch 2 first this works very well and it will keep your DHT:s very happy..
"As I mentioned in my reply, you'd have to find space for the extra transformer, and as you can see from the ST-120 photo below, there isn't any space."
Outboard the filament transformer and add 2 switches.
Creative thinking wins the day here.
Or buy a Class D amp with more power and smaller form factor.
DT667
ddt67Why are you promoting class D? This is nothing special except that this is a high efficient type of amplifier. The distortion figures of this amplifier are nothing special and in line with what has been made in the last 50 years.
Ohh, i forgot to tell that Clas D is frequenty limited. That it is like CD really bad in high frequencies
Edits: 09/23/21 09/23/21
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