5E3 Standby Switch Question

[lmolter]

Thanks to all who replied. I should have looked at the '57 Reissue schematic first (actually I did, but it was only a partial image that someone uploaded - I thought it was their own design). Anyway...does anyone have a source for those BYD33V (or was it BDY33V) diodes? And do they have to be that special? Can't I just use a couple of 1N????, 1000V, 3A rectifiers instead?

[fyl]

Anyway...does anyone have a source for those BYD33V (or was it BDY33V) diodes? And do they have to be that special?

BYD33V's are 1,400V fast recovery diodes. Quite difficult to find in consumer quantities and IMO very noisy in audio applications. I wouldn't bother sourcing them.

Can't I just use a couple of 1N????, 1000V, 3A rectifiers instead?

Small 1A diodes are OK in a 5E3, 1,000V PIV is too close to max Vout, one diode might work but you should use two 1N4007's in series per leg, for a 2,000V/1A max rating, or two 1N5408's for 3A.

Get some UF4007's if you fancy fast 1A diodes, or UF5408's for 3A parts. All are quite easy to find.

[lowell]

CT standby and fixed bias

hey yall,
I've got a new design that I'm ironing out some wrinkles on. I've used CT standby but the problem is that the bias supply is at 430vdc when the standby is in the "off" position. I'm guessing this is due to the CT not being grounded. When I first powered this amp up the bias cap exploded like a black cat firecracker!! It was wild! Tons of smoke and capacitor parts all over the inside of the chassis. Anywho the only way I've thought to deal w/ this problem is to have the standby also switch the bias supply on/off. I know this is technically a "no no" because of cathode stripping from the surge when the standby is engaged, but according to Enzo who seems very knowledgeable and experienced w/ these things it shouldn't matter. Anyone dealt w/ this issue on fixed bias amps w/ CT standby? If so, have any alternatives to offer?

[Enzo]

I would not tell someone to turn the bias off and on. I would say that a standby switch is not necessary for tube protection.

But all of that doesn;t tell us anything about your circuit. breaking the CT is a legitimate way to open the B+ circuit. If your bias supply is tapping off the B+ winding however, then you need to accomodate that. You might prefer to tap the bias off the B+ winding, but use a DPST switch for standby, breaking the two hot leads to the rectifier. Take the bias tap off the hot side of the switch.

A fixed bias amp with a separate bias winding won;t care what sort of standby you use.

[lowell]

I should've been more specific when referencing your previous post. If I leave this the way I have it w/ the bias being turned on/off via the dpdt standby is that gonna be hard on the power tubes?

[tubeswell]

I should've added that I get my fixed bias PTs made with a separate 50VAC secondary winding. I guess you could use a 500V bias cap (which shouldn't be a problem if you are using the standby a couple of times each time the amp is used).

[lowell]

duh, why didn't i think of that? This amp DOES have a separate bias winding of 60v, but is upwards of 400v when the ct is not grounded. I'll put a 500vdc cap in there!

[tubeswell]

Thinking further about this, adding (a) high-voltage cap(s) to the bias supply will prevent caps exploding, but the bias voltage will still go out-of-kilter when you flick the standby off (won't it?). (But I don't think this would be a 'bad' thing, because the plates won't be working anyhow).

When I said separate 50VAC winding, I meant that the bias supply on the PTs I get made are actually an entirely separate winding - not a tap from the HT winding at all. So the bias is unaffected by operation of the standby switch on the HT centre tap.

[lowell]

oh... nice! That sounds like a good feature to have on a PT. Do yall think the split second that it takes for the neg bias voltage to be engaged when the standby is flipped will cause cathode stripping or is it not REALLY that big of a deal. Just curious.

[Bruce / Mission Amps]

I don't know why you just don't use a conventional standby switch circuit and break the B+ lead from the rectifier cathode(s) to the first filter cap.
Any 2a to 5a SPST switch will work fine.

[lowell]

I wanted to try a new stby arrangement. Also this seems to have no popping issues when the switch is thrown, though that may be because of the choke. Is that likely? Also, I'm thinking not because there is a lot of voltage on the switch when the ct is not grounded, but is switch longevity improved when place on the secondary CT?

[tubeswell]

I don't think it really matters where you put the standby switch on a cathode-biased amp like 5E3 because output tube bias won't be affected, whether the stby switch is on the HT centre tap or not.

The only thing that possibly would be different in terms of putting it on the HT CT is that there is a much lower potential on either side of the switch contacts in that position, compared with putting it in series in the B+ wire. But as Bruce says, mains-type switches are robust enough to cope with this without arcing.

[Enzo]

I have not found switch life to be an issue no matter where you wire the thing. IN my experience the vast majority of switch failures are mechanical.

[Johnrcurry]

Does a standby switch that breaks the B+ lead have to be rated for the full B+ voltage?

John

[lowell]

No the 250vac rated switches work fine. These switches from Mouser are my favorite: 633-S6A-RO. 633-S6A-RO