A couple questions about an AB763 build

[turbofeedus]

Hey all, working on this delxue I'm building into a Sundown chassis.

DIYLC layout *there's a couple errors and omissions, I know*
Schematic off which I'm working
pics of the current state
A few pics from before the teardown

I have few questions;

1) Sundown originally had the heater center tap left unconnected. Is there some general reason for this, or was it maybe just specific to the original sundown circuit that yielded better results? My gut would tell me to just ground it by default. If I don't have to bother with the artificial ground trick I'd rather not.

2) The sundown didn't have a choke, so I'm using just a 1K dropping resistor in that "position". This is what the sundown schematic used, and the two power supplies were similar. Is this reasonable?

3) I'm using the bias circuit out of Merlin's article, included the diode/resistor drain. Sanity check on the wiring?

4) This is a more general question, but what's the consensus for the physical mounting of transformers? These are Schumaker transformers which have a nut that holds the iron together. The PT is "floating" on those nuts when mounted on the chassis. I have other transformers however, that are mounted with the iron straight touching the chassis, no nut. Is there a "correct" way to Z mount?

[pompeiisneaks]

Hey all, working on this delxue I'm building into a Sundown chassis.

DIYLC layout *there's a couple errors and omissions, I know*
Schematic off which I'm working
pics of the current state
A few pics from before the teardown

I have few questions;

1) Sundown originally had the heater center tap left unconnected. Is there some general reason for this, or was it maybe just specific to the original sundown circuit that yielded better results? My gut would tell me to just ground it by default. If I don't have to bother with the artificial ground trick I'd rather not.

2) The sundown didn't have a choke, so I'm using just a 1K dropping resistor in that "position". This is what the sundown schematic used, and the two power supplies were similar. Is this reasonable?

3) I'm using the bias circuit out of Merlin's article, included the diode/resistor drain. Sanity check on the wiring?

4) This is a more general question, but what's the consensus for the physical mounting of transformers? These are Schumaker transformers which have a nut that holds the iron together. The PT is "floating" on those nuts when mounted on the chassis. I have other transformers however, that are mounted with the iron straight touching the chassis, no nut. Is there a "correct" way to Z mount?

1. you can just ground the center tap BUT an artificial center tap actually provides a cheap emans of 'fusing' if anything goes too hot on the heaters, they burn up before anything else and end up saving your tubes and other parts of the circuit, so since its a. easy to do, and b. cheap, it's worth it to not use the center tap.

2. So long as that 1k dropper is the same resistance the choke provided, yes you can. It gives a slightly different response since you've lost a bit of the filtering the choke provided, but so long as the other CRC filters etc, work to filter the PS, then it should be fine.

3. Sorry at work don't have time to check - hopefully someone else can chime in.

4. So long as the transformer is solid, I don't see any consequences of the transformer sitting on the nut, and having another nut on the inside to bind it to the chassis. Others may have better input though.

~Phil

[turbofeedus]

1. you can just ground the center tap BUT an artificial center tap actually provides a cheap emans of 'fusing' if anything goes too hot on the heaters, they burn up before anything else and end up saving your tubes and other parts of the circuit, so since its a. easy to do, and b. cheap, it's worth it to not use the center tap.

Thanks Phil, that's interesting. I suppose the wattage of the resistors is important then, what do you recommend?

2. So long as that 1k dropper is the same resistance the choke provided, yes you can. It gives a slightly different response since you've lost a bit of the filtering the choke provided, but so long as the other CRC filters etc, work to filter the PS, then it should be fine.

I don't know what the DC resistance is of the deluxe choke. I'm thinking it should work either way, may just effect the voltages at the different nodes, which aren't necessarily going to be correct anyway because it's not the exact transformers.

4. So long as the transformer is solid, I don't see any consequences of the transformer sitting on the nut, and having another nut on the inside to bind it to the chassis. Others may have better input though.

Gotcha, I'm guessing it doesn't really matter too.

[pompeiisneaks]

1. you can just ground the center tap BUT an artificial center tap actually provides a cheap emans of 'fusing' if anything goes too hot on the heaters, they burn up before anything else and end up saving your tubes and other parts of the circuit, so since its a. easy to do, and b. cheap, it's worth it to not use the center tap.

Thanks Phil, that's interesting. I suppose the wattage of the resistors is important then, what do you recommend?

2. So long as that 1k dropper is the same resistance the choke provided, yes you can. It gives a slightly different response since you've lost a bit of the filtering the choke provided, but so long as the other CRC filters etc, work to filter the PS, then it should be fine.

I don't know what the DC resistance is of the deluxe choke. I'm thinking it should work either way, may just effect the voltages at the different nodes, which aren't necessarily going to be correct anyway because it's not the exact transformers.

4. So long as the transformer is solid, I don't see any consequences of the transformer sitting on the nut, and having another nut on the inside to bind it to the chassis. Others may have better input though.

Gotcha, I'm guessing it doesn't really matter too.

I've always used 1/2 watt ones. There should be very little current to ground. If there is, it's why they burn up.

As for the resistance on the choke, the only place I found an easy listing said 167 ohms for a replacement hammond 4H choke. So 1k is over that obviously, but I think your point of the voltages not being exactly to spec means it's not a big worry, I think that's a valid assumption.

now to the question on the bias. The one in the schematic of the deluxe you linked should work fine and has worked for a really long time for fenders. What's the impetus to change it?

~Phil

[turbofeedus]

now to the question on the bias. The one in the schematic of the deluxe you linked should work fine and has worked for a really long time for fenders. What's the impetus to change it?

So I don't think it's all that different. The biggest difference is the parallel diode/resistor drain for the bias supply filter caps. According to Merlin, the bias caps can drain faster than the PS filter caps, which could cause the tubes to draw excess current and blow the HT fuses if the amps is turned off and on quickly. I am using 1/2A HT fuses, so in theory this amp would be susceptible to this. Obviously the answer could be to just let the amp cool for 30 secs after turning off before turning on again, giving time for the PS filter caps to drain, but I figured why not experiment?
Other than that, there's an extra filter cap in the bias, but that's it.

[xtian]

now to the question on the bias. The one in the schematic of the deluxe you linked should work fine and has worked for a really long time for fenders. What's the impetus to change it?

So I don't think it's all that different. The biggest difference is the parallel diode/resistor drain for the bias supply filter caps. According to Merlin, the bias caps can drain faster than the PS filter caps, which could cause the tubes to draw excess current and blow the HT fuses if the amps is turned off and on quickly. I am using 1/2A HT fuses, so in theory this amp would be susceptible to this. Obviously the answer could be to just let the amp cool for 30 secs after turning off before turning on again, giving time for the PS filter caps to drain, but I figured why not experiment?
Other than that, there's an extra filter cap in the bias, but that's it.

Notice that in vintage designs, if the wiper of the bias pot fails, tubes red plate. In modern designs, like Merlin's, this cannot happen.

[turbofeedus]

Notice that in vintage designs, if the wiper of the bias pot fails, tubes red plate. In modern designs, like Merlin's, this cannot happen.

Wow, I'm glad you mentioned that, because I've realized I installed the wrong value resistor on the bias pot.

[pompeiisneaks]

Oh that is true, I'd forgotten the need for a minimum resistor that keeps some minimum bias range and the pot is just an adjustment only. Yeah true.

~Phil

[Tony Bones]

Most bias supplies come up pretty quickly (less than one second) so, as much as I admire Merlin, I wouldn't worry too much about tubes redplating before there's adequate bias. Just be careful not to go nuts adding massive amounts of capacitance to the bias supply; that will slow them down.

It's possible to model most bias supplies with PSUD if you're clever. OTOH, LTSpice is pretty easy to use. Either way you can get an idea how long it takes for the bias supply to come up.

[turbofeedus]

Most bias supplies come up pretty quickly (less than one second) so, as much as I admire Merlin, I wouldn't worry too much about tubes redplating before there's adequate bias. Just be careful not to go nuts adding massive amounts of capacitance to the bias supply; that will slow them down.

It's possible to model most bias supplies with PSUD if you're clever. OTOH, LTSpice is pretty easy to use. Either way you can get an idea how long it takes for the bias supply to come up.

I've already incorporated the slow drain (is there a better name for this?), so I'm just going to leave it. According to Merlin two 10uF caps is plenty, so that's what I used.