Need some 5f6a build advice, please

[schoolie]

I have a few questions, and some advice would be a huge help.  I really should have just built this stock, but I couldn't help myself.  This build is using the traditional 5f6a layout board.

1.  I want to remove the standby switch to free up some space, and I don't like the idea of B+ across a switch.  I'm using a GZ34 and 39uF Solen + 420KOhm bleeder for my first filter cap.  Is there any modification that I would need to limit current?

2.  I installed bias test point jacks very close (Hopefully they won't melt ) to the power tube sockets.  and have 1R resistor across the terminals.  Would having the filament wires in proximity introduce any noise?  I also have my bias adjustment pot in one of the speaker jack holes which I hope is OK.  There isn't much space in the chassis for more parts.

4. The third 20uF filter cap, for preamp doesn't seem to belong with the first two stages.  I really want to connect to the 4.7K or 10K voltage divider resistors and ground to the preamp ground bus. Would this be a bad idea?  I know Doug Hoffman says that it could be located on the back edge of the board, but is this OK, if I'm using the Fender layout?

5.  I moved the pilot lamp to the ground switch hole.  I don't think this is a problem, but wanted to make sure.


Thanks!

Rob

[sluckey]

1. no

2. no

4. no and OK

5. No problem

[schoolie]

1. no

2. no

4. no and OK

5. No problem

Thank you, SLuckey!  I wonder what question #3 was

[John]

  I wonder what question #3 was

3. Does Sluckey accurate, concise answers?

 

[schoolie]

  I wonder what question #3 was

3. Does Sluckey accurate, concise answers?

 

That was it!  Thanks ...and the answer is yes!

[Dave]

drgonzonm, standby switches were originally intended for the reasons that you have outlined, but......
general consensus seems to point (nowadays) to them being completely unnecesary.
Having high voltage on a tube with cold filaments has no detrimental effects or any effect on the life expectancy of the tube (so I have been informed).

Dave

[bluesbear]

Dave Allen sells "Carling 3-position progressive toggle switch (OFF-STANDBY-PLAY)".
1 switch does it all. I've been using them for a while, for the same reason you mentioned. Go to:

http://www.allenamps.com/parts.php

and scroll down to switches. These things are great.
Dave

[schoolie]

Thanks, Bluesbear!  I saw those switches at tubeandmore.com.  They look nice, and I'll definitely give them a try.

[jjackson7]

I've been reading The Guitar Amp Handbook by Dave Hunter and it says that during time it takes the rectifier's heaters to warm up they don't pass high voltages so the standby switch is unnecessary when using tube rectification. Thoughts?

[sluckey]

The standby switch was not originally used for tube protection. It was a convenience feature that allowed you to leave the amp idling with tubes heated up. Comes in very handy when taking a break. And when break is over, the amp will immediately snap back to life.

[Willabe]

Some of the literature out states the heaters  on the can draw 4 times the power when in standby.

I've never heard that, don't see why they would.

Are you sure you don't mean that the heaters can draw 4x the current at turn on?


                 
                               Brad     

[sluckey]

Some of the literature out states the heaters  on the can draw 4 times the power when in standby.

That statement is not true. I'd love to see a link to a credible web page that claims that. The reality is that a tube filament will draw much more current when initially turned on. This is because the cold filament has a low resistance. As it heats up, the resistance increases and the filament current decreases. You can actually see this in some tubes, ie, bright flash on the filaments when you turn the amp on, then the brightness decreases to some nominal level. And think about this... Ever notice a light bulb blow when you turn the switch on? Ever wonder why they don't blow when you turn the switch off? Tungsten has a positive temperature coefficient, meaning resistance increases as temp increases.

Standby switches typically switch B+ to a tube. Filament current does not change depending on whether or not the tube has B+ applied.

[spacelabstudio]

Some of the literature out states the heaters  on the can draw 4 times the power when in standby.

That statement is not true. I'd love to see a link to a credible web page that claims that. The reality is that a tube filament will draw much more current when initially turned on. This is because the cold filament has a low resistance. As it heats up, the resistance increases and the filament current decreases. You can actually see this in some tubes, ie, bright flash on the filaments when you turn the amp on, then the brightness decreases to some nominal level. And think about this... Ever notice a light bulb blow when you turn the switch on? Ever wonder why they don't blow when you turn the switch off? Tungsten has a positive temperature coefficient, meaning resistance increases as temp increases.

Standby switches typically switch B+ to a tube. Filament current does not change depending on whether or not the tube has B+ applied.

Until fairly recently I was pretty religious about standby procedures for powering on my amps.  I even went so far as to *add* a standby switch to an old Ampeg Reverberocket 2, many years ago.  Merlin Blencowe, in his power supplies book, makes a pretty eloquent and convincing argument that a standby switch is entirely unnecessary, so I've stopped worrying about it.  Not everyone agrees, but his explanation is good enough for me.  It also make sense to me from the point of view of designing an end user experience.  If it were true that you needed to perform an arcane power on ritual every time you powered on your amp or it would break, then your average guitarist would constantly be breaking his amp.  Tube amps in use by your typical touring band seem to be able to take years of daily abuse without much in the way of servicing, so they must not be nearly as fragile as the myth would seem to imply. 

The practical advantage of a standby, of being able to mute your amp with the tubes still hot, can be more easily and safely accomplished by grounding out a preamp tube grid somewhere along the line.

Chris

[schoolie]

I standby mod that I might try is putting a 3W, 100K resistor and .047uF cap in parallel with the switch contacts.    This allows the caps to charge up a bit and still mutes the volume. That's how Ceriatone wires the standby switch on their 5f6a model.

[sluckey]

Oh, you mean this statement from that pdf...

Plate and screen dissipations are

P = (432)(33mA) = 14 watts
P = (430)(1mA) = 0.4 watts
This compares to 6L6GC limits of 30 watts and 5 watts, respectively. The tube heater produces 6 watts, so heat production almost triples when the standby switch is on

That's not the same as your statement.

Some of the literature out states the heaters  on the can draw 4 times the power when in standby.

Here's my take on that discussion...

The author had just went through a lengthy explanation that shows how cold a 5F6A is biased. The 6L6 is only running at 14W but is capable of being run to 30W, so yes, pretty cold.

Then he continues to compare the tube power when the amp is operating filaments only (standby OFF) versus power with B+ applied (standby ON). The 6L6 tube filament uses 5.67W (6.3V x .9A = 5.67W) even when no B+ is applied to the tube (standby OFF). When B+ is applied, the 6L6 will idle at 14W plate dissipation and .4W screen dissipation. This 14.4W of power is in addition to the 5.67W already produced by heating the filament. So now the total tube power is 14 + .4 + 5.67 = 20.07W. Now the tube power consumption is about triple what it was when only the filaments were burning.

All that means that the tube is consuming about triple the power, not that the filament is consuming about triple the power. I'm not even sure why the author threw that into the mix. All it means is that the 6L6 tube wastes about 6 watts just to heat the filament! And that is a total waste. Thank goodness for the transistor!   

[Willabe]

power draw in standby

See Circuit Design of the Fender 5F6a (www.ampbooks.com/home/classic-circuits/fender-bassman-power-amp).  In the DC operating point section, heat production almost triples when standby is on.  Not quite 4 times.  (bad memory)

That is not written very clear to me.

I think he's talking about adding the plate and screens power disapation as heat (when not in stand by) to the heaters power disapation as heat.



                     Brad      

 Edit; Sluckey beat me to it. IMO he's right.

[sluckey]

I think he's talking about adding the plate and screens power disapation as heat (when in stand by) to the heaters power disapation as heat.

There is no power dissipated by the plate and screen when in standby. So the heaters are dissipating the only power. Then when you turn the STBY switch ON, you add the plate and screen dissipation to the total tube power.

Maybe I have a dislexic view of the STBY switch. To me, when the STBY swich is in the ON position, the contacts are made and B+ is applied to the amp. When the STBY switch is OFF, the contacts are open and no B+ is applied to the amp.

So, when you say "when in stand by", or "standby on", what is the state of the switch contacts? I see room for some more confusion here.    

[Willabe]

No it's me. I meant to say _not_ in stand by, power on, stand by off, tubes running at idle.

So, when you say "when in stand by", or "standby on", what is the state of the switch contacts? I see room for some more confusion here.    

I always thought of it as when _in_ stand by, the stand by switch is on, so the heaters are on and the B+ is off. But I think either way has been used to talk about it.      

When I first started to play out in the blues club I would turn my amp off in between sets. After a while a few of the older players told me to leave the amp in/on stand by between sets. They felt the amp would have a more consistent sound through the night, set by set, by keeping the amp/PT warm. I've always done so since then. Even if it's not true, it made the night easier by having the amp ready to go when the boss man got back on stage. They did not like to have to wait for the side band players. Can't say I could blame them. Doesn't look very professional to have the band hitting stage one at a time and late to boot. There was always 20 guys standing in line for your job so you hit stage, tuned up and ready to play, when the boss man did. And I'd bet it's still the same.    

                Brad      

[PRR]

> having the amp ready to go when the boss man got back on stage. They did not like to have to wait...

That's the way I see it too.

When the Tavern Owner glares "PLAY!" at the Band leader, and the Band Leader glares at the Band Members, we have to gulp our beer, dump the girl off our lap, crawl back on stage, grab the axe.... the additional 10-20 seconds to get sound may give Owner or Leader just enough time to pop a cork and not-rehire us.

Other special cases. Wedding, church service. The Pastor or Father may wish to speak from the floor (no mike) without amps humm/hissing behind, then get back to the music. Your cord shield craps-out 7 bars before your Big Solo: you slap standby to kill the BUZZZZ!, change cords silently, slap to Run, and shred.

[steven]

Merlin's argument convinced me, too, and I stopped using standby switches. I add a mute switch just before the PI to route the signal to ground. This can be a very tiny switch.
My biggest problem was psychological. We HAVE to have standby switches, don't we? For a while I felt the same way I feel when I forget to buckle my seat belt in the car - 'something's just not right here.'

just my 2 cents

steven