5F11 current question

[Randall]

A customer who is a tone chaser and quite knowledgeable wants to try a 5W4 rectifier in his 5F11 Vibrolux. This an excerpt from an email I received

"Also, is there a way to monitor an amps' total tube-current draw while in operation? I'm still chasing this "tone Quest" involving that tweed 5F11 Vibrolux, thinking of trying a 5W4GT in place of the 5Y3GT rectifier. I've been told that would work but only if the Total tubes' current draw ( 5-volt rectifier, 2-6V6GTs & 2-12AX7s) is under 100ma for all. If there's a considerable risk of frying the power transX &/or output trannie I won't risk it"----

I don't know where he is getting this information. Two things: Assuming he is concerned with the rectifier and not the PT, then filament current, which would far exceed 100mA, does not figure in here, am I correct? If this is correct, what would be the st way to measure this? would I have to measure all seperately and add them up? And what about the rectifier current, would this need to be measured as well?

Second thing, which may make the whole thing moot, is that this is a tremolo bias amp. Seems that turning the trem off is when it has the most bias voltage hitting the grids. And turning the trem depth up would cause the bias voltage to swing less negative and the cathode current up. How would one know what the highest it might swing if one only has a digital DVM that would track that fast?

Personally I don't like the idea much, but I would like to get my understanding cleared up before I speak with him about it.

Thanks.

http://ampwares.com/schematics/vibrolux_5f11.pdf

[Dave H]

A customer who is a tone chaser and quite knowledgeable wants to try a 5W4 rectifier in his 5F11 Vibrolux.

I seriously doubt the "quite knowledgeable". Tell him changing the rectifier will make naff all difference to his tone and he would be better off working on his playing.

[Enzo]

I agree:

A customer who is a tone chaser and quite knowledgeable

How knowledgeable can he be if he can't figure out how to check circuit currents? Or can't look up tube specs?

I've been told that would work but only if the Total tubes' current draw ( 5-volt rectifier, 2-6V6GTs & 2-12AX7s) is under 100ma for all. If there's a considerable risk of frying the power transX &/or output trannie I won't risk it"----

This makes no sense. the rectifier tube itself has a heater current draw of course, so the original 5Y3 needs 2A of heater, while the 5W4 needs 1.5A. So far so good. But you can't add up all the various tube currents into anything meaningful. You can ask what current demands will be placed on the rectifier - ie the B+ current. But that only applies to the signal tubes. If you exceed the current rating, I'd fear the tube burning up, rather than the power transformer, and I can't see how this would affect the output transformer.

You could break into the B+ line and insert an ammeter. But really, the ratings of these two rectifier tubes are so similar, I doubt there is a concern. All tube plate current comes from the rectifier, so here is no need to individually measure tube currents, unless you find it interesting. Just see what leaves the rectifier tube cathodes.

I'd not worry about the trem.

The sag ought to be similar between these two tubes. Might be a little diferent, who knows. I don;t see it as affecting tone, but maybe dynamics a little.

[Randall]

This guy is more on it than any of my other customers, he reads and understands and asks hard questions. That doesn't mean he has the skills to check his tube currents. And he is an outstanding musician, so let's cut the guy some slack, shall we?

He wants to try to dial back his old Vibrolux to break up sooner and be less loud in the process. Personally, I'm into repairs not re-engineering mods, so I'm not real excited about a guy who wants me to tweak an already working vintage amp to do what it wasn't meant to do. But you know some people get an idea in their head, and want to stick to it. As I said, I don't know where he is getting his info.

So may I ask this in a different way? Is this feasible, viable or worth the time to pursue this? Would you put the ammeter on the B+ feed or the ground? If not, why specifically?

[The Dude]

You could do that if you wanted, but I don't think it's necessary. As Enzo said, the tubes are rated so similarly that I can't see any slight variation making a difference. The only thing I might have looked into is filament current, and as already stated, the sub tube actually draws less filament current. I don't see any reason you couldn't just do the swap and see if the desired results are achieved- whatever those results might be.

[Chuck H]

According to the specs I've got here the 5Y3 will be dropping 15V MORE than the 5W4. So it's probably a case of people suggesting that have popped that tube into their amps because they could, rather than because they should and then decided that because they did something it was cool. Tell your guy not to drink the KoolAid!!! There's nothing more "tame" about a +15Vp and the added risk of over current.

Maybe try dropping his preamp volts a little by adding resistance to the HV rail. Unless he's in that camp that thinks the circuit should remain stock.?. In which case he shouldn't be Jonesing for a 5W4

[The Dude]

Side Note: I would certainly re-check bias with a different rectifier.

[Chuck H]

Ok... I just looked at the schem for clarity. Scratch reducing preamp voltages. That would affect the PI and trem voltages as well. Possibly in a negative way. Since they're all on the same node you would need to branch the HV rail to do it and I'll guess that's not in the plan.

Dude made a good call on the bias. It should be hot-ish. Which should actually happen on it's own with modern wall voltages increasing B+ if the stock value cathode resistor is in place.

You could try adding a bypass cap to the second gain stage. That would straight up increase the gain. Not the same as lowering headroom, which is how I tend to interpret "breakup sooner". Quieter can be achieved with an attenuator, a less efficient speaker or a small, overall reduction in HV. Regarding the latter... If the amp is stock it's currently running higher than stock plate voltages because of AC at the wall. Perhaps a Variac could be used to drop 10VAC-15VAC from the wall and get the amp down to stock plate voltages. Which would be some 35VDC-45VDC less than it has now! That would be more "tame".

[The Dude]

Not sure what he's using, but you might also try a less efficient speaker.

[d95err]

You could try using Weber Copper Cap rectifiers. You can custom order them to get the specific voltage drop you want. A few of those would make it easy for your customer to experiment himself.

[Enzo]

The data sheets I was looking at showed only a 5v difference, but 15v isn't that much either. A useful difference in volume probably won;t result in a few volts difference in B+. reducing power is never an effective way to make amps less loud. A 3db cut in speaker efficiency is the same drop as cutting power in half - not a lot.

[jimboyogi]

How about cathode biasing the output tubes, instead of fixed bias?

[DrGonz78]

How about cathode biasing the output tubes, instead of fixed bias?

Wouldn't cathode bias eliminate the functionality of the tremolo circuit?

[robrob]

I second The Dude's suggestion of a less efficient speaker. Your user will get less volume and earlier apparent breakup. He doesn't know it but he wants a new speaker.

[d95err]

Wouldn't cathode bias eliminate the functionality of the tremolo circuit?

Not necessarily. There are cathode biased amps using bias wiggler tremolos, for example some Matchless amps. However, since most bias wiggler tremolo amps use fixed bias, I assume it works better.