Virtual Center Tap Resistor Value

[boots]

I'm not sure how much it matters where you borrow the elevated voltage from. I have had good luck tapping off the output tube's screen supply with a resistive voltage divider. I also understand the exact value of the elevated voltage isn't too critical either, within reasonable limits.

Here's my question: Should there be any concern regarding how well regulated the voltage source is? For example, if you monitor your B+ or screen voltage while playing the amp, the high voltage "pulls" under a big load (sag!). So, in effect, the DC reference that the filament AC rides on is varying up & down as the amp is being driven.

I don't think I've ever noticed a problem due to this, at least not anything noticeable to my ears while playing. Is it a bad idea to borrow the filament bias voltage from the output tube's screen or B+? Perhaps the preamp B+ is a better spot to do it in terms of regulation?

And my 2 cent's worth: I don't think the value of the virtual CT resistors is critical at all - probably the matching is more important. It's probably mostly a matter of how much heat you want to dissipate. But 100 ohms works great, and I have a small drawer full of them, so 100 ohms is my choice!

[Enzo]

The point of elevating the heaters is to keep the heaters more positive than the cathodes they heat out in the amp. That way any electrons boiling off the heaters themselves will be attracted back to the heater and not the less positive cathodes. If electroncs flowed to the cathode, you would have a small AC current added to whatever else was in teh cathode. That would make hum. By keeping the heater more positive than the cathodes, the cathodes cannot be attractive to the electrons trying to escape.

So if the cathodes are at - who cares, 5v, for example - +5VDC, then leaving the heaters at anything over +5 would render them unattractive to cathode/heater electron current. +50, +500 won;t be any less unattractive really. We could make a case for the elevation to be greater than cathode voltage plus any positive signal peaks.

SO also then, regulating the offset shouldn;t much matter. You are either less positive than the cathodes or not.

[Chuck H]

SO also then, regulating the offset shouldn;t much matter. You are either less positive than the cathodes or not.

AND the amound of fluctuation on the cathodes is usually pretty small anyway. Also, any voltage variations that may affect the heater elevated voltage will also be affecting the cathode voltage, so... I wouldn't bother.

As Enzo points out, the only requirement is that the heater be more positive than the cathode. This will obviously be a higher elevation in an amp with a cathode follower type circuit. Otherwise figuring the heaters positive to the cathodes is just the heater voltage + the cathode voltage + the signal peaks. IMHE signal peaks rarely go over 10V in a preamp and most preamp tubes bias at less than two volts, so, 6.3+10+2=18.3, this is the voltage we need to be more positive than I suppose. And you probably don't really need to consider signal peaks since your not likely to actually hear any hum while the amp is playing. But just to be anal let's say that 20+ volts is probably enough if you don't have a cathode follower circuit in the amp. If you do have a cathode follower then even 1 one volt more than the peak voltage on that cathode is enough.

[Enzo]

And if there is a cathode follower, it is not likely to be the input stage. The high gain input stage is the stage that is most sensitive to the effect anyway. That is why many amps elevate only the first tube or two. SO said follower could be left out of the elevation program.

[Chuck H]

+++

Yes, it's been my experience that elevating above the heater voltage and preamp bias is enough IF elevating helps at all. So that's just 8.3V+. Using the top of cathode bias resistor for a pair of el84's will usually give you 9V to 11V, and that's how I know.

[jrfrond]

Hey people, I don't want to sound like a pompous ass or anything, but a lot of debating went on here just trying to "reinvent the wheel". This is stuff that we usually don't have to think about. It's all tried and true. Most Fender amps are dead-quiet, and they were using 100Ω 10% resistors. almost anything in the neighborhood will work.

[Merlinb]

Most Fender amps are dead-quiet, and they were using 100Ω 10% resistors. almost anything in the neighborhood will work.

I've never met a 'dead quiet' Fender...

[Robert M. Martinelli]

I did, but it was quiet BECAUSE it was dead.

Cheers

Bob

[Pedro Vecino]

About this issue ... A week ago I found an Engl Ritchie Blackmore with a related problem.
The amp worked fine except for a very intense noise (neeeeeee) immediately associated with a loss of reference between heaters and ground. Previously I had Screamers, Savage, etc. with similar symptoms and a damaged adjustable resistor.
After changing it (I use strong 250 Ohms Morganite pots direct to chassis) I did not get any result. The same noise.
And measuring PT secondarys I have obtained the real voltages (heaters, HT, bias, and switching line). All OK, but:
I have found crossing voltages between different lines (18VAC between heaters and bias, and also between heaters and HT).
This is the first time I see this, and the biggest surprise is that today I found a Savage Special Edition with the same crosses and the same noise (!).
In the Blackmore I disconnected the original heaters line and I have connected a 6,3 VAC external transformer. The noise is removed and the amp sounds fine.
Has anyone seen this effect before?
Thanks

[jrfrond]

I've never met a 'dead quiet' Fender...

Sorry to hear that.

In my experience, a properly-operating typical Fender BF amp is pretty darn quiet with all controls turned down. With input sensitivity ratings in the mid-30dB range, these aren't really "gainy" amps. I also have found that versions with hum balance controls sometimes perform quieter by switching to fixed resistors.

[Merlinb]

Sorry to hear that.
pretty darn quiet with all controls turned down.

You mean turned up, i.e., maximum, right?

I also have found that versions with hum balance controls sometimes perform quieter by switching to fixed resistors.

I'm afraid that's nonsense. A hum balance pot is two resistors, plus they're adjustable!

[mooreamps]

You mean turned up, i.e., maximum, right?


I'm afraid that's nonsense. A hum balance pot is two resistors, plus they're adjustable!

You know what ? He's an amplifier designer/builder/technician, and U R Not.. If he said sometimes they work better, maybe he has a point...

-g

[R.G.]

I have found crossing voltages between different lines (18VAC between heaters and bias, and also between heaters and HT).
This is the first time I see this, and the biggest surprise is that today I found a Savage Special Edition with the same crosses and the same noise (!).
In the Blackmore I disconnected the original heaters line and I have connected a 6,3 VAC external transformer. The noise is removed and the amp sounds fine.
Has anyone seen this effect before?

Never seen it, but the description indicates that there's an internal short in the power transformer between windings. I can well expect that it would offer a path for feedback and oscillation, and that an external heater transformer would fix it completely.

Quote Originally Posted by jrfrond
I also have found that versions with hum balance controls sometimes perform quieter by switching to fixed resistors.

It depends on the pot and the resistors. Pot sliders are not always the best contacts for things like this, possessing some inherent resistance of their own. Two resistors have a soldered connection between their junction and wherever else it goes, which can make it quieter.

Quote Originally Posted by mooreamps
... You know what ? He's an amplifier designer/builder/technician, and U R Not.. If he said sometimes they work better, maybe he has a point...

Hmmm. A real, no fooling amplifier designer/builder/technician!!

Did he pass his NACATDAB certification test?(For the uninformed, NACATDAB stands for "National Association of Certified Amplifier Technicians, Designers And Builders". )

And while we're on that, if you have to be a real he-manly, certified amplifier designer/builder/technician before you can post technical opinions, did YOU pass your formal certifications so that you're licensed to talk about amplifiers?



The devil is whispering in my ear to ask if you did OK on the fuses section of the test.

[Enzo]

I had a field service tech once who needed a 15A fuse for a piece of gear, and had none in his kit, so - honest to God - he wired a 5 and a 10 in parallel.

[jrfrond]

My credentials are not the issue here. Like many amp builders/techs or whatever, my opinions are the result of 30+ years of experience, and amp design/debugging is as much empirical and black art as it is science. Your mileage, however, may vary.