SE bias too hot?

[ronh]

I have a SE amp using an EL-84 tube. Using one of the bias spreadsheets found on the internet, I measured my bias at 11.5 watts and 39.09 Ma. This is at maximum plate dissipation. Am i running too hot? The amp has a certain sound I like. Goes from clean to mean with the volume control from the guitar. Should I rebias tp 70%?
Will this amp blow up at some point? Also the heater voltage is at 6.9Vdc. Can I use a 5W 6.3V zener to bring it down? Or would It better to put a 1N4007 diode in series to drop to 6.2 Vdc? Thanks

[g1]

Max dissipation at idle is normal for SE. It will not run any hotter during use.

[eschertron]

I have a SE amp [snip]

SE amps are 'typically' biased to 100% in order to get the maximum headroom out of the tube. If the amp is cathode-biased, there is an additional margin in that as idle current goes, the voltage drop across Rk increases, making the bias (voltage) a little colder in turn. Some call it self-regulating. as long as Rk was chosen by the designer to reasonable, of course.

you can modify the heater voltages, but beware that either of the methods mentioned will dissipate quite a bit across the diodes, power that the PT may not be able to afford. if all the voltages in the amp are high, there are threads that have dealt in detail about bucking transformers or replacements. Is this a vintage or collectible amp?

[mozz]

Filament is too high. I usually end up setting them 6.3v with a small resistor .22 or .47, whatever this calculator tells me.
mh-audio.nl - Home

[bob p]

In a related thread that has gone sticky, we've been discussing that when people ask for help, it would help to disclose as much about the amp as possible, including the name of the amp and a schematic if it is not a ubiquitous design like a Champ.

I hate to do this, but I'm going to answer your question with a few questions:

Is this a homebrew amp or a commercial product of some kind?
Is this amp new to you, so that you don't have any experience WRT the heater voltage, bias condition and tube life?
Is this amp going to be used at home or for gigging? ie: how paramount is the concern about reliability?
Are you willing to accept any possible changes in tone that might accompany restructuring the amp's voltages?


Regarding those "high" heater voltages -- excessive heater voltage will have a negative impact on tube life. You know about it, so you have the option to fix it or to do nothing. Depending on which data sheets you look at, heater voltage specs are allowed to vary 5%, 7% or 10% from the nominal 6.3 VAC design center value. Many data sheets don't even list a variance spec for heater voltage. 6.9 VDC would be at the upper limit of normal on the 10% scale. I've seen GE data sheets that allow a 10% variation, while the STC 6BQ5 recommends a maximum variation of 7% for a tube operating in Class A. Some data sheets may recommend 5%. I don't think getting the voltages to exactly 6.3 VAC is necessary. I try to design amps to operate within 5% of the nominal value and if I can get that close then I'm happy.

Here are a couple of reference pages that might be helpful regarding heater voltages. One of them is focused on radio trasmitters and covers the topic of commercial vs. intermittent hobbyist use. I think those considerations are meaningful because after all, this amp isn't a continuous duty radio transmitter. It will see intermittent use, most likely in the home. That gives us some leeway.

Tube heater voltage vs. Lifetime
Filament Voltage life


It would be helpful to know the range of line voltages that you normally experience and what the line voltage actually was when you took your measurements.

If you do nothing you'll continue to have the great tone that you enjoy, at the expense of shortened tube life. One way out it to just accept the situation. The typical SE amp isn't every expensive to re-tube.

If you decide to "fix" the heater voltages, then you have several options.

The best textbook approach would be a center-tapped bucking transformer or a new PT. The problem with those textbook-correct methods is that they will also change B+ which may effect the tone of your amp in a way that you don't like. If you want to go this route then keeping an eye on source voltage while you adjust it with a variac and monitor secondary voltages would be helpful in determining exactly what kind of voltage change works "best" for your application, and whether or not you like the tonal changes that accompany the voltage change. In the big scheme of things, having "correct" voltages may not be as important as getting good tone out the amp. There's a definite tradeoff to consider.

A bucking transformer for a small SE amp would certainly be an inexpensive and easy way to change voltages. It would be far cheaper than a new PT. And considering that a bucking transformer is a good thing to have around the house if you own old gear, I would build one just out of principle. Then you have the choice of whether or not you choose to use it. A good way to build a bucking transformer box is to wire it up with three outlets, one that provides line voltage, one that provides a 5% buck and one that provides a 10% buck. Google "Vintage Voltage Adapter" for more info.

If you decide that you do not want to change the global voltages (variac, PT or bucking transformer), then directing your fix to the heater circuit fix is a good option. Personally, I would not consider the use of diodes on the heater supply, I would just calculate the value of series resistor needed to bring the voltage down to "safe" levels. One thing to consider is that you don't have to bring the voltages down to exactly 6.3 VDC, even a 5% reduction would help the situation. A small change to an intermediate voltage would help the heater voltage problem while effecting tone the least.

If I were in your shoes, I'd start off by building a bucking transformer and wire it up to get your choice of voltage reductions. These things are incredibly easy to build, they're brain-dead simple, they're cheap, and they're very handy to have around for fast diagnostics/modelling in situations like this one. They're a plug and play solution to the voltage reduction question. Then, after using the VVA, you'll have better insights on how you want to address the voltage problem. The beauty of the VVA is that it gives you the right answers without requiring you to modify the amp.

[Dave H]

I have a SE amp using an EL-84 tube. Using one of the bias spreadsheets found on the internet, I measured my bias at 11.5 watts and 39.09 Ma. This is at maximum plate dissipation. Am i running too hot? The amp has a certain sound I like. Goes from clean to mean with the volume control from the guitar. Should I rebias tp 70%?
Will this amp blow up at some point? Also the heater voltage is at 6.9Vdc. Can I use a 5W 6.3V zener to bring it down? Or would It better to put a 1N4007 diode in series to drop to 6.2 Vdc? Thanks

I'd play it and enjoy it as it is. If it sounds good it is good. It ain't broke so don't fix it. 11.5W is fine for SE. 70% is for push pull. It should run for years without blowing up. You could use a resistor in series with the heaters to reduce the voltage to 6.3V but I wouldn't worry about it. It will be fine.

[ronh]

Thanks for the replies. I guess this is a case of much ado about nothing. The amp was a epi valve junior. Now it is a fender princeton 5F2-A with solid state rectifier and EL-84 output tube.
If my only concern is the tube wears out sooner, then I can live with that. Thanks again.

[Dave H]

The amp was a epi valve junior.

I have a VJ. When I first measured it the EL84 was running at 13W so I adjusted it down to < 12W, actually about 11.6W from the numbers I wrote on the schematic years ago and it still sounds fine.

[eschertron]

Keep in mind that if your stated dissipation figure is gross dissipation for the tube, then plate dissipation will be less. At idle, I'd expect the screen to be taking about 5ma out of the total that you measured. Just thinking out loud.

[Dave H]

Keep in mind that if your stated dissipation figure is gross dissipation for the tube, then plate dissipation will be less.

Good point, the 13W I calculated was plate dissipation only but we have no idea how ronh measured that 39.09mA or calculated 11.5W. How did you do it ron? Did you measure the cathode voltage and calculate the 39.09mA from that? And did you use the plate to cathode voltage or plate to ground voltage when calculating 11.5W?

[eschertron]

In any case, I wouldn't consider 11.5W to be too high for an EL84.