tube amp questions

[Tony Raven]

I'm conversant in electronics... barely. Therefore, I have some potentially silly questions that occur to me, & I figured I'd start a thread to collect a few.

So, to begin:

#1: if "automatic biasing" is a good thing, then why don't more amps have it?

#2: audiophile tube amps often have all sorts of trim pots. How often do the amps need to be adjusted by someone who knows what they're doing? Is this comparable to guitar amps?

#3: over the years, I've noticed that speaker impedance is a variable, not a precise constant -- an "8 ohm" speaker might be somewhere between 5 & 12 on the meter. If my amp has choices of 4, 8, & 16, should I be matching the speaker resistance to one of these, or just trust the labels?

#4: similarly, when a speaker is on a long cable run, that adds resistance. Does this cause impedance problems?

#5: is there such a thing as "no-load protection" for tube amps? Over the years, I've seen speakers come unplugged, & sometimes the speaker lead breaks or even burns out, but I've never seen these faults actually kill an amp. Still, I've always been paranoid that some "helpful" person wouldn't put a plug properly all the way in, & I'd end up with dead gear. Is this a serious concern?

[Folderol]

I'm conversant in electronics... barely. Therefore, I have some potentially silly questions that occur to me, & I figured I'd start a thread to collect a few.

So, to begin:

#1: if "automatic biasing" is a good thing, then why don't more amps have it?

Cheap and cheerful and no setup required. OK on class A amps, Dodgy on class AB, a definite NO on class B (might go into details later!)

#2: audiophile tube amps often have all sorts of trim pots. How often do the amps need to be adjusted by someone who knows what they're doing? Is this comparable to guitar amps?

As well as setting bias, some believe in critically balancing phase splitters (as if 1% resistors aren't good enough), changing the position on the conduction 'slope' of pre-amps, changing feedback characteristics. In fact if it can be tweaked then there must be a control for it. Personally I don't subscribe to that idea

#3: over the years, I've noticed that speaker impedance is a variable, not a precise constant -- an "8 ohm" speaker might be somewhere between 5 & 12 on the meter. If my amp has choices of 4, 8, & 16, should I be matching the speaker resistance to one of these, or just trust the labels?

Trust the label! Speakers aren't resistors. The are quite complex impedances, and your meter measures only resistance.

#4: similarly, when a speaker is on a long cable run, that adds resistance. Does this cause impedance problems?

If you've got a decent weight of cable the difference should be insignificant.

#5: is there such a thing as "no-load protection" for tube amps? Over the years, I've seen speakers come unplugged, & sometimes the speaker lead breaks or even burns out, but I've never seen these faults actually kill an amp. Still, I've always been paranoid that some "helpful" person wouldn't put a plug properly all the way in, & I'd end up with dead gear. Is this a serious concern?

No load on a valve amp is extremely unwise. Some amps have circuitry to detect this and shut the amp down. Some have spark gaps on the transformer primary to at least limit the damage. With many you're on your own

[ef37a]

Auto or cathode biasing can only set a valve current so low and even then it "robs" you of signal swing. An EL34 cathode* would sit at 32V+ and KT88's even higher so the power rating of the cathode resistors starts to get "silly"! A pair of 34's for example will, give only 25-30W in cathode biased mode but a very easy 60-80W fixed biased.( for guitar amps! The distortion is quite high).

Trim pots: Good amps (cough!) do at least have DC (hum) balance as well as bias setting. "Hi-Fi amps often have an HF balance pot as Will has said but if you design for a low resistance drive to the op valve grids (and keep the grid leaks low, another story!) this really is not needed because stray capacitance becomes less important. Tis gilding the lily on gitamps anyway!

Load specifications: These are determined by test. For guitar amps they are usually chosen to extract max power but with an acceptable overdrive sound but always with an eye to valve and other component safety. Decent amps will not protest at 25% off spec loads and you can go as LOW as you like but watch for "red plating" and big amps will pop fuses!
Protection? Yes,(cough again!) some amps bias off the op valves with no speaker jack plugged in. and the same brand have another "trade secret" that helps. I know of a 100watter that had the nuts driven off it with no load. Fuses and valves later...Fine...I'll be back

[ef37a]

The dreaded wrap struck again.
In two years I did not change an op transformer for no load damage or saw any evidence of valveholder flashover.
*There is a 20W EL34 amp that uses both cathode (110Ohms) and a negative supply bias. Gives a bit of the best of both worlds.

Lead resistance?: Don't sweat it, you need H of a lot of 5amp "garden orange" to get to even ONE Ohm! There will almost certainly be more resistance in the jack socket contacts than any sensible length of cable. Then, the op resistance of guitar amps is in Ohms to tens of Ohms, why worry over a bit o cable?

Dave.

[Tony Raven]

Thanks, all!! Succinct & insightful -- many doubts & arguments have been put to rest. I've been playing combos for most of my life, so recent moves into the heads-&-cabs arena is still in the experimental stage.