Tube Amp Archive

Posted at GroupDIY, link below. fazer ,Denver AmpGarage log in problem New « on: Today at 06:44:59 PM » I've been a member in the AmpGarage Forum. Is there something going on with the Forum I don't know about? Recently I was not able to log In and asked to answer a question which is, what is the pri...

I'm swamped by Real Work. As others said: The second side acts like a "grounded grid" amplifier, driven by the first side functioning as a "cathode follower". You don't see a GG stage anywhere else in audio, so you may have to brush-up on it. Or look at Differential Pair. > 82K plate resistor ... 10...

> The glass wall of one of the tubes was "pushed in" ... getting sparks near the on/off switches ... I had recently changed the PT... I think I have a burnt ..OT ... it was amazingly hot. ...the power tubes were almost read, a very dark orange. And no, strangely enough, the bias read normal, which i...

> Brown - 6V6GT pin 3 > Blue - 16/450 capacitor (positive side) That way or the other way. (I wish tranny winders would stick to standard colors.) Build the amp but leave one end of the 22K disconnected. Verify that it works, but a bit too brash/harsh. (You may like it.) Then connect the 22K. Be rea...

> says -34V bias pp tetrode vs -75V bias for p-p UL Look at the G2 voltage!!! > I hadn't heard of that very large difference in bias requirements before, and it seems odd to me, as UL really only affect the AC operation of the circuit, There's one big effect on DC operation: the G2 must sit at Plate...

> A 12ax7 has a gain factor of about 100. That means that the output voltage is roughly 100 times higher than the input. Define "roughly". The tube has gain, everything else sucks gain. Even the resistor you need to feed power to the tube without shorting its output. At best you get 50%-70% of the t...

> What is the theory behind tube spacing? > What should I take into consideration when choosing a material for my turret boards? You think too much. You should burn your ruler, grab a wood board, and some nails. Nail sockets and parts on board. The main thing in a guitar amp is: keep the dirty stuff...

> Given the dynamics of tubes, what would cause interference between them, thus defining this spacing? In audio amps, it is almost always about fitting the capacitors and resistors nicely. Vacuum tube computers have many more tubes and much simpler coupling networks between them. Some arrays were bu...

> gain explanation doesn't work without dividing the bottom line by a million anyway. And transconductance is in micro Mohos? Where "micro" is 1/1,000,000 ? Those funny greek letters have to be accounted. (And the US custom of using uMho is annoying.... zeros rolling off the paper in all directions....

> R-C tables ..... for the EL-34/6CA7 and 6L6? Ummmmm... "R-C Tables" are for Voltage Amplifier work. Generally taken to mean high voltage gain and reasonable audio voltage swing into "high" impedances. That's not what the Big Bottles were made for! They were made for POWER. Voltage gain is a minor ...

> 100k increase to 120k results in increase output from 20V to 24V. > Now, I assume this is a simplistic example Exactly true IF the input is a 0.1mA signal. We almost never control current directly. We wiggle grid voltage, which influences current. But in triodes, plate voltage also influences curr...

> a choke input filter .... They do provide the best regulation, though. I believe this is semi-true with vacuum rectifiers and costly caps, not at all true with sand rectifiers and big cheap caps. The cap-input pulls current in short spikes. The choke-input (when working right) pulls current all th...

> where are the tables you mention in many of your posts? Many collectors don't realize that the R-C Amplifier Tables are THE designer's crutch. Pete Millett has the Sylvania tube manuals posted, but has the R-C lumped in with other end-matter. Here is Sylvania 1959 Resistance-Coupled Amplifier Tabl...

> a champ and a princeton..., and they look dang near identical Princeton was often just a higher price Champ. Usually bigger speaker, sometimes bigger iron, sometimes an extra knob. Just as with Chevy-Buick-Cadillac, frills from the Cadillac trickle down. The Cadillac has to dream up bigger frills ...

> the 330k grid load resistor on the grounded grid side (1meg in Fenders) seems odd. It's insignificant. Maybe they had too many 330K resistors hanging around. This resistor should be 1Meg or less (tube rating), and much more than 10K so it doesn't load the 10K in the NFB network. Actually it could ...