Plate swing? - Page 1
Forum section: electronics / general

author: vlad777
author_rank: Frequent Contributor
timestamp: June 29, 2016, 06:48:39 pm
content: Hi,How much do you think the pentode plate voltage swings in this circuit? (Peek to peek)(as in absolute max value, because I think tubes can't swing rail to rail)+HT = 170VI only ask because I want to make this circuit.I happen to have PCL82 tube.Many thanks.

author: chris_leyson
author_rank: Super Contributor
timestamp: June 29, 2016, 07:56:09 pm
content: Good question, it's Class A so I'm guessing 25%, 30% maybe, depends on how much DC bias you put through the tube and transformer as well. You've got a screen grid to counteract secondary emission from the anode but in truth the sreen grid blocks seconary anode modulation or reverse intermodulation products and you can't do that with semiconductors by their very nature. I love electron physics so much simpler than semiconductor physics.

author: bktemp
author_rank: Super Contributor
timestamp: June 29, 2016, 08:09:18 pm
content: For a resistive load the voltage could vary between 10-30V and supply voltage (170V). For inductive loads it can even go higher. In a typical audio amplifier you stay way below the extreme values, because of the high distortions.Tubes can tolerate quite high anode voltages (much higher than the dc anode voltage). PCL82 allows up to 2500V peak anode voltage for a short time, so using this circuit the voltage will stay within specs.

author: vlad777
author_rank: Frequent Contributor
timestamp: June 29, 2016, 08:35:39 pm
content: Thanks, BTW I kinda want to hear tube distortions

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2016, 08:04:05 am
content: The "turn on" or "saturation" voltage drop can be as low as 10-30V, but is usually more like 30-100V.Obviously it depends on load resistance; with a real (loudspeaker) load, you can expect peaks will turn on quickly, then recover somewhat as the current gets flowing through the coil inductance.It also depends on screen supply. When plate voltage drops severely, cathode current remains more or less constant, but plate current drops, so, all that current dumps into the screen. A stiff screen supply will easily give toaster-grid in saturation.Usually, screen is supplied by an RC filter, so that some compression (with attack and recovery time constants) occurs, while the screen power is safely limited. This can be rather annoying, but is desirable in guitar amps (one of the many ways they achieve their sound).Turn-off peak could be quite large indeed, considering it's a transformer. If the load is open circuit, it'll only be limited by the R+C across the primary, and whatever losses the transformer has. Thousands, easily. You'll need to use high quality sockets, if you expect to operate with such high peaks for very long!Under normal conditions, with a reasonably stable load, it won't be much more than the supply voltage. That is, the amount by which the plate peaks above B+, so peak plate voltage is around 2*B+.Tim

author: vlad777
author_rank: Frequent Contributor
timestamp: June 30, 2016, 11:16:10 am
content: So it would be 70V min to 140V max. Sorry I didn't state why I need this.I need to know output transformer ratio, my speaker is 16Ohm 3W so i need 20V peak to peak on the secondary.

author: vlad777
author_rank: Frequent Contributor
timestamp: June 30, 2016, 01:07:03 pm
content: @chris_leyson25-30% of what, HT (170V) ?30% of HT in saturation , or 30% of HT peak to peak?Can you please clarify?What do you think about comparing screen greed in tubes with cascode in semiconductors?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 30, 2016, 10:31:51 pm
content: Oh, well you only need to know what load impedance to match to.I would guess 5kohms or thereabouts. Can draw the loadline to get a better idea.Tim