martin manning wrote:Dartanion, I think the result would be about the same. Chris, if you're trying to reduce volume you could add the so-called Lar-Mar post-PI master volume. Do you know that one?
Martin,
Trust me, I have a feature on my amps that I call the "Harmonics" control that is basically like half a ppimv. Mark Durham used this in his Vajra 18 watt amp and let the community use it. Cutting signal to one tube is much easier to negotiate than lifting the cathode, which takes the tube out of the circuit and introduces a change in reflected impedance, bias change, etc. Since the control is a potentiometer, you can go from full pp to full se. Granted, the amp should be cathode biased to work in the way I do it.
Chris, the Dumble schem is shutting of a pair of tubes, I think if your were to find a way to turn the push pull into parallel SE you would get the wattage drop I guess you are looking for? From what I know so far, I have no idea how that would be done!!
martin manning wrote:Dartanion, I think the result would be about the same. Chris, if you're trying to reduce volume you could add the so-called Lar-Mar post-PI master volume. Do you know that one?
Martin,
Trust me, I have a feature on my amps that I call the "Harmonics" control that is basically like half a ppimv. Mark Durham used this in his Vajra 18 watt amp and let the community use it. Cutting signal to one tube is much easier to negotiate than lifting the cathode, which takes the tube out of the circuit and introduces a change in reflected impedance, bias change, etc. Since the control is a potentiometer, you can go from full pp to full se. Granted, the amp should be cathode biased to work in the way I do it.
The control grid is fairly high impedance, what value pot do you use?
Here's the PPIMV I mentioned. The word is this is the most transparent type, and when turned full-up returns the circuit to normal. This has worked very well on 'wrecks, should be just fine on a D-type too.
davent wrote:Here's the link to the Vajra circuit that dartanian mentioned earlier. This amp also allows you to switch between fixed bias and cathode bias.
Is that half-a-PPIMV control supposed to be used in fixed-bias or cathode bias mode, or either?
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If you want to turn one side of the power amp 'off', you could always put a 250k volt pot on one (side) of the output stage (after the PI coupling cap) in place of the grid load resistor I guess.
Thanks for posting these references teemuk; interesting stuff. Chris, I missed your statement above viz. your motive not being to cut power... Looks like MESA's "Duo Class" is what you're after, but it appears that they shift the bias point to avoid the high distortion. Peavey's Texture control looks much like the Vajra circuit, but it's raison d'être is distortion.
martin manning wrote:Here's the PPIMV I mentioned. The word is this is the most transparent type, and when turned full-up returns the circuit to normal. This has worked very well on 'wrecks, should be just fine on a D-type too.
davent wrote:Here's the link to the Vajra circuit that dartanian mentioned earlier. This amp also allows you to switch between fixed bias and cathode bias.
Is that half-a-PPIMV control supposed to be used in fixed-bias or cathode bias mode, or either?
Funny, I expected the folks here to refer to that control as Fisher's MV!
I've used it many times and like it but I'm guessing Chris is looking for something else.
dynaman wrote:Funny, I expected the folks here to refer to that control as Fisher's MV!
It is basically KF's "Type 2". The subtle differences here are the use of safety resistors and matching exactly the value of the bias feed resistors it replaces. This way the PI sees the same load as before the addition of the MV, and so essentially no change to the original circuit when the PPIMV is maxed-out. KF just specified a 100k dual pot.
However I am not understanding what we have here. We got a 750K bias feed/leak resistor in parallel with a 500K pot wired as a rheostat. Why? Seems like the pot is just altering the bias feed resistor's value.
I thought you would keep the bias feed resistors the same then just add a variable voltage divider pot after them. After thinking about it some more that would still change the value of the bias feed resistor as the two are in parallel.
ChrisM wrote:Did it sound bad Tom? I never played an amp with only one tube or one pair running.
Hi Chris,
Here's the link to the Vajra circuit that dartanian mentioned earlier. This amp also allows you to switch between fixed bias and cathode bias.
TUT5 has a couple amp circuits that have a pot to dial out the input to half of the output tube(s). http://mhuss.com/18watt/schematics/Vajra18Scheme.jpg
Take care
dave
Hey Dave, thanks for the link.
However I am not understanding what we have here. We got a 750K bias feed/leak resistor in parallel with a 500K pot wired as a rheostat. Why? Seems like the pot is just altering the bias feed resistor's value.
I thought you would keep the bias feed resistors the same then just add a variable voltage divider pot after them. After thinking about it some more that would still change the value of the bias feed resistor as the two are in parallel.
In cathode bias mode the pot takes the grid of one half of the p-p pair to ground, "grounding out" the signal. In fixed bias mode it ties the grid to Vbias (by making the value of the bias feed resistor zero), which is the same as grounding it out. In fixed bias, the bias feed resistors make up most of the AC load seen by the PI stages.
