• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Little help for a newbie?

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#21
out of topic but again I am trying to learn something here
What do you think about the pseudo triode connection in this amp is it worth? I have to say that I am pretty happy with the present pentode configuration. Apart for the problems we talk about ..guys Jonny Cash is and his guitar is in the room !!
 
#23
That would be a great idea but don't know how to do...I believe it is not only a matter to add a simple switch to commute the grid to ground or to the plate
Am I correct saying that switching from pentode to triode require the bias resistor to be changed to?
So how I can I do this? I mean change the grid connection and the bias resistor in one go? Do you mind to post a simple wiring schema?
 
#24
ciccio64 said:
That would be a great idea but don't know how to do...I believe it is not only a matter to add a simple switch to commute the grid to ground or to the plate
Click to expand...
Triode connection is what is shown in the schematic, for pentode, connect one end of the resistor to the B+ (instead of the plate).

Am I correct saying that switching from pentode to triode require the bias resistor to be changed to?
Click to expand...
No, that is not necessary, the bias is still the same for SE operation.
 
#25
ciccio64 said:
Those have been both used to power the heaters is series. As such the twisting technique is not applicable here also I had to created an artificial centre tap to ground in order to minimize the hum as there was none. This was done for both secondary independently. Any suggestion on how to wire the heaters better in order to eliminate the residual hum?
Click to expand...
Twisting is still helpful for series heaters as it reduces hum due to magnetic induction. It won't reduce hum due to capacitive coupling, but still worth doing. AC heater wiring should be run right next to the chassis (so first to be installed) as this reduces capacitive coupling.

In addition, ensure that the -ve end of the bridge rectifier connects directly to the -ve of the first (reservoir) capacitor, then from there to the next (smoothing) cap -ve. Only from there should you connect to signal ground (or, perhaps, chassis). People often connect the rectifier straight to ground and then wonder why they get buzz.
 
#26
jazbo regarding triode connection it is clear as per schema however my pentode connection is done bringing the upper grid to ground (the grid with N1 in the schema) is that wrong?
Noted regarding the bias resistor

DF96 tks for yours I shall try to twist what is possible. Also noted your advice ref the grounding. My chassis is wood so at present I have all the ground connected to the -ve of the first reservoir capacitor kind of center ground and than to the main ground

Any of you have knowledge of a hum cancellation technique done by bringing the center tap of the heater supply to the cathode of a power valve? I sow it on some forum but reluctant to try if I do not know what it is exactly
 
#28
ciccio64 said:
jazbo regarding triode connection it is clear as per schema however my pentode connection is done bringing the upper grid to ground (the grid with N1 in the schema) is that wrong?
Click to expand...
If you mean pin 1 (the suppressor grid) is connected to pin 8/ground, then that is the standard pentode connection (if the screen grid is connected to the B+).
 
#31
huff I was starting to feel like an idiot!! good to know that connection is still good actually I was already surprised by how much volume I can get with such few watts so I should get even more in pentode
by the way what is the difference from the practical side from "my" triode connection and the one as per schematic?
 
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