The voltages were indeed the key - I swapped the 2k2 and the 10k resistors in the Concert's power supply and - presto! - there went the harshness! The tone now is smooth and singing just as I expected. On the contrary, in my modded Deluxe Reverb II the voltages were too low (170 and 190). I could raise them to 190 and 210 and again the sound improved. Thank you all so much for making me aware.Darkbluemurder wrote:Very interesting - I guess this could be the key why my modded Concert (70s style) sounds harsh. I haven't measured voltages yet. The schematic shows a B+ at V1 of 300 V which however should be correct by looking at the various schematics. However, the B+ at V2 is shown at 350 V whereas it should be close to 300 V. I guess I need to swap some resistors in the power supply chain.
This is why I love this site. I could probably have gone on forever tweaking in the wrong places ...
V1 & V2 Plate Voltages - A critical observation
Moderators: pompeiisneaks, Colossal
-
Darkbluemurder
- Posts: 584
- Joined: Tue Jan 03, 2006 7:28 pm
Re: V1 & V2 Plate Voltages - A critical observation
Last edited by Darkbluemurder on Mon Jun 12, 2006 6:06 pm, edited 1 time in total.
Re: V1 & V2 Plate Voltages - A critical observation
Teo,
How am I "not completely correct"? All I said was some gain stages are biased closer to cutoff than others. If I am not correct than all gain stages would be biased exactly half way between clipping and cutoff. Simple logic. Right?
Are you saying that's the case?
Look at V2A: 0.76ma. How is that not biased closer to cutoff than V2B at 1.10ma?
Running sine wave signal through this circuit and looking at it on a scope confirms this.
Ck reduces bias shift and increases gain, but it does not change the bias point. That's DC. Ck works on AC.
Smitty
How am I "not completely correct"? All I said was some gain stages are biased closer to cutoff than others. If I am not correct than all gain stages would be biased exactly half way between clipping and cutoff. Simple logic. Right?
Are you saying that's the case?
Look at V2A: 0.76ma. How is that not biased closer to cutoff than V2B at 1.10ma?
Running sine wave signal through this circuit and looking at it on a scope confirms this.
Ck reduces bias shift and increases gain, but it does not change the bias point. That's DC. Ck works on AC.
Smitty
Re: V1 & V2 Plate Voltages - A critical observation
Hi Smitty
Considering "correct" values the bias voltages are 1.7 for V1 and 2.0 for V2.
Cutoff considering 320-360 V at power supply node occurs usually at -4.5/-5 grid voltage and saturation (clipping) obviously occurs at 0 grid voltage.
So you can see that actually they are all biased more toward clipping than cutoff!! (suggestion take plate curves and draw load lines...)
Regarding dynamic behaviour connect a meter to measure voltage at V2b cathode, at idle it should be 2.0 V (or similar depending on tubes) as above, then play and you should see it rising at least .5 V -> the dynamic operating point has moved! and now it's almost in center between clipping and cutoff!! Ck act as an integrator here, it take some milliseconds to change the voltage across it, yes it's AC but its freq is very very low.
Some Hi-Fi gurus use this effect at their advantage by letting it works the opposite way so that at idle the "class A" stage is biased a little bit colder, which then means increased efficiency.
teo
Considering "correct" values the bias voltages are 1.7 for V1 and 2.0 for V2.
Cutoff considering 320-360 V at power supply node occurs usually at -4.5/-5 grid voltage and saturation (clipping) obviously occurs at 0 grid voltage.
So you can see that actually they are all biased more toward clipping than cutoff!! (suggestion take plate curves and draw load lines...)
Regarding dynamic behaviour connect a meter to measure voltage at V2b cathode, at idle it should be 2.0 V (or similar depending on tubes) as above, then play and you should see it rising at least .5 V -> the dynamic operating point has moved! and now it's almost in center between clipping and cutoff!! Ck act as an integrator here, it take some milliseconds to change the voltage across it, yes it's AC but its freq is very very low.
Some Hi-Fi gurus use this effect at their advantage by letting it works the opposite way so that at idle the "class A" stage is biased a little bit colder, which then means increased efficiency.
teo