Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Lectroid on February 03, 2024, 10:08:42 am
-
Still tweaking my new 18W for tone but mostly pleased overall. Thanks again to all who kicked in advice and encouragement along the way.
So--working with TSC to try to tighten up the TB tone stack. I got stumped over the standard 1M value for the "load" resistance, RL. What RL, exactly, does this tone stack feed into?
On my schematic, the signal comes out of the tone stack, through the 150K wet/dry mixing resistor, and on into the LTPI. What about the 330K resistor in the reverb return line, is it in parallel to the 150K? And does one or both of those together form a divider with the 1M grid leak resistor on the PI?
I know the short answers are "Yes" but I'm hoping to understand the total interaction so I can understand the tone stack's load. Can someone give me a hint as to how to visualize this beyond "you use Ohm's Law?"
A reverb question: I'm using a 12DW7 in one of tubenit's one-tube reverb circuits. The recovery stage has a D node voltage of 268V with a 100K plate resistor. That stage's plate voltage is 82V. It verbs okay, but I wonder if maybe it could be run a little hotter. The two preamp tubes are fed by the same D node, and all three tubes have a 100K plate resistor. Since both of the 12AX7 triodes have plate voltages of 170V, why does the reverb recovery plate voltage drop so low on the 12DW7?
Thanks in advance!
-
Your 12DW7 is wired wrong. Pins 1,2,3 (12AU7) should be used for the driver and pins 6,7,8 (12AX7) should be used for the recovery.
-
And so it seemed to me but I read a post here that seemed to call for the way I did it. It's easily changed, though. I hoped tubenit could weigh in if he's about.
sluckey, how does that affect my original tone stack load resistance question?
-
And so it seemed to me but I read a post here that seemed to call for the way I did it. It's easily changed, though. I hoped tubenit could weigh in if he's about.
sluckey, how does that affect my original tone stack load resistance question?
No effect on your original tone stack load resistance question.
Here's tubenit's 12DW7 one tube reverb showing the correct pins...
https://el34world.com/Forum/index.php?topic=30415.msg335174#msg335174
-
I seem to remember that one of the tube manufacturers at one point made a 12DW7 that had the triode pin-outs reversed (JJ maybe?)
-
I seem to remember that one of the tube manufacturers at one point made a 12DW7 that had the triode pin-outs reversed (JJ maybe?)
Yes.
https://www.tubesandmore.com/products/vacuum-tube-ecc823-jj-electronics-dual-triode
-
Thanks all
I found my original mistake when I was planning the amp. tubenit gave me a design that clearly read "12AX7" on the schematic, but in the same post he told me he had good luck with 12DW7 in that circuit. It's working okay, even using the wrong sections. Should be a nice improvement.
Anyone have a thought about figuring tone stack load? Any hints would be greatly appreciated.
-
Anyone have a thought about figuring tone stack load? Any hints would be greatly appreciated.
Can you post a schematic? Easier to calculate the AC load when the picture is clear.
-
You can download the schematic from my top post. Thanks.
-
You can download the schematic from my top post. Thanks.
Minor picky-point.. on the schematic, the AC Hot should first go directly to the fuseholder (inner / rear contact..), then the switch and finally onto the rest.
-
> On my schematic
Say 240k above 200Hz rising over 400k below 100Hz.
The real tricky bit is that the 1Meg is bootstrapped to many-Meg.
It is mostly a broad-band droop--- turn your VOL up a hair.
-
The tonestack (AC) load depends on the pot rotation (which also alters the frequency bandpass). 'Worst case' load with both pots fully cut; with bass pot fully cut, the 0.022uF bass cap and 0.022uF mid cap are in parallel (0.044uF) dumped to ground through the 6k8 tail resistor after having been attenuated by the 33k slope resistor. If the treble pot is also fully cut, then the 500pF is sonically 'out of the picture' and you have a Vout = 6k8/(33k+6k8) = 17% of Vin for the frequency f = 1/(2Pi.R.C) where R is 6k8 and C is 0.044uF (which has been cut to 17% of Vin). RL = 39k8 = 33k + 6k8 from the TS alone. If the reverb level pot is fully cut, you have 150k + 330k in parallel with the 6k8, which makes 6k7 on that 'bottom leg', so with treb and bass fully cut, Vout = 6k7/(33k+6k7) = 16.8% of Vin; not much difference.
With bass pot at full rotation and treble pot fully cut, the 0.022uF bass cap forms a C/R filter with the 500k pot and the 6k8 tail in series (506k8) for the frequency f = 1/(2Pi.R.C) where R = 506k8 and C = 0.022uF, and where Vout = 506k8/(33k+506k8) = 94% of Vin BUT the 0.022uF mid cap also forms a C/R filter with the 6k8 tail, which has been attenuated to 17% of Vin thanks to the 33k slope resistor (or 16.8%, once the reverb level pot load is added in). So 'RL' = 572k8 (for the bass frequencies) and slightly lower once the reverb level mixing resistance is factored in (with the reverb level pot cut).
With the treble pot and bass pot both at full rotation, the 500pF and the 250kL+500kA+6k8 form a C/R high-pass filter and RL = 756k8 for the treble frequencies. BUT there are 2 other C/R filters in parallel with this, these being 0.022uF/506k8 and 0.022uF/6k8 respectively (but these latter two are both attenuated by the 33k slope resistor). So the RL is different at different 'f's. Note that (with bass pot full rotation) even though the bass frequency RL = 506k8/572k8, this is not a major reduction and the effect of all that bass is more audible, while the mid frequencies are dumped considerably through the 0.022uF/6k8 filter (after attenuation via the 33k slope resistor)
The effect of the load on V1b from the LTP is minimised by the effect of the LTP's 1M grid resistor being bootstrapped to the 39k tail (as PRR already noted), so you might as well ignore that.
Note also that 'RL' is different from the 'output impedance' of V1b because the latter also requires that you factor in the parallel effect of the plate resistor of V1b, and the partially unbypassed plate resistance of V1b (the 0.68uF bypass cap on V1b noticeably favours treble-mid boost - somewhat counteracting the mid-scoop opportunity afforded by this tone stack).
-
@PRR,
Thank you for figuring the load resistance for me. I've been stuck on the TSC app without that number. I know it's not significantly different from the 1M value but more precision can't hurt.
@tubeswell,
Thank you. This is outstanding, way over and above requirements. I was hoping for just a bare-bones quick sketch of how the various parts worked together, but you've given me much more detail than I expected, and very clearly laid out. You've already answered three or four nagging questions of mine about tone stacks. I will be studying this in detail.
Thanks again! :worthy1: :worthy1:
-
Minor picky-point.. on the schematic, the AC Hot should first go directly to the fuseholder (inner / rear contact..), then the switch and finally onto the rest.
I know that's the best practice with actual fuse holders. I've done it that way. This time, since it was a Marshall-ish amp, I used one of those IEC power inlets where the Hot runs through a fuse in a built-in holder. I realize my schematic doesn't really portray that fact.
Connection points between the AC inlet, mains power switch, and PT are all covered with shrink wrap. It's impossible to change the fuse without first removing the power cord.