Hi Gents,
now I know why Mr. HAD used this specific trim (together with 220k.
I had, as usual, only 250k or 500k Pot/trim for this particular OD entrance. So I took the 500k. But with no satisfaction. The OD was allways too hot. At the trim as at the OD Level.
So I took a 1M2 parallel to the 500k (between Pin 1 and 3). So there is now aproximatly 350k. Put it 20-30 k to ground and there is a beautifull wide range for installing at the OD Level. From smooth crunch to enough haevy.
HAD done right with his 346k!
Great follow your amp research and findings - are you learning towards a low plate #124 amp now?
I never tried the (220k resistor into) 345k trim, but used (the ordinary 220k into) 100k trim, when I tried the low plate setup (100k/1.5k on V1 and v2). The OD tone was a bit too harsh and couldn't quite get the OD to be mellow enough for my taste. Maybe I should give it another try with the 345k trim.
Hi,
oh, yes, I´M digging and learning.
I had once a 100k there too. It was harsh sounding too. I changed it to 500k. still some harsh sound, than the usefull small cap here and there. I becomes better. But the way of the Trim/Pots where still short, or overloaded. But with this 1,2M over the 500k (in thruth its a 550k) reading something between 350k and 360k (I´m not at home now) makes OD Level like butter and wide range. The trim is around 25k to ground.
Its terrific, something like a key to the OD.
So if I read this right, you're saying that for you the overall resistance between the coupling cap and V2 is 220k + 320k(trimmer wiper to high side) + 68k = 608k, with 25k to ground.
I'll have to go back and see what I ended up using, but I think I ended up with something similar except that I used a lower-value pot (50k if I remember right) plus fixed resistors so that the trimmer was less touchy. So IIRC I ended up with 500k + 38k (wiper to high side) +68k = , with 12k to ground.
And my amp seems to prefer that 12k to ground. Anything over about 15k to ground and the OD sounds harsh. That low value surprised me given what other peoples' experience here is, but then this is a low plate amp and I don't know what to expect.
(PS - thanks for your service, vets! We honor you today.)
talbany wrote:I believe it's safe to say that this type OD entrance would only work (or work best) on a low plate w/ low value (5uf) bypass caps like 124..
Tony
Which are you referring to, Tony? Mine or the OP's? And why in your opinion is the utility limited to low plate amps?
greiswig wrote:
And my amp seems to prefer that 12k to ground. Anything over about 15k to ground and the OD sounds harsh. That low value surprised me given what other peoples' experience here is, but then this is a low plate amp and I don't know what to expect.
When I tried the 100k, low plate setup, I also ran the (100k) trimmer farily low... around 12k in order to get a usable OD tone.
Guess it depends on how hot your preamp tubes are - I used JJ 83S back then. ...Now, I'm a NOS GE ecc83 JAN, short plate convert....(IMO Dogears was right again) -
G
I was just referring HAD's usage of the 346K pot there..(Should have made that clear)..The bigger resistance to ground the lower the -3db break point = more bottom and somewhat less highs (which is where the smoothing effect comes in IMO)..You have to be careful where you set the breakpoint so early in the OD gain stages or it will get boomy (worse w/high plates) and lack focus and kill some sustain..I prefer lowering the break point adding bass later in the chain say at the drive pot but every amp is different and that's just me..
Tony
" The psychics on my bench is the same as Dumble'"
bluesfendermanblues wrote:...Now, I'm a NOS GE ecc83 JAN, short plate convert....(IMO Dogears was right again) -
That's what I've got in V2 as well. I never cared for the JJ's that much, but either the GE or an RCA black plate that I have work really well in that slot. I have yet to lay my hands on a Mullard to try there.
Oh, and thanks for the clarification, Tony. I always learn something from you.
This is interesting stuff. If you do the numbers the -3dB break out of Cl2 is around 9Hz for the more usual 220k + 100k trimmer (40k Zo, 0.05u Cout, 320k Rload). If you up the load to 570k with a 350k trimmer, the break goes to 9*320/570 = 5Hz. The response at these frequencies is important to the extreme low end (as the roll-off begins two octaves higher), but not so much for the OD tone, I don't believe.
IMO, the more significant effect of the load resistance is the positioning of the AC load line, and it's effect on the distortion content of the signal being fed into the OD. This effect is much more noticable for a high-plate configuration than a low-plate because the DC load line is already flatter. For example, I got 13% HD2 for a high-plate Cl2 with a 320k load, and that was reduced by about half to 7% with a 570k load. The effect on a low-plate configuration is to go from 7% down to 6%.
This is of course with a given tube characteristic and will vary in practice, maybe to the point that reports of this or that load being "best" is highly dependent upon the particular tube installed in V2.
Last edited by martin manning on Mon May 30, 2011 10:36 pm, edited 1 time in total.
martin manning wrote:This is interesting stuff. If you do the numbers the -3dB break out of Cl2 is around 9Hz for the more usual 220k + 100k trimmer (40k Zo, 0.05u Cout, 320k Rload). If you up the load to 570k with a 350k trimmer, the break goes to 9*570/320 = 16Hz.
Martin.. I am confused perhaps you can help.. The formula I use is..
fc = 1/(2*pi*R*C)
By using this I get..
fc = 1/(2*pi*320k*.047u) =10Hz
fc = 1/(2*pi*570k*.047u) =6Hz
The response at these frequencies is mportant to the extreme low end, but not so much for the OD tone, I don't believe...
Being virtually out of the audible range I didn't believe it made much difference until I tried it...FWIW I can tell..
IMO, the more significant effect of the load resistance is the positioning of the AC load line, and it's effect on the distortion content of the signal being fed into the OD. This effect is much more noticable for a high-plate configuration than a low-plate because the DC load line is already flatter. For example, I got 13% HD2 for a high-plate Cl2 with a 320k load, and that was reduced by about half to 7% with a 570k load. The effect on a low-plate configuration is to go from 7% down to 6%.
Great observation..Thanks!!
This is of course with a given tube characteristic and will vary in practice, maybe to the point that reports of this or that load being "best" is highly dependent upon the particular tube installed in V2.
Oops! I just discovered my error with the frequency shift and edited while you were posting. I started out in the right place, but went the wrong way... see the revised version above.
Re the effect of input distortion on the OD, you have probably observed that the OD character is changed depending upon the position of the clean volume, and whether or not the FET boost is used.
BTW, the difference between my 9Hz and your 10 is the Zo, Ra//rp, which is in series with the load so the R's you used should be increased by ~40k... Results in 8.8Hz for 360k and 0.05uF. The shifted frequency for the increased load would then be 8.8*360/610 = 5.2.
Yeah running it through the calculator real quick I get 10.6..
To give it a reference upping the load to 570 from 320 is the same as upping the V1b coupling cap to a .084
Tony
" The psychics on my bench is the same as Dumble'"
- are you learning towards a low plate #124 amp now?
.. Thank for the double check..