tweeking the OD

[Pete]

Hi
I'm new here and not very up on the technical stuff but I have been reading a lot.
It's all so very interesting to me, thanks to all the posters and all responsible for keeping the site going.
I haven't built a T'wreck type amp but am dabbling with the D stuff and now that I found this board I'm hooked.

I do have a working amp together and am doing the tweeking. After reading everything here some things are a lot clearer to me. Other things, not so much.
So perhaps I can get a few comments to the following (much thanks in advance).

Regarding OD level & Drive pots, I have read that "The pots are linear taper. The schematics were deliberately wrong according to my inside source"
Q: would this apply to the 250K and 1M values, or just the 100k?

Regarding OD input circuit/resister network, when it is said to "use a 220k over a 100k trim".
Q: would this be AFTER the 470k (w/47pf bypass) OR IN PLACE OF? i.e. the 470k/47pf is left out?

thanks

[dogears]

The OD drive and level pots are linear 100k. The treble pot is a 250k, I prefer audio taper fwiw.

The 220k feeds a 100k trim. The 47pf and other components are only for the HRM amps. Only use the 220k and 100k trimmer in non HRM amps. Fwiw, I prefer 180k or so instead of the 220k.

I also like a 470pf to bypass the 220k V2b grid resistor.

250Ka midpot is good.

500Ka bass pot is good.

Make sure to use the .001 cap across the bass pot.

100k over the OD drive pot and 150k over the OD level pot. The 330k on Hybrid A is wrong.

There is many more tips.... Let me know when it is up and running.

[Pete]

Let me know when it is up and running.

Thanks for the reply, been playing it this way for some time, sounds pretty good.
Started with s3988009 (I use 6L6s) the first ODS schem I found(has HRM). I have since become aware of some of the others. I like the HRM I think.

So re that schem here's what I have so far:
- a correction I've learned about (and applied) is V1B cathode should be 2k2 for the 150k plate.
- I changed to 1.0uf for V1B bypass (currently using 2uf & plan to try 1.5)
The only other place I've varied from the schem is: OD input circuit/resister network - see below

Now, after some reading here about other tweaks I made the following changes.

- 300pf treble cap
- 470pf bypassing the 220k V2b grid resistor
- changed .047uf V1B coupling to .02
- changed V2B bypass from 1uf to 4.7uf
- HRM treble cap changed to 500pf

WOW! it sounds great, I like it much more! Improved clean and OD sounds. Not much to complain about.

What I'm still messing with is:
- The OD input circuit pre V2A I use is ... from V1B coupling cap (.047mf in schem), to 220K over 100K trimmer (without the 470k/47pf shown in schem), then 68K grid.
Now I just don't know if I should have the 470K/47pf in there before the 220k/100k trim at the OD input per that schem or not.
Also, I'm using 250kA drive and 1MA level for the OD controls (per schem) should these be 250kB & 1MB??

I'm looking for a little more "clean OD volume" where you can hit it harder and not get so crunchy. It seems like if I turn down the trimmer I loose too much volume, turn it up gives too much break up.

Only use the 220k and 100k trimmer in non HRM amps.

So maybe I do it like the schem, 470k etc? This is one place I'm unclear.

P.S. Treble is 250Kb (I may try the audio taper), .001 cap across 500KA bass pot, 100k over the OD drive pot and 100k over the OD level pot (all per schem) (guess I could try 150k over the OD level)

Also, does the Master on all REAL ODSs control both clean & OD as in the Hybrid Rev A schem?
I think I prefer the individual (clean) "Master" and OD vol ("Ratio") controls. Is this considered NOT authentic? i.e. someone's tweak/mod?
looks like a lot of questions still
Thanks again for taking the time with this.

I also like a 470pf to bypass the 220k V2b grid resistor.

I'm assuming you mean V1b

The OD drive and level pots are linear 100k. The treble pot is a 250k, I prefer audio taper fwiw.

The 220k feeds a 100k trim. The 47pf and other components are only for the HRM amps. Only use the 220k and 100k trimmer in non HRM amps. Fwiw, I prefer 180k or so instead of the 220k.
I also like a 470pf to bypass the 220k V2b grid resistor.
250Ka midpot is good.
500Ka bass pot is good.
Make sure to use the .001 cap across the bass pot.
100k over the OD drive pot and 150k over the OD level pot. The 330k on Hybrid A is wrong.

[dogears]

Oops! Yeah V1B for the bypass. I also was referring to NON HRM amps with most of my comments.

On the HRM amp I may suggest the schematic low gain network.

47pf bypassing a 470k into a 120k into a 25k trimmer with a 4.7k ground tail.

No 220k/100k network. The above replaces it. But.... You always use the PAB boost! Set the tone using boost mode. That is the concept of the low gain wierd network that precedes the OD stages.

If you use the 250ka drive pot, the OD1 coupler must be smaller than .01 Depending on which post stack slope you choose, I like anything from .0022 to .0033 The amp tone is not nearly as good to my ears if I go bigger. I only run my HRM woth PAB boost though. It does get a better, less compressed clean OD when setup this way IMO.

Use a 150k grid resistor on V2a. There is a ton of Miller capacitance with the 250ka drive pot. This is ok btw. THe 100k resistor over the OD master is OK too.

I do not use that bright cap, at most a 10pf IMO.

Let us know how it works out!

[Pete]

Oops, just looked at the schematic again and noticed all of the first four stages are labeled B first then A (as in V1B, V1A, V2B, V2A)
I have always referred to them as A, B, then A, B. (other schems use CL1 & 2 and OD1 & 2) so anyone reading my last post should keep that in mind, sorry for the confusion.
So the 470pf to bypass the 220k grid resistor would be V1A (CL 2) of course.

On the HRM amp I may suggest the schematic low gain network.

47pf bypassing a 470k into a 120k into a 25k trimmer with a 4.7k ground tail.

OK, I'll put it in like that.

If you use the 250ka drive pot, the OD1 coupler must be smaller than .01

I'm using .0022 and 250ka, is 100kB worth trying (w/HRM)??

Use a 150k grid resistor on V2a.

I do have 150k grid @ OD2, I assume that's what you mean

Let us know how it works out!

shure, going back to the workbench now - THANKS again.

[dogears]

Whoops! Me bad again. I meant 150k on V2b!! You are correct.

You can try the 100k OD Drive pot. However, due to the much lower Miller Capacitance, the OD will be MUCH brighter. You may need to install snubber caps on V2. Dumble left the snubbers out of the HRM amps as the 250ka drive pot creates a HUGE high frequency rolloff. Remember to swap the .0022 to a .01 if you out a 100kb pot in. Also you can try and raise the V2b grid to 180k to smooth things a hair.

My current amp has the 100kb drive pot with HRM. I did this because the amp switches between stack or no stack. THe no stack tone was really dull with the 250ka pot. THe amp is FAR superior sounding in non HRM mode with the 100k. I can't honestly say the HRM tone is better with the change though. It is still really nice, but I run 270pf snubbers in HRM mode now. I also have a post stack treble bleed (250k pot and .001 cap to ground) attached before the 100k series resistor that feeds the OD master.

Try messing with the stack slope resistor too. I have tried 33k, 39k, 47k, 56k, 82k, and 100k It makes a huge difference.

[Pete]

I meant 150k on V2b!!

Well. like I said, (on that particular schem), OD2 is designated V2A. So to add to the confusion, you were right the first time.

You can try the 100k OD Drive pot. However, due to the much lower Miller Capacitance, the OD will be MUCH brighter. You may need to install snubber caps on V2.

Currently using 250ka drive pot AND have the snubbers on OD 1 & 2 (390pf). It's plenty bright so I may not be in a hurry to try the 100k
(also, I'm not using any bright cap at this time)

I also have a post stack treble bleed (250k pot and .001 cap to ground) attached before the 100k series resistor that feeds the OD master.

I've heard that folks find it necessary to do that w/non HRM so I'll do that if I go non HRM.

So I put the pre OD resister network & trimmer just like the schem (470k w/47pf BP -120k series over 25k trim 4k7 tail) and really like it. Cleans up better for me when playing lighter, just what I was looking for I think (even usable for me with PAB off).
Post OD tone stack is about as bright as I would want. I've adjusted it with Treble trim all the way off (or nearly), and most ANY other settings sound good ... OD trim sounds good at any setting (good variety)

Try messing with the stack slope resistor too. I have tried 33k, 39k, 47k, 56k, 82k, and 100k It makes a huge difference.

I have the 33k, anything bigger would be brighter right?? As mentioned above I don't think I need brighter so I may not experiment with the slope for now (but hey, if you insist, maybe I'll check it out when I get time)

Thanks for the tips, I think I'm really gettin' it dialed in, great fun.
Any other thoughts come up, lemme know.
P

[llemtt]

...due to the much lower Miller Capacitance...

I just can't follow you completely here... I guess you mean that the source impedance "into" the Miller Capacitance of v2b is higher and this obviuosly lowers the rolloff freq.

Miller Capacitance is given only by grid-plate capacitance of v2b (approx 3-5pf) multiplied by the voltage gain of v2b (approx 50-60).

Interestingly the rolloff freq changes as you rotate the drive knob because source impedance goes from zero (plus 180k grid resistor!) to approx 90k (plus 180k grid resistor!) and it's not linear inbetween! I would prefer a more predictable behaviour

Teo

[dogears]

Err..... Yeah... I am just a player! haha

I meant the source impedence with the 250k pot is much higher than when you use the 100k pot. I found myself setting the 250k pot on about 1-2pm. That is about 180k of series resistance in series with the 180k grid..... MUCH higher than the 100k pot half on. Hence, much more high end rolloff.

...due to the much lower Miller Capacitance...

I just can't follow you completely here... I guess you mean that the source impedance "into" the Miller Capacitance of v2b is higher and this obviuosly lowers the rolloff freq.

Miller Capacitance is given only by grid-plate capacitance of v2b (approx 3-5pf) multiplied by the voltage gain of v2b (approx 50-60).

Interestingly the rolloff freq changes as you rotate the drive knob because source impedance goes from zero (plus 180k grid resistor!) to approx 90k (plus 180k grid resistor!) and it's not linear inbetween! I would prefer a more predictable behaviour

Teo