Fine tuning/Blueprinting your amp-

[Fischerman]

The few tweeks that have stuck on mine:

HRM tweek: I think a 500K audio HRM bass pot works better than 1M linear...it just makes the control less of an ON/OFF switch and since it's just a variable resistor it has no effect on tone (unless you typically run the HRM bass trimmer above 5 with a 1M-L...which I never did). At the minimum I would use a 1M-A pot (is it possible HAD used linear here simply because linear trimmers are (or were) much easier to find and he was more or less forced to?).

I split the difference on the Skyliner mid cap...I have a .022uF cap with 250K pot. But I pretty much never use Strats through my amp (I like Strats fine...I just rarely play them through my ODS-type). I really just wanted the Mid control to have more range/more of an effect.

For amps with built-in D'lator: I like a 500K-A Send Level pot...seems punchier/livlier/more transparent.

[Funkalicousgroove]

- 150K post-OD2 (replaced 180K)

Man...I could hear the difference between the 1k and 500 screen resistors (less harsh with 1k), but on the resistor you're talking about here...?

I put a 1M resistor in parallel with it temporarily, and couldn't really detect much of a change. What should I be listening for?

You hear a difference in .25v (maybe more/less) in Screen grid voltage? I Sure can't hear that, but I hear a pretty big difference in the post OD2 resistor....Hmmmm....

[greiswig]

- 150K post-OD2 (replaced 180K)

Man...I could hear the difference between the 1k and 500 screen resistors (less harsh with 1k), but on the resistor you're talking about here...?

I put a 1M resistor in parallel with it temporarily, and couldn't really detect much of a change. What should I be listening for?

You hear a difference in .25v (maybe more/less) in Screen grid voltage? I Sure can't hear that, but I hear a pretty big difference in the post OD2 resistor....Hmmmm....

I questioned it, too. I may be imagining things on the screen resistors, but I'll stick by my statement about the post OD resistor in my experiment. But I went back and checked, and I think I didn't hear a difference there mainly because I'd picked up a 10M resistor instead of a 1M resistor to put in parallel. DOH! Okay, using the right value I can definitely hear a difference. Like I said, my amp doesn't need more edge, if by "edge" we're talking about that upper mid character like a Celestion Vintage 30 has.

['67_Plexi]

OK I'll give something up that some might find interesting. Here are some of the changes I made to the OD channel on Eric Johnsons OD-2 to get it where he was really happy with it.

Bear in mind this will only work correctly on a 100W EL-34 HRM style amp that is really cranked (master on eight).

1. OD Entrance - 15K to ground, no trimmer.
2. OD Treble Cap 250pF Silver Mica.
3. OD Slope Resistor 33K CF.
4. All Cathode bypass caps CF.
4. Coupling from the plate of the first OD stage 0.02uF
5. Cathode bypass cap on 2nd OD stage 5uF (also try it removed altogether)
6. Cap removed from 470K/47pF network on the input to the 1st OD stage.
7. V2 tube cover removed. (This was important to Eric and yes I could hear the difference when he was playing)
8. No snubber between V3 plates.
9. 1.5K grid stoppers on power tubes. (This was a happy accident and works well with JJ tubes)
10. Slightly more elevated voltages on both V1 and V2 than what most people aim for. (215-220V)
11. Fairly mismatched 12AX7 in V2.
12 7V difference between plates on PI, no trimmer (a high quality trimmer is equal to an average quality fixed resistor IMHO). 100K/110K plate loads.
13. I can't remember the feedback resistor, but I think it was 68K on the 8 ohm tap.
14. 2K Primary O/T.
15. 100pF switchable Brite cap on 'Pre-Gain' control. No other brite caps.
16. THe only screened cable used was from the input jack to the first grid.
This requires very careful lead dress, but is do-able, especially if you connect the masters to ground via a 10 ohm resistor.

Notice that these mods, especially compounded could make the amp a little less stable than a vanilla HRM style build. However, I think thats where the magic stuff really starts to happen.
With Eric playing this gave his signature lead tone without the BK Butler. With me playing this sounded like I had not quite figured out how to play a barred 'F' yet !!

I know some folks on here will see some of these mods as 'breaking the rules'. You must have an open mind when dealing with someone as skillful as Eric.

This is a fairly drastic variant from the 'Texas' version I make of the OD-2. The 'Texas' version somewhat emulates this at a lower volumes.

[Funkalicousgroove]

Sounds like in the wrong hands that thing would be an untamed beast!

[jelle]

7. V2 tube cover removed. (This was important to Eric and yes I could hear the difference when he was playing)

Hmmm, the only thing I can think of here is the decrease in Miller cap that occurs. I hear this effect too. How could we get this effect with the tube cover on? If the grid resistors are decreased, the lows can become farty, I'd guess and if we would add bypass caps across the old value grid stoppers than the upper freq of the highs would be far higher than when using the old grid resistor values and the cover removed. Isn't the Miller cap a function of the plate load resistor? Why not a smaller plate resistor for the same effect? The gain would change....

Just thinking out loud anyway....

Cool tweak! Thanks for sharing!

Jelle

[Tonegeek]

Did you try the 100K slope with the .047?

I did today and like it very much with my Strat (have not tried the Reverend HB yet). I thought my clean sound was good before, but it was a bit forward compared to what it is now. I had a .047 and .01 on a switch at one time and did not like the .047 but that was with the 150k sloper. With the 100k sloper the .047 really works well. It finally is the answer to some slight upper mid sharpness I have had all along with this amp. I can hear the change the sloper made on the top end in all modes (PAB, Mid Boost, OD). I get a very even, warm sound now on the clean channel. OD is benefitting too. I kept the 500k bass pot which is key to getting a fat OD (PAB + MID Boost) sound. I found out by accident that if you use a smaller pot on the bass, that there is not the right kind of lows in full OD even with the wiper lifted with PAB. Too much gets bled to ground because of the lower sloper unless you have a 500k pot there and even then there is some loss that I can detect. I may be able to readjust my HRM bass trim to compensate but I havent decided that it needs it so far. Thanks for pushing me to try that. I am taking this amp to Charleston SC tomorrow to demo for a friend of mine whose got golden guitar tone ears. Life is good!

[Fischerman]

Just curious Alan,
Which way was V2 mismatched? OD1 with more gain or OD2?

Thanks for posting that...very interesting.

[moj067]

Alright you went and did it.
Alan, how's Eric like the cleans when those plate voltages are highish or will he use the Fenders for clean work or do the other adjustments made compensate.
I know, I know, play what I like to hear when I'm playing, but Eric is one of those guys I got to ask about.

[ayan]

16. THe only screened cable used was from the input jack to the first grid.
This requires very careful lead dress, but is do-able, especially if you connect the masters to ground via a 10 ohm resistor.

Thanks for sharing. I wonder why connecting the masters to ground via 10 ohm resistors mitigates susceptibility to oscillations, which is what I infer you're referring to here. Any light to shed on this, Alan?

Cheers,

Gil

[heisthl]

I know Fender did this in some amps that had the ground plate on the front panel to get rid of ground loop hum. I think Glasman did some research on this.....

[ayan]

I know Fender did this in some amps that had the ground plate on the front panel to get rid of ground loop hum. I think Glasman did some research on this.....

How interesting! Were the 10 ohm resistors close to the brass plate, or close to the pot's lug? Fender amps were one huge ground loop though, with the signal grounds at the pots' cases, etc. Dumble's grounding is a bit more refined than that, IMHO.

Gil

['67_Plexi]

Just curious Alan,
Which way was V2 mismatched? OD1 with more gain or OD2?

Thanks for posting that...very interesting.

OD2 had more than OD1.

['67_Plexi]

I know Fender did this in some amps that had the ground plate on the front panel to get rid of ground loop hum. I think Glasman did some research on this.....

How interesting! Were the 10 ohm resistors close to the brass plate, or close to the pot's lug? Fender amps were one huge ground loop though, with the signal grounds at the pots' cases, etc. Dumble's grounding is a bit more refined than that, IMHO.

Gil

Hi Gil,

Sorry I never answered the original question, I just didn't come on here to see it.

Firstly I use a steel chassis and a multi-star grounding arrangement.
The amp has dual masters that feed in to a cathode follower via a relay arrangement for the efx loop.
It seems that input impedance of the cathode follower plays a big part in any oscillation issue. If you disconnect the preampand ground the imput to the cathode follower, the cable you use to ground it is very prone to creating the oscillation by it's postion. The closer it is to the chassis, the worse it becomes. I believe a resonant circuit is being created and the chassis is acting like a feedback loop. The 10 ohm resistor just stops the input to the cathode follower from ever being at a true ground reference, thus prevent the feedback loop. Is it masking the symptoms or curing the problem ? I would say the former was probably more true.
9 out of 10 amps don't need this resistor, but I use it anyway.
There are a couple of other observations I made.
The first is I have changed the 5751 efx loop tube on an amp that made that noise without the resistor and it cleared it and visa versa. THis suggests that the amp is always close to this stage being unstable.
The second is the fact that the signal coming from the clean stage is more susceptible to creating this oscillation based on its dress than the signal from the OD stage. I guess the extra legth on the cable may be the reason. Sheilding the clean master output definately helps, but I still like to add the 10 ohm resistor just in case.
Some Marshall designs employed a 10 ohm resistor to ground on a similar efx loop circuit, again definately making me feel the problem is with the input to that circuit.

Would love to hear your thoughtsw. There's nothing I like more than debating such things and I don't mind being proven wrong with another more plausible theory.

[Dai H.]

putting an R in a ground line sounds like a ground loop is being broken. Maybe the ground point needs to go somewhere else?