100K Plate Amp (non HRM Findings): The Verdict

[Luthierwnc]

Hi Gil,

Exhausting work! I'm curious how you think these changes would effect a 450VDC 2x6L6 amp?

Thanks, Skip

[Pete]

Gill, much thanks for the report. I've been interested in this subject since the info re: 124 was made available. I changed one of my amps to 124 (4X6L6) and have been evaluating for some time now. 335 (or LP) & vintage EVM12L or G12-65s.

Some observations are:
With no cap on the MV the clean seems to have plenty of high end and I'm considering disconnecting the LNFB @ CL2. I assume you have it. (??)

I have a 250K OD input trimmer (set pretty low, maybe 20k) and 250K "Level" control and no HF trim on the OD out. I prefer the OD not too "gainy" and this sounds pretty good to me, maybe a bit muddy. Perhaps I will revisit the HF trim and go to the 100K trimmer.

The settings of the controls you stated are very similar to what works for me other than I don't get the Master up quite that much and have to keep the Treb control too low.
Any tips for me??

[dogears]

Hi Pete,

Although 124 is a nice sounding amp, I would not use its values as gospel. We have several other degooped "grail" specimens to consider. They all have 100K drive pots and 100K pre OD trimmers w/220K feeding them. Several have fairly large bright caps on the master!

All in all, I prefer 100K drive pot and a 100K OD trimmer. YMMV.

[JimiB]

Several have fairly large bright caps on the master!

how large?

[ayan]

Hi Gil,

Exhausting work! I'm curious how you think these changes would effect a 450VDC 2x6L6 amp?

Thanks, Skip

No idea! My amp has higher voltages than that and 4 big puppies in the power section.

With no cap on the MV the clean seems to have plenty of high end and I'm considering disconnecting the LNFB @ CL2. I assume you have it. (??)

I have the FB loop on V1B and I have 30pF cap across the master volume. But yes, the amp is plenty chimey, so I am sure that taking the cap out wouldn't deprive it of any highs per se.

Hi Pete,

Although 124 is a nice sounding amp, I would not use its values as gospel.

I suppose the statement is true enough. However, how would you know what 124 sounds like? It was just Billy and me in that room with the amp, I could swear!

Several have fairly large bright caps on the master!

how large?

47pF is the largest I have heard of, but that is as printed on the disc... We all know those things should be measured, right? And they cannot be measured without disconnecting at least one of the leads.

Cheers,

Gil

[dogears]

Gil,

You and Billy both told me what it sounded like! And you are not using a few of those 124 innovations, as well as my testing with them. I prefered 100K drive pot and standard 100k gain trimmer. As have many others that have tried it. But no doubt, it seems (based on your comments) it worked in 124 for some reason.

To answer the bright cap question, as Gil says, a "printed" 47pf.....

[ayan]

Gil,

You and Billy both told me what it sounded like! And you are not using a few of those 124 innovations, as well as my testing with them. I prefered 100K drive pot and standard 100k gain trimmer. As have many others that have tried it. But no doubt, it seems (based on your comments) it worked in 124 for some reason.

To answer the bright cap question, as Gil says, a "printed" 47pf.....

Actually, 124 was great sounding but a brown sounding amp (at least before the cap job Billy gave it, I didn't hear it afterwards) and it had no clean tones at gigging volumes. However, as I recall, Billy LOVED that amp.

As for the loop bypass network, I always use the loop in my amp so I wouldn't have any use for something like that. The 345K trimmer... should have made the amp muddy, but it somehow didn't. I don't believe it would work in MY amp, because it would probably make the OD too bassy. But, Brandon has tried that and liked it, so it may be worth a try.

The master volume cap, totally. Those things are 20% tolerance, and the difference between a 38pF and 56pF (both of which would be printed as 47pF) is HUGE.

I have compared notes with several people, all of whom have copied Dumbles and tried values that would suggest an amp would sound "not so good," and our findings have been that the amps do not sound so good! Yet, the original Dumble which those values were copied from sounded great. That is the missing link, IMHO... somehow Dumble got combinations of parts to work in cases where no one has been able to get them to work. So, I encourage everyone to try the 124 schematic as is and see how it comes out yourseleves.

Gil

[heisthl]

I had a not so good sounding 50 watt Bassman Iron, skyliner stack, 220k/150k/180k/120k w/BM PI laying around so I went with the 100k plates(RK1k5 and 10uf) on V1 only and tried the clean, It seemed just OK until I converted the PI back to standard and then It sounded better than OK. Really good actually, in fact you really don't need OD at all if you can play louder - the mid boost and PAB are enough for really great OD sounds. To me, the cleans are a more sensitive version of the BluesMaster cleans. They get dirtier from going from light to hard pick attack than the BM stack with 220k/150k but have about the same fullness of tone. Don't get me wrong, the high plate w/ BM stack has nice touch sensitivity but the 100k plates have more (maybe too much). This amp has a 15pf on the master.
The OD (still at 180k/120k 220k into 100k trim, 100ka drive) is NOT as good as before but with the proper tweaking I'm sure it will get there, right now depending on 100k trim setting it's too wimpy or becomes harsh when turned up above 9 o'clock.
335 neck pickup w/120pf bright on, volume noon, tones t11,m1,b1 very Carltony.

[Pete]

Although 124 is a nice sounding amp, I would not use its values as gospel. We have several other degooped "grail" specimens to consider.

Yes, I suspected that may be the case.

They all have 100K drive pots and 100K pre OD trimmers w/220K feeding them. Several have fairly large bright caps on the master!

All in all, I prefer 100K drive pot and a 100K OD trimmer. YMMV.

Any of those with 100K plates and 1.5K cathode??

[heisthl]

Well I went ahead and tweaked the OD section. Out of laziness and curiosity I duplicated Ayan's work and am very pleased. OD is not bassy or shrill. I used a fixed 68k/.001 off of MVp1 instead of a trimmer. It's not the in your face non-HRM "talent in a box" OD like I'm used to but still very nice - notes can still do the sweet sustain forever thing. So now were up to 4 essential Clones everyone needs to build. High Plate non-HRM, low plate non-HRM, Bluesmaster and HRM. I will play with the MV bypass cap (30pf) tomorrow but I will say the 15pf is working well. I did a quick V check 204,204,206,204 309 304 B+ on this amp is 460 with the choke,2k2/22k/2k2 - 150k to ground string. Lately on the high plate amps I've liked lower V1 and V2 plate voltages but on this lower plate amp it seems like the 200+ is ok without being sterile.
Gil - what plate voltages did you end up with on V1 and V2?

[ayan]

Gil - what plate voltages did you end up with on V1 and V2?

Glad you found the amp to be cool after the 100K conversion. The ticket to this amp is volume, and with it you can stay on either the clean or OD channel and do everything you need to do by riding a volume pedal.

Voltages? I have no idea, I don't tend to measure those in the preamp tubes unless something sounds off. However, since I have about 480 VDC or so on the power tube plates plates, I would suppose everything is probably about 5-9% higher than when using Fender iron.

In the settings you mentioned, was that "o' clock" or not? I was suprised to see you had T11, M1, B1 with a 335 on the neck pickup. If those are o' clock readings, I guess your amp must be quite a bit thinner sounding than mine.

Gil

[JimiB]

with the treble bleed circuit installed are you able to leave the bright switch on all the time?

[ElectronAvalanche]

Gil wrote:

I have compared notes with several people, all of whom have copied Dumbles and tried values that would suggest an amp would sound "not so good," and our findings have been that the amps do not sound so good! Yet, the original Dumble which those values were copied from sounded great. That is the missing link, IMHO... somehow Dumble got combinations of parts to work in cases where no one has been able to get them to work. So, I encourage everyone to try the 124 schematic as is and see how it comes out yourseleves.

The few Dumbles I was able to play did indeed sound all different, or let me put it another way. Each amp had to be approached differently.

One very early one had almost NO clean headroom (this was an amp with one large eyeletboard with the PS filtering on the same board) and thus the player had to use the volume pot on the guitar to get a Clean sound, but the Clean "morphed" into the Drive very nicely. The OD was just an extension of the Clean so to say.

Since there is so much talk about #124, here is how #123 looked like. I saw the schemo only, but supposedly that amp was a great sounding one as well.

ODS 100W #123.

First stage:
220k, 3k3, 5uF
Tone control: as in the schemos available (330pF, 0.1, 0.047)

second stage:
150k, 2k2, 5 uF, local feedback: anode -> 22M -> 0.047 -> 22M -> grid
220k grid stopper, no cap across it !!!!
0.047 decoupling

OD section:

220k series, 500k (!) trimmer -> 68k -> grid
220k, 3k3, 5 uF, 250 pf anode -> cathode
0.01 uF -> 100k -> 100k pot -> 150k ->grid
150k, 2k2, 5uF, 250pF anode -> cathode
0.0047 uF decoupling
150k -> 1M (master) -> Pi
from the ccw of the 1M to 500k pot to 0.022 uF to ground (tone control of OD!)

PI: as in the schemos, 820, 24k, 1M, 0.1 uF, 0.01 uF (I am not sure about this one tho). Tubes were 7581. 420 V on the anodes.

The preamp had quite low voltages:

V1a: 190 V, V1b: 192V, V2a: 300 V, V2b: 320V

So Dumble seems to have used 100k/1k5 and 220k/3k3 depending on the customer or maybe even more importantly depdending on the OT used? Dumble seems to have used MusicMan, Dynacord (ST70; MKIII), generic Fender Bassman and Twin or Showman, Triad etc. Like Ken Fischer, he used what he could get his hands on (at least that is what me thinks).

He seems to have used the 6PS Sprague Drops throughout, then there are the blue low voltage 0.1uF caps in the NFB loop on some amps. Oddly enough, Dumble used CC resistors in the beginning and in some places even in later amps. Dale resistors and Carbon Film for the rest or a metal film for the 68k in the OD section.

To sum it up:

Gil´s findings are very interesting. But I guess not everyone will be happy with the 100k/1k5 in their amps, since the parts do seem to make the sum. On another note: before I even knew there was a BM PI, I played around with different PI values and found the Marshall-PI very pleasing. But I went back to the published circuit since I thought I would NEED to stick to the secret formula to get the sound. Seems like it would be best to stick with what sounds good for an specific amp (specific OT and circuit).

Hope this is of some help.

Cheers,

Dominik

[dogears]

Dominick,

So you are saying that the OD fed directly into the 1M master with no level? #123 would have a level control on the chassis. You sure your notes are correct? 150K into 100K pot w/HF trim control. Then into master?

BTW, 124 had the 500k pre-od trimmer too. It measured very low and had likely been opened and modded. Did you actually measure the impedence here?

Thanks for sharing!

[ElectronAvalanche]

So you are saying that the OD fed directly into the 1M master with no level? #123 would have a level control on the chassis. You sure your notes are correct? 150K into 100K pot w/HF trim control. Then into master?

Hi Scott,

I guess I was not quite clear enough with the "from the ccw of the 1M to 500k pot to 0.022 uF to ground" part in my posting. "ccw" would mean on the input side of the OD Master.

Look at the ODS101 schemo: 0.005 uF OD2 decoupling, into 150k, tone pot (500k, cap to ground) into 1M OD Master from there to PI.

The 500k reading of the input pot of the OD was measured by someone who I think knew what he did. I did not measure it myself.

Cheers,

Dominik