Wouldn't having no bypass on either of the V2 cathodes kill the gain?
No just the 1uf in V2b....Sure it lowers it a bit and you may have to run the drive up a notch.. Listen to the tone fatten up and throws in a bit more crunch to boot..Sounds great with the amp cranked up...The 1uf to me can get buzzy in the mid range..We set up Rick Derringers #1 this way and he loves it.. We call it the Derringer mod..
So all 5uf or 4.7uf bypass caps on V1 and V2 but nothing on V2b?
The reason I tried the 1uf was because I saw it on the Ceriatone S&M layout.
But they also have a 22uf on V1b.
But I had 4.7uf an all of them.
Not that that is a benchmark but it was something to try.
I think in a band situation, it might cut through a little better.
I have another question I can't seem to find the answer to.
Regarding the fizz cap on the PI plates, how does changing the value of that cap effect the tone?
In my case I had a 47pf there and on this last session I changed it to 100pf.
I made a few other changes this time so I can't tell exactly what that did to the tone.
So what does lowering the value or raising the value do?
Is there a formula or a known thing about that?
Such as, lowering it will kill lower frequency fizz and raising it will affect higher frequency fizz?
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regarding the fizz cap on the PI plates, how does changing the value of that cap effect the tone?
In my case I had a 47pf there and on this last session I changed it to 100pf.
I made a few other changes this time so I can't tell exactly what that did to the tone.
So what does lowering the value or raising the value do?
If memory serves the fizz cap originally was put there to suppress oscillations from entering the grids of the output tubes. The cap acts like a sort of negative feedback canceling out those frequency's that pass through it..due to the fact the plates are out of phase...This will also help to smooth the PI as well..Some amps you can really tell a difference others not so much...especially at low volumes...For a more intense effect you can also put it on the other side of the coupling caps..220k side feeding the grids..I believe they call that a conjunctive filter of some kind..To tame the buzzy'ness associated with some of the newer EL 84's these filters do help...022 w/250K pot in series w/the two grids...
The reason I tried the 1uf was because I saw it on the Ceriatone S&M layout.
But they also have a 22uf on V1b.
But I had 4.7uf an all of them.
Not that that is a benchmark but it was something to try.
Tom I don't want to come off here as someone who is telling you hey you don't have the right cap there or well Dumble did it that way so it's WRONG!!..Good tone is very subjective and elusive especially dealing with this topology..If you like the tone of a 1uf go with it..these are just recommendations...Cool!! My find is to try to do most of the tone shaping with coupling caps...Larger bypass caps promotes more harmonic content and richer overtones.. I always try to go with the largest bypass cap I can..Much of this depends on several factors which includes types of output tubes...Output wattage...Type of guitars speakers as well as type of cabinet..After you build several of these amps you will adapt your own recipe amp to amp based on the ingredients you have to work with...It's late I doubt any of this made any sense!! Good Night!!
So I tried putting a 250kB pot in as the level pot, and bypassed the 150k leading into that pot per Gil's suggestion. Definite bump in volume, no doubt. But it seemed to me that it also changed the tone substantially. To my ear, it sounded like I lost some of the midrange bark that I like, and highs were added...all at the same time that things got more muddy. Removing the short across that resistor brought things back tonally to where I wanted them, and the 250k pot gives me enough room to turn up the heat when I need to. It also seemed to help to bump the trimmer down to about 26k from where it had been, 32k.
Been there, I had one amp which had exactly your dilemma, and I went thru your hoops with the same frustration. In the end, I increased the supply
voltage to od1 and 2 and now have lots of room to work, and the tone is stable.
It's easy to f*** up the tone, sometimes the undo button is a blessing. The hardest part is getting the tone with just the right amount of top on it, and interesting how just changing the value of -say- a cathode resistor can affect the balance. I try to adjust the median od tone just a bit dark, so that i know i can get that sound for the studio, etc, but can bring in some top with the presence control if needed, that sounds right. If it's too bright to start with, then there's nowhere to go. If set up right, then with the 150k series r and a 100k lin pot, there can be a nice wide range of volume that doesn't change eq much from lo to hi volume.
The 100k/150-180k resistors that feed the drive and level controls are there to set up the frequency responce after each gain stage ..To my ears helps keep the low end in check and the top end from being buzzy.. These values work perfectly in 100 watt amps..In the lower powered amps 50's and 22's you can raise the value of these resistors slightly with good results...A little goes a long way here..
talbany wrote:The 100k/150-180k resistors that feed the drive and level controls are there to set up the frequency responce after each gain stage ..To my ears helps keep the low end in check and the top end from being buzzy.. These values work perfectly in 100 watt amps..In the lower powered amps 50's and 22's you can raise the value of these resistors slightly with good results...A little goes a long way here..
Tony VVT
Thanks, Tony. So if you've got too much bass in the OD section...instead of lowering the coupling cap before the OD stage (Most D'Lites that I know of have added a .02uF cap in series there), and you could bump the 150k (tweeked D'Lite value) up a little and get similar effect? Maybe the original 180k value was actually chosen for a reason in these 50W amps?
I've tried the hi pass series cap before the od and have mixed feeling about it. On one hand it's an easy shotgun fix to the 'too much bottom' issue, but also throws away some of the good stuff. I think the real challenge is to get a mid and top end reponse that balances the big bottom. It's harder to pull off, but more satisfying in the end.
Anyone know of a real ODS that has a hi pass input cap before the od? It wasn't a common thing imo.
Structo wrote:Actually the OD entrance mod uses a .05uf cap in series with the 220K.
I thought that was an agreed upon mod that a lot of guys did.
I think it may have been Gil Ayan that came up with that. Could be wrong.
I installed that on my amp and liked the result.
I also lowered the value of the coupling cap on V1b from .05 to .02uf and the clean channel still has enough bottom end with that change.
1. I started using a smaller cap (in parallel with a 4.5 Meg resistor to keep the cap referenced to ground at all times, thus avoiding a popping sound when switcing to OD mode) to shave off some of the bass in the amps with 220K/150K plate load resistors.
2. No longer have any of the above amps and I don't think that mod is warranted for amps with 100K plate load resistors, so I don't implement the mod any more.
3. Other people use the same thing to have a full clean sound but an unbuzzy OD, but choose to use a larger second cap, like a .05, .02 or .01 uF.
4. Dumble didn't use anything like that, per se. You could argue that the 470K resistor bypassed with a 47pF cap found in most HRM amps does something similar. However, I see Dumble's network as being a gain reduction scheme where highs are preserved. My approach was to have a coupling cap for the clean channel and another for the OD.
I think you're right in that regard, and it's a balancing act to get the whole od
sounding right. I like having no input hi pass cap, for the snarl and squish, but then the input resistor, trimmer, and overall master bypass cap needs to be just right, and the snubbers can be selected for many different sounds. The od master bleed plays a minor part also.
Iv'e tried the OD entrance cap mod on several amps and didn't care too much for it.. I thought it changed the character of the breakup of the OD.. Just me being weird!!
talbany wrote:Iv'e tried the OD entrance cap mod on several amps and didn't care too much for it.. I thought it changed the character of the breakup of the OD.. Just me being weird!!
Tony VVT
Amen bro, the sound of the OD is the best w/o an entrance cap especially in an open back combo. I finally get the coolest bottom on the sound once i got rid of that cap-- just have to use a bigger entrance resistor to get the gain down--- the 'feel' of the playability is better also
