bass attenuation

[iknowjohnny]

Like Jana said to me recently, "you can't add a frequency thats not there to begin with". Very true, but heres a related question. My amp has always had a degree of loose fuzzyness in the low end. In recent times it's been much better but still it's not perfect. It's w/o a doubt the distortion generated in the high gain pre thats causing it, and the prevailing attitude is always been to use a very small first coupling cap. Even with no high end attenuation and a couple 470/470 cap/resistor networks (since changed even higher to 1m/470uf) this fuzzy low end happens. But if i drop the first coupler things get thin sounding by the time the bass becomes right.

So the question is, how do you go about getting it back later on in the non distortion stages, or is that even possible? I would like to do this again maybe going as low as .0047uf, but how to i get back not just the low end but the low mids it kills? If i take those frequencies out in the beginning, is there still enough of them left at a very attenuated level but not so low that i cant retrieve them? How do other amps accomplish this, or do they?

[Structo]

What amp are you talking about?

Is this a 50 watt or 100 watt amp?

What kind of output transformer are you using?

People including me, always miss the simple stuff first.
Make sure your voltages are inline with what the amps should be at.

When voicing an amp, try to test it at the volume you will be playing it most at. In other words, if you play gigs, tune it at gigging volume, not at bedroom levels.
Loudness effects everything, from tubes to speakers.


What screen resistors are you using on the power tubes?

Is the loose/ fuzzy bass on the OD or on both OD and clean?

To tighten the bottom end on 6L6 tubes use 470R screens.
The bigger the screen resistor the mushier the bass will be.

Piggyback a 50uF filter cap to each 100uF main filter cap to tighten the low end. This will be more of a feel type than an big audible mod.

Remember to test each tweak thoroughly before proceeding.
IOW, don't make a bunch of adjustments then test.
If something is bad, you won't know which one caused the bad result.

This takes some serious time and if you approach it systematically, you can get the amp to sound much better but you have to evaluate each change before proceeding.

Here is a copy of some notes I took several years ago.
This was offered to me by either Scott or Gil, sorry I didn't write that down.

Before tweaking make sure you are using an appropriate guitar and speaker combination. i.e. don't tweak with a Jensen if you amp will have a Celestion.

1. Do the voltage changes first - i.e. dropping string adjustments (don't forget to re-bias after each change). Lower = smoother

2. Coupling caps next - if you make a change and like it but think you lost something try changing the bypass value accordingly on the stage ahead of the coupler. i.e. if you gained highs but lost lows increase the bypass cap.

3. Bypass caps next - in addition to tone these caps affect gain too, so you can add or take away "character" as the tone changes. i.e. changing to a 10uf (from a 05uf) will add bass and gain, if you like the gain but think it's too bassy try reducing the coupling cap to the next stage.

4. RP and RK values - I've learned not to mess with the ratio of these much, changing the ratio more than a few % hurts the "linearity" for lack of a better word. 220/3k3 180/2k7 150/2k2 120/1k8 100/1k5 pairs can be tried in any location. Sometimes replacing both pairs in the OD section is just what an amps needs i.e. V2 180/2k7 and 120/1k8 changes to 220/3k3 and 150/2k2 or vice versa.

5. Bleeders and filters - you can place a cap and resistor in series to ground anywhere in the signal path to bleed off highs or do a resistor/cap parallel pair to increase highs i.e. a .001 - 330k series from OD2 CC output to ground gets rid of "OD fizz" or i.e.2 lowering the 500p in the 220k/500p filter on the input to CL2 for less mids.

6.PI and output tweaks - I saved the best for last - you can really make or break an amp in this section. For max headroom try and use the smallest PI entrance cap you can (a lot of Fenders used .001!) and larger output tube grid resistors (I like 5k1s better than 3k3s) Screens on 6L6s sound better to me at 500 rather than 1k. If you have a trim pot to balance the plates make sure you can adjust them to within 5 volts of each other if not change one RP until you can, start at 5 volt difference and then very slowly increase as you strum a chord through the amp - you are trying to get maximum overtones - you'll know it when you find it.

The NFB area is especially fun - max the presence control and cliplead in various values of caps and the FB feed resistor to tailor the gain and NFB cut off frequency.

[ckpop]

Structo,

Great description of tuning !!!! Same process and rules I follow. You must look at the factors of the big picture and remember for every change you make how it may change something else in the circuit and tone.

Also the trick is sometimes is knowing where to make a change for a specific result. There may be more then one location to somewhat give you what you are looking for tuning an amp but at the same time they can all be slightly different overall.

[iknowjohnny]

I've tried most of what you described to one degree or another, but i should mention this isn't a case of bad fuzzies or anything like that. It's sorta subtle, but there. Just enough to warrant looking into it still but not necassary. It's not even an issue at stage volume, but at home it is a bit, and i've had plenty of amps where the lows stay the same at home or at a gig.

What amp are you talking about?

homebrew EL34x2, high gain pre thats based on a jcm but changed a lot, and a cathode biased output with 220uf bypass caps. (and tried many more smaller ones down to 22uf). Typical PI and CF values.

What kind of output transformer are you using?

Hammond marshall 50w replacement. I think if i recall it was a 1750n, but don't quote me. It's 3.2k

Make sure your voltages are inline with what the amps should be at.

i dunno.....i've had the voltages every which way imaginable and that never seems to matter as the low end's fuzz is concerned.

What screen resistors are you using on the power tubes?

1k

Is the loose/ fuzzy bass on the OD or on both OD and clean?

OD only

Piggyback a 50uF filter cap to each 100uF main filter cap to tighten the low end. This will be more of a feel type than an big audible mod.

I hate to keep saying this, but been there done that. The amp becomes sterile thin and horrible with bigger caps. I've tried a 50/50 then a 100/100 for the can, and 50 then 80uf for the pre nodes. Nothing sounds near as good as the small caps i have in there. Not even close. As they get bigger the amp is worthless to my ears. then again, A/B'ing a JCM 800 with mine made me thing JCM 800's are horrible. That may be one reason why ! (i've owned 3 and clipped pot caps and all that) This amp sounds far far better with the 32/32 and 22uf nodes than with 50uf or more. Not to mention the bass is still fuzzy sounding no matter how big the caps. It just sound less obvious due to the much thinner tone.

Remember to test each tweak thoroughly before proceeding.
IOW, don't make a bunch of adjustments then test.
If something is bad, you won't know which one caused the bad result.

Thats a rule i'm very aware of i assure you. I never do more than one at a time under any circumstances.

The NFB area is especially fun - max the presence control and cliplead in various values of caps and the FB feed resistor to tailor the gain and NFB cut off frequency.

I've tried several and i agree. Lots of variation to be had. I always seem to go back to .1 tho. I've tried a series cap to tighten bass ("resonance") too. Sorta cool, but the bass stays fuzzy.

Thanks for the long post, i do appriciate the time you spent. And there are several points that have me thinking where i wouldn't have before. i will let you know if i find something in there that helps.

[iknowjohnny]

geez.....i just had me a moment. I have a 1.8k/.68uf on V1A and i just stuck a 2.7k/.68uf on V1B. I did this as a way to increase highs in hopefully a good way and hopefully tighten the lows. I never did it before because i thought two .68's on both sides of V1 would be too much. What can i say....it worked and then some. Honestly, the amp has never been this good. The lows are far better, the tone now sounds similar to a really good JCM800. I have one part i have to choose a value for now and at that point it's going back into the cabinet. For good? I hope so, but i'm not stupid.

[Structo]

Congrats!
It's always nice to be able to get satisfaction with a tweak.
There are so many variables in tube amps, I mean after all, they are powered by light bulbs.

What I hate is when I get an amp sounding great with a particular set of tubes then have one or more go south. Then the hunt begins anew.

I think the biggest challenge I have had since I got back into tube amps is knowing which direction to go in when I have an issue.
Really it's a matter of experience and a smattering of good sense to go along with it.

The guys and gal here can be very helpful and it's hard not to ask too many questions. After All they had to learn all this stuff too, so there is a certain amount of learning that you (we all) have to do on your own.
Because once the training wheels are off, it is so cool to be able to voice and amp to your preferences and solve the myriads of problems that can plague these bits of archaic technology.

[iknowjohnny]

The guys and gal here can be very helpful and it's hard not to ask too many questions

don't i know it ! There are times i have so many but can't ask them all at the same time w/o coming off like i'm asking too much. So i often have threads going at several forums. Whats funny is that i rarely really get the perfect answer that makes a big improvement in my amp, but rather things are suggested that get my mind going in that direction. I discovered this very successful tweak due to one thing that jana said. She said in a hi gain amp you want to boost high end at the beginning so as to keep the low end from getting muddy in the gain stages, then cut excessive highs later on. I have since been trying to do that and this was the one thing that really worked. Other things helped, but putting another .68 over a larger resistor to get a bigger high end boost did the trick. Fortunately my amp for some reason is a bit dark to begin with, so once i did this i didn't have to cut highs later. sounds good as is. Thanks again to everyone's help.

[vibratoking]

Not even close. As they get bigger the amp is worthless to my ears. then again, A/B'ing a JCM 800 with mine made me thing JCM 800's are horrible. That may be one reason why ! (i've owned 3 and clipped pot caps and all that) This amp sounds far far better with the 32/32 and 22uf nodes than with 50uf or more. Not to mention the bass is still fuzzy sounding no matter how big the caps. It just sound less obvious due to the much thinner tone.

geez.....i just had me a moment. I have a 1.8k/.68uf on V1A and i just stuck a 2.7k/.68uf on V1B. I did this as a way to increase highs in hopefully a good way and hopefully tighten the lows. I never did it before because i thought two .68's on both sides of V1 would be too much. What can i say....it worked and then some. Honestly, the amp has never been this good. The lows are far better, the tone now sounds similar to a really good JCM800.

Ok, I am stumped. You just contradicted yourself the way i read it. Before the tweak the amp made JCMs sound horrible. After the tweak, it sounds like a good JCM? If I understand what you wrote, you have made your amp sound worse. Sorry, but the facts are the facts. Anyone reading this would have to completely disregard your conclusion, unless you clear it up.

[iknowjohnny]

I never said they sound horrible. I said IN COMPARISON to my amp it sounded horrible. Just a way of saying how much better i liked my amp even tho I LIKED 800's a lot in the past. I wouldn't have had 3 if i didn't. A good sounding 800 can sound killer. But of the 3 i had only one did. the others were good but nothing like that one.

Don't concern yourself with my semantics....it's not important. What is is that the amp now sounds the way i always wanted it too, or at least far closer than ever.

[iknowjohnny]

By the way Structo, how did you arrive at those voltages for the preamp and PI? Is that from personal preference for hi gain amps or are they numbers that a lot of people agree on or what? I just want to get an idea as to how people go about determining optimal node voltages. i do it the only way i know which is the old ear test. But i'm sure thats not the best way to say the least.

[martin manning]

Here is a copy of some notes I took several years ago.
This was offered to me by either Scott or Gil, sorry I didn't write that down.

Those are from HH, Nov 2007:

https://tubeamparchive.com/viewtopic.php?t=3779

...just to give credit where credit is due!

MPM

[Structo]

Here is a copy of some notes I took several years ago.
This was offered to me by either Scott or Gil, sorry I didn't write that down.

Those are from HH, Nov 2007:

https://tubeamparchive.com/viewtopic.php?t=3779

...just to give credit where credit is due!

MPM

Thanks Martin.
I found those notes at the Brown Note forum but it didn't mention the author.
Thanks goes to Henry from Redplate Amps for concise intructions for tweaking an amp.

[megemen]

hi all, where is this "OD2 CC output" really?
Br

[surfsup]

structo thanks for that repost of info. Its appreciated.

Would it be considered good practice to solder up all the stuff except cathode RCs, coupling caps, etc and use alligator clips with long leads to clip in different stuff before soldering?

would the long leads cause more of a problem or would it be better to just solder the components in? I'm thinking about doing this on the new build I'm working on as I'm using some unconventional rail caps (630v solen fast films)...thx

[Structo]

Yes I usually just tack solder different components into the circuit rather than use long test leads.
Been shocked too many times trying that.

It kind of depends whether it is an eyelet or turret board but just lightly solder the new part in so you can remove it easily until you find the right value.