6L6 Cathode biased advice / ideas

[studiodunn]

Hey all! It's been a minute since I've been around or done any tinkering, but I was jonesing to fire up the iron and got myself back into some builds.

I found a deal on some local amps and picked them up this week. I was really just looking to play with some circuits I haven't built and ease into the next ground up build. The two amps I got were a Trinity SIII and a Mahalo DR40.

The trinity is really well done and I don't know if it was a kit or from Trinity, but It does what I want an 18watter to do. I'm not going to mess with it. I think it will make a great grab and go when my new cab gets here from sourmash.

The second amp is were things get interesting. It's a Mahalo DR40 with a DR20 faceplate. From what I found, this amp was sold as an EL34 amp, but this one has 6L6's and is appropriately set up for them. I would start with I do not have much experience with bigger cathode biased amps, so what I'm about to describe may be normal.

Classictone iron - 40-18006, 40-18004, 40-18003

After firing the amp up I instantly noticed the following:
-an ugly decay on the notes
-woofy flabby bass
-extremely bright amp that reacted heavily to minor tone stack adjustments
- a lot of swirl...too much swirl

I knew the decay was an actual issue, so I started there. After much research I went straight to the bias section. The amp had 22uf/300R in the cathode bias and was putting 456v on the plates. Cathode-to-plate voltage was 414v and the amp was sitting at 57mA at idle. This looked very cool. From my reading the larger bypass cap would help tighten the bass and potentially help with the decaying notes. I put in a 220uf / 250R and Decay was gone and the bass was indeed better...not great but better. The amp now had 66mA at at idle.

At this point I just play the amp for a couple days to just get use to it before I start changing stuff. I'm using different guitars, different cabs and working the controls to just find something I like.

What I like:
- this amp has some jangle to it and can produce some killer "voxish chime" but is always at the edge of harsh
- With the gain on 2 to 3 there are great edge of break up tones
- with the gain up around 6 the single note sustain is really nice and the amp has the faintest swirl going on
- amp reacts pretty good to guitar volume

What I don't like:
- gain need to be on .5 to get any real clean tones and just sounds thin without pushing the volume to piss off band mates levels.
- everything besides single note sustain and jangely edge of break up tones just suck to my ears. Bass is flabby and weak, the mids are either too little or too much and never sit right in the mix. Once the gain comes on chords are a mess and all the lows and mids are mushy and hard to distinguish. Notes on the E and A strings are nowhere to be found in chords.

My first thought is, I don't like Cathode Biased 6L6's and will convert this amp to Fixed Bias. But before I do such a thing I want to play with the design as is and try to find something that works, so lets here those ideas and experiences.

IMG_1519.jpg

[stephenl]

If you want to try something a little different, try a combination of Fixed/Cathode.
Size the cathode resistor for 15-20 % of what you would typically use for a common cathode resistor and generate the rest of the bias with the fixed bias circuit.
Do not bypass the common cathode resistor - it will add a little feedback. If you do this, reduce or axe the global feedback if used.
I've used this on the last few builds and really like it.

[xtian]

Cathode bias is not the cause of your complaints--changing to fixed bias won't solve any of your issues.

You report 66mA current--is that for one tube or the pair? I hope it's for one tube only, because you need twice that current for the pair.

The lead dress looks ok. "woofy flabby bass" can be too little filtering, or too much bass passing thru the circuit. Several ways to attack this. If we had a schematic, we could make suggestions.

"extremely bright amp" -- could be bright cap on Volume control. Looks like there's a master volume? Evaluate tone with MV wide open.

Don't know what you mean by swirl.

Again, a schematic is worth a billion words.

[Reeltarded]

fat SM bright cap.. That is your overt brightness with VERY LOW gain rotation. Is it 500p? Try 100p. If it is 250p still too large for low gain rotation in that circuit, look at peaker over the bright channel mixer. uhh.. what are mixer values? Looks unequal. My glasses are not on.

Aha! So, this is a psuedo 800. That mixer/divider is making too much gain for your taste. Try 470k/470k with 470p over signal side. Change bright cap over gain to 100-150p. The gain will light up later in rotation and the bright peak will be less just from lowering gain slightly at divider before V2 since the gain needs higher rotation to take up headroom slack you add.

This might also alleviate the gacking lowend.

[studiodunn]

Cathode bias is not the cause of your complaints--changing to fixed bias won't solve any of your issues.

You report 66mA current--is that for one tube or the pair? I hope it's for one tube only, because you need twice that current for the pair.

66mA is Per Tube

Again, a schematic is worth a billion words.

This is a pretty straight forward 2204 preamp. I'll try an annotate the image or finish the schematic, but other than the Cut control it's all pretty generic circuit from what I can tell.

Screenshot 2023-03-17 at 11.30.31 AM.png

I've built and modded many amps that closely resemble this amps circuit and have never experienced such a muddy, undefined tone and feel. Thats why I pointed to the cathode vs fixed idea.

[pdf64]

The power amp schematic may be more relevant here.

[studiodunn]

The amp is pretty straight away an 800 clone. Even my voltages are close to this schematic. I’ve just plugged my values in to the below schematic.

Screenshot 2023-03-17 at 5.01.25 PM.png

[10thTx]

I would also be surprised if cathode bias is the cause of your problems. I've had numerous cathode biased 6L6 and 5881 amps and they did not have the characteristics you are describing. In fact, I think someone would be hard pressed to do a blind fold A/B listening (using different guitars and A/B testing each one thru the amp) and tell which is cathode or fixed biased with any consistency at all.

I've greatly enjoyed the cathode bias 6L6 amps I've had and currently have.

With respect, 10thtx

[xtian]

I love the 2203/2204 platform, but hate the bright cap and treble peaker--these only sound good when you have the Volume control up towards max. Anything less, and the amp sounds brittle and harsh, as you describe. So first, clip the bright cap and see how it sounds. Next, remove the treble peaker and repeat.

[Reeltarded]

7E0C48E4-E829-441A-A1F3-E127F7AA6020.jpeg

I don't like the 100k ground leg of the divider. If the circuit needs more attenuation I do that on the gain pot with a larger value.. 220k probably.

I like the amp. Looks good.

[TUBEDUDE]

Just for s#:ts and grins, replace that 220uF final cathode cap. I had a funky one gnarl up the bass once.

[bepone]

add litle bit carbon films inside, and silver mica change to random cheapest ceramic cap, too much metal films for marshall type of amp..no wonder that is harsh

[studiodunn]

Thanks for all the suggestions!

The amp is getting better and I would say into a "tunable" state as opposed to I can't use this.

It's interesting how the amp clearly has that 2204 thing going, but has allot of chime and this really nice harmonic bloom. I guess the combination of Cathode biased and no NFB are what makes this amp unique to my ears.

[TUBEDUDE]

That's a time honored recipe for sure. Negative feedback is evil!