New Build Idea: 6G4 Based, Cathode Biased

[Chuck H]

Sounds like your on it. I'm surprised to not read the usual "This amp rules!" that most guy's post at first working start up. Perhaps your immune to that sort of thing. But I hope it doesn't mean that you don't really like it. Anyway...

Tweaking will bring it all in. A neat trick for the bright switch is to leave the stock 47p cap in place and have the switch add another 100p in parallel. You'll hear that fo shizzle and it'll help you get those snappy cleans.

I think the PI voltages are fine without being raised. The preamp will probably benefit from getting the voltage down to spec. It will be "brown"er as they say. Less stringent and not as spikey on attack. It'll also soften the bass a little. Just stick a bigger resistor in there. Your amp doesn't have all the other tubes pulling current through that 56k resistor so there's less voltage drop. Reducing the voltage on the pre and adding more bright switch should be a nice combo.

Power tubes are running pretty hot for idle. May actually be OK for cathode bias but you will probably get better clean volume and tighter bottom end with a cooler bias.

Something I do when I'm designing with split plate loads is temporarily wire a 100k pot in as the plate load taking the signal from the wiper. Then I can choose what I want, remove the pot without turning it and measure each leg to wiper. Then choose fixed resistor values that are close.

Kudos

[Rocketrob]

Well Chuck, after plaing through tube amps for about 40 years I guess I don't get too excited about it. This is only my fourth scratch build though, so it is a big deal that it is working. I just know I can get it to sound better. This is a good sounding amp, no doubt about it. I really appreciate all the help you and others have given here. You guys are really helping to get me up that steep learning curve. RD.

[Rocketrob]

Update: After playing this amp for a few days it's clear that there is too much gain going on. So I tried a couple of changes; cathode resistor on output tubes is now 330 ohm, split load plate resistors on v1b now 68/27. Still too much, so I tried some lower gain tubes for v1 and v2. 12ay7 in v2 sounds pretty good, but I need to bias it correctly. I was thinking maybe 820/33k for the cathode and tail resistors. Questions: Are those values workable, and how should I adjust the NFB circuit? Thanks, RD.

[Chuck H]

Update: After playing this amp for a few days it's clear that there is too much gain going on.

You don't have any preamp overdrive because of the design. You may be overdriving the PI some, but not much because of the padded signal from V1B. So much of any distortion you hear is likely the power tubes. If the bias point is set right then you will get your maximum clean headroom. Doesn't much matter whether the volume knob is on three or eleven, what you hear is what you get. Reducing gain in this circuit unfortunately isn't going to give more clean headroom. It'll just move the point of clipping to higher volume knob setting and limit how much overdrive is on tap. So you could just turn the amp down to where it's not clipping. No matter what mods you do, that's all the clean headroom your going to get anyway.

That said... I have built single preamp tube plus PI tube single channel BFish type amps. Long ago before I was using the split plate load. I remember that I had to pad the signal from V1B quite a lot otherwise the amp seemed too gainy (sound familiar) I think I ended up with a 15k load resistor on one amp and a 22k on another right behind the PI input coupling cap. Also, in preamps where I generate a lot of voltage gain I've used the split load feeding the PI with 18k/82k, with the smaller 18k resistor as the load. So that's quite a bit of attenuation.

With this circuit the only places where your likely to get overdrive is the power tubes and PI in that order. If you trim the signal going to the PI more this could do what you want. Easy enough to try it. You could also use a pot temorarily as the split plate load to dial in the ratio and then replace it with fixed resistors.

There is a point of diminishing returns with the NFB loop. Too much NFB and the tone becomes sterile and weak. Too little and it can be brash and unruly. Your guestimate of 10k to 22k off the 8 ohm tap with a 1.5k dropper is actually very conservative. 22k to 33k for the feedback resistor with your 1.5k dropper would make a more typical ratio seen in guitar amps. Also, the dropping resisto in the NFB loop sets the impedance of the circuit. This has a profound effect on the frequency knee of the cap in the presence control. The .1 cap your using now gives a typical presence when using a 4.7k dropping resistor. But with your 1.5k dropper that .1 cap is only "presence"ing the very very high frequencies. A .33 cap used in your presence circuit will give a more typical effect.

Nothing wrong with lower gain tubes. I actually like the sound of 12ay7's and think they sound different than the typical 12ax7, not just lower gain. So use 'em if you like 'em. But... Your not getting any preamp overdrive anyway. So all a 12ay7 in V1 will do is reduce drive to the PI. You can do that by adjusting your split load ratio. All the 12ay7 is doing in V2 is reducing drive to the power tubes. You should be able to adjust the split load and get a good balance of OD between the PI and power tubes without the need for different type preamp tubes.

[Rocketrob]

Chuck, thanks for the thoughtful reply. I'll keep tweaking to find the best balance for headroom/distortion. I'm really surprised at how easily the power tubes are breaking up. I've had many old fenders, and still have a couple, that don't break up half as much. Is it because there is nothing between the pre amp and output? I thought the tone stack would provide enough signal loss, but I guess not. It's like I have a boost pedal in front of the amp all the time. At very low volume the tone is pretty nice and warm, but at band volume it's Billy Gibbons time . Maybe a bigger OT would help? I'm using a smallish 30w OT now, and the chassis already has the holes for a larger two hole mount OT. BTW, I'm using a pretty efficient EV SRO speaker. More tweaking in order, RD.

[Chuck H]

First off, bias the power tubes so they idle at about 80% of max rated dissapation in cathode bias. This will help keep the power tubes from breaking up too early. Then...

It's worth noting that the BF (as in AA or AB763) Fender "normal" channel is attenuated greater than 50% by peripheral circuitry that performs voltage division prior to the PI. Early "tweed" type Fender amps have both less loss in the tone stack AND less attennuation due to peripheral circuits. Many tweed Fenders do start breaking up at about two on the volume knob with dual coil humbucker type pickups. And I too was surprised once upon a time with snigle channel "clones" of BF Fender type amps I built that needed a load resistor of 15k to 22k to present the same final gain as a BF "normal" channel. Just wire in a 100k pot where the split plate load is and put the amp through some paces as you dial in the amount of drive by adjusting the pot. When you find a good balance pull the pot and measure it to establish your fixed resistor values for the split plate load.

once you have the bias and gain level dealt with you can turn your attention to tweaking the preamp with bright caps, bypass and coupling cap values and tonestack values.

Also, as you noted, if your OT is cheesy and you want more bottom end headroom you may want to upgrade.

[Rocketrob]

Well, I finally got around to working on this amp after the holidays. I’ve made a bunch of changes, so new schematic and layout drawings are posted. It’s been to a few local jams and the results have been pretty good; although this may not be the final version, it’s working well enough that I thought I would post it here, if anyone is interested.

Some of the changes: All of the coupling caps have been changed to different values, as well as the cap on the treble pot, and the cap on the presence pot. I kept the early Brownface tone stack and volume control. The PI now has Tweed type values for the cathode and tail resistors. Added 1K5 resistors to pin 5 on the output tubes. Rectifier tube is now 5AR4. Plate v = 430, screen v =425, cathode v = 34, so the output tubes are “seeing” a little less than 400 volts, and dissipating around 25 watts each. This seems to work OK with the 30 W OT I’m using. The tone at this point is some where between the 5E3 head I built and my old Vibrolux Reverb; more muscle and headroom than the 5E3, more mids and bark than the VR.

I want to thank everyone here that I am learning from, especially Chuck H, who pretty much guided me through this one. RD.

p.s. updated voltage chart on the way soon.

DCA_Type3_Schem_REV4.pdf
DCA_Type3_LAYOUT_REV4.pdf

[Chuck H]

A couple of things I notice...

Your mid dump cap (.01 in the tone stack) is only affecting a very narrow band of mids and only a small amount. If it gives you the tone you like then thats great. I usually prefere a deeper cut down into the lower mids (like a .022 cap in this position) as too much low mids seems to muddy the tone. This is a very subjective tweak and tastes will vary.

Your presence circuit with the .047 cap is only effecting frequencies above the audible range of the speaker. Amp function is certainly affected, probably audible, but with your lower value dropper in the NFB circuit (which affects the impedance of the circuit) you would need a cap around .22uf (not .022) to affect the same frequencies as a typical presence control. In fact I wonder if you even have much noticable effect when adjusting the presence as it is.

At 25W dissapation you have the tubes biased in class A. Nothing wrong with that as I see it if it gives you what you want but it may be worth trying a cooler bias to increase available power.

[Rocketrob]

Thanks for the suggestions Chuck.

Your mid dump cap (.01 in the tone stack) is only affecting a very narrow band of mids and only a small amount. If it gives you the tone you like then thats great. I usually prefere a deeper cut down into the lower mids (like a .022 cap in this position) as too much low mids seems to muddy the tone. This is a very subjective tweak and tastes will vary.

I'll try this to see how it sounds. There are a lot of mids now that I changed the treble cap to 500pF (from 250pF), and the cap at the PI input to .022 (from .001).

Your presence circuit with the .047 cap is only effecting frequencies above the audible range of the speaker. Amp function is certainly affected, probably audible, but with your lower value dropper in the NFB circuit (which affects the impedance of the circuit) you would need a cap around .22uf (not .022) to affect the same frequencies as a typical presence control. In fact I wonder if you even have much noticable effect when adjusting the presence as it is.

You are correct that the Presence control is very subtle. It seems I should go back to a .1 cap and a 4K7 resistor on the Presence pot and keep the 33K feedbadk resistor. I need to study some more NFB resources; I still don't understand how to tweak this circuit in spite of all the info you've given me.

At 25W dissapation you have the tubes biased in class A. Nothing wrong with that as I see it if it gives you what you want but it may be worth trying a cooler bias to increase available power.

Maximum plate dissipation for these tubes (SED 6L6GC) is 32 watts, 80% of that is 25.6, so with cathode bias I thought this would be OK. I had a 330 ohm cathode resistor in there foe a while but took it out. Maybe I'll try it again now that I've tweaked the sound of the amp abit.
Thanks, RD.

[Chuck H]

If max diss on that tube is 32 watts I take back my comments regarding bias. FWIW a typical 6L6 is 20 watts. A 6L6GC is 30 watts. What you have now is fine.

You could just do the NFB and presence circuit rote as described. Change the presence cap to .22uf. This should give you about the right amount of feedback for that circuit and a functioning presence control.

Changing to a 4.7k shunt would mean changing the feedback resistor to 100k, changing the pot to 25k and changing the cap to the typical .1uf. Changing just the cap in the existing circuit to .22uf seems simpler. A 100V or 200V rated cap is fine for this circuit so it doesn't have to be expensive or overly large.

You will notice less mud in the tone by changing the mid shunt cap to .022uf. You'll still have plenty of upper mids with the 500pf treble cap. Low mids always seem to take the clarity right out of an amp so it's worth experimenting.

There's no need to have a PI coupling cap greater than .0022uf. Anything bigger than that is letting in lows that the speaker can't even use and only serves to limit available clean watts because lows will make the power tubes overdrive sooner than higher frequencies.

[Rocketrob]

Update: I changed the .047 cap on the presence control to a .22 cap. Much better. The presence control actually does some thing now The amp sounds good with the NFB switched off too. I like having the option of the two different modes. Still have the .01 cap on the bass pot; I'll try the .02 cap there as soon as I get to it. On the PI input I see where the Tweed Baseman uses a .022 cap, so that's why I put it there when I changed the cathode and tail resistors to TB specs. I'll try the .0022 to see if I like it.

See attachment for current voltage chart. I changed the 80K dropping resistor (two R in series) to 82K standard value. This dropped V1 about 10V. I might change the 470 dropping resistor to bring the V1 and V2 up a little; I need to play the amp some more to decide if it needs a little more "clean".

DCA_Type3_Voltages_1-17-2011.pdf

[Chuck H]

Great. Your down to refinements... Cool. Trying different things now is just gravy on the top.. Yumm.

There is some debate about balancing output from the PI. I notice a pretty big Vp difference between the two halves. You don't need to change the circuit really. Just try different tubes in that socket to see which one sounds best. Balanced or unbalanced, whatever sounds best is right.

Yup, nothing wrong with the NFB or presence circuit. Just needed a different bypass cap value for the presence to accomodate the the impedance of the circuit.

[Rocketrob]

Regarding the PI balance, I have read a few threads were the designer just put 100K plate resistors on both sides of the PI, and just let it be. Some thing about more complex harmonics generated that way. Isn't the 82K + 100K setup supposed to balance the PI? Also, in this amp Vp at V2a (82K) is higher than Vp at V2b (100K), in this case by about 20 volts. I thought that it would be the other way around. Maybe it's my thinking that's backwards. Maybe just the difference between the two triodes? RD.

[Chuck H]

I go to some trouble by using a split plate load on the input side of an LTP to get the voltages close and still balance gain. But I've used highly unbalanced preamp tubes at times and it's always a crap shoot what will "sound" best with any amp so it's hardly worth the trouble. I just do it because it appeals to me somehow having close Vp on both plates.

The 82k load on the input side of the PI drops fewer volts but the higher 100k value on the cathode coupled side has higher gain. The reason is that these resistors do double duty as a load because they are coupled to the power supply and the bypass caps in the power supply do double duty in shunting any AC to ground. So the higher the load from ground, the higher the gain. Thus the 100k value increases gain for that side. The actual tube characteristics will vary greatly and different tubes that have dissimilar characteristics will show different voltage readings too.

I once built an amp that sounded real fine with a particular 12AX7 in the PI socket. All my other tubes sounded similar to each other and not as good. I reasoned that the one tube must be imbalanced because in general 12AX7's are reasonably well balanced so it was likely that IT was the odd one and not the dozen others. I changed some values in that amps PI to give the same results using any of my other tubes as it had with the un balanced tube because I didn't want to hunt for a special tube whenever I had to change the PI tube. Made sense to me. Ordinarily though the difference is subtle. But it's still worth plugging in different tubes to find one that sounds best.

[Rocketrob]

REV 5 MODS

Hello again all. After playing out with this head several times I decided that somechanges were needed. While the amp sounded good, there were problems with low notes washing out and a bit too much compression. Here's what I did; the first filter cap was changed from a 40uF/500 to 2x100uF/450 in series. Then I changed the filter caps for V1 and V2 from 33uF/500 to 16uF/475 for each tube. Finally, the OT was changed from a Allen TO30D to a Heyboer/Weber HY404248. Now there is no problem with low notes, as the power supply seems to be able to keep up with demand much better. Changing to less capacitance in the filtering for V1 and V2 seem to have helped with dynamics and touch, and the bigger OT gives better balance and some added "beefiness" and presence to the sound. I'm liking this setup much better; although there is one problem: it's freakin' LOUD. Even with cathode bias it's really in your face if you're not careful. I think I'll try 5881s with a 5U4GB for a little less output for smaller rooms.
I have included the latest drawings if anyone is interested. RD.