70's vs. 80's power amp...

[wjdunham]

I've been experimenting with the power amp section in the last few weeks. I wound up chasing a problem I thought was the amp, but was in fact something in my rack causing a gravely noise but only at high volumes. Coincidentally at the end of our gigs we're usually pretty loud, I figured it was the amp/tubes having some bias recovery problems after running long and hot and late in a gig. Aiken's site has pretty good article on blocking distortion and I thought for sure I had stumbled onto something. Even though I found the problem wasn't in the amp, the whole investigation raised a few questions...

While cranking the sh*t out of the amp trying to find the crackle, on the clean channel I noticed that the low end was not very tight. I mean cranking, Pete Townshend style playing. With my Marshall, there's not hint of mud with this style of playing. I spent a lot of time tweaking the OD couplers and whatnot to get the OD nice and tight at medium volumes, but this is all power amp. After reading the Aiken stuff, I got to thinking why not try something closer to the Marshall values (which turn out to be more like the 70's amp values), 150K bias feed resistors, and .05 couplers from the PI. I didn't have a choke lying around, so I lowered the 300R 10W resistor down to 150R. Man, what a difference, much tighter low end without any loss in low end at lower volumes. I did swap the 300/150 in and out and it seemed to have a fairly pronounced effect, I didn't go back a put the original 220K/.1uF back in with just the 150R change to see what was really having the biggest effect. For reference, I set the bias on the TAD 6L6 at 40mA, 435V on the plates.

Most of the discussion around here seems to be about the preamp, and from the sound of most of the clips that are posted, not much driving the poweramp into clipping with all the associated side effects. What's the general wisdom here? Most puzzling to me is why did The Creator switch back to the blackface poweramp values, given the known issues - seems like it was common knowledge at the time that twins got "farty" when being overdriven. Because none of his customers were playing that style? Hard to believe. Based on my admittedly little experience here, seems like a #124 with the 70's values would is a pretty good combo and can handle a wider range of playing styles. I'd love to hear what people think here.
thanks
Bill

[bluesfendermanblues]

I've been experimenting with the power amp section in the last few weeks. I wound up chasing a problem I thought was the amp, but was in fact something in my rack causing a gravely noise but only at high volumes. Coincidentally at the end of our gigs we're usually pretty loud, I figured it was the amp/tubes having some bias recovery problems after running long and hot and late in a gig. Aiken's site has pretty good article on blocking distortion and I thought for sure I had stumbled onto something. Even though I found the problem wasn't in the amp, the whole investigation raised a few questions...

While cranking the sh*t out of the amp trying to find the crackle, on the clean channel I noticed that the low end was not very tight. I mean cranking, Pete Townshend style playing. With my Marshall, there's not hint of mud with this style of playing. I spent a lot of time tweaking the OD couplers and whatnot to get the OD nice and tight at medium volumes, but this is all power amp. After reading the Aiken stuff, I got to thinking why not try something closer to the Marshall values (which turn out to be more like the 70's amp values), 150K bias feed resistors, and .05 couplers from the PI. I didn't have a choke lying around, so I lowered the 300R 10W resistor down to 150R. Man, what a difference, much tighter low end without any loss in low end at lower volumes. I did swap the 300/150 in and out and it seemed to have a fairly pronounced effect, I didn't go back a put the original 220K/.1uF back in with just the 150R change to see what was really having the biggest effect. For reference, I set the bias on the TAD 6L6 at 40mA, 435V on the plates.

Most of the discussion around here seems to be about the preamp, and from the sound of most of the clips that are posted, not much driving the poweramp into clipping with all the associated side effects. What's the general wisdom here? Most puzzling to me is why did The Creator switch back to the blackface poweramp values, given the known issues - seems like it was common knowledge at the time that twins got "farty" when being overdriven. Because none of his customers were playing that style? Hard to believe. Based on my admittedly little experience here, seems like a #124 with the 70's values would is a pretty good combo and can handle a wider range of playing styles. I'd love to hear what people think here.
thanks
Bill

What you are describing is 'sag' in the poweramp, which is the effect blues guitarist, who play fender tweed style amps, chase often by selecting the right tube rectifier (5U4, 5Y3, GZ34 etc) for their preferred volume level.
Your amp has the tube rectifier subsittute known by some as the 'Robben Ford mod' (300R 10W resistor, now 150R) instead of a choke. And as you describe changing this resistor from 300 ohm to 150 has an impact on the feel of the amp at higher volumes.

I like my amps with the master between 1 and 5 after which the amp will produce distortion in the power amp and not be really clean in clean channel. If you want power overdrive buy a marshall, fender super og bassman.

[Structo]

Is your amp a 100w or 50w?

I'm not sure how much it will change but did you check your plate voltages after lowering the choke resistor value?

Something else you could try is to increase the filtering on the power tubes.

At one point I piggy backed a couple 100uF caps on the existing 100uF caps.
50uF caps might be a better choice there.
Seemed to tighten up the bottom end in tone and feel.

Do yo notice it is flabby only in OD mode or in both OD and Clean?

As you mentioned you can try lower value coupling caps off of the PI.

I have a 50w 2 x 6L6 amp and the few times I turned it way up I didn't care for the tone I got.
I really don't think these amps are designed to be totally cranked.
If your amp is a 100w, I would think it would get you loud enough to gig way before you are past half on the gain and volume.

[wjdunham]

I guess I could have summed it up in one sentence, in stead of writing a book Really the question should have been: is this more likely due to sag from the power supply and my RF mod resistor, or "sag" from the overdriven grids pulling the bias supply down, and the 220K/.1uF having too slow a recovery causing a "mushy" feel?
It's a 50W amp, and I only have 100uF in the first filter stage, per most 50W schematics (vs. 220uF for the 100W). I'm not hearing any ghosting or other ugly distortion that I understand comes from inadequate power supply filtering.
I'm talking about power amp at 6 or 7 so the clean channel is starting to break up, but mostly in a good way to my ears. Maybe Marshall is a bad comparison here, but any other schematic I've looked at has at least half the time constant feeding the power tube grids, including early D style amps. Why?

[dogears]

In my experience, the choke replacement resistor is the primary cause of mush.

[angelodp]

Ok, not to derail, but have any in the Dlite camp jettisoned their choke resistors in favor of a real choke.

[Structo]

I did.
But I can't really recall the difference it made since I had done some other things as well.
If it is doing what I think it is doing, I would say the amp is a bit smoother to my ears.
I also moved where the standby switch is in the circuit.
The first power supply caps now are energised when the power is turned on.

This is the one I bought.

http://www.tubesandmore.com/scripts/fox ... em=P-C194B

[wjdunham]

Good question Ange, I'd like to hear if anyone's done that as well. The stock resistor with the D'Lite kit is 500R, which I never used. I used 300R to start with, but I always felt the amp was a bit mushy. Sounded absolutely awesome at practice and at home, but when pushed at gigs, it seemed to loose something.
I'm going to play with 150R for a while, and see how I like that. On my #124, I'm going to change back to the 220K/.1uF, stick a choke in there and see how it compares. I've never played an amp with an "induced" sag, my Prosonic which I used for years, has a switchable tube or solid state rectifier, and I always left the switch on solid state, now I know why
Bill

[angelodp]

I know my RedPlate has that " thing " going on all levels. We know how much Henry has give to all of us as and is one of resident "creators" here, perhaps H you can shed a little light on this matter without giving away too many proprietary ideas.

[talbany]

The main difference with resistors vs chokes from a functioning standpoint (all filtering aside) is how each manipulates voltage/current.. Under extreme conditions resistors experience larger voltage swings/drops than chokes which have very little DC resistance.. At some point the resistor saturates the voltage drop is too great and you starve the PS of the proper current.. The effects felt are the ones requiring the most power to generate mainly lower frequency's.. 100 watt don't feel the effects as quickly since 2 tubes divide the load.. This will be real obvious though in a D-Lite especially w/6V's..
The .1/220k's are a slightly different effect.. This will attack the bias supply so the symptoms will be more like cold bias.. loss of sustain a gravely breakup power loss etc etc..
At very high volumes you can be experiencing both especially if it's a 50w..
If you play at very high volumes for long periods (w/50 watter) I would lose the resistor altogether since power supply's sag naturally when pushed even w/ choke and use the resistor if your a bedroom player..If you suspect the bias going cold you can replace the 220k's w 100k. since this doesn't effect tone but helps speed up recovery time.. JMO..

Tony

[wjdunham]

I finally got my choke and had a chance to swap it in last night. What surprised me was not that it was different, but how much of a difference there was. The amp is only 50W, and from reading Tony's previous post, it sounds like this is going to have a bigger effect than on a 100W. From reading lots of other posts in other threads, I would have expected a much more subtle effect. Granted, I was not playing in the typical more jazz oriented style of a most of the clips posted here.

I'll be able to confirm why my amps sounded great at home or even in our practice facility, but at gigs (loud) always seemed to loose something and get muddy on certain parts. The leads always sounded great, but other parts weren't quite so tight. I have a gig tonight and I'm really interested to see how it goes. I also changed the bias feed to 180K/.05uF, hopefully it will sound as good at the end of the gig as at the beginning this time. Always learning here... If you have a 50W DLite and haven't yet tried a choke, I'd recommend that you do, it's one of the simpler mods to try out. I'll post a follow up after the gig.
Bill

[greiswig]

I finally got my choke and had a chance to swap it in last night. What surprised me was not that it was different, but how much of a difference there was.

I look forward to hearing more, and to seeing how you added it. Also, did you play with the dropping string afterward? If not, I would expect the voltages on all segments to have risen somewhat, which could be at least part of the reason it sounds different.

[phsyconoodler]

My last D style build uses the 150 R resistor and it sounds killer good when the amp is dimed.I played it live through a 4-12 cabinet loaded with Warehouse speakers G12-65's.Wow! was all I could say about the sound.
It does not do the tight chugga-chugga of a Boogie or a Marshall,but what I heard was killer awesome.
It really depends on what kind of music you play.
Mine was a 50 watt version,NOS paper in oil caps,Hammond Fender replacement OT and a 272JX PT.
425v on the plates.I don't care for higher plate voltages in these amps.But again,that's me.YMMV.
I don't like Orange drops and prefer the clearer sound of the paper in oils.
I like the choke too and will try one in my next build.
A tight bottom and fast reaction makes some people really happy and other's don't like it.It's all relative.

[BobW]

Tony, I agree basically with what you're saying but a dropping resistor limits current, and the voltage drop is a result of the current limiting.
A choke does have less resistance but it's the created magnetic field that supports a resist in the voltage change on the load side whenever a curent flows. As the load increases, a differential voltage is attempted across the coil, but the magnetic field, initially setup, will partially collapse causing a reverse voltage on the load side (better regulation than a resistor). You can't really compare resistors and coils without considering the regulating effects of the coil. (tomato , tomahto)

[Structo]

I thought a inductor opposes a change in current, which is why it is used in a power supply, keeps the current from dragging down too fast, the result being that it helps the filter caps maintain a current source for the amp.
Is that wrong?

Bill,
Which choke did you install?
I am very interested in your bias splitter and cap results.

I installed a choke in my D'lite several months ago and liked the results.
Although I am no currently gigging so I don't have a lot of opportunities to crank it.

But I have been struggling a bit with the bass response of this amp for quite a while. Just not very defined and mushy.
That may be desirable in these types of amps, I don't know but for hard rock, I think it needs to be a bit tighter in the low end.
My amp uses 6L6's and I use a open 2 x 12 with two EVM 12L Classics for a 8 ohm load.

What you said about the time constant and quicker recovery makes complete sense.
I still have the 220K/ .1uF caps
Most Marshalls use .022uF couplers there and I know the JCM uses 150K splitters there.

Hmmmm, I may have to try some .05uF caps and 150K.
Probably try the .05uF's first with the 220K.

Please post your caps and resistor change results.