Tweaking an amp

[wicker]

Hi all good people
I'm nowdays working on a clean only, reverb amp. I know that there are lot of people here that know how to tweak amp to sound good, the amp is kind of mix of fender, dumble and traynor (mosty those 2 first now). I'm wanting to get that smooth, rounded a bit (but not too much) sweet sound clean we all know (Fender guy), here's the schematic I've got now:
http://imageshack.us/photo/my-images/24/schemat1c.png/
The voltages are a little too low imho - ~170V on first triode, but I suffer to a lack of proper resistor to change it (damn).
Any suggestions ? I'm very intrested what values do you suggest in NFL, also, I really feel that in this configuration, presence pot does not affect on a tone that much (nearly nothing)

[David Root]

NFB ratio of resistors is 10:1. If you are taking the NFB off the 4 ohm tap that sounds about right, but your schematic seems to indicate it coming off the 16 ohm tap, way too much NFB voltage if that's the case.

If it were my build I would be using a 2K2 cathode resistor on V1B, not 4K7, with the 150K plate as that will keep the Q point on the load line more or less central, Fender and Dumble style. It will also boost the DC current some, you are a bit light there I think with the 4K7 cathode.

Why drop half the gain going into the 12AT7 reverb driver, with that 100k/220K voltage divider, when you are putting a huge amount of gain on the reverb tank output?

The following, to me, come more under a matter of taste, tweaking, if you like:

Why 100k grid leaks on the power tubes? I could maybe see it if they were 6550s but they're 6L6GCs, you're trading off output for stability more than you need to in a guitar amp.

The reverb recovery tube is paralleled, and with the 150 K plate you are going to get huge gain there, why not remove the voltage divider into the reverb driver and lower the reverb recovery gain accordingly, say 82K plate with 1K2 cathode, or are you using some unusual type of reverb tank?

I might put a 5K trimpot on the PI plates B+, would give you more control over the AC balance of the PI outputs.

Oh, and I agree that 170V is a little bit low on V1, I would want 180 to 190 there. That would also help the current on V1B a bit too.

[wicker]

NFB ratio of resistors is 10:1. If you are taking the NFB off the 4 ohm tap that sounds about right, but your schematic seems to indicate it coming off the 16 ohm tap, way too much NFB voltage if that's the case.

It's going from 4 ohm, just my mistake on schematic (sorry), but I have changed resistor R9 (near presence) from 470 to 4k7 as VacuumVoodoo sugested me, so now I'm supposed to change R41 and R42.

If it were my build I would be using a 2K2 cathode resistor on V1B, not 4K7, with the 150K plate as that will keep the Q point on the load line more or less central, Fender and Dumble style. It will also boost the DC current some, you are a bit light there I think with the 4K7 cathode.

I will try this as soon as possible, for now on, I've got only 1k5 and 2k7 resistors from this range.

Why drop half the gain going into the 12AT7 reverb driver, with that 100k/220K voltage divider, when you are putting a huge amount of gain on the reverb tank output?

Due to EQ behind 2nd stage it was just to much for me on input, yes I've got dwell, but I wanted it to be more safe, my philosophy (maybe not correct) was - "If I were a reverb tank (not mentiong how stupid that souns), I wouldn't like to get so much on input", however, since I've added local NF which lowered the gain of 2nd stage, I will probably change this divider a bit.

The following, to me, come more under a matter of taste, tweaking, if you like:

Why 100k grid leaks on the power tubes? I could maybe see it if they were 6550s but they're 6L6GCs, you're trading off output for stability more than you need to in a guitar amp.

What do you suggest ? standard 220k ?

The reverb recovery tube is paralleled, and with the 150 K plate you are going to get huge gain there, why not remove the voltage divider into the reverb driver and lower the reverb recovery gain accordingly, say 82K plate with 1K2 cathode, or are you using some unusual type of reverb tank?

It's standard Fender tank you can meet on Reverbs, as soon as I'll have 82k 2W resistor I will install it.

I might put a 5K trimpot on the PI plates B+, would give you more control over the AC balance of the PI outputs.

I'll consider that.

Oh, and I agree that 170V is a little bit low on V1, I would want 180 to 190 there. That would also help the current on V1B a bit too.

Small update, found better value, and now got 246 on V1 and 275 V1B, too high ? Or maybe change Rk on first stage ? Now it's sounds better to me, but its too late (11 pm here) to check it better.

Really thanks for your feedback.

[David Root]

Now that you have changed the supply voltage so dramatically, my recommendations for V1B plate & cathode will also change. Are 246 and 275 now the plate voltages on V1 A & B? If so, they're too high, need to be at least 50V less, I would go for 60V less myself.

If you can get B+4 to 300V, then the reverb recovery tube in parallel, with 82K plate and 1K cathode, should give about 190V plate with 1.5 mA, and is fairly well centered, not perfect but OK, there will be a little 2nd harmonic which is nice.

[wicker]

Now that you have changed the supply voltage so dramatically, my recommendations for V1B plate & cathode will also change. Are 246 and 275 now the plate voltages on V1 A & B? If so, they're too high.

Sir, yes sir, more, I've got ~260V on PI plates and 116V! on cathode what is not very healthy, since datasheets says that max cathode to heater voltage is 90V. It happened, cos I've changed 22k ressistor i PSU to 10k (cos ~200V on PI previous seemed to me to low). I'll give some more info tomorrow.
B+4 is ca. 360V, I'm goind to change this 2k2 res.

[David Root]

I wouldn't worry about 116V on the PI cathode, Fender ran the 12AT7 PI cathode in some Blackface amps above 130V, look at the old schematics on Schematic Heaven. OTOH tubes were better made in the '60s than they are now.

Maybe try 2K7 or 3K3 on that dropping resistor if you have one or other of them. That should drop that 360V B+ some. Aim for 300V on V1A & B, don't worry about the reverb recovery tube because that 150K plate resistor should come down at least to approx half that value anyway.

When you run a dual triode in parallel, you are halving the effective plate resistance. This should be reflected in the plate resistor and cathode resistor values to keep gain at the same level as a single triode and maintain the Q point on the load line, more or less.

[David Root]

Here's what you should get with the reverb recovery tube and V1A & B if you can get the B+ to 300V on them. V1B will be a little different as I did not account for the internal NFB.

[wicker]

Ok, found 82k res, now I've got 214V on rev recovery, but I feel that there's not as much reverb as I would like to have, I mean, when I set it max i have enought, but I would like to have a little bit more just in case. More, as I said, I'll change the divider 220k/100k. I have changed NFL resistor to 47k + swichable 10k, however due to noise that comes when you switch it, I think I'm going to change it a bit, one option is to add a cap in series witch resistors, or switchable cap across 10k resistor.

But I must say that I like how does it sound now, I've puggled the Strat today (which sound completly different than my Tele).

I'll lower the voltages a bit cos I've got 400V 22uF caps and I'm a little afraid of 365V on B+3 (or change cap !).
Also, this presence does suck still, I'll have to do something that will make it more efficient.
Also this Mid boost does not sound that good as i thought.

Here's actual schem:
http://imageshack.us/photo/my-images/15/schemat1.png/

[David Root]

Don't worry about 365 V on a 400V cap at this point, you're still too high on V1A & B plates. 190 and 210 or so will improve tone.

You have only about 3.7mA coming up thru the B+4 node, so I would try a considerably higher dropping resistor to V1A &B, but at the same time take the B+ for the reverb recovery stage directly off the PI node with a smaller dropping resistor to the reverb recovery stage. That should work a bit better I think. Also I would change the V1B bypass cap to 2k2, that will give you a more centered Q point on the load line.

You MB switch is only changing the mids ground reference from 10K to 22K, that's not enough difference to make it work well. I would try cutting the connection to the 22K pot and putting a 4M7 resistor on the MB switch so that you are switching the mid pot ground reference from 10K to 4M7. That will make a difference.

[wicker]

Thanks for response, give me 2 - 3 days to deal with amp, cos I've got a lot other work to do now, hope to go tomorrow to local electronics shop and get all I need.

[wicker]

Btw. what do you suggest in V1A cathode ? I'll try to tweak the amp this evening.

[David Root]

V1A cathode is fine at 1K5. That follows the
"rule of 66" which is a rule of thumb that will keep the DC operating point pretty much in the center of the loadline on a 12AX7.

Divide the plate resistor value by 66 and that is the cathode resistor value.

So, 100K/1K5, 120K/1K8, 150K/2K2, 180K/2K7, 220K/3K3 all work that way.

[wicker]

Sorry that a little bit late, I've been busy, I've got 216V ona V1A and 202V on V2B, 193V on Rev recovery, did not tried rewiring reverb recovery to PI voltges, it won't be that easy cos there will be a bit of soldering
Should I try to go down with votlages still ?
Btw. what's with this presence, it just doesn't work, I hear no diference if it is set max or min, I even consider changing to high cut or something more usefull.

[Structo]

Move the cap on the presence so it goes from the wiper to ground.
Do not ground the left lug of the pot.

[wicker]

It didn't helped.