Basic tube amp question(s)

diagrammatiks · Post by **diagrammatiks** » 12 Mar 2012, 18:00

it's a differential pair. that's reasonable.

not sure how well it functions as a phase inverter.

As another note, that's the only marshall schematic that shows the particular PI circuit.

The other 20 watt marshalls are different...

but people have built the 2061 of that schematic so it ought to work.

Rolbista · Post by **Rolbista** » 14 Mar 2012, 17:56

Dang... Guys, I should probably lie now for my own good or else you ain't gonna help me with anything ever again... Turns out the PI wasn't connected to the power tubes at all, I missed two wires from 22nF caps! On the way though i increased the 130R resistor to 210R to save tubes a little and redid the heaters wiring so it's tidier now. Anyway, thanks for all the input, at least I learned a little about tube amps and didn't get electrocuted. I want to build a power break for it now and put a PPIMV so it's more recordable. It isn't as loud as I expected it to be though, easily bearable when maxed out, definitely enough for jamming with drums though. Sounds like a million dollars, I played it with a 112 cab with an Eminence V12 inside, can't wait to plug a 412! Anyway, gotta grab a Gibson, and learn to play the Bluesbreakers' Beano Album!

deltafred · Post by **deltafred** » 14 Mar 2012, 18:58

Glad to hear that you got it going ok.

Hey if you never make any mistakes it's because you never do anything.

Rolbista wrote:... Anyway, gotta grab a Gibson, and learn to play the Bluesbreakers' Beano Album!

Now that I like the sound of.

juanro · Post by **juanro** » 14 Mar 2012, 19:04

Ah, the silly miss! Who didn't, anyway? Glad it was something easy to fix. Regards,

Juanro

Rolbista · Post by **Rolbista** » 15 Mar 2012, 15:44

One thing surprised me though, everywhere on the net I read about the 20 watter's clean headroom, and on mine there is none in fact, there always is slight crunch present, even with single coils. Roars with a Superfuzz!

ansil · Post by **ansil** » 26 Mar 2012, 05:52

phatt wrote:
Rolbista wrote:Ok, I'm gonna grab some 1/2 watters or bigger, shops in my town don't have all the values in 1/2 but i was able to locate some in 1 and 2 watts ratings. Soo are these caps for 400V expensive? will 4$ be enough for all (approximately)?
As mentioned already the issue is the modern 1/2 watt/ 1/4 Watt Resistor is *The wire* is way too fine to be of practical use in an eyelet board layout.

i read this and giggled all the way home. too fine in an eyelet board layout?? damn all of us tube guys better hit the wood shed. well as for me i am going to throw away my Marshall and go by some two watt resistors and start over. lol its all tongue in cheek don't get to serious here

ansil · Post by **ansil** » 26 Mar 2012, 06:00

Rolbista wrote:One thing surprised me though, everywhere on the net I read about the 20 watter's clean headroom, and on mine there is none in fact, there always is slight crunch present, even with single coils. Roars with a Superfuzz!

allow me to help you friend.
r11 r 14 increase to 5k
r16 decrease to 1k
change 12ax7 to 12at7 or 5751.
c1 to 2.7uf for freq response at 70hz instead of .6hz
c8 to 25uf.

BTW its a Marshall good luck on clean

but that will get you very close and it will still crunch out when cranked

kleuck · Post by **kleuck** » 26 Mar 2012, 09:32

12AT7 are harsh when pushed (and in most circuit, they exhibit the same gain as a 12AX7 they are just harder to drive, i'd rather put a 12AY7 in the Pi position.

Rolbista · Post by **Rolbista** » 26 Mar 2012, 11:48

It's not like I miss the clean here, it was just an observation.

DrNomis · Post by **DrNomis** » 26 Mar 2012, 11:48

You should be able to use pretty much any twin-triode 9-pin base tube (provided they have the same/compatible pin configuration/pinout) as the phase-inverter in a class-AB guitar amp, you'll just get differing amounts of overdrive from the power tubes, higher gain tubes, like the 12AX7, will overdrive the power tubes more than lower gain ones, also if your amp uses negative feedback, the type of tube you use as the phase inverter will affect the tone and general feel of the amp too....long story short, give the tube a try and see what works for you.....

kleuck · Post by **kleuck** » 26 Mar 2012, 12:43

DrNomis wrote: if your amp uses negative feedback, the type of tube you use as the phase inverter will affect the tone and general feel of the amp too

Same with no feedback you know.

ansil · Post by **ansil** » 26 Mar 2012, 22:58

kleuck wrote:12AT7 are harsh when pushed (and in most circuit, they exhibit the same gain as a 12AX7 they are just harder to drive, i'd rather put a 12AY7 in the Pi position.

hmmmm perhaps less circuit simulator more oscilliocsope time.... but hey we all have our opinions. also there is an online rca tube manual as well.

phatt · Post by **phatt** » 27 Mar 2012, 01:26

@ ansil
Unless I'm mistaken? Neither a sim nor a scope come with EARS!

But your right if folks actually read the data sheet they may notice that an AT7 has 9~10 times the current gain compared to an AX7.
So yep a very different Valve and likely a very different outcome.

FWIW, to *Rolbista* at least, I have built similar circuits and did try an AT7 in the PI and DID note that it had a little more clean headroom.
But NO it's not a mind altering experience.

You may also notice that a lot of Fender circuits use AT7 PI whereas most British amps use AX7.
Tetrodes need a lot of wiggle on the grid to make the power tubes go fast but pentodes only need a little wiggle.

Check the Transconductance numbers for clues. hint.

TC of common Power Valves
EL84 = 11,000
EL34 = 11,000
6L6 = 8,000
6V6 = 3,800
For the non tecky guitar players Transconductance just means more built in gain.

From memory a 6V6 needs a ~70 volt wiggle on the input grid to deliver full power whereas an EL84 only needs about ~30 Volts wiggle.
So it becomes obvious that True pentodes are a lot easier to OVERdrive.

If you where to swap those El84 power Valves over to 6V6 and leave everything else unchanged.
The result would be a much cleaner Amp.

Sadly these days a lot of Valve gear is all multi stage triode fizz distortion with master volumes holding the power section back so it's debatable as to whether you can hear much of the magic of the power valves as they compress.
If that is the kind of Amp you have then it matters little which power valves are used as the power tube quirks are much less obvious.

Phil

diagrammatiks · Post by **diagrammatiks** » 27 Mar 2012, 04:41

The TC rating doesn't mean anything.

those power amp stages are configured for voltage gain.

if they were current drive or transconductance amplifiers it would be a different story.

kleuck · Post by **kleuck** » 27 Mar 2012, 07:50

ansil wrote:
kleuck wrote:12AT7 are harsh when pushed (and in most circuit, they exhibit the same gain as a 12AX7 they are just harder to drive, i'd rather put a 12AY7 in the Pi position.

hmmmm perhaps less circuit simulator more oscilliocsope time.... but hey we all have our opinions. also there is an online rca tube manual as well.

I just tried a bunch of 12AX7 vs 5751 vs 12AT7/6201 vs 12AY7 with my ears in my Push-Pull.

phatt · Post by **phatt** » 27 Mar 2012, 08:56

diagrammatiks wrote:The TC rating doesn't mean anything.

those power amp stages are configured for voltage gain.

if they were current drive or transconductance amplifiers it would be a different story.

I'm all ears,,,, ready to be enlightened?
Phil.