HIWATT LEAD

[TIMBO]

Hi guys, The hiwatt JP did not go quite to plan. It worked but the fizz/hiss was just too much to be able to get rid (too many gain stages i guess  )

So not to be bummed by it i pinched some other bits from hiwatt and put this together, BUT...................

Again with this many gain stages it does get hissy at hi volume but can be reduced if the with the right ballance between the volume pot and master volume.

The OD does not help much as it is part of the circuit, so i think i will add a switch to take it in and out (and add some other bits to it )

I looked at the AX84 hi octain and it has a resistor combo (voltage divider i think) in front of each gain stage, Is this to bleed off some of the GAIN  could this help with the hiss or is the hiss a power supply problem 

Any ideas would be great, thanks

[tubenit]

That amp should not be a really high gain "hiss" amp?   It's like a Dumblish style with four gain stages.  So are you saying it's got alot of hiss at the volume you normally play at?  

IF I dime everything on my amps, yes .......... I will have hiss.  However, at the volumes that I normally play or would play with a band, my amps are very very quiet at idle.  In other words, at an intolerably high volume that I will never play at ............ I will also have intolerably high hiss.

Have you chopsticked the wiring to see if moving a wire reducing hiss or hum?  Are you using shielded wires on long runs?

Tried a 5751 or 12AY7 anywhere?

You can use some more smoothing caps from plate to cathode or across plate.

Your mid pot is really like a dial in PAB  (preamp boost) separating the treble and bass pot the way a PAB does.

I would add a grid resistor between each gain stage like something between 22k - 68k. Look at the Tweed BluezMeister or D'Mars ODS and look at how high the grid resistor values are.  

I would not use a 1k cathode resistor with the 220k plate OD resistor.  Maybe try something like 2.2k to 3.3k with a 1uf or .68 cathode cap?

With respect, Tubenit

[TIMBO]

Thanks Tubenit, I think i have tried everything that i am able to think of but that is very limited. HBP has also thrown a few things my way as well with the other circuit but the hiss is still much the same 

If compare this to my TOS with nothing plugged in the TOS has no hiss with controls at 0 but a small amount of hiss with controls turn to max, with guitar pulgged in and controls at max also some hiss but acceptable. This circuit has without guitar plugged in has no hiss with controls at 0, loud hiss at max and even louder hiss with guitar plugged in at max more than i think is acceptable 

I thought by changing the preamp circuit this may fix the hiss, so would i be right in thinking that it may be a PS problem as this is the bit i haven't changed.Thanks

PS sheet 2

[HotBluePlates]

Best approach to fixing this:

Trace out the schematic for the entire preamp when it has the maximum number of gain stages running (I'm a little too lazy right now to go find the schematic).

Now cut the number of stages in half. I mean if there are 4 gain stages, temporarily disconnect the coupling cap from stage 2 to stage 3 and tack in an input jack at stage 3. Play through it. Hiss level okay? Okay, move back one stage and repeat.

It seems to me that either a listening amplifier or this approach will sort out where the hiss is coming from.

[TIMBO]

Thanks guy's, HBP i did the connecting the input jack to each triode grid and got this :-

V3b was the quietest with no hiss w/without guitar plugged in
V2a no hiss without guitar, faint hiss with guitar
V2b Bad hiss w/without guitar
Bypassed V2b clip lead off 470k resistors to V2a bad hiss w/without guitar
Bypassed 470k resistors clip lead from vol pot wiper to V2a hiss better but not much
anything north of the 470k resistors just add to the hiss but not having the 470k resistors don't allow the signal to mix

I will redo the OD circuit with a MOSFET CF and add a foot sw.

I'm going to order some new pots so that i can solder a buss to the back of them (can't do that with the pots that i've got as they have plactic stems and innards )

Is there a possibility that i do not have enough PS filitering as i have 5 triodes hanging off one (E) node????

Thanks again 

[SILVERGUN]

Hey TIMBO, glad to see you're having some real fun down there   
If it makes you feel any better,,,,,I've changed the circuit in my amp 3-4 times in the past couple months....
I'm just looking through your post and thought i'd give my 2 cents....I'm obviously no expert, but here's what I picked up on...

I would not use a 1k cathode resistor with the 220k plate OD resistor.  Maybe try something like 2.2k to 3.3k with a 1uf or .68 cathode cap?

I think tubenit is hitting on a good point here....He's referring to V2b,,,,and it stood out to me as well...
The way that tube is set up, you have a ton of gain there (with 1K cath. res.), and the bypass cap value (.047uf) is boosting a range of higher freq. that could be exaggerating the perceived hiss.....combined with the same cap value on V1b and it would seem like you are trying to boost a selected range of high freq. that would probably exaggerate your hiss....(because you're making the highs, including the hiss, a lot louder than the mids and lows)
If you'd like to see a huge change (just for reference + experimentation),,,put a 4.7K (or larger value) cathode resistor on V2b and see what happens (much less gain, AND much less amplification of the hiss?),,,,while you're there, change out the V1b + V2b bypass caps for anything up to 4.7uf, so you're boosting more of the entire freq. range

Is there a possibility that i do not have enough PS filitering as i have 5 triodes hanging off one (E) node????

YES, 
I think you could be picking up some hiss there,,,,and now that you've identified that the hiss is being amplified in that V1-V2 area,,,you're onto something..
Squeeze another filter cap in ther and create another node....good idea

Wait for someone to confirm or correct my thoughts,(because I have been known to say some dumb sh!+  ) but sometimes it's just in the wording of things that helps me get past something

I created this cathode board in my project to be able to make cathode component value changes quicker,,,and I really appreciate having pots for cath. resistors, so I truly can dial in the gain stages, and hear and feel the results of the interaction of stages....(the 2 trim pots on the left act as variable cathode resistors for my 3rd and 4th stage)

For the record,,,how attached are you to the 2 input topology?

Keep having fun ......I am