I've never seen an *SA412* but with your amp the ONLY thing that is some semblace of HiWatt is the PI topography ,,,datz it.

AFAIK,, No Hiwatt used the extra front end. It's dumb noise prone stupidity.
Marshall hot rodders found out the fatal flaws of wiring up the extra triode like that. they just osscilate and any extra gain is rendered useless.

My advice for you is just loose the first AX7 and wire the preamp back up to standard hiwatt specs,, you will thank me one day.

If you wish to underatnad how hard it is to get that many stages to work without issue then download the *Carvin Legacy* schematic and have a squizz at the Hot channel and count the triodes.  ALL heaters are DC,,, only way you can make it happen.

Cheers phil.

Try looking at the *Insert Jack*. (a common place for this kind of issue)

Bridge R41 to C26, thereby bypassing any possible intermitant connection issues.

If you have no need for EFX loops or insert jacks then consider disconnecting them from your circuit. IMO the dumbest thing to have on a guitar amp.
If the problem persists, then it maybe a switching issue. Check voltages at D25/D26.
Phil.

Hi some schematic's,,
There may well be errors but others will soon pickup any oversight.

If your Heater circuit *is* Center taped then consider the ground plane situation I mentioned before.
I've found two ways for that if you are interested.
Hope it helps,, Phil.

Hi Jack,
Yes frustrating as no one mentions that these things need a well regulated PSU.
You can add another Electro can right before your amp.
Maybe add a 100 ohm resistor in series right before the cap.
have fun, Phil.

One thing I just noticed,
The wire jumper next to the notch does not line up with your previous picture.
The little yellow wire crosses to ground track on pic.

Which raises an interesting thought,, hum.

The ground of that input socket would be better connected straight to the Main Green Ground wire.
The way it's pictured the small signal high imp path will certainly pick up unwanted noise as it passes all the *high current* components on the long way back through
the *Earth Track* to ground.

This is bad practice if you wish to reduce noise/hum.

The efx loop is a dozzy as it takes straight off the tone stack,, ouch don't think that's a good idea either.  but no matter,,, hum problem first.
phil.

Remember it's not the bias it's the *screen*
Go track down some data.  you will find that working with *High B+* needs the screens *LOW*.  Often at HALF B+  If you want them to live.
Yes your kind of right in that Vavles are by and large *overvoltage tolerant* but often little consideration is made for *Screens*.
You can't just bias colder if you keep uping the B+ you have to back off the voltage on the screens as well as rebias.

Phil.

Two things that need to be looked at before you go to far;

1/ AC heater wire up and layout. If AC and no CT (centre tap) then you are looking for two small value R's (50 to 100 ohms) across the heater circuit going to ground.

Without a ground reference then there is *going to be excessive hum*.
These R's (If present) maybe on the Valve PCB. (remember the Valve pcb is double sided) look on the Valve side of the board.

My mate's Fender Pro Junior has these hidden on the valve PCB and you have to pull it right out to see them.

The only other way is to lift the CT of heaters to a reference voltage but I doubt this is done on this amp.

2/  Transformer leakage. Check how close the power transformer is to the Preamp valves. If the valves have no shielding cans and PTr's are close then it quite possible to have induced hum.

You can ground a flat plate of steel via an aligator clip and slide it between the Tr and most sensitive preamp tubes to see if it reduces the hum.
Some tube geeks build the whole circuit and then move the Tr's around untill they find the best mounting point.

old Tr's came with shielding bell ends but these days they don't seem to bother.
Some TR's have a *copper strap* wrapping around the core to help reduce EMF but the old bell ends where the best way to go.

Yes JMF is on the ball all those connectors are painszzzz.
"If you want it to be bullet proof then Solder everything"
That comment came from an old chap who spent his life in AM radio.

If I get time then I'll have a crack at a schematic as well but the 2nd pcb would help,,,
just a pic of the other side with valves removed would help out.

Good work on the picture :tu:  Phil.

Hi Anthony,
           King TUT's book has some good clear explainations about the common issues of tube amps. I liked his simple approach to valve life expectancy.
When it comes to power tubes in guitar amps,,The higher the voltage the shorter the life span.
Guitar amps (old and new) generally run the screen volts very close to plate volts.
This gives more power but leaves less margin for safety.
Some older tubes handled it but often the older amps used Valve rectifiers and also ran lower HT or B+.

Nowdays it's not uncommon to find a little EL84 amp with a HT of 370VDC or more ,,,now add SS diode rectification (less sag). So the tubes are coping a belting and something has to give.
Again King TUT; "Most common cause of tube failure, *Screen grid* over dissapation".

The plate is huge and can cope with some abuse but the Screen grids are delicate little wires.
Now add it all up; An overly high B+,,, plus screen volatge very close to plate voltage,,,,add a tight SS Rect.
Well those poor little screen wires are going to start rattling around inside (where you can't see) then start arcing out and before long you are shopping for new tubes.
*The Screen is the weak link*

It is actually possible that under heavy load the *Plate voltage* can drop a few volts and be at a lower positive potential than the screens,
A momentary situation but if it keeps on happening,,$$ ouch.
When that happens the screens actually become the most positive element in the Valve and hence cop a belting.
Remember that negitively charged electrons coming from the cathode will be attracted to the most positive element, they can't tell the difference between the plate or screen. (No safety margin)

TUT again; "If you run high B+ ***Drop screen voltages*** Be kind to your screen grids and Your Valves will reward you with a long and tonefull life span.
Your schematic shows R58 being 400R 5watt. Now again I'm guesstamating your B+ is well over 300VDC (The -14 bias volts hints there is a high B+) So I would just up R58 to 1K,, even 1k5.

This will do 2 things it will help prolong tube life and also add a bit more
compression to the output section.
Not only does it help the tube and give a better safty margin it makes the amp much more responsive. (BTW this is an old blues trick, I guess it depends on style of playing)
Most folks I've helped out with these simple changes like the wider dynamics you get from the powerstage.
I doubt you will even notice the slightly less output wattage.

I may well have missed something you need to understand, so go find the *Valve wizard site* good valve talk.
Also *Tone Lizard Amps*,, checkout his *Tone Lounge* pages good simple explainations there also.
One other option is to hunt down the *Dana VVR Kits* (A High voltage regulator for amps under 40 watts). that will allow you to set the Max HT or B+ . I know little about them but they do exist.
Or look at King TUT's ideas on regulators, also sells Reg kits .
His Site is called *London Power*
Or look at the old brute force approach, a big 10 watt drop resistor in the B+ path.
Phil.

Thankz fur pointin out dat I'z carn't even speeel in me own language. :lmao:
Yes, it's  High *Tension* with an S, not a T.  :grr
I'll have to visit "Miss Palomba's" house won't I?
Phil.

Hi JMF, Thanks for the vote of confidence, My brain was switched on I guess, lol.
Brymus, Thanks for the thought also, maybe I don't deserve to much credit as I just made a complete stuffup drawing a simple diode circuit.  :duh
Phil.

Quote from: Brymus on November 18, 2009, 11:09:32 PM
Interesting as the amp I have that makes that noise is still using the original PCB and connectors VS an eyelet board like I normaly use.
I will check the connectors on mine as well.
Glad you got it fixed.
Did you bypass the connector or use DeOxit or something similar to solve the issue?

Consider the *Climate* you live in. Humid climates wreak havoc on cheap connections,
spare a thought for poor sods like me. I live in SE Queensland, Australia,,, 5 khm from world famous beaches. Sounds nice for tourists but had work for us musicians.
It's Hot, Humid,, now add salt air. 

Life span of components is definitly reduced where I live. I'm constantly replacing "green gunk coated" output sockets on guitars due to the added humidity.
Phil.

Quote from: awdman on November 18, 2009, 11:20:14 PM
That's good, right around target.  Second question is what is VHT short for. That is kicking my butt.

Sorry mate,, wrote it in a hurry.
VHT,, Volts High Tention. Probably not the right way to state it but even the experts tend to shorthand a lot of words.
Phil.

Hi Ant,
Once again, no one even notices the outright lies that some Big names clain an no one ever bothers to read the Valve charts.

Generally speaking,, 4xEL84's will deliver approx 28 Watts. (the classic Cathode biased Vox circuit)
Your amp maybe Fixed bias which will add a few extra Herbs but NO your amp does not put out 50 watts.

Between 14/17 watts per pair in PP mode.
And before you ask that Amp is not class A.
You need to run them at around 250 VHT for that magic stuff.

As with most of these tubes running on lower HT produces the better sounds but all the amp makers know that if the Amp says 50watts then most will buy the BIGGER amp,,, regardless of tone.
Phil.

Simply put,,, ""Audio is an *AC signal* riding on a *DC potential*.""
You have to isolate the DC.
Took me quite a while to understand it but once I read that simple line it made a lot more sense.
So when you look at circuits you need to think in both terms.
Phil.

Quote from: 5thumbs on November 17, 2009, 11:37:13 AM

Quote from: phatt on November 17, 2009, 08:53:40 AM
Forgot,,
Re Bridge for clipping?? I breadboarded a few a lot of those clippers and the extra diode did little for me but heck try it out and make your own judgement.

Marshall used this idea in JCM 900's I think. The only way they could get more crunch from the amps as triodes alone can't do that trick.

The diode polarities on your diagram do not match mine.  On the two "left side" diodes, they need to "point" inwards towards each other (and the + side of the "middle" diode).  On the two "right side" diodes, they need to point outwards away from each other (and the - side of the "middle" diode.)

The diode arrangement in your diagram will block flow from the signal-side through the bridge rectifier to Vref/GND (except once reverse-breakdown voltage is met in the diodes, but I doubt you'd ever hit that voltage in this scenario. )

EDIT:  If you "rotate" the 4 bridged diodes 90-degrees to the left (and reposition the middle diode accordingly, but flip its polarity), your bridge rectifier clipper would work as expected.

Whoops!! yep right you are ,,, I've gotta stop drawing stuff late at night.
Phil.