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Author Topic: Hiwatt custom 20 tube hum problem  (Read 47822 times)

tonyharker

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Re: Hiwatt custom 20 tube hum problem
« Reply #60 on: January 08, 2010, 02:55:24 PM »
Just looked at the characteristics for the Philips EL84.  For class AB Push pull grid bias should be -14.7 with B+ at 300v and -11.6 with B+ at 250v. 
-30v is sending your EL84s into cutoff which is why you are getting so much distortion.  The quiescent plate current with no signal should be 7.5mA in each tube.   For 17W output the plate current should be 46mA.  so your original bias of -14 was correct and you should get your tech guy to put it back to that value!  Alternatively get someone else to look at it who knows what they are doing!  ::)
Regards Tony

J M Fahey

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Re: Hiwatt custom 20 tube hum problem
« Reply #61 on: January 08, 2010, 03:03:08 PM »
Hi billyJoe, calm down.
I bet the technician is as angry, confused, pissed, out of his mind, whatever,  as you.
You two are killing yourselves over a terribly designed piece of cr*p.
Consider pulling the PCB and building something like Marshall 18 Watt, Soldano Atomic 16, a Tiny Terror, an AC15, or even a mini-JCM800 with 2xEL84.
You can later add as much gain as you wish, but start with a no nonsense working amp.

phatt

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Re: Hiwatt custom 20 tube hum problem
« Reply #62 on: January 09, 2010, 09:08:13 AM »
Just looked at the characteristics for the Philips EL84.  For class AB Push pull grid bias should be -14.7 with B+ at 300v and -11.6 with B+ at 250v. 
-30v is sending your EL84s into cutoff which is why you are getting so much distortion.  The quiescent plate current with no signal should be 7.5mA in each tube.   For 17W output the plate current should be 46mA.  so your original bias of -14 was correct and you should get your tech guy to put it back to that value!  Alternatively get someone else to look at it who knows what they are doing!  ::)
Regards Tony

I agree with it ,,,cept the 48mA @300VHT.
Assuming the plate voltage is normally close to B+

Then 300 x 048 = 13.66 ? Ouch!
Last time I looked Max plate dissapation was 12 Watts for EL84?

A vast majority of these copycat import amps have overly high B+
and this amp has the screens running at close to B+ so even at -15volts Bias
The Valves have little safty margin.

My bet is that this amp will have a B+ in excess of 350 VDC so my first port of call on this amp is insert larger value screen drop.
This concept is now becoming almost a unversal understanding as more people realise the screen grid is the weakest link in the valve.

It's interesting to note that there is only a couple of watts difference in output power from 250 to 320VDC and then an inaudible difference from 300 to 350VDC.

The funny part is that a pair of EL84's sound so much sweeter running at 250VDC.
Bigger voltage = less tone,, but no one gets the joke. :duh

Cheers, Phil.

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #63 on: January 09, 2010, 04:17:36 PM »
Checked it again; still hums and buzzes.
With gain high and master volume pretty low it's okay. The guitar covers it and it's a ral rocking sound.
Should I leave the pot in or put back in the resistor?
The resistor would make  -14 again.
Did some tests.
Pulling v3 and v4 nothing connected to input,
no buzz hum or anything.
V2 pulled out, no buzz, some hum.
V1 out, buzz some hum.
Plug a guitar cable to  input but not to guitar buzz goes up.
Could the input jack be part of the problem here as well?
« Last Edit: January 09, 2010, 04:47:27 PM by billyjoe24 »

J M Fahey

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Re: Hiwatt custom 20 tube hum problem
« Reply #64 on: January 10, 2010, 09:14:56 AM »
Hi billyjoe.
What are you talking about?
We are talking -14 *VOLTS* bias *VOLTAGE* for  pair of 6BQ5.
You just referred to 25 to 27 *MILLIAMPERES* cathode *CURRENT* (which is quite low), yet set the bias VOLTAGE to that value. :loco
You are looking everywhere on the world to blame the hum and hiss, beyond the real suspect: poor design and layout, hidden behind a label they don't deserve. .
*If* the bias supply is poorly filtered (which I don't think is) , the hum will be inaudible, because it is balanced out; and in a *very* poor situation, it might be heard as 50Hz tremolo, generating "ghost notes", but anyway, it would be *constant*, not depending on gain or any control, which is not your case.
If you wish to do something with that amp (besides selling or junking it), pull the board and build a Tiny Terror, that has gobs of gain yet is simple enough to be built with reasonable possibilities of success, or even better, a Marshall 18 which is killer, although you'll have to drive it with some pedal of your choice for the high gain sounds.
Don't waste any more time and money on that nightmare.

Brymus

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Re: Hiwatt custom 20 tube hum problem
« Reply #65 on: January 10, 2010, 04:28:20 PM »
What caps are you talking about ?
And seriously the wiring on that is a mess,and probably part of the reason for your hum.
I cant believe how they did that,It was obviously to save labor,using harnesses to connect the tube pcb to the main pcb.
But the lead and dress is suffering horribly from it.
Not to mention the tubes look way to close together.I would guess they are part of the problem as well.
You already have the most expensive parts needed to build a good amp.
The tubes ,chassis,and most costly the trannies.
You could salvage the pots and the "candy" caps (the brown ones) as well.
Maybe even the filter caps too.
Ditch that and build an 18 watt Marshall,or something similar.
Using an eyelet board and some proper lead dress.
Otherwise I doubt you will ever be happy with that.
Yeah they (techs) try to fix it for you.
But if they told you,a new amp was needed you would just think they were trying to scam you (probly why they havent said that here)
So rather than risk losing some work in todays economy - they do what you ask and try.
You can polish a turd all you want....but its still a turd.

J M Fahey

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Re: Hiwatt custom 20 tube hum problem
« Reply #66 on: January 11, 2010, 12:03:06 AM »
Hi billyjoe.
Then your *only* chance is to borrow your friend's silent one, open both chassis, place them side by side on a table, get *good* lighting, and compare them component by component, track by track, wire by wire.
I understand his is an original (or correctly repaired), yours has been heavily abused, get both exactly alike and they *probably* will be and sound alike.
Not even that is a guarantee of success.
Gremlins do not exist (that coming from somebody having Irish ancestors is a hard thing to say), but they *should*, to explain many electronics things.
I understand how you feel, in fact all we do here in SSGuitar, you are aware of that, but only so much can be done on "remote control".
Good luck.

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #67 on: January 11, 2010, 10:14:14 AM »
I certainly appreciate your help and everyone else who's chipped in.
It's a frustrating amp this thing.
I think the amp had a somewhat better sound at -14.
Could the bias affect the loudness?
It seems louder than before.
But the output transformer is very, very close to tube 1.
I think the hum is tube 1 position, buzz from tube 2 position.

You know the problem with the guy who has the amp, he's way over in California.
If he could check the parts on the board and shoot some photos maybe you could see something but  I don't think he can.
I have one last fellow lined up, who is a retired tech,
so I think he's the best man for this troubleshooting job.
After that :'(

« Last Edit: January 21, 2010, 02:01:14 PM by billyjoe24 »

phatt

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Re: Hiwatt custom 20 tube hum problem
« Reply #68 on: January 11, 2010, 09:47:23 PM »

It's a good hi-gain sounding amp, on the noisy side.
I will probably try to sell it at a good affordable price for someone.
Like I said, turn it up and it roars, noise and all.
It will kick the crap out of a vox night train or Haze or tiny terror any day, believe me!

Get your facts sorted if you want good advice!

From the very start I made the comment that this was a high gain monster but you totally denigned that stating it was way to clean and nice.  So make up your mind :duh

I've done enough simulations to know that this preamp is capable of swinging in excess of 150 volts at the master volume knob at which point the amp has gone way past distortion and into crud tone,,, Far out!!! :o Ouch and plain stupid circuit.  :loco

An EL84 only needs 30Volts signal swing on its input grid to produce full power.

Put an audio signal (CD cable) into efx return and see if you can get **Clean music** coming out the speaker.  If you still have crossover distortion there then the bias is way off.
Untill you define the B+ voltage there is no way to establish a bias point anyway.
I and others here have made several intelligent suggestions,,,, most of which seems to get fobbed off and you then become fixated on another part of the circuit.
So you are really adding to your own confusion. :-*

I'll leave to you to go back and read the many comments that have been laid down,,otherwise I'll just be repeating myself  forever ever ever.
Have fun,,, Phil.

J M Fahey

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Re: Hiwatt custom 20 tube hum problem
« Reply #69 on: January 12, 2010, 05:10:25 AM »
That makes two of us .....  >:(
Specially after fresh comments like:
Quote
I spoke with my tech a very knowledgeable and great guy.
Here we go again
 
Quote
   
There is room to move the transformers, but someone would either have to use, say every second socket on the existing pcb and add in a new one, to space them out,
or make a new pcb, punch in some holes and move the transformers back.
Sounds easy , try it.  :tu:
EDIT: I found this, a few posts behind:
Quote
Well they should be honest and say "i can't fix it" and that 's fine.
Don't know why, but I believe telling you that is impossible.
In a nutshell: after 6 pages of answers, over 80 replies, over 3800 reads, we are back on square one?
Count me out.
« Last Edit: January 12, 2010, 05:21:01 AM by J M Fahey »

phatt

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Re: Hiwatt custom 20 tube hum problem
« Reply #70 on: January 12, 2010, 09:51:59 AM »
When you get off the high and mighty and act in a rational manner you will get all the help I can give,,, till then bash a wall.

"You think it can be fixed? Then fix it!",,, IF you Give me the info I ask for then I can improve it somewhat.
But without modification this amp will always HUM.

The Amp DOES have major design flaws and if you keep insisting it must be a quite amp because your mate has one then I doubt I can help you.
AS is the Amp is a dud.

YES!!! the Tubes Are to close together.***
YES the power transformer has no shielding.
YES it has to many gain stages.
YES the EFX loop is killing it.
NO!!!! the board is not right. (Ground plane is wrong.)

NO Without knowing the B+ Voltage I have no way of knowing the correct bias.

NO I can't do a FRIGGIN thing to help you if you get stroppy.

BTW I think there may be a misunderstanding about bias voltage,
that being the bias of -35VDC or whatever,,, was being taken while the amp was in standby,,,,,You need to know the **PLATE Voltage** and the bias voltage while the amp is ON,, not on standby.

Don't get angry get even,,, Educate yourself and learn how to fix/build these things yourself,,,, but that requires a lot of patience and reading. another hint ;)

Do the ground work and I'll help,, go off half cocked and get mad again and I'll leave.
Cheers Phil.

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #71 on: January 13, 2010, 11:01:02 AM »
How about getting the ceriatone 504 head and using it with the amp?
I don't think i can fit the chassis in here.
It's too big. The board alone is not expensive.
But can it fit in the existing chassis?
Can the wattage be lowered on it from 50?

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #72 on: January 13, 2010, 12:25:38 PM »
Is it possible to change the mounting bracket on these transformers?
I f the bracket had longer slots I could move the shielded transformer further from v1, probably a good 1/2 inch.There's plenty of room on that side.
What about flip flopping the transformers? Changing sides?

 Changing value of gain or master volume have any effect?
The other transfomer is a bit more of a problem, but I think I can get bell ends.
It's really  a last try, after that...

A 12ay7 in v1 lowers the hum somewhat, about same effect as cutting traces.
Zero effect on buzz.

phatt

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Re: Hiwatt custom 20 tube hum problem
« Reply #73 on: January 13, 2010, 09:45:45 PM »
Delete V1 per my previous recommendation, Then take it from there.
Phil.

billyjoe24

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Re: Hiwatt custom 20 tube hum problem
« Reply #74 on: January 14, 2010, 08:34:53 AM »

If there is info you need I could ask someone to check that and just go with a list:
this is the info we need.
There are some guys who don't charge or charge very little for looking it over.
 Do the caps have to be removed from the board to be checked?




You're right there is a lot of gain here :)
(stop laughing) :) I admit defeat.
In my defense I haven't heard the thing in months.
For me it's not a super high gain amp, but you don't need a distortion pedal.
Perhaps it's high gain 7- 7.5 out of 10? with 10 being out of control hi gain monster?
Maybe I can post a power chord so you can hear it.Or the hum.
But it's the crap tubes they put in too.
Sovtek wb are not helping. Yuk.

The clean sound is (from 0 to 10)
Mv at 3 gain at 3.
Going up with MV control gets more hum. Gain up, some buzz.
Can the buzz be filtered out as before with filters,rather than hunt down the cause?

You could use the amp with mv very low (below 1 really) and use the gain at whatever setting. Not very loud but no buzz or hum.

I'm trying to get the transformers out of there but the one thing they did right was glue the bloody bolts on like there's no tomorrow! How do these bolts work?
If moving the transformers has no effect then :-\
But now I can't get them out.


What about a 470k resistor on tube 2 where the trace was cut?
I'm going to try this.The solder is there.
I see it on a lot of Hiwatt amps including the sa412.
This should cut down the gain somewhat?
I'm going to try it tomorrow just to see what happens.
Dumb idea maybe but what the hell.Why not?
What about a 68 k resistor on tube one? A grid stopper. What exactly is that?
« Last Edit: January 21, 2010, 01:58:32 PM by billyjoe24 »