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Messages - phatt

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Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« on: March 09, 2019, 11:09:20 PM »
Another bad idea :duh

**IF** the circuit is not be designed to run at higher voltage then expect tears when the smoke escapes from the power amp :'( :'( :'(

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 09, 2019, 11:06:26 PM »
Answer; You just removed the ground loop. 8|
If you ground the case there are two ground paths which will cause hum. >:(
When looking at schematics the ground paths are hardly ever shown as they are all assumed.

Translating that drawing onto the actual circuit can be a challenge especially if you don't have a good understanding of ground paths.

I fixed a circuit years back and forgot one small fiber washer and hum was a problem.
I had created a ground loop as the fiber washer isolated the PCB from chassis.

i've seen reverb tanks where the rubber grommets had rotted away and gave intermittent hum problems as the tank would make contact with chassis as it vibrated.
Stuff like that can drive you insane if you don't know about grounding techniques.

Amplifier Discussion / Re: HELP Marshall 5203 Master Reverb 30
« on: March 09, 2019, 01:25:25 AM »
No don't do that as you will exceed the voltage rating of the main filter caps which you say is 25volts.
Your transformer should have close to 15-0-15ACV which will reap 15 x 1.4 = 21VDC.

Never run Caps above the rated voltage otherwise they will fail. 8|
Always wise to have some head room as well so +/-21V rails leaves a few volts safety margin for the filter caps. ;)

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 07, 2019, 03:56:03 AM »
Check the reverb wiring, you may have re-connected the tank backwards which would induce the hum symptoms you describe.

Reverb circuits are normally wired with only the pickup side grounded to the Tank Case. (Not always but most often)

If the tank connections use RCA plugs then swap them over and see if the hum improves.

Input and Output are often marked on the tank case but some folks get them mixed up.

Schematics and Layouts / Re: Ibanez TSA30 Schematic and PCB Layout
« on: March 06, 2019, 07:11:50 AM »
well you had me wondering as I noted a posting and then it was not there. :duh
Not to worry, in one sentence;
If you want lots of distortion then don't bother with NFB. :-X

The NFB of AB165 bassman is not going to work without major mods. you need those 220k resistors from the plates of power tubes. It was not a good design. :loco
(look up Rob Robinette site for lots of clues on what works and what is a waste of time)

I'm not a big fan of presence systems as you need NFB and that will run the amp cleaner so if you want lots of OD then you have to run the amp at much higher level.
If that is what you want then go for it.
If you just want more treble then lower the coupling cap in front of the PI stage.

Look at some fender schematics for clues,, a lot of the AB763 circuits have very small value caps at the PI input.

Those big fender twins are so loud due to the 1nF cap at PI input, Change that to 22nF and they would just fart out at high volume.
Remove the NFB and they would not be so clean at high volume. xP

Regards to Bassman,,, Which one???? :loco

Old valve amp schematics might look simple but you need to build a few to realise that a couple of changed values can make a huge difference. 8|

At the heart of most of them is the Power supply resistor values, they are often very different and that has a massive impact on the sonic result.

As example;
Your Amp circuit has R3 (22k/2Watt) now go look at most Bassmans and you will see they are 1k to 5k.
That alone makes a big difference as to how early the PI distorts.
Your circuit is closer to Plexi Values which means the PI OD's early.

Most Bassman's use AT7 while nearly all marshall's use AX7. Again that changes how the amp responds.

I could go on here as the options are endless but maybe you can just print out a dozen Schematics and look closely at the Power rail values.
Meantime read up Robs pages.

At 750 pages you might be there for a long time ;)

HTH, Phil.

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 02, 2019, 07:56:52 AM »
Well obviously there is an issue with the limit control system and as *Tonyharker* has noted it's likely simple.

I would think that the Limit leds should only light up when the signal reaches the set threshold. maybe a short flash at power on until the voltages settle but as they all come on then something is wrong around Q26.
So check the voltages around Q26,, all the test voltages are on the schematic.
Check you have -9VDC at ALL points marked (H). See if that voltage changes over time.

Namely points (H) at
Q3 & Q4
Q10 & Q11
Q23 & Q24
And also Q15 & Q16, the trem circuit.

My first suspect would be that Q26 has a problem,, maybe the Zener diode (D3) has died.

At Q26 also check you have -8.4VDC at points (G), as Q26 also supplies the neg rail for the whole preamp section.
HTH, Phil.

Amplifier Discussion / Re: Help with Gallien-Krueger 200GT guitar amp
« on: March 01, 2019, 07:44:26 AM »

This is very confusing,,
You say the OT is shot, and you hate the Current limiter but if the OT is dead then the Current limit would not be working as it only limits the power amp section.

How about you explain the symptoms and let the more qualified folks here help you find out what might be a simple fix.
 I'd guess the limit circuit might be faulty.

Looks like the Limit voltage controls the Emitters of Q1&Q2 at the power amp input. Probably works like a master gain control for the power amp.

I maybe wrong but likely the Power Tx has 30/30VDC split supply the negative side likely has a much lower current ability. So It would not be able to power a 50 Watt Chip Amp.


Amplifier Discussion / Re: Trace Elliot Tramp 40w more gain in preamp mod?
« on: February 27, 2019, 12:16:07 AM »
You could strap a pair of back to back diodes across R39 for more dirt but you would likely loose volume on the boost.
Frankly I'd just use a pedal for the extra crunch. 8|

Amplifier Discussion / Re: Marshall 5210 problem. Loud hum.
« on: February 23, 2019, 11:55:57 PM »
If the pot covers are not grounded to chassis then yes you will get hum.
The tone controls are high Z type and as such are highly prone to pickup hum.

You may find that there is corrosion between the chassis and pot mounting holes and that can cause the pot covers to loose contact with chassis.

You can solder a jumper wire across the back of each pot case and then ground it back to chassis. Don't ground the pot cases back to *Circuit Ground* as that may cause other problems, you want to ground those pot cases to *Chassis*
Just by undoing the pot nuts and re-tightening them will likely scrape off any corrosion and return the pot case to chassis ground.

Schematics and Layouts / Re: Ibanez TSA30 Schematic and PCB Layout
« on: February 23, 2019, 06:50:50 AM »
You have 2 Cathode followers, one at V1 and another at V2.
Implementing a Cathode follower at V1 is likely just a hair brain idea by someone in design who reads too much internet gossip. probably no great advantage over a single triode. :-\

Obviously you did not google Valve wizard. :trouble
It's all there for you to learn. Hint!!! 8|

Main Index;

Regards bypass caps at V1&V2,, yes try some values and see what happens.
Not a lot of advantage at V1 but at V2 will pull more tone change.

As it's an oddball circuit (with the SS preamp)
It's anyone's guess how much advantage adding Cathode caps will help?

Schematics and Layouts / Re: Ibanez TSA30 Schematic and PCB Layout
« on: February 20, 2019, 06:49:27 AM »
Yes highly likely you will have to cut tracks.
As I don't have a track layout I have no idea of how easy that would be.
I've found with pcb's you often have to get inventive with solving issues. I find a week may pass sometimes before an idea comes to mind, other times it comes to me straight up.
If the tracks are wide enough you may be able to cut and drill mounting holes.
but a lot of those pcb's have very narrow tracks which makes it hard to find a spot.
If tracks are too narrow I drill beside the track and solder the pig tail along the track.

No down side to grid stop resistors that I've read about.
I think you will find it's Mandatory on nearly all good designs. 8)

As to why they omitted those resistors,, anyone's guess. xP

Be aware a lot of these amps are just whipped up in a mad frenzy with little or none R&D.
The whole amp is likely out sourced to a far off country,, read cheap labor and no QC

There was a HiWatt Amp question on here long ago that was so badly designed it was unusable because of noise due to bad PCB layout. :duh
turns out HiWatt had little to do with that Amp and hence had no records. :loco

AFAIK, Marshall still make amps that use EL34 tubes and run them at elevated voltages,, which is no problem for the plate of EL34 but the screens are then way past their max voltage limit. For most brands of EL34 the screen limit is 420volts.
They don't last very long with the screen at 470volts.
Do they care? I doubt it. :trouble
The RCA manual states quite clearly that AX7 will last for 10,000Hr's and power valves about half that.
The Valves in a well designed amp would/should last for at least 10 Years. (with heavy use)
I've witnessed Amps that are 30 plus years old and still using original Valves.
Today they just slap it together and know that the valves will last at least a year.

A chap who came to me years back had asked the local music shop,, How often do you replace the valves?
The shop owner replied,, oh every year or so. :lmao:

He was so glad he came to me for some advice. 8|


Schematics and Layouts / Re: Ibanez TSA30 Schematic and PCB Layout
« on: February 19, 2019, 06:47:20 PM »
Yep a fair point. :tu:
As I don't have that Amp I'm only guessing.

One thing I should add is that the Schematic shows no Grid Stopper resistors on the power tubes and although you won't notice much at low volumes when the Amp is working at high volumes it will most likely spit,, sometimes called power stage choking.
I'd suggest at least 5k and right at the pin if possible. you may have to cut tracks to do it but worth it if you play with a lot of heavy loud distortion.
This very problem drove me nuts on some of my early valve amp builds years back as I only had 2k2 resistors. sounds gawd darn awful. :grr
I replaced them with 10k and bingo no more harsh blocking distortion.  <3)

Explained well here if interested;

The Newcomer's Forum / Re: Carlsbro Hornet (1970s)
« on: February 19, 2019, 06:29:24 PM »

What an eejit. It so clearly has.

Just goes to show I need those readers or a magnifying glass. Age has not only caught up but overtaken me this last year... 

Thanks, again, Phil.

Yep I know the feeling, my eyes also struggle to see fine detail. :'(
I don't even bother to try and fix anything that uses SMD's.  xP
I agree with *galaxiex* about replacing Electro's in old gear but I would add this;
Only replace the obvious first and get the amp working,,, then go back and replace the others.
If you shotgun the whole lot and it still does not work then you have no idea if you added yet another mistake. :-X

Amplifier Discussion / Re: Ampeg SVT450
« on: February 19, 2019, 06:09:10 PM »
Hum? That is not a definitive observation.
Try this, Plug your guitar into the *POWER AMP INPUT*
You should hear clear undistorted signal.
If there is still bad distortion then the power amp has issues.

To test the preamp;
(Assuming you have another amp?)
plug a lead into *EFFECTS SEND* and send that signal to another amp.
You should be able to engage the Graphic EQ,, but master will be bypassed.

If it's clean and all controls work (Except Master volume) then plug into the *PreAMP OUT* and send to another amp.
Now the master should work as well as all preamp controls.
Report results.

The Newcomer's Forum / Re: Carlsbro Hornet (1970s)
« on: February 19, 2019, 02:37:49 AM »
C16 has popped,,so replace for sure.
What's the bet that is part of the trem circuit. ;)

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