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1
Amplifier Discussion / Re: Ampeg SVT450
« Last post by aruption on Today at 05:43:30 PM »
Both 16v rails are right in the 15-16v range.

If I am reading the pinouts correct, then on IC1 there's -14v on output 2/pin 7 (0v on the other three outputs), on IC2 there's -14v on output 1/pin1 (0v on the other three outputs), and no voltage on the output pins of the IC3 and IC4.

At the test point near D4, I am reading 14v on one side of D4 and 0v on the other side. Is that correct? Because it looks like the side that is reading 14v is the side that the test point should read 0v, because that is the side that connects to R13.
2
Schematics and Layouts / Re: Ibanez TSA30 Schematic and PCB Layout
« Last post by Mprall00 on February 20, 2019, 01:20:24 PM »
Another question while I am at it - V1 and V2 don't have a cathode bypass cap.  Worth adding one to pin 3 of both?  Or at least V1?  would raise gain... and depending on value, on specific frequencies?
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Schematics and Layouts / Re: Ibanez TSA30 Schematic and PCB Layout
« Last post by Mprall00 on February 20, 2019, 10:08:26 AM »
I'll have to look tonight but i feel like C13 and C5 being so close to pin 5 means i could almost cut the trace and then just solder the resistor to the existing hole that the leg of C5 is already in then directly to pin 5 without modifying the board at all.  Any problem not using the board at all here?  More noise?
4
Schematics and Layouts / Re: Ibanez TSA30 Schematic and PCB Layout
« Last post by phatt 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|

Phil.
5
The Newcomer's Forum / Re: Carlsbro Hornet (1970s)
« Last post by psafloyd on February 20, 2019, 01:09:49 AM »


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
Phil.

Will do, Phil. Simple process of elimination far more sensible than having to pick the bones out of wholesale change. Thanks again.


Sent from my iPad using Tapatalk
6
Schematics and Layouts / Re: Ibanez TSA30 Schematic and PCB Layout
« Last post by Mprall00 on February 19, 2019, 11:15:40 PM »
Good advice!  Didn't notice that.  I bet it was never a problem with the original design because it was never getting loud enough to get the power amp cooking anyways.  it was so quiet stock...

I doubt I will get it cranked that loud anyways but in case I do...

Would that mean essentially cutting the trace between R14 and pin 5 of v4 and R10 and pin 5 of v5 then adding something between 1.5k and 10k where i cut the trace?  Is there any downside?  Nothing mentioned in what I am reading... did ibanez just design it without to save $.05?
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Amplifier Discussion / Re: Ampeg SVT450
« Last post by Enzo on February 19, 2019, 10:13:22 PM »
SO check the preamp, are both 16v rails clean and more or less the same voltage  ANything in the 15-16v range is OK.

Check all op amps for unwanted DC offseet on teh outputs.  It looks like they use quad op amps, TL074, so the four corners are the outpu tpins.
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The Newcomer's Forum / Re: Carlsbro Hornet (1970s)
« Last post by galaxiex on February 19, 2019, 09:08:59 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
Phil.

Yep, very good point. Fix the obvious first.
9
Amplifier Discussion / Re: Ampeg SVT450
« Last post by aruption on February 19, 2019, 07:48:35 PM »
Sorry, that was unclear. I was saying that the problem is definitely in the preamp, because I did the tests as you described, and had a clear undistorted signal from the power amp input. The rest was still super distorted. There's no issue with hum, just distortion of any signal being input.
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Schematics and Layouts / Re: Ibanez TSA30 Schematic and PCB Layout
« Last post by phatt 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;
http://ozvalveamps.org/choking.htm
Phil.   
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Ampeg SVT450 by aruption
[Today at 05:43:30 PM]


Ibanez TSA30 Schematic and PCB Layout by Mprall00
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Carlsbro Hornet (1970s) by psafloyd
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Frontman 212r by dlbraly
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Power transformer questions by Loudthud
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