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September 28, 2020, 04:44:56 PM

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21
Tubes and Hybrids / Re: VHT Classic 18 Preamp Modification
« Last post by joecool85 on September 21, 2020, 09:20:23 AM »
Quote from: Tomsouthtrader on September 21, 2020, 08:32:11 AM
From what I can tell the circuit only utilizes one side of at as 3 pins are just left floating..
And I figured the power for it on the schematic is 'IC1-2'??

You should be looking for IC4 (or IC4a/b).  I'm only seeing IC4 at the beginning of the schematic, makes me wonder if there is a typo there.  How many IC's do you see on the PCB?  My guess is that IC4 (the one you are trying to disconnect) is still connected at one or more of the pins and causing an issue.

**edit**
To keep it simple, I suggest trying the mod this way (cut trace on red line, add jumper on green line):
22
Tubes and Hybrids / Re: VHT Classic 18 Preamp Modification
« Last post by Tomsouthtrader on September 21, 2020, 08:32:11 AM »
From what I can tell the circuit only utilizes one side of at as 3 pins are just left floating..
And I figured the power for it on the schematic is 'IC1-2'??
23
Tubes and Hybrids / Re: VHT Classic 18 Preamp Modification
« Last post by tonyharker on September 21, 2020, 08:22:42 AM »
You have forgotten that the TL072 is a dual op-amp.  What does the other half do?
24
Tubes and Hybrids / VHT Classic 18 Preamp Modification
« Last post by Tomsouthtrader on September 21, 2020, 08:14:49 AM »
Hello There
I have been trying to modify my VHT Classic 18 (basically a cheep Chinese amp based loosely on a Marshall 18w circuit).
Essentially I am trying to completely bypass the awe full input gain stage that uses a TL072. And wire the guitar straight to the input of the first preamp tube..
I have found the schematic for the amp, and working from that I got the soldering iron out..
My aim was to completely remove the TL072 from the circuit and wire the input straight into the tube.
The schematic I have attached shows what I have done to the circuit.
The amp didn't blow up thankfully.
And it does make a sound... And The tone itself is not to bad (at least through the LO input).
However it now has a couple of issues  :-[

The input is now incredibly noisy. There is some sound even when the master is at minimum.
Also the treble control when turned all the way up makes make an awful noise (almost like some self oscillation). However I had never turned this all the way up before attempting this modification.

So I am asking for you knowlage and advice on how to fix my 'mod'  <3)
Link to the schematic and my 'MOD'..
https://drive.google.com/drive/folders/1Q-AdViC60HTd6p0KYzZEajgf1M5A8tYZ?usp=sharing
25
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by shasam on September 21, 2020, 06:13:15 AM »
Hi Phatt!

Quote from: phatt on September 18, 2020, 10:19:23 AM
Likely over my head but I'll ask.
                              What is it you are trying to design?

I am trying to design a discret class B power amp, with CFP output. Something in the 15 -20 Watts range for beginning.


Quote from: phatt on September 18, 2020, 10:19:23 AM
Re the last part of the link to SOA by Rod Elliot, (Section5  Protection Schemes)
To my mind that is how you cover the extremes.

So, I should place the number of output devices to endure most real life situations, and put I-V limiter for protection against anything worse?

Quote from: phatt on September 18, 2020, 10:19:23 AM
I doubt any circuit is totally bullet proof but if it's just a 100Watt guitar amp then considering how many millions of 100watt amps get flogged and survive for years ,,, maybe you are over thinking the design. :-\
Or have I completely missed the point? xP

I am probably over thinking it  ;)
26
Amplifier Discussion / Re: Peavey VSS-20 amplifier with chorus noise problem
« Last post by edvard on September 19, 2020, 03:38:00 PM »
Make sure you buy from a reputable seller; there are some counterfeits ever since the MN-series bucket-brigade delay chips went out of production, though on eBay you're more likely to encounter salvage chips, which are a good deal if they aren't dead.  If they're actually listed as salvage and are cheap enough, I'd order two or more to make sure you get at least one good one. 


YouTube link: (Part2) Detecting Fake Ebay ICs Chips - Sound Test




Smallbear carries genuine NOS stock at somewhat of a premium ($10.95 at the time of this posting), but at least you know you're getting the real deal. 
Xvive is making re-issues, but they may be a little hard to find, and are on the expensive side ($25 and up from what I could find).  They sound as good as the originals, from what I hear.

27
The Newcomer's Forum / Re: Roland JC-120 with weak reverb
« Last post by KattMat on September 19, 2020, 09:16:57 AM »
Thanks for the suggestion!
I checked before but re-checked it now. Can't pick up any DC or AC at R96 with the tank connected. 

With the tank disconnected I get about 10Vdc that immediately starts dropping.
It reaches 3V after about 10sec of measuring and then starts jumping around between 0 and 3V.

What do you think? Faulty cap somewhere?
28
The Newcomer's Forum / Re: Roland JC-120 with weak reverb
« Last post by Jazz P Bass on September 19, 2020, 12:45:40 AM »
A TA-7200 P 10 watt power amplifier is used to drive the reverb pan.
It appears to be a 1.1 Vac output signal on the schematic.
Measure it and see what you have.
29
The Newcomer's Forum / Re: Roland JC-120 with weak reverb
« Last post by KattMat on September 18, 2020, 03:03:08 PM »
Serial is 901576
220V model

I've been using this one for troubleshooting:
http://guitar-fx-layouts.42897.x6.nabble.com/file/n46737/JC_120_Schematic.png

Not positive it's the right one but it mostly seems to check out
30
The Newcomer's Forum / Re: Roland JC-120 with weak reverb
« Last post by DrGonz78 on September 18, 2020, 02:33:32 PM »
What is the serial number on the amp? I ask only that we can first help identify the correct schematic to use.
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