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Topics - Littlewyan

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Amplifier Discussion / FET Self Biasing Method
« on: March 23, 2021, 08:39:54 AM »
I've been building myself a simple amplifier (Of which I will post more details about in a separate thread) which uses a TDA2050 chip of the power amp and a few JFETs and NPN transistors in the pre amp. I've breadboarded it and it works fine, but I don't believe the bias of my FETs is correct. I've googled and googled, read various websites and have gotten nowhere, everyone seems to have different methods of finding the correct bias. Can someone please help? I'm using J202 JFETs, datasheet is here:

My supply voltage is 35VDC. I'm currently using 33K Drain and 1K Source. My VDS is 20V which is roughly ok, but the JFET is only drawing 0.46mA. The datasheet states it should be between 0.9 and 4.5mA.

Amplifier Discussion / Old SS Project Rebuild
« on: March 18, 2021, 05:35:49 AM »
Hello all!

It's been a rather long time since I posted here, but recently I decided to get back into building Solid State amps. The last one I built was......well no other way to describe it, a death trap! So I've gutted it and am starting again applying all the lessons I've learnt building various valve amps, mainly in safety! I will post project photos etc to show progress but I need some help please with applying current feedback to my design.

You'll see in the attached screenshot that I'm basically using the standard TDA2050 (Ignore the fact the model says TDA2030) circuit from the datasheet, but am trying to add current feedback to it. As I'm using it in a Non Inverting configuration I'd have thought it should be applied to the voltage feedback loop, but this gives me positive feedback? Is this right?

Amplifier Discussion / Two Channel Amp Design
« on: September 26, 2014, 03:36:55 AM »
Hi Guys

I'm rebuilding an amp I built ages ago into a Two Channel Clean/Distortion amp! I've put together a design and I just want to know if anything looks like it won't work to you guys. I've attached a screenshot of the LTSpice Sim that I've made and the Sim itself. Both channels in the sim give out near enough the same output, although I can obviously measure and adjust this when I build the amp. The chip amp is a TDA2050.

I've made the distortion side near enough like the Marshall JCM800 2203 Pre Amp, mainly because you'll get distortion with it at all levels on the gain knob. The clean is just a two stage pre amp with a fender tonestack. I've biased the FETs using a guide from to make them sound more valve like. Thats why they aren't bypassed at all and they don't need to be anyway as there is plenty of distortion.

I should mention that the input will always be connected to both channels, the switching will be done just before the master volume.


Tubes and Hybrids / New Single End Amp
« on: April 22, 2014, 03:47:46 PM »
Hello Everyone

Today I started looking at building myself a very small Valve Amp that would give me good tone and distortion, but obviously at a low volume (well lowish :D). Anyway I had a look at AX84 and looked at their P1 Extreme. I knew I wanted to use a 6V6 in the Output Stage and a 12AX7 in the Pre Amp so this was a good start.

So I played around with their design and come up with what I've attached. Now pretty much the only things I've changed are moving the Tone Stack to after the first triode, changing the bias of the 2nd triode and lowering the B+ to 250V (should give me 4.5W according to the 6V6 Datasheet).

So what do you guys think? I'm hoping that this will give me nice power amp distortion, however the original design looks like its based on a mix of Pre Amp and Power Amp distortion so not sure if I'll get much distortion until I really turn it up. Also I've never touched a single end amp in my life so I don't know how hard to drive it or anything!

Tubes and Hybrids / Marshall DSL201 OT Possibly Blown
« on: January 14, 2014, 06:20:59 PM »
Hi Guys

My mate has a Marshall DSL201 which I think may have a shorted Winding in the Output Transformer. Now the issue he gets is when he turns the volume up above a certain level the volume suddenly goes down real low, so low you can only just hear sound coming out of the speaker. Doesn't matter if you have the pre amp volume up and master down or vice versa.

So I've gone through the amp with my oscilloscope and found the volume stays constant all the way up to the EL84's Grid Stoppers, however on the Anodes of the EL84's is where the drop occurs. Soon as you go above a certain level the output from these valves drops significantly. So I believe it could be the Transformer as I would have thought a shorted winding would have the effect of pulling down the Output of the valves. Also when the amp is played at low volume with distortion the distortion doesn't sound as good as I think it should.

Now I've checked the voltages when the volume drop occurs and the B+, Anode and Bias all stay at the same level when the issue occurs. I've also changed the valves which didn't make any difference. My mate did say that the problem started occurring when he lent the amp to a local band who cranked it for a gig, although I don't think this should have killed it as Marshalls are designed to be run at full volume. I should also add that the board inside this amp is actually for a DSL401, looks like a tech changed it a while back, most likely to overcome the Bias Drift issue that these amps are known for (among many other issues!)

Sound like Output Transformer to you guys?

Tubes and Hybrids / Marshall 1987xl Bias
« on: December 21, 2013, 01:50:41 PM »
Hi guys

I had to buy new EL34s for my 1987xl recently as one went bad and have had a few issues with the new valves. The valves are Svetlana Winged C EL34s from Karltone and get burned in for 30-40 hours. Now when i first received them they werent matched as they were about 4ma out. I didn't think that was toooo bad so biased up to 70% which was 38ma if i remember rightly and then played through it. After I played through it I found one valve had drifted and was now 10ma out, also noticed that the HT voltage was going up and down by 4 volts by itself, with no drive. So sent the valves back, got new ones and these biased up perfectly. However after a bit of playing same thing happened! Altho this time it was only 4ma and the HT Voltage didn't appear to go up and down at first but originally it was 440volts and now its 449volts. Also I found that the plate current had drifted up by 5ma. So rebiased, Swapped the valves and the readings follow the valves. When I first turn the amp on I do hear a very slight hint of motorboating, however it doesnt increase with volume and the valves arent redplating. Played with the amp for another hour, rechecked the bias and its now gone down a bit and both valves are matched again!!!! Any ideas?

Tubes and Hybrids / 18W Amp Build - Few Minor Issues
« on: November 11, 2013, 06:55:11 PM »
Hello everyone

You may or may not remember I mentioned that I was looking at building myself a valve amp a few months back, well I did and it works! I've attached some pictures and a few sound clips. I have taken photos throughout the entire build process but obviously cannot post them all here so I've just posted a few of the parts pre build and then the amp post build.

Now this is an Ampmaker PP-18 amp kit which sounds alright as it is but has a slight issue with the overdrive. When the amp goes into overdrive it seems to have a fizzy/buzzyness as the note decays. You can hear it in the sound clips I've attached to this post, especially when I switch to the neck pickup during the Tone 0 clip. Unfortunately I haven't got immediate access to a scope to see where this is happening and have read MANY different causes of buzzing and fizzing in these 18watt builds. So I thought I'd let you guys have a listen to see what you think. Some people believe that this sound is a characteristic of these amps, HOWEVER, I have heard 18w Amps not have this problem so I'm sure it can be fixed.

I should also mention that I changed the V1 Coupling Cap to a .01uF which made the issue worse (I am going to change this to a smaller value but thats a story for another day).

Here is the schematic.

Any input would be greatly appreciated.


P.S. I've also attached the schematic I drew up of the Pre Amp in LTSpice along with the Triode ASC File that I used. I've also used a potentiometer symbol in LTSpice but I imagine a lot of people already have this.

Amplifier Discussion / Serious Blocking Distortion
« on: July 05, 2013, 10:54:13 AM »
Hi Guys

I have built the circuit in this link:

And it gets pretty bad blocking distortion when you go above half way on the volume control. On full volume its so bad the amp cuts out and then comes back. Now I have tried just connecting my guitar to the 2nd JFET in the preamp and that works fine, so I think its probably the Pre Amp driving the Power Amp too hard. Any ideas? You can hear the blocking distortion start to come in a bit with the volume at half way by the way and its not the speaker as I'm driving a Marshall 2 x 12" speaker cab.


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