Phil

Now this makes complete sense as I have been looking at schematics for the Marshall 1974 (Pretty much the same amp with a few exceptions) and the 2061. Now the 1974 doesn't have grid stoppers but it only has 330VDC on the Power Amp and the Power Amp is biased a bit different (125 resistor instead of 150). The 2061 has a higher voltage on the Power Amp BUT it has grid stoppers and the PI is a bit different.

I am going to reduce the 47uF Cathode Bypass Cap to 1uF as at the moment there is so much bass even with the tone on 10 which should be really trebly! If I still have too much bass then I'm going to reduce the Coupling Capacitor back to .005uF or worst case .0022uF. I shall then look at putting in a grid stopper of 470K with a 470pF Bypass Capacitor. I plan to do it in this order as the cathode bypass cap and coupling cap are easier to change than adding in a grid stopper and should reduce the gain a bit. What do you think?

Ryan

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.

http://www.ampmaker.com/images/ak03kit/ak03sc2.jpg

Any input would be greatly appreciated.

Ryan

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.

Ah right I see I shall look at painting the bolt for the transformer and using a P Hoop.

With regards to the sockets conducting with the chassis that could explain something, as I have found that I sometimes get a bit of hum until the lid is on, would this be connected?

I shall keep you all updated.

Thanks Roly

Thanks Roly

I will get some heatshrink on that as I must admit I do get bit nervous when I have to go near that switch when working on the amp whilst its on .

Now do you mean the bolt and the washer so it doesn't conduct with the cover on top of the transformer?

I'll take a look at that as I'll probably have to drill a small hole for the P Hoop.

I can get hold of star washers so thats not a problem .

With regards to the Amp common it is only connected to the chassis near the PSU, unless I'm missing something?

I did have some issues with heat in this amp with the voltage regulator, it seems to be coping now however I obviously haven't tested it in muggy, sweaty gig conditions. I did try and find a chassis with vents but struggled surprisingly. Will definitely be considered next time though.

My next project is actually going to be my first ever valve amp, although it won't be from scratch it will be a kit from ampmaker.co.uk, the PP-18 to be precise. I want to start off with a kit as I've never built a valve amp before and making mistakes in valve amps can be very expensive. If you make a mistake in an SS build then you might blow a couple of pence of components (Unless of course you cook the mains transformer or somehow destroy the chassis), if you make a mistake in a valve build and blow components there will be tears (Valves, Output transformer etc). Also if I start with a kit then I'll have a good sounding amp as a starting point for many modifications.

Like you said, amp building is as addictive as crack

Hmm I'll have to have a think as I don't really have enough room to add an extra three knobs for tone controls as I wanted to keep the amp side of things as far away from the PSU as possible. You'll see what I mean in the photos attached. I could perhaps double up the switch so as I switch between different JFET Source Capacitors it also switches between different RCs for tone......

Roly how did you get the TDA2050 chip into LTSpice? Been trying to figure a way for ages!

Worked out how to include an opamp so I can see the extent of the issue and now it has all become clear why the amp sounds like it does! So to begin with I'm going to change the shunt resistor to 1K and the Feedback resistor to 18K and see where that gets me. I should have about 0.8V before clipping then.

In the end I just replaced the 680ohm shunt resistor with a 1K5 and now the Chipamp can take a nice 1 volt before clipping. The result......a now fantastic sounding amp! It stays pretty much clean with no source bypass capacitor, starts to distort slightly at about 9 on the volume but not much. Then with a bypass capacitor it starts to distort at about midway on the volume. Perfect .

Thanks Phil, means a lot .

Roly, as usual very informative. I will say that the amp did sound best to my ears when the capacitor wasn't in the circuit with the volume turned up, I guess thats because it isn't driving the chip quite so hard. Now if I change the 22K resistor to 18K then that will lower the gain to about 26.4 which will then give me 0.56818 volts peak, however I'd probably lose a bit of volume so I'd then increase the supply voltage to perhaps 32volts to make up for it so then I'd have 0.6 volts peak before the chipamp starts clipping. How does that sound? Am I going in the right direction?

Hang on a second, according to the datasheet in the single supply circuit if I decrease the size of the 22K resistor then that increases the gain? So either the datasheet is wrong or we're using the wrong equation to work out the gain.

Ah no apologies Roly, whoever wrote that datasheet got it wrong. Checked one of my books and checked the TDA2030 Datasheet and decreasing the 22K resistor does decrease gain.

So looks like if I decrease the gain slightly and then up the supply voltage by 2 volts then I could reduce the clipping on the input of the chipamp.

How would I lower the distortion in this circuit? Is it just a case of trying different JFets? Or if I increase the IDSS Current then the JFET will be able to handle more before distorting?

Brand new, electrolytic by the way, the .22uF is a Polyester I think. Its Orange and Rectangle shaped

Well guys I created a few sound clips, unfortunately they were a bit rushed and recorded using my iphone. But they should hopefully give you an idea of how the amp sounds. These recordings are only of the .68uF capacitor setting.

Also it no longer pops or clicks when I switch between capacitors, added a 100K resistor between the source and .68uF Cap and a 820k between the source and .22uF cap. Reason for the different resistors is the 820K worked for the .22uF but wasn't enough for the .68uF, so I just changed the .68uF's cap and left the .22uF's as is.

Ah good idea Roly. I was going to ask about the clicking, I thought it may of been due to the capacitors quickly charging up and could be solved by having a small resistor in series with the source and switch, but your solution sounds like it'll actually work . I'll implement this fix and will get the sound samples soon as possible. Unfortunately as I still live with my parents and they aren't big fans of loud guitar so I'll have to wait until they're out.

I believe I finally have the amp the way I want it. I tried the gain control Roly but didn't like the way the sound went very bassy when I reduced the gain. So I used a switch to go between two capacitors. I'll attach a schematic to this post. Basically one capacitor gives me less bass and in turn less distortion, whilst the other gives me pretty high distortion. Also the switch I used had a middle position and this just leaves the source resistor by itself without a bypass capacitor.

O right sorry didn't see that!

This is very true. When it comes to overdrive/distortion no one can really describe it as it'll be an opinion not necessarily shared by others.

Ah i see now. Shouldn't i use the configuration hes using on his JFET source? I see what you mean about replacing the JFET Source with a pot which would work, until the pot is turned all the way and the cap is effectively shorted. I'm on holiday this week so can't try anything until i'm back on Saturday but will definitely give this a go. Also i've done a bit of reading about valve sounding JFET preamps and it seems the best way to achieve this is by using multiple JFET stages all softly clipping the waveform.

I tried this in LTSpice, starting from 6K2 on the JFET Source and then lowering the resistance as a Pot would and this just seemed to increase distortion. I think I may just put the BJT back how it was minus the Emitter Bypass Capacitor and just keep the tone control where it is. As in this configuration I lose very little gain and the tone control works well. I've attached the circuit I used in LTSpice to test this.

I think i want to remove the Tone Control Stormbringer. Do you think what i proposed will work?