Sorry for the very late response, been so busy recently. Ok I finished modifying the amp the other week and now it sounds great. I've attached a modified schematic to this message, the only thing I didn't add to the schematic was the Paul Ruby Buzz Fix which I implemented. If you google that then you'll probably find it and for the record I used 15.1V Zener Diodes with 1N4003 Diodes. Also I added a 270pF Bright Cap to the Volume Control.

Well definitely improved the sound anyway, the capacitors i ordered still haven't arrived yet so can't do anything with the PI anodes yet. Was thinking about getting extra distortion out of this amp by cascading the first preamp valve but due to the layout it wouldnt be an easy task.

Also need to lower the bass a bit, always got the tone control on 10 and on input 1 (single triode) its still too bassy! Going to try lowering the .1uF in the PI to .01uF and maybe the .022uF to .01uF and then may change the .0005uF on the Tone Control to 10nF. I think the first two cap changes will just tighten up the bass a bit and the tone control cap change will make the sweet spot a bit more central on the knob

This one is just before the 22nf cap going to the PI, would it not be called a Grid Stopper there?

Also i put the resistor there to stop high frequencies above 5Khz being as present as they were, so with the 470pF it wouldnt do that. So i'm just wondering why you'd bother having it there in the first place? As it wouldnt be doing it's job anymore surely

Thanks for that Roly I'll give that a read as I do plan on building another amp (Like you said addictive as crack) next year from SCRATCH! So as you can imagine I've been looking at plenty of different circuits working out what does what.

Now an update on the amp I think the Paul Ruby Fix helped lower the buzz although its still there, I then added a 470K Grid Stopper and I think this improved the amp sound overall. Unfortunately its difficult as once I've finished testing the amp I have to wait for the EL84s to cool down so I can take them out otherwise I can't turn it upside down to work on it, by which time I've almost forgotten what it sounded like! Anyway the amp sounds better to my ears, probably because the 470K should stop a lot of the higher frequencies from getting through so there is less going on if you know what I mean. Once the capacitors that I've ordered arrive then I will be able to cancel out the higher frequencies in the power amp even more so hopefully this will improve the sound further.

Now one thing that puzzles me is looking at Marshall's 1987xl (one I own myself) they have the 470K Grid Stopper in the Pre Amp bypassed by a 470pF capacitor. Now according to LTSpice this basically allows everything through that the 470K Grid Stopper is put in to stop. Either way it shouldn't really need bypassing, sure the 470K does cut into the 5Khz range a bit but in my case the amp sounded better. Any ideas?

Phil, quite an essay there, must admit I read just after I woke up and couldn't take it all in so had to reread it after breakfast .

With regards to the guitar pickups I did actually install Tonerider Rocksongs in my guitar not too long ago and immediately found that my Marshall distorted more at lower volumes. As for the grid stoppers those are on my list of things to try, but first i'll finish trying the Paul Ruby fix as i'm halfway through that.

I'm using a Marshall 2x12 cab for this and when I scoped the amp I used one of my Grandad's Hi Fi speakers and found the fizz to be a LOT worse. Only issue with trying a different OT is I'm not quite sure what I'm looking at yet with OTs, haven't got that far yet!

One thing I did find that sounded nice last night was the conjunctive filter in place with the Paul Ruby Fix. However I've disconnected the filter for now as I just want to try the Paul Ruby Fix without it. I'm getting the correct Zeners tomorrow so tomorrow night I'll be trying all of these mods.

With regards to the layout you're definitely right, I managed to find a drawing of the Marshall Layout and whereas they do have two ground lugs in the chassis, there is clearly one main ground lug that most of the ground connections go to.

I did think about a Cap on the PI tails, however I was finding it a bit difficult to find a cap with a voltage value that high, going to have a look on ebay again. 100pF and 200pF with 1KV rating on order

In the meantime I tried out the Paul Ruby Zener Fix on the amp, but unfortunately.............I only had one 8.1V and one 9.1V Zener Diode, whereas really I need two 13.1V (minimum) Zeners. So as you can imagine this added a bit of distortion (which didn't sound too bad tbh!) making it hard to hear if I'd fixed the issue or if the extra distortion has just masked it somewhat.

Going to get hold of the correct Zeners on Monday to try it again. With regards to the grounding however, basically I've just got to rewire the amp so it uses only one ground lug, is that right?

Also Toby I forgot to mention, I know you used my audio clip in Cool Edit to get that WaveForm, however, I did record that audio clip with my iPhone and additional buzzing could be caused by the microphone on the iPhone clipping as I did have it a bit close to the amp. Although I must say I was quite impressed by it!

Just realised this topic is in the wrong place! Could a moderator move it please?

Good to hear from you Roly!

Off the top of my head I believe there are three grounding lugs in this amp which are per the kit instructions. The Grounding Lug near the Input Jacks grounds the ground line on one half of the turret board and there is a ground cable going from the input jacks to that side of the turret board. I did check all of the connections as I built the amp and all were good, also checked the grounding again on the input jacks and they are grounded when nothing is plugged in.

I did read a few topics over at 18watt.com where a few people had similar issues but they'd done different things to fix it. Barry at Ampmaker believes that it is a characteristic of the amp but it can't be, I've heard Amps that are of very similar design and they sound nothing like this amp. The only thing that does sound similar to this amp is one of the sound clips on ampmaker.com, you can hear the fizzy noise in it.

I've been comparing my amp with the Marshall 1974x which I've heard clips of and has no fizzyness.

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/marshallamps/marshall_18watt_schem.pdf

Only real differences that may make a difference are the HT Voltage, Tone Stack, Capacitor between Tone Stack and PI and the capacitor on the otherside from the Tremolo Channel (to ground in my case).

 <3)I've checked the Input Jacks and they are fine, it does this fizz noise on Input 1 as well. Now I've tried the conjunctive filter and it does seem to help a bit. Also tried a Zobel network but that didn't seem to do much.

I might try lowering the Plate Voltage from 350V to 280V just to see if i still get the issue. Other than that looking at other schematics my amp isn't much different from others so cant see why it has this issue. Unless its something like the EL84s or the OT

Right we scoped the amp and there is no oscillation. Altho we did get it to oscillate a few times with no input and the volume turned up but that disappeared as soon as we plugged something into the input. We found The signal didnt distort until the PI output and then again a bit more on the Power Amp output. Going to try the conjunctive filter when ampmaker send the parts.

Ah yes that was what I needed to look at, what probes I can get for it. I would go for a proper scope to sit on my desk but I just haven't the room and I don't think the cheaper ones include as many features as the USB Picoscope.

Yes he has a signal generator as well. I'm actually looking at getting a Picoscope 2204 for xmas as these have a signal generator built in. Do you have any experience with these?

Toby

My grandad has a scope, i'll see if i can pop over there tomorrow night to use it. How did you put a scope on my audio clip?

Ryan

Sorry I had part of that conversation in my head, I was talking about the 470K Grid Stoppers you sometimes have before the PI. Am I right about grid stoppers not reducing gain?

I changed the first stage Cathode Bypass Cap to 1uF and now the amp has just the right amount of bass, although I've only tested it at bedroom volume levels so far with an attenuator. I've left C2 as .01uF for the moment. I see what you mean about C5, not entirely sure about that as at .022uF its not really doing anything in the way of tone shaping as its letting the entire guitar freq range through it.

The guy who designed the amp is sending me the parts for a conjunctive filter on the OT as he believes this should tame the fizz. I have asked if he could also send me a 100pF Cap in case the conjunctive filter doesn't do the trick and I've asked for a 120ohm resistor in case I want to experiment with the bias of the power amp.

Ryan

Although Phil it was my understanding that Grid Stoppers dont tend to reduce gain due to the miller capacitance. They might reduce treble slightly but not normally gain.

Changed the Cathode Bypass cap btw to 1uF and now the tone is great! Its just the fizz thats the issue now -_-