Marshall 18 watt. One tube red plating

[evirob]

Hi, I'm new here although I have spent some time reading topics on this forum, this is my first post. I have a problem with one power tube red plating, I hope someone with more experience can help me

A little (OK, a lot of) background. I recently completed scratch building a clone of the Marshall 18 watt (lite IIb variant) and I've been through a number of issues. Please excuse the length of what follows, but I want to attempt to share all the pertinent information in the hope it helps someone make a remote diagnosis.

After performing all the usual power-on tests, I was initially pleased with the sound of the amp. It has a lovely clear open tone at low volume settings. Unfortunately, problems started when I cranked the volume. The overdrive sound was, in a word, horrible. In detail, it was thin and raspy, or you might say buzzy. It had a distinctly fuzzy edge, with an octave overtone (it literally sounded like an octave fuzz pedal) and the harder it was pushed, the worse it got, especially when the tone control was set to the treble side. Maxed out, not only did the volume seem underwhelming, there was also a metallic sounding rattle. I actually tried it with another speaker because the rattle sounded so mechanical, but to the same result.

Of course, I have read a lot on this subject - the 18 watt buzz is well documented and so I set to work trying the various mods and fixes people have suggested.

Before that though, I should say when I first ran the amp, it had one old stock Mullard EL84 and a brand new JJ - just because those were the tubes I had to hand. When taking plate voltage readings I noticed the Mullard was running about 5 volts higher than the JJ. I thought this was down to variation in current draw, and the poor sound could be attributed to the wildly mismatched tubes so I ordered a brand new matched pair. But they would take a few days to be delivered and I couldn't wait.

So first up I installed a conjunctive filter. Lots of people swear by this as a buzz fix, but all I can say is they must have a different type of buzz, because it made no appreciable difference.

Second I dropped the grid leak resistors on the output tubes from 470k to 200k. This actually helped, but only because it prevented me from driving the power tubes as hard. A sticking plaster solution you might say.

Third I increased the cathode bypass capacitor from 47u to 470u. Some folks said this was a buzz-killer, whilst also tightening or "stiffening" the bass response. It did the latter but not the former. Still, I liked the stiffer amp and left it in.

Fourth, after reading somewhere that EL84's like to be run hot (and seeing it in many EL84 power sections, old and new) I dropped the shared cathode resistor from 150r to 130r. I think this actually made the amp sound a little better, but not much. I also thought I might be detecting a little red glow from the plates of the EL84's, so I replaced the 150r.

By now my matched tubes arrived. Whack 'em in, now the amp is maybe a little louder... but still a buzzy, rattly, stinkin' mess

I didn't have any zener's to hand so had to order some up, and while waiting it occurred to me I hadn't checked to see if the EL84 plate voltages were now in sync. They weren't. One was still running 5 or so volts higher than the other 345v and 350v.

I swapped the tubes over and discovered it was definitely the socket at fault.

My first port of call was to replace the 10n coupling caps between the pi and the power section as I have read that if they are leaky, DC can get onto the grids of the EL84's and put the bias out of whack. No joy.

I thought - I could chase down every component in the amp, but that could be a wild goose chase - so I just switched the wires coming from the OT primary to the EL84 plates, and hey presto - now it's the other socket running the higher plate voltage. So it's definitely an imbalance of some kind in the OT. But, I read that this can be expected within reason. Is 5 volts difference within reason? I know people run EL84's way higher than 350v so I figured it was safe enough. Was it affecting the sound though? Which still stinks.

Ok, so I know if you've read this far, chances are you're going to be thinking one thing - Ruby mod. Just to say I was saving this as a last resort because I have a power scaling solution and the Ruby mod loses effectiveness as the power reduces. But what the heck, I've tried everything else I can think of and I want to rule it out if nothing else.

I am happy (sort of) to say the Ruby mod had a huge effect on the sound. The amp now sounds great at all volumes (unless I scale down the power) and is loud as hell. My amp was clearly suffering from a bad case of crossover distortion. But, if I'm brutally honest, although it sounds great, it's still not the amp of my dreams that I hoped for.

Anyway, I was testing recently and by coincidence turned the lights off while the amp was on and idling, only to see what seems to be a faintly glowing plate on one of the EL84's. It's so faint I wasn't even sure of what I was seeing so I took some pictures with the intention of reaching out to the interwebs for advice, and on-camera the red plating is considerably more obvious than to the naked eye, which I found odd and alarming. Of course, the tube that is red plating is the one that is running at 350v.

Here are a couple of photos:

[img1824]http://www.shynola.com/chris/stomp/rpdark.jpg[/img]
[img1824]http://www.shynola.com/chris/stomp/rplight.jpg[/img]
So finally to my questions!

Firstly the output transformer. Could it be faulty or is this within expected margins? For the record, it's a Hammond 1760e, designed as a replacement for Fender Princeton's/Tweed Deluxe's. Maybe 6V6 tubes don't mind the extra voltage so much, so the margin for error is greater? I don't know enough about transformers.

I know the 1760e is not ideal for an 18 watter, but I had it for another project that never materialised, and my research (googling) led me to believe it was usable. It has an 8k primary, which seems to be the accepted norm for 18w variants, and while only being rated at 15w, I have read various accounts of people pushing it to 18 or even 20 watts with good results.

What's up with the EL84 red plating whilst idling at 350v? Could there be something else going on in the amp besides the slightly high voltage that is causing this?

Any light that can be shed on this would be most gratefully received. Because at this point, all I can think of are drastic solutions such as replacing the output transformer or even modding the amp to take 6V6's, and I'd really rather not if possible!

Thanks for reading.

[xtian]

Gotta run, so just a quickie: Tubes redplate from too much current. Two EL84s in my 18-watt style builds usually get 180R or higher cathode resistor to keep them from burning up!

Cheers, welcome to the board, more later.

[labb]

I don't think that the 350 vdc plate voltage is a problem. Peavey runs EL84's at 400 vdc in their classic 50's. Screen voltage should be looked at. You can check the winding resistance in the OT and see how close they are. Check the 10n coupling cap that goes to that socket. Also check to see if you have any DC voltage on the grid in that position

[menger66]

Or the tubes may be weak, depending on voltage @ el84 pin3 an. ything under 11 volts is a bit too hot for some tubes though ive never had lssues with jjs, ?

[Phil_S]

You have a lopsided transformer. It is normal for one half to have higher DCR than the other. That's because the outer half of the winding needs more wire length to cover the same number of turns. It is the number of turns that govern operation. I would contact Hammond and tell them your findings about the 5V difference and swapping the wires. See if they will replace it.

Short of that, you may want to bias each tube with a separate cathode resistor, which should be 250-300 ohms for each tube and tweaked to make them balanced.

As for the buzz, I recommend the zener fix. It worked very well on mine.

Reducing the 470K to 200K changes the character of the amp considerably, but I doubt it will address what you describe.

[Richie]

If you swap the EL84s in the sockets, does the red plating stay with the same tube? Or does it make the other tube in that socket replate?


When the tube red plates, the amp will start to drop in volume, and sound dull or fuzzy/distorted sounding.

Got any pics of the chassis and wiring? Voltage chart? Have you moved the chassis out of the combo. Sometimes the caps can vibrate and cause the metallic buzz sound,or loose connection.

Best to find the problem causing the redplating tube.

[tubeswell]

What Richie said. First thing to do is swap the output tubes around to find if its the tube or the circuit - before you go any further.

[romberg]

Gotta run, so just a quickie: Tubes redplate from too much current. Two EL84s in my 18-watt style builds usually get 180R or higher cathode resistor to keep them from burning up!

Cheers, welcome to the board, more later.

+1. The el84 has a maximum rating for 12 watts of plate dissipation. I know it is considered fashionable to run them in excess of this. But you are over the line. Even though your amp is cathode biased you can still check at set the bias by adjusting the cathode resistor.

What is the voltage across this resistor? You can use this measurement (and the resistors value) to find the combined plate and screen current. With the current, plate voltage (and cathode voltage) you can find the power being dissipated. My guess is that it is way over 12 watts.

Increasing the cathode resistor will bring it down to something more reasonable.

Mike

[pdf64]

Maybe the amp is oscillating?
Have grid stoppers been implemented and if so, how?
The g2 screen grid is a grid and the regular current limiting resistors also act as grid stoppers, so consider using normal good practice of mounting them at the tube socket.

[evirob]

Thanks everyone for the helpful responses.

I fired up the amp before working on it and cranked it, and was suprised/disappointed to discover the horrible buzz was back even with the Ruby zener fix in place. Considering the buzz had mostly gone away immediately after installing the zeners, I'm a bit mystified, but there must be an explanation, so we press onwards!

labb, thanks for the tip on the Peavy. I checked the screens and while I can't remember the precise reading, they were safely below the plate voltages.

Phil_s, thanks for the info on transformers. I bought it some time ago (over a year I think) for a project that never materialised, and it has sat on my bench til now. I will contact the reseller (I'm in the UK so will go through them before I go to Hammond direct) and see if they are sympathetic, but since it's well outside their usual returns period I am not optimistic.

In the meantime I tried your suggestion of individual cathode resistors to account for the lop sided transformer. I started with 300 ohms each, and with these in place I did the maths for each tube. The one with the 350v plate was indeed exceeding the dissipation limit. In fact they both were - one was at 12.5 watts, but the hot tube was over 13 watts, so I guess that's the root of the red plate.

So I tweaked the resistors and got the dissipation down to 12W on the colder tube and 11.5W on the hotter tube. I will tweak the values further until they match, but for now I wanted to see if either tube was still red plating. Which is was. Frustrating. I swapped the tubes over and this time the red plating followed the tube, not the socket - I call that progress

These are brand new matched tubes, so either I have one that was faulty out of the box, or it has been damaged at some point by the excessive dissipation. Anyway, as luck would have it I had another JJ EL84 hanging about. I swapped it for the red plating tube and hey presto - no red plates any more. Unfortunately by this point it was late in the night so I had no chance to crank the amp and see how it sounds for fear of waking up the whole the neighbours kids, never mind my own.


Richie and tubeswell - I think it was the tube AND the socket causing the problem. I don't have any good shots of the wiring. I will take some this evening. I hope they are useful - I have used my own layout.

pdf64 I have grid stoppers and I did indeed wire them directly to the tube sockets. One thing I was going to try but haven't yet is to increase their value as some suggest that 20k or upwards could be appropriate. Otherwise, how would I tell if the amp is oscilating?


Thanks again for the help. I will report back when I've had chance to crank the amp.

[romberg]

got the dissipation down to 12W on the colder tube and 11.5W on the hotter tube.

This may still be biasing too hot. Usually you see the rule of thumb to bias push pull tubes around 70% which for el84s would be 8.4 watts). Merlin says you can go up to 85% which would be 10.2 watts.

http://www.valvewizard.co.uk/pp.html

Since the amp is already exceeding the max plate voltage (300V) why push the max power dissipation too? If it were my amp, I'd increase the value of the cathode resistors further.

Mike

[randalp3000]

got the dissipation down to 12W on the colder tube and 11.5W on the hotter tube.

This may still be biasing too hot. Usually you see the rule of thumb to bias push pull tubes around 70% which for el84s would be 8.4 watts). Merlin says you can go up to 85% which would be 10.2 watts.

http://www.valvewizard.co.uk/pp.html

Since the amp is already exceeding the max plate voltage (300V) why push the max power dissipation too? If it were my amp, I'd increase the value of the cathode resistors further.

Mike

I've got a 100+ hours on a set of JJ's running at lease 12 watts/100%, 330vdc. I think the 70% rule is geared more towards fixed bias amps.

[romberg]

I've got a 100+ hours on a set of JJ's running at lease 12 watts/100%, 330vdc. I think the 70% rule is geared more towards fixed bias amps.

Good point. I stand corrected. Here is a bit from someone who has more experience than I:

http://www.aikenamps.com/index.php/the- ... on-biasing

Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.

If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation.

So maybe these particular tubes just can't be biased that hot (try a new set) or the bias needs to be backed off a bit.

Mike

[evirob]

Yes, I was going by the pp 18 guide in this link:

http://www.ampmaker.com/pp-18-final-testing-1045-0.html

I don't actually have one of his kits, but the schematic is the same and he recommends anything up to 13w, and in fact he suggests not to go below 10w for the sake of tone.

This is my first pp build and I'm happy to be corrected. I do want to preserve the tubes but above all I'm trying to make a great sounding amp.

[evirob]

I had the chance to crank the amp this morning, and now the tubes are dissipating at a more reasonable level I was hoping for improvement. Well, the sound is much better. Back to how it sounded right after the Ruby mod went in, which is to say the buzz is gone.

But I still don't believe this is quite how a Marshall 18 watt should sound. I have played a Watkins Dominator and the tone (subjective judgement) was in a different league. While the worst artifacts in my amp have been dealt with, I would still describe it as harsh and brittle, and just generally not nice to play. Clean still sounds fine, BTW.

Here is a picture of my wiring in case anyone can spot any obvious issues. As I said, I used my own layout - please ask if anything needs clarification.

[img1824]http://www.shynola.com/chris/stomp/mar18.jpg[/img]

The big ugly bundles of resistors in the middle are the bias resistors. I was tweaking the values by adding more in series, and I haven't got round to tidying that up yet.

I am beginning to wonder if this is all down to the output transformer. Does the lopsidedness indicate a fault? Are there any tests I can perform to check the integrity of the transformer? I don't have much in the way of test equipment though, basically just a DMM.

My other thought, assuming it's not faulty, is it's just not fit for purpose. The Hammond 1760e is sold as a replacement for Fender Princeton/Tweed Deluxe amps. It was presumably designed to work with 6V6 tubes with much less overdrive in the power section. I don't know much about transformer design. I thought the 1760 might be ok for the job having 8k primary, but I have read in a few places that the 18 watt buzz has been tracked down to the transformer and the Heyboer design and its derivatives have been identified as eliminating buzz issues. The only technical explanation I could find is here:

http://www.ampmaker.com/store/18W-push- ... ormer.html

The seller describes his transformers, which are based on the Heyboer, as having "grain oriented lamination". Can anyone shed any light on this issue?

Unless someone can see a glaring issue with my layout or lead dress, I think my only recourse at this point is to try a different OT, which is a pain in the wallet