Modified Weber 6M36 questions

[afinitemind]

Hi y'all,


A friend bought a 6M36 kit on a whim a couple years ago...after having it sitting in a box in his garage, he enlisted my help to build it in trade for some design work. So I started by kicking all the jacks and pots to the curb and getting Switchcraft/CTS parts. Orange drop caps, CC resistors at his request. Not the best iron, but it's what we've got.


I've built a super simple 18W with no preamp tone control, just a cut control after the PI, and he really likes that sound. Any reason that wouldn't work here?


Also, to that end, what about grabbing the AC30's PI values? Schematic attached, my possible changes in red.

[afinitemind]

Reading around the site, I found the link to sluckey's 1960 AC15 and some discussion about making a simple AC30 in the same vein. Looks like this has been done before, so I've got plenty of info to refer to as I start this build!

[afinitemind]

Alright, got this thing up and running(ish). Used the schematic on the original post. I measured & matched all my resistors before soldering them in.


Both channels sound like I'd expect at low-to-medium volumes. When I start to turn the volume knob up into overdrive (60-80% rotation), there's a really harsh, buzzy, almost crackly tone. I can make the trouble go away on the TMB channel by turning the bass way down.


Voltages attached. What am I missing?

[shooter]

almost crackly tone.

the crackly is  pretty typical of something not making good contact, cold solder, loose pin, bad wiper.........

using a wood stick, wiggle/giggle wires around the bass pot, thump on the pot, jiggle tubes

[afinitemind]

I've poked and prodded it quite a bit...all solder joints are tight and shiny. And I've tried a different set of known-good tubes throughout. The chassis is on my bench and not physically coupled with the speaker cab in any way, so it's unlikely that vibration is causing it.


The issue happens on both channels when I turn the channel volume to 60-ish percent. It's consistent enough that it seems like something circuit-wise is off, not just loose. It's not actually a crackle, really just sounds like bad distortion rather than good overdrive.


I'll make a quick audio recording this afternoon...probably way easier than trying to describe what I'm hearing.

[shooter]

like bad distortion

so how much signal are you slamming the 84's with?  It doesn't take much, if you're feeding them square waves they're probably are getting really cranky

[afinitemind]

so how much signal are you slamming the 84's with?

I wish I could answer this question, but I don't have an oscilloscope. Any way to check this without one?

[sluckey]

You can if your meter can measure AC RMS voltage. Multiply reading by 2.828 if you want to convert RMS to peak to peak. You'll also need a sine wave source.

[pdf64]

How about if the instrument volume control is turned down - down the nasty sound stop or is it still there? If yes, there may be an inadequate margin of stability.

[afinitemind]

You can if your meter can measure AC RMS voltage. Multiply reading by 2.828 if you want to convert RMS to peak to peak. You'll also need a sine wave source.

Alright, so I'm inserting a 1Khz sine wave at the input, measuring 235mVAC (665mV peak to peak?) just after the 68k input grid stopper. When I turn the volume up into the range that's problematic with my guitar, I'm measuring ~22-23VAC at the power tube grid stoppers (diming the amp's volume takes it to ~27VAC).


So, assuming I measured correctly, does that seem pretty dang high? Multiply 23x2.828 and I get 65V peak to peak...

[afinitemind]

How about if the instrument volume control is turned down - down the nasty sound stop or is it still there?

Roll back the guitar volume and the nasty sound goes away. It really only happens when I’m wide open on the guitar and digging in with the amp volume past 60%.

[shooter]

look at the voltage at the Cathode of the 84, I think  you had ~ 9.1v, anyway that is basically where clipping/compression starts, by double that you are squared out.
A big bottle sometimes sounds ok at square, I've not liked it in small bottle tubes ike the 84

[afinitemind]

Alright, I think I might be learning what blocking distortion sounds like... I've never heard it before, so I'm kinda guessing based on reading various forum discussions on the subject. Hoping y'all can advise me on what might work.


Based on some reading, I just tried lowering the grounded resistors before the power tube grid stoppers from 220k to 110k (paralleled another 220k on top of each) and the ugliness is *almost* non-existent now. I'm in the 'know enough to be dangerous' situation here - I don't entirely understand how that changes the function of the amp. Can anyone enlighten me?


I might push the grid stoppers back up to the schematic's 5.6k (currently 1.5k installed) and see if the last little bit of trouble goes away.


Thanks for all the help so far - I'm learning and I appreciate everyone's insight and guidance!

[shooter]

read this (a gazillion times:) ;
http://www.valvewizard.co.uk/acltp.html

Then you will be dangerous WITH understanding 

[afinitemind]

read this (a gazillion times:) ;
http://www.valvewizard.co.uk/acltp.html

Then you will be dangerous WITH understanding 

Will do, thanks for the link.


Are you thinking the PI is pushing the PA too hard?

[nandrewjackson]

My 2 cents


Change c1 and c14  to something higher. Try 0.01uF or 0.02uF.  Let more of that 1st triode sound come through.


Add a post phase inverter  master volume.


With  bigger c1 and c14 values, you'll get the right preamp overdrive, and the PPIMV will keep the 84's reigned in.

[afinitemind]

Finally got a chance to record the noise - Listen Here. Using the non-TMB channel, Volume is at noon.



I've been digesting that Valve Wizard LTP article. Went ahead and replaced the PI tube with a 5751 prior to making this recording - my understanding is that the 5751 should put a lot less into the EL84s, so I'm still confused. For the sake of testing, all resistor values were set back to those on the above schematic.


I get the same distortion/splat with a different speaker cab, so it's gotta be something in my circuit.

[nandrewjackson]

That splat sure sounds like a busted speaker, but you said you tried different speakers. 


 

[tubenit]

Your chassis photo shows two 50ohm/10w resistors.  What does each of those go to, please?

And am I understanding that you built this with two EL84's and not four?  Is that correct?  So, are you running 100 ohms for a pair of EL84's?

With respect, Tubenit

[Ed_Chambley]

Are your power tubes red plating? Tubes crackle when hot. What dissipation are you biased?

[sluckey]

Those output tubes are biased extremely hot. You're killing them. That splat is likely a death cry.

That 50Ω cathode resistor (the one on the end of the board is meant to bias 4 tubes, not 2. Change that to 130Ω or 150Ω and recheck voltages. Does the amp sound better?

[afinitemind]

Sorry for the confusion - it is indeed 4xEL84 in the power amp. Tubes haven’t been redplating.


Will recalculate bias and report back.

[afinitemind]

Attached tonight's voltage measurements. Schematic is the same as noted in the first post on this thread, except I've got a 12ax7 in V1, 12AT7 in V2, 5751 for V3 (PI).


Cathode resistor measures 54ohms - shared between 4 EL84s. Cathode is at 9.47VDC, Plate-Cathode measures 289.5VDC.


My math shows ~12W/tube. 42ma plate current. So definitely hot at 100% max dissipation, maybe not 'splat & fart' off the edge? I'll try backing that down.

[sluckey]

it is indeed 4xEL84 in the power amp

Then that's OK.

[afinitemind]

Finally sorted it out, and of course it's much simpler than I expected - I just had a bad set of EH EL84s. Didn't expect it as they were new when I installed them, but I dropped a different set in the amp and the problem went away immediately.


All good with a replacement set of tubes...Lesson learned: new doesn't always mean the tubes are good.


Thanks for all the help working through it!

[afinitemind]

What about a switch to kill one pair of EL84s for half-ish power operation? Obviously it'd kill one tube on each side of the PP pair.


So, split the EL84 quad into pairs (one tube from each side), each pair gets its own 120-150ohm cathode resistor. Then a DPDT switch to a) lift the ground on one cathode resistor and b) kill the B+ to that tube pair?

[tubeswell]

You can do that by using a DPDT switch to disconnect the cathodes of 1 pair (i.e. to stop them conducting). (Disconnecting the B+ for the pair is more difficult because you need to switch off the plates and screens for one pair.)


You also probably want to alter (double) the speaker impedance to get the appropriate load resistance seen through the OT for the remaining 'on' pair.


As you've seen, since the 6M36 uses a shared 50R screen resistor for all four EL84s, so you probably should install separate 100R - 1k screen grid resistors on each EL84.


At the end of there day, you will only get a volume reduction of about 3dB by switching off 1 pair.


A better (more effective) type of simple switch attenuation is a '1/4 power' switch using a pair of big power* resistors bolted to the outside of the chassis, like Fender does on their EC-series tweed amps. - See the EC tremolux schematic for details. This way you will get audible attenuation without having to worry about changing the load resistance (or the overdrive characteristics of the pre-amp and output stage).


*In the EC tremolux, Fender uses a pair of 25W wire-wound ceramic resistors. The SPDT puts 8R in series with the 8R speaker in the amp, and 16R in parallel with both of those (meaning the speaker sees 1/4 of the output power). In the Twinolux, they use 3 x 25W resistors and a 3PDT switch to get a 1/2 power and 1/4 power thing going, so its a tiny bit more complex

[Tony Bones]

Or just buy an L-Pad from Parts Express for $12.95.

https://www.parts-express.com/parts-express-speaker-l-pad-attenuator-100w-mono-3-8-shaft-8-ohm--260-262

[afinitemind]

you probably should install separate 100R - 1k screen grid resistors on each EL84.

Will do regardless of any half power switch. Know if I can install them directly on the EL84 sockets? On the datasheet for my EH EL84s, pins 1, 6 & 8 say "don't connect" but show nothing of note on the diagram. I'm thinking no, but man that would be the easiest place to mount those Rs.

[sluckey]

I would not mess with the spare pins on an EL84/6BQ5. There never has been a consensus on how to treat those extra pins. Some are internally connected to other pins. See attached pic.

As far as half power switch, look at page 3 of this pdf. Da Geezer’s Bypass Cap attenuator may be an option worth considering...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[afinitemind]

Excellent, and easy. Thanks for the suggestion!

[afinitemind]

As far as half power switch, look at page 3 of this pdf. Da Geezer’s Bypass Cap attenuator may be an option worth considering...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

I'm going to try this tomorrow. I settled on a 150ohm cathode resistor for each pair of EL84s. Looking at that Bias Cap Attenuator, seems a pair 100ohm resistors with an on-on DPDT would get me in the neighborhood of half power on this setup?

[afinitemind]

Checking back - the 100ohm R's for bypass cap attenuator work perfectly alongside the 150ohm cathode resistors. The switch provides a noticeable drop in volume and a nice change in saturation. Lots of very useful tones on tap!


And with that, I'm done with this build. I'm introducing my buddy to his new amp tomorrow night. Pretty sure he's gonna dig it.