Taming the 18W

[Lectroid]

Last winter, my build was a Marshall 1974-ish 18W design from tubenit, with added PPIMV and 1-tube reverb.  I'm after a blues-oriented sound with some Marshall overdrive goodness.  Think Eric Clapton/Bluesbreakers and you'll get what I'm aiming for.  Attached is the schematic showing my amp with component values and voltages, and a copy of tubenit's original design.

I like the amp overall--lots of OD available, and lots of overall power.  Its one big flaw is that it pushes into overdrive sooner than I like--not much useful clean volume at all unless I keep my guitar volume very low.  It starts breaking up around 2 on either Volume or MV.

On the 'scope the waveforms slip from smooth into distorted very fast when turning up either volume pot.  If I keep both volume knobs low, I can get a good clean sound, but only at a low volume.

Question: Why does it overdrive so soon?

Is this just how this little Marshall normally behaves? 
Is there a way to increase the range of clean sounding volume before it breaks up?
Is this effect related to the MV at all?
Would adding NFB help tame this thing?
Any other thoughts/ideas from those more familiar with this circuit?

Let me know if anyone needs more; I'm happy to run tests or try out suggestions.

Gracias, amigos!.   

[pdf64]

Try inserting a 1M volume pot at V, trimmer or full size pot, and tweak the system gain down to taste.

[bmccowan]

Have you visited the 18 watt forum - https://www.18watt.com/ There is a lot of discussion on this topic. Also the Trinity 18W kit site has a good troubleshooter. I built an 18W TMB and had a similar experience. I ended up doing what PDF suggests but with a fixed voltage divider. A pot would have saved some time.
I also found the MV to be problematic - hard to find the sweet spot.
I never did bond with the amp and eventually reconfigured the power amp for 6973s - still too much of a gain monster for me though.

[kagliostro]

As to tame gain excess you can try a Split Load Resistor

The sum of the two Plate Resistors must be the same of your V1b Plate Resistor value (or close to it)

The bigger is the lower resistor the higher is the signal drop



Franco

[ac427v]

It would help if you knew what stage was over-driving.
Is it in the reverb circuit? Turn reverb to 1 and see how loud the amp can play before distortion.
Are bass frequencies causing too much distortion? Turn the bass to 1 and the treble up to 5 or more and listen.
Other observations:
Tubenit's plan has several differences that may allow less distortion. He has a Volume control, a pre PI Master Volume, and the post PI Master Volume (LarMar). Previous posters have recommended an additional volume pot or a fixed voltage divider to duplicate the third Master Volume.
You could significantly reduce preamp gain by removing the cathode capacitor (2uf) on the second stage.
I have seen the 12DW tube used in reverb circuits before. Those circuits used the low mu triode to drive the tank and the hi mu triode to amplify the recovery stage. The result is cleaner reverb. An additional benefit is you then could increase the reverb dividing resister.  That sends more signal to the reverb circuit and less of the preamp signal to the pi and power amp, allowing cleaner amplification at a higher volume knob setting.Where to start???

[AlNewman]

Fixed bias maybe?

[tubeswell]

Remove the cathode bypass cap from the output stage. (Or put a switch there to lift it out)

[Merlin]

Remove the cathode bypass cap from the output stage. (Or put a switch there to lift it out)

+1. The 18W is a minimalist circuit, so you don't have a lot of options. Removing cathode caps from the preamp or power amp is where I would start. You can also increase the 820R bias resistor on the LTP, which is a surprisingly effective way to reduce the LTP gain.

Also, the 12DW7 seems to be mis-labelled in your schematic; the reverb driver is the low-mu half.

[tubenit]

12AY7 in V1      12AU7 in LTPI position (which I have done in several amps with results I'm pleased with)

The previous suggestion of the split load resistor is also a good one.


The suggestion of changing the LTPI 820R to maybe something like 1.2k or slightly higher is also a good one.   



With respect, Tubenit

[acheld]

Easiest thing to do is substitute a 5751 or 12AT7 in the V1 position.  This will reduce your "gain" (meaning distortion) as it hits your LTP and the EL84's. 

Tubenit's suggestion of subbing in a lower mu tube in the LTP is also a great suggestion.

The difference is that a lower mu tube in V1 will have less distortion and lower Vp-p in the signal as it hits the LTP, where the lower mu LTP takes the distorted signal and lowers Vp-p as it hits the "sensitive" EL84.

Either approach can work, but they can sound different. 

I'd try this before getting the soldering iron out.   

[tubenit]

Take these as an approximate gain factor for each tube. 


With respect, Tubenit

[pdf64]

Take these as an approximate gain factor for each tube. 


With respect, Tubenit

That doesn't represent the real world, 'swapped into a typical 12AX7 circuit' effect though.
See https://www.thegearpage.net/board/index.php?threads/updated-tube-comparison-table.2279173/

[pdf64]

Easiest thing to do is substitute a 5751 or 12AT7 in the V1 position.  ...
I'd try this before getting the soldering iron out.

If this was a commercial amp built on a pcb / Lectroid had poor soldering competence, then yes, valve type substitution should be much preferred.
But regardless of any gain changes, different valve types will tend to different tonal balance, and as it's a DIY amp build, sticking a pot in there to investigate how much the system gain needs trimming back seems the most efficacious first step to me 

[kagliostro]

If I can say


Using a Split Load Plate Resistor don't affect in any matter the actual "tone" of the preamp, it only reduces the swing that is transferred to the following of the circuit


And, yes, with the necessary attention the lower plate  resistor of the Splitted Load can be temporarily swapped with a potentiometer to establish the preferred voltage divider values to achieve your preferred work conditions


Franco

[sluckey]

Rather than a split load resistor you could just replace the plate resistor with a high quality locking pot. Then just dial in the gain you want and lock it down. Kinda like the BAL pot in this circuit...

     https://sluckeyamps.com/warbler/warbler_rev2.pdf

[RoadShow]

Rather than a split load resistor you could just replace the plate resistor with a high quality locking pot. Then just dial in the gain you want and lock it down. Kinda like the BAL pot in this circuit...

     https://sluckeyamps.com/warbler/warbler_rev2.pdf

[Lectroid]

Thanks to everyone for all the suggestions.  You've shown me how to approach the problem, and ways to change the areas I need to change.  Lots of good stuff to ponder.  I'll post what I find.

The suggestions so far:

• Remove one or more cathode bypass caps in the 1st/2nd stages.
• Remove cathode bypass from the power amp cathode.
• Split the load going to a preamp tube or (or the LTPI? to lower its output
• Swap in lower-mu tubes as an easy way to lower a stage's output.
• Remove the bias resistor on the LTPI.

Regarding the split load resistor technique, is it usually applied to one particular amp stage? Or the LTPI?  Both?

Removing bypass caps caught my attention.  Intentionally removing components just to reduce an amps's available power seems a little weird, like I'm crippling a perfectly good amp on purpose.  But it all makes sense.

@ac427v
Very good tips on how to troubleshoot the distortion systematically. I will work up a table to  see whether the tone controls might influence it. 

[tubeswell]

Removing bypass caps caught my attention.  Intentionally removing components just to reduce an amps's available power seems a little weird, like I'm crippling a perfectly good amp on purpose.  But it all makes sense.

You said you were after less distortion and a bluesy sound, so…

EL84s have a lowish (10 to 12ish volts) bias voltage so it doesn’t take much preamp gain to reach signal clipping.

Removing cathode bypass caps can reduce the gain of a stage by allowing cathode current feedback. If you start with the EL84s you will immediately notice an effect on how much you can dial up the clean volume.

And easy enough to make the Ck switchable.

(FWIW, Supro T-bolt has no Ck on the 5881s, and it still kicks ass)

[AlNewman]

Probably the best thing to do is to try things.
Have you tried removing the bypass capacitors?

They're easy to switch, and you can dial in a certain flavour.

You have a scope, so you should be able to see where the distortion starts.

[Lectroid]

First of all, concerns about the reverb have been corrected--a matter of last minute change that never made the schematic.  It's one of tubenit's circuits and it works great.  I run it though an 8A- tank with the short box and 3 springs.  It's definitely different from Fender reverb.  It's less wet, more feathery, less clattery, more echo-y.

I changed the LTPI to a lower-power tube, 12AU7, and it made a noticeable improvement.   It has some issues with voicing I need to address later, but it’s already better behaved.

Volume = 2, guitar = 4:  Produces a good clean preamp signal.  Turning up the MV delivers clean amplification, and the tone stays pretty consistent.  There is now a good usable range of clean signal by keeping the Volume below 4.  The strings sound balanced in volume. is definitely better.

Volume = 5+, guitar = 5:  Smooth overdrive appears gradually as the Volume knob is raised above 4.  The tone and character of the overdrive can be modified with the guitar’s tone stack.  Once the tone is set, then the MV can be turned up pretty loud.

The tone of the overdrive can be modified pretty widely with the tone stack.

Removed V1b cathode bypass capacitor.  This did reduce overall power somewhat, the effect wasn’t large. Also, it seems to make the sound just a little bit “grittier.”   But I’m leaving it removed for now.

Tomorrow I'll continue swapping preamp tubes and looking at bypass caps.

First time in its new home, rough fit.

[kagliostro]

See here an option to have a gain control (by our friend Sluckey on his Smoky)


Franco

[tubenit]

I changed the LTPI to a lower-power tube, 12AU7, and it made a noticeable improvement.

Glad that was useful to you. I would continue to suggest a 12AY7 in V1.  You might consider a 12DW7 (or reverse 12DW7 depending on how you wired the pins of the socket) for your reverb.  You'd want the low gain side as the driver.


With respect, Tubenit

[Lectroid]

Glad that was useful to you. I would continue to suggest a 12AY7 in V1.  You might consider a 12DW7…
With respect, Tubenit

I would like to try the 12AY7 at V1, just don't have one on hand. Per acheld I will test it with a 5751 and 12AT7 today.  I did use the 12DW7 for your 1-tube reverb; you suggested it to me a while ago. Strong reverb effect, mellow sound. Not quite drippy enough or deep enough for good surf rock, but good enough for jazz, rock, blues, even with a 470k dwell resistor

Are the tubes in the 12A_7 tube family so widely interchangeable in standard preamp circuits like these?  Can we substitute any of the tubes from the 12A_7 family or their analogs, for any of the other 12A_7 tubes, assuming it had the value of mu we wanted?

[acheld]

Can we substitute any of the tubes from the 12A_7 family or their analogs, for any of the other 12A_7 tubes, assuming it had the value of mu we wanted?

Sorta, kinda.  Yup. 

Many of us focus on mu as the big difference between the various tubes.   However, there are (several) other considerations, including output impedance, to keep in mind.    That said, I haven't caused any catastrophes by trying different tubes.  They may not work well . . . but no harm has been done.   

[Lectroid]

Can we substitute any of the tubes from the 12A_7 family for other 12A_7 tubes?

Sorta, kinda.  Yup.

Many of us focus on mu as the big difference between the various tubes . . .They may not work well . . . but no harm has been done.

EDIT (6/26/24): pdf64 has a post above that includes a link to a great analysis from HotBluePlates over at TAG about swapping within the 12xx7 family.  Anyone interested in this should go there now and read it.

[Lectroid]

Yesterday I swapped out the 7025 in V1, replacing it with a 5751 (mu=70).  This yielded another big improvement with the amp.  Cleans are stronger, and the amp is less brassy, less percussive. Overdrive and clean power are both readily available. 

I have still have a hum in my reverb recovery stage, that changes volume as the Intensity knob is turned up or down.  The recovery tube's plate coupling cap is 0.01uF, and there's a DC voltage across it between 40mV and 70mV.  I'll replace the cap.

Otherwise, this thing is ready for final voicing.

This amp is sounding much better than when I opened the thread, thanks to everybody's ideas.  Thanks again to everyone.

[tdvt]

Otherwise, this thing is ready for final voicing.

I would be interested to see where you land at the end, so please update or post a final schematic.

I am still fooling with an amp that is close kin to your circuit; notable differences being a Fender pre (same tube but tone stack between stages) & 2 tube reverb. But otherwise very similar with LTPI & EL84s & no trem.



Check out this recent thread where reverb hum/noise was discussed. LINK

The amp I mentioned currently has a 220uF bypass temped in on the driver cathode, seemed to help with noise, going to permanently add it.

[Lectroid]

Thanks, I will look at the link.  The new reverb driver cap solved my only hum problem, so that's one thing down.

Here's the latest schematic. It should be close to the final version.  (Translation: I want to get this thing into a box and start showing it around.).  I might need to change a resistor or two in the power supply, but all the components in the signal path should remain the same. 

[tdvt]

Thanks for posting the revision. I like how basic it is all is.


Don't think I already asked, but what program did you use to draw the schematic?

[ac427v]

I'm surprised at the cathode voltages on the reverb tube, V2.
V2a of 2.16 volts is about half of what I would expect on a 12AU7.
V2b of 3.27v is about twice of what I would expect on a 12AX7.
Are you using a JJECC823? That is the JJ version of a 12DW7 (ECC832) with the pins reversed. If so, there is less drive to the tank and more hum when the lowered tank output is amplified in recovery.

If my theory is wrong then double check the actual values of those cathode resistors. If the resistors are correct then...nevermind

[Lectroid]

I like how basic it is all is

Me too.  It's so minimal, it's amazing it sounds so good.

Thanks for the suggestions.  I will double-check the cathode resistors for sure.

I'm surprised at the cathode voltages on the reverb tube, V2.
V2a of 2.16 volts is about half of what I would expect on a 12AU7.
V2b of 3.27v is about twice of what I would expect on a 12AX7.
Are you using a JJECC823?

For the one-tube reverb, I'm using a GE 12DW7, old stock, but still good.  (Says so, right on the box!)  Many people use a 12AX7 in that spot, but tubenit suggested the 12DW7 so I tried it.  Good option, I think.

Pretty sure the tube socket is wired correctly, too after several people here corrected my original hookup, lol.

For now, don't take any of those weird voltage numbers too seriously. I changed the reverb coupling cap yesterday, and have to troubleshoot some stray popping this morning.  After the reverb circuit is stable again,  I'm going to tweak the PS to boost  power to the C & D nodes.  I'll try to post a new schematic later today.

I use DIYLC for my schematics, (and all my board layouts too.).

 

[sluckey]

Something that helps me with the 12DW7... Low pin numbers are low gain triode. High pin numbers are high gain triode.

[Lectroid]

Oh, now, that's just too easy.       But a great tip.  Thanks.

[tdvt]

Started thinking about it, you've kind of got a tidied-up Blues Jr Lite, w/tube reverb.

Just missing the un-bypassed 3rd stage really..

[Lectroid]

It does indeed.   I've heard so many bad things about Blues, Jrs. and Blues Deluxes that I've never looked at either schematic.  Interesting.

One big difference is in the tone stack. It has similar values to the stock AB763, more than any Marshall tone stack. Except that the Volume is moved before the 2nd stage, and I dispensed with the cathode follower. 

Did you build that Blues, Jr. circuit?

[tdvt]

Did you build that Blues, Jr. circuit?

I didn't build it, but I have one that I bought used & have serviced over the years (so I have that schematic saved).


Not a bad amp really, but pretty marginal construction as far as the electronics. The "FAT" feature is just a switchable by-pass cap, reverb is passable but not as nice as real tube reverb.


There was/is a YT video where the guy (StuartUK) does a voltage leakage test on the separate tube board & the leakage between traces is pretty substantial. As those boards burn up anyway, I replaced mine & also ditched the ribbon cables connecting that board to the main board, which quieted things up dramatically.


Anyway, I like it enough that I would like to try your circuit (being close) for a better-built unit, keeping what I DO like about it.