Can I make an 18 watt EL84 amp more like a blackface.

[Petedub]

Hello, can anyone offer some advice please.
I built a Marshall 18 head, it sounds great but the sound is not for me.
I mainly use a princeton RI and a Supersonic 22.  I'm wondering is there a fender style circuit that I could build onto my 18watt (Hammond) transformer set and chassis.

[sluckey]

Sure. Just replace your Marshall normal preamp with a Fender AB763 preamp.

[Loomer]

In the 18w there is a lot of distortion coming from the power amp section, the consensus seems to be that implementing the so called Jason PI mod helps keeping that distortion down - this is me guessing you might be looking for a cleaner tone anyway, since a lot of the 18w character seems to come from those saturating EL84s

[jordan86]

Sluckey and the team helped me convert an old organ into a guitar amp. Its now a favorite of mine.

For that I wanted a single tone control that would be not too gainy but not too lossy either. I went with the Dr Z style tone control that he puts in Carmen Ghia. Online the amp schematic is called the Phoenix. I could dig up schematics if you can’t find them online. I assume your 18w has a single tone control. It might be easier to just try that tone stack first than gut your whole preamp. It’s very scoopy in the mids. Will be more Fendery. Sort of fender meets vox, just cleaner.

[bmccowan]

I built a Marshall 18 head, it sounds great but the sound is not for me.

Same here. What I did was to rewire the power amp for 6973s/6CZ5/6CM6. Was simple to do, I just followed schematics for Valco 6973 amps. It was a TMB model and I also found the gain too high and bright in that channel and cut the gain some with a voltage divider and changed the value of the bright cap. To my ears the 6973s are smoother and have a broader/fuller sound.

[Petedub]

Many thanks for your replies and suggestions. I have read some posts and had a look at the clean lite dual and bright schematic. 

I can see how it might be possible for one channel of a Marshall 18 watt amp to have an ab763 style preamp (V1) and then maybe leave the second channel as it is with the tremolo intact. 

I notice on the diagram the grid resistor on the EL84 is 1.5k I think the usual value for marshall here is 8.2k.  Does this affect headroom somehow?

I can't post a photo as the amp isn't here this week but thanks again for the replies, I should add I am a novice builder and if I do post a picture its not as pretty as some of the lovely amps I see on this website.

[sluckey]

I notice on the diagram the grid resistor on the EL84 is 1.5k I think the usual value for marshall here is 8.2k.  Does this affect headroom somehow?

I don't see it. What diagram? 8.2K is usual. 1K to 10K probably won't make any sonic difference.

This is how I would do it. Easy mod on paper. May be a bit more of a challenge in reality, especially for a novice. For example, if you have a standard chassis and faceplate, where will you mount the extra knobs?

[Petedub]

I have attached a copy of a diagram from the el34 site,  I have looked at and I think it answers my question. 

It looks similar to my 18watt amp at the power amp and phase inverter stages (with some extra bits and different values).

However the preamps dont look anything like my standard marshall channels.

So in theory we can just replace any one preamp section with another and leave the rest of the amp the same.

The difficulty I have found when building a custom preamp is to get the output level of the signal to the correct gain level when it reaches the phase inverter.


I have a standard faceplate so just a volume and tone control for channel 1 which would be the channel I want to blackface.

I need to look at the tone control circuit next as has also been suggested on this thread. 

 

[sluckey]

The schematic you posted is quite different from the Marshall 1974 18 watt amp.

The schematic I posted is a Marshall 1974 18 watt amp but the normal channel has been replaced with the Fender preamp. The Fender preamp will have plenty of gain when it reaches the phase inverter.

[Esquirefreak]

If I'd wanna change a preamp to AB763 specs and only had room for 2 pots - I'd make an internal R-network/divider instead of a bass pot and have the treble pot mounted on the faceplate. I rarely adjust the low end, but occasionally I find some guitars too bright.

Think of it as bass pot but in fixed form. Solder in a A250k pot, adjust the bass to find your sweet spot and measure resistance from the wiper to both pot ends to find out what R values you need for the divider.

At least that's how I'd do it.

/Max

[mresistor]

 A 2 x 9 pin power section can be converted to a 2 x octal section  if care is taken to get some space between the two larger sockets and tubes.   Then you could have a 2 x 6V6 power tube section which is what many fender power sections are.

[bmccowan]

A 2 x 9 pin power section can be converted to a 2 x octal section  if care is taken to get some space between the two larger sockets and tubes.   Then you could have a 2 x 6V6 power tube section which is what many fender power sections are.

Very true, but that is part of the reason I mentioned the option of rewiring for 6973s/6CZ5/6CM6 tubes. The 6CM6 tube is a 9 pin version of the 6V6. Not produced today, but easy to find, and not too expensive. But I also like the idea of trying to make the EL-84s meet expectations. Strangely, EL-84s are my favorite in Hi-Fi amps, but despite many tries, I just do not like them in guitar amps. I have tried 18W, AC-15, Spitfire, and Carmen Ghia and they never thrill me.

[66Strat]

A dual concentric pot can take the place of two pots in the tone stack. You could use a 250K/500K pot for the treble and bass pots in the AB763 schematic.
https://www.allparts.com/collections/potentiometers/products/ep-4585-cts-250k-500k-concentric-audio-pot

You can substitute capacitor and pot values in the tone stack calculator for comparison of roll off frequencies.
https://www.guitarscience.net/tsc/fender.htm#RIN=38k&R1=100k&RT=250k&RB=500k&RM=6.8k&RL=1M&C1=250p&C2=100n&C3=22n&RB_pot=LogA&RM_pot=Linear&RT_pot=LogA

[Loomer]

I have attached a copy of a diagram from the el34 site,  I have looked at and I think it answers my question. 

It looks similar to my 18watt amp at the power amp and phase inverter stages (with some extra bits and different values).

However the preamps dont look anything like my standard marshall channels.

So in theory we can just replace any one preamp section with another and leave the rest of the amp the same.

The difficulty I have found when building a custom preamp is to get the output level of the signal to the correct gain level when it reaches the phase inverter.


I have a standard faceplate so just a volume and tone control for channel 1 which would be the channel I want to blackface.

I need to look at the tone control circuit next as has also been suggested on this thread. 

 

Hey there, the schematic you have posted was drawn just the other day by myself, for my still-in-the works project, so some caution should be used.

A handful of values are actually wrong in the 12AX7 side preamp as well, please refer to the thread to follow this WIP - your contribution would be hugely welcome of course should you choose to go down this path!

I'll be correcting that and posting a V2 but keep in mind the power section is still in the works (though your standard 18w-spec power section should do the job no problem, so this might not be too big an issue for you) it's still worth noting neither myself nor anyone else has built it up to that spec, so just how clean it might or might not be is still up for debate!

Also the voicing should be in the ballpark of being in between Marshall and Vox, and wouldn't expect it to exhibit any Fender-y character. Should be a departure from the stock 18 watt though, so possibly worth a shot as it might add something new to your amp lineup!

Lastly, in terms of the tone control you could look at the Duncan TSC - the Dr Z tone control is quite simple but effective, uses one knob and through the calculator you should be able to tweak and compare it to a curve you might like the look of or simply sounds and feels more familiar to your ear and what you are after

[Petedub]

Thanks for all the input here.

Some of the suggestions are really interesting and offer food for thought, however . .

I can implement the fender style preamp without rebuilding the whole thing as per the diagram from slucky, and use a dual concentric pot for treble and bass as suggested by 66strat.

This allows me to keep the trem channel of the marshall intact, which is great, and use the original faceplate. 

Also loomer suggested the jason pi mod.  It looks like it has been implemented on the 18watt schematic I posted earlier in the thread. This has to be worth a try as well.


again, many thanks for all your help! !

[Loomer]

Thanks for all the input here.

Some of the suggestions are really interesting and offer food for thought, however . .

I can implement the fender style preamp without rebuilding the whole thing as per the diagram from slucky, and use a dual concentric pot for treble and bass as suggested by 66strat.

This allows me to keep the trem channel of the marshall intact, which is great, and use the original faceplate. 

Also loomer suggested the jason pi mod.  It looks like it has been implemented on the 18watt schematic I posted earlier in the thread. This has to be worth a try as well.


again, many thanks for all your help! !

Yup, the Jason PI mod is there. Sounds like you've got a plan then 👍

Looking forward to seeing how this one develops 🙂

[kagliostro]

Before to swap EL84 with one other tube, I'll give a try to a Split Load resistor on the PI, really simply to be done


Franco

[jordan86]

Yeah, I’m glad Loomer chimed in. I didn’t want to be disrespectful but was gonna say there’s not much Fender at all in that schematic. It’s all Marshall & Vox topology.

I do like 66 strat’s dual concentric pot idea!

[66Strat]

A 2 x 9 pin power section can be converted to a 2 x octal section  if care is taken to get some space between the two larger sockets and tubes.   Then you could have a 2 x 6V6 power tube section which is what many fender power sections are.

Very true, but that is part of the reason I mentioned the option of rewiring for 6973s/6CZ5/6CM6 tubes. The 6CM6 tube is a 9 pin version of the 6V6. Not produced today, but easy to find, and not too expensive. But I also like the idea of trying to make the EL-84s meet expectations. Strangely, EL-84s are my favorite in Hi-Fi amps, but despite many tries, I just do not like them in guitar amps. I have tried 18W, AC-15, Spitfire, and Carmen Ghia and they never thrill me.

Maybe cathode bias is part of the limitation? IME, fixed bias ala Blues Junior may get a bit closer.

[kagliostro]

Other 9 PIN 6V6 is the 6P1P

Franco

[DummyLoad]

A 2 x 9 pin power section can be converted to a 2 x octal section  if care is taken to get some space between the two larger sockets and tubes.   Then you could have a 2 x 6V6 power tube section which is what many fender power sections are.

Very true, but that is part of the reason I mentioned the option of rewiring for 6973s/6CZ5/6CM6 tubes. The 6CM6 tube is a 9 pin version of the 6V6. Not produced today, but easy to find, and not too expensive. But I also like the idea of trying to make the EL-84s meet expectations. Strangely, EL-84s are my favorite in Hi-Fi amps, but despite many tries, I just do not like them in guitar amps. I have tried 18W, AC-15, Spitfire, and Carmen Ghia and they never thrill me.

I am on the fence with EL84s as well. After many experiments, I find that the tone is tolerable if you use large-sih screen grid R, control grid stoppers, and delete the bypass cap if running them in autobias mode. They do sound a LOT better running in fixed bias, also with large-ish screen R (1-2.2K per tube) and control grid stopper R (10K-22K).

 
Your voltage divider did not reduce gain (if sum was same as original load), your voltage divider reduced the DRIVE (signal level) to that following stage.

--Pete

[bmccowan]

I find that the tone is tolerable

  I found that strangely funny - not a typical asperation. Anyways, if wanting to try fixed bias with EL-84s I would also look at the version of the 6G9 Tremolux that was produced with EL-84/6BQ5. It was short-lived. but combining the power amp with the preamp that Sluckey posted would be close to what you are thinking.
BUT - you'd be looking at a more complex project that a novice might want to attempt, and chassis space, etc.
Anyways - if you built your current amp, do you have a schematic of the actual amp? Built from a kit? I think we are dancing around this subject without a real understanding of what you now have. It's fun, but not very productive for you.

[Petedub]

Hello again, and thanks again for all this amazing info.

The 18watt amp in question is a marshal 1974. It is built exactly to normal specs.  The only change is the addition of an imbalance on the V1 a and b sides to emulate a jumpered plexi (by changing the cathode bypass components)  This does sound amazing but I don't really play a lot of rock music.

Hence the desire to make the amp more Fendery.

[sluckey]

Since you already have a Princeton RI and are currently building a Deluxe Reverb, you may want to just abandon the idea of making this Marshall 1974 more like a "blackface". Sell it if you don't like it. Probably be worth more as is than it will be after mucking it up with a Fender preamp.

[Petedub]

This advice does make sense, however I've rebuilt that 18watt 3 times now, each build has been quieter although it's still a bit noisy,  I have used it for tiny little bar gigs and it sounds excellent but lacks bottom end sometimes.  Also my Dad built the cabinet.  I think of the amp as a part of me, I could never part with it .

I also own a Hot Rod Deluxe combo, now that I could sell once the deluxe reverb is built as I have no emotional attachment.

Does anyone else form a bond with their DIY amps or am I just an idiot lol.

[jordan86]

 guess it may be worth clarifying...in what way do you want it more Fendery? Just cleaner with more low end? Less mids? Do you really just want a clean amp? That’s a different approach and potentially simpler than a full overhaul into ab763 territory. Shoot...an EQ pedal in front of the amp might be the simplest solution. And something you could use in other contexts as well.

It’s an EL84 amp so there’s not much fender in that arena, save for a blues junior which is hardly a sought after circuit.

What about the “Fender” sound are you looking for?

[bmccowan]

Well this does not prove anything about the idiot deal, but yes I build such bonds. A Princeton Reverb because its the first amp I rebuilt. Ok, maybe it proves that we are both idiots 
Sluckey is too practical. I would keep playing around with it until I got a sound I liked. I think you are more experienced than I first thought. The chassis and iron are likely good bones and there are many things you could do with that.
I would agree with not going the Fender route again. So many options with good schematics and layouts available - Gibson, Valco, Sluckey and Tubenit designs on this site, etc. If you do not play much rock, I expect you play blues and jazz. If so, the Gibson GA-16, 18, 20, 30, and 40 are all cathode biased 6V6 amps that sound great and could use that iron. And with help here, you would tame that noise you are talking about.

[Platefire]

Petedub
Been reading your post with interest because I'm starting a project with PP 6973's that I can use or go to EL-84's or 6V6's. I'm thinking I'm wanting mine to be cleanish also. I can certainly understand your attachment to your amp withyour Dad being involved. It's got me wondering what it looks like? I couldn't fine any pictures or links to pictures.I'll be looking forward to finding out what series of changes you go through to get your sound out of this one. Platefire

[bmccowan]

Not that this is what either of you might want to build - just food for thought - an amp I built a few months ago:
Mojo NC3015 chassis and faceplate only  https://www.mojotone.com/British-Style-NC3015-Amplifier-Kit
Iron from a Hammond AO-39 amp
5V rectifier
modded the Mojo schematic for 6973/6CM6
Changed some cap values. Still tweaking.
Its basically a top-boost circuit, but the 6973s are not a sparkly as EL-84s
It does cleans, but only at lower volume levels.

[bmccowan]

Hey Petedub,
I'm wondering what you might have decided on that amp? The reason I ask is that I was watching this vid of the fantastic Kenny Vaughn https://www.youtube.com/watch?app=desktop&v=sb4JKW4bs7M
He plays through and talks about a Vox Cambridge Reverb. The tone he gets is great. you can skip ahead to about 1:15 to the section where he talks about the amp, but the whole video is worth watching. It could be worth considering the preamp circuit, without the reverb of course.
I have a Hammond AO-35 (the basis for both the Dr Z Carmen Ghia and the Goodsell 17) gathering dust and I think I'll use it to build a non-reverb Cambridge.
BTW, Kenny bought that amp at a store named Hum Depot - cracks me up.

[thetragichero]

what about something like a brown tremolux 6g9? fender flavor but a little different, get to use those lovely tapped treble pots from weber

[bmccowan]

I've long been curious about that 6g9 with EL-84s. I was going to build one a few years back but in researching it the amp owners thought its unique sound came from the fact that it used plate and screen voltages way above spec for the 84s. I never built one. The PT Petedub has in his amp would be well below the 6g9 voltage levels and I imagine would have no bias tap. Of course that doesn't mean the preamp circuit could not be used.

[Petedub]

I have the 18watt head home again now. It's sitting beside me in the front room on top of a 2 x 10 cab.  In terms of tone it is the sweetest amp, although I can't quite turn it up loud enough in the house to get to the marshall fun zone, it has a lovely clean sound.

It is not an exact clone of a 1974 but its similar.

Differences include a ss rectifier. 

Channel 1 has the bypass caps separated and modded to simulate a jumpered input.

I've tried a 1.5k resistor in the el84 grid (insted of the 8.2) and it feels a bit sort of tighter/cleaner. I might increase this a little and see what is a good fit for me.

I have tried using this at gigs with various cabs and speakers but it's never clean/loud enough or at least it's way more dirty than I want.

Aside from this, a real neon roach with a silver dot came thru my letter box today and I'm looking at my deluxe reverb board again, I think this is the final component for the board.