I get frequent questions about this schematic on other sites.  I am bumping it here so people can find it easier. 

This schematic is a faithful reproduction of the Certiatone 18W TMB Layout 23.   Other than a note about an optional PPIMV it should match exactly.   Came in handy as a platform for modding my Ceriatone 18W TMB.  Scott

I just use an Avatar G212 traditional cabinet (open back) with a couple of Scumback 12" speakers.  The speakers are rated 60W each and 16 ohm each wired in parallel giving effectively 8 ohm load.  I use other higher power amps as well with these speakers but sound great with the 18Watter.  The cabinet was delivered with Celestion mix rated 30W and 60W respectively and sounded OK but a little rough on the top with any amp.  Not sure how the EF86 comes into play with your amp.  With EF86 you may prefer Alnico magnet speakers which heighten the upper end response and good for "clean" distortion.  Power wise a couple of 12" speakers in the 15 to 60W range should work.  Celestion Greenbacks @ 25W each may work as well as Celestion Alnico Blues @ 15W each.  The Blues are pricey though.   Anyway to 18W TMB responds well with two 12".  I can get controlled feedback easily with this amp and speaker combo.

I like buying 16 ohm speakers as they seem the most versatile.  Two in parallel yield 8 ohms or four in parallel yields 4 ohms.  I generally don't like wiring speakers in series like two 8 ohm speakers (8 + 8 = 16) as one could fail and you would have no sound.  Speakers usually open circuit when they fail electrically.

Good job!  Tube amps are such high impedance that it doesn't take much to be conductive and throw off the circuit.  I worked on an old amp once where a dust trail was conductive.  Birthing amps can be tedious.   2 out of 4 of mine had start up problems that were unique in some way.  And like you have been doing dig a little deeper and think outside the box.  But great job on hanging in there and not giving up.

Some other things to check.  V2 10nF coupling caps can be leaky.  What is the B+ voltage measure feeding the two 100K plate resistors?   Also detailed pics would help.   It usually something constantly overlooked or assumed worked or was correct prior.  But on a new amp it can be anything until proven to work.   Broken wires, poor soldering, component out of tolerance, etc.

Good job Wyatt on compiling a parts lists.   There are also the potentiometers listed below.

VR1, VR2, VR4    500K ohm, audio taper  (500K-A)
VR3, VR6, VR8    1Meg ohm , audio taper (1M-A)
VR5                    250K ohm, linear taper (250K-L)
VR7                    25K ohm, linear taper (25K-L)

And I put the schematic notation "CCW" near the appropriate pin of each pot for reference.  This means counterclockwise or across the pond anticlockwise position of the pot knob. 

Been a while since I was around this forum but still using and liking the 18 watter represented in the schematic.   The open loop gain between 12AX7 and 12AU7s is large but in this circuit yields only about a 10dB difference.  See the note on the schematic. Yes it is just a tube swap.  If you don't like the sound or need more gain pop a 12AX7 back in.  The EL84s need less drive than EL34s so the TMB stage adds unnecessary gain IMO and was hard to control.  Plus a lot less circuit noise and more clean range with a 12AU7 cathode follower.  Not sure about the EF86 preamp 18 watter but give it a try (V3 is also cathode follower in the EF86 version).  

I don't use old stock tubes as I don't think it buys me much in these amps.  For the 12AU7 I use a JJ ECC802 which is a long plate variant of the 12AU7.  All I had at the time but I like it and works well here.

Interesting about the ECC832 but the cathode follower section (2nd stage of V3 or V2 in regular Marshall amp speak) are almost unity gain no matter which tube type selected.  I would just use a standard tube for desired gain/sound provided by the 1st stage of the cathode follower circuit and the 2nd stage driving the tone stack will be near unity gain regardless.

Wyatt has some good ideas about bass response.  I changed the V1 cathode bypass on the TMB channel from 1uF to 2.2uF to add a little extra bass for a Fender strat.  Gave it a little more balls.  But I wouldn't say the amp is overly bassy by any means with the 12AX7 front end.  The EF86 version I haven't heard.  But of course I encourage experimentation so go for it!

Just Google search "rotary switch 6 position " and you will see options like this below.  This one has two poles but he price is right.   Scott
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=023-664&utm_source=googleps

That is fun to do and you gain a lot of experience doing it like that.  Give Nik a shout.  He usually gives me what I want at a really decent price.  If you want a more traditional board material then check out somthing like turretboards.com.   You will find board material, stakes, and staking tools for a truly roll your own amp.  The 1/8" board material is really nice.  I generally though like to buy pre-punched chassis in the configuration I need and pre-drilled board material.  Going totally raw is too much work but you will learn a lot doing that way.  Scott

That is an excellent suggestion for debug and tune up.  I have an aluminum cookie sheet (baking sheet) that is the perfect width.  Scott

There is sometimes a lot of drama when a tube fails catastrophically.   Fireworks, noises, etc.  So did the rectifier just blow or did something else take it out?  So put on your CSI cap.  I would pull all the tubes and with no power applied ohm out the high voltage caps and resistors in that part of the circuit.  Drain the electrolytic caps first.  Check for shorts or low ohmage across the filter capacitors and resistors that may be burned or out of tolerance.  If it looks good at this point put in the new rectifier (no other tubes) and check voltages.  They will read high without a load if all is normal.  Then install the remainer of the tubes and see if it functions OK.  Flakey power tubes could cause the rectifier to pull too much current and shorten its life.  Check for red plating in a dim light.  If something does look right stop and analyze, get a 2nd opinion, or ask a question.   I trust the tech that worked on the amp is competent.  Fresh power tubes could draw more power and stressing the old rectifier I suppose and hastened its failure.

No, it is a mistake.   The missing terminal is the ground normally close section that is opened when a plug is connected.  In effect this is not used at all.  Just run the ground bar all the way across like the speakers and other effects jack.

You certainly can source all the parts and build your own but will spend a lot more cash.  For my blackflag JTM50 I purchased the chassis, faceplates, and hardware from Nik.  The Ceriatone plexi type faceplates are just simply gorgeous and the chassis a perfect match.  I supplied the transformers, tubes, caps, resistors, turret board, top coat wire, and cabinet.  The amp turned out spectacular.  Just ask Nik and he will bend over backward to satisfy your needs.  Again I paid well over $1K doing it separately but in the end got what I wanted.

With two FX jacks installed the impedance selector switch is removed to make room for one of them.  As a result the 16 ohm tap that is hard wired to the speaker jack is just an example.  OK to connect to desired impedance.  Just make sure the unused transformer taps are insulated and not touching anything. 


These are line level passive FX loops.  The pedal levels are lower gain but no problem with something like a Kleinulator or C-lator to boost the levels on the RETURN.  There are line level effects processors that work great in the passive loop.  My old TC Electronics G-Force is one example.  But if you plan to use pedals you need additional gain.


That would require a redesign to make the front end of the amp more like a Plexi with a mixer circuit to combine both inputs before the cathode follower driving the tone stack.   It is possible.  Shoot me a PM if you would like a concept schematic.  I was going to rewire my 18W TMB but found I didn't need it.  I like the non-TMB channel the way it is and driving the PI stage directly.  I also like the TMB channel for the scooped mid-range.  Best of both worlds but had to balance the overall gain between the two channels when jumped.  Easily done by using a lower gain cathode follow tube like a 12AU7.   Scott

I think you have it.  Wired just like to normal channel.  Scott

In standby my Ceriatone JTM50 hummed quite noticably from the speaker when first built.  It was low-level but loud enough to be heard across the room.  Drove me nuts for six months.  To make a long story shorter the issue is caused by the power transformer coupling magnetically directly to the output transformer.  I also experienced slightly less hum when the standby switch was closed and the tubes supplied HV.  So was a real head scratcher for a time.  I tried everything to remedy the problem but the hum remained (different transformers, grounding, rewiring, mu-metal shielding).  I finally fixed the problem for good by physically relocating the output transformer.   The output transformer has moved 1/2" further away from the power transformer and, at the same time, aligned the OT to the PT centerline (and still orthogonally of course).  Now hum free.

It was a major pain to relocate the OT (drill new holes in the chassis) but gave me a chance to remove all the temporary mods and now have clean wiring again.  But it seems the Ceriatone chassis, JTM50 in my case, locates the OT closer to the PT than other amps of similar layout.  A little too close in this case.  If you are sure grounding/wiring are good, pull all your tubes, and if hum is present in standby then you probably are experiencing cross coupling of the PT magnetic fields into the OT primary.   And don't be confused by mechanical hum where the PT vibrates the chassis.  In my case the hum would disappear if I pulled the speaker cable.

Really love the amp now but bugged me to no end initially.  Best of luck,  Scott

Thanks HH.  The tube placement was a head scratcher for me as well as I followed the Ceriatone layout which swaps V2 and V3 for some reason.  So that is the reason for swap in the schematic.

I did not try a 12AT7 in the PI as that would effect the normal channel's gain as well as the TMB.  The 12AT7 is only about 0.2dB less gain than the 12AX7 anyway in this PI circuit with the selected components.  I did some gain calculations and it boils down that the 12AU7 cathode follower gain is about 9 dB less than 12AX7.  This lower gain helps balance the overall amp gain, drive the FX loop close to nominal levels, and keeps noise low. 

For some reason this 18W amp gets the trebles right for me.  So it has gone from a ragged sounding amp to a lovely gem.  Thanks for the kudos... Scott 

Edit: A year after gain balancing and mods I have nothing but praise and love for this amp.  I have used Mercury Magnetics OT transformers in the past with OK results but this one is exceptional and perhaps a large reason I like this amp so much.  The OT full stack with added iron increases inductance so it is harder to saturate the core.  Core saturation distortion is not my thing.  This amp does not have the fizzy sound you hear with so many low wattage amps.  You know, the fizzy decay of notes when turned up.  So does it sound stiff and overly clean as a result?  Not on your life.  It is big and open with a warm sound at any volume that begs to be cranked up.  Bass is tight, upper notes smooth, and quite loud before full on distortion.  With Nik's nice head case and super looking panels it is hard to tell what is under the hood of this amp.  Friends are amazed it is only a lowly 18 watt EL84 amp.