Dual Lite JTM45 worth a try?

[Jennings]

After my 18w build conked out a couple of years back I resurrected it into a variation on the Sluckey Dual Lite...and it became my go to amp ever since (cheers again for the suggestion!), being used live and on my last two EPs.


Anyway, being an addicted tinkerer, I’ve been itching to build something else...I was thinking the preamp section of the Dual Lite (18w and AC15 sections with switching) but this time married not to the classic EL84 power amp, but the KT66 power amp of a GZ34 rectified JTM45 with Vox style Cut control. Anyone tried anything along those lines? Worth me attempting to try putting together a schematic and giving it a go do you think?

[sluckey]

Sounds like a fun project that will be powerful and versatile. I look forward to seeing this project develop.

[MFowler]

Yes this has been done many times by us guys on Ampgarage


Trainwreck Rocket preamp (bright side of Vox AC30) with cut control into the power section of pair of KT66's or 6L6GC or EL34 etc with GZ34/5AR4.


Very good amp and my main gigging amp for years.  I added a subtle reverb in my amp.


Mark

[Jennings]

Thanks guys...reassuring to know I’m not barking, and similar projects have been successes. Just a thought, but what are the opinions on running the KT66 as cathode biased instead of fixed?

[sluckey]

I like fixed bias because it's easy to adjust and the tubes run cooler when idling. Cathode bias (AKA self bias) requires no adjustment but the tubes run hot all the time. These ideas apply to all tube types. If you want to hear the difference for yourself then make the bias mode switchable. Pretty easy and cheap to do. I did this on a couple of my earlier amps but soon found out I was no longer using the switch so I quit installing the switch. I mostly copy old amp circuits and just build whichever bias circuit the original had.

[Jennings]

Great suggestion! Thanks! Mostly I tend to come back to cathode biased EL84 based amps...and generally I find all amp designs work better for me with low filtering. Seems most people like to beef up the filtering for a tighter and less hum filled feel. Anyway, I’ve drafted an idea of a circuit...fixed bias, dual adjust for non matched tubes. Quite tempted to try a cathode bias, and possibly a switch, as I’m only really familar with the EL84 results currently and I’m tempted to try other valves in that arrangement.

[shooter]

I’m tempted to try other valves

one of the nice things about the 84, your drive signal in can be "reasonable". they are easy to drive, switching to other tubes, you wanna leave enough available signal to make them happy also.

[Jennings]

I was going to run the two single valve preamps (the EF86 AC15 and ECC83 18w) of Steve's design into the JTM45 PI and output section.  Although I was toying with whether to cathode bias the KT66s or not.  Would this perform in a similar ball park?  Or would the signal from the preamp need some adjustment to match up better?  I tend to flavour vintage amp levels of gain and volume, and everything else I add with pedals if I need, so from a sonic perspective I don't need to worry about adding gain or cascaded stages for flavour...but I do want a lively amp with a good colour and flavour to the tone as you crank it.  I'll try to get a mo in the coming days to draw out my scribbles and share the circuit diagram I've sketched out, just to see if I've missed anything or anyone has any tweak advice.

[Jennings]

Here is all I have so far...I'll be using a JTM45 transformer set and board etc.  I'll ditch the presence control, and ditch B/M/T circuit and ECC83.  In the board space I'll fit the EF86 valve and components.  The JTM45 PI I'll leave as-is, as well as the KT66 (though I'll probably add dual pots for adjusting the fixed bias independently, and split the 220k resistor join).  I'll add in the cut control too.  The filtering cap values I'll reduce too.  The lower filtering I've already tried in my last Dual Lite build and like the feel and sound of that amp.  Sorry I haven't drawn it out yet...I'm away from home without software or an A3 sheet of paper!  You'll need to refer to Steve's diagram for the drawn out preamp section until I do.  Any glaring issues anyone can spot on what I've thought so far?

[sluckey]

Those simple preamps will make that EL84 power amp roar but I don't think they can drive your JTM power amp to glory. Probably need one more triode between the preamps and LTP PI.

[Jennings]

Great, cheers for the tip...I'll add another stage inbetween the two preamps and the PI, and route an addition hole for the EF86.  Also, if I use an ECC83 or similar I'll have an extra triode to either ignore or maybe use.  Food for thought!

[shooter]

add another stage

use both halves, wire up a DCCCF (dc coupled cathode follower)
it's a nice stage to get good gain and less loading at the next stage.

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

and go with self biased, You'll be the cool kid that goes against the flow!!! 

[Jennings]

Great tip and link, cheers 👍 That’s settled then.


Not sure I’ll ever be the cool kid, but I’d settle for quirky oddball😂 Haha!!! If I went self bias I wouldn’t have to order the bias pots, which is a saving, and have three matched pairs of KT66 in the cupboard to pop in. I also seem to go against the tide with what I like he sound of as it happens...for example I never did like the stock JTM45, and always favoured a Selmer TnB 50 mkII over that amp model any day of the week!

[shooter]

seem to go against the tide

then self bias is a MUST 
from one contrarian to another 

[Jennings]

Hahaha!!! Solidarity brother! Self bias it is then 👍

[shooter]

welcome to the dark side 
hunt down a datasheet for your PA tubes, get a "ballpark" idea where you expect the plate volts to "land".  the datasheet will give you a "typical" Rk value for that voltage.  the example I looked at was 450vdc plate, recommended 500 ohms for Rk, but that is for 2 tubes (I think).  once you have the start Rk value, have on hand 4-5 around that value, so if 250 is your start (one/tube), I'd have a 220, 270, 330 handy.  I also prefer each tube get it's own Rk and bypass cap.  Makes the math easy, twice as hard to tweak though. also get new batteries for your math box.
enjoy

[Jennings]

Hahaha!!!  Thanks...I think most of my self made/modified gear goes against the grain.  My wind/magnet choices for example in most of my pickups, my FX usage, and my retro amp tastes I rarely deviate from, even though my main band play heavy and fast rock...who says I have to use a Mesa and active pickups with a 7 string?  Haha!!!  Equally I play mandolin using a modded mandocaster with a self-conceptualised strat-sized rail pickup and a modded ProCo Rat!  Anyway, I digress...

...The Shaugang datasheet isnt the best, but seems to suggest the 0.5 MOhm for self bias.  I presume that's for 2 valves then, especially as most of the other data sheets I've seen seem to quote similar for two valves and recommend separate resitors too.  What cap values should I try?  I've seen a couple of circuit diagrams using 47 - 80uF caps with 200k-540k 10-25w resistors.  Of course, I don't know what the success of the builds were, but I thought I'd start lower and work up if I need to.

[shooter]

seems to suggest the 0.5 MOhm

That probably refers to the grid (G1) resistor.  You're looking for Rk, (cathode resistor)

47 - 80uF caps with 200k-540k 10-25w resistors

I typically use 47 - 100uF.  again, the 200k - 540k is WAY off on self biased, I can see that for the grid R's where you inject the FIXED bias.  I would be looking for values ~~ 250 OHMS/ tube, double that for a single R

[Jennings]

Ah yes, my bad! The tired eyes misread “R” as “K” on those schematics I was looking at...I usually use a small “r” myself to avoid that 😂


I can’t find much data for the Shuguang tubes sadly...and what I did find was sparse and had spelling mistakes...I’ll use ballpark and adjust if required if I can’t find anything more definitive.

[Jennings]

OK, I've had a go at drawing out the schematic for this project...I'd be really grateful for any comments, tweaks and corrections, please.  Apologies for the quality of the software used, the fact that it doesn't have all the symbols quite right, and my novice electronics knowledge level!  May I direct you to two areas I'd particularly like a bit of double checking on.  Firstly, I'm sure you can't just combine the two preamp channels at the DCCCF like I have?  Do I need to in some way separate them to prevent interference issues?  Also, Does my power supply look about right?  Thanks in advance!

[Auke Jolman]

Hi Jennings,

I'm following this build with interest.

I think that in your schematic HT - D the 1M resistor schould only connect to the 0.1 uF cap under it and not to the HT 220k resistor. Futhermore you might need 2 mixing resistors at the junction where both channels come together. An option is to use a switch to select the right channel input. You can combine it with the input selection switch.

[sluckey]

I've added some suggestions to your schematic...

[tubenit]

IF you have some interest in fixed biased for KT66 tubes, this schematic may be of interest to you?  It's a proven build.

http://el34world.com/Forum/index.php?topic=23437.0

With respect, Tubenit

[Jennings]

Cheers chaps...much appreciated, and thanks for the help.  I'll check out the fixed bias suggestion and make a final decision.  Then I'll whip up a BOM and see what I've got in versus need to order.  I'll also post some pics when I'm building, and maybe some sound clips when I've recorded with it.  My original Dual Lite (just as Steve's circuit, but with lower filtering and EZ81 rectifier), was used for all the lead/melody and riffage on the last two or so Fatal Crowbar Injury EP releases (1x12" combo with a Weber Jensen style alnico).  Before that I used my home made AC30 (just the TB channel in combo form with Weber alnico blue).  Hoping to use this one live later in the year, again in 1x12" combo form with a Celestion Century Vintage.  Plus then I'll use it for recording in the autumn for a late year release.

[DummyLoad]

OK, I've had a go at drawing out the schematic for this project...I'd be really grateful for any comments, tweaks and corrections, please.  Apologies for the quality of the software used, the fact that it doesn't have all the symbols quite right, and my novice electronics knowledge level!  May I direct you to two areas I'd particularly like a bit of double checking on.  Firstly, I'm sure you can't just combine the two preamp channels at the DCCCF like I have?  Do I need to in some way separate them to prevent interference issues?  Also, Does my power supply look about right?  Thanks in advance!

on the triode preamp: i believe that the volume pot is wired incorrectly. e.g. output should be taken off the wipers.


--pete

[sluckey]

on the triode preamp: i believe that the volume pot is wired incorrectly. e.g. output should be taken off the wipers.

yep

[Jennings]

Thanks again folks...OK, I think I've made the corrections, revisions and suggestions.  I've decided to keep with cathode bias for now, simply for the novelty if nothing else.  All the other KT66 and EL34 amps I've made or modded have been fixed, so I fancy seeing what the difference might/might not be.

[shooter]

wait for conformation, but I believe the 180pf needs to be left of volume pot, or more like switched as Tubenit has in his post.  because you have no grid reference to ground on your DCCF

EDIT:  I did find a path on the bottom volume!

[DummyLoad]

perhaps attached is what was intended.


--pete

[Jennings]

😊👍 That’s it exactly! Cheers!

[shooter]

 
yup, doing 12 things at once and failing miserably 

[Jennings]

Story of my life  I'm halfway through a refinish on a 50's Selmer lap steel.  Multitasking!

Here's the edited schematic for completeness.

[Jennings]

Full size on-on-on switches seem to be a rare and expensive beast 🤔

[sluckey]

That 7211 is a mini switch but it is available with several different toggle handles. Here's the 1/2" fat bat...

https://www.ebay.com/itm/Guitar-toggle-switch-ON-ON-ON-Fat-bat-toggle-1-2-hole-C-K-7211-HIGH-QUALITY/252267874328?hash=item3abc564818:g:Z3EAAOSwxN5WWJGg

You can get the 7211 cheaper from an electronics supplier that isn't advertising these as guitar switches...

https://www.digikey.com/products/en/switches/toggle-switches/201?k=c%26k+7211&k=7211&pkeyword=c%26k+7211&sv=0&sf=0&FV=ffe000c9&quantity=&ColumnSort=0&page=1&pageSize=25

...or digikey direct link to 1/2" fat bat...

https://www.digikey.com/product-detail/en/c-k/7211T1ZQE/CKN11690-ND/2783906

[Jennings]

That's great...thanks :-) 

[Jennings]

Well I've completed the preliminary build...and yes, I do come from the Fender school of wire dress rather than HiWatt    Anyway, she powers up and works, but does seem to need a spot of final trouble shooting.  Even with both volumes completely off the amp is still on...loud enough to play your guitar in your bedroom that way!  Plus the hum is louder than normal, and the channels are a bit sputtery as you turn them up...particularly the 18w.  I'll sit down and go through it as and when time allows, but in the mean time can anyone spot anything obvious to hone in on?

[Jennings]

Tried a trace through with the schematic, and have yet to spot the issue...although I might try a spot of rerouting the ground line and some lead directions. Plus I’ll try to multimeter the voltages this week. To me the hum sounds like DC in the line, so either a short or grounding issue...and that might explain the fact that there’s significant playing volume even when both volume pots are turned right down.

[sluckey]

Use a gator clip lead to ground the junction of the 220K mixing resistors and V3 grid. Does the hum disappear? Does that totally kill the guitar sound?

[Jennings]

I'll try and do the checks tonight if I get time after work...otherwise I won't be home until Sunday.  I've drawn up my voltage sheet ready, and I'll add the check you suggest too cheers Steve  . 

I am little suspicious of channel interplay issues as I had a similar problem on a build some years back.  Also it's the first DC couple cathode follower I've used, and the valve position makes more noise than I'd expect when the chopstick test is performed.  I tried a couple of tubes in the dcccf V3 position with the same result when I powered up for the first test.  The guitar tone when both volumes are off is quite bright.  As you turn up the volume the fuller normal amp tone comes in as you'd expect, but very early on in the volume range it goes into instability and bassy hum/feedback issues.  On the plus side the KTs behaved perfectly with the cathode bias circuit as spec'd, and the GZ34 is doing dandy.  My money's on a wiring mistake, unseen short, or some channel interference in or between the preamps.  Just to double check, what's the hook-up look like on that on-on-on switch?  I currently have the lower middle wired to the input jack, the left pins (top and bottom) wired together and off to the AC15 pre, and the right pins (top and bottom) wired together and off to the 18w pre.  Should I have a ground somewhere?

[sluckey]

Look at this to see how that switch needs to be wired...

     http://sluckeyamps.com/phoenix/phoenix.pdf

[Jennings]

Well my switch is definitely not hooked up right!  Thanks for the direction.  Could well be a big part of the problem!  I'd looked at a diagram which obviously has a different configuration.

[Jennings]

Switch wiring adjusted.  Tested the amp after that:

Switch right
- both vols down gives the guitar sound quite loudly.
- AC15 vol up gives guitar volume increase.
- 18w vol up reduces guitar volume.

Switch middle
- both vols down gives the guitar sound quite loudly.
- turn either volume up and the guitar volume increases.

Switch left
- both vols down and no guitar sound at all.
- AC15 vol up gives some guitar volume after about half way.
- 18w vol up gives rapid increase in guitar volume.

Something not quite right there still.

Measured the voltages as per the attachment.

[Jennings]

I got a bit of time over my lunch break to pour over my schematic, and when I get back to the amp later at the weekend I want to double check my DCCCF wiring...I want to check that I haven't missed the connection between the diode and the 47k resistor.  I'll report back with what I find when I get my hands on the amp.  Just looked back at my photo, and I think my hunch is right on that!

[sluckey]

Why do you even have a cathode follower in that amp? It certainly isn't needed. That LTP PI has a very high input impedance and can easily be driven from the plate of the triode that is currently feeding the CF.

[Jennings]

I could take that stage out...I only popped it in after an early suggestion, and I was curious as I’ve not used one in a build before.

[PRR]

> Why do you even have a cathode follower in that amp?

It is seriously mis-biased. It distorts. If it copied Fender's values the distortion is mild at high level. Many folks mis-copy it and then it may distort at low level, may even be in grid-current at no-signal so everything is bent.

It really wants Careful Design, scaling to the rest of the circuit. However Design By Soldering Iron keeps the builder busy.

[Jennings]

I’ll look at it over the weekend. Cheers folks 👍

[Jennings]

I've dispensed with the cathode follower and routed the first triode of that valve to the PI, leaving the second triode unused.  I've had to repair an amp for a gig tonight, so that's all I've had time for on this project.  I'll target hunting for the issue in the preamp at a later date.

[Jennings]

Did some section checks yesterday afternoon. Since power up with all valves gives a loud and low frequency constant hum I powered up with only the GZ34 and KT66s in. Result was completely quiet and stable running. So confirmed that the power amp section seems fine. Plug in the PI valve as well and the loud low hum is back, even with no life in the other preamp positions. So something’s going on in my PI section there. Start plugging in the other pre amp valves and it just adds in the fact that you can adjust the knobs and induce feedback and squeal. With all preamp valves in there’s a microphonic effect if you tap the amp...and if you tap the triode prior to the PI it’s the loudest. Like ringing a bell!

[sluckey]

With just the rectifier, power tubes and PI tube plugged in you have loud low hum.  Disconnect the NFB wire. Any better?