Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Jennings on May 05, 2019, 02:57:42 am
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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?
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Sounds like a fun project that will be powerful and versatile. I look forward to seeing this project develop.
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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
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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?
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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.
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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.
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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.
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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.
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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?
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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.
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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!
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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!!! :icon_biggrin:
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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!
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seem to go against the tide
then self bias is a MUST :icon_biggrin:
from one contrarian to another :occasion14:
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Hahaha!!! Solidarity brother! Self bias it is then 👍
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welcome to the dark side :icon_biggrin:
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
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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.
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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
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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.
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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!
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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.
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I've added some suggestions to your schematic...
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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
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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.
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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
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on the triode preamp: i believe that the volume pot is wired incorrectly. e.g. output should be taken off the wipers.
yep
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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.
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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!
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perhaps attached is what was intended.
--pete
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😊👍 That’s it exactly! Cheers!
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:think1:
yup, doing 12 things at once and failing miserably :cussing:
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Story of my life :laugh: I'm halfway through a refinish on a 50's Selmer lap steel. Multitasking!
Here's the edited schematic for completeness.
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Full size on-on-on switches seem to be a rare and expensive beast 🤔
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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
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That's great...thanks :-)
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Well I've completed the preliminary build...and yes, I do come from the Fender school of wire dress rather than HiWatt :laugh: 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?
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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.
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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?
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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 :smiley:.
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?
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Look at this to see how that switch needs to be wired...
http://sluckeyamps.com/phoenix/phoenix.pdf
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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.
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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.
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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!
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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.
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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.
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> 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.
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I’ll look at it over the weekend. Cheers folks 👍
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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.
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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!
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With just the rectifier, power tubes and PI tube plugged in you have loud low hum. Disconnect the NFB wire. Any better?
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I dont have any NFB on this build...so no wire, resistor etc.
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I would check for a cold solder joint in the wiring around the PI..Maybe the grid of the PI tube. Also the grounds of the PI stage.
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Good suggestion...I did reflow one connection I was suspicious of where the cut control meets the 220k resistors to ground, but I'll go over the whole stage. I'm also wondering whether there's an issue in the power stage feeding the PI or V3 plate, and the next link down to the triode feeding the PI.
On a separate note, I'm debating later on whether to dispense with the preamp 220k mixer resistors and use the spare tride in V3 to make each preamp have it's own dedicated stage prior to the PI, then feeding each side of the PI separately.
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Brief update - Part way through separating my ground bus into more of a star ground configuration. My original bus design worked fine with other builds, but separating the control array onto its own ground line seem to have helped a bit on this one. When I get more time I'll continue so that the power line, each preamp channel and the main amp circuit all have separate lines. I've also done away with the mixer resistors and used one half of V3 in each preamp channel, putting them into either side of the PI separately now. The main issue is the channel interplay still...the slightest nudge of one of the volumes and the amp is very loud instantly. Turning them up more leads to feedback and squeals.
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After some busy months and a family bereavement I'm aiming to get back to finalising and finishing my KT66 Dual Lite build...first step is to confirm the schematic. I'd love it if the more experienced could kindly cast a knowledgeable eye over the attached schematic for any comments, corrections, ideas or recommendations. All gratefully received!
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Schematic looks fine. I do wonder if you need the extra gain stage between the volume controls and the PI. I'd try pulling that tube and connect the volume wipers directly to the PI input caps. Should probably eliminate the squeals. How does that sound?
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the EF86 and paralleled 12AX7 share the same power node: speaking from much experience building similar front ends, you really want a seperate supply node for each of those stages and a 100nF HF shunt on the EF86 B+ supply.
grounding is critical. ALL preamp part ground connections should follow one bus to a single ground point at the input jack, IF you are using a switchcraft jack, if you're using a cliff jack, take the jack's ground lug to V1 and V2 bypass caps ground and V1 and V2 power supply caps grounds then wire those to a ground lug near the input jack.
don't bypass gain stage that drives the CF - if you want more gain - bootstrap the CF as shown in the attached. you don't have a gain sucking tone stack after the CF, so you're slamming the LTPI VERY hard. consider divider with a fixed R and pot to tame that - again, see attached. R15 can be 1M or more or make VR4 smaller.
use coax from VR1 and VR2 to the 220K mixing Rs and mount the mixing Rs as close as possible to V3 pin 7 - use a terminal strip.
--pete
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Thanks guys...much appreciated. And that Dual Lite 45 schematic is a great reference for me. I’ll do the minus gain stage check, and then move on to separating the power node for the EF86 and first ECC83. I’ll also review the grounding etc as suggested.
Once I get this running without the hum and volume/squeal issue I’ll then work out whether I want more or less gain form the pre-PI stage. I’m aiming for a ballpark AC30 response as the volume goes up.
I’ll post the result in due course. And a sound clip to help show what the problem sounds like. I’m still slightly suspicious there’s a wiring fault/short that I just can’t trace or see yet...or a dodgy component.
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I’ve split the ECC83 / EF86 power node with a 1.5k resistor and another 8uF cap, plus tided up the grounding as well. While I was there I found I’d accidentally switched the OT and ground connections to the speaker jacks 🤦🏻♂️ Can’t believe I’d missed that! Stopped to test the result, and I’ve succeeded in fixing the hum/issues and the amp controls all work fine, tone is good etc, and its no longer clouding the issue.
So now on to the gain...turning up either volume even slightly gets way too much volume too fast...and the amp just starts feeding back...so definitely the extra pre-PI gain stage either wants removing or reigning in. Before I start, I should just whip it out, or maybe try dialling it down by inserting resistors between the gain stage and the PI? Thought I’d ask before I start on it.
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suggested mods attached: individual drivers with added complexity -or- plate summing is second.
before you hack, try removing the bypass caps on the LTPI drivers and replace 12AX7 with 12AU7.
--pete
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I would temporarily bypass the extra gain stage. Just pull the tube, remove the 100K plate resistors, and put a jumper between grid and plate pins directly on the socket. Play it a bit to see if you're happy without the extra gain. If you find you still want more gain put everything back as it is now. Then put a voltage divider just prior to the PI input cap. You will need a coupling cap between the plate and the two resistor voltage divider. You may also need a voltage divider prior to the grid of the extras gain stage.
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Thanks for the guidance...much appreciated as ever...I’ll give that a go 👍
Update: I’ve removed the tube and bypassed the extra gain stage. Quite like the result, and it definitely makes the amp useable now. I’ve only tested it through a bass speaker I had to hand this far, but tomorrow I’ll pop it in it’s 1x12 combo can and try Celestion Century for a bit. Could be a little tame, but it’s hard to be sure on a brief test through a bass heavy cab. Great progress...and thanks for the assistance!
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The standby switch arrangement is ‘hot switching’, not good for good service life from the rectifier tube.
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Crikey...I'd completely over-looked that! Thanks! I normally never use standby switches, and only included one here as I had the chassis hole...I'll move it. Actually I might disconnect it and just leave a dummy switch to avoid the faceplate hole.
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I just noticed that you used a CK 7211 (on-on-on) switch. I would think that when you select both channels the sound would get thin because the signals will be out of phase and do a lot of cancelling. The signals are in phase up to the PI inputs, but the "out of phase" condition is due to the fact that each signal is applied to different inputs of the PI. The LTP PI is really a differential amplifier, meaning it amplifies the "difference" between the two input signals. If the two signals were exactly the same the PI output would subtract those signals, producing zero output at the plates.
BTW, I just used a simple DPDT switch on my Brit Dual Lite for this very reason. So, how does your amp sound when both inputs are selected?
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That makes sense...it does indeed sound thin on the middle setting. I might consider mixing the channels earlier and feeding into one input of the PI instead as part of considering Pete's mod suggestions. I might also replace the switch anyway though, as I'm probably fairly unlikely to use the middle setting much really.
Out of interest, would there be any benefit in me mixing the channels via resistors into one input of the PI, then referencing the other to ground over my current set up (or vice versa)? Other than saving one half of an ECC83, and thus also needing one voltage divider only rather than dual...thinking of if I do go back to having the extra gain stage in there and a voltage divider as per Pete's schematic mod suggestion.
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Out of interest, would there be any benefit in me mixing the channels via resistors into one input of the PI, then referencing the other to ground over my current set up (or vice versa)?
Absolutely. Doing that keeps everything in phase and will allow you to select both channels. I like Pete's mod3 idea. I would probably experiment with replacing his 220K and 100K voltage divider with a 220K and 250K pot. Might make it easy to dial in just what you want.
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Thanks Steve, much appreciated. I’ll try your and Pete’s suggestions out for size and see where we settle out at.
I like the trim pot idea a lot combined with mod3 if I put the gain stage back in...one daft question though, would I need to add summing resistors in when bringing the plates together, or are they just fine as drawn in mod3?
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would I need to add summing resistors in when bringing the plates together, or are they just fine as drawn in mod3?
No summing resistors needed. The tubes provide the mixing.
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if you try mod3 - try with both plate resistors first, and then with one plate resistor.
you could also add switchable bypass to either or both stages individually for more bite from a specific channel.
suspect 1 plate resistor with both triodes unbypassed will sound best overall.
--pete
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Thanks for the help and guidance...that all makes sense, and is much appreciated. I've removed the standby switch, as I never use them anyway. I've also tried removing the gain stage, after some tube rolling with ECC82 and 81...then rebuilt the gain stage to Mod3, one plate resistor, with an ECC83, and as predicted it's bob-on what we need...the right level of gain etc. I was going to insert a trim pot in case I had to tune it, but stopped short as it's behaving just fine as-is and I'm happy with the result. I stopped short of adding switching for gain levels for now...probably too many options for my simple guitarist's mind! Haha!!! Might try some more switching options on future builds though...I'm learning a lot from you folks, and these builds, and this is the first time I've experimented with dedicated gain stage options like this. One last task for me before I complete this project and build up the combo to completion...there's a return to a little bit of channel inter-play that I just need to stop. Turn up the volume on the channel that isn't selected, and you get some noise. Not like before...this time it's much less and lower in volume. Sounds like a grounding thing to me, so I'll check through my wiring and review where I've grounded the voltage divider 100k resistor on the preamp ground rail.
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Had a quick run through with the amp last night...all looks in order, and it sounds great in its 1x12 birch ply cab. This one I've put with a Celestion Century Vintage, mostly for weight saving. Very quiet at idle, and very low hiss/hum in operation too, so I'm happy the basic circuit is in order. Just the one niggle remains. Select the 18W channel and turn the Vox volume up, and you get a bit of hum when you reach 12 o'clock on the volume knob. Select the Vox channel and turn the 18W volume up, and you get a sort of out-of-phase (hollow, fairly thin) sound of the guitar...almost like it's a deliberate feature, and it's as loud as a deliberate channel on the 18W side. No crackles, no buzz/hum associated...it's pretty cool actually, like when you have two pickups out of phase on a guitar...but it's clearly not supposed to be happening. So long as you keep the volume at 0 on the unselected channel this issue doesn't cause a problem when using the amp. I've tried running the control pot gounds into their respective tube cathode grounds with the selector switch ground joining the preamp bus where it leaves the board and chassis-gounds at the input. I've tried running the controls on their own bus (Vox to switch to 18W to input jack to main gnd point). I've tried running a ground from each control side to the switch, and then from switch to the jack ground (current set up). None of these approaches were noisy, but none seem to impact the channel issue either. I can live with it for now and use the amp, but it obviously niggles me a bit. What do folks think?!
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the second gain stage (the summing amp) is picking up noise from the PS of the inactive channels input tube when the volume of the inactive channel is turned up. IOW, noise is leaking through the anode resistor of the input EF86 or input 12AX7, whichever one is the inactive device and being amplified by the inactive channels summing amp. this is one of the drawbacks of this scheme. to effectively deal with problem, you need to short the input of the summing amp of the channel that's inactive. splitting the PS node of the input tubes to separate PS taps MAY help with this and cross-talk, e.g., each channel input tube and summing amps get their own respective PS tap.
for this scheme you have the solution - turn the volume down of the unused channel.
if you decide to walk down the path of electronic switching, then ground the input of the preamp and input of the summing amp of the inactive channel.
--pete
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I think this will solve your problem...
Remove the switch from the input, wiring the input jack to both 33K resistors. Use a SPDT (center off) switch, outer lugs connected to the wiper of each volume control and center lug connected to ground. Should fit neatly on the control panel.
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Thanks again gents...not only for the build help and patience, but also the switch suggestions. And thanks to all the folks from earlier on with various suggestions, hints and input. I'll probably get some time over Christmas to open her up and try Steve's switch suggestion. I'll order one to have a play with, and let you know what happens. In the mean time I'll enjoy her in her current configuration...it's a compact, open back, birch ply 1x12" combo cab (a couple of inches deeper to accommodate the KT66 with speaker clearance), Celestion Century Vintage speaker (60W). Valves are two TAD KT66, a Mullard GZ34, Telefunken EF86, and the ECC83s are all Electro Harmonix. I might. This build and combo combination gives a bigger, fuller amp than my previous 2xEL84 build in a normal depth cab with alnico speaker. I like the contrast, and I'm really pleased with the tone. I've got some practices and gigs in the New Year to try it out in anger. Here are a few pictures of the completed build:
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And here is a revised schematic with the final (for now!) revisions, a couple of slight value corrections here and there for trueness to the build, plus I've indicated the grounding groups.