New amp build - Modded Robinette JCM800 EF80 + switchable mods

[NickK-UK]

So yesterday was the best type of day - I got a performance related bonus so I have some budget and the significant other has approved expenditure.

I've been mulling this planned design over for several years and with the almost completion of my guitar, my mind is switching back to some form of amplification. I've designed and built hit tube amps (640Vdc!), I have an old 1957 tube based entertainment system I've restored back, plus I have luxury of a scope, sig get and bench supplies.

So what do I want this amp to be able todo in the end (after iterations):
* Able to take a single 7 string 28.625" scale length (actually 29" for low B) drop tunings - allowing both old metal (JCM/Fortin) and new prog metal (abasi) cleaner tones.
* Cope with the Sentient and Pegasus pickup outputs (which are medium hot output) in humbuck and split levels.
* Play at full volume in the following scenarios (ie have an inbuilt speaker load to attenuate for):
* * Connect via DI to be completely silent but allow recording of the sound when using volumes to saturate the output transformer
* * Connect via headphones (mine are 55 ohm AKGs) - pure analogue
* * Creamback G12M-65 - pure analogue
* take upfront pedals well - this should be reasonably easy
* have a pedal effects loop
* have reverb tank

Getting a good power transform is a pain, especially one that supports iterative changes. I have two 117V secondary tranformers I'll use and a 22V transformer - I'll build the power supply with those and then after I'm comfortable switch to a single transformer.

So my starting position was the Robinette jcm800 micro 1W using EF80 output tubes.
A design decisions:
* use the original B+ voltages for the JCM800 preamp section - for this I will have two B+ lines so that the preamp can sit at higher voltages, the down side is less sag up front in the pre based on the power tubes current draw. To maintain the character both B+ lines will have a choke.
* use a solid state rectification - there will be two power supplies. I've looked at EZ81 as a minimum voltage drop but it just seems to increase the power and transformer requirements - especially with additional stage driving etc.
* Use elevated heaters (~75V) with DC rectified 6.3V supplies (the EF80 needs 6.3V anyway) I could do 12V for the preamp section at a later date. This should save the poor cathode follower that is notorious for destroying valves I'll also put a diode over to protect the grid.
* turret board - this allows me to make the stages and then move from the development chassis at a later date. Point to point preferred.

Now for the abasi cleans.. I may look at having a switch over from a 12ax7 for the LTP to something with less gain - two options here 12au7 or a 12bh7a-str which I have. The 12au7 is less linear than the 12bh7 but both will do a fender clarity.

Anyway - that's it so far (I have modelled this in LTSpice to get a finger in the air but that does nothing a soldering iron and a few shorts will teach you).

I will use this thread as my build thread as I progress.

 

[NickK-UK]

The bad news is I've dug out my transformers and it appears I have one 2x117V secondary transformer instead of two transformers, and even stacking the transformers I have would not provide the required voltage (and I don't want the hassle of doubling or tripling ladders).

The good news is I've found a suitable single toroidal transformer that is designed for a 50W Marshall head:
* 2x 0-340Vrms 150mA HT secondaries, 300mA in total (65x 12ax7!)
* 2x 3.15Vrms 5A heater, 10A in total (33x 12ax7 tube heaters)
* 120/230/240V primary - my wall voltage is 253Vrms which typically pushes heaters to their limits with 230V primaries, so 240V primary is a godsend.
* 137VA shouldn't need an inrush control. Over this and I like using a PTC on the primary.

The HT DC output of the power supply is approximately 430Vdc. So nicely below 500V which simplifies filter cap selection.

I will connect the heaters direct to the 6.3Vrms and keep them AC. Previously I had intended to run DC heaters but this will do.
I had a concern that connecting the HT and heater to elevate the heaters (75Vdc) would provide a path for AC hum. However I think the solution for this is simple as I have two independent HT power supplies, one preamp and one LTP+power amp. If I use the elevation tap on the HT for the push-pull power section, any common HT B+ 50Hz hum introduced will cancel out in the output transformer.
Modelling the amp with zero input I can see at the moment provides a little confirmation of that solution:

In reality I'd take this with a grain of salt but it's promising - the 100Hz hum of the rectified HT DC is higher than the 50Hz that would appear if it was an issue. Eitherway at below 120dB I'm not going to hear it.. moving on..

If I want to play with EZ81 rectification I can do at a later date but I would need to add a 1Amp 5Vac supply for each rectifier tube added. However let's get the thing working and I can play at a later date (I have a EZ81 in the old 1957 tube radio I can experiment with).

Next is to create a simple passive reactive load for silent operation, model that in LT spice with the amp.

For DI connection I have a couple of options - a passive divider, or, use an opamp to buffer which would prevent any change in the frequency response. However for now I may opt for the passive. The input of the DI should have a opamp and I can keep the cable short between the two to reduce inductance and capacitance.

I also measured the peak-to-peak voltage of the guitar pickups - 2.58Vpp neck and 1.3Vpp bridge (that bridge may get reseated a little).

 

[NickK-UK]

So.. I've been woking through the amp in terms of B+ voltages, power transformers etc.

I've decided to go for a 300-0-300 150mA transformer. This has the added benefit of reducing power loss but also allows the use of a EZ81 rectifier. That in turn means a standby switch is less of a requirement. This means less recovery rate but the caps will allow an attack rate, in theory but it does allow an element of tweaking of B+ voltages with resistor changes without changing the rectification. I don't want to go lower than 300V secondaries because it's more difficult to fix any ripple.

If I want to change/add tubes then I may need to change to solid state. I'll worry about that when I get to it.

	B+1	B+2	B+3	B+4	B+5
Rvalues 1	295 (EF80)	292 (EF80 screen)	282 (V3 - LTP)	350 (V2 - Pre3+CF)	335 (V1 - Pre1+CC)
Rvalues 2	250 (EF80)	250 (EF80 screen)	330 (V3 - LTP)	291 (V2 - Pre3+CF)	281 (V1 - Pre1+CC)

So you can see I can vary the voltages quite flexibly with just a couple of resistors. I think I'll go for the lower voltages initially until I know the actual physical voltage appearing.

Let's start there.
Next step is to build a bill of materials.

 

[NickK-UK]

So based on that initial modelling. I've made my bill of materials. It's annoying that I can't really reuse my existing transformers for this but they're just a little too lower voltage. Also with the sink inductors that would take way to long to be shipped they'll be added at a later date.

Resistors
For the power resistors I'll be using wire wounds - TE
For the majority of resistors they will be metal foil, a combination of TE and Vishay.

Caps
For the electrolytic caps I'm going to use F+T caps 500V and 450V.
For the decoupling/foil caps I'm going to use Vishay MKTs

Tubes
EZ81 - JJ, and I have a old 1957 EZ81 in my old receiver.
EF80 - Mullards
12ax7a - JJ but I want as Phillips MilliWatt for the main gain.

The preamp tubes will be shielded and the EZ80s and EZ81 will have spring retainers.

Transformers
139VA 300-0-300 and 6.3V toroid
Hammond 155H 5H 50mA chokes
Hammond 125B output transformer

Indicator lights
I'll use a fender style B9A pilot light off the 6.3V but I need to isolate this because it will be running elevated.
The HT supply will also have a low current LED internally.

Power socket
I'll use the IEC socket I have already that has a mains filter, fuse and switch inbuilt.

Chassis
Aluminum chassis, with a baltic plywood case added at a later date and I will add a aluminium power transformer shield.

 

[NickK-UK]

Ok, I have my final list of items for the build and where to purchase (some prices are without tax and delivery, some with so it takes time to work out the best location to order from) and some final decisions:

* EZ81 rectifier is confirmed I have some 1N4007s or UF4007s if need be but I'll need to adjust the resistors in the B+ lines to reduce the B+ to acceptable levels but that's a later mod
* It will have a clipping switch for 20V diodes and 2N5400 based clipping.
* I decided - blow it.. it will have a 16ohm G12M-65 initially and then I'll build a reactive load once I've taken some readings

I note that the Hammond 125B is a 100Hz to 30kHz so I may have to change the bass tone to provide a lower frequency boost, especially as the lower frequency drops with higher speaker impedance. This may be ok if you're running a 6 string but for a 7 string it may need a little more current handling for the lower end. In the longer term I may end up finding a better transformer but that's a fight for another day.

 

[NickK-UK]

Parts finally all ordered - turns out I've had to source the EF80s from a different seller bringing it to a total of 4 different sites for ordering. (I wonder how long it will be before JJ start selling them too)
I will use the vintage EZ81 I have already rather than use a JJ for the time being - just to reduce costs (£625 including delivery and sales tax on both parts and delivery). I had a bonus due to working my butt off so this works (96 hour weeks anyone?)

I have a 1/4 sheet of baltic ply left over, so I can make up something to take the speaker easily enough - although it's just occurred to me I missed the speaker mounting screws.. well there's always going to be a follow up order somewhere.

 

[NickK-UK]

Parts have started arriving... (teaser) still waiting for the main delivery..

 

[spilla]

Sounds like you got a good plan in the works. Will be following. Ive heard good things about robs jcm micro

 

[NickK-UK]

Final package finally arrived.

 

[NickK-UK]

So I've been working on the layout in DIYLC, and have some decisions:

1. Heater wiring layout - do I (a) use the vertical wiring against the shield plate rather than (b) hifi style wiring where the wiring its in the corner of the chassis and run out to each tube socket. The benefit of the verticals that it's still against the grounded metal plate but is further away from the choke and output transformer that are sat on the flip side of the top.
2. Switch to mini-switches for the following mods, as the switches will not be carrying high voltages, it's relatively safe todo:
* V2 mod bypass - this would switch the signal for the standard preamp/CF V2 stage between the V1 stage and V2 mod stage. This has some possible impact on the signal.
* V1a cathode select - between Marshall/Soldano/grid clip cathode resistor values. Ideally this would be on the far left of the panel.
* Clipping select - diode/off/bjt post tone stack clipping (originally I had a large switch for that planned hence the square between the preamp and master volume.

Here's the layout - I tested the transformer-to-choke, transformer-to-OT and choke-to-choke interference. That big schaffner IEC filter I stole off one of my experiments has the IEC socket, filter, mains switch, two fuses in one big metal box. Still have a few additional things to add to the layout (speaker output etc and the switches) but it's good start:

I've missed the 4k7 presence resistor off the order by accident, so I need to order some additional bits - I may as well order the mini switches and resistors/coupling caps for the mod stage.. so I may order the mini-watt ecc83 as I'll need another 12ax7. Oddly I dislike 12ax7s and prefer the lower gain and more linear 12bh7a-str in multiple stages but this s a guitar amp which is a different beast.
I'm also tempted to add a socket for a 12bh7a-str (ie a slightly more linear 12au7) output tube whilst I'm at it - the option to use pentode or triode would then be a case of pulling the output tube(s) and installing whichever I wanted. I've also got 3W screen resistors - which should be fine for the 2W tubes, but I may add a pair of backup 5W flameproof to the order. The 3W for the rectifier are high surge so they can take 7W for a short time on startup, but continuous rating is 3W which is more than enough for the amp. The minor peeve is they supplied a blue jewel and not a purple.. will contact them and see what happened there..

Also it's apparent that getting a tube-driven reverb and effects loop stages may be difficult to squeeze in. I'd have to use the space either side of the LTP by positioning the valves there (ie the reverb would require a drive amp and a buffer, the effects loop would require a driver and buffer to prevent the external circuits having an impact).

I also need to design the reactive load - now Aiken's design is for 16R 100W.. we only need 5W. My thinking here is that I can make a cheaper design with smaller sized inductors (possibly self winding with copper wire).

Either way - the new purchases don't really stop the next stages of the amp - other than mini-switches - let's get that built first then worry about the second output tube, etc. There's space and that part of the amp isn't going to be seen in head or combo forms anyway.

I have also had a thought that the Mrs' sublimation printer could be used to make the markings for the front panel and then use a plexiglass front to protect it. Not thought about that too much to be honest but with all the switches.. it will need some front panel description so I remember

 

[NickK-UK]

Update so far as the final candidate.. looking for feedback.

Note I get a wall voltage of 253V which results in a 5% higher secondary voltage when I use the 240V primary.

Some differences to Rob's JCM800 which is based on:
* 0-300V secondaries means burning some wattage and less sag. It's built using the 240V primary and I have 253V wall voltage hence everything gets a 5% uplift.
* Two chokes - one for pre and one power stage.
* EZ81 - I will add a pair of 1N4007 as backups just incase..
* V2 mod built in with the extra modifying the waveform rather than needing direct gain. I can then switch this in/out.
* the LTP is current sat on the pre-stage supply but could be moved to the power stage but run at a lower voltage
* 16R speaker using OT 1-4 with 4 being ground (yellow) and 1 being signal (black), then the NFB is using the 8R tap (orange)

 

[NickK-UK]

And so it begins...


Just to say I had a bit of mad session this morning - in the ADC I've build I obsessed over current loops. So this is the update with regard to that obsession - this makes the power/rectification loop and cap loops much smaller:

 

[NickK-UK]

Missed an update yesterday...

I tried overcomplicating it slightly with attempting mil spec wiring - so that has thermal expansion (both contraction and expansion) built in which is why the wiring looks a little odd. It should work but in the end it's good enough and it's clean. I found I'm missing a 4k7 resistor - so for a follow up order I think plus I'd missed the 270K/220K attenuator but managed to squeeze that in with an extra turret.



I need more turrets for the power board, and a I messed up missing out a 25K pot. etc etc.. so there's a few things to add.

SO today was random stuff and finally get around to some chassis work.

My punch is 21mm so a little large for the Belton sockets (works well with ceramics) and rather than wait for a smaller one I got a step bit. A bit of a faff and not quite as pretty as the puch hole but it's effective.

The chassis front is offset down to cope with being in a combo or a head. Microphonics to determine. Also there will be switches above the sockets. I have a plan to put a face plate on as aluminium scratches easily so I'm not too worried if there's any. Rectifier and HT fuse next to it - idea being if it's blown then something needs investigating and not simply change the fuse only to create more damage. V2 mod socket hole made but I'll add a Belton socket to the next order.
The power transformer will end up flipped, once I have worked out the best hole for position for the transformer primary/secondary. Once flipped I'll re-confirm the transformer and choke positions. There's ~1.5 tube widths between each tube which should be good with the little tubes. Chickenheads - you have to love these, for me they scream DIY (mainly because my did had chicken heads on his old builds etc).





I thought I had found a 4k7 resistor on a low noise current shunt project prototype board.. but alas it was 2k49.

Anyways - this should allow me to crack on with the wiring etc whilst that second order comes in.

 

[NickK-UK]

Ok.. so things have moved on

I'm deliberately starting the build up with larger droppers than I would use normally as I don't know the transformer (it's not an Antec or Hammond hence details are a little scarce).






Bode plot using a 16R resistor as a dummy load. This also is using the low gain input:



Turning up the presence pot (I have a 1Meg instead of a 25K and a 5k6 instead of the 4k7 resistor):


The high gain is even better and I can see on the scope that it's distorting as you'd expect at high volume.

It's late and I'll sort out some more tomorrow.

 

[NickK-UK]

Note this is running with reduced voltages initially - I will order some new resistors to increase the voltages and that will give it some additional bite. I will also add the V2 mod and that will also provide bite (although at near unity gain).





So initial impressions:
* Can sound bluesy
* Can bark
* Chuffing loud (97dB/W and 2W)
* Does have some bass but it's not a bass amp.

 

[NickK-UK]

Lots of hum finding (mainly ground loops)- so the hum is mainly behind the hiss now - I do have this really annoying 20kHz noise (I think it's feedback through the heater wires through the heater elevation.
This is full pre-amp, full master volume and full presence, plus full tone stack:

 

[NickK-UK]

So some background noise, power on, then pre and master volume noise floor now I've had a go at reducing the hum. It's probably still uploading but has about 5 mins left..



In short
* open chassis with out full shielding or shielding on the power cabling or EF80s.
* still has the mains 20khz noise - that I'll have to think of someway to low pass that or notch it.
* the mains him is now very very low, below the hiss of the amp reallly - so full volume pre + master + presence and tone stack is now humless but instead has hiss.
* the hiss is the next target.. although we're limited in what we can do (no running dual triodes for each stage to reduce the noise floor.

I also did a 20V shottky mod to test that.. hehe that gave it 1980s metal vibes all over. so that's going on a switch.

 

[NickK-UK]

An update on the sound - I've switched the NFB from the 8R tap to the 16R tap alongside the speaker itself, the sound speaks for itself. I've also made a number of other mods in terms of reducing noise:



A far better sound and I get feedback at the end of long sustained notes. That seems to come in because the NFB can't keep up.

 

[NickK-UK]

I was playing around with drop tunings on the baritone to see what the response would be:

The amp and speaker have no problem with B1 and I know from experience that the guitar with 62 gauge strings works to F#1 but not E1, I suspect with 68-72 gauge strings that it would do D1 however at that point (around 30Hz) the small output transformer isn't able to drive the speaker and the frequency response roll off hits hard.

Perhaps something for direct interface connection when done