So with the tube circlotron HPA in progress I'm researching my next project - my first and small guitar amp for small room/practice.
Used to have a solid state Marshall practice amp back in the 90s (IIRC a Lead 12) with a Boss DS-1 and Boss Pitch shift & Delay. I still have my old Japanese fender in good condition but over the years my amp/pedals disappeared (I think during house moves). So with a good dose of Maiden, Megadeth, etc and Police etc I'm quite interested in building a guitar amp. Yet I know nothing of getting amp tone/distortion.
Researching - the JCM800/DSL seems to be a good tonal fit and there's examples of people creating 1W and 2W - although I note that the 2W differs quite a bit (including tube rectifier). I'm also thinking of going down the EF80 output stage but keeping the three stages of the larger JCM as most seem to want some form of tube screamer or distortion pedal head of it. The later DSL seemed to have a nicer tone to my ears bit that's what I'm sort of interested in dialling into. I was thinking of a Vintage 30 (60W) which may be too underpowered by the EF80.
Rob has a great page on this: https://robrobinette.com/RR2104_Master_Volume_Micro.htm#JCM800_Micro_EF80
M. Huss has some good information https://mhuss.com/MyJCM/
I understand that the original Drake OT supposedly gives a better sound than the modern transformers (I see amp garage has a Drake teardown here).
Has anyone here any experience/advise on this?
Used to have a solid state Marshall practice amp back in the 90s (IIRC a Lead 12) with a Boss DS-1 and Boss Pitch shift & Delay. I still have my old Japanese fender in good condition but over the years my amp/pedals disappeared (I think during house moves). So with a good dose of Maiden, Megadeth, etc and Police etc I'm quite interested in building a guitar amp. Yet I know nothing of getting amp tone/distortion.
Researching - the JCM800/DSL seems to be a good tonal fit and there's examples of people creating 1W and 2W - although I note that the 2W differs quite a bit (including tube rectifier). I'm also thinking of going down the EF80 output stage but keeping the three stages of the larger JCM as most seem to want some form of tube screamer or distortion pedal head of it. The later DSL seemed to have a nicer tone to my ears bit that's what I'm sort of interested in dialling into. I was thinking of a Vintage 30 (60W) which may be too underpowered by the EF80.
Rob has a great page on this: https://robrobinette.com/RR2104_Master_Volume_Micro.htm#JCM800_Micro_EF80
M. Huss has some good information https://mhuss.com/MyJCM/
I understand that the original Drake OT supposedly gives a better sound than the modern transformers (I see amp garage has a Drake teardown here).
Has anyone here any experience/advise on this?
Ok. Two comments. Maybe three.
A JCM800 or similar master volume Marshall is probably a good fit. It's "RAWK" not super high gain. You may still want a clean (or treblish) booster in front but that's a $50 investment. Perhaps a trip to the music store or hockshop (depending on where you are) might be in order, just to confirm (lots of videos show a Marshall Stack but the player's got something completely different actually making tone)
Also, the amp's tone contains two elements: the amp and the speaker. I've just boxed up an appropriate Marshall 12" appropriately to get the right tone (or near enough) for my 2204. Yes, it's still stupidly loud with a single 12". So factor in a sealed back Marshall-ish 12" on your shopping list.
Finally, starting with a known-good build (like RobRobs) is a pretty good place to start. You still may want a "soak" to get it down to the milliwatt level you'll want at home.
A JCM800 or similar master volume Marshall is probably a good fit. It's "RAWK" not super high gain. You may still want a clean (or treblish) booster in front but that's a $50 investment. Perhaps a trip to the music store or hockshop (depending on where you are) might be in order, just to confirm (lots of videos show a Marshall Stack but the player's got something completely different actually making tone)
Also, the amp's tone contains two elements: the amp and the speaker. I've just boxed up an appropriate Marshall 12" appropriately to get the right tone (or near enough) for my 2204. Yes, it's still stupidly loud with a single 12". So factor in a sealed back Marshall-ish 12" on your shopping list.
Finally, starting with a known-good build (like RobRobs) is a pretty good place to start. You still may want a "soak" to get it down to the milliwatt level you'll want at home.
Just a quick update on the speaker piece - that took me down a rabbit hole. I spent last week watching comparisons between the Celestion Creambacks, great backs and the Alnico cream etc. I was thinking of a 10" cream back although the Alnico seemed to have more detail etc but perhaps not surprising given the double the price tag! The only thing with comparison vids is you don't know how long they've been using the speaker.
The G12-70, Vintage 30, Neodymium and greenbacks, blue etc seem to be quite lead/treble focused. I want crunch and good detail. The creamback seems more rhythm guitarist focused.
Now I suspect there's creambacks and creambacks with the different power handling models.
I suspect that a good power sink/soak may be an option to allow the tubes to run at full but remain with hearing in tact. I remember one of the videos using one and it seemed like a great solution.
The G12-70, Vintage 30, Neodymium and greenbacks, blue etc seem to be quite lead/treble focused. I want crunch and good detail. The creamback seems more rhythm guitarist focused.
Now I suspect there's creambacks and creambacks with the different power handling models.
I suspect that a good power sink/soak may be an option to allow the tubes to run at full but remain with hearing in tact. I remember one of the videos using one and it seemed like a great solution.
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RobRob has some simple options, to really soak without messing with tone you need to emulate the impedance curve of the speaker (and maybe the thermal change in R )
Examples here (AGGH)
So I've been chewing through this prep/design before ordering the BOM.
There are a couple of points that prior builders have noted:
1. Power supply choice
The original design from Rob Is a 190V or 200V power supply transformer, many have noted this provided lower figures than actually stated on the design's B+ values. Some builders have increased this to 220V or 240V transformers, the result from the latter is very close to 300Vdc which is the EF80's design and also puts the output dissipation from the little pentode very close or even beyond it's limit. Given the pentode sits in class AB, this is operating in class B at this point but still operates very hot. Hence the original move to a lower B+ voltage.
The 200V transformer gives about 270Vdc B+ for the EF80s but then reduces the voltage further for the preamp stages. Compare this to the original JCM that has 390Vdc for the front end stages.
The output of the front end will be clod clipped, and typically tubes like more voltage for a cleaner sound so perhaps dropping ~80V isn't a big thing if it's still producing the Marshall sound.
2. Discussion around the use of tube rectifier
The current draw of the EF80 and the rest of the amp is very small, thus a EZ81 tube rectifier has no problems supplying 50mA to an amp pulling 10-20mA max with capacitance to match means the B+ rails don't really sag that much. An option here is to tune the power supply to provide some sag ability to mimic the JCM sag but for the smaller EF80 current draw.
So I am considering running a hot power supply - possibly a 250V tube town power supply. Then use both solid state for the front end to provide a higher voltage front end, but use an EF80 purely for the back end to drop the voltage by ~20Vdc and use a resistor to help provide some sag.
For the speaker choice - there's a reality check that 2W into a 100dB/W/m is going to be loud, very loud. So the search has taken me away from the alnicos and towards something with a lower dB sensitivity. In this case the current plan is a G12M-65 8R which is 96dB/W/m.
I'm aware that this is never going to get anywhere close to breakup, but I can look at attenuating the output transformer to reduce this further. The
Current options I'm investigating:
There is another facet that I'm interested in here - speaker frequency response. So my thinking is that the same switch could switch in a frequency response shaping network so that the resulting speaker frequency response at 10% is similar to the speaker running when the amp is running at full volume in a vain attempt to preserve the tone. However the attenuation and shaping is post build.
The key concern is the power supply at the moment.
There are a couple of points that prior builders have noted:
1. Power supply choice
The original design from Rob Is a 190V or 200V power supply transformer, many have noted this provided lower figures than actually stated on the design's B+ values. Some builders have increased this to 220V or 240V transformers, the result from the latter is very close to 300Vdc which is the EF80's design and also puts the output dissipation from the little pentode very close or even beyond it's limit. Given the pentode sits in class AB, this is operating in class B at this point but still operates very hot. Hence the original move to a lower B+ voltage.
The 200V transformer gives about 270Vdc B+ for the EF80s but then reduces the voltage further for the preamp stages. Compare this to the original JCM that has 390Vdc for the front end stages.
The output of the front end will be clod clipped, and typically tubes like more voltage for a cleaner sound so perhaps dropping ~80V isn't a big thing if it's still producing the Marshall sound.
2. Discussion around the use of tube rectifier
The current draw of the EF80 and the rest of the amp is very small, thus a EZ81 tube rectifier has no problems supplying 50mA to an amp pulling 10-20mA max with capacitance to match means the B+ rails don't really sag that much. An option here is to tune the power supply to provide some sag ability to mimic the JCM sag but for the smaller EF80 current draw.
So I am considering running a hot power supply - possibly a 250V tube town power supply. Then use both solid state for the front end to provide a higher voltage front end, but use an EF80 purely for the back end to drop the voltage by ~20Vdc and use a resistor to help provide some sag.
For the speaker choice - there's a reality check that 2W into a 100dB/W/m is going to be loud, very loud. So the search has taken me away from the alnicos and towards something with a lower dB sensitivity. In this case the current plan is a G12M-65 8R which is 96dB/W/m.
I'm aware that this is never going to get anywhere close to breakup, but I can look at attenuating the output transformer to reduce this further. The
Current options I'm investigating:
- 10% switch mod - This is a pure resistive option to drop the current to the speaker but keeps the amp seeing a DCR of 8R.
- impedance current mirror - this uses semiconductors to mirror the current flow to shunt current past the speaker.
There is another facet that I'm interested in here - speaker frequency response. So my thinking is that the same switch could switch in a frequency response shaping network so that the resulting speaker frequency response at 10% is similar to the speaker running when the amp is running at full volume in a vain attempt to preserve the tone. However the attenuation and shaping is post build.
The key concern is the power supply at the moment.
For some reason I can't quote today. So, excuse me paraphrasing.
Certainly there's something to be said for providing a load which looks like a speaker, as valves behave differently to high-NFB amps (SS or otherwise).
The speaker still matters but you've a lot more choice if you're only playing with milliwatts: any driver with a paper surround is worth listening to. And the smaller it is, the more likely you are to get "cone cry" (i.e. significant distortion at non-destructive levels). Whether it's musically useful is up to you!
Certainly there's something to be said for providing a load which looks like a speaker, as valves behave differently to high-NFB amps (SS or otherwise).
The speaker still matters but you've a lot more choice if you're only playing with milliwatts: any driver with a paper surround is worth listening to. And the smaller it is, the more likely you are to get "cone cry" (i.e. significant distortion at non-destructive levels). Whether it's musically useful is up to you!
I've read about adding a small 6" speaker into large stacks with an attenuator for practice levels. The combo I have will be quite large (26"Wx16@H internal air volume) so there's aways an option to have two speakers (ie switch between them).
I did look at Jenson which is less sensitive - awesome blues and jazz tones but the celestion voicing for rock/metal I liked better.
However I need to stop analysing an put a stake in the ground for the V1 build with the option of a simple resistor attenuator. It can then iterate over time after that. Once built, I can play with the active load as the next stage.
So once I have the power supply design finalised then I'll order the BOM which should be relatively soon.
I did look at Jenson which is less sensitive - awesome blues and jazz tones but the celestion voicing for rock/metal I liked better.
However I need to stop analysing an put a stake in the ground for the V1 build with the option of a simple resistor attenuator. It can then iterate over time after that. Once built, I can play with the active load as the next stage.
So once I have the power supply design finalised then I'll order the BOM which should be relatively soon.
So I've worked through the power supply.
I found a 62VA transformer with 250V 120mA, 6.3V 3A and a 12.6V 1A. If I use a split rectified B+ (one for the pre and one for power) it means I can supply high voltage to the 12ax7 and LTP but supply a dropped B+ to the EF80s through a separate supply to allow for designing in some sag ability.
I'll run the pre section on the 12.6V as regulated DC heaters, but then leave the 6.3Vac heater to the EF80s and the EZ81 I'll add as part of the power section supply. This means the amp shouldn't be running the transformer over 75% on any of the secondaries.
There was a lovely 260+VA transformer with dual sets of 250V 400mA, 6.3V 4A and 50V secondaries.. but way overkill for what is needed.
Next up I was thinking if I'm going to hand build this.. do it properly with some nice multi-colour-coded wiring, so I have a wiring colour scheme worked out this morning
AC mains - follow UK colours: brown (live), blue (neutral) and striped green/yellow (protective earth). The PE will have a grounded bolt to each detachable metal surface - the chassis, the speaker frame etc.
B+ red, 22/24AWG multistrand, 900V insulation, for the pre-amp section running at the higher B+ power supply. This will include B+, between Rplate and plate pin and screen.
B+ orange, 22/24AWG multistrand, 900V insulation, for the power section running the second lower B+ power supply. This will include B+, Rplate-plate pin.
Ground - black 22AWG multistrand/18AWG solid core, the lower the resistance and impedance the better. This could be bare bar or black wire.
Heater 6.3V AC - green, 18AWG solid core, twisted pair. Larger cross section for higher current.
Heater 12.6V DC - white(+)/green(-), 18AWG solid core twisted pair (note this may not be at ground hence not using black). Larger cross section for higher current.
Signal LV <30V - white 22/24AWG, shielded, 300V+ insulation. This should mean any front face controls should be connected with white wire apart from the power switch and 6.3V light. There's a trade off between capacitive increase in the wire that could harm frequency response and insulation/shielding to reduce noise.
Cathode follower elevated cathode - blue 22AWG multistrand. 600V+ insulation. The danger here is that the cathode follower cathode is sat at an elevated voltage V+ so I don't want black and I'm in two minds about blue too.
NFB signal - white or purple, 22/24AWG multistrand. 900V insulation to prevent any accidents inside the chassis then shorting to the output side of the transformer
Output transformer to the speaker jack - purple (+) and black(ground), 16AWG, 300V insulated. Larger cross sectional area for current capacity.
For the wire insulation I'm looking to ensure the wire is 85degC+ to prevent the tube pins form melting it.
The speaker wire, as it's external I'll connect with red/purple and black but connect through a 1/4" plug to the speaker itself - the wire will be probably the same 16AWG wide that is used inside the chassis. Depending if I get a pre-made cable or make one.
I found a 62VA transformer with 250V 120mA, 6.3V 3A and a 12.6V 1A. If I use a split rectified B+ (one for the pre and one for power) it means I can supply high voltage to the 12ax7 and LTP but supply a dropped B+ to the EF80s through a separate supply to allow for designing in some sag ability.
I'll run the pre section on the 12.6V as regulated DC heaters, but then leave the 6.3Vac heater to the EF80s and the EZ81 I'll add as part of the power section supply. This means the amp shouldn't be running the transformer over 75% on any of the secondaries.
There was a lovely 260+VA transformer with dual sets of 250V 400mA, 6.3V 4A and 50V secondaries.. but way overkill for what is needed.
Next up I was thinking if I'm going to hand build this.. do it properly with some nice multi-colour-coded wiring, so I have a wiring colour scheme worked out this morning
AC mains - follow UK colours: brown (live), blue (neutral) and striped green/yellow (protective earth). The PE will have a grounded bolt to each detachable metal surface - the chassis, the speaker frame etc.
B+ red, 22/24AWG multistrand, 900V insulation, for the pre-amp section running at the higher B+ power supply. This will include B+, between Rplate and plate pin and screen.
B+ orange, 22/24AWG multistrand, 900V insulation, for the power section running the second lower B+ power supply. This will include B+, Rplate-plate pin.
Ground - black 22AWG multistrand/18AWG solid core, the lower the resistance and impedance the better. This could be bare bar or black wire.
Heater 6.3V AC - green, 18AWG solid core, twisted pair. Larger cross section for higher current.
Heater 12.6V DC - white(+)/green(-), 18AWG solid core twisted pair (note this may not be at ground hence not using black). Larger cross section for higher current.
Signal LV <30V - white 22/24AWG, shielded, 300V+ insulation. This should mean any front face controls should be connected with white wire apart from the power switch and 6.3V light. There's a trade off between capacitive increase in the wire that could harm frequency response and insulation/shielding to reduce noise.
Cathode follower elevated cathode - blue 22AWG multistrand. 600V+ insulation. The danger here is that the cathode follower cathode is sat at an elevated voltage V+ so I don't want black and I'm in two minds about blue too.
NFB signal - white or purple, 22/24AWG multistrand. 900V insulation to prevent any accidents inside the chassis then shorting to the output side of the transformer
Output transformer to the speaker jack - purple (+) and black(ground), 16AWG, 300V insulated. Larger cross sectional area for current capacity.
For the wire insulation I'm looking to ensure the wire is 85degC+ to prevent the tube pins form melting it.
The speaker wire, as it's external I'll connect with red/purple and black but connect through a 1/4" plug to the speaker itself - the wire will be probably the same 16AWG wide that is used inside the chassis. Depending if I get a pre-made cable or make one.
Don't worry I'm slow as this is at the end of a number of DIY priorities (mainly involving concrete at the moment).
I've already costed the amp up - in the region of ~650-700GBP in parts+tax+delivery. Thanks to Brexit - it's cheaper to buy in the EU even with VAT (sales tax) and higher transport costs! Hence taking a little time to get it right and for it to be more of a long-term keeper amp.
I'm still not sure if I want the controls on the front (Marshall lead 12 like) or on the top (fender tweed like). I'm leaning to the having the controls, recessed on the back. So I need a metal plate to provide a top shield / panel too.
I did have a think about if I want to real point-to-point or current board. I'm opting for a turret board but I will probably split the pre and post sections so I can add a reverb section between, or run an external the power stage for experimental playing.
Someone on the telecaster forums made a very good point about heater hum - the 6.3Vac heaters are optimised for AC being bifilar (?) but the 12.6V not having this, hence using a rectified and regulation on the 12.6Vdc heater line. Works for me - I've built many regulators, so should be able to knock something up easily that will support the higher current with minimal voltage drop to fit.
I'm slow as this is at the end of a number of DIY priorities (mainly involving concrete at the moment). I've already costed the amp up - in the region of ~650-700GBP in parts+tax+delivery. Thanks to Brexit - it's cheaper to buy in the EU even with VAT (sales tax) and higher transport costs! Hence taking a little time to get it right and for it to be more of a long-term keeper amp.
I'm still not sure if I want the controls on the front (Marshall lead 12 like) or on the top (fender tweed like). I'm leaning to the having the controls, recessed on the back. So I need a metal plate to provide a top shield / panel too.
I did have a think about if I want to real point-to-point or current board. I'm opting for a turret board but I will probably split the pre and post sections so I can add a reverb section between, or run an external the power stage for experimental playing.
Someone on the telecaster forums made a very good point about heater hum - the 6.3Vac heaters are optimised for AC being bifilar (?) but the 12.6V not having this, hence using a rectified and regulation on the 12.6Vdc heater line. Works for me - I've built many regulators, so should be able to knock something up easily that will support the higher current with minimal voltage drop to fit.
I've been working through this a little more ...
So this uses a 2x0-300V 150mA 139VA transformer that has more heater current - 3A of 5V (SS rectified to 6.3V) for the two EZ81s, plus 5A for the 6.3Vac for all the tubes and the reverb tank option. It also has a 50V so I could elevate the heaters too. The 5V winding would be attached to the rectifier cathode so it would be elevated to about 360V so that a heater to cathode wouldn't see a large voltage difference.
The issue is that the cathode follower cathode sits at 194V and the JJ 12ax7 datasheet states Ukf max is 180V. So it needs a good 70-100V elevation to survive long with JJ tubes (thinking ahead for ease of replacements). A cap can ensure that the CF heater is kept elevated during dips or warm starts where the rectifiers will charge B+4 fast. I can also make the 50V elevation a slow bleed (a few minutes) to match the EZ81 cool down.
So I need to find another option to light the power light on the front panel I can't use the 6.3V as the bulb is grounded one side and the heater supply is elevated and the planned fender gem bulb holder isn't going to stand 70Vdc isolation to chassis.
So this uses a 2x0-300V 150mA 139VA transformer that has more heater current - 3A of 5V (SS rectified to 6.3V) for the two EZ81s, plus 5A for the 6.3Vac for all the tubes and the reverb tank option. It also has a 50V so I could elevate the heaters too. The 5V winding would be attached to the rectifier cathode so it would be elevated to about 360V so that a heater to cathode wouldn't see a large voltage difference.
The issue is that the cathode follower cathode sits at 194V and the JJ 12ax7 datasheet states Ukf max is 180V. So it needs a good 70-100V elevation to survive long with JJ tubes (thinking ahead for ease of replacements). A cap can ensure that the CF heater is kept elevated during dips or warm starts where the rectifiers will charge B+4 fast. I can also make the 50V elevation a slow bleed (a few minutes) to match the EZ81 cool down.
So I need to find another option to light the power light on the front panel
I can't use the 6.3V as the bulb is grounded one side and the heater supply is elevated and the planned fender gem bulb holder isn't going to stand 70Vdc isolation to chassis.
So I've sorted out a problem with my ltspice model (thanks @OldHector) and I've now added the volume pots and the tone stack into the model. Before the amp had way too much gain but the volume pots and tone stack now suck that gain out:
This also has all the grounded configured as I intend to connect it. So I'm feeling happier with this configuration. Ideally I wanted to model this ready for ordering the BOM Monday.
This also has all the grounded configured as I intend to connect it. So I'm feeling happier with this configuration. Ideally I wanted to model this ready for ordering the BOM Monday.
Rob has suggested using a 190-0-190 or a 200-0-200 toroidal. Robs build uses SS full wave rectification but the quoted max B+ is 289Vdc, whereas the JCM front end needs 350V (if the ltp is using a lower voltage).
If full a full bridge SS is used without the CT, then a single secondary of 0-275 or a 150-0-150 (ie 0-300) could be used to give 350Vdc B+.
A hybrid bridge could give that too. In either case the current capacity drops..
If I used a CT full wave like Rob, then I need a 275-0-275 (SS) or 300-0-300 for the same 350Vdc B+.
As I was going to purchase the majority of the BOM from one place, and I’d like to use a toroidal, to minimise emi as it will be used for low volume and high gain. Then I’m limited in choice :/
The ez81 is a good way to provide both drop but also load sag, my understanding is the SS will drop from 1.414 to 1.3x under load, but the tube will drop to ~1.2x.
If full a full bridge SS is used without the CT, then a single secondary of 0-275 or a 150-0-150 (ie 0-300) could be used to give 350Vdc B+.
A hybrid bridge could give that too. In either case the current capacity drops..
If I used a CT full wave like Rob, then I need a 275-0-275 (SS) or 300-0-300 for the same 350Vdc B+.
As I was going to purchase the majority of the BOM from one place, and I’d like to use a toroidal, to minimise emi as it will be used for low volume and high gain. Then I’m limited in choice :/
The ez81 is a good way to provide both drop but also load sag, my understanding is the SS will drop from 1.414 to 1.3x under load, but the tube will drop to ~1.2x.
Where I am at the moment - using a 490x200 chassis. I have the luxury of an oversized cab for the 12" so this will fit easily. I'm trying not to go above 500mm so I have options for the cab if it's too large (currently ~26" horizontally).
Still work in progress but the size of components means I can see it will fit. For now I have sacrificed the second pair of sockets for the output tubes (I was going to try a some experiments but I can modify the chassis at a later date and patch holes. I have space to give 40mm between the tubes. I have switch to a two tube reverb and lose the octal socket as I'm not using the 6k6gt. For now these will simply be left empty but I wanted the space in place.
I'm also tempted to shift the tubes along one (I have a reserved spot) to make space for an in-guitar distortion tube (a tube version of a tube screamer) to add a little more optional 'chug' to the JCM sound.
Still work in progress but the size of components means I can see it will fit. For now I have sacrificed the second pair of sockets for the output tubes (I was going to try a some experiments but I can modify the chassis at a later date and patch holes. I have space to give 40mm between the tubes. I have switch to a two tube reverb and lose the octal socket as I'm not using the 6k6gt. For now these will simply be left empty but I wanted the space in place.
I'm also tempted to shift the tubes along one (I have a reserved spot) to make space for an in-guitar distortion tube (a tube version of a tube screamer) to add a little more optional 'chug' to the JCM sound.
I was thinking about a dc preamp heater power, and this transformer is an option but it complicates things:
Pre power:
250V 400mA + 50V 100mA in series = 300Vrms 100mA, which gives us our 424Vpk 70mA once FB rectified. Not a problem for the pre.
Power stage:
250V 400mA = 353Vrms 282mA once rectified which suits the lower voltage power stage. Not a problem for the power section.
Heaters
6.3Vac 4A => 6.3Vac 2.6A (2xEZ81 + 2xEF80)
6.3V (4A-2.6A) + 6.3V (4A-2.6A) in series => 12.6V 1.4A = 17.8V 990mA once rectified we can then regulate down to 12.6Vdc, we need 5x150mA = 750mA.
I can then use the remaining 50V, rectified to 70V to elevate the heaters and with a 3R7 1W resistor in series to power the pilot light. I will steal a few mA for the heater elevation.
I have remove the tube screamer tube otherwise the heater supply would only have 90mA margin. The 50V line is only 100mA so it's not going to power heaters - I do have a 24V CT 5A (ie 2x12V 5A) toroid that I could use for 12V heaters but it's a bit overkill in this situation! An alternative is to use a small micro toroidal transformer (we're talking 2" across and about 7VA) if I want a effects tube screamer.
I suppose in the end it's better to reduce the heating in the cab.. and remove that socket entirely.
Pre power:
250V 400mA + 50V 100mA in series = 300Vrms 100mA, which gives us our 424Vpk 70mA once FB rectified. Not a problem for the pre.
Power stage:
250V 400mA = 353Vrms 282mA once rectified which suits the lower voltage power stage. Not a problem for the power section.
Heaters
6.3Vac 4A => 6.3Vac 2.6A (2xEZ81 + 2xEF80)
6.3V (4A-2.6A) + 6.3V (4A-2.6A) in series => 12.6V 1.4A = 17.8V 990mA once rectified we can then regulate down to 12.6Vdc, we need 5x150mA = 750mA.
I can then use the remaining 50V, rectified to 70V to elevate the heaters and with a 3R7 1W resistor in series to power the pilot light. I will steal a few mA for the heater elevation.
I have remove the tube screamer tube otherwise the heater supply would only have 90mA margin. The 50V line is only 100mA so it's not going to power heaters - I do have a 24V CT 5A (ie 2x12V 5A) toroid that I could use for 12V heaters but it's a bit overkill in this situation! An alternative is to use a small micro toroidal transformer (we're talking 2" across and about 7VA) if I want a effects tube screamer.
I suppose in the end it's better to reduce the heating in the cab.. and remove that socket entirely.
This has moved on a little:
So this is perhaps a little cover complex but:
So this is perhaps a little cover complex but:
- it has to cope with my 253V wall voltage into the 240V primary - a 5% uplift of voltages.
- copes with the EF80s being low voltages but the pre-stages being true to JCM voltages
- has mods - specifically V1b cathode switch between soldano, Marshall and grid clip, V2 mod (it has an additional tube stage - specifically for modification of the wave form rather than gain) and finally a diode/bjt clipper switch.
- 16R speaker - which along with the low frequency shy OT, is going to make it a puzzle to give it some EL34 slam in a micro format. I'm thinking here possibly making it hybrid to solve or replace the OT later.