JCM800 Micro EF80 Build

[mountainhick]

apparently it can?
in the description for this product, it's claiming that these two windings have a full 160ma each.
View attachment 989362
and even if it's actually just 160ma over both the windings, surely 160ma is more than adequate?

I made the same mistake by not looking carefully at an ANTEK transformer diagram last week or so.

The high voltage outputs are not two separate windings. The 275 is an extension off the 190V, or you could look at it as the 190 is a tap off the 275. the difference between them also offers 115V.

I don't know though, can two bridges be used on a tapped coil setup?

EDIT: Hmm, discussed here: https://antiqueradios.com/forums/viewtopic.php?t=378174

 

[owlexifry]

I made the same mistake by not looking carefully at an ANTEK transformer diagram last week or so.

The high voltage outputs are not two separate windings. The 275 is an extension off the 190V, or you could look at it as the 190 is a tap off the 275. the difference between them also offers 115V.

I don't know though, can two bridges be used on a tapped coil setup?

EDIT: Hmm, discussed here: https://antiqueradios.com/forums/viewtopic.php?t=378174

ahhh i noticed that too, and i suspect it’s likely that the windings are not seperate.

how much or whether that matters is beyond my understanding
but reading through that thread…looks like im boned again

thanks - i appreciate your input

 

[pavel]

Meat & Potatoes . Presumably you could use a bridge on the tap and a half wave rectifier on the HT winding? The bridge would give less ripple for the higher draw of the EF80 power stage.

 

[wangdaning]

Can we wire it backwards so B+ goes preamp then to power?

 

[owlexifry]

Meat & Potatoes . Presumably you could use a bridge on the tap and a half wave rectifier on the HT winding? The bridge would give less ripple for the higher draw of the EF80 power stage.

im trying to visualize what this looks like in schematic format..

Can we wire it backwards so B+ goes preamp then to power?

haha im glad you asked this... (considered this, but yet to ask, ive already put out enough of my silly questions out there...)

- the idea i had for this, was to run B+ (with filtering), direct to phase inverter (V3), then V2, V1, and then from V1 to OT CT, choke, then screens.
will this work? i have no idea.

assuming it's even feasible, should the choke remain between OT CT and screens? or elsewhere?

 

[wangdaning]

I am guessing we are just looking silly because the preamp tubes are perfectly fine running on lower voltage. The one I did sounds fine, but I am not sure if it sounds like a bassman.

 

[2L man]

apparently it can?
in the description for this product, it's claiming that these two windings have a full 160ma each.
View attachment 989362
and even if it's actually just 160ma over both the windings, surely 160ma is more than adequate?

This 160mA is 160mAAC and when it is bridge rectified and filtered its max current drop using multiplier of "square of 2"

160mAAC / 1.41 = 113mADC. This is because filter capacitors fill the area between sine wave forms increasing RMS voltage 1,41 times higher and then RMS current must drop same amount! "This square of 2" rules always when sine wave is manipulated.

When a Center Tab secondary is full wave rectified and filtered the DC current potential increase by 1,41 because current flow alternating between secondary halfs and althougs other half heats delivering double the current other half deliver no current and cools

I think in practice quite a lot more current is possible to take out of this Antek 190V coil than 275V coil because its voltage is lower and total power with same rated current would be 70%.

Antek is good when they list AC output voltage for no load and one or two currents. It again does not lead to DC because filtering "loads" mostly the sine wave tops and peak current comes higher than RMS current and higher peak current drop voltage more.

 

[2L man]

I don't know though, can two bridges be used on a tapped coil setup?

It is not possible to use two pridge rectifiers when two coild are tied together. When output is exactly in the middle it can be used to have exactly half voltage output what bridge rectifier produce installing two same value filter capacitors series so that their junction go to CT

I think this kind power supply is called "twin rail". It is very common in solid state electronics where symmetrical "plus and minus" power supply is needed.

 

[owlexifry]

I am guessing we are just looking silly because the preamp tubes are perfectly fine running on lower voltage. The one I did sounds fine, but I am not sure if it sounds like a bassman.

i guess my point is, after having built one of robs designs (6L6 40W SLO), and finding the ‘target’ plate voltages i was initially getting were quite off, and in the process of correcting that, i found that even a 10V-20V shift in the plate voltages altered the sound significantly.

i’m surprised people aren’t noticing it as much.
perhaps this is less noticeable in lower gain amps?
or maybe, it’s because they’re ‘micro amps’, where concessions are already being made?

This 160mA is 160mAAC and when it is bridge rectified and filtered its max current drop using multiplier of "square of 2"

160mAAC / 1.41 = 113mADC. This is because filter capacitors fill the area between sine wave forms increasing RMS voltage 1,41 times higher and then RMS current must drop same amount! "This square of 2" rules always when sine wave is manipulated.

When a Center Tab secondary is full wave rectified and filtered the DC current potential increase by 1,41 because current flow alternating between secondary halfs and althougs other half heats delivering double the current other half deliver no current and cools

I think in practice quite a lot more current is possible to take out of this Antek 190V coil than 275V coil because its voltage is lower and total power with same rated current would be 70%.

Antek is good when they list AC output voltage for no load and one or two currents. It again does not lead to DC because filtering "loads" mostly the sine wave tops and peak current comes higher than RMS current and higher peak current drop voltage more.

It is not possible to use two pridge rectifiers when two coild are tied together. When output is exactly in the middle it can be used to have exactly half voltage output what bridge rectifier produce installing two same value filter capacitors series so that their junction go to CT

I think this kind power supply is called "twin rail". It is very common in solid state electronics where symmetrical "plus and minus" power supply is needed.

i have so much to unpack here..
i’m just gonna stop now and try understand 10% of this..
greatly appreciate your input, thank you.

 

[pelayostyle]

You're right, there's a lot to unpack here. Perhaps we should get @robrob in here to chime in on what he thinks? In the meantime, I'm just going to enjoy the amp as is.

 

[andrewRneumann]

Presumably you could use a bridge on the tap and a half wave rectifier on the HT winding? The bridge would give less ripple for the higher draw of the EF80 power stage.

This is not a completely unreasonable idea. You will end up with some DC in the transformer, which it is not designed for and can lead to undesirable results. But... it's a small amount of DC--just the current supplying a few preamp valves--so you may actually be able to get away with it.

As someone else noted, you cannot wire up 2 bridges to that PT and get what you want.

Can we wire it backwards so B+ goes preamp then to power?

This multiplies the power supply sag because the power tubes have to get their current now through all the power supply dropping resistors. Not suitable for Class AB. Could work for Class A--but have to increase the wattage of all your dropping resistors.

I am guessing we are just looking silly because the preamp tubes are perfectly fine running on lower voltage. The one I did sounds fine, but I am not sure if it sounds like a bassman.

It does work fine I'm sure. But I will say one thing I know--the DC Cathode Follower performance is dependent on B+. The other valves not as much. So there is a small mark in favor of keeping the preamp voltages as designed.

i found that even a 10V-20V shift in the plate voltages altered the sound significantly.

Elaborate, because that is well within the tolerance of these amps.

 

[pavel]

You will end up with some DC in the transformer, which it is not designed for and can lead to undesirable results.

Curious about this -- is this because the half way rectifier only loads up the PT on one half of the waveform?

 

[mountainhick]

Can we wire it backwards so B+ goes preamp then to power?

Another problem is filtering. The preamp stages have more RC filters in normal order.

ahhh i noticed that too, and i suspect it’s likely that the windings are not seperate.

how much or whether that matters is beyond my understanding
but reading through that thread…looks like im boned again

thanks - i appreciate your input

I still haven't yet finished a project with a dual voltage PS for higher voltage to preamp tubes than the 6AK6s, but here's the design. The PT itself is only 180V for rectified of about 250V. Single ended 6AK6s will run on 185/180V plate/screen. This is an inefficient way to do it, and you would have to dump a lot more with higher value resistors with your desired voltages. The preamp tubes run off the last node of one of the lines for two stages of filtering (hopefully enough). The power tube plates run on the first tap after a dropping resisotr, then another node to screens.

 

[owlexifry]

View attachment 989567

looks a lot like that drawing i posted above

[which ive learned is ok for class A, and not for class AB]

Elaborate, because that is well within the tolerance of these amps.

i’ve caused enough trouble already…

but i’ll refer to this post from my old build thread..

View attachment 838627
so i changed the dropping resistor (B+2 to B+3) from 3.6k to 4.7k
(highest value i can get from the local electronics shops, they don’t do 3w resistors, so used a 5w wirewound)

preamp plate voltages are now ~10v closer to target.

measurements:
B+1 443/444(unchanged)

B+3 was 413, now 403 (target 378)
B+4 was 393, now 383 (target 350)
B+5 was 399, now 390 (target 363)

still quite far off target, but holy crap, it sounds so much better, even with this small change.
the low end is passing through better, more “chonk”, the overall tone is smoother, and with the gain set high it’s not so harsh. gain ‘range’ and control is more spread out too.
what a fun afternoon.

have now ordered some higher value dropping resistors (5k6, 6k8, 8k2) to try out and see which will get closer to target and/or sound best...

edit:
i should really clarify that i meant 10-20V shift in B+3,4,5 preamp supply, not preamp plate voltages.
(i’m an idiot and i’ve been referring to B+/plate voltage as the same thing, when they’re defintely not)

 

[andrewRneumann]

Curious about this -- is this because the half way rectifier only loads up the PT on one half of the waveform?

Yes, exactly. I run a test PT with about 5mA DC coming out of the 50VAC bias tap using a half-wave rectifier. It works without hum, buzz, or overheating.

 

[Len058]

i’m surprised people aren’t noticing it as much.
perhaps this is less noticeable in lower gain amps?
or maybe, it’s because they’re ‘micro amps’, where concessions are already being made?

Well, the JCM get's is agressive tone because V1b is biased cold. This will give it asymmetrical clipping. They call it a 'cold clipper'. Maybe I'm wrong but high voltage is what prevents early clipping. Everybody is obsessed by voltages but again, could be wrong, it just increases headroom and volume. We do not use a wall of stack's to make ourselves heard anymore, or do we?

 

[pavel]

Yes, exactly. I run a test PT with about 5mA DC coming out of the 50VAC bias tap using a half-wave rectifier. It works without hum, buzz, or overheating.

Cool, thanks for trying that! I have yet to finish my JCM 800/EF80 in the stock config, but this sounds like a cool avenue to experiment with down the line.

Or maybe it will just go straight to eleven as-is right off the bat, cold clipper and all

Either way I need to get on with it, you guys are a true inspiration!

 

[Paul-T]

I am guessing we are just looking silly because the preamp tubes are perfectly fine running on lower voltage. The one I did sounds fine, but I am not sure if it sounds like a bassman.

I have been mulling over this for a few months now. I built a 5A3 EF80 and found it a bit fizzy - which could be for a variety of reasons. But I now have a 200V Primary Windings and a 250V Tubetown toroid which I can use for a JTM45 micro and I'm slightly torn on EF80 vs 12AU7.

Do we know where the effects of lower preamp voltage are felt? The LTP? What are those voltages on Marshall-style builds?

I note that Sluckey's 18W Marshall has 295V on the Phase Inverter. Isn't that pretty close to the Bassman Micro EF80? Or is the problem that we simply don't get the B+ shown on the EF80 schematics? Sorry if that's a naive question.

 

[Len058]

I have been mulling over this for a few months now. I built a 5A3 EF80 and found it a bit fizzy - which could be for a variety of reasons. But I now have a 200V Primary Windings and a 250V Tubetown toroid which I can use for a JTM45 micro and I'm slightly torn on EF80 vs 12AU7.

Do you mean some kind of distorted harmonics, on top of a clean signal? No matter how clean you try to set the amp, it always has some fizzy overdrive in the background?

I had the same problem, some annoying distortion, always in the background. It disappeared after I mounted a bigger OT. I'm waiting for the Hammond 125D, which will hopefully come this weekend. Than I can confirm for certain that the 125B is too small and will not produce good tone.

I don't think higher voltages in the pre-amp will be very noticeable, especially when using the 125B OT. You can try a 125C,D or E in the 5a3, then choose between the JTM EF80 or 12AU. You'll already have an OT, or maybe you'll toss the 125B

 

[pelayostyle]

Here's another idea. I have an AS-05T240 in my junk drawer. It has both a 240V and 220V HV tap. I can run the 240V tap in the amp and beat the crap out of the EF80 plates at ~300V. I think Rob has stated before that he's run them at 300V. This will also bring up the rest of the voltages at the B+ nodes. I could also use the 220V tap and raise voltages marginally.