High Voltages After Headfirst Origin 50 Mod

[Inspector #20]

World Peace...

I was hoping for a ragged hole in the time/space continum...

 

[syscokid]

I had to go back & find the pre & post mod pre-amp schematics to be sure, but in following the Jose V1a/V1b theme, the amp is actually developing "less" gain from the first two triodes than it would have if he (Jason Tong) had left the cathode R/C combinations as they were.
V1a previously had 1k5/1uf, he's biased the tube cooler & with less gain in the low end by using 2k7/0.68uf (though IIRC @syscokid used 2k7/1uf).
V2b previously was biased quite warm, with an 820 ohm cathode resistor.

This Origin 50 originally had both cathodes of V1 with an 820 ohm resistor and a 2.2uF bypass cap.

 

[syscokid]

I just lowered the voltages a tiny bit to V1 & V2 with bigger valued resistors. This is it, and I shall not molest this poor amp anymore. The numbers in red represent the new voltages after increasing R9 & R10:

To make the boost function more effective, increase the value of that 47k that is in series with the 0.68uf capacitor (try say, 3 ~ 5 times its current value, 150k ~ 250k).

I just tried different resistor values at this suggestion, and nothing seemed to make any difference for the better. So I guess there will not be any World Peace... bummer...

 

[Inspector #20]

@syscokid - how do you feel about the sound???

 

[old rocker]

I just lowered the voltages a tiny bit to V1 & V2 with bigger valued resistors. This is it, and I shall not molest this poor amp anymore. The numbers in red represent the new voltages after increasing R9 & R10:

View attachment 73644



I just tried different resistor values at this suggestion, and nothing seemed to make any difference for the better. So I guess there will not be any World Peace... bummer...

Hi syscokid, do you mind if i ask a couple of questions about the voltage lowering & the R9/R10 resistors ?, i've done the same HF mods on my ORI50 & after reading your (& Roberts) posts i'd like to check my voltages & bias.
I did the Mods myself, i'm ok with working on PCB's & changing components etc following instruction, as many moons ago i built Hi Fi amplifiers for a small UK manufacturing company(i was a co-owner) , but i'm not an Amp tech & have not fully understood where you measured the voltages & i wouldn't be able to work out replacement resistor values.
So my question is would you be comfortable giving me some guidance or would you rather not do this via the internet, i can take it to an Amp tech an ex working colleague, but after doing all the Mods myself i'd like to complete it myself .
I fully understand if you may not comfortable with this & i may be able to figure it out myself by searching out all your notes & posts, but it could take a while .
Cheers, Graham (aka old rocker

 

[syscokid]

Hi syscokid, do you mind if i ask a couple of questions about the voltage lowering & the R9/R10 resistors ?, i've done the same HF mods on my ORI50 & after reading your (& Roberts) posts i'd like to check my voltages & bias.
I did the Mods myself, i'm ok with working on PCB's & changing components etc following instruction, as many moons ago i built Hi Fi amplifiers for a small UK manufacturing company(i was a co-owner) , but i'm not an Amp tech & have not fully understood where you measured the voltages & i wouldn't be able to work out replacement resistor values.
So my question is would you be comfortable giving me some guidance or would you rather not do this via the internet, i can take it to an Amp tech an ex working colleague, but after doing all the Mods myself i'd like to complete it myself .
I fully understand if you may not comfortable with this & i may be able to figure it out myself by searching out all your notes

Ha ha... You British truly are too proper and polite! Ask away my friend...

 

[old rocker]

Ha ha... You British truly are too proper and polite! Ask away my friend...

, ok thanks, so if i'm reading your chart correct you have replaced R9/R10 with 10K resistors to lower the voltage as shown by the Red numbers ?, if so can i ask:
Do R9/R10 control the voltages for all 5 Valves & all pins ?, so changing the resistors affects all 9 pins on the 5 valves?.
How is the value of the resistor calculated to get the required Voltage ?
Is there a chart/table showing what the voltages should be ? or is it experience based ?, in which case i have a bit of a problem .

 

[syscokid]

, ok thanks, so if i'm reading your chart correct you have replaced R9/R10 with 10K resistors to lower the voltage as shown by the Red numbers ?, if so can i ask:
Do R9/R10 control the voltages for all 5 Valves & all pins ?, so changing the resistors affects all 9 pins on the 5 valves?.
How is the value of the resistor calculated to get the required Voltage ?
Is there a chart/table showing what the voltages should be ? or is it experience based ?, in which case i have a bit of a problem .

Here's a pic of Rob's Origin 50 before any mods, highlighting R6, R8, R9, & R10:


Power supply is on the right. Thinking from right to left:
R6 will only affect V3, V2, & V1. I never messed with R6. I never measured the voltages going into R6.
R8 & R9 will only affect V2 & V1. R8 was increased to 10k, and R9 was increased to 15k, to bring down the excessive voltages hitting V2 (Which also affects voltages at V1).
R10 will only affect V1. R10 was increased to 12k.

Yes, voltage drop can be calculated but you would need to know the amount of current going through the resistors. Perhaps others here can explain the calculating formulas.

AFAIK, there is no voltage chart of this amp that's available for an amp hack like me. But most tubes available today have a datasheet that will show maximum operating values and specs.

 

[old rocker]

Here's a pic of Rob's Origin 50 before any mods, highlighting R6, R8, R9, & R10:
View attachment 73670

Power supply is on the right. Thinking from right to left:
R6 will only affect V3, V2, & V1. I never messed with R6. I never measured the voltages going into R6.
R8 & R9 will only affect V2 & V1. R8 was increased to 10k, and R9 was increased to 15k, to bring down the excessive voltages hitting V2 (Which also affects voltages at V1).
R10 will only affect V1. R10 was increased to 12k.

Yes, voltage drop can be calculated but you would need to know the amount of current going through the resistors. Perhaps others here can explain the calculating formulas.

AFAIK, there is no voltage chart of this amp that's available for an amp hack like me. But most tubes available today have a datasheet that will show maximum operating values and specs.

Thanks again, i asked about calculations because if i had needed to do that i wouldn't know where to start, so it's a relief i don't need to do that so would it be reasonable to assume that if i replace R8/9/10 with your posted values on my ORI50 pcb, i would see the same voltages as Roberts board ?(mine is also a V1 board).
If this is the case could you tell me the spec of the resistors i would need , then all i need to do is look up how to adjust the Bias .
Thanks again, Cheers Graham.

 

[syscokid]

o would it be reasonable to assume that if i replace R8/9/10 with your posted values on my ORI50 pcb, i would see the same voltages as Roberts board ?(mine is also a V1 board).

Yes

If this is the case could you tell me the spec of the resistors i would need

And make sure that the resistors are "metal oxide" and have a 2-watt rating!

then all i need to do is look up how to adjust the Bias

Eazy Peezy... See the square pad in CN32?


Amp warmed up for at least 10 minutes. Set the meter to read DC. With one lead of the meter, probe it at that square pad of CN32. The meter's other lead should be grounded anywhere on the chassis, OR... grounded at the circle pad of CN32. Marshall recommends 70mV (35mV per EL34 tube), and this equates to a 60% or "medium" bias.

Yes, this is the trimmer pot for the bias circuit:

 

[Inspector #20]

Picked up my Greg-Modded Origin 50 today. It was a 330 mile round trip and definitely worth it. My oldest Son and my Dad accompanied me on the trip.

Let me say that the "Greg-Mod" goes beyond the Headfirst Mod and corrects a lot of issues.

Playing through Greg's 2x12 loaded with Scumback M75-LD's it sounded amazing.

The tone and response is otherworldly. Where in stock form, I had to max out all my controls, after the Greg-Mod, everything is about in the middle and far more responsive than stock.

The amp has much more clarity and dynamics and its much tightervthan it was even with a TS-9.

For example, cranking the bass knob gives totally usable low end without ever intruding into the other frequency ranges. No muddiness even with bass full-up.

The amp's volume production is way beyond stock and it actually has LESS noise than it did in stock form with the "boost" feature engaged.

I am beyond stoked and impressed.

I'll try to get some video up on Saturday...

 

[ivan H]

How is the value of the resistor calculated to get the required Voltage ?

You first need to know the current drawn by the tubes being supplied HT via the resistor(s). Suppose we use Sysco's pic etc above, R10 feeding V1 as our example.
Start by measuring & noting (writing down) the voltage on the cathode of each triode of V1. Now divide the voltage on each cathode by the value of the corresponding cathode resistor "in ohms".
Suppose V1a's cathode resistor value is 2k7 & the cathode is at 1.6V.
1.6/2700 = 0.000592A.
Do the same for V1b, then add each triode's current draw together for V1's total (rough) current draw. (I say "rough" because to get an accurate figure we'd need each resistor's "exact" value).
For convenience sake, let's suppose both triodes are drawing the same current of 0.000592A. Times two = 0.001185A (or 1.185mA) total current for V1.

You know how much HT voltage you need to be dropped by the resistor, so the formula to use is from ohm's law;
R = V/I
where
R = resistance value required
V = voltage you want to drop
I = current drawn by V1

Example; you want to drop 10V from V1'sHT node, so
10/0.001185 = 8,438 ohms resistance needed.

Sorry for the long-winded rant, you're most likely even more confused now. Cheers

 

[Inspector #20]

You first need to know the current drawn by the tubes being supplied HT via the resistor(s). Suppose we use Sysco's pic etc above, R10 feeding V1 as our example.
Start by measuring & noting (writing down) the voltage on the cathode of each triode of V1. Now divide the voltage on each cathode by the value of the corresponding cathode resistor "in ohms".
Suppose V1a's cathode resistor value is 2k7 & the cathode is at 1.6V.
1.6/2700 = 0.000592A.
Do the same for V1b, then add each triode's current draw together for V1's total (rough) current draw. (I say "rough" because to get an accurate figure we'd need each resistor's "exact" value).
For convenience sake, let's suppose both triodes are drawing the same current of 0.000592A. Times two = 0.001185A (or 1.185mA) total current for V1.

You know how much HT voltage you need to be dropped by the resistor, so the formula to use is from ohm's law;
R = V/I
where
R = resistance value required
V = voltage you want to drop
I = current drawn by V1

Example; you want to drop 10V from V1'sHT node, so
10/0.001185 = 8,438 ohms resistance needed.

Sorry for the long-winded rant, you're most likely even more confused now. Cheers

I appreciate these explanations, because I am learning something new everyday!!!!

 

[old rocker]

Picked up my Greg-Modded Origin 50 today. It was a 330 mile round trip and definitely worth it. My oldest Son and my Dad accompanied me on the trip.

Let me say that the "Greg-Mod" goes beyond the Headfirst Mod and corrects a lot of issues.

Playing through Greg's 2x12 loaded with Scumback M75-LD's it sounded amazing.

The tone and response is otherworldly. Where in stock form, I had to max out all my controls, after the Greg-Mod, everything is about in the middle and far more responsive than stock.

The amp has much more clarity and dynamics and its much tightervthan it was even with a TS-9.

For example, cranking the bass knob gives totally usable low end without ever intruding into the other frequency ranges. No muddiness even with bass full-up.

The amp's volume production is way beyond stock and it actually has LESS noise than it did in stock form with the "boost" feature engaged.

I am beyond stoked and impressed.

I'll try to get some video up on Saturday...

View attachment 73696

View attachment 73697

Hi Robert, I'm almost afraid to ask what's the "Greg Mod" .

You first need to know the current drawn by the tubes being supplied HT via the resistor(s). Suppose we use Sysco's pic etc above, R10 feeding V1 as our example.
Start by measuring & noting (writing down) the voltage on the cathode of each triode of V1. Now divide the voltage on each cathode by the value of the corresponding cathode resistor "in ohms".
Suppose V1a's cathode resistor value is 2k7 & the cathode is at 1.6V.
1.6/2700 = 0.000592A.
Do the same for V1b, then add each triode's current draw together for V1's total (rough) current draw. (I say "rough" because to get an accurate figure we'd need each resistor's "exact" value).
For convenience sake, let's suppose both triodes are drawing the same current of 0.000592A. Times two = 0.001185A (or 1.185mA) total current for V1.

You know how much HT voltage you need to be dropped by the resistor, so the formula to use is from ohm's law;
R = V/I
where
R = resistance value required
V = voltage you want to drop
I = current drawn by V1

Example; you want to drop 10V from V1'sHT node, so
10/0.001185 = 8,438 ohms resistance needed.

Sorry for the long-winded rant, you're most likely even more confused now. Cheers

Hi ivanH, ugh! thanks a lot , yes now confusion & out of my depth reigns .

 

[old rocker]

Yes

View attachment 73688

And make sure that the resistors are "metal oxide" and have a 2-watt rating!


Eazy Peezy... See the square pad in CN32?
View attachment 73689

Amp warmed up for at least 10 minutes. Set the meter to read DC. With one lead of the meter, probe it at that square pad of CN32. The meter's other lead should be grounded anywhere on the chassis, OR... grounded at the circle pad of CN32. Marshall recommends 70mV (35mV per EL34 tube), and this equates to a 60% or "medium" bias.

Yes, this is the trimmer pot for the bias circuit:
View attachment 73691

Once again, this is great thanks , yet another question, how do you draw the red markers & printing on your pics ?, just curious.

 

[old rocker]

Hi Robert, just in case you missed my reply , i'm almost frightened to ask , but what's the "Gene Mod" !.

 

[syscokid]

Hi Robert, I'm almost afraid to ask what's the "Greg Mod" .

That's just Robert showing his appreciation for the help in getting this amp to fly with some added mods based on common sense by humorously calling the extra mods the "Greg Mod"... or the "Iverg Mod"... or even the "Ivanberg Mod"! BTW, I'm Greg.

But the extra mods is simply composed of bringing the high voltages to V2 down quite a bit. And instead of replacing the two V1 cathode bypass caps with .68uF's, we felt that 1uF's was more appropriate. Even installing the new fx loop board had to be adjusted accordingly. You got to remember that Jason Tong modded and documented an Origin 20. The Origin 50 is a lot more different than Jason acknowledges.

 

[syscokid]

Once again, this is great thanks , yet another question, how do you draw the red markers & printing on your pics ?, just curious.

With my PC, I print out the blank voltage chart. Then I hand draw the added info with a pencil or a red marker onto the chart. Then I scan the chart and save the new pic as a jpeg or pdf copy back to the PC. I could also add all the text, the lines, the circles, and colors, using my PC's Paint program.

 

[Inspector #20]

Ivanberg is awesome

 

[Inspector #20]

Impressions:

The amp after the Ivanberg mod is at least twice as loud as it was before the mod. Volume dial '3' is now giving you '6' and 6 is absolutely unattainable.

It's crystal clear at 6, bit the sheer volume at that setting is incredibly loud.

The EQ knobs are much more sensitive.

Gain NEVER gets muddy...ever.

The amp has a tone that's hard to explain. The best way I can explain it is that it seems as though a noise gate is connected. When you let go of the strings, it gets quiet immediately.

The guitar responds to pick attack and you can "clean up" just by picking more lightly.

Here's a shot of my settings from a February 4, 2021 performance



And the settings we dialed in at Greg's lab yesterday:



The tone from the bare amp sounds studio produced. Totally balanced across the tonal spectrum. Almost like it has a compressor running.

The gain production is very similar to George Lynch's tone on 'Breaking The Chains.' (Which we know was an extra gain stage Marshall hit with a Tube Screamer.)

The voicing of the amp is night and day different. It's got a sort of Friedman quality to it. It's not quite like any Marshall I have heard.

Maybe Greg can describe it better. Gonna be at least Saturday before I can get soundclips recorded...

Here's a shot of the rig from a 2021 performance.



More to follow...