I plan to increase R10 but that’ll have to wait.
The amp works just fine for the moment.
I’ll report back when I’ve adjusted R10.
High Voltages After Headfirst Origin 50 Mod
- Thread starter Inspector #20
- Start date
I plan to increase R10 but that’ll have to wait.
The amp works just fine for the moment.
I’ll report back when I’ve adjusted R10.
OP
You need a really good tube in V1....ideally a short plate with 330 volt capability.
JJECC83S, JJECC83MG, 12AT7, 12AU7 are rated at only 300 volts.
Genelex Gold Lion B759, 7025-WA, Sovtek 12AX7LPS and JJ5751 are spec'd for 330 volts maximum.
Here are my voltages from the initial HF mod:
And voltages after the "Ivanberg" reliability mods:
All the voltages shown are with the power scaling switch set to "HI" and the plate voltages are actually higher on "LOW" power mode.
As you can see, this amp is really pushing most 12AX7s that have a 300 volt ceiling...
Last edited:
ivan H
Ambassador of Tubes & Grooves
Hi @Polifemo, I'll try to be of assistance.
I'm ina bit of a rush (at work), so haven't read all of your posts, but believe you want to drop the V1 HT node voltage. I would suggest also dropping the V2 HT node voltage so it doesn't take out cathode follower tubes too. Rather than take an experimenter type "suck it & see" approach, it's best to "calculate" the resistance values that you'll need to add to the HT dropper resistors.
So firstly you need to (roughly) calculate the total V1 & V2 current draw. You'll be doing seperate calculations for each of the 4 triodes of V1 & V2.
So with the amp powered up & stabilised, use your volt metre to measure & note (write down) the voltage on each of the 4 triode's cathode.
Now, divide each cathode voltage by the corresponding cathode resistor's value in ohms. This will give you each triode's (rough) current draw in amps.
Now add the current drawn by V1's two triodes together for V1's total current draw (again, write it down), then do the same for V2.
So now you have the total V1 & V2 current draw, & you know how much voltage you want to drop from first V2's HT node & then V1's HT node, you can calculate how much resistance needs to be added to each of the HT dropping resistors.
Starting with V2's HT dropper;
First, add together V1 & V2's total current draw (in amps).
Now, take the amount of volts that you wish to reduce V2's HT node by & divide the figure by V1 & V2's total current draw in amps to get the resistance value in ohms that you need to increase V2's HT dropping resistor by (to achieve the desired voltage drop).
For V1's HT dropper you will need to do the same sort of calculation, only this time you will divide the desired voltage drop by V1's total current draw only.
Hope that I made this understandable & hope it helps. Cheers
Last edited:
syscokid
Ambassador of War & Peace
Cool info, Ivan...
Thanks a lot for your detailed reply!
Much appreciated.
My amp isn’t modded (Headfirst Mod) and V2 is still in use for the loop, and there’s been no problems in that position.
This far I’ve only done minor tweaks to the circuit (well, ripping out the useless “Tilt Control” circuit and replacing it with a resonance control was a bit more work)
I do plan to mod this amp into a 3 gain stages + cathode follower circuit, but not to “Headfirst specs” as I’m not interested in that amount of gain, and when I do this mod I will increase several dropping resistors.
This mod also will have to wait quite a while I’m afraid...
Thanks once again for taking your time to reply to my post
Last edited:
ivan H
Ambassador of Tubes & Grooves
Do you have a specific circuit in mind, or are you planning one of your own design? Cheers
Well, you never really know until you're able to tweak and listen, but compared to the "Headfirst mod" I'll most definitely keep the voltage divider (probably a standard "470k/470pF-470k") between V1a and V1b.
The cathode on V1b will probably end up somewhere in between a standard "2.7k/0.68uF" and a "10k cold clipper".
I'll keep a bypass cap (unsure about the value) to be switched in/out with the foot switch, and the resistor will be "higher than 2.7k but lower than 10k".
I'll have to try different values and listen.
I'll probably skip the 470pF on the voltage divider between V1b and V2
Apart from that it'll probably end up quite close to the Headfirst mod, but no clipping diodes.
In an earlier post I state the tweaks I've done this far, and that I'll most likely will keep.
Last edited:
OP
Well, Guys and Gals, the Ivanberg Modded ORI50 is still blowing people's minds. Every time I fire it up, people hear it and the questions start.
One fellow I know quipped about the size of my pedalboard and I quickly disconnected it so he could hear just the amp alone. He was floored.
This guy is a gear connesuer with a thin body, Gibson Les Paul (Custom Light?) and a Friedman BE-50, so I just let him play on the Origin50H for a while.
He told me, "Whatever you guys did to that amp it's as good, if not better sounding than my Friedman..."
When I showed him the 70/80's in my cabinet, he just shook his head.
I've never had more fun with an amplifier or really loved the tone I was getting out of it.
When you break it down and really think about it, I've got (2) 25 foot cables going in and out of the loop, a 25 footer from pedalboard to amp and a 25 footer from the guitar to the pedalboard and I still have my tone controls close to the middle.
Super pleased with this...
One fellow I know quipped about the size of my pedalboard and I quickly disconnected it so he could hear just the amp alone. He was floored.
This guy is a gear connesuer with a thin body, Gibson Les Paul (Custom Light?) and a Friedman BE-50, so I just let him play on the Origin50H for a while.
He told me, "Whatever you guys did to that amp it's as good, if not better sounding than my Friedman..."
When I showed him the 70/80's in my cabinet, he just shook his head.
I've never had more fun with an amplifier or really loved the tone I was getting out of it.
When you break it down and really think about it, I've got (2) 25 foot cables going in and out of the loop, a 25 footer from pedalboard to amp and a 25 footer from the guitar to the pedalboard and I still have my tone controls close to the middle.
Super pleased with this...
ivan H
Ambassador of Tubes & Grooves
The 470k/470pf/470k treble peakers/voltage divider plays a part in the classic Marshall's upper mid characteristics. If we calculate the frequency response it comes out around 720hz (starts rolling of just a tad above this frequency), & this is a common theme in the classic Marshalls. If we calculate the frequency response of the classic first stage (2k7/0.68uf cathode, 100k plate, 1 meg volume pot), again it's rolling off at 720hz. If we calculate the frequency response of the superlead bright channel first stage 0.0022uf coupling cap (100k plate, 1 meg pot, 0.0022uf), to we see it's rolling off at 795hz. So if one wants to modify their Marshall & retain the classic Marshall vibe, frequency roll off points in this range are important. Learn to calculate things like this.
Same if you are going to get into the biasing of gain stages etc, learn to draw the various load lines etc. You could take a "suck it & see" approach, but it's a whole lot better to really know what you are doing from the get-go.
All this type info is out there if one really wants to learn.
Oh, & never underestimate the importance of interstage attenuation, especially when cascading multiple gain stages.
Glad you are liking it. @syscokid really came through on this.
It's really not surprising that ol' mate likened your amp to his Friedman, as some of the Friedman amp models also have roots in Jose mods (Dave Friedman is commonly known as the expert on Jose mods).
Jose had no standardized modification, rather, would do them to suit the specific players needs. He definitely had a "theme" that was common to most all of them though. Cheers
Last edited:
syscokid
Ambassador of War & Peace
The Headfirst mod for the Marshall Origin is based on the Origin 20. The voltages are much higher on the Origin 50. IMO, the voltages on V2 after being reconfigured through the Headfirst mod, are way too high for any 12AX7: 385v at the plate/pin 1, and 244v at the cathode/pin 3. I did the "suck it & see" approach till the cathode voltage dropped to just under 200v, which also brought down the pin 1 plate to about 305v. Even though the new dropped voltage numbers are still high for the old school Marshallites, the new voltages are in spec for certain 12AX7's. We could have easily brought the PI & preamp voltages down more, but I thought that the amp sounded very interesting with the voltages on the high side.
There's also a slight difference between the O-20 and O-50 with the stock V1 cathode bypass caps. Jason Tong tuned his mod to the O-20... Would he have changed anything had he modded an O-50 instead?????? Anyways, the .68uF's that I needed for the HF mod were out of stock from Mouser. I ordered a couple .47uF's and a couple 1uF's instead. I first tried the .47's, but as soon as I switched to the 1uF's, I heard something that I really was digging even more... V1 cathode circuit of 1uF's/2.7k combo is the shiitkizzel...
I do agree that it’s important to know how to calculate the frequency response of cathode bypass caps/resistors and coupling caps, BUT one always have to listen in order to fine tune the final tweaks.
Ps In builds/mods I’ve done where V1a is shared by the clean and distortion channels I usually go for 1.5k/2.2uF on the cathode.
Works well as a “compromise” (but actually doesn’t sound like one) for both type of sounds ds
Last edited:
syscokid
Ambassador of War & Peace
IMO, because the whole point of the HF mod for the Origin is to turn it into a fire breathing high gain beast, a 2.2uF cathode bypass cap will bring in the mud and reduce the availability of "useful" gain.
old rocker
Well-Known Member
The Headfirst mod for the Marshall Origin is based on the Origin 20. The voltages are much higher on the Origin 50. IMO, the voltages on V2 after being reconfigured through the Headfirst mod, are way too high for any 12AX7: 385v at the plate/pin 1, and 244v at the cathode/pin 3. I did the "suck it & see" approach till the cathode voltage dropped to just under 200v, which also brought down the pin 1 plate to about 305v. Even though the new dropped voltage numbers are still high for the old school Marshallites, the new voltages are in spec for certain 12AX7's. We could have easily brought the PI & preamp voltages down more, but I thought that the amp sounded very interesting with the voltages on the high side.
There's also a slight difference between the O-20 and O-50 with the stock V1 cathode bypass caps. Jason Tong tuned his mod to the O-20... Would he have changed anything had he modded an O-50 instead?????? Anyways, the .68uF's that I needed for the HF mod were out of stock from Mouser. I ordered a couple .47uF's and a couple 1uF's instead. I first tried the .47's, but as soon as I switched to the 1uF's, I heard something that I really was digging even more... V1 cathode circuit of 1uF's/2.7k combo is the shiitkizzel...
Hi, syscokid. if i'm using Marshall ECC83 & EL34 valves, should i be ok using the value of resistors you've used for Roberts Amp in R8/9/10 ?(which i've now sourced at last).
Thanks Graham.
OP
It's just an incredibly loud, clear amp with huge gain potential, that never gets muddy. It is at least 2-3 dial positions louder than it was before the mods.
Gain is usable all the way to '10' on the dial, but I seldom run it past 10am.
My Ivanberg Modded ORI50 puts out more gain - hooked up without any FX - than a JCM800 2204 with an extra gain stage AND a Tube Screamer.
Be sure to check voltages before and after.
I only wrote about the 2.2 uF cap as an example of a larger bypass cap - that I’ve used in 2 channel amps that share V1a - that still can work in distortion circuits.
The HF-mod is most definitely aimed at really high gain, and a tight first gain stage is necessary.
Personally I’m not looking for that type of tone, which is why I’ll mod my amp in a different way.
Last edited:
Amp Mad Scientist
Ambassador of Heresy
Well it kind of goes like this:
adjust the output tube bias before any other voltage adjustments.
Because when you set the bias, the B+ voltage can go up or down.
If the bias is set cold, the B+ voltage is higher than normal.
Then when you set the bias correctly, that will make the B+ lower.
It's probably not a good idea to set the output bias by ear.
Ideally, the bias should be adjusted with an O scope.
But you can use a bias meter...just not as accurate.
I always use an O scope.
Last edited:
old rocker
Well-Known Member
So is a multi meter not advisable for adjusting the Bias ?.
OP
Tomorrow we are recording and I will try and get a really good recording if the Ivanberg Modded ORI50
ivan H
Ambassador of Tubes & Grooves
Yes, a multimeter is fine. There are those who say that using a function generator & 'scope is the best method to bias output tubes, but there are just as many who say it is not (like Mr Aiken for example, who definitely knows his stuff).
Biasing simply means setting the "quiescent" (no signal) plate current.
There are a couple of different ways you can use a multimeter to properly/successfully bias your output tubes.
My prefered method is as follows (amp chassis out of cabinet & well supported in an upside down position). I'm assuming that you already know how to calculate your intended bias "plate current". If not, yell out & we'll cross that bridge later.
With the amp "off" & the filter caps drained, first measure & note (write down) the (DC) resistance of each half of the OT primary winding. This is measured between pins 3 of octal type tube sockets & the OT primary's Center Tap.
Now, with the amp powered up & stabilised, measure & note (write down) the voltage drop across each half of the OT primary winding.
Now divide the voltage drop across each half of the OT primary by the resistance of the corresponding primary half to get the plate current.
What I do is to bias to about 60% (or a bit less) of the tubes maximum plate dissipation, then note the negative voltage at the output of the bias circuit (or at the junction of the power tubes bias splitter resistors, which is the same thing).
Then I bias to 70% of the tubes maximum plate dissipation & again note the negative voltage.
Then, with my multimeter hooked into the bias circuit & reading the negative voltage output, I set the bias by ear, by playing through the amp at normal (loud) volume to find my prefered setting.
So long as the negative voltage is within the 60% & 70% parameters I know the output tubes are safely biased. Hope this helps. Cheers
Edit:
Here, from the Marstran site.
Brian Wallace of Marstran also knows his stuff regarding tube amps. Cheers
Last edited: