B+ low on my chupa 50. Change PT for higher

harddriver

Well-known member
420DCV B+ is not low for a 50 watt Marshall amp. My 1972 Marshall 50 watt super lead is 400DCV B+. The average 100 watt Marshall is 450-485DCV with some early 80's JCM800's clocking in at 500DCV-510DCV for whatever reason.

Higher B+ would give you slightly more headroom and wear the tubes faster. Anything above 450DCV really stresses the new production tubes especially EL34 types. You can use 2.2K screen grid resistors instead of the traditional 1K value which gives less screen grid current giving more reliability using modern EL-34's. It does reduce the overall output 1.5 db. George Metropolous is using this in his builds now.

The difference from 420 to 450DCV would be that the amps response would get tighter with less sag kinda like Eds' brown sound in reverse. If you like the super tight sound then you may enjoy that. I think Wizard amps are in the 500DCV range.


Post by VelvetGeorge » Mon Sep 12, 2022 11:08 am

Those Welwyn 1k are actually 7 watts. Surprisingly. I stopped using them because the working voltage ratings are too low.

FWIW I now use 700V rated 2.2K Less screen current results in about 1.5db less overall output, but far better reliability with modern EL34's.
 
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FourT6and2

Well-known member
420v on the plates is fine. But the Chupa's preamp voltages are kinda low for my taste. If you want the sound/feel of higher voltages (tighter response, less fizz, less sag, more authoritative), you can do a few things in that amp. That TAD transformer has 347v secondaries, which is a step in the right direction. The Chupa's iron has 325v secondaries. So you'll see a bump up to about 450-460v or so on the plates. You wont have any issues with power tubes. You can also try:

1. Properly decouple the PI and CF stage by installing missing 10K dropping resistor, if you haven't already.
2. Lower V1a plate resistor to 330K
3. Play with first B+ dropper. If it isn't already 10K, lower it to 10K. If it is 10K, you can try something like 8k or so. Make sure to rebias the amp. But if you install new transformer, keep it at 10k.
 
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Nigel

Well-known member
420DCV B+ is not low for a 50 watt Marshall amp. My 1972 Marshall 50 watt super lead is 400DCV B+. The average 100 watt Marshall is 450-485DCV with some early 80's JCM800's clocking in at 500DCV-510DCV for whatever reason.

Higher B+ would give you slightly more headroom and wear the tubes faster. Anything above 450DCV really stresses the new production tubes especially EL34 types. You can use 2.2K screen grid resistors instead of the traditional 1K value which gives less screen grid current giving more reliability using modern EL-34's. It does reduce the overall output 1.5 db. George Metropolous is using this in his builds now.

The difference from 420 to 450DCV would be that the amps response would get tighter with less sag kinda like Eds' brown sound in reverse. If you like the super tight sound then you may enjoy that. I think Wizard amps are in the 500DCV range.


Post by VelvetGeorge » Mon Sep 12, 2022 11:08 am

Those Welwyn 1k are actually 7 watts. Surprisingly. I stopped using them because the working voltage ratings are too low.

FWIW I now use 700V rated 2.2K Less screen current results in about 1.5db less overall output, but far better reliability with modern EL34's.
I ran 2k’s in my old Zinky Mofo with 550v and KT77’s.
 

glpg80

Well-known member
It doesn’t need a new PT. 420V isn’t insanely low as said before. What people seem to forget is that you’re also going to take gain out of the preamp side by increasing plate voltages - you walk the Voltage on the IV curve upward which is no different than decreasing plate resistors across the entire preamp. What you get in headroom you lose in distortion. The only time a new PT is needed is when you’re actually low because of a scenario similar to mine where big bottle tubes with large heater requirements drag down the main B+ and when you also install more tubes for features or distortion. My amp is supposed to be at 460-480 and I’m at 440.

To go from 420V to 440V I’d recommend changing dropping resistors from 10k down to 5k and also adding what has been said before about adding the 10k tail resistor for the phase inverter back in.

Adding a new PT is a PITA and should only be done if your voltages are low across the entire amplifier which is not the case here.
 

harddriver

Well-known member
It doesn’t need a new PT. 420V isn’t insanely low as said before. What people seem to forget is that you’re also going to take gain out of the preamp side by increasing plate voltages - you walk the Voltage on the IV curve upward which is no different than decreasing plate resistors across the entire preamp. What you get in headroom you lose in distortion. The only time a new PT is needed is when you’re actually low because of a scenario similar to mine where big bottle tubes with large heater requirements drag down the main B+ and when you also install more tubes for features or distortion. My amp is supposed to be at 460-480 and I’m at 440.

To go from 420V to 440V I’d recommend changing dropping resistors from 10k down to 5k and also adding what has been said before about adding the 10k tail resistor for the phase inverter back in.

Adding a new PT is a PITA and should only be done if your voltages are low across the entire amplifier which is not the case here.
As glpg80 and 4T6&2 have said I also would increase the B+ rail voltage to your preamp tubes either at the dropping resistors and/or the plate resistors to V1 mostly rather than swap out a perfectly good power transformer, this is going to have more of a tightening effect than going from 420B+ to 450+ on the overall B+. If you do replace the power transformer....yes the overall B+ will go up maybe giving you what you want...maybe not or maybe not enough without those preamp section tweaks. I would listen to 4T6&2 since he has modded his Chupas to what you are describing and I think he has posted numerous threads and clips as well....maybe give those a listen and see what you think.

That amp not being tight enough is due to the B+ on the preamp section mostly as well as other component values so as these guys have recommended you need to fine tune the B+ you already have in the right places if that make sense. Alot cheaper than a new Power transformer which may not still give you the overall response you want specifically.
 

FourT6and2

Well-known member
What people seem to forget is that you’re also going to take gain out of the preamp side by increasing plate voltages

Maybe this amp isn't lacking in gain with 470K V1a plate + bright caps + diodes + 820R/.68uF cathodes in every stage.

What you get in headroom you lose in distortion.

I don't think it will be a drastic difference. This amp has plenty of distortion and giving up a little to get a more responsive, tighter, aggressive, and authoritative feel can be a worthwhile trade off.

PT with higher voltages: good
Lower V1a plate to 330K: good
Install missing 10K B+ dropper between PI node and V2b node: good

These three things are what I consider bare minimum to get a Chupacabra sounding/feeling healthy.

To go from 420V to 440V I’d recommend changing dropping resistors from 10k down to 5k and also adding what has been said before about adding the 10k tail resistor for the phase inverter back in.

I'm talking about the missing B+ dropper between PI and CF. Without it, the PI and V2 share the same filtering and the stages are not properly decoupled. The PI tail resistor is another topic, but not needed in this amp because it has the correct presence circuit.

Adding a new PT is a PITA and should only be done if your voltages are low across the entire amplifier which is not the case here.

Seems to be the case to me if he wants higher plate voltage on the power tubes as well as higher preamp voltages. I truly think this kind of amp benefits all around from a PT with higher secondaries. That is exactly why Ceriatone sells the Yeti with 350-0-350 PT. The chupa's PT has 325v secondaries. Maybe Ceriatone wanted more "brown sound"? I dunno. In its stock form. V1a sees about 97v. That's suuuuuuuuper low. Way too low.

I'm running close to 500v on the plates in my builds with a custom Merren PT and the amp sounds incredible. Running typical 1K screens and 10K grids. I use JJ E34L because they are one of the few modern production tubes that can handle 500v.
 
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jasonP

Well-known member
If this was my amp the last thing I would do before anything was to replace power transformer in my opinion. I would verify voltages in the pre amp stages, make sure that they were in range where I’d want to be. FourT6and2 talks about that in his earlier post and I’m in agreement with him on V1a plate resistor.
Having built the chupa100 and finding that there is a missing dropping resistor between the PI and CF again as FourT6and2 describes that changes the feel and tightens of the amp greatly in my opinion. I would work on fixing that.
Next for a tighter amp you can play with PI coupling caps and bias resistors. I like 100k/.1uf here like Fortin likes to use but for a tighter response you could go with more traditional Marshall setup, with 220k/.022uff.
I run 550v on the plates with Mercury transformers and it’s one of the best sounding amps I own. Run el-34ls and they just take a beating.
 

scottosan

Well-known member
Raising it will yield a stiffer feel and more headroom. My favorite Jose mod was on 395v Roccaforte Custom 80.
 

FourT6and2

Well-known member
Could always go with a PT that has both high and low voltage secondaries and put it on a switch. My Merren has that. But I prefer the sound/feel with higher voltage. It's all personal preference.
 

RedPlated

Well-known member
420v is not low. I've modded 800's with high 300's B+ that ended up super tight, punchy, and super aggressive. Set your PI node to 320-330v with appropriate dropper(s), and adjust the preamp component values to get the feel/ response you want. .022/ 150k is actually a good setup for the PI coupling / bias splitters on a 50w. .022/ 220k is also fine.
 
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FourT6and2

Well-known member
I'm not well known in names for parts. Where are the PI coupling and bias splitters? :)
The phase inverter (PI) is V3, the third tube socket from the right. Follow the red wires coming off pins 1 and 6 (the plates). Pin 1 connects to an 82K plate resistor. Pin 6 connects to a 100K plate resistor. Each of those resistors meet at a single point. That is what's called the PI B+ Node. To the left and right of those plate resistors (100K and 82K), there is a 0.1uF cap. Those are coupling caps. Each of those coupling caps connects to a 110K resistor. Those are PI Grid Leak resistors. Some call them Bias Splitters. Those resistors have a J and K label next to them. J and K are wires that connect to the output tube sockets via 5K6 Grid Stopper resistors.

Here are two calculators to determine what various combos of coupling cap and grid leak/bias feed values do:

But basically... stock Marshall value 0.022uF + 220K, with 5K6 grid stoppers. The Chupa uses 0.1uF + 110K, 5K6 grids. This passes more sub bass frequencies and the excursion time is slower. What does this mean? I means flubby and loose lol. Also the lower the value of the grid leaks, the more you lower the output signal/volume. But the easier on the tubes. Also with regard to the grid leaks, grid stoppers, and part of the bias control: tubes have a maximum rating for how much resistance they see in this part fo the circuit. So I wouldn't go higher than 220K + 5K6. If you go lower, like 110K or 150K grid leaks, you can bump up the grid stopper to 10K.

Common values for the grid leaks you can try: 220K, 180K, 150K, 110K.
Common values for coupling caps you can try: 0.022uF, 0.047uF, 0.1uF

Chupa50-28-Jan-2021.jpg
 

FourT6and2

Well-known member
Also what's cool about this kind of circuit is it's great for learning what everything does. A lot of guitarists look at a guitar amp and think it's magic. It's mojo. There's something each "amp guru" does special to the amp to make it sound like it sounds and they must possess some mystical ability nobody else has. And that's complete BS. Once you learn what each part of the circuit does, like oh hey if you change the 0.1uF PI couplers to 0.022uF, it does this. Or oh hey, if you change the 47K slope resistor to 33K, it does that.

Once you learn what alllll these parts of the circuit do and how they interact, you start to realize it's not magic or mojo. And what Cameron does, or what Fortin does, or what Friedman does, or Bogner or Soldano or [insert magical metal guru] does... well it's no longer hype or mojo. It's cooking. And then you can start to season your own amp to how you like it, rather than relying on a chef to cook your steak for you.
 

glpg80

Well-known member
I’m a big fan of stock PI values for those that are curious. We all have our preferences - big emphasis on preference. I don’t like large PI coupling caps - makes the tone sound too far inside the cabinet and less forward If that makes sense.
 
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