Two nearly identical heads with voltages differences at v1

erkl

Member
Messages
100
So I have built two nearly identical heads though there are some differences in some of the brands of components…. for example, head 1 uses Sozo caps and a mixed-bag of resistor types while head 2 uses Jupiter caps and mostly DALE metal film resistors.

Both amps have similar voltage readings on pre and power amp tubes with ONE exception. Pin 1 on v1 reads 213 VDC on head 1 and 152 VDC on head 2.

Head 2 sounds more distorted and a lot brighter - overly aggressive and a bit harsh… even with treble pot set at ZERO. My intention is/was to have them sound as close as possible.

I have been over everything multiple times, everything seems to be where it should and I can’t even figure out why pin 1 has such a difference between the heads. Same tubes even.

Any ideas why 1 amp would run in a difference of some 60 VDC? ANd what would be such an effect of running 150 VDC on pin 1 V1???

Thanks, as always!
 

PremiumPlus

Member
Messages
1,150
You could also help locate the cause if we knew the voltage on the other side of the plate resistor on both amps. If it's similar, then you know that the tubes are conducting differently...therefore causing a larger voltage drop on the plate resistor. Trace the power supply through the plate and cathode resistors to ground and see where the difference is.
Also look for DC on the grid.
 
Messages
6,835
One would want to be looking at the cathode voltages there, too. You don't tell us what amps these are or what voltages should be expected there????
IF that one triode is grossly misbiased....to much positive voltage on the cathode, it will be drawing a lot of current and the plate voltage will drop. The reverse is also true...if the plate voltage is too low, the bias is affected as well.
 

DGDGBD

Silver Supporting Member
Messages
7,804
Assuming you have identical power transformers, filter caps and circuitry, maybe there is a wiring discrepancy in the power supply?
 
M

Member 1963

With less voltage on head 2 i would expect the opposite as brightness goes....i would expect head 1 to be brighter. Can i assume the sozos are in head one? I find sozos are dark and lack top end. maybe thats why. I tried both the standard and vintage sozo yellows and returned to mallory and got my top end back, not to mention the harmonics that come with that. maybe had i built the amps i tried them in around sozos in the first place they'd have been ok because they would have had me designing more top end into the circuits.
 

VaughnC

Silver Supporting Member
Messages
19,035
I'd check the plate resistor resistance, the voltage from the power supply node, and the cathode voltage. The plate resistor or power supply node resistor could be out of spec or the cathode bypass cap could be shorted. Or, if that triode is #2 in the chain, the coupling cap could be leaking DC from the previous stage.
 

erkl

Member
Messages
100
wow - thanks for all the suggestions - and I do apologize for the gross under-information. I am waiting for my wife to wake from her turn to sleep in and I will be getting into day FOUR of troubleshooting. Within an hour I will be trying some of the suggestions until I nail this thing....

more very soon...
 

erkl

Member
Messages
100
Hi again - okay so just for the record, this amp has some switches to re-create in as close as possible most mainstream Marshall circuits. i.e. v1 cathode has three switchable options - option 1 is .68µF/2.7kΩ, option 2 is 330µF/820Ω, and option 3 is 220µF/820Ω. Obviously covering a JTM50'ish/plexi Bass/plexi Lead/2204... the amp sort of goes on and on with these switches, v2 has it's era cathode options, tone stack options, NFB/tap options, bright switch options, and cascade ala JCM800 options.

It seems that on v1, with 330µF/820Ω O 220µF/820Ω cathode options active, plate (PIN 1) measurements fall in the 154-158 VDC where the .68µF/2.7kΩ changes the pin reading on PIN 1 to be 205-215 VDC.

Does this make sense?

So for the readings that I took between yesterday and today... I swapped tubes so that both amp 1 and amp 2 were using the exact same preamp tubes in the same order and all with the same options enabled and the readings are extremely close now and that part seems okay. So I obviously took readings on one amp setup one way and the other amp with a different set of cathode options.

As for voltages I have used mainly METRO amp charts and nothing really seems to wildly differ.

Can i assume the sozos are in head one? I find sozos are dark and lack top end. maybe thats why. I tried both the standard and vintage sozo yellows and returned to mallory and got my top end back, not to mention the harmonics that come with that.
That is correct, Sozos are in head 1, Jupiter in head 2. And head one is a bit overly dark, but head 2 is like MB Dual Rectifier bright - even with treble off.

I will create a few small clips to post for comparison.

I'd check the plate resistor resistance, the voltage from the power supply node, and the cathode voltage. The plate resistor or power supply node resistor could be out of spec or the cathode bypass cap could be shorted.
Since the amp was done on Christmas Eve - I have spent hours everyday reading values to make sure it is all correct - I even had to unsolder a few components to get a proper reading - it seems that the master volume pot is one of them. Certainly possible that there is a mistake but I have been over it several times both wiring and readings of every component I could think to check.

I will make a few clips - you can certainly tell they are the same family of circuit - just one sounds like I cranked the treble pot on it when it is in fact off.... be back in a short while.

AND THANKS FOR YOUR HELP!!!
 

erkl

Member
Messages
100
annnnnd...

here is a link to an 11 second clip from each amp...
soundcloud.com/guitzero/sets/99-praalums/s-o14p5
Amp 1
  • Sozo caps
  • mix bag of resistor types and brands (some DALE metal film, some carbon film, very few carbon comp)
  • JJ multisection filter cans (50+50µF, 32+32µF, and 16+16µF)
  • FRED diodes
  • treble control at half
Amp2
  • Jupiter caps
  • 95% DALE metal film, a few carbon film
  • F&T single section filter caps (2x 47µF, 2x 33µF, 2x15µF)
  • regular old IN4007's
  • remember amp2 treble control is on maybe 2
And again now that I had discovered my error, the voltages are damn near the same.

I absolutely love amp 1 and hopes amp 2 would come out the same if not better, but amp 2 just seems to sound like a raunchy overly distorted version.

Could caps make this kind of difference????

I can also post full voltage charts of desired.

THANKS!
 
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Blue Strat

Member
Messages
30,722
Have you tried swapping V1s between the amps and rechecking the voltage yet?

Caps are unlikely to have any effect on the DC voltage readings unless you have an electrolytic cap installed backwards, or one is leaky.
 
M

Member 1963

Could caps make this kind of difference????

I can also post full voltage charts of desired.

THANKS!

Well, they did in mine. When it comes to cap and resistor type/brand i think a lot of people make the mistake of looking at the difference in the wrong way.....you can't just change one or 2 and easily notice a difference. But when you compare them with with MANY OR ALL different, there can be a big difference. I CAN hear slight differences with one depending on the location, but it's subtle and i'm maybe even being fooled by placebo effect. But when i put all sozos in an amp it changed quite a bit, no placebo effect there. And your amps are VERY different as brand/type of both caps and resistors go, so i'd look to that.
 

erkl

Member
Messages
100
Have you tried swapping V1s between the amps and rechecking the voltage yet?

Caps are unlikely to have any effect on the DC voltage readings unless you have an electrolytic cap installed backwards, or one is leaky.
yes, same preamp tubes are used between the amps and I have consistant voltage between them as well... and the .68µF isn't electrolytic... it is a Jupiter

as for the resistors and caps... I am replacing the resistors with cf or cc at the moment while i figure out what do do with the caps>>>
 

Blue Strat

Member
Messages
30,722
Do you have a schematic of what you have? Maybe some photos of your work? The best we can do given the information we have is throw darts while blindfolded.
 

Blue Strat

Member
Messages
30,722
Since you're looking at DC voltages right now, I'd disconnect the cathode CAPACITOR and the coupling cap to the next stage. You need to isolate the circuit.

Also, if you haven't already done so, MEASURE every resistor in the circuit and power supply voltage divider chain.
 
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Messages
6,835
It seems that on v1, with 330µF/820Ω O 220µF/820Ω cathode options active, plate (PIN 1) measurements fall in the 154-158 VDC where the .68µF/2.7kΩ changes the pin reading on PIN 1 to be 205-215 VDC.

When you are switching from the 820ohms of resistance to the 2.7K ohms, you are rebiasing that triode. The 2.7K resistance will reduce current draw compared to the 820 ohm value and thereby increase the plate voltage. I would expect the 820ohm situation to yield a gainier situation.
 

TimmyP

Member
Messages
2,488
Why such large bypass caps on the 820s? Caps 1/10 that size would still give more low frequency extension than one can make use of.
 

erkl

Member
Messages
100
Do you have a schematic of what you have?
No, a few rough layouts with values placed... I had to design the board layout to accommodate so many extra components. I have, however, made a rough sharable draft from my Google Drive... https://drive.google.com/open?id=0B4L1sdPIk2ODX2RGUE9GODRZeE0

It's rough and I wouldn't normally post it as those who haven't read any of my previous posts above would only comment how wrong it is ;-)

But it's mainly usual Marshall stuff...

Since you're looking at DC voltages right now, I'd disconnect the cathode CAPACITOR and the coupling cap to the next stage. You need to isolate the circuit.

Also, if you haven't already done so, MEASURE every resistor in the circuit and power supply voltage divider chain.
I have, in addition I have measured every resistor in the amp, even pots and isolated than where needed to do so.

When you are switching from the 820ohms of resistance to the 2.7K ohms, you are rebiasing that triode. The 2.7K resistance will reduce current draw compared to the 820 ohm value and thereby increase the plate voltage. I would expect the 820ohm situation to yield a gainier situation.
Ahh yes, I suppose so as well. I assume this is why there is some 50 to 60 more volts running through when switched to 2.7kΩ.

Why such large bypass caps on the 820s? Caps 1/10 that size would still give more low frequency extension than one can make use of.
Because that is what Marshall has historically used. I am not reinventing the circuit, just giving more options so that I do not have to build multiple heads - I love them allllll.


At this point I have ripped out the entire preamp section all the way to the phase inverter. I will replace what I can with other brands and see if it yields a different result.

For what it's worth - early on in this amps re-design, I replaced all sozo caps with super cheap Xicon polypropelene film caps to see if there was some actual hype to paying up to 10x more for Sozos. The result (which I still have in my files) wasn't as drastic in the overall brightness as much as I am seeing (possibly) with Jupiters... the result was waaay less clarity and overall smearing of sound with the Xicons - in such case the cost of Sozos was warranted. But the difference between these amps is more like sweet creme vs razor blades.
 
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6,835
just a side note, but....Marshall used those big caps because the 5F6A Bassman used a large cap there.
I have to go back and reread this thread everyday. Today I caught that you think that you are in the 2204 preamp zone here....no way. Take another look at the 2204 preamp. The input stage in the MV amps is much different than their predecessors.
I am also confused....are these two amps exactly alike? Same switching, same circuit, same components values, etc? And the amps sounds totally different with different voltage results in that preamp stage when the two amps are in the same ''mode'?
 




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