John Mayer Signature New Build with Reverb Issue

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dorrisant
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Re: John Mayer Signature New Build with Reverb Issue

Post by dorrisant »

I tapped it from the screen supply node.
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martin manning
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Re: John Mayer Signature New Build with Reverb Issue

Post by martin manning »

Is this like SSS CF driver stage or something different? if there's a scheme you can link I'll have a look for myself...
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Re: John Mayer Signature New Build with Reverb Issue

Post by dorrisant »

I shoehorned the driver from Mike Hartman's take on SSS into my John Mayer Signature using these two schematics...
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Re: John Mayer Signature New Build with Reverb Issue

Post by martin manning »

Thanks Tony, so you are getting negative voltage from the HT, ala the SSS schematic. That puts a lot of voltage across the CF stages IMO. I've been thinking about trying this, and worked out a layout, powering the CF stages using only the 60V bias tap. Interesting to hear about the tonal effect.
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Re: John Mayer Signature New Build with Reverb Issue

Post by dorrisant »

I built a slightly different power supply... bias was off a separate tap. I was using the Epi SoCal iron so I used the same DC supply for the filaments and the same setup for bias, etc.. I will look up my notes and post back what you need. This works well. I had the JM built and liked it a lot. Never played it much because I wanted it to sound like Hartman's SSS. When he posted about it here, I took some notes and followed them to a tee. I figured the filament winding that supplied the power tubes only could handle the extra load from one 12**7. It was begging for the mod (or was that just in my head). Anyway, when I installed it, it sounded soooo much better!! There was some newfound hum that I had to eliminate and BAM!!! "I can't wait to play it again" tone... Makes me want to put in a PAB switch just to see, if you know what I mean. I kinda need to leave the front panel alone on this one but I have my eye on a Super Twin Reverb that is just begging for some attention. Maybe not... I need to find an SSS chassis and build one I can tinker with (look in the for sale section).

If you want, I will post voltages and everything on this particular one. I took all the info earlier today... It is just that I got my Yamaha CA610II back in service... the vinyl is spinning and the whiskey is flowing... Time for restoratives. :D I will post that info and pics till your eyes pop out if you want. I thought I had a build thread for this amp but I couldn't find it earlier... maybe it's just Mandela Effect... :wink:
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martin manning
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Re: John Mayer Signature New Build with Reverb Issue

Post by martin manning »

Cool, a little celebration is good! Yes, I'm interested in your voltages and the details of your negative supply. What are the values of the resistors on the driver tube? Looks like 220k for the cathodes and 2M grid leak, with no grid stoppers?

My goal was something that could be done with a standard Twin PT without the high voltage or an extra transformer. I posted it here somewhere, but I don't know if anyone has built it.
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Re: John Mayer Signature New Build with Reverb Issue

Post by dorrisant »

Sorry Martin, the shop was super busy today and it took me a while to get to this...

John Mayer Signature Prototype
***Schematic Date = 16-07-12***
Used PT, Choke, Rev TX and OT from Epiphone SoCal 50
Built 5/14
Voltages take @ 120vac line

B+1 = 434 OT CT
B+2 = 432 Screens & Cathode Follower
B+3 = 337 Reverb Driver
B+3a = 331 Reverb Input & Return
B+4 = 260 Mixer
B+5 = 256 Stages 1 & 2

V1 - 12AX7 Stages 1 & 2
P1 = 165
P3 = 1.34
P6 = 166
P8 = 1.33

V2 - 12AX7 Reverb In & Return
P1 = 219
P3 =1.63
P6 = 195
P8 =1.34

V3 - 5751 Reverb Driver
P1 = 331
P3 = 3.77
P6 = 331
P8 = 3.77

V4 - 12AX7 Mixer
P1 = 177
P3 = 1.23
P6 = 176
P8 = 1.30

V5 - 12AX7 Phase Inverter
P1 = 241
P3 = 48.8
P6 = 228
P8 = 48.8

V6 - 12AX7 Cathode Follower Driver
P1 = 434
P3 = -44.4
P6 = 434
P8 = -44.4

V5 - 6L6GC Power
P3 = 434
P4 = 431
P5 = -44.5

V6 - 6L6GC Power
P3 = 434
P4 = 431
P5 = -44.5


"One thing to try on yours is going by the schem for the bias section -- (i.e., bypass your bias pot so you get the full negative voltage going to the 220k CF resistors. Then adjust your power tube bias with the 130k resistor going to ground in the PI). That worked for me to get the CF and power tubes working correctly." - M Hartman

Page 2 here: http://ampgarage.com/forum/viewtopic.php?f=4&t=20662


I realized you were right about the lack of grid leak resistors and figured out why Hartman said the above. I installed the grid leaks and found the bias supply was not putting out enough negative dc for the job. I pulled the bias pot and resistor to ground out to get the voltage as far negative as possible and it wasn't working. The bias supply that was I using was derived from a separate PT tap. So I used the HT tap and dropped voltage with a 3.3k resistor in place of the dedicated tap.

HT winding = 347 VAC to ground.
After 3.3k resistor = 333 VAC to ground.

This feeds the diode and a pi filter consisting of 10µF 500v caps and 1k resistor. I removed the bias pot and the resistor to ground from the original design.

Filtered Bias DC = -387

Referencing M Hartman's SSS Poweramp & Power Supply schematic, I replaced the 130k resistor to ground with a 250k trimmer pot. I kept the original JMS PI and Cut control values. As it is right now the bias settles in at roughly 21 watts per tube at the given voltages.

I will post a schem in a few minutes...

***Edited to show final voltages... I think.***
Last edited by dorrisant on Sat Sep 16, 2017 7:37 pm, edited 3 times in total.
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Re: John Mayer Signature New Build with Reverb Issue

Post by dorrisant »

Schematic and updated pics...

I don't really want to change anything right now but I wonder what difference the PI values in Hartman's schematic would make.

***Edited to show correct pictures and update schematic values.***
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Last edited by dorrisant on Sat Sep 16, 2017 7:49 pm, edited 3 times in total.
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martin manning
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Re: John Mayer Signature New Build with Reverb Issue

Post by martin manning »

Fantastic! Thanks for posting all that. Negative bias supply is basically Marshall type from HT leg, 1M grid leaks (the brown band looked red to me in the photos). No grid stoppers on the CF like Hartmann scheme... if it's stable then I guess they are not needed.

The 820k resistor between the bias pot and the 220k cathode resistors in your schematic should be 120k.

I only see 18k vs. 24k in the tail of the PI, which is not a huge difference, and also that you omitted the PI balance trimmer.

One more question, if I may, what value of feedback resistor and presence cap did you go with? They're not marked in your schematic... 4k7 and 1uF?
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Re: John Mayer Signature New Build with Reverb Issue

Post by dorrisant »

martin manning wrote: Sat Sep 16, 2017 10:08 am Fantastic! Thanks for posting all that. Negative bias supply is basically Marshall type from HT leg, 1M grid leaks (the brown band looked red to me in the photos). No grid stoppers on the CF like Hartmann scheme... if it's stable then I guess they are not needed.
1M grid leaks... in the PI? Yes. The power tubes have 220k grid leaks though. Hartman's PA shows 470k for each tube. They are in parallel there so I just used 270k at each of the two tubes... Sound right?
martin manning wrote: Sat Sep 16, 2017 10:08 amThe 820k resistor between the bias pot and the 220k cathode resistors in your schematic should be 120k.
Hartman's layout and subsequent schematic both show 820k there... :shock:
martin manning wrote: Sat Sep 16, 2017 10:08 amI only see 18k vs. 24k in the tail of the PI, which is not a huge difference, and also that you omitted the PI balance trimmer.
Yes
martin manning wrote: Sat Sep 16, 2017 10:08 amOne more question, if I may, what value of feedback resistor and presence cap did you go with? They're not marked in your schematic... 4k7 and 1uF?
I updated the hand-drawn schematic in my above post to show those values... Also edited a few of the pics I had up. They were showing some mid-transition stuff that I removed and is irrelevant.

You are very welcome Martin. I am glad to be able to help with what I know... How many times have you answered my questions now? You get the idea... :)
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martin manning
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Re: John Mayer Signature New Build with Reverb Issue

Post by martin manning »

This pic shows a 120k, no? That's the one labeled 820k on Hartman's scheme, I think? Yes, the power tube grid leaks are in parallel, and were shown that way on the hand-drawn SSS 002. Other SSS schemes do not have them, so their purpose is a bit of a mystery.
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Re: John Mayer Signature New Build with Reverb Issue

Post by dorrisant »

OMG... You are right, it is 120k. I changed it to 820k and now will have to revert to the old bias supply circuit.
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Re: John Mayer Signature New Build with Reverb Issue

Post by martin manning »

But it's working, right? I like 120k there... makes perfect sense with your -100V bias supply and 250k trimmer.
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Re: John Mayer Signature New Build with Reverb Issue

Post by jam-mill »

dorrisant wrote: Sat Sep 16, 2017 3:55 am [...snip...]
V6 - 12AX7 Cathode Follower Driver
P1 = 434
P3 = -44.4
P6 = 434
P8 = -44.4

[...snip]
434 volts on the plate of 12AX7?

Doesn't that exceed the max of 300-330V?
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dorrisant
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Re: John Mayer Signature New Build with Reverb Issue

Post by dorrisant »

I reworked the bias supply. Made it more like Hartman's except one more dropping resistor and filter added. Voltages changed ever so slightly so I updated them and a few of the pics as well.
jam-mill wrote: Sat Sep 16, 2017 7:49 pm
dorrisant wrote: Sat Sep 16, 2017 3:55 am [...snip...]
V6 - 12AX7 Cathode Follower Driver
P1 = 434
P3 = -44.4
P6 = 434
P8 = -44.4

[...snip]
434 volts on the plate of 12AX7?

Doesn't that exceed the max of 300-330V?

Yes... and with the cathode at a negative voltage to ground it is 477v from plate to cathode. Makes you wonder about longevity. Also, what happens to the bias if that tube does go bad?
"Education is what you're left with after you have forgotten what you have learned" - Enzo
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