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PostPosted: Thu, Jul 05, 2018 10:34am 
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Hey guys after a long time I finally finished my 1969 Super Bass build, it has a Metro chassis, period correct fiber circuit board and NOS Iskra reisitors and NOS mustard caps. I formed all the caps while bringing the amp up to line voltage on a variac. All the voltages fall into line except the V2 Pin 1,7,8 voltage is about 30 DCV too high at 205 DCV, this is the cathode follower tube with the 100K resistor from pin 6 to 1 and then pin 7. The Normal spec voltage rating for V2 on a vintage 100 watt Marshall is supposed to be, pin 1 168 pin 6 303 pin 7 168 pin 8 168 according to voltage charts I have. I thought the vintage Iskra resistor might not be dropping the B+ from the rail correctly so I changed it out with a modern 100K and it had the exact same reading. Should I up the resistor to to 150K or so to get my V1,7,8 closer to spec or live with it? From my other readings the 100K should be giving at least a 100 DC volt drop Anyway took some reading from the incoming B+ along the dropping resistors to the B+ rails to the preamp tube sockets to give you an idea of the overall voltages. the amp plays and sound great, I did try a 12au7 in that position and the Pin dropped to like 111DCV on pin 1,7,8, but it sounded like shit but the it dropped the votage.. Could the preamp tube itself be causing the voltage shift?, I used good JJ12ax7's on the iniital start up after cap forming. Here's what I have for voltages interested in your thoughts o the matter. The AC line voltage was swinging yesterday due to grid load so the ACV line in was 115 to 118ish. At 123ACV the B+ is a healthy 491DCV.


Last edited by harddriver on Sat, Jul 07, 2018 11:50am, edited 1 time in total.

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PostPosted: Thu, Jul 05, 2018 11:02am 
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Different tubes can throw those numbers off. You're close enough. You can try different brand 12ax7's and you'll see what I'm talking about.
If memory serves me, I think current production tung-sol drops pretty good.

Basically different 12ax7's conduct different amounts of current. the ones that draw more, will pull that voltage down more.


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PostPosted: Thu, Jul 05, 2018 11:21am 
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Cool build! Do you have grid load 'safety' resistors in parallel with the dual pot MV? Often they are 2M2 with a 250k dual pot.

Also, it seems many Marshalls have a 0.68uF Presence cap even though all the schematics say 0.1uF. I usually like somewhere in the middle.


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PostPosted: Thu, Jul 05, 2018 12:08pm 
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CrazyNutz wrote:
Different tubes can throw those numbers off. You're close enough. You can try different brand 12ax7's and you'll see what I'm talking about.
If memory serves me, I think current production tung-sol drops pretty good.

Basically different 12ax7's conduct different amounts of current. the ones that draw more, will pull that voltage down more.


I was kind of thinking this when I mistakenly put a Sylvania 12au7 in there instead a the 12ax7 and the 205DCV volts on pin1 dropped to 111DCV. I will definitely swap some other tubes in and out of V2 and record some more voltages to put my mind at ease. I just wasn't comfortable if the 205DCV volts on pins 1,7,8 would start to chew up 12ax7's in a bad way. We all know V2 cathode follower voltages are hard on modern preamp tubes.

I really like the Tung Sol 12ax7's but unless things have changed they aren't recommended for use in the cathode follower V2 positions due to alot of failures there when they first hit the market a few years ago. I have an RFT to try in V2 which are robust as hell but I remember it being kind of harsh in a plexi, for some reason I keep coming back to sylvanias in plexis. Thanks alot for your suggestions. I hear great thing about the new Mullards long and short plate tube even though they are reworked Sovteks IMO they are supposed to be pretty good, I think George Metropolous is using them in his Metroplex and Superplex amps and I know George knows NOS and Modern tubes well.


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PostPosted: Thu, Jul 05, 2018 12:29pm 
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SpiderWars wrote:
Cool build! Do you have grid load 'safety' resistors in parallel with the dual pot MV? Often they are 2M2 with a 250k dual pot.

Also, it seems many Marshalls have a 0.68uF Presence cap even though all the schematics say 0.1uF. I usually like somewhere in the middle.


Thanks, it was half built for a long time until now, I wish I had used a steel chassis but they weren't available when I started this. I based this build off the famous 1969 Purple superlead which was shared cathode but super lead .022uf output PI coupling caps and a regular Super Bass. Like you guys I've been hobby building for quite awhile. I've built a 68 Metro plexi, 2203, 2203/slo/cameron aldrich high gainer and some other amps in the past few years. I have a 1983 JCM800 2204 that is next up for some mods. I've been watching your BE100 build... which is turning out great BTW. :rock:

Yes, the LAR/MAR PPIMV has the 2.2 meg safety resistors and I used the $30 PEC 250A dual pot to compare to the Alpha pots I have used in the past and it does seem to be better, at low volume setting it sounds less harsh than the Alphas and has a smoother taper so I guess $30 might be worth it in this application. I used the traditional .1uf and the 5kL pot on this 69. THe .68uf cap was used with a resistor on the pot to shunt the value down I think a 25KL pot in the late 70's because it took away the scratchiness of the presence pot. My 1979 2203 had the .68uf cap and the resistor on the pot, I guess the presence less harsh with the .68uf value.

This amp is kind of a mishmash of ideas, I have the 5000pf bright cap on Vol1 switched pot but i retained the 47K feedback on the speaker jack like a Super Bass. I had to get another PT for it as the old 2009 Magnetic component PT B+ was drifting up and down and running super super hot, it was junk, I knew it but i tried it anyway. I purchased and new 2017 production Classictone 40-18053 with the M6 laminations designed to run cool and it does, thank god. So far it seems to be breaking in nicely it's a cool departure from my 68 evh plexi, it's got a darker growl to it.


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PostPosted: Thu, Jul 05, 2018 12:53pm 
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Cool stuff, gets me wanting to do another one. :rock:

Regarding the V2 tube, so many of the current production Russian made tubes have been deemed risky in the CF spot I just avoid anything Russian made there. I do like the Mullard reissue long plate 12AX7s especially in the PI, they look almost identical to Sovtek LPS internally.


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PostPosted: Thu, Jul 05, 2018 12:55pm 
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harddriver wrote:

I really like the Tung Sol 12ax7's but unless things have changed they aren't recommended for use in the cathode follower V2 positions due to alot of failures there when they first hit the market a few years ago.



Run elevated heaters, and that problem goes away.


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PostPosted: Thu, Jul 05, 2018 1:07pm 
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SpiderWars wrote:
Cool stuff, gets me wanting to do another one. :rock:

Regarding the V2 tube, so many of the current production Russian made tubes have been deemed risky in the CF spot I just avoid anything Russian made there. I do like the Mullard reissue long plate 12AX7s especially in the PI, they look almost identical to Sovtek LPS internally.



Really the only thing safe in CF, new production wise is JJ's, and Chinese. Unless you run elevated heaters, then pretty much everything is safe in the CF.

I've ran a lot of amps, and tubes. I've actually only had a few fail in the CF spot, that is using the non CF safe tubes. So it's not necessarily guaranteed to happen, theres just a slight chance that it might.


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PostPosted: Thu, Jul 05, 2018 1:38pm 
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CrazyNutz wrote:
SpiderWars wrote:
Cool stuff, gets me wanting to do another one. :rock:

Regarding the V2 tube, so many of the current production Russian made tubes have been deemed risky in the CF spot I just avoid anything Russian made there. I do like the Mullard reissue long plate 12AX7s especially in the PI, they look almost identical to Sovtek LPS internally.



Really the only thing safe in CF, new production wise is JJ's, and Chinese. Unless you run elevated heaters, then pretty much everything is safe in the CF.

I've ran a lot of amps, and tubes. I've actually only had a few fail in the CF spot, that is using the non CF safe tubes. So it's not necessarily guaranteed to happen, theres just a slight chance that it might.


I ran elevated heaters in my 2203/Aldrich build, I had to build a standalone board on the end of the chassis. As you state, it also is supposed to help protect the preamp tubes. If I remember right your sending like 40-50VDC into the PT heater center tap wire which floats the heaters with DC. Aside from that I just know it is supposed to beneficial especially for high gain build for noise and other issues. I' m trying to figure out a way to install it without building a standalone board on the JCM800 build... we will see. Any suggestions or layouts?


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PostPosted: Thu, Jul 05, 2018 2:04pm 
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Between your screen filter cap balance/bleed resistors. Use that as a voltage divider to get 250ish volts. then come off that into a 470k / 82k (to ground) divider, and a cap to lower it in the correct range.

I just wire these on a little tag board, and bolt it to one of the cap can retainer bolts. Adjust that 82k if you need more/less voltage.

Image

I'll take a picture of one when I get home.


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PostPosted: Thu, Jul 05, 2018 2:17pm 
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Thanks alot I appreciate it. I forgot mention in the 69 build I'm running some JJ6CA7's power tube and so far I'm really liking them. THe bias is staying tight and not drifting but we will see and I continue to burn it in.


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PostPosted: Thu, Jul 05, 2018 2:39pm 
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One way I saw elevated heaters in a Friedman modded Marshall was to use 100x100 cans for the screens. The screens get 100uFs in series which leaves 2 unused 100uFs. The can that is actually grounded has a 1M resistor to the other unused 100uF then a 100k (or 82k) to ground (in parallel with 100uF). No extra caps just 2 resistors and they are mounted on the can.

EDIT: If you like the JJ 6CA7s you might want to try EH 6CA7. I like those a bit more than the JJ but just a personal preference.


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PostPosted: Thu, Jul 05, 2018 2:45pm 
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^^^ The EH 6CA7's are superb!


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PostPosted: Thu, Jul 05, 2018 3:03pm 
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Ok found a pic, this is how I do it:

Image


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PostPosted: Thu, Jul 05, 2018 3:29pm 
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SpiderWars wrote:
One way I saw elevated heaters in a Friedman modded Marshall was to use 100x100 cans for the screens. The screens get 100uFs in series which leaves 2 unused 100uFs. The can that is actually grounded has a 1M resistor to the other unused 100uF then a 100k (or 82k) to ground (in parallel with 100uF). No extra caps just 2 resistors and they are mounted on the can.

EDIT: If you like the JJ 6CA7s you might want to try EH 6CA7. I like those a bit more than the JJ but just a personal preference.


I have used them in the past, they were ok but I was comparing them to actual Sylvania 6ca7's at the time. I picked up the JJ6ca7 just to try them out and compared, they have abit more EL34 feel to them but I really like the thick glass on them they really feel substantial and heavily built, we'll see how long they last. The EH6ca7 feels kind of cheap but I read they are a true beam forming tetrode like the Sylvania/GE6ca7, I have a low hours quad in their boxes here as well. THe JJ is still a power pentode but with some spine which i like.


Last edited by harddriver on Thu, Jul 05, 2018 3:36pm, edited 1 time in total.

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PostPosted: Thu, Jul 05, 2018 3:30pm 
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Thanks for the pic CrazyNutz that is exactly what I was looking for... I snagged the picture for reference when it's time.

What was the recommended DC voltage range you needed to go into the center tap? I remember there was a minimum DCV needed. I believe it was 40-50DCV that stands out. I think I had like 45-47DCV and it was enough and I didn't have to adjust the voltage divider more than once.


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PostPosted: Thu, Jul 05, 2018 4:14pm 
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30-60vdc is the typical range for hum reduction. The closer to 60vdc you get, the better protection from tube death.

You can use a smaller 100v rated cap. The 500v is just what I had on hand.

The Friedman style SpiderWars mention is nice and clean. However that is really only cost effective if you planned and bought the 100/100uf caps ahead of time. For modding an existing amp I like just using the tag board


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PostPosted: Sun, Jul 08, 2018 3:31pm 
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CrazyNutz wrote:
Ok found a pic, this is how I do it:

Image


Isn’t this called the “hum dinger” ?

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PostPosted: Sun, Jul 08, 2018 7:57pm 
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psychodave wrote:
CrazyNutz wrote:
Ok found a pic, this is how I do it:



Isn’t this called the “hum dinger” ?


No, a "hum dinger" is a 500 Ohm pot that takes the place of a faux center tap resistors. Hum is usually introduced asymmetrically so the pot allows you to balance the faux center tap. You just turn it one way or another until you have less hum.

You guys can read everything you need to know about heaters here:
http://www.valvewizard.co.uk/heater.html


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PostPosted: Sun, Jul 08, 2018 9:35pm 
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An example of how Friedman uses one half of a can cap for the elevated heaters in a Dirty Shirley.
Pretty ingenious actually.


Attachments:
Friedman DS elevated heaters.jpg
Friedman DS elevated heaters.jpg [ 82.23 KiB | Viewed 548 times ]

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