Everything We Know So Far

The man, the band, and everything else

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Tazin
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Re: Everything We Know So Far

Post by Tazin » Wed Aug 01, 2018 10:43 am

I don't see any 1 meg resistors on the Input jacks so I assume their low sensitivity.

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Re: Everything We Know So Far

Post by jnew » Wed Aug 01, 2018 12:46 pm

I've scoured those pics. How can you see down in the shadows of those pics. The input jacks cannot be seen. And of course the schematic you pointed out has both. :bang: :(
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Tazin
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Re: Everything We Know So Far

Post by Tazin » Wed Aug 01, 2018 1:34 pm

I've seen other pictures of this model Master PA and they didn't show any 1 meg resistors.

Regarding the NFB resistor...These Master PA amps followed suit with the changes Marshall did with the other PA models. So, for the '69 version they used a 27K resistor and that changed to a 47K in 1970. The end of '71 brought in the 100K resistor.

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Ralle
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Re: Everything We Know So Far

Post by Ralle » Wed Aug 01, 2018 3:12 pm

Tazin is right, there's not any 1M on the inputs on this model... the schems aren't showing this exact modell even if they're named the same, the ones in the schems are PA heads in a Marshall top cab, and they have 4 channels but with both high and low inputs... here's a link to a pic I found showing the internal of this model we're talking about:
https://www.flickr.com/photos/wild_guitars/7615332414
and this one is the same as Ed has, and the one I showed in the previous link...
I hope this link works... :wink:

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Re: Everything We Know So Far

Post by jnew » Wed Aug 01, 2018 3:56 pm

Yes. That link works and the pic is great. So channels 1&2 are shared cathodes as is 3&4 with the typical 820 Ohm/330uF. Pretty straight forward Marshall. :thumbsup: I'll refer to the schematic to figure out how the Master Vol and Tone are factored in.
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Ralle
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Re: Everything We Know So Far

Post by Ralle » Wed Aug 01, 2018 6:36 pm

That's fairly easy; the master is simply a 1M pot on the common point of the two 470k mixer resistors, tapping into the seccond stage ( refering to a regular head, that is ).
The tone control is a 1M pot with a 5nF cap in parrallel across the regular gain 1M pot...

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Re: Everything We Know So Far

Post by jnew » Wed Aug 01, 2018 7:50 pm

Ok, so the usual VOL knob that a plexi has will translate to the equivalent of either VOL 1, VOL 2, VOL 3 or VOL 4 on the Master PA head. Just make sure to remove any bright caps.

The 1 Meg at the common of the two 470K mixers is the Master and is basically between that common point and pin 2 of V2, which would put it before the tone stack like you previously mentioned. Bypass cap to be lifted.

And the Tone is just another 1 Meg pot, paralleled with a 5nF, across the input VOL pot.

So if this is all correct, I just need to make sure there are no 1 Meg's are on the inputs, any additional 68K's are lifted, output couplers are .1's and tone stack is changed to 250pF/56K. ala bass spec.

How we doing? :lol:
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jnew
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Re: Everything We Know So Far

Post by jnew » Wed Aug 01, 2018 9:27 pm

I got the soundclick thing sorted out. Here is RWTD with everything I could possibly squeeze out of a 50 watt Plexi. Original GE 6CA7's, 820/.68 on V1B, 330uF on V2, NFB resistor completely lifted and every knob completely max'd. Head on variac at 90VAC and then into another variac between head and cab. Definitely some sauce to this little gimmick. Pickup is an original Super 70.
https://www.soundclick.com/bands3/defau ... ID=1214336
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Ralle
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Re: Everything We Know So Far

Post by Ralle » Thu Aug 02, 2018 8:17 am

jnew wrote:
Wed Aug 01, 2018 7:50 pm
Ok, so the usual VOL knob that a plexi has will translate to the equivalent of either VOL 1, VOL 2, VOL 3 or VOL 4 on the Master PA head. Just make sure to remove any bright caps.

The 1 Meg at the common of the two 470K mixers is the Master and is basically between that common point and pin 2 of V2, which would put it before the tone stack like you previously mentioned. Bypass cap to be lifted.

And the Tone is just another 1 Meg pot, paralleled with a 5nF, across the input VOL pot.

So if this is all correct, I just need to make sure there are no 1 Meg's are on the inputs, any additional 68K's are lifted, output couplers are .1's and tone stack is changed to 250pF/56K. ala bass spec.

How we doing? :lol:
Seems about right... just to be absoloutly clear; The tone; 1M pot with a 5nF cap across the center and end tap of the pot. This configuration is to be mounted in parrallel with the regular gain 1M pot. Be sure to connect this pot in the right direction ( I think it's connected the other way around )... when using it, it's suppose to CUT what's left through the cap to ground, in other words, the opposite way of what the regular bright cap does in a volume pot. Either way, it works no matter, you just have to turn it one or the other way to hear what I mean... mine is actually connected inaccurat, so when cutting the highs, I have to " turn" it up... I'm cool with that, it doesn't sound or act any differently, but you might want it hooked up correctly... just like to mention that :wink:
One more thing ( if you want to, not nessacarally ), if do have an input jack left over, you can use that for the line in input that's in the schems for PA head; what you do is just hook it up directly to the same point as the master vol; the common point of 470/470k. Here you can use the main head together with a clean powerstage, with out the PA preamp section... Not sure if Ed ever used it like this, but I find it cool to use sometimes when I want more of a naturall marshall sound, if you know what I mean...

Looking at this, this is ( to me ) THE most perfect way to use a marshall, I mean just look at what you can do... I can't see how Ed ever could be with out this setup... other than it'll be one more amp to cary... but hey... :wink:

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Re: Everything We Know So Far

Post by jnew » Thu Aug 02, 2018 9:20 am

Good point. Very clever. So you can have the option of a straight, more normal type tube power amp or the Master PA with the tone. (although wouldn't a normal power amp arrangement be straight to the PI? :scratch: ) I'm thinking about building a whole new amp just for this. Kind of been itching for another amp project but I already have 3 plexi's. 50 watt and two 100 watters. But this would be a very cool and very fun build.
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Ralle
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Re: Everything We Know So Far

Post by Ralle » Thu Aug 02, 2018 10:06 am

jnew wrote:
Thu Aug 02, 2018 9:20 am
Good point. Very clever. So you can have the option of a straight, more normal type tube power amp or the Master PA with the tone. (although wouldn't a normal power amp arrangement be straight to the PI? :scratch: ) I'm thinking about building a whole new amp just for this. Kind of been itching for another amp project but I already have 3 plexi's. 50 watt and two 100 watters. But this would be a very cool and very fun build.
Yes that's correct, but the buty of this arrangement is that you have this master before the eq, so you can tune in any sweetspot you prefere, or to fit any venu... He he... I can't find any downsides to this setup... it's like a bad commersial movie... :wink: " not only do you get this product, BUT you allso get this fine product for ONLY half the price " :lol:

Allso one more thing that's fairly important; the load/line box: I have a resistive 8 ohm with a 0.68-1 ohm tap to the PA head... don't know what's the best, but I think that somewhere around these values would be the best... I think that the resistive part is crusial here...

jnew
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Re: Everything We Know So Far

Post by jnew » Thu Aug 02, 2018 10:38 am

Many years ago I built a line out box that has that Jensen 4:1 step-down transformer that Jose was said to have done for Ed at some point. I can adjust its level out so probably something I'll have to do by ear. I use that on one of the amp speaker outs. The other speaker out is for a load box switchable between 16 and 20 ohms.

So in your opinion, this arrangement proves the most accurate so far? I'm guessing we're talking VHI tone. :D
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Tazin
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Re: Everything We Know So Far

Post by Tazin » Thu Aug 02, 2018 11:04 am

Ralle wrote:
Thu Aug 02, 2018 10:06 am
jnew wrote:
Thu Aug 02, 2018 9:20 am
Good point. Very clever. So you can have the option of a straight, more normal type tube power amp or the Master PA with the tone. (although wouldn't a normal power amp arrangement be straight to the PI? :scratch: ) I'm thinking about building a whole new amp just for this. Kind of been itching for another amp project but I already have 3 plexi's. 50 watt and two 100 watters. But this would be a very cool and very fun build.
Yes that's correct, but the buty of this arrangement is that you have this master before the eq, so you can tune in any sweetspot you prefere, or to fit any venu... He he... I can't find any downsides to this setup... it's like a bad commersial movie... :wink: " not only do you get this product, BUT you allso get this fine product for ONLY half the price " :lol:

Allso one more thing that's fairly important; the load/line box: I have a resistive 8 ohm with a 0.68-1 ohm tap to the PA head... don't know what's the best, but I think that somewhere around these values would be the best... I think that the resistive part is crusial here...
I asked this before but it got overlooked...How exactly did you setup the 0.68 - 1 ohm line out?

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Ralle
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Re: Everything We Know So Far

Post by Ralle » Thu Aug 02, 2018 4:55 pm

There's a Fender amp that has a line out with a .68 ohm output... don't know wich one, but I talked with a friend of mine who's very good at this, and he said it's perfect value for a lineout. And what would be a better comparison than something from Fender amps? I mean if it's in a Fender, it gotta work, right? I know that it's a better way to tap a signal than through a let's say 47k pot in parralell with a smaller resistor such as one of the lineout that's been posted here somewhere ( wich is a very common way to do it ), but using simply a small resistor such as this, decreases the loss of signal comparerd to the 47k/small r... The best way to do it would be to just have a pot with a value of 1 ohm. Everything between zero -1 ohm gives a perfect level of the signal into the seccod amp/powerstage or what ever you're driving...

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Ralle
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Re: Everything We Know So Far

Post by Ralle » Thu Aug 02, 2018 5:01 pm

Sorry, I didn't read you question correct; I hava a big 8 ohm effect resistor as a load for the main amp, and after that the signal is tapped across this 0.68 ohm resistor into the seccond amp. Use a speaker cable into the load and a guitar cable out from the lineout...

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