70s Marshall - North American to Brit Conversion (6550-EL34) Project Help

[BlackCrowe604]

Hi everyone! I am hoping to get a bit of guidance here. I've had this 70s 100w Marshall 1x10 Combo sitting around. It came into my hands with a torn grill, trashed tolex etc. My original plan was just to restore the tolex and grill, but now I am feeling a little more brave after giving the amp a "once-over" and reading the schematics. My main questions are around the rotary impedance switch, and NFB. However, I will start at the beginning.

To do this conversion I noticed the values for the UK/US Spec are noted on the Schem. I plan on using 1/2watt 1% metal film. Is 1/2watt OK? The stock resistors on the board look slightly larger, possibly 1watt? - See pictures below. I held up arbitrary values for a visual reference. Blue resistor is 1/2w, tan 1w.

Bias feed:
R24- change to 220k (from 150)
R25- change to 220k (from 150)
R26 in series with bias trimmer - change to 56k (from 47)

*RB - change to 27k (from 15k) on the power side. I am not clear what this does? This amp has a seperate fuse board. Perhaps this was for Canadian market amps? Can anyone clarify?

Now, after I change these values, the next step is the NFB. This is where I am unfamiliar with these rotary switches. The impedance selector on this particular amp, has 4 positions, two of them being "vacant" in that there is no indicator on the switch.

From full L to R it goes: blank//4ohm//8ohm//blank.

Per the schem, it has the NFB on the  4ohm tap. I have heard that Marshall has been inconsistent over the years for putting the NFB on the 4 or 8ohm tap. How can I confirm which tap it is on? (pictures attached).

I believe I need to relocate the NFB wire, and tie it in to the 8ohm tap? Is this true?
On the schem, this looks like wire "E" (yellow in reality). Is this correct? I am unclear as to the "pinout" of these rotary switches. Does the stock wiring in this match the schematic (purple NFB on 4ohm, and yellow on 8ohm?)

(Aside... I guess tying to the 4ohm would drive the tube more? And the opposite true for the 8ohm? How would this affect the presence control?)

I plan to try my new (to me) scope & 200w dummy load to do my first scope bias on this project to finish it off.

Thanks in advance to anyone who is kind enough to shed some of their knowledge on this one!

I've attached some pictures to hopefully clarify!

Schem:
http://www.classictubeamps.com/schematics/Marshall/jmp_powercell_100w_2150.pdf

[PRR]

Welcome.

> North American to Brit

It is NOT NA to Brit. It is EL34 or 6550. Apparently they discriminated by destination. You can do whichever you want. (Especially in Canada....) Most folks run EL34 in Marshalls.

So what bottles are you going to use?

I don't see why you want to move the NFB. The change is small. Which is maybe why Marshall dithered.

[BlackCrowe604]

Thanks for the warm welcome PRR!

I know it's not technically NA > Brit. I thought that the history behind it was no reliable EL34 supply at the time?

For bottles, I have a matched quad of new Winged-C EL34's that I would like to try out. Once I get this amp up & running with them, I will probably try them in a couple other amps as well and compare.

As for the NFB, once I figure out which lug is what on the rotary impedance switch, I will likely try both and see what kind of difference there is, if any. I've never done this before, so just looking for opinions and other's experience.

Welcome.

> North American to Brit

It is NOT NA to Brit. It is EL34 or 6550. Apparently they discriminated by destination. You can do whichever you want. (Especially in Canada....) Most folks run EL34 in Marshalls.

So what bottles are you going to use?

I don't see why you want to move the NFB. The change is small. Which is maybe why Marshall dithered.

[robrob]

The 8 ohm tap will provide about 41% more NFB voltage over the 4 ohm tap and Marshall typically used heavier NFB so I would use the 8 ohm tap too.

The extra NFB voltage from the 8 ohm tap will give the presence control more authority because it has more voltage to play with.

I would mod the bias and make it adjustable.

[BlackCrowe604]

The 8 ohm tap will provide about 41% more NFB voltage over the 4 ohm tap and Marshall typically used heavier NFB so I would use the 8 ohm tap too.

The extra NFB voltage from the 8 ohm tap will give the presence control more authority because it has more voltage to play with.

I would mod the bias and make it adjustable.

Bias is adjustable see RV1 - 22k bias pot.
Hold on one sec, the 6550s need to be driven harder than EL34s.... So wouldn't the 8ohm tap be less?

[BlackCrowe604]

The 8 ohm tap will provide about 41% more NFB voltage over the 4 ohm tap and Marshall typically used heavier NFB so I would use the 8 ohm tap too.

The extra NFB voltage from the 8 ohm tap will give the presence control more authority because it has more voltage to play with.

I would mod the bias and make it adjustable.

After some digging, here's what I found:

100k on the 4ohm tap is standard, and would probably be the most gain you would want on the power amp (depending on the load the amp sees).

Marshall also did 47k on the 8ohm, and 27k on the 16ohm.

I will probably leave it on the 4ohm tap as-is and see what it sounds like. If it's too harsh, maybe drop it to 50k to smoothen the edge. Won't know until I get there!

Soldiering on, I've made the changes to the bias circuit. Tested the ESR of the Frako caps, it was through the roof, and they are leaky (as seen on the back of the board). I ordered replacements and they are on their way. Moving onto stripping the cab and doing a re-tolex while I wait.

Stay tuned!

[sluckey]

Tested the ESR of the Frako caps, it was through the roof

Did you check the ESR with the caps removed from the board?

[BlackCrowe604]

Tested the ESR of the Frako caps, it was through the roof

Did you check the ESR with the caps removed from the board?

I pulled the leg of one, and tested it. Very high. The other still on the board with a resistor on it, even higher (of course). If I remove them from the board fully to test, I would just replace them.

Regardless, you could visually see the dried up dielectric on the tip of the + side and crusty in the insulation. Also, dried up dielectric on the back of the board from them. That's enough justification for me. Out they go!

[BlackCrowe604]

UPDATE:

I completed the project recently. My health hasn't been the greatest, but I had some energy to finish it off. I'd like to get back on here again. Thought I would share the outcome!

I was really nervous, but everything biased up with no problems, and the new tolex and grill turned out perfect. I did switch the NFB to the 8ohm with 100k. It seems the presence is very lacking now. I may switch it back to the 4ohm tap with 100k resistor when I get a chance.

And thanks Rob for sharing your write-up on the NFB loop! I found the article on your page. Great stuff! You should publish a book, I would buy it!

[BlackCrowe604]

Last one! FWIW, I took a lot more pictures, but will save you guys the agony!

Overall, really happy with how it turned out.

[Ritchie200]

BlackCrowe604,

This has to be a model #2150, correct?  The info I have shows it to be a 1x12" not a 1x10" as you mentioned?  Also the speak should be a Celestion Powercell.  These were unique short-lived (in manufacture) with huge magnets and a 2" voice coil.  Very high power handling for their time (for Celestion).  Powercells were also rumored to be in Clapton's 4x12 cabs and Bruce's 4x15 cabs for the last Cream tour.  Known to be very sensitive (LOUD!) but very unlike the other Celestions as they were super clean with a very flat response, almost high-fi'ish.  There are rabid fans and other's who hate them!  Regardless, they are VERY RARE!  Marshall called the 2150 their "Rock and Roll Baby".  That's a lot of umpf in a small package!

I hope you are feeling better!  Amp porn is NEVER bad!  Post away!  Very nice job on the re-cover and restore.  Now you MUST post a soundclip!  That is the unwritten law of all who post a rare bird like this on the forum!

Jim

[Willabe]

Very nice job on the re-cover and restore. 

Yep! Very nice! 

[BlackCrowe604]

BlackCrowe604,

This has to be a model #2150, correct?  The info I have shows it to be a 1x12" not a 1x10" as you mentioned?  Also the speak should be a Celestion Powercell.  These were unique short-lived (in manufacture) with huge magnets and a 2" voice coil.  Very high power handling for their time (for Celestion).  Powercells were also rumored to be in Clapton's 4x12 cabs and Bruce's 4x15 cabs for the last Cream tour.  Known to be very sensitive (LOUD!) but very unlike the other Celestions as they were super clean with a very flat response, almost high-fi'ish.  There are rabid fans and other's who hate them!  Regardless, they are VERY RARE!  Marshall called the 2150 their "Rock and Roll Baby".  That's a lot of umpf in a small package!

I hope you are feeling better!  Amp porn is NEVER bad!  Post away!  Very nice job on the re-cover and restore.  Now you MUST post a soundclip!  That is the unwritten law of all who post a rare bird like this on the forum!

Jim

Thanks Jim & Willabe!!

I guess since you've wrote it, it's now official. I did this for a dear friend of mine who paid for the materials, and I donated my time. It really does sound great. He is currently overseas but when he's back I will see what I can do! (We've been planning an unrelated amp shootout anyway).

You know Jim, without having the speaker in front of me to physically measure it, you may be absolutely correct of the speaker's size. Perhaps it was just the massive magnet that dwarfed it into a 10in optical illusion? haha.

Here's a couple more pics I found on our "real" SLR camera to share.

Will post a follow-up soundclip soon!!