Help with my 1978 Mark I

[mark1406]

The original picture does not look like the "Post FX" loop mod that was done to my 1978 model, at MESA. It worked very well at controlling the volume, and a Boss OD-1 in the front of the signal chain.

Hi Joey,

What is the post FX loop in your amp?

Gut shots?

Regards

Mark

 

[jimkeaney]

I recently bought an original Mark1 60w head, sn204x, w/reverb, loop, geq. It is dated June78. Interesting that the OP's mk1 is only 3 months older and didn't get the GEQ. Was the graphic eq an available add-on with many markI's? Or is it because the Mkii's were brewing in late 78?

Can I ask a couple questions as a new owner of an mk1?
- Is it normal that when the GEQ is engaged(with all sliders at unity), the overall volume of the amp drops?
- Is it normal that when the CH1 Boost is pulled out (engaged) the bass/mid/treble knobs are defeated?
- Does anyone have a manual for the original mk1? Not the Re-issue MK1 manual.

Thanks!

 

[bluebug]

I recently bought an original Mark1 60w head, sn204x, w/reverb, loop, geq. It is dated June78. Interesting that the OP's mk1 is only 3 months older and didn't get the GEQ. Was the graphic eq an available add-on with many markI's? Or is it because the Mkii's were brewing in late 78?

Can I ask a couple questions as a new owner of an mk1?
- Is it normal that when the GEQ is engaged(with all sliders at unity), the overall volume of the amp drops?
- Is it normal that when the CH1 Boost is pulled out (engaged) the bass/mid/treble knobs are defeated?
- Does anyone have a manual for the original mk1? Not the Re-issue MK1 manual.

Thanks!

Hey Jim, congrats on the nice score feel free to put some pics! What speakers does it have? Is it still running the fetron in V1 position?

The options were available and you had to choose whatever you wanted and it was built to order. 60 or 100w, reverb, GEQ for instance were all options. Mine is pretty much the bare bones model.

I can’t comment on the effect of the GEQ other than my guess, a passive tone stack robs some signal level, so maybe this is what is happening (the GEQ I think can be seen as an advanced tone stack). Someone might correct me on that, so please take this with a grain of salt!

The pull boost does put the tone controls out of circuit, the result is stronger signal for the following amplification stages, but to me it is pretty much useless (for my taste anyway) since it becomes very flubby.

This maintenance manual looks to be the same as my original one:

https://mesa-boogie.imgix.net/media/User Manuals/Mark I Maint-Repair.pdf

 

[jimkeaney]

Hey Jim, congrats on the nice score feel free to put some pics! What speakers does it have? Is it still running the fetron in V1 position?

The options were available and you had to choose whatever you wanted and it was built to order. 60 or 100w, reverb, GEQ for instance were all options. Mine is pretty much the bare bones model.

I can’t comment on the effect of the GEQ other than my guess, a passive tone stack robs some signal level, so maybe this is what is happening (the GEQ I think can be seen as an advanced tone stack). Someone might correct me on that, so please take this with a grain of salt!

The pull boost does put the tone controls out of circuit, the result is stronger signal for the following amplification stages, but to me it is pretty much useless (for my taste anyway) since it becomes very flubby.

This maintenance manual looks to be the same as my original one:

https://mesa-boogie.imgix.net/media/User Manuals/Mark I Maint-Repair.pdf

No speaker, it's the head version. Not sure about the Fetron, I'll have to take a look but I think it's all newer tubes. The tech who sold it to me did a re-cap. So far it's a great sounding amp. The GEQ definitely gives it a lot of tone options.

 

[electric mayhem]

......

This maintenance manual looks to be the same as my original one:

https://mesa-boogie.imgix.net/media/User Manuals/Mark I Maint-Repair.pdf

Thank you for posting this manual. And to everyone posting.

I have a '76 OG project Boogie so this will come in handy. There is a lot of great info in there that also should translate well to the follow on Marks.

Mine has no fetron- do you all find the load R's need to be checked for 12ax7?

I had never seen Mullard tropical fish caps before owning this A70x. Pretty wild.

 

[James]

No speaker, it's the head version. Not sure about the Fetron, I'll have to take a look but I think it's all newer tubes. The tech who sold it to me did a re-cap. So far it's a great sounding amp. The GEQ definitely gives it a lot of tone options.

Interesting how it has five preamp tubes. They usually seem to have three or four (depending on if it has reverb)

 

[bluebug]

I wondered that too about the 5 preamp tubes, maybe the EQ requires an additional tube? In mine there are 4 and it’s because of the fx loop. No reverb or eq.

 

[guitarman59]

Thanks for opening the amp and taking the pictures of the first volume control. I couldn’t see in the pictures, but what value did you see written on the cap when you had the amp apart?

I still find it hard to believe that an 0.047uF cap would be in that position, I could believe a 470pF (471), or maybe a 4700pF (472) as those values would be tone shaping. I see a 0.047uF (473) affecting the taper of the pot making it almost reverse logarithmic.

Thanks again for all your assistance, I appreciate the effort you have gone to on my behalf.

Regards

Mark

I have seen 47pf on the #1 gain/volume pot. It adds a little sparkle to the dark sounding circuit.

 

[guitarman59]

Also adding the IIC+ loop made a huge improvement to my ear. I even figured out how to make it channel switching. Ill look for a video and post if you are interested

 

[guitarman59]

Also adding the IIC+ loop made a huge improvement to my ear. I even figured out how to make it channel switching. Ill look for a video and post if you are interested Its actually a Rivera Era Fender Princeton completely rebuilt. 6L6's with 50 watt power/output transformers that dropped right in. Push/Pull pots everywhere. IIC+ loop, reverb also IIC+, it's a bit bright cuz my G&L Comanche Strat was a bight guitar, and EVM-12L 1-12 cab (Dumble Style 100 year old pine). Took some Fender HRD channel switching ideas.... My phone at the time had crap sound quality. Oh, the clean was more like the HRD Clean channel and I put a Cathode bias/negative bias switch on it to go from Classic Tweed breakup to modern Negative bias headroom. The volume was low as it was late at night and had to keep family happy, lol! Sorry for the sloppy playing, must have had too much coffee,

 

[bluebug]

Hi all,

I came back to this thread to ask a question, but started by just re-reading the whole thing as I think I know a bit more now than I did back then, I'd like to say thanks again to everyone who helped me build up a bit more knowledge, little by little. An Happy New Year everyone too!

So on to the questions Recently I built a couple amps from old tube stereo consoles. My more recent one is from an Admiral stereo which has the power amp and supply in a separate chassis from all the remaining AM FM, turntable and input selection stuff. That made for a cleaner chassis to start from. And I was pleased the power amp is a push pull EL84 pair (per channel) with a cathodyne phase inverter, instead of single ended like my other stereo project.

I grafted a clone of my Mark1 to that power amp, using the closest value components I had in my leftover parts from dismantling the previous stereo consoles. I simplified a bit, combined the two gain knobs into a double pot (stereo consoles are full of double pots, to affect the two channels at the same time), and eliminated the second input, having all 4 stages always in use. I like a lot how the amp sounds, the controls have quite a drastic range of control but I can find a lof of very good lead tones. And like my real Mark 1, the presence control is of great use to fine tune the tone (which is more like a high frequency roll off control as we discussed previously).

But now I'd like to add an effect loop to add some reverb. I plan to use my Mk1 circuit, but looking at it now, I wonder if I should eliminate the 0.12uF cap at the start of the circuit. It is in series with the existing 0.047uF coupling cap, so it seems like it would do nothing, since it is already coupled, and moreover with a cap of lower value. As I understand it, the 0.047uF already blocks the too low frequencies, while the 0.12uF lets pass more (but the frequencies are already removed).

Maybe it is there because the person designing it as a stand alone FX loop circuit, without consideration of what is connected before and after it? Having high coupling cap value to pass the frequencies mostly unaltered. I re-drawn the 4th stage of the Mk1 circuit on the right, but the FX loop circuit really starts between the 220k and 0.12uF cap.

The rest of the circuit I am pretty happy with, so I plan to reuse as is, located between 4th stage and master pot.

I added some pictures of my frankenamp Since those pics I added some shrink tube to exposed wires and solders, replaced the old filter caps. The wooden box I made out of the wooden panels from the stereo consoles, made it pretty rough just so it is not open chassis anymore.
Thanks all!

Fred

 

[mark1406]

“I wonder if I should eliminate the 0.12uF cap at the start of the circuit. It is in series with the existing 0.047uF coupling cap, so it seems like it would do nothing, since it is already coupled, and moreover with a cap of lower value. As I understand it, the 0.047uF already blocks the too low frequencies, while the 0.12uF lets pass more (but the frequencies are already removed).”

Having the two caps in series will result is a lower capacitance value. In this case 0.033uF.

https://www.digikey.com.au/en/resou...sion-calculator-series-and-parallel-capacitor
Regards

Mark

 

[bluebug]

Hello Mark,

Oh, I forgot about that, right. So even if they are separated by a resistor this rule still stands? Thank you for highlighting that for me!

Fred

 

[jimkeaney]

Fred, do you know what that single RCA jack is for (above your amp's date)? My Mk1 head has a 1/4" jack there (the two chassis holes to each side are populated with RCA's for my reverb tank). Is it a Power Amp input maybe?

Markings on the chassis:

 

[jimkeaney]

I have an odd sound coming from my MK1 (see short video clip). I've tracked the issue down to the Reverb Tube (I'm 99% that is the reverb tube - see blue arrow). It's a high-pitched laser-beam harmonic distortion that sits above an otherwise clean tone. It is most prominent when playing lower notes. When I use an old GrooveTube 12AT7, the noise doesn't appear, but when I put a new JJ ECC81 (which I understand is supposed to be interchangeable with an 12AT7), I get this noise. I've tested with 3 different new JJECC81's and they all give this noise (with or without the reverb tank connected / and with the Reverb Volume all the way down). Anyone here have any ideas why the Groovetube is happy in that socket but the new JJ's wouldn't be? Thanks