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.

 

[user 41014]

......

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

 

[CrustyBooger]

Fred,
the .12 is part of the RC network driving the send triode. Leave it unless you are implementing a different loop circuit such as the Mk1 Reissue or Mk2 loops.

 

[edge59AlnicoIV]

Hi all,

I have just joined the forum, this is my first message. Reading other's stories and posts made me think this was a nice place to talk about our amps and try to understand more things about them. Anyways in 1997 I bought my 1978 (3-78 written on the chassis) Mark I in my first year of college and still have it today. It is serial number A1747, it is a 60w without reverb nor EQ. I talked to the original owner and it originally had a JBL K120, which he replaced with a Celestion Vintage 30 that was there when I bought it. At some point the Celestion started to fail, then I used a spare G12B-150 I had, then a JBL E120 and I now have a EV12L in it which I like best. It was obviously modified to add a tube effect loop with an additional 12AX7, clearly not factory, plastic jacks, tiny pot, and done in a weird floating point to point kind of way.

I have quite a bit of questions about it. First I have to say I do have an electrical background, I studied electrical engineering, but what I actually do is mostly automation and programming and I do not have a very deep knowledge of tube amps compared to some of you guys. So please bear with me while I fail to use the correct terms for the function of the various components in the little Boogie. Reading your discussions of the circuit variations of those amps I feel there is quite a knowledge gap with me, but I would certainly love to learn more, and I enjoy learning complicated stuff so who knows, maybe the new information will sink in better than I expect :lol:

I am having some issues with it so I started searching. It seems in those years it likely originally came with a Fetron. I tried to search for clear indications of that but I am not 100% sure. So I realized maybe I am running it less than optimal since the beginning since I am using two 12AX7 at V1 and V2. I saw some references of how to modify to use regular 12AX7, for instance this thread:

http://www.grailtone.com/forum/viewtopic.php?t=4319

and it pointed to the resistor values and replacing the TCR5302 things on V1 with resistors amongst other things. The fact I have those things below, do you think it means it is still set up to use a fetron?

- TCR5302 on 2 places on V1, I read that those should be replaced by 1500 ohm 1/2 watt resistor parallel with the existing 22uF, 25V cap...
- There are two 82k resistors on V1, I read that should be 100k for a 12AX7...
- There is a big 60uF 350V capacitor next to the first stages board, not every Mark I seems to have that.
- The first tube is identified as a 12AX7/AT7 with permanent marker (seems like the original markings, same as the date and signature). Is this like that when it actually comes with a Fetron? The manual used to say that it could use either Fetron or 12AX7, but I read the result was sub par when using a 12AX7 without modifying the circuit.

Source of the 82k vs 100k and TCR5302 vs 1.5k resistor comment:
https://steelguitarforum.com/Forum11/HTML/000274.html


So, does it looks to you it is still made for fetron? If so I would certainly like to modify it to optimally work with 12AX7, but would need some help to figure this out.

Some maintenance I have done:

- Replaced one cap (don't ask me why I only did one, I replaced only the one that appeared to have swelled, did not know back then that recap was a thing and should have done all)
- Change the tubes a few times
- Corrected bent chassis (around the middle, close to speaked jacks, probably previous owner dropped it with a long jack in there instead of the angled one).
- Replaced cracked 2W resistors on the power tubes (called screen grid resistors I think?)
- Did a hack job of replacing the power on light that was not working anymore, and also the power switch that failed. I say hack job because I did not use Mesa components, and I am not that proud of how did the job back then.

An actual problem I have is the sound tends to cut off pretty much completely (only a very faint and trebly sound) and it would typically come back to life if I put into standby and back on after a few seconds. It seems to do the problem less when I play fairly loud for extended time (for instance for a gig), and more when I play at lower volume, and maybe higher gain. I'm not quite sure since it is quite random. But more recently it started to not return after the standby sequence and I have to do it many times, or shut off the amp completely. I am sure the amp is due for a complete re-cap now anyways, but are the old caps the plausible cause for that kind of issues, or is there likely something else going on?

If you have ideas of measurements to do (I only have a voltmeter, no fancy equipment) while the problem is occurring let me know. I tried to measure voltages at the different pins of the power tubes to compare between working and failed state but found no discrepancy. It does not help that I don't really know the meaning of my measurements

Other questions:

- Why is there no 60uF 350V caps on the Mesa store? Do I need to get rid of that cap anyways for the fetron to 12AX7 mod? If not, where is a good place for caps other than from Mesa directly?
- What do you think of the effect loop mod and how it was done? I use a Lexicon LXP15-2 multi effect in it for my reverb so I don't really want to get rid of it, but I would be curious if you guys think it is garbage or asking for trouble to have it.
- Do I need to change more caps than the big ones, maybe the two silver ones next to the two 220uF 300V ones? (50uF 25V if I recall correctly)

Sorry for the long text, hopefully some of you made it to here I will post some more picture in the next posts, somehow adding pics makes the post appears like spam and I cannot post.

Thank you for any help!

Frederik

 

[edge59AlnicoIV]

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
View attachment 3806
View attachment 3807

Can you post a pic of the inside so we might see what the extra tube stage is for, is it a fx loop?

 

[bluebug]

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?

I'm sorry I missed your post, I too always wondered about that RCA jack, there is absolutely nothing tied to it inside the chassis. On yours, inside is there anything connected to that 1/4" jack?

 

[bluebug]

Thank you, I missed your post too sorry for not replying before. On V1 I already replaced the diodes by 1.5k resistors, and also the large orange filter cap, but did not replace the bypass caps yet. I ordered some waiting for the next bench time with the boogie. Right now what's on the bench are a couple of Mark1 semi-clones I built from leftover stereo console parts Trying to figure out how to design the sound I want by playing around, learning a bunch in the process!