Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Opry Audio on July 20, 2020, 03:09:14 pm
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Hey y’all,
I was brought a 1979 Marshall JMP MKII 50w. It has the blue prototype board. So, this amp really went through the ringer by some tech who was paid to mod it. It’s just a complete mess and will need a full rebuild, basically every inch of this amp needs work. This customer initially wanted the #34 frank Levi mod done.
What should I consider doing with this? The board is a complete mess and burned up, damaged, leads melted into it. The underside looks ok. I had the idea of putting the ceriatone AFD35 in this one. It’s a great sounding amp and would be more reliable with a new turret board. How does the Hoffman plexi hold up compared to this? Just looking for some ideas of where I should even start. It’s so goofy. Will attach pics :help:
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Yikes - and I think some of the amps I drag in are a mess. Anyways I doubt you could go wrong with a Hoffman circuit. I built the Plexi 6V6 (I know you are talking more wattage here) and the circuit is great. I much prefer it to an 18watt I built. It looks like the chassis, pots and sockets are ok - transformers too? If so I'd snip off the leads close to the board connections, make, or have made a new turret board and build something that fits the chassis and is close to the layout of the sockets and pots. Likely have a nice amp when you are done.
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I bought a 1974 Marshall 100 Watt Superlead that was heavily modified (new 3rd party PCB board, etc) that was a complete mess. Before I bought, no other amp tech in town was willing to try to repair it so I talked the owner into selling it to me cheap. I gutted the amp, built and installed a 100 watt plexi turret board and completely rebuilt the amp. The iron was good as well as the pots, sockets etc. Also installed new tubes and finally sold it for a nice profit. I would do the same using Doug's 50 watt plexi board and making any modifications to it you want. This way you are starting with a clean slate. This would be a lot easier in the long run. The person I sold it to was very pleased with the amp.
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Going with the Hoffman plexi is essentially what I’m thinking. I really like the sound of that ceriatone a lot, but it has so many bells and whistles that I don’t want to bother with. I might just copy some of the stages from that onto the Hoffman board and see how it comes out. He also asked me to put in a pair of nice 6550’s so I have something to look forward to haha. It’s really a drag about this one. The pots and the iron are all good! Whoever did this terrible work put in new F&T caps so I don’t need to rebuild the power supply, just reflow terrible solder joints
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Here are before and after pictures of my Super Lead re-build.
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The pots and the iron are all good! Whoever did this terrible work put in new F&T caps so I don’t need to rebuild the power supply, just reflow terrible solder joints
Good pots, good filter caps, good iron, good sockets. Just what do you have to "bitch" about? :icon_biggrin:
I actually love amp saving projects like yours. Its just satisfying to make something useful out of what might have become junk. I picked up a Blackface Concert chassis recently - it had burned. Then I found an empty bandmaster head cabinet (same size) in good condition. So I will make a BF Concert head - an amp Fender never made, and I plan to try Sluckey's trem thing.
And dpm, nice clean work on that Superlead.
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That super lead looks great! Ah I don’t mean to bitch... I just hate to see people spending money on their amp getting destroyed. Even at my worst I never did anything like that. Anyway, I am very happy to save it, that’s a good outlook. I haven’t built a plexi yet and seems relatively simple as far as builds go. Good way to spend covid days
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I know you don't mean to bitch, I'm just busting on your handle which really made me chuckle. So great to see a woman working on tube amps. Pretty rare I think. I own an engineering firm and 25 years ago it was damn near impossible for us to find female hires. It is wonderful the change that has taken place.
Some "techs" do some terrible things to customers' amps. Connections twisted and covered with masking tape! Tacking leads to tube sockets with a glob of solder, etc.
I also do woodworking and the trade has a saying, "Keep craft alive." We must try to do this with tube craft too. I hope you keep us updated on your project.
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Alright, I got my plexi 50 underway, drilled mounting holes in the board, and took care of installing and soldering all the buss connections on the board! I have a few questions with the best way to proceed with the following connections.
Can anyone offer a good solution for connecting the wiper of the bias pot to the turret for the control grid resistors under the board? I usually like wrapping the the wire around the turret through the top from the bottom of the board but there’s not enough room for the wire and the pot to fit that way. The next logical solution I can think of is using some length going into the turret for both to fit from the top and bottom.
Same question with the 3A diodes for the rectifier. They won’t fit in the same turret. I love the way components look from the top but I guess I have to wrap those. Correct?
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Drill a hole in the board adjacent to the turret for the pot. Drill another hole adjacent to the turret for the bias resistors. Run the wire under the board and pass the end through the hole adjacent to the turret for the pot. Wrap the end around the turret and solder. Repeat for the other end of the wire.
Wrap the big diodes around the turret. But why 3A diodes? 1A diodes are all that's required. And two 1N4007 leads will fit nicely into a Hoffman turret.
Here's an example... http://sluckeyamps.com/phoenix/p11_big.jpg
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Sluckey, that’s a good suggestion, just seeing if I can find a way that makes sense with the way the layout is drawn up, the trim pot fits into 3 turrets near the top of the board.
IN504’s were included as the rectifier. I pretty much always use 1A IN4007’s but I wasn’t sure if the design benefited from 3A vs 1A. If it doesn’t matter then it’s more important to me that I can put them into the turret. Thanks for your reply!
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Wow Sluckey, that amp looks incredible!
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IN504’s were included as the rectifier.
Never heard of such. Do tell more.
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IN504’s were included as the rectifier.
Never heard of such. Do tell more.
Sorry I meant IN5408...
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Almost done populating the board. Sluckey’s suggestion for the wire under the board came out great. Trying to keep the components nice and clean, leads straight. Can anyone offer some techniques for bending the leads at the right place so that you get a good looking component consistently? I’ve sort of been eyeballing it and having good luck. But there has to be a tool or something. Right?
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Yup - the lead benders on this page work great.
https://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Tools
{EDIT: when deep-linking to Doug's store, omit the "&ORDER_ID=!ORDERID!" because it is IP-specific, won't work for others. -PRR}
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Thanks! I can’t believe I haven’t seen these before
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Ok y’all, I’m almost done with the plexi, I’m feeling confident that this amp is going to turn out great! I have a few questions about the PT however. It’s a drake 1202-164. Does anyone have the datasheet? I just want to make sure I know what the color codes for the primary and secondary are. There are some leads I obviously won’t need that were originally wired onto the original board.
I still have to remove the ground switch as well. Something else I’m wondering about is the way the HT fuse is wired originally. The A PS node and the choke are both wired to it, so it’s fused a bit differently than the Plexi 50 layout, and might leave it as is if it won’t interfere with proper fusing.
Attaching an image of the test fit.
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I don't have one
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Color code ; some answer here ?
http://forum.metropoulos.net/viewtopic.php?t=7681
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Thanks Latole that had the answers I need
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Almost done. Just need to do the input jack assembly and it will be ready to test
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nice temp you have set, I think I will :icon_biggrin:
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Perfect for capillary action :)
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You've really done a nice job with the layout board and wiring! :thumbsup:
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I do have just a few more things on my mind regarding the last steps.
Should I ground the bus wire soldered to the back of the pots to a point on the chassis? Or are the pots sufficient to ground the preamp section on their own? I know it technically would be a ground loop, but Marshalls historically have done this. Will I get better results either way?
And for the input jack assembly, I need to basically redo all the wiring because of how poorly it was originally done. My plan is to wire 2 68k resistors in parallel for the dual HI LO arrangement from the input jacks. What is the best way to wire the shielded wire in this case? Wire it to the 2 resistors coming off of the jack, ground it to the jacks ground connection and then run the wire under the board to the grids?
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Take a look at the Hoffman grounding scheme. https://el34world.com/charts/grounds.htm
Are you using Switchcraft input jacks or cliff input jacks (plastic) ?
When possible, I like the grid resistors mounted on the tube socket pins 2 & 7.
Wire it to the 2 resistors coming off of the jack, ground it to the jacks ground connection and then run the wire under the board to the grids?
I take it you mean ground the shielding to the jacks grounds? Yes, that should work fine.
With respect, Tubenit
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I looked at your pic Opry. Nice neat work! Congrats on turning a mess around. I like the looks of that rework station - I think I need to get one. I work on a lot of old amps - and I LOVE tools! Looked up Circuit Specialists - nice stuff.
BTW when wiring up the shielded wire, I used to do it simple (sloppy) but those dang shield wires are tiny, wispy, buggers, that can go astray. So I now do a good solder connection of the shield to hookup wire and cover the connection with heat shrink. There are some good threads here on different approaches.
In the pic - is that duct tape covering an unused chassis hole? McMaster-Carr, Newark and others sell nickel plated snap in hole plugs that work slick.
Your work is so neat, that I'm probably telling you stuff you already know.
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Looked up Circuit Specialists - nice stuff.
In the pic - is that duct tape covering an unused chassis hole? McMaster-Carr, Newark and others sell nickel plated snap in hole plugs that work slick.
Your work is so neat, that I'm probably telling you stuff you already know.
I really like circuit specialists! I originally got this because I had some SMD repairs I needed to get overwith and it was within my budget, but it turned out being great as anything else! It has a nifty sleep function to extend the life of the iron tip, or if you leave it on by accident overnight :) haha.
I do know about the nickel plated covers! To be honest I haven’t investigated that tape too much.. maybe I’ll report back. It doesn’t feel like it’s covering anything or serving any purpose but now I’m interested to know
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Could be a map to buried treasure on the other side of that tape.
Thanks for the thumbs up on Circuit Specialists - the prices are really good I think.
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Some good news and bad news upon completion!
Bad news: the amp did not work the first time I fired it up. I didn’t even notice until now but I guess on one of my CE distribution orders, there were quite a few 100ohms in my order of 100ks... so I had to correct those values. Super frustrating but it happens! The band colors are almost the same.
The good news is that it works now! And it’s extremely quiet, even cranked it’s like almost no hiss, so that’s a positive. But it’s not sounding too good at lower volumes. I would describe the tone as grainy and square wave like faintly under the signal that does sound good! I hope that makes sense. It will be continued on Monday with a scope to see where that crap is originating from! If anyone is familiar with this symptom please chime in!
Here’s a pic of the finished amp
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I gave up on color bands when everything went East, ALWAYS ohm 1st, solder 2nd :icon_biggrin:
the scope knows :laugh:
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Have you recorded voltages? I seem to recall a sound like you describe and when I checked voltage I was way off on one of the preamp tubes for the reason you mentioned - confused a resistor color band.
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Have you recorded voltages? I seem to recall a sound like you describe and when I checked voltage I was way off on one of the preamp tubes for the reason you mentioned - confused a resistor color band.
It's absolutely possible. Whatever it is I'm positive it's coming from either the CF or phase inverter since it's happening on both channels. When the output section breaks up it totally covers up whats happening before it (extremely nice cronch! we love that!) I'll check back with some plate and cathode voltages on monday when I'm back in the shop. I'm hoping there will be an obvious issue :) I'm honestly just grateful for the amp to be sounding almost how it should. It already sounds worlds better than it did!
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I'm honestly just grateful for the amp to be sounding almost how it should.
Oh yeah, whenever I fire up a new build or a major repair and it makes sound, I shout out my best Gene Wilder, Young Frankenstein; "It's Alive!" impersonation, take a break and set out making it sound good.
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Here’s a list of some operating voltages.
V1 -
Pin 1 - 128v
Pin 3 - .72v
Pin 6 - 164v
Pin 8 - 1.3v
V2 -
Pin 1, 8 - 135v
Pin 3 - .8v
Pin6 - 228v
V3
Pin 1 - 177v
Pin 3, 8 - 29v
Pin 6 - 174v
V4
Pin 3- 377v
Pin 4 377v
Pin 5, 6 -44
V5
Pin 3 - 378
Pin 4 379
Pin 5, 6 -44
I think it maybe be a bias issue. The signal looks perfect up until the plates of the output sections. For now I have the amp off without tubes installed until I figure this out. Any tips for what to look at?
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Also another issue I’m having is no light from the power switch LED
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-44v is also the most negative the bias voltage will go, which seems like not negative enough for max and not the normal range.
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Also another issue I’m having is no light from the power switch LED
I doubt it's an LED. See if this helps...
(https://el34world.com/charts/images/mar_switch.gif)
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Also another issue I’m having is no light from the power switch LED
I doubt it's an LED. See if this helps...
(https://el34world.com/charts/images/mar_switch.gif)
Thanks Sluckey, the switch isn’t wired properly... now onto the bias.
One thing I noticed about this amp when I first got it is that the socket was wired to run a 6550 but there were EL34’s in it. Could the EL34’s I’m using have been damaged. I don’t have a tube tester or any extras that aren’t in other amps.
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Maybe it is wired correctly.. this is one section I didn’t touch, but I did remove the .047 cap the for the grounding switch but that shouldn’t affect this right? Here’s a pic.. the blue lead for the PT primary is jumpered to terminal “11” and the yellow lead is to the fuse and then to the other PT primary lead
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I think this may be the right voltage chart.
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The signal looks perfect up until the plates of the output sections.
your #'s line up good with bmc's chart
sometimes scoping output tubes can be misleading, scope the speaker instead. put 1 ohmers to ground on the cathodes to "know" what -44 = in current, makes the math sooooo much easier
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The signal looks perfect up until the plates of the output sections.
your #'s line up good with bmc's chart
sometimes scoping output tubes can be misleading, scope the speaker instead. put 1 ohmers to ground on the cathodes to "know" what -44 = in current, makes the math sooooo much easier
Shooter I know this is a basic skill but just read current across 1R to ground? Or should it have a mv reading?
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mV = mA when r = 1, other than 1.... :icon_biggrin:
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So that’s a yes?
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Put in the 1 Ohm. Set your meter to DC Volts, scale 0-100mV.
For a first whack, 40mV or 50mV (really 40mA or 50mA) per tube is about the right ballpark.
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Ok, so my issue seems to be no bias.. only reading .2mv on the 1R. But all my voltages should = bias :w2:
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One thing I just observed is that when I first flip the HV on, I get up to about 20mv of bias and then drops immediately to 0 after it warmed up
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One thing I just observed
monitor the "TAP" for the PA tube's VDC, should see the opposite "effect", vdc low, then does up.
the -44 stays solid?
with no current, plate vdc should be about the same value as with no tubes in. IF not, i'd look at G2 and filament wiring
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I was a big dummy and just needed to make a bias pot adjustment. The bias actually works very well now, and the amp sounds excellent.
Just a few more thoughts before I figure out the switch situation. Channel 2 sounds much darker than channel 1 does... is this by design? It almost sounds too dark. I’m getting some oscillation in channel 1 at high volumes but I need to take it off my bench to see if it’s the lights / lack of shielding
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the quick n dirty "fix" for dark sound, decrease the cathode Ecaps, starting with 1st gain stage, example if it's 25uf, go to 10, evaluate, repeat. once that's done, coupling caps. but the real answer lays in an oscope, a sig-gen that you can set for specific frequencies and "see" where treble ain't n bass is to much or visaversa. I typically use 80hz, 200hz, 400hz and 1k (4k when i'm really bored :).
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Well, just to assume it is correct, what I would say about it is that I would probably not use it on it’s own. Jumpering both channels together makes magic happen.. so I will keep it as is. I think that’s probably it’s intended function.
I’m still a bit confused about this switch.. according to the sheet sluckey linked, my switch should be lighting up. I have a feeling that there is a connection to ground that it needs but I’m not sure which terminal. I don’t want to short out the mains voltage
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Channel 2 sounds much darker than channel 1 does... is this by design? It almost sounds too dark.
Look at the schematic of the Hoffman Plexi50. It's pretty obvious that CH1 is very dark and CH2 is very bright. That's just the way a plexi is. Neither channel really sounds great. But magic happens when you jump the channels together and use the volume controls to blend the dark and the bright together.
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Thanks for your help everyone, this amp came out great!
Any ideas about the rocker switch? Apparently the LED needs a ground connection but not sure where. The sheet that was linked is for a switch that doesn’t require it. This one has 6 tabs on it. Last thing I have to do before I close her up! :)
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That's just the way a plexi is.
:BangHead:
thought it was the jcm 900 amp
+1
took me some time to figure out the blend on a plexi, but when I did, i stole the preamp 3 more times
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Do not connect a ground wire to any lug on that power switch! There should be some numbers for the switch lugs. Look closely. Draw a pic of the switch lugs showing the numbers. Post the pic. Or post a good closeup of the actual switch so we can see the numbers.
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Here’s a pic. Sorry it’s gonna be sideways.. pics always do that from my phone to threads here.
Neutral is wired to 12
Blue PT primary jumpered between 11, 13
Hot going to 25
Fuse and brown Pt primary on 24
26 is vacant
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You only need four wires connected to the switch. What are the two extras connected to?
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The other two is for the polarity switch. I cut out the cap so it’s not doing anything
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After jumpering 26 to 24, it lights up. Case closed :laugh: awesome build, I might try this one again. Thanks y’all! I finally built something good haha
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Just curious but is there anyone who’s built a plexi into a bassman chassis?
Will it fit the transformers needed for this kind of build? I would love to build this amp again but as a combo. I’m really glad I got a chance to build this with JMP drake iron.. probably why it sounds so good. Any recs for transformers that would be similar?
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Just curious but is there anyone who’s built a plexi into a bassman chassis?
Will it fit the transformers needed for this kind of build? I would love to build this amp again but as a combo. I’m really glad I got a chance to build this with JMP drake iron.. probably why it sounds so good. Any recs for transformers that would be similar?
...to help you get an answer:
Which Bassman chassis?
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fwiw;
I put this 5 tube, 6 knob, 4 switch, handyman special in a 12"X8"X3" and had room to dance, IF i was neater :icon_biggrin:
do the square foot math to be sure :laugh:
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Sorry I meant a tweed bassman chassis. It has the right amount of input jacks and tube socket holes... haha