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Old 17th February 2012, 06:54 PM   #1
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Default Dean Markley T120-R fixer-upper project.

I have been researching the Dean Markley t120R i own, since I inherited it and no little about it. Until recently when with the help of StephaninMelb I hadn't a clue! Hey I was a toddler in the 80's....

Still, I plan to 'restore' it, for want of a better word, and I will document as much as I can in pictures, and component values,(for those, like me cant read DMs schematic).

If this interests people then its a bonus, but I think this restoration may be slow

Here is some pic of my 120R, s/n t120r-047:

Front View:

Click the image to open in full size.


Rear View:

Click the image to open in full size.

The next photos show the output valves. All of the valves in general seem fairly old, though not really smokey.

Click the image to open in full size.

The writing on all the output valves, as well as the pre amp ones is very hard to read. The pre's are all " GE 'Electronic *something* " 12AX7, The left pair of output tubes are " PM '6L6GC' ", and the right hand side pair read something like " PhillipsECG '6L6GC' ", and also the letters " ha AGX "
If you look closely they have radically different structures inside, and the Phillips ones look very interesting. I know little about valves but I look of them better.

Click the image to open in full size.
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Last edited by mondogenerator; 17th February 2012 at 07:17 PM. Reason: Im hopeless at writing and being prepared with pre uploaded pictures....sorry
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Old 17th February 2012, 07:39 PM   #2
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Cool,-- Do you have the schematics in digital form? If so, post them. This is the quickest way to get folks to chime in. Does the amp work now? Humm 4 6L6GC's humm. What power do the claim? I say that because they sould be good for 50W - 100W but the higher figure is one that I have always found optimistic.
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Old 17th February 2012, 07:44 PM   #3
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That thing is sweet.

I still use Dean Markley strings to this day.
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Old 17th February 2012, 08:38 PM   #4
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Haha thanks for the interest. Yeah it works fine, probably due for new valve in the output side since the 2 pairs seem so very different considering they're both supposedly 6L6GC's...lol

Basically, as I shouldve said in the beginning, this thread was the spawn of an earlier thread:

dean markley valve head 120 watt

where I was seeking advice in general. using that advice, I'm hoping to renovate the amp a little. The pots are a little dodgy, it had some surgery to the internal wiring. The auxiliary mains output has been disconnected, which is probably no bad thing since I wont use it. Im planning on replacing the electrolytics too due to the age of the amp.

As for power, its a claimed 120watt, and from what ive read, or been told, it has quite a heavy bias. The stubby PM valves seem to show the dissipation, and the Phillips seem far more capable of tolerating the biasing. I now suspect these either to be the originals, or early replacements. either way older than the PM's. The pre tubes are almost certainly original GE's to the best of my knowledge.

Its a stunning guitar head, I think it deserves a fix up, so I can see what it can really do

RE: the DM strings....I dont buy them. I find they sound like no other string, in a good way, but the tone is gone SOOOO quick. I always end up going for my old 'safe' option, slinkys, whether the LP or strat just coz the tone seems to last 2 or 3 times as long.
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Old 25th February 2012, 05:37 PM   #5
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From what ive been able to gather info wise the pots im planning to replace are:

VOL: 1Meg
BASS: 250k(A)
TREBLE: 250k(A?)
MID: 10k
REVERB:1Meg

This info all from the DM schematic, however 'VOL' could be 'PRE' or 'MASTER', its unclear which, and the 'PRESENCE' is also not shown (or not obviously) SO those values will have to wait until I begin dissecting the amp......

Also the PSU schematic is not available, and I hope to draw the schematic given time.

So far the PSU caps are:

1 of 22uF 500V
2 of 100uF 350V
2 of 33uF 350V
3 of 100uF 80V
2 of 470uF 25V

also on the Pre board:

1 of 1uF 50V
1 of 22uF 350V
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Old 25th February 2012, 05:55 PM   #6
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Default Next: components which CAN overheat

Reading some of the advice and literature, the latter of which there is little about, I checked for brown board, and found diode on the PSU board, and a couple of resistors overheating.

1N4746A (2 off)
1k 'RCD' type W/W resistor guessing 500V and 2-5W
1.6k 2-5W +/-5%
150 Ohm 1W +/-5%

All these components within a small area of PCB discolouration, the RCD resistor and diode being the main contributors, by appearance. I plan to replace these all just in case.

the underside of the board, another resistor_47k????(added by SOMEONE) connected to one leg of a 5k preset pot(bias?), and the original track scratched out. Unless this is bias related I plan to reinstate the track using 0 Ohm resistor. Not quite sure of the 'new' resistors' purpose....

Also small ceramic cap soldered directly on the leads of the 220k R near the 'Bass' pot, presumably the 220k on the wiper of the treble circuit.....another mod that im not sure is necessary and am a little mystified to its purpose.

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Last edited by mondogenerator; 25th February 2012 at 05:59 PM.
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Old 25th February 2012, 06:25 PM   #7
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Default Schematic for those who requested it

Her is the 'official' Dean Markley schematic, from their site.

Click the image to open in full size.

However, from a quick butchers' Im not entirely sure that it is correct in its entirety, since 2 pots are either missing, or are there but no values are attached so they arent obvious. The cap mod I mentioned above, I believe is on the 220K R in the top right hand part of the schematic.

At any rate this schematic is not nearly as complete as I would like! also It would appear to be ONLY the preamp section!
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Old 26th February 2012, 01:39 PM   #8
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Default !!!

On my T-120R in the PSU section I replaced the electrolyic capacitors with RIFA brand. They are one of the few that are high voltage and appropriate quality. While I was upgrading the amp I had started to build a duplicate using a form of tag-boad construction. The idea was to end up with a Dean Markley clone. Ideally it would use turret board or eyelet construction. Also I would add any improvements the the clone amp. It is much easier to clone an amp when you have an original amp.

Your left hand pair of 6L6GC power valves look like a 1st generation Chinese type because of the coke bottle shape. Mine came with 4 Sylvania 6L6GC. Philips 6L6GC are also original quality type. When I retubbed after studing all the available new 6L6 types I ended up using Winged-C (SED). I wanted a tube that reproduced sound accurately and that was had good bass reproduction, since I occassionaly played bass through the amp. I found the Winged-C also handled the mids and upper frequencies well. I was especially impressed with the top-end frequency tone. It outlined the 2nd-order harmonics with a sweet sound but not cutting edge glassy. Almost a floating bubble 3-D effect. Mine also had GE 12AX7 in the reamp section, which would have been the original valves.

As far as the powr output it is rated at 120W with bias set at 100%, which no one would do on any amp. The T-120R has at least as much power output as a Fender Blackface Twin Reverb (which was rated at: 85W) and Fender Silverface Twin Reverb (rated at 100W to 120W). The T-120R can be set at very high B+ current, but the convention for setting all amps is 70% of tube maximum rating. Regardless of what level the T-120R is set at it is one of the loudest 4 x 6L6GC amps ever made. It will keep up with a Fender Twin Reverb, which are well know for being a loud pro gigging amp. With modern PA systems these types of amps aren't as critical as in the past, but still it is nice to know it has plenty of sonic volume reserves.

These Dean Markley amps were mainly sold to professional musicians for rack-mount in a large rig. The Eric Clapton and The Police rig were fairly typical configurations for this amp.

I had a pair of T-120s - a reverb model: T-120R and a non-reverb: T-120. These were loaded in a rack with Perfect Connection GP-1000 preamps made by Lee Jackson. I know of no better preamp and this model of preamp is now has a cult following because of rarity and sound quality. They rarely come up for sale and getting expensive. If you can find one - buy it!
Attached Images
File Type: jpg Rifa Axil Electrolytic Caps.jpg (19.0 KB, 26 views)
File Type: jpg T-120R - T120 Back.jpg (86.8 KB, 30 views)
File Type: jpg Lee jackson GP-1000 Stack.jpg (37.5 KB, 28 views)
File Type: jpg Dean Markley Guitar Amplifiers.JPG (81.9 KB, 26 views)
File Type: jpg Corrine Lynn and Dean Markley Signature Tube Amp.jpg (25.9 KB, 17 views)
File Type: jpg Eric Clapton T-120 pair.jpg (97.0 KB, 21 views)
File Type: jpg Eric Clapton Dean Markley rig 2.jpg (30.1 KB, 24 views)
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Old 26th February 2012, 03:01 PM   #9
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I have tried over several years to locate the potentiometers used in the T-120 and never succeeded. The closest I could find were Peavey "spider" potentiometers. I believe there are correct matching values if you draw from the whole range. A small hole will need to be drilled in the T-120 preamp PCB to accept the lower middle leg. This is used to just stabilize the pot. These vertical PCB-mount pots are hard type to locate. Marshall, Fender and other mount their preamp PCB horizontal and this is the most common PCB mount pot type. It is the same issue for the vertical PCB mount input jack, but I did manage to locate the OEM source for that part. I've found that particular part is rugged and haven't had to replace them.

The transformers were made by Woodward Schumacher, who also for many years made all of Fenders transformers. Schumacher is still in business under a new name and can supply transformers if necessary. I've found them to be rugged and also parts I haven't had to replace. The only issue is on International transformer getting the values and in general the schematic lacks voltage readings for the transformer leads. It would be nice to get readings from that area and apply to a new schematic.

I too had brown board and the same 1N4746A 18V - 1W diodes overheating. With one of the two amps the board had been really heated up to where it damaged one trace. I had to solder in a patch lead to bypass the damaged trace. I understand the trick to replacing the parts is to raise to bead the diode leads into a L shape to raise the diode above the board and allow some air space. Use a spot of silcon to stablise the diode.

Those diodes need to be upgraded to 18V 2 watt in DO-41 axil package (2EZ18D5). Microsemi Commerical Components Group (VA) is one manufacturer. It should be common part to locate.

When replacing the resistors keep the same ohm value, but increase the watts rating to the next value up. Take a close picture of that whole problem area. The type doesn't matter as long as +/-
5% or better.

On the power lead going into the amp and before the power transformer connection check that there is no death capacitor bridging the leads. If you see one clip it out.

To replace electrolytics you can use Illinois Capacitor, which is original and still available or swap out where possible most of the values for a higher quality cap.

1 x 22uF 500V
.....Evox Rifa Peg124 is not available in this value
.....Illinois Capacitor 22uF 500V axil is same as original and still available - common in guitar amps
.....F&T Capacitor (Germany) 22uF 500V axil is a bit higher quality option
2 x 100uF 350V
.....Evox Rifa Peg124 Series is not available in this value
.....Sprague Atom 100uF 450V should work
2 x 33uF 350V
.....Evox Rifa Peg124 Series 33uF in 400V or 450V or equivalent
3 x 100uF 80V
.....Evox Rifa Peg124 Series 100uF 100V or equivalent
2 x 470uF 25V
.....Evox Rifa Peg124 Series 470uF 25V or equivalent <--- I've put an axil value here but was thinking this might be a vertical mount cap? I forget...

Take a pic of the PSU cap area.
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Old 26th February 2012, 03:30 PM   #10
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Note everyone. There has been a Dean Markley T-120R (Serial number: T1203-969) on eBay that was just passed in at USD$379 after not hitting reserve price. It was earlier offered at around $1000 Buy It Now Price. The problem with Dean Markley T-120 amps is that very little information is available on the Internet, so most people know little or nothing about these amps. Also until now through people like us very little technical information available. Because of the lack of historical and tech information people don't know how much bid on these amps. Also so few were produced that hardly anyone knows how they sound.

Only a few come up for sale each year Internationally. These amps at a minimum are worth $500 to $1000 (in that lower range because of the lack of brand awareness). In actuality these amps should be selling at the $1000 to $2000 level if they have been serviced to bring up-to-date. An equivalent new head built would cost over $2000.

I expect this Dean Markley T-120R will be relisted on eBay USA within the next few days. Good to be aware of it since these amps are rarely offered for sale. Most amps of this type would have had a pro player owner at beginning.

Here is the eBay listing:

http://www.ebay.com/itm/Dean-Markley-Signature-Series-120-Guitar-Amp-Head-W-4x12-Cab-T-120-Clapton-/190644279136?pt=LH_DefaultDomain_0&hash=item2c6348 d760#ht_684wt_954

An interesting technical fact learned from this sale is that JJ 6L6GC are too tall for this amp. The current owner then describes how the protective grill that covers the top and back has to be left off. He then goes on to say that he read that many people leave the grill off and it is only needed in a rack-mount installation. I've never heard of this before and more than anyone I have trolled the Internet for info on this amp. Quite simply one shouldn't use JJ 6L6GC tubes if they are too tall to fit inside the DM T-120 enclosure. He should have returned the tubes to the supplier and purchased another brand of tube. Personally I wouldn't have selected JJ tubes even if they fit.

This sale also includes a DM 412 cab, which needs to be considered as it adds to the value of the amp.

This sale also includes the T-120 User Guide. It would be good to try and get a copy of this from the seller before this amp finally gets sold. This is likely to be only a simple 2-4 page document, but still be good to have. I have only once before seen a sale that included the T-120 user guide.

Even with the JJ tube issue this looks like a desireable amp as it has the 412 cab (with dean Markley Magnum Model 1280 8 ohm 80W speakers) and is the Reverb version of the T-120 = T-120R.

Last edited by StephaninMelb; 26th February 2012 at 03:56 PM.
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