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Old 13th May 2011, 03:32 AM   #11
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Default bumpage

hey folks. It turns out I was blind and the schematic for the 120R head IS on the DM site......oooops.

From what little I have found documented online, these amps were built and designed by a couple of famous amp designers for DM....not made by crate.... sorry Geek. Happily I have found that they are somewhat respected for their warm bluesy sound, but unhappily they are also compared to marshalls.......I certainly have no desire to part with the amp since it is about the best thing I have heard, beating any Marshall ive played, and any other Hi gain amp ive come across.

All this leads me to a final question. Im thinking of replacing the output valves, does anyone here have a favourite 6L6 bottle they can recommend? at the moment, im leaning towards some Harma ones. Also, would it be worthwhile replacing the capacitors on the HT side of things?

Since the pre amp valves are the original GEC 12AX7's, I am not planning to replace these. That is unless someone can give me good reason to!

Many thanks in advance for any help those 'in the know' can give.
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Old 4th January 2012, 02:33 PM   #12
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Winged-C 6L6GC work extremely well in a Dean Markley Signature Series 120 Model T-120R. It produces good tone across the entire spectrum with a nice fat bottom end and sparkly highs. I know of at least two people using modified T-120s as part of high end audio systems. They are a very good sounding guitar amp if serviced properly. Since they were all made around 1984 - 1985 it is important to swap out all the electrolytic capacitors, as they usually have a service life of 10 years for any electronics use. Once a T-120 / T-120R is properly serviced it makes a damn great sounding amp. There are a few places in the schematic that need some improvement, but any good valve techo should be able to see what is needed... IMO one of the best sounding guitar amps ever made.
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Old 14th January 2012, 04:02 PM   #13
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wow thanks for the advice! I must confess I havent been on the forum in a while, and I thought this thread was gone and certainly forgotten!

replacing the electrolytics sounds like a good idea....would there be any merit in replacing ceramics/polyester, with polystyrene or another dieletric? size permitting obviously.

whilst im not an amp expert i work in a HVAC environment, so small mods and component replacement is within my scope of ability In fact I think the amp has been modified somewhat, since the FX loop send and return have been removed from the circuit, probably crudely at best, by a previous owner....

I got the amp from my late father wife, shortly after his passing, and it wouldnt surprise me if he 'modified' it....its a bit of a hack n slash job

Ill have another look at the winged-C types you mention....I vaguely recall looking at them before. The pre-amp are all GEC 12AX7s if i recall, and likely the original valves. Im not planning on replacing these yet, BUT since ive heard good things about the GEC 12AXs, is there anything worth considering as a high end pre-amp valve replacement?
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Old 19th January 2012, 08:51 AM   #14
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Default T-120 / T-120R capacitor swap

@mondogenerator

I wouldn't bother replacing the ceramic or polyester caps, as odds are they will be fine. It is useful to go through the resisters and check against the colour bands.

I noticed on the power board that there are some large diodes that can run very hot. I have seen were they can even start cooking the PCB surface to the point of looking a bit burned. In my case even to the point of damaging circuit traces. This is a know issues I have seen documented on the Internet. Of course it would only show up on amps run over many years rather than ones with light use.

Other than that the only main weakness it the tone control pots wear out and vertical-mount potentiometers are hard to source. The most effective way to deal with that is to use Peavey type vertical-mount pots with "spider" legs. Just leave the legs long enough to make up the difference in space between the PCB and control panel face. You can read more here on what pot values are needed for the different T-120 / T-120R controls.

http://deanmarkleyvintageamps.weebly.com/comments-on-t-120-amps.html

Below are some examples of Peavey pots. The ones with four legs is what you use and just select for values that match Signature schematic. For the fourth leg you drill a small hole in the PCB to accept the leg. It is just used to stabilise the pot. Check to make sure you don't drill through any traces.

http://www.tubesandmore.com/scripts/foxweb.dll/catalog@d:/dfs/elevclients/cemirror/ELEVATOR.FXP?PAGE=SUBCAT&SEARCH_TREE01=POTENT&SEAR CH_TREE02=10_PEAVEY

The solution there is to mount new diodes but leave the legs long enough to raise the body of the diode above the PCB. After installation give them a shot of silicon to stabilise each diode in place.

There were a couple of other components that are getting hard to find, but I'd have to look at the schematic to tell you which ones. Odds are they are OK anyway.

Yes the GE 12AX7 it comes with standard are very nice. No reason to swap those out. It is sensitive to tube in the preamp section. It also runs well with the original Sylvania 6L6GC valves, but I think the Winged-C tone is better.

Be good if people can look at their Dean Markley T-120 / T-120R and remake a schematic from visual inspection. The schematic currently is very hard to read and missing some voltage values. I thought it would be useful to copy the schematic to make a turret-lug construction clone. Somewhere I have made a schematic of the preamp and power PCBs but only for T-120 non-reverb version.

When I remember any other know issues I will post them in this thread...

The only other problem I had was converting an American sourced T-120 from 120V AC to 240V AC operation in Australia. The schematic lacks the transformer details. Would be good if people with working T-120s have read the voltage values at various points. In particular B+ plate voltage & bias levels.
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Old 19th January 2012, 09:38 AM   #15
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The T-120 / T0120R use 1N4148 / 1N4001 and 1N4001 diodes in the preamp schematic


Replacing diodes


1N4148 is very common small signal diode which should be easy to get almost everywhere. 1N4148 has the following specs:
Diode type: Fast switching diodeVoltage rating: 100 VCurrent rating: 0.2 ASwithing time: < 4 ns
If you can't get exactly this part, replace 1N4148 with 1N914 or 1N4448.


1N4001 is a very common rectifying diode with the following specs:
Diode type: Rectifying, general purposeVoltage rating: 50 VCurrent rating: 1 AYou can replace 1N4001 with almost any general purpose rectifying diode with similar or better specifications. You can use 1N4002 diode (same as 1N4001 but increased 100 V voltage rating.

1N4003 is a very common rectifying diode with the following specs:
Diode type: Rectifying, general purposeVoltage rating: 100 VCurrent rating: 1 A
You can replace 1N4003 with almost any general purpose rectifying diode with similar or better specifications. You can use 1N4004 diode (same as 1N4003 but increased 200 V voltage rating.
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Old 19th January 2012, 10:37 AM   #16
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T-120 / T-120R Preamp PCB

If the 2N4148 (Silicon Controlled Rectifier (SCR) 0.8 Amp Sensitive Gate, TO92) goes bad it can be a hard to find obsolete transistor. The NTE Electronics component: NTE5404 is an equivalent part.

http://www.nteinc.com/specs/5400to5499/pdf/nte5400_06.pdf

The other two transitors on the peamp PCB: 2N4001 and 2N4003 should be widely able. If not subsitute with:

2N4001 = NTE128 (NPN)
2N4003 = NTE129 (PNP)

They are also made by NTE Electronics

TI TL072 Operational Amplifiers (Op Amps) were used in the T-60 / T60R / T120 and T120R pre-amp PCBs. A good upgrade for these components is the Burr Brown OPA2134 op amp, which increases the tone detail. A good advantage of Burr Brown OPA2134 is it gets great low distortion (0.00008%) from 8MHz bandwidth, is real stable and introduces less noise compared to TL072. There are two TL072 on the T120 / T-120R preamp PCB.
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Old 19th January 2012, 10:49 AM   #17
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For the front vertical PCB input jack on a T-120 / T-120R use one of the Cliff Electronic Components, Ltd " S1VS Vertical PCB Mounting Jacks. Six different switching circuit options are available. One of them should match the original component function.
  • • Compact vertical PCB mounting with 24mm stand-off
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Old 28th January 2012, 10:54 PM   #18
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I have a pdf of the schematic somewhere, I think of the DM site, and its a little hard to read i agree. I haven't even attempted to see just HOW its been modified as yet lol, but Ive had a quick poke around. enough to notice a pair of JJs and some other random brand in the other 'half' of the output stage. As for overheating diodes...I think i have a 690V 35 A bridge lying around....although that's a little overkill methinks. Its a 120R and I haven't really looked at the PCB at all, but the pots are definitely due for a change. If I cant get these Peavey ones, cant I just use chassis pots and 3 core + screen for each one?
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Old 31st January 2012, 06:04 AM   #19
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@ mondogenerator

You could use chassis pots, but keep in mind that the pots hold the PCB. It should be possible to use L shape brackets attached to the chassis floor and stand-offs to hold the pre-amp PCB in a vertical position. Single core wire run from the PCB to the pot terminals should work OK.

As you mentioned the hand drawn schematic from the DM site is very hard to read as it is a low resolution scan. I am trying to draw a new schematic using a circuit layout package, but the process is going slowly, because I can't interpet parts of the schematic.

It would be interesting to find out if the DM T-12OR circuit layout matches any popular amplifiers. A few years ago I had two DM Signature 120 (a T-120R and T-120) and got 1/2 through process of building a clone using turret board construction. I've since been side tracked by other projects. My impression was that it is a decent amp and worth copying the design.
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Old 25th February 2012, 06:09 PM   #20
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Default For those who are interested...

Ive started a 'refurb' thread, with the aim of documenting the refurb of this amp:

Dean Markley T120-R fixer-upper project.

If possible I will sketch a PSU schematic, and try to measure/read any components on the original DM schematic for those who are interested.

Many thanks to the contributors in this thread, who helped ignite my passion to get this project underway.
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