JMP 50 watt. Add Tube rectifier or not???

[fixin-man]

So I have an old JMP 50 watt combo master volume chassis and was wondering if anyone has any advice about converting it to a tube rectifier. pros or cons? I am trying to get a better tone that is not quite in your face. A little warmer and usable.

[jezzzz2003]

I guess if youre looking into JTM-45 tone then yea, it would drop the output volume by about 10 watts or so too,
Give you some sag and earlier compression.
I think I would prefer it stock though but thats just me.

[908SSP]

Go to the Metro amp board they will have the recipe for you. I don't think you need to change the rectifier. Change the caps to match a bass head like the model 1986. First thing is get rid of the bright cap on the volume pot.....

[fixin-man]

I have been playing a 1946 sound craft twin 6V6 output lately and did not realize how much I love the sag and compression. I went back to my Hotrod deluxe and could not believe the differance. So that is why I am on the quest. I already have the JMP that had mass mods to it and was going to take it back to original anyway. So I thought what the hay.....

[fixin-man]

Thanks for directing me to the Metro amp board. I found a ton of great info.

[fixin-man]

Is the JTM-45 a simular amp without the master volume? I don't use all the volume anyway. To loud to crank it were I play.

[Ian Anderson]

I have a '67 JMP with a tube rectifier. The PT had been replaced when I got it and it did not have a tap to power the tube recto so a ss was plugged into the tube socket.
I put a Mercury PT in there and went back to the tube recto.

I am no amp tech so keep that in mind but you may either need to replace the PT with one that has a tap to power the tube or install a 2nd small PT to power it.

Here is the amp.
http://marstran.com/67SP10415.htm

[John Phillips]

You can achieve most of the characteristics of a tube rectifier by fitting a large power resistor in series with the solid-state diodes - a 50 ohm, 25W high-power chassis-mount type would be a good choice to roughly simulate a GZ34. The audible effects of a tube rectifier are purely due to its resistance.

No, it isn't exactly the same (a real tube doesn't quite have a constant or linear resistance), but it's close, and only needs two small screw-holes drilling in the chassis and a couple of wiring connections. Especially on an amp which isn't intended to be an exact replica of an old one anyway, I don't think the extra work and cost of fitting a real tube is worth it.

The resistor will give you about 95% of the result for 5% of the work.

[scopeboy]

Yes, tube rectos always struck me as one of those things, a nice extra touch but you can live fine without it. When building an amp from scratch, it's just another source of heat in the chassis, another thing to make room for, another load on the PT, and another tube that wears out and needs replaced :(

Possibly the only good thing about a tube rectifier is that, if it's an indirectly heated type like the GZ34, you don't need a standby switch. The GZ34 is designed to take a long time to warm up, to let all the other tubes get warm first. Well, I think NOS ones were, I can't vouch for new production tubes.

Put another way, I don't think a Mesa Dual Rectifier is twice as good as another amp with only one rectifier tube :P

[pokey]

What about one of these-

http://www.musictoyz.com/guitar/amps/thdrect.php

[walterw]

those pre-jcm800 2204's are starting to become collectible, so i would think really hard about altering it. of course, i wired a relay-switching second master volume into my '78 jmp 100w 2203, so what the hell, go for it. (i like john p's idea of a big resistor to cop some of the vibe of a tube rectifier without major surgery, and i wonder what value would work similarly on my 100 watt?)

[El Caballo]

What about one of these-

http://www.musictoyz.com/guitar/amps/thdrect.php

Or a Weber Copper Cap. Same thing but under half the price:
http://www.webervst.com/ccap.html

I think a WFW is what fixin-man needs: it's the version with leads, not the one that goes into a tube socket.

[John Phillips]

i like john p's idea of a big resistor to cop some of the vibe of a tube rectifier without major surgery, and i wonder what value would work similarly on my 100 watt?
If you want to approximate a real rectifier type, some rough resistance equivalents are:

GZ34/5AR4 - 40-50 ohms
5U4 - 120-150 ohms
5Y3 - 400-500 ohms

You don't have to use these values, but they give a useful guide to the range you'll want.

The next thing you need to consider is power rating. In a more powerful amp, the current draw is greater, and the power dissipation in the resistor is the resistance multiplied by the square of the current (in amps). A typical 50W amp draws about 250mA at full power, and if it has a HT fuse the value is usually 500mA (0.5A), so really to be safe the resistor should be capable of handling the current before the fuse blows, or if it's not fused, a safety margin of double (which is the same thing in this example). So for this example, a 50-ohm resistor will need to be a minimum of 0.5*0.5*50 = 12.5W. So a 15W resistor should be safe, and a 25W one completely certain.

But for a 100W amp, you'll need a much bigger resistor - if the maximum current is 1A, the minimum power rating (at 50 ohms) is 50W... and that's even just to give GZ34-type sag. Going higher will quickly become difficult as the resistor requirement rises - 100W resistors are the largest easily available.

Finally you need to consider voltage rating. In order to be properly cooled, these high-power resistors are bolted to the chassis (remember to use a little heatsink compound), and since the resistance element is carrying the full B+ voltage of the amp, the resistor must be rated for at least that, and preferably more. 1000V would be a good safe rating. Generally the larger power ratings also have higher voltage ratings, which is why it may be a good idea to use a bigger one even when the power dissipation doesn't really warrant it.

So, to answer the question directly :), a 50 to 100-ohm (depending on how much sag you want), 100W, 1000V resistor would be ideal.

[Giraffecaster]

correct me if i'm wrong but doesn't the jmp 50 not have the 5v taps for the rectifier tube?? that means changing to a tube rectifier means not only cutting a hole and mounting the socket but also either replacing the power transformer or adding on an external 5v transformer.

if you don't like your amp sell it and buy a clone of a jtm45

[JimmyR]

I tend to agree with Geoffrey. Keep the 50W as a 50W - clip the bright cap, etc and either use a good pedal or sell it. I can recommend a couple of pedals that will work beautifully with that amp. The Menatone Working Man's Blue or the Bad Cat Two-tone. The Menatone now has a knob called "sag" and the Bad Cat uses two 12ax7s at real voltage to get you there.

Put back to near stock the amp you have is a good amp! If you want to experiment, build a clone and go for it - that's what I do!

[epluribus]

If you want to approximate a real rectifier type, some rough resistance equivalents are:

GZ34/5AR4 - 40-50 ohms
5U4 - 120-150 ohms
5Y3 - 400-500 ohms

You don't have to use these values, but they give a useful guide to the range you'll want...

<Snip>

...So, to answer the question directly :), a 50 to 100-ohm (depending on how much sag you want), 100W, 1000V resistor would be ideal.

Sounds like a cool candidate for a switchable system, presuming the switch is stout enough and safe. Feasible/useful?

--Ray