18 watt Marshall 1974 reissue cathode resistor question

[Planobilly]

Hi guys,

I have a reissue 1974 18 watt Marshall I have been working on. I am the first person to work on the amp. It has a 97 ohm cathode resistor from the mfg. The output tubes run hot as hell. They run at 50mA. The output transformer failed a few months back. I replaced it with a Mercury Magnetics radio spares. The power transformer failed last week. I replaced it with a Mercury Magnetics Radiospares P18RS. The plate voltage is 330 VDC.

The amp gets used 16 to 20 hours a week at near max volume.

I understand these amp run hot but 50mA is way too much. I was considering changing the 5 Watt 97 ohm cathode resistor to around  150 to 200 ohms. Is there anything else I need to adjust... screen grid ? Screen grids are currently at 100 ohms.

Thanks,

Billy

[dude]

Change the cathode resistor to 150 ohms and check bias, if still too hot use a 200 ohm. Go higher till correct bias is seen. The 100 screens are fine. BTW, 5 watts is fine. Watch for red plating on the el84's.


al

[Planobilly]

Thanks for the feedback.

Billy

[labb]

I believe that the "stock" cathode resistor for a 18 watt Marshall with a Va of around 340 vdc is 130 ohm. You may be a little on the cold side with the 150 ohm. Take a look at this. It might help. http://mhuss.com/18watt/schematics/18wattLite2b.gif         As Dude said, check it and see.

[sluckey]

Here's the original schematic that I used to build my 1974. It calls for a 100Ω. I changed to a 150Ω for the same reasons y'all are talking about.

[PRR]

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[Planobilly]

Thanks guys

I need to order a few resistors. I don't normally stock a lot of 5 watt resistors. I had a 200 ohm 5 watt but that was too cold...too much crossover distortion.

The way these amps are set up from the mfg does not look like a good situation for a working band. I have read that power transformer failures are pretty common in this model amp. I  only assume the Mercury Magnetics PT is better made. I would expect any amp that cost over $2000 new should not exhibit these type of failures. Well...perhaps that is just wishful thinking...

The secondary issue with this amp is the fact it is too small for the use it is being put too. Just not loud enough, and no sound man and no good sound reinforcement system. This is a typical issue with bands going through a transition from smaller venues to larger venues.

My on take on 18 watt Marshalls is that it is not a very versatile amp to begin with. Great for certain styles of music but limited for many other styles.

The owner of this amp played through the Deluxe Reverb I just finished building and liked it much better than the Marshall. The Hoffman Deluxe I built is not at all "stock" and the main difference is channel one was converted into a lead channel and wired to access the reverb and tremolo circuit. I may wind up building one for Vic but with a little more power and designed to be more road worthy.

Cheers,

Billy   

[labb]

The AB763 preamp with any power end is hard to beat if you like the Fender sound. Leo knew what he had with that one. Reason it is found in so many different Fender amps

[Planobilly]

Hi Labb,

I like the sound of Fender amps in general. I also like the sound of the 18 watt Marshall. I just think the Fender is a good bit more versatile.

I also got to play a small Two Rock amp today for the first time. That was one of the best sounding amps I played in a long time.

Cheers,

Billy

[dude]

The secondary issue with this amp is the fact it is too small for the use it is being put too. Just not loud enough, and no sound man and no good sound reinforcement system. This is a typical issue with bands going through a transition from smaller venues to larger venues.
My on take on 18 watt Marshalls is that it is not a very versatile amp to begin with. Great for certain styles of music but limited for many other styles.

A stock 1974 has two channels, first - tone/volume, second the same plus tremolo. That's why most build the 18 watt TMB, the first channel, is the same but the second drops the tremolo and adds a full tone stack, treble, mid, bass and a master volume and preamp volume. The second channel is much more versatile than the stock 18 watts. The master volume and preamp volume can be blended to get a variety of tones from spanking clean to Marshall crunch. Just Google "18 watt TMB schematic", many vendors sell these 18 watt TMB kits too.


As far as volume for bigger gigs, there's the same version of the 18 watt TMB with four el84s which puts out 36 watts and the same tones. I have  a Weber 18 watt TMB kit I bought years ago and have played it for hours at a time, 100's of times with no problem with transformers or anything for that matter except tubes. I can get sparkling cleans to crunch and highly recommend a 18  or 36 watt TMB (treble, mid, bass, with master and preamp volumes). You can get these amp kits with a two 12 cab too, the 36 watt with two twelves and the right speakers will do a bigger gig, you won't  have 100 watt output but much better cut through than the 18 watt.  al