Project suggestion needed - Which Marshall??

[Janglin_Jack]

I haven't built an amp in quite a long time. My JTM45 is sounding better than ever. I sold some of my other creations to friends and bandmates, (which game me an excuse to build others). I won't bore you with all the other amps I own, but the two I want to note are my JTM45 kit that I housed in an 18w 1x12 combo cab with a Scumback speaker. The other amp I picked up a while back was a modded JCM800 4010 1x12 combo w Heritage G1265. The guy claimed a well known amp builder put in a footswitch mod to make it a two channel amp. I put it back to stock, and then opted to make it a two channel amp, by using all shared controls for tone, but two dual concentric pots for the volumes. That way, I can set a clean tone and a lead tone and not compromise with the volume and preamp levels. The mod that was in there was cool (after I figured out how it was done). It used a transformer to and the footswitch LED would work. I forgot the kit I got, but there was some footswitch mod I got that didn't require the transformer and was much cleaner. In the end, I probably could have left it alone, but I was confused by the mod and needed to revert to stock to know what I want to change about the JCM800 stock circuit, then wire in the switch mod.

Anyhow, to get to my question. I have always wanted a head with a 1/2 stack B cab. When I owned them I rarely took them out to gigs as I always opt for combo amps. That said, I dig Charlie Starr's tone and playing. He moved away from Orange Rockerverb and used Plexi style amp with a 2x12. This is a rig I can see myself lugging around. I love the Greenback tones so I think I can get away with a 2x12 and use Greenbacks.

So where I am at, is to find a model to make as a head. The easy route is just to make a 1987 50w and be done. But the wheels are turning and I start to consider a 1986 spec. Then I start to contemplate a JTM50, (SS with EL34).

What do you guys think would be a good compliment to the two combo amps JCM800 and JTM45. Which circuit to go with a 2x12 with Greenbacks. I usually play a Les Paul with humbuckers. I play some blues and funk, CCR to AC/DC to Stones to the Black Keys BlackBerry Smoke, ZZTop Skynyrd, etc so mostly Classic Rock and some Hard Rock. No metal, (other than Living after Midnight...LOL).

Suggestions on a build????

Mike

[Freebird]

Hi Mike

Think also on the needed tube output power. Is 100, 50, 40, 30 or even 20 Watt enough? Of course it deppends how loud your drummer is playing or how big the stages are.

When I had to think it over, I came to the end that for small clubs, 20 Watt are enough for me. So I decied to build an amp with 2xEL84. With a 2x12 or a 4x12 cabinet I get enough power. And one advantage is, the EL84 tubes are easy to bring to saturation.

Good look to find out
Dani

[neikeel]

I agree for most venues 18w with a 2 x12 is enough nowadays, particularly mic'd up and run to the desk through the PA, just decent in ear or fold-back wedges. I use my 18w trem 2x12 a lot but for some venues I use the beater (1968/9) 1935A 4x12 with G12Ms and a JTM45 or 50w for the headroom I need for the funk/pop stuff with delays and compression pedals and accept I need to use a pedal for dirt.
Depend what you are playing and how flexible you need to be. Your idea of 2 independent channels is a good one and I keep meaning to build one!

[Janglin_Jack]

Thanks Neikeel and Freebird...I opted to go with a 50w Bass Spec 1986. Initially I am going to go with El34, SS rectified shared cathode design. Parts on the way. Will keep you updated.

Mike

[Janglin_Jack]

I hope the parts come this weekend, but I think it is going to be next week. Those of you with some experience with 1986 Bass Specs, what tweaks do you suggest. I am going to build it basically to stock specs, but I tend to use 25uf for the V1 vs 330uf cap. Suggestions on volume bright cap, (I usually leave out). Tips on negative feedback, I was thinking 47k off the 8ohm tap.

Mike

[neikeel]

I hope the parts come this weekend, but I think it is going to be next week. Those of you with some experience with 1986 Bass Specs, what tweaks do you suggest. I am going to build it basically to stock specs, but I tend to use 25uf for the V1 vs 330uf cap. Suggestions on volume bright cap, (I usually leave out). Tips on negative feedback, I was thinking 47k off the 8ohm tap.
Mike

Both ideas sound good to me and what I would use.

[Janglin_Jack]

What filetering do you suggest on a 50w Bass Spec? I am also confused on the diagram/schematic for the dual can that is attached the the plate resistors on V1. The two halves are connected by a 10K resistor (balancing resistor?). Does this mean they would be in parallel and combine the capacitance? Dual 16uf cap in this position connected by 10K resistor = 32uf filtering? Dual 32uf in the same way = 64uf?

I am trying to figure out how to best use the caps I have, (I have some dual can and some single cans).

So the first question is to understand the wiring with a dual can with 10K resistor. I don't have much space on my board for a giant dual can here. But I do have 16uf and 32uf single capacitors that will fit. I also have the option of surface mounting the dual can on top. Just trying to uderstand the wiring.

The second is what filtering to use. I have seen 16uf up to 50uf and not sure what the '68 style was using, (16uf / 32uf??)

Thanks in advance,
Mike

[neikeel]

Short reply as long one just got lost on submission

10k decouples the stages so 32uF V1 and 32uF V2, nicest filtering in 50 and 100w plexis IMO

Use 100uF for rmains (50+50 dual can)

32 PI and 32 screens (Goldilocks combination not to high or too low!)

[Janglin_Jack]

Roger that! After pouring over the layouts I started to notice how the dual cans were attached with the 10K resistors. I realized that they were not connected and being used as separate halves. Thanks for the confirmation !! I will go with your suggestions!!

I don't have all my parts so I am just populating the board tonight.

Mike

[Janglin_Jack]

I finally wrapped up my build the other night. I am going to double check all my connections. In the past I have tested voltages as the Metro instructions recommend. Once I had something whacked out and fried a PT.

What is the best way to power up a new build the first time? I have heard of using a light bulb. I have a variac.

Suggestions on how to test a new build without frying a transformer?

Mike

[neikeel]

Light bulb is ok
I fire up without tubes and test voltages, but then I've made a few by now, often I do a cap reform first in circuit with 100k current limiter in HT line.

[Janglin_Jack]

I think I have made about 20 amps by now. The first 10 I used to just plug it in and turn it on. I finally started testing without tubes and checking voltages. A while back I was trying to bring it up slowly and have the caps form and I was turning the variac about 10v every 10min. I was glancing at the meter and watching TV at the same time. I smelled something and the current was climbing. Too late...lost a PT. I am thinking the Dim Bulb current limiter would be a good way to avoid killing a transformer. I am going to try and get one built and then give it a go this weekend. I know there are no issues, but for some reason I am a chicken to turn this one on.



Mike

[Janglin_Jack]

Holy Hell....I didn't know I built my amp 3+ years ago. I never tested it until the other nite. Got it running and it sounds stellar. Getting motivated again to play guitar and I love tweaking amps and gear.

Mike

[Freebird]

Yes time flies! Cool to read you got your amp working.
Have lots of fun with that tube driven and self-made amp.

Daniel