What PT for this SS amp?

[daz]

I was thinking about building a marshall 30 watt SS amp from the old 80's series, the master lead 30. But i don't know what PT to use. Can anyone suggest what to look for judging by the IC's/transistors in the schematic?

http://www.schematicheaven.com/marsh...d_30w_5010.pdf

[olddawg]

I was thinking about building a marshall 30 watt SS amp from the old 80's series, the master lead 30. But i don't know what PT to use. Can anyone suggest what to look for judging by the IC's/transistors in the schematic?

Seriously, save yourself the headache: EARLY 80'S MARSHALL MASTER LEAD COMBO NICE CONDITION!!! - eBay (item 290352350883 end time Jan-19-10 18:24:04 PST) If you take some time and look around you will find one even cheaper and you won't have layout issues.

[daz]

Thanks, but i have a combo cab, speaker and chassis from one from years ago, and i have most of the components i'd need to build it. I'm trying to save money and i imagine a PT is gonna be probably only $25-40. Total cost is going to be way under $100 vs $435 for that amp.

[loudthud]

A 24VCT at 2A will probably get the job done. The MC1458 will be close to it's voltage rating. Use a MC4558 if MC1458s can't stand the voltage or increase R23 and R24 to 4.7K.

[daz]

Thanks. What do you think the original one was rated at given the working voltages on the output trans? Wouldn't they be the deciding factor as to the PT's voltage rating? I may be able to find an oddball if marshall used a custom PT.

[tedmich]

the $13 Antek AN-0512 should be plenty, or double its VA rating for $5 more (AN1212)

I am a little confused, there is no OT (output transformer) as its SS...just an PT and some silicon

[loudthud]

Thanks. What do you think the original one was rated at given the working voltages on the output trans? Wouldn't they be the deciding factor as to the PT's voltage rating? I may be able to find an oddball if marshall used a custom PT.

A quick calculation on 30W at 4 ohms gives slightly less than 11 volts RMS which is about 15.4V peak. The Rails need to be a volt or two higher than that. A 24V transformer will put out 26V-27V or so under light load so the rails will idle around 18V. The output transistors are rated at 80V so they aren't an issue. The MJ2500/MJ3000 are a 60V rated version of the same parts that may be cheaper if you can find them. The MC1458 is rated at 30V rail to rail which is what concerned me.

[daz]

the $13 Antek AN-0512 should be plenty, or double its VA rating for $5 more (AN1212)

I am a little confused, there is no OT (output transformer) as its SS...just an PT and some silicon

By "trans" i meant the output transistors. i will look at that model, thanks.

A quick calculation on 30W at 4 ohms gives slightly less than 11 volts RMS which is about 15.4V peak. The Rails need to be a volt or two higher than that. A 24V transformer will put out 26V-27V or so under light load so the rails will idle around 18V. The output transistors are rated at 80V so they aren't an issue. The MJ2500/MJ3000 are a 60V rated version of the same parts that may be cheaper if you can find them. The MC1458 is rated at 30V rail to rail which is what concerned me.

got it, thanks.

[J M Fahey]

Hi DAZ:
Short answer: 32V CT, 2 A ; that's to say 16+16V , 60VA.
+/-B rails will be +/- 22 or 23Vdc ; the preamp will get around +-/15V (well within specs) thanks to those 2K7 resistors.
I have used successfully for ages the old "look Ma, no Zeners" trick, trusting the 3 mA current consumption of typical dual op amps (RC4558, LF353, TL072/82)
I've never used LM1458's, considering them "cheap" working, but Mr Marshall seems to favor them a lot.
Don't you have the old PT on that chassis?
The schematic has an error in the input connection; you can (should) use Tip 142/147 for the output devices.
The amp is a nice sounding practice amp, powerful enough for small gigs; the big selling point is the speaker.
The EBay seller is crazy, over 400 U$S for that?
"Vintage" my grandmother!!!
What was actually left out of that amp?
Can you post some pictures?
Designing a new PCB from scratch , with the constraint that it must fit into a predrilled chassis, is not easy; yet, it's your time.
I suggest you should concentrate on the preamp only, and build any power amp you wish or can fit there.
An LM3886 or similar would be interesting, cheap, and lower your workload by half.
Good luck with your project.

[daz]

Theres nothing left of it, as i shitcanned the tranny years ago when i used it to build a tube amp i no longer use or care about. So i figured i'd gut it, or maybe just make a new chassis.

As to the board, i was going to use a generic one and just cut traces as needed. I've built pedals that were almost as much as this amp, so i figure it shouldn't be too bad.

[imaradiostar]

This begs the question- why wouldn't you build a tube amp in the chassis?

This summer I built a simple tube amp in a valvestate chassis. The guy went from two channels with verb and stuff to the simple single channel four knob tube amp I built him-and he loves it. His amp sounds so much better than it did so he's totally happy.

I used an antec PT on that built too.

jamie

[daz]

I DID ! Thats what i'm dumping in preference of the SS amp ! Confusing? I know, but thats because i haven't said why i want to do this. I build another amp that i love. It's my main amp and it's a cathode biased marshall sort of amp. When i play out, which i haven't for several years but am going to be starting again, i always used a 2nd amp as my effects return. DI from main amp>effects set with zero direct signal>2nd amp. So the tonal quality of the 2nd amp is not important as long as it's even decent, as all it's doing is amplifying my effects with no direct signal at all, which is just a bit of delay or verb. The tube amp i built into it is pretty darn heavy, and at 56 i'm unwilling to lug more gear than necassary. A master lead 30 weighs about 2 lbs more than the amp's cab, speaker, and chassic combined. The tube amp i built into it....well, you know what i'm saying. the pt and ot are by themselves about as heavy as the cab and speaker. At the moment at home i'm using a SS peavey thats one of the worse sounding peaveys ever, yet it works fine for this purpose. But being 8 watts with a 8" speaker it's too small for a gig.

reason #2 is that i always liked those master lead 30's more than any other SS amp. So i think it woul also make for a fun change at home to plug into it.

[J M Fahey]

Well, then you may build the Lead 12 preamp that's an absolute killer and slightly simpler, and an around 50 watt chipamp (LM3886) "downgraded" to 30W for eternal life.
You could build the *power supply* of the Marshall 30.
I'm not sure about the power handling of your speaker though. Obviously well over 30W RMS. Good luck.