Hi guys,,,

[knockbill]

just found/joined your forum,,, I am a member of AK,, and DIYaudio, and saw reference to this forum in an AK (IIRC) post...
I picked up my first Marshall amp last month at a radio show, while looking for guitar speakers to use for build projects... Its a SS G30RCD, with 6 pots and 2 jacks snapped off the PCB, which I read is a problem with these due to using the pots/jacks as the board mounting system...
As I'm into tube amps, and got this for its 10" speaker,, I figured I'd try to replace the broken pots and see if it worked at all... I am amazed it still does!!!

I ended up using a different speaker for the latest Champ build,,, and am going to try to build an 18W Marshall tube amp for the cabinet, but in a new chassis and preserve the original SS chassis...
Here's a schematic of a Marshall PA amp that was recommended for the project...
https://el34world.com/charts/Schematics ... w_2022.pdf
Thanks, John

[crgfrench]

I like that circuit.

[knockbill]

I like that circuit.

Thanks for replying... It was sent to me by a well know guitar tech on DIYaudio, here is his quote...
"Build one of the 20W Marshalls. Although they are labelled Guitar/Bass/PA they are basically the same. he only other Marshall amp which can fullly reproduce all sounds achieved with a 100W "Plexi" Marshall head."
The schematic doesn't show specs for the OPT, tho... I could probably dig up a usable replacement for PP 6BQ5s, but just wondering if there are any output transformers you can recommend?

Regards,
John

[JMPGuitars]

Hi John, welcome to the site!

You can see more drawings of that amp here, including newer/clearer versions (and the variations): viewtopic.php?f=25&t=24425

I've never seen transformers specific for the 20W. I'm guessing they use the same transformers as the 18W, but use the solid state rectifier for a higher B+.

Personally I like the Heyboer versions GDSAmps.com sells, but other Heyboers, and ClassicTone are good too.

Thanks,
Josh

[crgfrench]

The Mojotone British 18 Watt OT is a Heyboer that sounds great.

[geoff 1965]

hello John,
i think like Josh that they would have used the 18W transformers,the only 20W OT i've seen is the fender but that has a lower 6K6 primary impedance.
have a browse through the downloads,there is an 18W plexi as well as the 20W's if that is the type of amp you want.
your location comes into play as well for the OT's if like me your'e in the UK the heyboers and classic tones are pretty costly if you add on oversea's postage and custom's fees.
i've got a hammond 1750PA and it tick's all the boxes!

[crgfrench]

Chris Merren makes a JTM45 Drake repro OT with an 8k plate load that would probably be amazing with that.

[knockbill]

Hi John, welcome to the site!

You can see more drawings of that amp here, including newer/clearer versions (and the variations): viewtopic.php?f=25&t=24425

I've never seen transformers specific for the 20W. I'm guessing they use the same transformers as the 18W, but use the solid state rectifier for a higher B+.

Personally I like the Heyboer versions GDSAmps.com sells, but other Heyboers, and ClassicTone are good too.
Thanks, Josh

Thanks for the welcome,,, and the info!! I looked at the ClassicTone site, and got some specs...Seems 20W and 18W use the same OPT as ClassicTone says" approx 18W" But I'm open to all info, looking fwd to this as I winter project... It will be my first Marshall tube amp!

[knockbill]

hello John,
i think like Josh that they would have used the 18W transformers,the only 20W OT i've seen is the fender but that has a lower 6K6 primary impedance.
have a browse through the downloads,there is an 18W plexi as well as the 20W's if that is the type of amp you want.
your location comes into play as well for the OT's if like me your'e in the UK the heyboers and classic tones are pretty costly if you add on oversea's postage and custom's fees.
i've got a hammond 1750PA and it tick's all the boxes!

Thanks for posting,,, I'm in PA USA... didn't get to fill out all the info yet!!! Kinda hoping to gather some OPT inf/specs,, build a test mule,,, and see what OPTs I have in stock may work with this first...

[knockbill]

Chris Merren makes a JTM45 Drake repro OT with an 8k plate load that would probably be amazing with that.

Thanks,, I have a couple 8K PP OPTs that may work then!! Never built with 6BQ5s tho,,, closest I've come is a pair of 6973s in a Supro 1624T clone....

[JMPGuitars]

Thanks for the welcome,,, and the info!! I looked at the ClassicTone site, and got some specs...Seems 20W and 18W use the same OPT as ClassicTone says" approx 18W" But I'm open to all info, looking fwd to this as I winter project... It will be my first Marshall tube amp!

The difference in wattage is almost nothing in reality. I've seen plenty of "18 watt" amps biased closer to 24 watts (or even a little higher - which shouldn't be done for the tube's sake) with typical 18 watt transformers. Good EL84s can usually handle a little abuse. The sound/feel of the amp is what you're going after in biasing the output power correctly, or choosing the specific circuit.

I hope it's a fun / educational project. Keep us posted on your progress!

Thanks,
Josh

[knockbill]

Thanks for the welcome,,, and the info!! I looked at the ClassicTone site, and got some specs...Seems 20W and 18W use the same OPT as ClassicTone says" approx 18W" But I'm open to all info, looking fwd to this as I winter project... It will be my first Marshall tube amp!

The difference in wattage is almost nothing in reality. I've seen plenty of "18 watt" amps biased closer to 24 watts (or even a little higher - which shouldn't be done for the tube's sake) with typical 18 watt transformers. Good EL84s can usually handle a little abuse. The sound/feel of the amp is what you're going after in biasing the output power correctly, or choosing the specific circuit.

I hope it's a fun / educational project. Keep us posted on your progress!

Thanks,
Josh

Thanks for the info,,, I try to build most of my project with parts I already have,,, needed a couple pair of OPTs over the years,,, but I have a lot of PTs, and chokes, along with somd orphan OPTs taht got me started with git amps!!!...
I will update when I actually get around to building this amp... I expect it to be a winter project... I'm still waiting for two pots to finish repairing SS amp that was in this chassis...

[knockbill]

P5200015.JPG

I found this guy with nothing to do!! Vintage Thorardson 6694... 8K/4-8-15R, 40W max output...

[crgfrench]

What resistances does the primary measure on that?

[knockbill]

What resistances does the primary measure on that?

392R end to end...

[crgfrench]

Marshall loves 20W

Screenshot_20191023-172250_Facebook.jpg

[crgfrench]

392R end to end...

Are you sure that's not from center tap to an end? That seems low compared to the usual 18W iron, which are around 325R per side. What do you measure from center to each side?

[knockbill]

392R end to end...

Are you sure that's not from center tap to an end? That seems low compared to the usual 18W iron, which are around 325R per side. What do you measure from center to each side?

190R to ctr and 202R to ctr....

[knockbill]

What resistances does the primary measure on that?

392R end to end...

Spent the morning bending a chassis for the tube amp to fit the Marshall cab I have,,, came out just right... Then the pots I ordered for the broken SS amp that was in it came, so I got that back together and re enforced the chassis so it and teh PCB wont flex...
Really hope the new tube amp sounds better than the SS one!!!
Cant figure out how to send pic here from the camera,, or I'd share...

[knockbill]

What resistances does the primary measure on that?

392R end to end...

Spent the morning bending a chassis for the tube amp to fit the Marshall cab I have,,, came out just right... Then the pots I ordered for the broken SS amp that was in it came, so I got that back together and re enforced the chassis so it and teh PCB wont flex...
Really hope the new tube amp sounds better than the SS one!!!
Cant figure out how to send pic here from the camera,, or I'd share...

Since the tube chassis came out good, and is mounted in the cabinet,,, I built a box for the SS amp kinda like a smaller version of a big Marshall head,,, I'll either Tolex it or maybe just give it a couple coats of texture paint to try to match the original cabinet,,, it will sit on top of it, with rubber feet deep enough to span the handle...
Project is moving along,,, just not real fast!!!!