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Hey guys, I've built the 18W in LTSpice to test out some tonestack mods and I think I know what I'm going to change. When I've got the amp open I kind of wanted to add a switch for a solid state rectifier.

If you didn't already see my build I have an extra torroidal transformer jammed in there to give me some more juice for a pile of relay switching and LEDs that I used in the preamp:


NAS_18W0006 by LITEFORCE, on Flickr

It would be nice to have a beefier filter cap section for the silicon rectifier but there's just no room left. Are you guys just using the same filter caps when you use solid state rectification in the 18W? I could add the switch in that little space between the PT, Torroidal Transformer and the tube rectifier I think. Or should I try to get a 9-pin solid state recitifier that plugs into the socket? I think "the company who shall not be named" (Trinity for life ) has some copper cap rectifiers but they also have drop down resistors to emulate tube sag so I don't know if these would give the "fast" response I'm looking for?

Any suggestions would be appreciated!

Thanks,

Nathan

With SS rectifiers your B+ voltages will come out higher than if you were using an EZ81. A 100ohm 10W sag resistor will provide the necessary drop, with about the same level of sag as an EZ81. If you don't want the sag, then you may need to put some zeners into the B+ line instead to et the voltages down to 18W spec levels. I can't be more specific than that, since I'm not familiar with your PT in real life. I also haven't heard of anyone particularly changing the stock filter cap values, and I suspect it wouldn't provide any real benefit. The BBQ option has been known to have reliability problems in the past.

You can also stiffen up the amp further by using a huge cap in the 1000uF to 2200uF range on the EL84 cathodes, along with a 180 ohm cathode resistor to cool the bias down a little to suit.

HTH

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Thanks man. I think a switch with some zeners and the diodes is what I want then. Or just build that 100W SS rectified amp I've been wanting to build...

The PT is the Trinity PT, the other one is a 12V PT for the relays etc...

Relays and LEDs are fun

I should have just looked at the picture!

OK, you will need to place a few zeners in series in the B+ rail somewhere between the first filter cap and the OT centre tap. I suggest using as many 6V 5W zeners as it takes to get the B+ voltage down to around 330V.

IMO real tube amps don't need any relays and LEDs.

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Great tones come in small glass jars!

So all we care about is the voltage on the plates (pin 7) of the power amp? The B+ part feeding the rest of the preamp is okay to keep high? I thought the zeners went in series with the standby switch.

Yeah it's not a real "pure" tube amp but it is what it is. I think it's a great practice amp for getting a lot of different sounds at a decent volume. Maybe I'll take your advice when I build a 100W head, go single channel. Mmm but channel switching, effects loop so tempting .

When did I ever say that? If you get the main B+ voltage right then all the other voltages will fall in line relative to it.


Yes, as long as the standby switch comes after the first filter cap.


I assume by "decent volume" you mean ridiculously loud. Because that's what Marshall 18W amps are like - to a lot of peoples' surprise.

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Great tones come in small glass jars!

Amp is wired like this:

http://www.trinityamps.com/ForumGallery ... ematic.jpg

I've gotta read up on my zeners again.

The amp is super loud but not loud enough

Okay I get it now. Thanks man.

I don't think there's enough room in there for multiple zeners and heatsinks...

Could you move the transformer to the outside of the chassis to free up some room?

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Or change to a smaller one! That's a monster in there!

Ya or that,
I've powered relays, LEDs and BJTs with transformers half that size!

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I forget what the current handling is on it. It's definitely over spec'ed. I thought about putting it up top but decided I didn't want to run the power through to the top. I think this is really just a good excuse to build another amp. I did 5W and 18W, I need to do 100W now!

If you break up the voltage drop you need into 6V chunks, then you can use 5W zeners that don't need any heatsinks. So you basically will have around four or so diodes wired in series with their stripe ends all pointing back towards the first filter cap and rectifiers. Alternatively you work out what drop you need and use one big fat honking zener of the required voltage, bolted onto the chassis with an insulating washer.


Nobody actually needs a 100W amp unless they're playing stadium gigs or they want to kill small birds flying overhead. Those amps are just stupid loud, and you end up never being aable to run them above '2' on the dial.

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Great tones come in small glass jars!

What's the advantage of the big zeners over a VVR? If I use the zeners I'll have to switch the tube / ss rectifier and zeners on the OT CT all at the same time.

As for the 100watter, I had a 100W twin reverb I really liked, volume was never a problem. I also had a 50W JCM 800 that was WAY too loud. Was thinking of doing a 100W 6L6 head with twin reverb transformers.

Depends if you want to change B+ voltages are just set them to some level you want.


AFAIK those twin reverbs actually put out quite a bit under 100W. Plus the speakers are probably quite inefficient. OTOH many Marshall amps put out more than their spec rating, depending on the model and year of manufacture. For example 60W (clean) out of a 50W Marshall isn't uncommon.

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Great tones come in small glass jars!

Ya still like 85W but the difference between 85W and 100W is nothing. It's bloody lould.

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Yeah my 2204 50W JCM 800 was pretty clean and insanely loud, the twin reverb could never keep up and was overdriving like crazy.

Okay, I just finished fixing up my LP Studio, gave it a new nut, bridge saddles, fixed the frets with a major level and recrown. I've been having too much fun playing to mess around with amp modding.

I have to finish building an overdrive for my friend and then I'll get around to tinkering with the 18W. I'll keep you posted.