Marshall 18watt Reverb

[SkilpaddaN287]

I have read in the Marshall litterature that the original 1958/1973/1974 18 watt could have optional reverb from factory.
Have anybody here ever come across an original amp with reverb? Or seen the actual schematic with reverb?
I consider building an 18 watter with reverb and would be very thankful for any help with such schematic

[CurtissRobin]

The topic came up long ago and the result was that no one here had ever come across an original Marshall 18 watt amp with reverb. IIRC it was available from Marshall but was not shown in the catalog, being a custom order. I also believe that no Marshall schematic was ever published for it. If there had been one it surely would have been included in Michael Doyle's book The History of Marshall (Hal Leonard Publishing). It's not there and not elsewhere that I've found. If your search is successful we'd be thrilled to see it posted here.

Good luck in your quest.

[JMPGuitars]

Yeah, I'd love to see that if you find one. If you're just looking to add reverb to an 18 watter, the "single valve reverb" method works very well.

[SkilpaddaN287]

Thank you for the replies
Will that method work on a tmb channel with the cathode follower...?

[JMPGuitars]

Thank you for the replies
Will that method work on a tmb channel with the cathode follower...?

Probably. It depends how you wire it. I usually only build single-channel amps, so it's really easy to add in the reverb signal path. With a two channel amp using both halves of the PI, you'd have to wire it differently to have reverb on both channels. Maybe KennyO (CurtissRobin) can help you with that.

[SkilpaddaN287]

So if i make the other channel into a reverb circuit it will be some easier...?

[JMPGuitars]

So if i make the other channel into a reverb circuit it will be some easier...?

It's easier if you pick one channel versus having it on both channels.

[SkilpaddaN287]

Hmmm with a princeton reverb circuit (or similar) is it possible to not use the 3.3meg mixer resistor, and take signal between first and second preamp tube and inject the reverb signal directly at the PI? I really not want to ruin the tone of the tmb channel...
i did not even consider the phase relationship
But correct me if i am wrong, is not the 2nd and 3rd gain stage in phase with eachother and out of phase with 1st...? As on 2nd signal is taken from plate which is out of phase with its grid and on 3rd signal is taken from cathode which is in phase with its grid...?

[JMPGuitars]

Hmmm with a princeton reverb circuit (or similar) is it possible to not use the 3.3meg mixer resistor, and take signal between first and second preamp tube and inject the reverb signal directly at the PI? I really not want to ruin the tone of the tmb channel...
i did not even consider the phase relationship
But correct me if i am wrong, is not the 2nd and 3rd gain stage in phase with eachother and out of phase with 1st...? As on 2nd signal is taken from plate which is out of phase with its grid and on 3rd signal is taken from cathode which is in phase with its grid...?

If you were making a single channel version of the 18 watt amp that only used half of the PI, then you could use the 2nd half to return the reverb. AFAIK, when you're using both halves, you need the resistors as they are in the schematic.

[SkilpaddaN287]

Yes it makes sense
But lets say i were to use a jtm45/5f6a preamp...
Then it would be even easier as it not have 2 channels in the same sense as the 18 watt tmb...
And signal is only input at one side of the PI....? Without NFB at the PI at least...

[JMPGuitars]

Yes it makes sense
But lets say i were to use a jtm45/5f6a preamp...
Then it would be even easier as it not have 2 channels in the same sense as the 18 watt tmb...
And signal is only input at one side of the PI....? Without NFB at the PI at least...

Look at the SuperLite TMB preamp in the download section (it's a single channel amp). That's the one I'm referring to. It only uses half of the PI (no NFB), so I sent the reverb return to the 2nd half. I'm not well acquainted with the other preamps you referenced, but if they're similar enough, you'd probably be okay.

[SkilpaddaN287]

Ah yes
But where do you take the signal for the reverb circuit?
After inputs and before 1st gain stage or between 1st and 2nd gain stage?

[JMPGuitars]

Ah yes
But where do you take the signal for the reverb circuit?
After inputs and before 1st gain stage or between 1st and 2nd gain stage?

Experiment.

[SkilpaddaN287]

Ah that is possible too off course

[fiat_cc]

This is what I've come up with. I have tweaked the schematic a little bit (and the layout a bit more) since my first prototype, but only due to tremolo thump. The reverb worked nicely straight out of the box (well once I moved all things tremolo away from all things reverb).

18 watt reverb - 6v6 schematic v1-4.pdf

18 watt reverb - 6v6 layout v1-4.pdf

[JMPGuitars]

This is what I've come up with. I have tweaked the schematic a little bit (and the layout a bit more) since my first prototype, but only due to tremolo thump. The reverb worked nicely straight out of the box (well once I moved all things tremolo away from all things reverb).

18 watt reverb - 6v6 schematic v1-4.pdf
18 watt reverb - 6v6 layout v1-4.pdf

You should have read a little more.

We were discussing removing some of the reverb circuit's tone-sucking resistors and sending the return through the unused triode of the PI on single-channel, non-NFB amps (especially our 18 watters)... Your signal origin point is 1 option, but it's nice to experiment with the signal before and after the tone stack to see how that affects the reverb tone. I've already done all this (including removing the extra resistors), now it's his turn...and maybe yours too, if you want to improve the reverb tone even more.

[fiat_cc]

Well, since I’m here now...
If I wanted to bring the reverb return directly into V3b in my schematic (spare triode of phase inverter), do I change (or delete) any of my resistor values?
Also, is there a benefit in tapping the reverb signal before the tone stack? I imagine that would leave you with less possibilities in terms of reverb sound.

[geoff 1965]

hello pilgrim's
this might be useful/interesting to the topic? it's the hoffman 18watt lite reverb which i saved for the V1 cathode biasing "has anyone tried this?"
note how the reverb circuit uses both halves of the phase inverter mentioned earlier in the post.

18w lite reverb.GIF

[SkilpaddaN287]

Yes exactly
And reverb circuit look much easier to do than the blackface reverb cicuit
Is there any info somewhere about what reverb tank(pan) to use and other stuff...?

[fiat_cc]

It looks, at a quick glance, to be the same reverb circuit I used (and posted a few posts ago). The main difference being I fed it in on the other side of the phase inverter. Tank model is on my schematic, and the transformer is the same as the standard Fender reverb transformer. Also, I don’t see it using both sides if the phase inverter. ‘C’ refers to a power node. R2 feeds back into 1 side of the phase inverter (the opposite side to the rest of the amp).