Cascaded Gains or extra gain mods anyone ?

[pedro]

Hi Folks,
Ive just finished building my second 18 watt copy - this is my 7th DIY tube amp now....
the Last 18w was built for a friend who plays second guitar in our band. The new amp is mine !
I want to add some versatility to the existing amps so we have switchable channels when playing live. Ive digested Kevin O Connors chapters on switching in "TUT" and I'd like to try switchable cascading gain stages with as little modification to the pure 18 watt layout as possible. Has anyone modified their original 18w copy to get switchable channels ?
Any advice ? - I dont want to completely change the circuit board when its so recently built but I'll try most things if they work.
Incidentally I have already fooled with a diode bounding circuit after the input stage on one channel ( a la Jubilee which I used to own ) and it sounds fantastic in conj with p.a. distrotion when the amp is cranked !!!
( just with 2 IN4001 reversed paralleled and a variable resistor to ground )
Suggestions awaited !
Rgds
Pedro

[loverocker]

Yikes. First posting and you're already confessing to putting clipping diodes in the venerable 18Wer!

Just funning I think the footswitching'll be the tricky bit - the 18W circuit is so simple that I bet the relay/opto/etc circuit would probably need more components that the entire 18W's signal path I can see why people opt for a cheap and simple stomper...

[pedro]

Thanks ....
just seen the mod for clean boost in the pro jnr posting - think I'll try that and see if I get the footswitchable boost I need

I feel very guilty about sullying the pure circuit with the diodes now !
( but I'll keep it as aswitchable option - its pretty noiseless with a 5v DC relay. )
Rgds
Pedro

[loverocker]

5V relay... now you've given me an idea. My first 18Wer is a not-entirely-faithful version, so it wouldn't hurt to do away with its EZ81 and use its heater supply for 6V relays... that'd make things easier...Hmmm... the Mesa/Soldano opto switching looks like hard work to me.

By the way, do you get a volume drop when you switch in the diode clipping? Or do you switch in an extra stage at the same time to compensate/boost?

[pedro]

Hi,
I run the relay(s) from the 6.3V heater filament supply , using a couple of silicon rectifiers ( IN4007) and 220uF cap to ground from their junction. Reasonably smooth relay DC at about the right voltage to trip relay when footswitched etc !
Also, if the switchable DC rn from a jack socket into pedal there is supply for LED with series resistor in the footswitch.
Keep the EZ81
( actually both 18w's Ive built have used Hammond mains TX with 5V rectifier supply so Ive used 5Y3s - sound pretty good. Plate voltae is 301V with 9.29V across a 125ohm cathode resistor ( actually measured at 123.5ohm.
I reckon that this gives me : 75ma for the two tubes at idle with 301V on the plates = 11.3W per tube - this seems high but its the first time Ive really checked an amp Ive built for the current draw !!! Any views anyone ?

To answer question on diode clipper - yes there is discernablr volume drop but actually when amp is loud the distortion tone makes up for slight volume loss. Ireally should have simutaneous gain stage switch in at same time to make it a lead boost.

Incidentaly - I tried the clean boost mod earlier tonight - OK except that with channel volume full up I started gettinghigh frequency feedback from the amp - even with guitar input volume right down. I then tried another filter stage in front of the "boost" triode section using a 2.2K 2W resistor and a 10uF 450V cap but got a VERY distored tone so undid this power supply filter stage. Have reverted back to the clean boost but cannot use the amp at full volume without unwanted squeal - Any thought anyone ?

Cabinet nearing completion for this amp so I want to finalise circuit before the queen bee gets put into the hive !

Rgds
Pedro

[loverocker]

I did have squeal on my first effort at that cascade. Went back in, fully expecting to have to put in a 470k+470k or 270k+820k divider - I tried tidying up the lead dress first and away the squeal went. Caveat: I might have changed valve at the same time

Have to say that although it's occasionally fun to dime both controls, the tones I like best are with the cascade (gain) pot between 9-12 o'clock and the 18W volume pot dimed. Still cleans up nicely with the guitar volume and gives a good range of clean through edgy blues to classic rock tones (with an LP). So a divider before the cascade pot might be a good idea anyway...

Thanks for the info on relays - will plan it into a future build.

[zaphod_phil]

Yikes. First posting and you're already confessing to putting clipping diodes in the venerable 18Wer!

Yes, those diodes kill the benefit of having a tube amp, and it's tube overdrive characteristics! You also loose the tube amp's lively dynamic fell. Is it any surprise that the most popular mod to the Marshall JCM900 is to remove it's diodes. The cascaded boost is a much better idea.

[loverocker]

To be fair, though Phil, if you already have a pure valve amp circuit that sounds great, getting some extra tones out of it by adding bypassable diodes (or pickled eggs for all I care) is a great idea. I can see no downside. At all.

As I've said more than once when valve amp fans get a bit preachy: "Let he who is without diode cast the first stone" And that includes those guys who claim to love the sound of valves, but insist on plugging into TS9s, Guvnors, V-Twins, etc.

[zaphod_phil]

That depends on where in the signal chain you put the diodes. They may be OK out in front where the signal levels are very low, so you can clip without taking away all the dynamics of your guitar and the following amplifier. In the middle of the preamp somewhere it's not so smart. You'll just get a very buzzy distortion - which is fun if you like the 'wasp in a jam jar' tone added to your 18W amp's capabilities. Ever wondered why the JCM900 is so widely (though not quite universally) disliked? And BTW I don't use those particular pedals you mentioned, either....

[loverocker]

Wasn't accusing you, Phil! But all of those FX use solid-state amplification stages *before* the diodes to boost the signal enough for the clipping to do its stuff. Even Marshall's much-loved JMP-1 does the same. As do the 2205/2210... and the Silver Jubilee... Gary Moore's "Still got the Blues" tone is a JTM45 according to Marshall's advertising for the Reissue (though they conveniently forget to mention the tone is 90% Guvnor).

Of course, one man's wasp-in-a-jar is another's metal crunch tone (esp when EQd to control the top end). Remember - there are people out there who like the FuzzFace

The JCM900 is widely disliked because today, people buy it hoping to get non-MV Marshall tones/response and can't. This is an indicator that it's a *far* better idea to take a great non-MV design and add a diode clipper (switchable, natch) to it than to remove the diodes from a circuit designed with them and hope it'll sound OK.

Anyway - do your pedals have diodes or not

[zaphod_phil]

Well, let's look at it another way - it's making a great valve amp sound like one of those cheapo transistor amps they sell to kids for Christmas. This is a free(ish) country, so if anyone really seriously wants to do that, good luck to them! Of course everyone prefers the sound of a Plexi Marshall to a dioded-to-death JCM900 - 'nuff said! The simple reason Gary Moore use the Bluesbreaker (not Guvnor) pedal is to boost the signal level into the JCM800, which doesn't have sufficient gain on it's own to hold a nice singing lead guitar tone (mainly due to that unbypassed 10k cathode resistor)...

Anyway - do your pedals have diodes or not

And no they certainly don't - at least not anywhere in the signal chain...

[pedro]

Interesting stuff - will try not to spark off such contentious debates with future posts !

Lets just put it down to search for additional versatility - having owned Park 1210 Rock Head, Silver Jubilee 1x12, Ampeg Reverborocket, Kitchen Marshall 100W PA Head ( military style green rack mount beast) , Champs, and current Anniversary I know the range of tones and I just wanted to see if clipper worked for some additional tones when needed. ( remembering that some of the most legendary tones on record were actually straight into desks or compressors without amps !!!

I'm probably a fairly pure tubeophile though (having also DIY'd SE 300B stereo and OTL headphone amps) so I can appreciate where the purists are coming from...

The clipper "appendix" will stay - but only for rare recording tone occasions... I do just need to get the cascade working well though...

Also I'm more concerned with getting the amps noisefloor and slight squeal down to minimum and still have a lot to learn about grounding placements, lead dress and component placement. All advice greatfully received. Any useful links to articles etc ?

PS I love this amp in its raw form anyway ....

[loverocker]

Yeah - the plain 18W circuit dimed is great I have a love-hate thing with valves. I'd only want a valve hi-fi for cosmetic reasons On lead dress, I had nothign special, just the usual routing of inputs away from outpus and shielded cables from both the jacks. Dunno if that helps

Phil - Re Gary Moore - it was a Guvnor (mk 1). From the horse's mouth: http://www.gary-moore.com/The_Warehouse ... swers.html I've got two, and they do exactly the SGTB tone (and a few others). I think GM is commendable for using "whatever works" tone-wise - he's used digital modelling stompers and even the Pod in the studio.