18 watt Marshall type circuit.

[guitardude57]

I have a bud that has an Avatar 18 watt amp head, and is dissatisfied with the tone control
on the one channel that is not tied into the TMB circuit.
Has anyone messed around with some component values that would sweeten up the thuddy midrange... or maybe tied that channel into the TMB?


And if anyone has a schematic of the Avatar 18 watt....it would be awesome.


Thanx

[silverfox]

They may not have followed Marshall’s schematic and changed some preamp bias values or even the design. Have you compared schematics of the two amps in question?

silverfox.

[jojokeo]

I have a bud that has an Avatar 18 watt amp head, and is dissatisfied with the tone control
on the one channel that is not tied into the TMB circuit.
Has anyone messed around with some component values that would sweeten up the thuddy midrange... or maybe tied that channel into the TMB?

Thanx

Yes, IMHO change it to the standard Fender Tone control instead.

[guitardude57]

They may not have followed Marshall’s schematic and changed some preamp bias values or even the design. Have you compared schematics of the two amps in question?

silverfox.

I am looking for a schemo...I don't have the amp yet...later this week.
I was just trying to do some research to get a leg up.

[guitardude57]

I have a bud that has an Avatar 18 watt amp head, and is dissatisfied with the tone control
on the one channel that is not tied into the TMB circuit.
Has anyone messed around with some component values that would sweeten up the thuddy midrange... or maybe tied that channel into the TMB?

Thanx

Yes, IMHO change it to the standard Fender Tone control instead.

I believe he is happy with the channel with the TMB... he likes to A/B switch the 2 channels, and the one with the single tone control...he is not happy with.  I'm sure he is not going to want to add any further controls on the amp.  I am thinking maybe replace the tone control pot with a switching pot, and maybe stick a couple different value caps on it... and/or once I see the topology, maybe swap the cathode cap on that triode, if there is one...lol

[sluckey]

Here's an option I really like... 18W TMB + EF86. The EF86 channel is basically an AC-15 normal channel circuit. Very different sound from the TMB. I like the AC15 sound a lot.

http://www.ceriatone.com/images/layoutPic/marshallLayout/TMBEFceriatone.jpg

[guitardude57]

Thanx Steve,


That looks like an interesting version with the 86 in there.
The Avatar is a 3 bottle preamp though.  V1a is one channel, and V1b is the other. 12AX7.
Don't know as of yet how much he wants to alter his amp, other than component values and maybe a switch...

[guitardude57]

Yes, IMHO change it to the standard Fender Tone control instead.





Just looked over the 5e3a drawing, I will see how far or close the Avatar is from that.
That amp channel does have some meat to it.

[jojokeo]

You will get more gain and treble response with the Fender tone control. The Marshall is not as bright, more apparent bass when dialed down, and not as much gain. It's an easily reversible mod that's fast, effective, no need to change the component board. I'm pretty sure you won't be reversing it once you both hear & play it?

[guitardude57]

I believe you are right there... once I see what is there to begin with,
it will be a quick change.  The 5e3a tone circuit will be an easy mod.

Thanks man!

[dude]

http://www.ceriatone.com/images/layoutPic/marshallLayout/TMBEFceriatone.jpg

What's with the two diodes on the EZ81?

[Paul1453]

http://www.ceriatone.com/images/layoutPic/marshallLayout/TMBEFceriatone.jpg

What's with the two diodes on the EZ81?

Safety feature in case the tube shorts through, no smoked PT.
Search the forum for more info, it is there.   

[mresistor]

Can anyone provide a better explanation as to why one would use two 1N4007 diodes on the plates of a rectifier? Is this just for EZ81s or other rectifiers too?

[sluckey]

It works for all tube rectifiers. Let's say one of the rectifier plates shorts to cathode. The PT is not in immediate danger but the filter caps are because now they will have AC on them. The negative half cycle of the AC will destroy the first filter cap, and probably the second cap. If the caps should fail shorted then the PT is at risk also.

Now if you have a diode in series with the tube plate, a plate to cathode short becomes a moot point because the diode still prevents any negative half cycle AC from reaching the filter caps. It's got yo back so to speak.
 

[HotBluePlates]

Can anyone provide a better explanation as to why one would use two 1N4007 diodes on the plates of a rectifier?

EDIT: I typed a longer answer while Sluckey was posting. But yes, this trick applies to all tube rectifiers; the solid-state pre-rectifies the PT voltage so the tube adds sag/voltage drop, but can damage anything should it short plate-to-cathode/filament.

[mresistor]

Thanks guys. Are there any factory built guitar amps that employ the diodes on the rectifier plates, I mean off the top of your head? 1N4007s are so cheap, one would think many would do this. Cheap insurance is a bargain. So why don't we see this employed more often? Interesting that Ceriatone does this. Sluckey, say we have a rectifier plate to cathode short , have you ever seen what happens? Does the cap explode?

[HotBluePlates]

Thanks guys. Are there any factory built guitar amps that employ the diodes on the rectifier plates, I mean off the top of your head? ...

I can't think of any off the top of my head, but I don't have every amp from every manufacturer memorized either. Gerald Weber recommended this addition since at least the late-90's.

... 1N4007s are so cheap, one would think many would do this. Cheap insurance is a bargain. So why don't we see this employed more often? ...

A manufacturer would probably hope/assume/know the fuse would protect the amp in the case of a rectifier failure.

But that's not why Weber suggested it: What happens if you're playing a show and the rectifier fails? Okay, the fuse pops and you put another in and that pops. Got another rectifier handy? Even if you do (and most won't) how long does it take to do the swap & replace the fuse? What if you're in the middle of a song? What's the embarrassment worth to you?

So Weber was thinking of this from the standpoint of bullet-proofing the amp so it won't fail during a performance. And you might not even know your tube rectifier failed unless you hear the missing sag.

On the other hand, a manufacturer might only look at how many complaints they get (in the form of warranty repairs) for a shorted rectifier. It's a "user replaceable part, manufactured by someone else. And their fuse may be sized to prevent damage to the PT. So adding the diodes may not make economic sense to them after building the 10,000th amp with those diodes.

Thinking about this some more, I can't say I've seen any guitar amp using fixed bias which has some mechanism to protect the amp in the event you lose bias voltage to the output tubes. Could be as simple as a socket with contacts loosened over years of use. It would probably take some form of solid-state servo circuit to monitor tube current & understand whether proper bias is present. So parts & engineering required for a failure mode which is low risk/high impact, but very unlikely to happen within a warranty period...

... Interesting that Ceriatone does this. ...

Well, Ceriatone doesn't "build amps" as far as I know. They draw layouts and sell kits. And they can incorporate anything that the buyer thinks is necessary, maybe because the addition was discussed a lot in books/forums.

[mresistor]

Well I can't say that I've ever had a rectifier fail on me. I've recently had some JJ 6V6 that went pop pop pow! and I think damaged the rectifier in the process. But the iron was unharmed. Thanks for the toughtful reply HPB. What a concept, a rectifier with a backup rectifier.

[sluckey]

Sluckey, say we have a rectifier plate to cathode short , have you ever seen what happens? Does the cap explode?

I've seen it only once, back in '67 or '68. It happened to another band member while we were playing. The amp was a combo Sunn Solarus. 5AR4 went sparky. Filter cap can shorted (did not explode). Killed the PT. The line fuse finally blew, but it was too late. I did the repairs on the amp. There was no way to prove if the tube went first or if the cap went first. But I had to replace the PT, cap can, tube, and fuse. Nothing else was wrong with the amp.

[Paul1453]

Sluckey, say we have a rectifier plate to cathode short , have you ever seen what happens? Does the cap explode?

I've seen it only once, back in '67 or '68. It happened to another band member while we were playing. The amp was a combo Sunn Solarus. 5AR4 went sparky. Filter cap can shorted (did not explode). Killed the PT. The line fuse finally blew, but it was too late. I did the repairs on the amp. There was no way to prove if the tube went first or if the cap went first. But I had to replace the PT, cap can, tube, and fuse. Nothing else was wrong with the amp.

That was quite an expensive repair, compared to a couple of cheap diodes. 
Having learned about this here, I don't think I will make another tube rectifier circuit without those backup diodes.

[jojokeo]

Talk about a timely subject. Over the weekend I was looking for a drawing or schematic showing the diodes installed for an either/or type of situation- but not put on a switch. I thought I remembered that they could simply be soldered across the recto socket pins and one could choose to run a tube or remove it and the diodes assume the job?

I searched the heck out of the forum without any luck. It's too late for me anyway cause I removed the whole damn tube & socket anyway. For my situation I was hoping for a nice voltage increase on a bad wreck clone re-hab & renovation brought to me but didn't get it. The main positives that came out was losing the tube recto (not used on a wreck anyway) for a stiffer supply, re-wiring of the entire triple cap can filters, & new badly needed grounding scheme.

[MFowler]

Here is an example.

[HotBluePlates]

Sluckey, say we have a rectifier plate to cathode short , have you ever seen what happens? Does the cap explode?

I've seen it only once, back in '67 or '68. ... 5AR4 went sparky. Filter cap can shorted (did not explode). ...

I'd think it would be hard to blow up a can cap, what with the thick aluminum shell and phenolic endplate.

However, when I took my first electronics class ("this is a resistor. This is a cap. Here's how to solder...") the teacher showed us what happens to a polarized cap when a.c. is applied. He clipped the ends of a 16v electrolytic to the bare wire ends of a power cord, then plugged that in an outlet (after we had our goggles on). Blew up like a small firecracker, and sprayed little bits of metal & plastic everywhere.

I think it's easier to blow up a smaller electrolytic with a rubber endcap and/or thinner metal sidewalls.

... Over the weekend I was looking for a drawing or schematic showing the diodes installed for an either/or type of situation- but not put on a switch. I thought I remembered that they could simply be soldered across the recto socket pins and one could choose to run a tube or remove it and the diodes assume the job? ...

You'd need a plug-in rectifier replacement which was bare wires from the tube's plate pins to the cathode pin(s). The added diodes we're talking about don't span from the high voltage a.c. input to the output to the filter cap. You'd also like to do this on a rectifier socket which has an actual cathode not connected to the 5v/6v winding.

Better idea would be a simple solid-state plug-in which has diodes spanning the socket pins I just noted. That is, if you just can't bear having a switch.

I say all that because if you simply adding the solid-state diodes across the rectifier socket from plate to cathode, charging current should flow through the lower resistance path through the solid-state, even if you have the tube rectifier in the socket. So you'd never get any sag/voltage drop.

[jojokeo]

Thats what I was thinking too HBP and switches get flipped either on purpose or not. But when running fixed bias it may not be a smart decision especially when chassis is hanging tweed style in the cab.

Reminds me of when an aftermarket alarm system was installed in one of my cars. When flipped it would go into auto-lock mode. It was down low sort of under the dash mounted toggle down. Well at the car wash one day the guy hit it while wiping things down prior ti the wash and after the car cane out the keys were locked inside. Pretty funny as they has mire cars bearing down on them finishing the wash too. All hands on deck were summoned like a cinese fire drill. 

[drew]

R.G. Keen article about the diode mod: http://www.premierguitar.com/articles/the-immortal-amplifier-mod-1

Doesn't really add anything to the above posts; just linking it because I happened to have it open in another tab in the browser.