Another "spare triode" idea - Vox normal channel

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potatofarmer
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Another "spare triode" idea - Vox normal channel

Post by potatofarmer »

I've searched but I don't think I've seen anyone do this. I have been converting a Crate Steath combo into a rocket-inspired amp, and I kept seeing people talk about how great the "normal" channel is on AC30s. It's out of phase with the 'top boost' channel, so I just ran it into the (normally-grounded) second grid of the phase inverter.

Which, of course, is what the AC30 did with the reverb/trem channel.

I bridged both 'channels' at the input jack so you can use either or blend them just by adjusting the volume controls. I've been considering adding switching to ground out either grid, but something about a channel-switching Rocket just feels so wrong.

Anyway, it gives you a whole bunch of new tones, and you still get plenty of overdrive with just one gain stage before the PI. Hit it with a treble booster and get your Brian May on!

Using both 'channels' at the same time really fills in the mids. Ups the gain a fair amount too which isn't necessary, but what the heck.

Apologies if this has been done thousands of times before, it just seems so obvious but I haven't found any mention of it. I'd have clips but an old power tube shorted on me and took out my power supply and possibly the Crate OT as well. :(

On an unrelated note, an Antek AS-1T250 does have plenty of juice for a Rocket. At least a solid-state rectified one pushing a quad of 6V6s! :lol: There was some speculation a couple years back here that it wouldn't be enough, but the math looked like it should and I can confirm experimentally that it works just fine. B+ doesn't hardly sag at all, though people might consider that "oversized" for a guitar amp, I'd rather heat up a sag resistor than a transformer. :)
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rooster
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Re: Another "spare triode" idea - Vox normal channel

Post by rooster »

Yes, I've done this and also added an EF86 for the AC30/4 flavor. All good. But in my case, I did not add the other channels to the PI as you did. I'm not into multi-channel gain in this amp I guess. But if you like it, cool.

Remember, too, that as you add the Normal Channel the gain of the TB Channel is reduced by almost 3db. Which is to say this is not a stock Rocket at that point. Here I added a switch to lift the connection to the PI - but, in fact, I like this channel left in.
Most people stall out when fixing a mistake that they've made. Why?
potatofarmer
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Joined: Fri Dec 02, 2011 2:54 am

Re: Another "spare triode" idea - Vox normal channel

Post by potatofarmer »

Yeah, that slight drop in volume is what was making me consider adding a switch to ground (wrt AC at least) for at least the normal channel, if not an on/off/on SPDT to pick. Plus, then it could go back to stock also. I really do like the bridged tone, but yeah it is a noticeable departure.

Now you've got me wondering what trouble I can get into with these 6GH8's...
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rooster
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Re: Another "spare triode" idea - Vox normal channel

Post by rooster »

Yeah, if you use a SP/ST switch you can just lift the 220k resistor of the Normal Channel and get what you need. I'm actually using a switch type like the Bright Switch on the TW amps for switching between the EF86 Channel - the OFF position - and the Normal Channel. I just attach the 220K resistor to the switch connection (one resistor for each channel) and the center position goes to the PI coupling cap. I really thought there might be some popping or such but it's completely quiet as it switches between all three positions.

Funny thing here is that I was always under the impression that the EF86 Channel would be hotter than the TB channel. It's not. In fact, it's VERY similar to the Normal Channel gain. But I really wanted the three distinct channels of the AC30 versions and I do have that. Haha, Ken knew the TB channel was the one.

Nice to meet you, BTW. :)
Most people stall out when fixing a mistake that they've made. Why?
potatofarmer
Posts: 50
Joined: Fri Dec 02, 2011 2:54 am

Re: Another "spare triode" idea - Vox normal channel

Post by potatofarmer »

rooster wrote:Yeah, if you use a SP/ST switch you can just lift the 220k resistor of the Normal Channel and get what you need. I'm actually using a switch type like the Bright Switch on the TW amps for switching between the EF86 Channel - the OFF position - and the Normal Channel. I just attach the 220K resistor to the switch connection (one resistor for each channel) and the center position goes to the PI coupling cap. I really thought there might be some popping or such but it's completely quiet as it switches between all three positions.
Pretty clever way to switch 'em. Course, you don't get to blend them but you could just sidle that switch over to the other grid... :D
Funny thing here is that I was always under the impression that the EF86 Channel would be hotter than the TB channel. It's not. In fact, it's VERY similar to the Normal Channel gain. But I really wanted the three distinct channels of the AC30 versions and I do have that. Haha, Ken knew the TB channel was the one.

Nice to meet you, BTW. :)


Likewise!

Yeah, you'd think pentode amplification factor of 200 or whatnot would be insane but it's still less than 60 * 60 = 3600! I do love the huge sound of preamp pentodes though, and supposedly if you keep the A.F. under 100 microphonics don't become such a huge issue. Kinda skeptical on that point myself. Was the bright cap the added mod they made to the TB model to try and recreate the EF86 sound?

That TB channel is the winner, no doubt. Every once in a while though, the OD you can get by pushing the normal channel is great. I can't imagine selling an amp with just that normal channel though. Even the Tweed addicts would be disappointed with the lack of versatility. :P
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