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Old 10th March 2007, 05:17 PM   #1
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Default Slightly Different guitar amp question

I am pretty good with this tube stuff, and I've been able to successfully build and modify several different hifi/and guitar amp circuits. I'm in the process of working something out.

I have figured out that I like the sound of a 6v6 power amp I have.

What I do not understand is all of the impedence matching stuff.

I used the preamp of my blues Jr amp to drive the 6v6 PP power amp I have, and it is fairly loud, but not really that loud. I think that it sounds much better than the El84 power amp that the fender came with.

My intention is to build an entire new circuit, but I am wondering about how to make it more powerful. The power amp is not going into distortion at all, if it is, its minute. Can I add a driver tube before the phase splitter to boost overall output?

If so.. Can I just duplicate one of the other 12ax7 stages? It seems like this would work, but I do not know how to implement it.

Aaron
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Old 12th March 2007, 08:41 PM   #2
Merlinb is offline Merlinb  United Kingdom
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While a 6V6 is a little harder to overdrive than an EL84, it should still be well within your means. What 6V6 amp are you talking about, and how do you mean you "used the preamp of the Blues Junior"?

If you're talking single ended, even two stages of an ECC83 cascaded should be easily capable of giving you appreciable overdrive, and three stages plus cathode follower would be hard rock territory!
If it's push pull then you'll probablybe talking about a long tailed pair driving the power stage.

I think you need to be a bit more specific in your request.
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Old 12th March 2007, 09:31 PM   #3
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I appologize for not being more specific. I had previously (recently) posted about this amp i'm building, and I've sort of switched gears. You may want to take a look at that thread:


But, I will explain what I did here.

I have an old, probably 50s or very early 60s mono p/p 6v6 hifi amplifier. The brand was Electrohome which I found out is a Canadian make. It was pretty high quality, from the looks of things, separate power supply, and big iron. It used a 12ax7 phase splitter, and had a 12ax7 and 6av6 in the preamp.

The preamp was certainly setup for hifi, the tone controls and whatnot were not sufficient for a guitar amplfier.

I first tested this with a CD player and a turntable.. I bought it with the intention of turning it into a guitar amp though. IF... I could have had two, I'd have used it for hifi, it's that good.

I then hooked the amp up to my Marshall 4x10 cabinet, and plugged a guitar into a line section, and it was what you'd expect, weak. Then I plugged into the phono section. This was the old type that had something like 8 or 10 different EQ settings for the phono, and one was a non EQ'ed setting. It sounded ok. Not a lot of power, but a major amount of gain, tons of overdrive. Almost a Marshally sound.. nothing like a fender.

So I gutted out the preamp and never got around to finishing it.

I never bothered to draw the schematic of the preamp, becuase I wasnt going to use it, having had a very lucky guitar preamp schematic that used a 6av6 and a 12ax7. Perfect, i wouldnt have to drill holes or anything. Then my computer crashed and I lost it.

So now i'm looking for a suitible master volume preamp to use with this 6v6 power stage.


As for using the blues jr as a preamp.. I disconnected the preamp at the phase splitter, and installed a .022uF cap and connected it to the phase splitter of this amp, and it sounded fairly good. But there is a lot of SS stuff in the blues jr, and I'd like to just come up with some, decent master volume preamp with a fair amount of gain.

I hope that clarifies, if you have any further questions, I'll try to answer them.

Thanks,

Aaron
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Old 13th March 2007, 12:42 AM   #4
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I forgot the link to the previous thread:

Master Volume preamp.. simple?

sorry about that!

Aaron
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Old 14th March 2007, 04:39 AM   #5
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Hi akimball, I noticed your thread a few days back, and was hoping you'd get a lot of good answers.
But basically, look at the classic guitar amp schematics and find a preamp stage you like, using equivalent tubes you have (for example the 6AV6 instead of the 12AX7, etc.). That would create one extra socket.
But you should be able to chain them, 2 or 3 sets of dual triodes, and try to keep the pair (2 tube elements) if using a 9-pin tube, together.

As far as a basic volume control: this can go anywhere, unless it's between a stage wth feedback. Usually there's a volume control early on, going into the first grid just after the jacks, and you won't need to worry about another again until after the tone stack., a 1Meg or 500k audio. You can add more later. The "master" generally refers to the one just after the tone stack and before the phase splitter.

Now, basically, as far as impedance goes, all pre-amp stages done the normal way will give a high impedance. You are only concerned with having a lower impedance when your output is going into the tone stack. For this you can use a preamp stage that has feedback, like the pre-amp I mentioned, but I think that's more a hi-fi tweak and you could do without it for guitar, but it does increase gain a bit and lower the output impedance coming out of it so you would not need a later cathode follower. or else use the second triode of the tube as a cathode follower. This is obvious on the fenders, early marshall circuits etc., but that's to help the tone stack have better response. Then the output of that will go into the phase splitter to drive the final tubes. I can post a few clips if you like.

So I would suggest: a dual triode first pre-amp stage (12AX7, 12AY7-quieter, or 12AV7 - quieter but a real cheap NOS tube)
Put an input volume control on this, at the very first grid. Then do a second stage (two triodes) in series with the first, the same way.
Then do another 12AX7 wired with the first stage as a preamp and the second as a cathode follower. See the classic schematics.
Put the tome stack after this.
Then, just after the tone stack, at the output of the treble control, you can run a "master" 1Meg audio pot form that signal to ground, and you should be good to go, for going into phase splitter, etc.
Hope this helps
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Old 14th March 2007, 12:41 PM   #6
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I am learning!

In that amplifier I mentioned, which has become my test-bed for learning a lot the last few weeks the 6av6 is placed directly before the phase inverter, which is a 12at7. The volume control was directly after the tone stack.

In front of the 6av6 was the tone stack, which was preceeded by a 12ax7. The switch was one of those typical 50's 60s nightmares with phono EQ and whatnot. I got into this and really looked at it.

From what I can tell, line stages were fed directly into the second stage of the 12ax7, and the phono stages were fed into the first stage, and all EQ for the phono was done between the stages of the 12ax7.

I was originally feeding the signal directly into the phase splitter, but i decided to connect the 6av6 back to the phase splitter the way it was, and feed a signal to that.

WOW. what a lot of gain. I plugged my telecaster right into the 6av6 (just for the heck of it). No tone controls, no level control.. except what was on the guitar, and it was really an amazing tone. Full tilt on the guitars volume control gave a pleasant overdrive.

Of course a tone stack will reduce gain by a long shot, so the other tubes are needed.

My guess would be that the signal I was feeding this amp from the blues jr's preamp (disconnected at the PI) was the wrong impedance. Because with all of those stages, even though it sounded ok, wasnt half the gain I got just plugging into this thing with one driver tube.

I'll keep you posted. It's interesting, and coming along.

I've also decided to build this monster on a new chassis, I work in a shop with all the tools needed to do it, and have someone there who is more than willing to give me a hand building the chassis. Now to decide which metal to use Probably steel... although we do have aluminum.

Thanks..
Aaron
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