EL84 and 6V6 in parallel??? - diyAudio
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Old 5th April 2010, 06:58 PM   #1
youngb4 is offline youngb4  United States
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Default EL84 and 6V6 in parallel???

Hello everyone,

I've been working on some simple single ended guitar amp designs over the past few months and I really want to try running a 6V6 and an EL84 in parallel for the power amp section! I've drawn a rough schematic and attached it to the post, so can someone take a look real quick and let me know if I'm on the right track and if this can be done?! I know I'll need an ideal primary impedance of 3000 ohms on the OT, but the hammond 125ese can be had with a 2500 or 5000 primary so that should be close enough right? As I said before the schematic is purely conceptual, I haven't breadboarded anything yet, I just want some people's opinions on whether this can be done with no issues and also if there are any recommended changes to my design.

Thanks for your help and comments in advance!
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Old 5th April 2010, 07:50 PM   #2
el156 is offline el156  Portugal
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hello there !
I think the 6v6 and el84 in parallel is not a good idea, because to parallel tubes ,they should be the same type and matched , to have the best results.
Good luck
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Old 5th April 2010, 08:02 PM   #3
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This is for a guitar amp where distortion and sonic signature rule. Certainly they would not share the load well but thats the idea right? I would try it. Worse thing that will happen is to have a little magic smoke escape.
The only thing I would do is kick up the value of the bypass caps unless you want them to add to your distortion characteristics and you don't need the gain.
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Last edited by SGregory; 5th April 2010 at 08:05 PM.
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Old 5th April 2010, 08:15 PM   #4
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Hey,

actually there are commercial geetar amps that put dissimilar tubes in parallel, though I've only seen push pull so far. Mesa Boogie for example, 6L6+EL34. The mismatch in plate resistance between one of the tubes is a sonic feature for guitar, rather than a 'problem'.

So I would suggest just to try it out!

Kenneth
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Old 5th April 2010, 08:46 PM   #5
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by youngb4 View Post
Hello everyone,

I've been working on some simple single ended guitar amp designs over the past few months and I really want to try running a 6V6 and an EL84 in parallel for the power amp section!
In your schematic the tube with the lowest impedance will carry nearly all of the load and the other will just be there and look cool but do little else.

You will need to put power resistor in series with each tube where each resistor is about the impedance of the other tube. This will cut the power of the amp in half and wate a lot of heat but then each tube will be sharing the work equally. Also each output tube will need it's own grid resistor
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Old 5th April 2010, 08:48 PM   #6
cerrem is offline cerrem  United States
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In Parallel, both tubes will become a new "effective" tube...
The transformer sees this "effective" tube as a single unit...it does not care ....
These are not BJT's where you have to worry about Vbe and other matching issues... SO you will not get into the whole current sharing or runaway issues you get with PTAT devices...
You may want to try cap to cross couple the cathodes so they are joined in the AC sense but still have DC isolation for independent biasing... This is if you want to use the feedback of the cathode resisitors, if not , then stick with the independent ones you have now...

2K plate load should be close to where you need... Going to high a plate load will cause much earlier square waving and kill the power output and loss of headroom will occur from the load line intersecting the 'triode" region of the curves, when it should be just to the right of the knee of the 0-bias curve, thus staying in the saturation region of the curves, therefore maintaining the plate resistance from significantly droping off...

Best Regards
Chris
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Old 5th April 2010, 09:31 PM   #7
youngb4 is offline youngb4  United States
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Wow thank you everyone for the quick responses! I'm glad to see that the general consensus is "go for it"!

cerrem - can you please explain what you mean by " You may want to try cap to cross couple the cathodes so they are joined in the AC sense but still have DC isolation for independent biasing... This is if you want to use the feedback of the cathode resisitors, if not , then stick with the independent ones you have now... " Although I've been using tube guitar amps for several years now I'm kind of a newbie to tube audio design. What advantages/disadvantages are there to what you suggested doing/my original design in regards to the sound the amp will produce when amplifying my guitar? Is what your suggesting simply having both cathodes share a single resistor and bypass cap?

ChrisA - What values should I use for the grid resistors that you suggested?

SGregory - How much do you think I should increase the values of the bypass caps? I just used what was common in some single ended amps that use each of the tubes (6v6 or el84) in the power section.

Once again thank you for all the replies!
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Old 5th April 2010, 10:25 PM   #8
cerrem is offline cerrem  United States
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When it comes to guitar amps...do whatever you want to make it sound good, so long as it is safe... The classic guitar amps were "monkied" to get working...there were no scientist or engineers doing any hard core analysis or design process...It was all clue-less mishaps and dollar signs in the eyes of these amp companies that just copied from Leo's stuff... 90% of todays amps are all just a mix of copied circuits, copied effects loops, copied master volumes, copied gain stages with little or no engineering behind it..
As far as paralleling tubes... the first order overly simplistic way to see this as two current sources in parallel working into the load... The transconductance of the tube makes it a non-linear voltage dependent current source..
Both tubes contribute current into the load, thus the current from each tube sums to make one contribution into the load...
Looking at this in a more realistic manner, the tubes are not ideal current sources and therefore contribute some some load to the other tube in parallel to the transformer load... So you have a current divider effect...
But this is OK, since the plate loading of one tube in parallel with the transformer load still constitutes a valid load for the other valve to work into..
El84 figure roughly 38k Ohms of plate resisitance and the 6V6 at roughly 50K ohms in your operating range....
Now figure a plate load of 1K ohms ...... Since a 2K plate load is close to optimum fit for each valve if it were independent Single-Ended amp... Since you have two valves in parallel, thus your plate load will half as well..
A 38K ohms current source in parallel with a 50K ohm current source in parallel with a 1K load.... Give me a break, there is not much current divider action going on between the tubes, 99% of the sumed AC current will contribute into the 1K Plate load... This of course refering to mid band performance...

Best Regards
Chris
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Old 5th April 2010, 10:47 PM   #9
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Youngb4,
20uF will work but values such as 220uF or higher are not uncommon at that value of cathode bias resistor. Experiment to get the sound you want.
This amp sounds fun! At first go I wouldn't worry about feedback too much. Make that version two. It can be rewired and added later if this concept pleases you.
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Old 5th April 2010, 11:15 PM   #10
youngb4 is offline youngb4  United States
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Yes i cant wait to start tinkering and experimenting with it! and im still going back and forth with the pre amp! Im thinking ef86 to TMB tone stack to a couple of 12AX7 gain stages, of course i wont know how that sounds till i get all my parts sourced and get a lil more free time on my hands!
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