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Old 17th October 2012, 02:39 AM   #1
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Default Help needed on a DIY guitar amp

Newish guy here with a few questions and doubts. After checked a few schematics of some low budget around 5W SE guitar amps (like Peavey Valve king Royal8, Marshall Class5, Epiphone Valve Junior,TT-sam-markII) i decided to try to build myself an amp using 2 EL84 (or EL90 which is 6V6 in a smaller package) in parallel instead of only one EL84 hoping to get out a bit less than 10W. So i combined some sections of already working amps toghether and i would like you EXPERT FRIENDS to have a look and tell me if it's doable and what's wrong (i'm sure you'll find a lot of mistakes).
At least this is how i immagined it could be.

By adding one output tube in parallel the OT should be easyer (and cheaper) to be done as the impedance needed would be of 2.6k Ohm instead of 5.2 (if i understand correctly). Correct me if i'm wrong - in theory the signal could be devided to op to 4 EL84 in parallel, but i guess some buffers should be put to provide strong signal. As i marked on the schematics i made, the position of the buffer(s) should be just before the output tubes or am i mistaken?

I also marked possible positions of tone stack but IMO it should be put in J1 position right after the first signal amplification of ecc81 - reason for that is the loss of signal of about -20dB, which is quite a lot, but the following stages should be able to re-gain the lost signal?

All B+ voltages are subject to change if needed (everything is still only on paper). Feel free to modify if you wish. Any good suggestion would be greatly appreciated!

Thanks in advance for your help!!!
Attached Files
File Type: doc Guitar amp 2xEL84 parallel SE.doc (81.0 KB, 38 views)
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Old 17th October 2012, 03:36 AM   #2
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Could you post it as a picture file?
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Old 17th October 2012, 07:54 AM   #3
Enzo is offline Enzo  United States
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Agree^^^^

I myself prefer the pdf format.

Not sure what your transformer difficulty might be. A single tube? One option is simply to buy the transformer from Peavey or whoever. It works in their amp, why wouldn;t it work in a single output tube amp for you? Peavey, for example, sells their parts to anyone direct, and their prices are reasonable.

As to the difference between 5w and 10w output, the difference is not much, 3 decibels if all else is the same.
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Old 17th October 2012, 08:04 AM   #4
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yea difference between 5w and 10w won't be much depending on what speaker you will be running and its sensitivity 5watts can still be loud as hell lol
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Old 17th October 2012, 10:44 AM   #5
sreten is offline sreten  United Kingdom
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Hi,

I can't see why the output transformer would be easier or cheaper.
The right impedance might be more difficult to find, and of
course it needs twice as much iron in it for 10W versus 5W.

rgds, sreten.
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Old 17th October 2012, 11:52 AM   #6
tinitus is offline tinitus  Europe
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the way you paralel el84 does not look right
looks like you have just doubled a simple SE output

but it should not be hard to find a good schematic
and I guess 6V6 is a very good guitar tube

tried a quick search, and nice looking Angela super-single-ended came up
and on Lone Wolf site an interesting 3x6V6SE
(but more tricky with the fixed grid bias)
but also have an easier 2x6V6SE
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Old 17th October 2012, 12:15 PM   #7
sreten is offline sreten  United Kingdom
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Hi,

FWIW you will be lucky to get near 10W out of an EL34 on a good
day, single ended with optimum bias and a really good transformer.

Two EL84's are near universally used push pull with some class A
bias for about 10W clean, not sure about overdriven power output.

The transformer for this will be a lot cheaper as there is no DC
current required to be supported as in the single ended case.

Similarly the less stressed mains transformer can be smaller.

rgds, sreten.
__________________
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 17th October 2012 at 12:29 PM.
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Old 17th October 2012, 04:04 PM   #8
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Hi to all,

Yes i was wrong saying it would be easyer and cheaper to make an OT like that for you do need to double the iron and yes, it would be much easyer to put the two EL84 as push-pull and get rid of DC component. But my reference schematics was the attached TT-sam-mkII, and i just thought of modifying it IF possible.
What about the rest of schematic (values of resistors, position of tone stack)?

I'm having second thoughts...maybe my idea is totally wrong and is not worth even trying...
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Old 17th October 2012, 04:10 PM   #9
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sorry my attachments did not go through
Attached Files
File Type: pdf tt-sam-mkii.pdf (334.0 KB, 23 views)
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Old 17th October 2012, 04:10 PM   #10
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One real cool single ended amp OT that hammond makes is a 125 ESE. Multiple primary impedeance taps, And you can use it for several different projects till you find what you like.
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