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Old 14th January 2007, 10:22 PM   #1
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Default Favorite easy 6V6 push-pull circuit?

One of my questions lately too, besides others, does anyone have a great link to a DIY project or schematic or brand/name/model of any great amp using 2 6V6's in push-pull, with some audio control pots, etc for a decent guitar amp?
I'm seeing at lot of circuits on places like Duncan's & MachMat, etc, but what's someone's real fave for sound simplicity, etc. ?
Don't need quite the power of 6L6's, and would like to keep the project more retro, and use a single Jensen P12R speaker as well.

From all my schematic-searching travels, this one seems easy, diodes for the power supply, a single 12AX7 and two 6V6GT's, and it's an original Fender schematic, a Musicmaster Bass produced from 1970-1982.
The only new thing to me is the 026847 interstage audio transformer, is this unique, or are there other items that will do for this?

Also, any critiques on the design would be appreciated, I'm hoping to use it for audio / treble guitar as well, just mostly as a fun project.
Do you think this is a good one to try?


Click the image to open in full size.

Also if this seems good to go, can you suggest any simple easy mods for more audio control, like "bright", etc....

Thanks
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Old 15th January 2007, 01:37 AM   #2
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Wow!!! transformer coupled, open loop pentode amp...I'd bet that is one sweet amp for lead 'tar work.
cheers,
Douglas
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Old 15th January 2007, 03:05 AM   #3
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Hi FRANK 754 ,

I don’t have any site or link to suggest , BUT I strongly
recommend that you go fast to buy the book :

“The Tube Amp Book “ , written by Aspen Pittman , a
very nice guy , from California , that knows almost
everything about tube amps for guitars .
He is the owner of ” Groove Tubes” , too .

The book has two parts : 1st A comprehensive history
of tube guitars amps and 2nd A complete schematic
archive with A LOT of schematics from many manu-
facturers and a CD with 800 schematics in pdf files .
Most of them , using 6V6 GT tubes .

My post , is not an advertising BUT only a “tip” to
suggest an excellent bibliography .

Certainly you will find there a good design .

Regards ,

Carlos
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Old 15th January 2007, 03:46 PM   #4
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Frank,

It's a fact of life that the transformers ("iron") are a major part of the cost of any tube power amp project. Even IF it's available, that phase splitter trafo is going to put some sort of dent in your bank balance.

Those of us involved in the "El Cheapo" project went to GREAT lengths to control "iron" costs. Advantage was taken of an inexpensive replacement O/P trafo for Fender Deluxe guitar amps in the HIFI amp. "What goes around, comes around." You use can most of the "El Cheapo" circuitry to advantage in your MI project. Not having to worry about deep bass, allows the use of the available ultralinear taps. The 'AQ5 "El Cheapo" uses is electrically "equivalent" to the 6V6. So, component value changes are not necessary. Whether or not you retain the loop NFB is up to you.

Since a MI amp has only 1 channel, you could use a Triad N-68X in the voltage doubler. Other savings are possible too. 470 muF. 'lytics in the doubler stack are but 1 example.



edit: fixed typo
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Old 16th January 2007, 12:51 AM   #5
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Thanks, that's some great info, and I do like el cheapo info as well,
seems like a great circuit.
I did a bit of research on the original xfmr on that schematic I posted above
and it is obsolete and unavailable. One one usenet board someone had
exactly the same question, and it was suggested to check the antique
electronics site for similar items, I found these that might work, only
thing is that I can't really guess at the turns ratio, but these are interstage
xfmrs for phase splitter circuits with a secondary center tap:

Hammond P-T124B 10K PRI 90K SEC CT $24.95
http://www.tubesandmore.com/scripts/...P?item=P-T124B

Hammond P-T124D 7K PRI, 15.8K SEC $18.95
http://www.tubesandmore.com/scripts/...P?item=P-T124D

Hammond P-T124E 15K PRI, 33.8/135K S $23.95
http://www.tubesandmore.com/scripts/...P?item=P-T124E

Hammond P-T156 $15.70
Equivalent to Stancor A53-C. 1:3 ratio overall. Center tapped secondary may be used for push-pull grids. Max. Primary Current: 10 mA Impedance Primary to Secondary (Ohms): 10,000 to 90,000
http://www.tubesandmore.com/scripts/...XP?item=P-T156


Is this question related to the characteristics of the output of the 12AX7 or is it
anyone's choice based on the factor you want. In this case what would be anyone's
guess as to the original xfmr? Would the 10K to 90K be a good bet?
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Old 16th January 2007, 02:37 AM   #6
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In my opinion ,

No doubt !! The P-T 124 E would be the best choice !

Carlos
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Old 16th January 2007, 07:50 AM   #7
Merlinb is offline Merlinb  United Kingdom
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I am concerned with that schematic- the second triode of the 12AX7 is running extremely hot- I would expect it to burn out!

Why not replace the interstage transormer with a valve phase inverter? Would probably be a lot cheaper, and you only need one or two extra triodes...
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Old 17th January 2007, 12:20 AM   #8
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OK, am researching, will look for something similar with perhaps one more tube,
this one seemed real "smooth" in design because of the transformer as a feature,
as I hadn't seen that design too often.
On overtaxing the second triode, are you saying this because the voltage is high,
and perhaps the 15k resistor across the interstage primary could stand to be
boosted up a bit to lower the plate voltage?
Also, though, on the plates of the 6V6's, isn't the 290v a bit conservative and
could stand a bit more to boost final stage power, perhaps a bigger power transformer
with more going to the last stage, but a bit less to the second triode?
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Old 17th January 2007, 01:22 AM   #9
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Sticking with the authentic Fender designs, for example, there's this one
(the Harvard 5F10) at 10 watts which seems very do-able and simple with a straight
3 element all-tube preamp section. This seems the most straightforward:

Click the image to open in full size.

The only other in this simplicity range is the vibrolux 5F11 which seems
to have some nice tweaks, but not a huge project, maybe this is the better bet:

Click the image to open in full size.
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Old 17th January 2007, 08:13 AM   #10
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by frank754
OK, am researching, will look for something similar with perhaps one more tube,
this one seemed real "smooth" in design because of the transformer as a feature,
as I hadn't seen that design too often.
It is a simpel design yes. Build it if you like! don't let me stop you. I was just offering an alternative to expensive transformers.

Quote:
On overtaxing the second triode, are you saying this because
the 15k resistor across the interstage primary could stand to be
boosted up a bit to lower the plate voltage?
Not the voltage: the current. That 15k resistor is letting way too much current through. I'd make it more than 47k, or maybe leave it out altogether.

Quote:
Also, though, on the plates of the 6V6's, isn't the 290v a bit conservative
Not really, I guess it depends how much power you want out, but there's not a lot in it. A lower anode voltage allows you to run more in CLass A, which is reputed to have the sweetest distortion.
There seems to be a bit of a vogue for very high anode voltages at the moment, there's no real need for it. If I want more power I don't thrash my wee little valves, I just move up to bigger ones. Use whatever voltage you can buy (What I mean is; build the amp to suit the transformers available, not the other way round.)
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