SoLow Watt 12A_7

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10thTx
Posts: 1863
Joined: Sat Dec 30, 2006 1:13 am

Re: SoLow Watt 12A_7

Post by 10thTx »

Wow, your chassis interior turned out alot neater than mine did. Nice job!

I notice your amp (similar to mine) has a nice bass tone to it also. I have been pleasantly surprised how much bass response these small tubes have.

Enjoyed the sound clip. Nice tone to it.

With respect, 10thtx
jamdigs
Posts: 9
Joined: Sat Mar 19, 2005 3:45 pm
Location: Queens, New York

SoLow Watt Transformer Change

Post by jamdigs »

10thTx;
I really like the sound clips of the amps you've posted,the solowatt clip was the stuff! You recommended a 250-0-250 transformer change with 5y3 rectifier. Would you use the same power rail as you posted for the ecl86 version? What voltages would you like to see on the power amp and pre amp tubes??

Thanks! jamdigs
10thTx
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Re: SoLow Watt 12A_7

Post by 10thTx »

Would you use the same power rail as you posted for the ecl86 version?
I don't understand this question? In regards to what type of power tube?

If you are referring only to the change in the PT, then yes ....... I'd use the same power rail.

Are you asking about the ECL84? If so, the schematic with voltages are on the first page of the thread.

With respect, 10thtx
jamdigs
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Location: Queens, New York

SoLow Watt ECL84 Power Trans. Change

Post by jamdigs »

10thtx
yes I was refering to the ECL84 tubes. So, I should adjust the
power string resistors to maintain the same voltages on the tubes
as for the 196-0-196 volt trans. as for the 250-0-250 volt tube
rectified trans.; correct!? Are you still going with the 16k load
from the pentodes into the output trans.?? e.g. 8ohm speaker on
4ohm tap.

Respectfully! jamdigs
10thTx
Posts: 1863
Joined: Sat Dec 30, 2006 1:13 am

Re: SoLow Watt 12A_7

Post by 10thTx »

The 196v PT w/ss rectifier = 255v per schematic

The 250-0-250 w/5Y3GT will probaby be around 270v?

I'd build it just like original & see how it sounds. Then you can increase dropping resistors if needed?

with respect, 10thtx
jamdigs
Posts: 9
Joined: Sat Mar 19, 2005 3:45 pm
Location: Queens, New York

Re: SoLow Watt 12A_7

Post by jamdigs »

10thtx
What size chassis would you
recommend for this build.
Thanks! jamdigs
Synchu
Posts: 522
Joined: Sun Mar 11, 2012 5:24 am

Re: SoLow Watt 12A_7

Post by Synchu »

Digging this up, as I know looking for my next build and this seems a great candidate ( thanks a lot 10thTx).
I have a hammond 1750e OT with a single 8 Ohm tap. Would this one work as I see ECL84s will expect higher load?
Thanks in advance
Niki
10thTx
Posts: 1863
Joined: Sat Dec 30, 2006 1:13 am

Re: SoLow Watt 12A_7

Post by 10thTx »

I used an 8 ohm speaker on an 8 ohm tap. Sounded good and worked fine for me.

I did not see the previous inquiry about chassis size until now.

I used a 12.5" x 6.5" x 2.5" from Watts Tube Audio. It was somewhat cramped. I'd probably be inclined to use an ebay inexpensive tweed 18 watt style chassis and maybe cut an inch or so off the length?


with respect, 10thtx
Synchu
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Re: SoLow Watt 12A_7

Post by Synchu »

Thanks for the response and the wonderful schematic - so I built it and it sound great. Changed the layout to accommodate a spare board I had from my previous project.
No clean sounds though, voltages are fine more or less (7-10 volts lower to be exact), except the plate of v2a where I have 67v instead of 133v. V2b is fine at 198v. But it seems to have drive and then more drive when I turn up the volume. :)
I plan to install Dumble style nfb loop to try and tame the little beast a bit.

Here is how it sounds with an LP (will have to take some pictures of the crammed little chassis I used).
http://soundcloud.com/synchu/scrap-o-ax ... emo-gibson
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Reeltarded
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Re: SoLow Watt 12A_7

Post by Reeltarded »

What a surprise! I could blisted Trower through that thing, love that!
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
10thTx
Posts: 1863
Joined: Sat Dec 30, 2006 1:13 am

Re: SoLow Watt 12A_7

Post by 10thTx »

Thanks for the response and the wonderful schematic - so I built it and it sound great. Changed the layout to accommodate a spare board I had from my previous project.
No clean sounds though, voltages are fine more or less (7-10 volts lower to be exact), except the plate of v2a where I have 67v instead of 133v. V2b is fine at 198v. But it seems to have drive and then more drive when I turn up the volume. Smile
I plan to install Dumble style nfb loop to try and tame the little beast a bit.

Here is how it sounds with an LP (will have to take some pictures of the crammed little chassis I used).
http://soundcloud.com/synchu/scrap-o-ax ... emo-gibson
Nice tone and playing! Thanks for sharing the results. IF you try the dumblish NFB, please let us know how that works out.

Has a big tone for a low wattage amp. Sounds like yours also ended up with a decent bass response.

With respect, 10thtx
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Structo
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Re: SoLow Watt 12A_7

Post by Structo »

Synchu wrote: Here is how it sounds with an LP (will have to take some pictures of the crammed little chassis I used).
http://soundcloud.com/synchu/scrap-o-ax ... emo-gibson
Liked the Oceans Theme, RAWK!

Amp and playing sounds awesome. :D

BTW, what gear did you play on the Fhr Jam?

https://soundcloud.com/synchu/fhr-jam
Tom

Don't let that smoke out!
Synchu
Posts: 522
Joined: Sun Mar 11, 2012 5:24 am

Re: SoLow Watt 12A_7

Post by Synchu »

Thanks for giving it a listen and the good words and again to 10thTx for the design :)

@Structo - huh, I've almost forgotten this Oceans Theme thing - it was recorded 7 years ago me thinks.

FHR jam is a backing track provided by a forumite as she recorded it for her band, but something didn't work out, so she (yeah this girl rocks :) ) posted it for others enjoyment and jamming around. Sounds like Marshall sort of amp was used for the backing track.
I played the lead guitar only (to my humble abilities) using a #102 clonbe, built by Howlin' amps, Mirc Cinku, which I acquired before getting into this all amp building thing. It is a Suhr humbucker equipped guitar and I used a Dallas Rangemaster (sort of) booster, I built, to tighten it a bit.

As for the amp, I did quite a bit of experimenting on NFB last night and currently it seems that 100k/4.7k Marshallish/Liverpoolish, if I am allowed to say so, combo works best (usual Fender/Dumblish values sounded either dull or needed at least 1uF cap to prevent oscillations). I bypassed the 4.7k resistor with a 2.2nf cap (which actually measures 2.8nF) and I get excellent tonal balance. I will try today the SSS 2.7K/270R combo, cause I didn't have this values on hand, but my guess is that I will revert back to the 100k/4.7k.

Reducing gain did happen but not to a great degree, so will have to explore other options as well.

I will post some samples when I get to the final useable combination.

To be clear - I am doing this out of curiosity - amp sounds great to me as it is and it is not being designed (as far as I can see with my yet limited knowledge) as a clean machine :) That said I will put the NFB on a switch :)

Niki
Synchu
Posts: 522
Joined: Sun Mar 11, 2012 5:24 am

Re: SoLow Watt 12A_7

Post by Synchu »

Update:
I did a whole weekend of experimenting and measuring NFB. While the best combo was as described above, at the end I really felt that the change was not so significant to justify the mod and the effect on "cleaning" the amp was not as big as expected (to the extent possible in a cathode biased configuration).
I used the NFB switch hole drilled at the back for adding a Cut control (post PI 0.002 nf+250KA pot filter) which I find much more useful, especially with Strat single coil at bridge position). It backs off very successfully the excessive high frequencies of the Strat's bridge pick-up.
One shot sample where lead is played on Strat bridge pickup with Cut at about half and backing guitar was on 4-th position, I think, with Cut off (i.e. full bandwidth).
http://soundcloud.com/synchu/scrappy-2-strat-diz-cut


I think to call it done at this stage and make a headshell :)

Though I've been looking through the new inventory of a ebay store I am using for odd ball tubes and saw a bunch of EL90/6AQ5/61P1 tubes averaging at $1.50 (shipping almost the same amount :D), so I was wondering whether to take the hassle and drill another socket for a standard 12A_7 PI and then wire the power amp with these pentodes :)
10thTx
Posts: 1863
Joined: Sat Dec 30, 2006 1:13 am

Re: SoLow Watt 12A_7

Post by 10thTx »

saw a bunch of EL90/6AQ5/61P1 tubes averaging at $1.50 (shipping almost the same amount Very Happy), so I was wondering whether to take the hassle and drill another socket for a standard 12A_7 PI and then wire the power amp with these pentodes Smile
6AQ5 is a great tube! 6V6 in a smaller bottle 7 pin socket.

You will need a PT that has 90-100ma minimum. (6AQ5 tubes will probably draw about 80ma?)

I'd make some changes to the SoLow Watt values to get a better tone with the 6AQ5.

Remove 470k resistor wiper to gnd on the volume pot
Change V1-3 cathode cap to 5uf
Change LTPI cap values from .0047 to .01 & .02 to .047
Maybe change coupling caps post LTPI to .01 -.03 range?
Remove the 120p snubbling cap
Remove the 100k resistors going to the PPIMV
Change the PPIMV resistors from wiper to ground to 2.2M.
Change the power tubes cathode resistor to 270 ohm.

With respect, 10thtx
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