SoLow Watt 12A_7

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

Re: SoLow Watt 12A_7

Post by Synchu »

Great! Thanks a lot for the hints :)
I will try to do a LTSpice simulation to play with the currents.

Current PT is a Hammond rated - 75ma, hence 85-90 will be a bit of a push, but I think it will handle it .
Synchu
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Re: SoLow Watt 12A_7

Post by Synchu »

Alrighty. I did order some Sylvania 6DX8 just to test against the russian ECL84 equivalents, but didn't manage to wait them to get through the customs and redid the amp for 6AQ5 (actually for the russian equivalents 6p1p:) ) with a separate PI tube of'course.

10thTx many thanks for the suggestions on the changes - I used most of these with a few changes. I left the 120pF across the coupling caps at the end. I ended up using 25uF cathode bypass cap (and made it switchable from 2.2 uF).
I ended up using 300 Ohm for the cathode bias resistor on the power tubes, since a quick simulation in LTSpice showed a bit safer plate current (given the PT is rated for 75mA only). Now the plate voltage is 243V and cathode is 13.22V on the power tubes.

I am very happy with result. The amp has more clean headroom now; bass response is fuller, highs are a bit more raw and open.
Few things left to be done, though:
- will have to play with the tone stack to dial out a little bit of 300-350 Hz-ish frequency (or build a separate notch filter)
- there's some sort of crackle/scratchiness at the notes tail (you can hear it 0:58-0:59 from the sample below and further on) which I have to find the reason for and will appreciate input. Couple of thoughts are - I got as a gift a bunch of russian military PIO 0.01 caps that I used as coupling caps to the power stage. They usually have some small DC leakage; Second is that I put 12AT7 as a phase inverter since I liked the sound quite a bit (I have some 12AV7 on order and will try it as PI as well), so probably will have to re-calc the plate resistors. Didn't have the time to replace it back to 12AX7 and find out whether crackle disappears or not, but will check today.
Other thoughts will be much appreciated :)

Here's the sound bit (just hit record and didn't play anything in particular, mostly different notes/chords to check the amp's response). It is a strat.

http://soundcloud.com/synchu/scrappy-61p1-12av7-12at7pi
10thTx
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Re: SoLow Watt 12A_7

Post by 10thTx »

I think your amp sounds quite nice. Lovely tone. Great playing also.

My guess is the crackling may be from leaky caps?

with respect, 10thtx
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M Fowler
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Re: SoLow Watt 12A_7

Post by M Fowler »

Don't you guys think that it's just the speaker not being able to handle that low end?

Good sounding amp just needs more treble and mids to my ear. :)
Synchu
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Re: SoLow Watt 12A_7

Post by Synchu »

Fair point.
It is a 4x12" closed back Diezel custom cab. It handles bone rattling 7-8 string guitars (if you still call them guitars) and much more bass than what is coming out of this one.
The amp itself has plenty of mids and high end - the recording is done with mids at around 9 o'clock, bass 1:00 and treble at 11:00.
And as mentioned above, it has a lot more in the 300Hz region, than my liking is - this is why I tried to dial this out with the current implementation of Mid, but didn't manage.
300-350 Hz-ish region is what sounds boxy (whatever this means) to me at least. YMMV :)

But this crackling is not out of the cab. I tested it with open back Two Rock cab with a WGS speaker - still there and twisted the knobs quite a bit.

After I track it down, I will do proper samples (hopefully).
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xtian
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Re: SoLow Watt 12A_7

Post by xtian »

Wow, Synchu, I am very impressed by your clips on Soundcloud. From fingerpicking acoustic with lots of nice harmonic variation and invention, to TEH METALZ!@! Talented guy.
I build and repair tube amps. http://amps.monkeymatic.com
Synchu
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Re: SoLow Watt 12A_7

Post by Synchu »

Thank you xtian. Glad you like it. I still like to think that these may sound much better than I play it, but wife, kids, work and hobbies do not allow for much playing time anymore :)

I continue to play with the amp tune up. Changed PI coupling caps to 0.022uF, bypassed the V2B 68K resistor with 0.01uF cap, and replaced the tone stack bass cap to 0.1uF from 0.01uF. I am getting much closer to what I would like to hear from the amp.
Crackling is still there, but much lower in volume and can be barely heard. The original ECL84 design was much gainier and smoother, while now the amp is cleaner and more raw sounding.

I will play tonight with the output section biasing by changing cathode resistor going from 220 Ohm up.
The restriction I have is the Hammond 369 PT rating of 75mA current and the Hammond OT is rated 15W, and although simulations look fine on paper, measured values do not always agree :)

That's a short (bass-less ;) ) sample:
http://soundcloud.com/synchu/scrappy-61 ... 2at7pi-hvy
surfsup
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Re: SoLow Watt 12A_7

Post by surfsup »

"Crackling is still there" - this is usually due to a poor solder joint or DC where it's not supposed to be. (imo)

The earlier clips sounded a bit bassy - even on laptop speakers. This last clip sounds a lot better. I am building a home but once I'm done this is one of the first projects I plan to do with my own little twist to it. Hopefully next year this time. Please continue to post your progress and changes. Sounds good!
10thTx
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Re: SoLow Watt 12A_7

Post by 10thTx »

The original ECL84 design was much gainier and smoother
I agree that the previous (below) was smoother. I actually liked the tone of it better, but that is just a personal preference thing. I do think the "new" one you just posted is grainier.

http://soundcloud.com/synchu/scrappy-61p1-12av7-12at7pi

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

Post by Synchu »

surfsup - thank you.

10thTx - I meant the Solow Watt ECL84 design. It appeared to have more and much smoother drive on tap than what I ended up with all the mods. Agreed - the current incarnation sounds a lot more grainier.

I triple check each soldering connection at soldering time and a fourth time before first power-up. It can be the case, but I believe it is more like a DC leaking somewhere it shouldn't be. I don't have a scope (yet), but becoming highly interested in acquiring one :).

Now the first build had a very smooth background buzz like sound (almost sort of a backing/doubling note, it is definitely not a "ghost note" issue). It was just part of the overall tone and wasn't irritating or something, so the other option I am thinking about is that with the gain decrease this might have turned to the mentioned crackle like sound.
Synchu
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Re: SoLow Watt 12A_7

Post by Synchu »

Alright, I consider it done, and will do my "permanent" soldering for all the tweaks :)
I did change the cathode bias resistor from 300 to 270 Ohm.
The voltage drop on the cathode was around 0.7V, but the effect on the tone was quite a bit - better frequency response and dynamics. According to my calculations current draw should be at around 75-80 mA. PT stays cool so seems normal.

I have 250 Ohm being shipped and will try it for comparison, but now I do like quite a bit what's coming out of the cab :)

It reacts great to the guitar volume knob. For the following sample the beginning and later the backing for the small lead were recorded with the neck pickup of a Strat with guitar volume lowered to about 6-7. Then lead was recorder with the guitar volume full way up. Pre Volume is about half the way up.

http://soundcloud.com/synchu/scrappy-61 ... -12at7pi-1

Didn't have the time to check it with an LP, but will do this tonight.
It turned out great for my liking.
Synchu
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Re: SoLow Watt 12A_7

Post by Synchu »

Here are the pics. Due to reused main board and the number of mods done it doesn't look as neat as I would like, but still can be of use for some one.
I put the last mod - LNFB on a switch (v2A anode to V1 common cathode) - sounds good to me :)
Telecaster with bridge (mini)humbucker
http://soundcloud.com/synchu/scrappy-61 ... -12at7pi-2

[img:500:292]http://farm3.staticflickr.com/2836/1121 ... e24607.jpg[/img][img:500:321]http://farm6.staticflickr.com/5548/1121 ... 2a0634.jpg[/img] [img:500:374]http://farm3.staticflickr.com/2894/1121 ... d64c1a.jpg[/img] [img:500:374]http://farm4.staticflickr.com/3761/1121 ... abe533.jpg[/img]

[img:500:374]http://farm8.staticflickr.com/7378/1121 ... 8afc6f.jpg[/img] [img:500:374]http://farm4.staticflickr.com/3809/1121 ... 73ce1d.jpg[/img]

I will look to update the schematic and post it here, if 10thTx is fine with that?
Last edited by Synchu on Thu Dec 05, 2013 1:55 pm, edited 1 time in total.
10thTx
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Re: SoLow Watt 12A_7

Post by 10thTx »

I will look to update the schematic and post it here, if 10thTx is fine with that?
Yes, please do so. That would be great!

With respect, 10thtx
Synchu
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Joined: Sun Mar 11, 2012 5:24 am

Re: SoLow Watt 12A_7

Post by Synchu »

Here it is.
Please, excuse my rather limited skills with ExpressSCH. I've been using LTSpice for some time already and still trying to get a grasp with the ExpressSCH.
I am attaching the editable file as well. Please, compare it with a schematic and use common sense as always.

Few notes:
- I removed the voltage measurements from most of the places except the PS section, since these are not relevant
- you can play with the cathode bias resistor with values from 200 to 330 - measure current. Safe values should be around 40-50 mA per tube
- I removed what I called FAT switch and wired the 25uF cap there permanently
- LNFB can be done on a switch as indicated on the schematic

Please, note that this is much cleaner amplifier (in the realm of over pushed 5E3/tweed bassman territory, but with quite a bit more clean headroom) than the original SoloW Watt - so probably invites for a new name :) NotSoLow Watt is a good candidate cause it should be 12-15 Watt output )
I am getting slightly more low freq hum than I'd like, so I will redo the grounding and lead dressing and then try to elevate the heaters, if no luck.
You do not have the required permissions to view the files attached to this post.
Synchu
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Re: SoLow Watt 12A_7

Post by Synchu »

While waiting parts to arrive for the next project I made few more experiments with the amp.
Implemented 2 x 220k resistors to ground after the coupling caps to the power stage.
Put an inductor at the cathode bypass of the paralleled triodes on the first stage. Now, the initial idea borrowed from the excellent Manzamp schematic and none less than amazing work done by ic-racer.
However the 1mH inductor didn't made any significant difference in the sound. Then I put a 500mH one (while waiting for some other values to arrive) and I think it sounds great. Band freq. should be around 43Hz with the current cap values I am using (27.2 uF to be exact) which is well bellow both low E string of the guitar and the output transformer's freq response. Now, it of;course has effect by changing the freq. response in the lows (tighten'em a bit).
I quite like the result and decided to post it should it would be of someones use.
couple of samples
http://soundcloud.com/synchu/sensitive- ... -lfb-strat

and this one is after changing the first dropping resistor in the PS to 3k3 (I don't seem to like it that much - the voltages in this amp are rather low and dropping further while makes it easier to play (i.e. feels more compressed), does impact freq. response in the highs, harmonics and esp. dynamics)
http://soundcloud.com/synchu/sensitive-tbc-3-3k-psu
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