(almost) first build advice needed - TW inspired

[flood]

ok guys, thanks for all your inputs... i finished it off and started it up, only to be greeted by a HUGE hum. my first suspicion turned out to be correct, and like in my firefly bild, i had forgotten to connect the filter caps to GND... i rectified this, and it was quiet as a mouse. a bit too quiet. i had no output whatsoever, except for faint scratching when turning the volume pot.

after a lot of measurements and conductivity tests, i finally located the source of the probem - i used a shorting jack and connectedthe 68k input resistor to - you got it - the shorting lug! i jumpered it (i will connect that lug to GND later...) to the tip lug, and plugged it in again. grabbed my bass, because it was the first thing i had lying around, and got a fairly quiet output using a WGS reaper (left it in the box while testing). it works! however: i could only really hear it with the output dimed, and it was VERY quiet. this was, however, with my downtuned bass guitar. i did get a nce peter gunn theme bass sound though.

it is surprisingly quiet as a mouse. no hum, noise etc. even with the output maxed.

extensive testing to folow this week along with photos and clips... thanks a TON to all you guys who wrote in here, as well as to everyone else on this board! i've learned tons here.

[flood]

ok guys, thanks for all your inputs... i finished it off and started it up, only to be greeted by a HUGE hum. my first suspicion turned out to be correct, and like in my firefly bild, i had forgotten to connect the filter caps to GND... i rectified this, and it was quiet as a mouse. a bit too quiet. i had no output whatsoever, except for faint scratching when turning the volume pot.

after a lot of measurements and conductivity tests, i finally located the source of the probem - i used a shorting jack and connected the 68k input resistor to - you got it - the shorting lug! i jumpered it (i will connect that lug to GND later...) to the tip lug, and plugged it in again. grabbed my bass, because it was the first thing i had lying around, and got a fairly quiet output using a WGS reaper (left it in the box while testing). it works! however: i could only really hear it with the output dimed, and it was VERY quiet. this was, however, with my downtuned bass guitar. i did get a nce peter gunn theme bass sound though.

it is surprisingly quiet as a mouse. no hum, noise etc. even with the output maxed.

extensive testing to folow this week along with photos and clips... thanks a TON to all you guys who wrote in here, as well as to everyone else on this board! i've learned tons here.

[flood]

so... did some testing with guitar last night and a plethora of output tubes. couldn't tell the difference between most of the tubes - 6L6 (brimar big bottle), 6V6GTA (tung sol) and 6K6 (RCA JAN) were all quite similar. the only real tonal difference came from 6F6, which was louder and a bit raunchier. i attribute this to the fact that i have only 258V on the plate and a 470E cathode resistor.

i also tried using a mazda 6CA7/EL34, but heard absolutely nothing when i turned on the standby switch. i began to smell melting plastic and shut it off immediately. i suspect the PT cannot supply the current required, or the OT impedance is simply too high (5k) for the EL34. .

BUT: it is still TOO quiet. it is quieter with the volume maxed than my firefly on half the volume. i am beginning to suspect that the voltage divider that i have just before g1 is the cause of this problem. i am currently using 100k/470k which would cause roughly an 18% drop. also, i suspect that using such a high resistor value is defeating the purpose of the cathode follower preceding the output stage. i'm thinking of putting a 10k and 6.8n in parallel with the 100k resistor...

also, i drove it with my red llama clone, which caused a noticeable increase in volume and output. i'm wondering if my EQ is too lossy, or if i'm not getting the best gain out of V1 (currently using 220k/1k5). or both.

any further ideas?

[stubbyfex]

Hello! I applaud you for trying a new design! I know that it takes me a long time to get all the bugs out of it whenever I've tried something new. One thing that I would try, is to raise the value of R4, the cathode follower's load resistor. Marshall uses a 100k in that spot. Even though cathode followers don't have gain, it might not load it down as much. Another thing I would maybe try is to lower the value of R10. I think it is acting as a voltage divider there. It maybe should be 2.7K? Then instead of cutting down the signal to the power tube grid, it would act as a parasitic stopper. And one thing that has given me problems in the past is the direct coupling between V2A and V2B. In some amps I've had to add a blocking cap there. I'm not sure why, but sometimes I can't get it to sound good until I do that. Good luck, and keep soldering!

[flood]

hey stubbyfex! thanks a lot for your input... this amp has been giving me a few problems, still trying to work them out.

i put a 1k in parallel with R10. this has increased the volume and output somewhat... but:

1. there is, ever so often a crackling and popping noise. at first i suspected arcing, but the noise increases when i touch the chassis around the input jack. am going to resolder the jack and see if this helps.

2. like i said, there is very little power and output, the amp stays fairly clean with the volume maxed. however, a couple of times, i knocked on the chassis and was greeted by a HUGE output that lasted less than a second.

am beginning to suspect that there is a cold solder joint somewhere, or that a signal wire is shorting against the chassis or something. am going to resolder all ground connections, i think. something is quite wrong here.

will definitely follow up on the points that you mentioned and make those changes. maybe there will be an improvement in performance...

[Markusv]

Flood

You mentioned that your OT was rated for 8 watts max and thas the PT used to run ecl82's?
You might be better off running those kinds of tubes as I wonder if the trannies can give wht the tubes demand

I have done a number of builds with radio donor chassis and you can make a pretty friggin loud amp with ecl82/ecl86's but then the PT at least is mathed to current demands

Other, more experienced people should weigh in

Cheers.
Markus V

[Markusv]

I just realized you are building an SE amp
Disregard my previous post please!

Markus V

[flood]

No worries Markus

will not get any work done tonight... am still in the office :/

EDIT: stubbyfex, when you say cathode coupling, do you mean just replacing the jumper from V2a to V2g with a coupling capacitor (let's say 22n) and resistor (say 470k) to ground? no further modifications required?

[stubbyfex]

Yes, try adding a cap and resistor, those values you said are good. After breaking your amp up into "building blocks" which by the way is the way I design amps, you have a parallel gain stage, then tones/volume, then into another gain stage direct coupled into a cathode follower and then the power tube. If you compare that to, let's say, a regular champ, the champ would have a higher gain due to it's two gain stages.
After hearing that you got a Very loud signal after moving something in the amp, it almost sounds like you have a bad solder connection, and none of the other "fixes" should be done until you get it to "stay" loud. All I'm saying is that the amp might be fine already, if the bad solder connection was found.

[flood]

sooooo....

haven't hanged he cathode follower to an AC-coupled stage yet, but did make a couple of repairs/changes:

1. i resoldered the input jack. i had a hunch that it was contributing to the breakfast syndrome (snap-crackle-pop) mentioned earlier. also removed the second 47pF cap since i didn't see the point of having it there. that helped things a lot.
2. changed Rk on V2b, as advised earlier, to 100k instead of the 56k vox value. there seems to be more gain now. the amp is louder at the maximum setting and distorts a bit too.

it is quite dynamic on the max settings - good attack, good harmonics. suppose that's whati was going for in the first place - not hardcore preamp gain.

also, i'm beginning to wonder if everything is ok after all when i take the following into consideration:

1. output increases greatly with a pedal in front of the amp. also, when i dig in very hard, it does get loud and nicely distorted.

2. plate voltage is 256V, Rk is 470 ohms. OT is 5k instead of the 4k optimum for 6L6.

3. guitar pickups are not very powerful, EQ could be a bit lossy.

problems yet to be sorted out, or mods to be made:

1. there is definitely a problem with the EQ section. i goofed up - the treble increases CCW, decreases CW. if the knob is fully CW, the output is gone altogether! i seem to have shorted something to ground. will have to check that again. i doubt this is nromal behaviour in a TW/Marshall tone stack.

2. am i getting optimum gain on V1? is this arrangement (220k/820R) ok? would i expect more gain from a traditional 100k/1k5 arrangement, or 100k/820, especially considering that i am running triodes in parallel?

3. might give the AC coupling a shot too.

pics and clips to follow on the weekend.

[flood]

sooooooooooooooooo....

many apologies fror the long, long, delay. finally got down to taking decent pictures and recording a half-decent sound clip. guitar was a fernandes revolver pro 7, speaker is a WGS reaper, a TS-I0 was used in some sections.

http://rapidshare.com/files/223263364/I ... n.mp3.html

sorry, but i couldn't find any other hosting service where the connection didn't time out during the upload.

here's a link to the somewhat chaotic making-of album on facebook, it's public so anyone can access it even without a facebook account.

here is a pic for your viewing pleasure:

[img]http:604:403]http://photos-a.ak.fbcdn.net/photos-ak- ... 188573.jpg[/img]

finally, i'd like to thank everyone who contributed to this post, and everyone else on the board too - i've learned tons here, and would never have ventured on an original design without the knowledge i got here. hope i can contribute more in the future - am planning to build an eighteen watter on the basis of this amp once i'm done clearing out my backlog. kudos to you all.

tube complement - RCA 12BZ7 in V1, Mullard 12AX7 in V2 and RCA 6K6 in V3.

[Structo]

Good for you getting your amp going.

Without being too critical of your playing, it sounds like your guitar is not tuned up properly.

[flood]

dear tom, i'm tone deaf - i probably wouldn't realise until i snapped a string or felt it flop

...buuuut seriously folks, i suppose i was too desperate to record and "feel" the amp cranked completely that i just disregarded using that wonderful invention, the tuner, prior to letting go. maybe i'll do an overdrive shootout sometime with this amp.

[flood]

Tom (and others), any comments/brickbats on the tone itself? Thanks!

[flood]

so, kicking this dead horse after a LONG time - i rewired the input stage so that i could get a bit more gain and crunch, low and behold, what do i find?

it turns out that instead of putting a 220k resistor for the V1 plates, i'd used a 2k2 instead. how the hell did it work for this long?

anyway, V1 is now wired as follows: first half - input to 68k to pin2, 220k Rp, Rk stay the same at 820/0.33u, EQ/MV section follows, wreck style. MV return goes to a make up stage now - Rp 110k, Rk 4k7//100nF, coupling cap 22nF directly to cathode follower with no grid stopper.

this opened up a world of gain, but is unfortunately extremely bright too. with the strat i had to roll the treble down to zero, with the les paul, it was ice-pick treble beyond 30%.

clips (strat 5W + LP 5W) are posted here: http://www.soundclick.com/bands/default ... tent=music