New Circuit, New Build, New Questions...

[flood]

hello all,

did the resistor drop method using one 220 ohm 2W metal oxide resistor on each leg of the filament windings - works like a charm. also wired in a pentode/triode switch for the power stage. swapped out the entire power supply PCB for a free-wired PSU. also changed the master volume pot since the earlier pot was defective and shorted to full volume every now and then.

however, another problem has cropped up that i can't wrap my head around - the power and volume seem to have dropped massively. earlier, it was impossible for me to getpast the 12 o'clock stage without fearing permanant hearing loss, now i need to turn it up that far to get an appreciable volume. suspecting the pentode/triode switch was to blame, i removed it and wired the screen grid resistors directly to the corresponding voltage, but to no avail...

there is nothing abnormal going on - the voltages check out fine, the power transformer runs cool to warm to the touch, but the power and headroom are definitely off. the cleans don't slap me in the face anymore, they're way too compressed.

at first i thought it was my imagination or temporary hearing loss due to a month of partying. now i'm pretty sure that something is amiss... this is definitely the final stage though. i suspect that the master volume pot could be the culprit... perhaps not 1M at all, or with a strange taper.

you guys should be really happy that you don't live in india. amp and stompbox DIY is a nightmare here with regards to parts - you get everything per se, but the quality might not necessarily be what you want.


another thing i noticed - i had been using a mislabelled 12AU7 instead of the 12AX7 in V3 (cathode follower). could this drive the power stage better? i noticed some tonal change after the switch to 12AX7.

here's a pic - looking pretty now - the white LED on the top of the chassis is the "charge indicator" - an LED in series with a 220k 3W resistor that drains the caps in standby. i initially thought this was the culprit, since it was attached to the same B+5 that fed V1a. disconnected but no change.

[flood]

a BIG thank you to all you lovely people on this forum - without your input, this amp would definitely have not been possible.

it's a crazy little beast now. shipped off to the buyer who is currently speaker-less and cannot use it at the moment... but wll be heading for a project to the states and picking up an avatar 2x12 with hellatones...

so, here's the description of the front panel (from left):

- H input (only to the 6SJ7 preamp, with mode engaged in "daytripper" green)
- Coke input (accesses the 12A*7 preamp, all modes)
- Mode switch - daytripper green, speedball yellow, overdose red (discrete, parallel and cascade input)
- coke volume and coke purity (cathode boost/fatness)
- H volume and H skwish! 6-way switch (kills gain and increases compression in 6 steps by changing the screen grid bias. LED lights brighter with increasing compression)
- BMT tone stack, marshall values with mid shift and bass extension switches
- master volume
- pup/mutt half-power switch (triode/pentode power stage)
- standby and power

back panel:

power input and fuses, cathode bias test points, empty hole for future VVR retrofit, 5-way cathode resistor selection switch, 3-way OT primary selector, 3-way OT speaker impedance selector. speaker jacks, line out with volume. there's a 10 ohm 50W dummy load installed in case someone forgets to plug in a speaker, or wants to drive another amp.

white LED on top of the chassis shows the capacitor charge so that my buddy can poke around the guts knowing he's safe...

it looks a bit skanky right now with the marker scribblings, but the buyer seems to like it. once i have a few more amps in the pipeline, i'm going to do a chrome/plexi faceplate for this one using the original CAD drawings, make it a bit spiffier.

i want to hear my friend'S reviews and clips once he has his stuff set up. i personally loved it to death while i had it and worked on it, and actually found it very difficult to part with. but there are other amps to be made waiting

if anything, this has definitely given me the confidence i needed to experiment with my own designs and ideas and work with oddball tubes too, of which i have a decent amount. who knows, maybe i'll come up with something unique sounding... over and over again.

THANK YOU again - your inputs were all invaluable, i've learned more about amps on TAG than all the other music forums combined! clips coming soon.

[Andy Le Blanc]

Thats sharp looking... Cant wait to hear clips

[iknowjohnny]

Can i ask how you did the mid shift? is it just switchable mid caps in the tone stack or something else? I've played with the tone stack endlessly but changing values there never really seems to shift the mid control's center in any way that accomplishes what i'm after. So i've always wondered if theres a better way.

[flood]

thanks andy! iknowjohnny, take a look at the schematic... it's not the mid cap i wired the treble cap to a DPDT so that it can switch two 470pF caps in parallel or in series. this has a pronounced effect, most noticeable in the quasi-djent areas of the cascade mode...

from my simulations in TSC i saw a shift in the midrange towards the 600-700 Hz area. almost sounds like a spiky narrow peak in the 1-2kHz area when the caps are in series.

[iknowjohnny]

Ahh, ok. yes, i've tried that and i didn't like the effect of the treble cap when bigger than 500pf. I did like the sound when halfed at 250pf or thereabouts, but i much preferred 500 so i just left it there. One thing i have NOT tried but might be worth a look is to change both treble and mid caps at the same time and see if the interaction creates a whole new midpoint that i have yet to hear.

[iknowjohnny]

I take that back about the bigger treble cap. I may not have tried that, as it was long ago. Or I may have not given it a chance. (tried smaller many times but decided in the end it's to skinny) In any case, i tried it tonite and at first i liked it tho it did take away something in the highs i missed. It added some good and some bad. This was a 500pf in parallel for a total of 1000pf. I then tried a 270 for a total of 770pf and that was better. What i'm liking about it is that it really tightens up the low end better than anything i've tried so far, and trust me, i've tried everything. But with the better lows comes highs that aren't quite as nice. So i'm on a quest now to find the perfect value to give me some of that better low end w/o any noticeable sacrifice of the highs. 770 may be it but i will try others. But chances are whatever i do i will also have a switch, possibly a rotary, because theres a lot of good tones to be had. A nice change of pace at times or a change that could turn a bad room into a decent one.