Modded 2061x build thread

[flood]

OK, not too many questions this time, but some amp pictures for y'all from sunny Bombay.

the circuit is pretty much stock 2061x from the infamous layout doing it's rounds on teh intertubes, with a couple of modifications:

1. switchable tube or diode rectification using the mesa boogie dual rect circuit.
2. dual 470k PPIMV and doubled PI coupling cap values
3. 3PDT cascade mod switch to channel the lead into the bass.
4. possibly redundant, but i'm considering sticking an IRF740B VVR in there.

the iron is locally wound and weighs a ton. also, the store follows some weird principle to wind the OT 4k-0-0-4k and have two wires as a center tap. i don't get it, but hope it's all right. am going to measure the values to verify symmetry...

PT: 275-0-275 150 mA, 3.15-0-3.15 5A
OT: 8k Pri, 0-4-8-16 Sec, 25W

the woman offered me a 600 ohm out as well, which i turned down - is this of any notable value at all? could it make for any applications, like a line out or something?

anyway, here's a picture of the powder coated (white! ) chassis. i got the "distressed" (unintentional ) lettering on using the toner transfer method i use to make PCBs (i kid you not) and it didn't work out that well. for some reason, i quite liked the messed up look it had. this one is called the English Mastiff. more pics to come soon...

[flood]

ok, it's done, i need to shout HALP!

i had a pair of "sacrificial" BEL EL84s that i put in. everything was working fine right off the bat - never had that immediate success before, so i was rather pleased... until! the sound started to deteriorate - sounded like bad biasing and i noticed that one of the EL84s was redplating (never seen that before either).

i shut it off and decided to try a pair of zaerix gold pin E84Ls that i had... same problem, same tube starts redplating. i strongly believe that it has to do with the fact that the OT isn't wound 4k-0-4k but is actually 4k-0-0-4k and is not symmetrically wound.

stupid of me to pop it in there without measuring the DC resistance of each half. i do have some more 25W 8k CT trannies in reserve. safe to say that this is the last time i'm procuring OTs locally.

i'm also going to split the 130 ohm cathode resistor on the output tubes - i've heard it's a bit more beneficial if each EL84 gets it's own Rk. i'm planning to use 250 or 270 ohms rated for 5W on each cathode. is this ok?

thanks in advance for opinions and advice!

[Structo]

Yes on the individual bias resistors.
I did that on a little 6v6 amp I have.

[flood]

hey structo,

thanks for chiming in.... will splitting the cathode resistor help even if the OT primary is not symmetrical?

importing OTs is forbiddingly expensive... which is why i get mine custom wound locally.

[Structo]

hey structo,

thanks for chiming in.... will splitting the cathode resistor help even if the OT primary is not symmetrical?

importing OTs is forbiddingly expensive... which is why i get mine custom wound locally.

I'm not sure about that at all.
I think with a push/pull amp that if the center tap of the OT is not centered well then your plate voltages will not be equal.

[flood]

so... i pulled the EL84s and measured the DC resistance of the windings to the center tap, and there's a whopping difference of almost 40 ohms, with one side measuiring 195 ohms and the other 149 or so. i'll be chewing the winder's brain out shortly.

i also checked the spare trannies i have from another winder and all of them measure 98 ohms and 126 ohms. another difference of almost 30 ohms. spoke with the manufacturer this morning, who said he'd get back to me.

[breakfastbuddy]

because youre not measure impedance but ohm . with ac current it equals out . put a 1 ohm resistor on each catode and select a resistance that equal each winding in the ot if you want , but its not important .

[Firestorm]

OTs are symmetrical with respect to turns ratio, not winding length so they are all off in terms of resistance (sometimes by a lot). I doubt that would be the source of the problem. What's the voltage on each plate? On each cathode?

You can use separate cathode resistors, but when you do, you lose the local feedback that helps balance the two tubes.

[flood]

hmmmm.... you guys are making me think, a hitherto unexplored domain...

will measure the voltages and cathode current using the one ohm resistor method tomorrow. i'm not sure why it's only one EL84 that redplates though. for the 5 minutes or so that it works, it sounds great, so i'd love to have this baby up and running soon. thanks to all here... will post tomorrow.

[flood]

ok, i tried checking the voltages across the newly installed hundred ohm resistors... i had to be fairly quick about this, because the redplating was quite nasty quite fast this time, and i didn't want to kill goldpin E84Ls on a current test....

the cathode currents, as far as i could measure them, were a whopping 130 and 140 mA. i will repeat this test with a pair of sacrificial EL84s, i have some used ones somewhere.

the voltages on the plates (tubes pulled) struck me as being a bit on the high side too, about 380V and 379V on g2. i wonder how far this drops with the tubes in place, will find out soon enough...

comments so far?

[drew]

Close-up photos of the innards, along with the schematic/layout you used (including the modifications you say you made), would be helpful.

[flood]

hey drew,

will take photos of the innards today (forewarned, it's a rat's nest in there, but the amp is actually quite silent). will draw up a schem right now.

[flood]

sorry for the long delay... i figured out what the problem was. i had forgotten to install the 470k grid bias resistors that shunt to ground on the output tubes! since then, the redplating has stopped and currents have stabilized well. right now, the draw is about 45mA per tube.

a new problem rears it's ugly head though - noise and hum. the PPIMV sound extremely nasty for some reason, and turning the knob seems to contribeute so i'm going to try bypassing it, probably removing it altogether and trying a VVR instead.

i still haven't taken pics yet mayn apologies. things have been rather busy and confusing of late.

[Colossal]

the PPIMV sound extremely nasty for some reason, and turning the knob seems to contribeute so i'm going to try bypassing it, probably removing it altogether and trying a VVR instead.

Flood, my vote is definitely for replacing the PPIMV with a VVR. I installed both on one of my amps and while the PPIMV works correctly (no buzz or nastiness), I very much prefer the VVR for preserving tone while reducing volume. The PPIMV was an experiment largely and I would not use it again. I use the VVR more than anything. Highly recommended. Just be sure to add the additional cap and 1M to ground in the appropriate locations (depending if you regulate the whole amp or just the power tubes).

Good luck!

[flood]

am back after a long hiatus... put the soldering iron down for a bit and went of to north india on a holiday.

this amp has been a bit of trouble, but i'm not giving up yet. before i pulled the PPIMV, the amp stopped working altogether. i assembled the circuit back as per the stock soultone schematic and flipped it on - nothing. no sound, no overheating, no smell, no burst caps, just... nothing at all. i racked my brains for a while and after a quick visual inspection, figured that there wasn't any damage to the amp so far... a voltage check showed 0V on the plates of all 12AX7s. quick check showed that the 470E resistor had come unsoldered.

resoldered it.... still nothing. measured the resistance both in-circuit and pulled, and it was too large to measure (i didn'T go beyond the 20 kOhm range. i figured the resistor - which is a "fusable" type - must have blown.

replaced last night with a 470E 20W (overkill, i know, but it was all i had lying around). haven't tested it since it was midnight and i do love my mum and dad too much to crunch them out of their sleep... so more details tomorrow.

have to dig up a semi-scaled VVR schematic from somewhere, i think i have the old "power scaling the 18W amp" from the 18watt.com prooject with me. would an IRF740B handle this amp? that's what i have with me at the moment.

thanks again!

EDIT: Colossal, I have no idea what you mean by the 1M resistors and caps... did i miss something?