Metropoulos Forum

How big is that bright cap on vol 1 pot? My split cathode amps with a 5000pF bright cap will start to break up at 2 on the volume but my '67 and 45/100 start to crunch/break up at 5 and 6 on the dial respectively.

It's 100pF.

One thought - the volume pots are audio, just as the originals, from what i have read. Do you get as much clean headroom of up to 5 on the dial with audio pots too, or maybe only with linear?

yes, audio pots work well in giving clean tones. you should try to measure how much power you amp has. it should be 65w or more with 490v on the plates. how is the voltages in the preamp btw? and the screens?

ok, thanks! me or Crane will post the voltages as soon as i get a chance to get the amp to him and he has a free moment to look into it.

Karlis

Looking at the hig res pics looks very good, I would twist and tidy the PT primary wires a bit neater and same with the OT secondaries (both minor points).

I cannot see the OT primary wires properly, the pic seems to show a brown wire looping across to somewhere between cap board and output sockets, presumably that is the primary centre tap. Usually hook that up to the spare turret on the board with a choke wire and HT feed from fuse holder - where is this one going?

PS do you want some paler pink wire for the pots

When we get it to sound right i won't bother about the color of the pink wire
I guess Crane will be able to answer your last question when he gets a moment to chime in.
We should be able to check the voltages and bias on friday - then we'll see where it takes us.

neikeel wrote:Karlis
I cannot see the OT primary wires properly, the pic seems to show a brown wire looping across to somewhere between cap board and output sockets, presumably that is the primary centre tap. Usually hook that up to the spare turret on the board with a choke wire and HT feed from fuse holder - where is this one going?

PS do you want some paler pink wire for the pots

The brown wire you are talking about is OT center tap - as I did not use the spare turret it goes straight to PSU board.
Karlis will post the voltages soon (I hope sometime today) - unfortunately they are all correct (much closer than I would thought) except one - let's discuss after he's posted it.

Wow, Crane was fast. Ok, so here are the voltages that we got from the amp today (it has a marstran PT on the 490v taps):

pin1 pin2 pin3 pin4 pin5 pin6 pin7 pin8 pin9
v1 246 0 1.8 241 0
v2 204 0 1.28 362 205 206
v3 274 28 47 277* 30 47
v4 494 492 -64 491
v5 493 492 -64 496
v6 495 495 -64 495
v7 496 495 -64 493

I wrote down all the voltages that we actually measured.

* this was the only voltage deviating noticeably from George's voltage chart with his 500v mains tranny. After we swaped around tubes, we got 264v on v3's pin 6, which means that also this voltage is pretty much spot on in our understanding. Or do you see any issues with any of the voltages guys?

Crane installed the 1ohm resistors on 2 output sockets and measured bias. Unsurprisingly it was on the cold side. He brought it up to 30mA if i got that correctly. The amp sounded noticeably warmer and it got also more breakup on the same setting, but it didn't do anything to the issues:

Amp still has very low headroom - amp starts to break up with a les paul copy (paf style pickups, gibson classics, if i'm not mistaken) on the hi input on bright channel at vol. 1.5, gets crunchy at 3. Hotter bias gave actually even a bit more break-up.

The amp seems to work annoyingly well (for a troubleshooting situation) - no unexpected noises, crackles, it is also not unexpectedly low in volume, it has little background noise - not more what you'd expect from a good tube amp and it seems to take volumes of over 8 very well without any technical problems.

I tested ALL resistors, all mustards, micas and pots before it was put together. Crane measured and formed the electrolytics with Larry's method. All rifas went fast under 5v and the nos 32uF@450v Erie got under 4.5v after a while and then Crane stopped forming it (Larry wrote that anything under 5v is good). He even measured the ESR on the old and some of the new filter caps. All was fine - the old one had almost the same ESR.

The issues seem to be also very consistent - there are no noticeable changes.

I rechecked all trannie color codes - compared them with Marstrans diagrams and the amp - the OT taps look correct to me.

Crane tried to spot some mistakes, but all was done correctly. He might be able to tell you more on what else he did and checked.

We even tried the amp with 2 tubes only, to see if some from the quad might be causing the issues, but it was pretty much the same thing (with just a tiny bit shriller and slimmer overall tone).

So the situation is this - amp seems to work like it should, but has low headroom (it is probably not more silent than it should be, it just breaks up much too quickly for this model) and from 3 on vol. it starts to add a bit of ghosting, at a bit higher settings the ghosting becomes VERY appearant, pretty much all over the fretboard, not just past the 12th fret - it sounds really like a harmonizer effect on the higher settings - 7 and upwards.
The distortion, especially on the bass strings is very farty, undefined, flabby, what ever attribute you want to throw at it and the distortion has some degree of fizziness that i don't hear in other people's clips, but that might be a recording-reality-issue. Or that farty bass problem.

Sorry for the long message, but i tried to squeeze in what we've done so far.

Any advice would be greatly appreciated, guys! You guys here have seen almost every imaginable issue with these amps and i hope you can give us a few directions for further troubleshooting.

p.s. i don't recall exactly how much the PT's "490v" taps were forkin out, but i think it was 300 something - they are called 490v taps because they help achieve ~490 volts further on in the circuit, not becasue they fork out 490 v right away?

The same color code for the marstran trannies were used that i posted earier in this thread.

i have no clue, and i wish i could help, as mine will be some kind of younger brother to this one, but something perhaps stupid : why not trying to swap every component that isn't the same as one clone that is known to work very well ? i don't know, from Shakti, Neikeel, Roe ... and see what it does ? and then, progressively put back , step by step, what was there originaly. this could be a rough way to discover where it goes wrong when you're stuck ?

I think you should verify that the OT taps are correct and the OT has the correct primary impedance. If the colours are mixed up, it's not the first time that happens (see my Black Flag build thread). Just guessing out loud here, but if they are mixed up and, say, the 4 ohm tap has been labeled as a 16 ohm tap, you have much less NFB, resulting in earlier breakup and looser bass.

Thanks Thorleif! What was the best and least intrusive way to check the OT?

There are some details in this thread:

http://www.vintageamps.com/plexiboard/v ... 0&start=90" onclick="window.open(this.href);return false;

Thanks! This might be the next thing we will check out. Any other suggestions - what else would you recommend to look for, given the mentioned symptoms?