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Author Topic: ampeg vt-22 PI  (Read 8155 times)

J M Fahey

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Re: ampeg vt-22 PI
« Reply #15 on: July 15, 2014, 04:25:06 PM »
OK, now go through that amp and put there what the schematic asks for, don´t mess with odd non original tubes.

As of the 7027, modern 6L6 will NOT last doing that kind of heavy job.

*Somewhere* (google it) I saw a page explaining how to replace a 7027 tube with a 6550, only reasonable tube strong enough for the job.
It definitely involves rebiasing and *maybe*  rewiring or replacing a couple resistors, but it´s the right way to do it.

Replacing 6L6 sets every few months soon becomes expensive.
« Last Edit: July 15, 2014, 04:26:13 PM by J M Fahey »

ilyaa

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Re: ampeg vt-22 PI
« Reply #16 on: July 15, 2014, 05:31:10 PM »
sounds good - ill order some tubes and report back once ive replaced everything.

that sneaky 832!

in the process of trying out which tubes were good and which were bad i blew a fuse (doh! why didnt i have the limiter inline?) and after some searching around (now with the limiter in place) found that D9 and D10 had gone bad/shorted out....the tube that did the job kind of sparked inside before everything went south....im hoping this was just a bad batch of tubes (who knows what condition they were in before this amp came to me) - without power tubes in all voltages and stuff are normal -

what are those diodes doing, anyway??

(they are not in the original schematic i posted - see a slightly different schematic attached)

Enzo

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Re: ampeg vt-22 PI
« Reply #17 on: July 15, 2014, 06:01:23 PM »
D7-8-9-10 are called flyback diodes.  They protect your output transformer from voltage spikes that occur if you play without a speaker connected.  (or a speaker cord or connection fails)

ilyaa

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Re: ampeg vt-22 PI
« Reply #18 on: July 22, 2014, 02:24:46 AM »
cool -

replaced the flyback diodes - got all right tubes and put them in -

im a little nervous about putting in the power tubes because of what happened before - i put them in with my limiter inline and i have to say it seems a little brighter than i thought it should be once i take the amp off standby - its a dull red but definitely glowing enough to where the filament is casting a red glow on the bulb itself -

is that an indicator of normal current conditions for a 100 watt tube amp?

edit: its a 200W bulb
« Last Edit: July 22, 2014, 09:12:24 AM by ilyaa »

Roly

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Re: ampeg vt-22 PI
« Reply #19 on: July 22, 2014, 01:15:27 PM »

Dull red with a 200W globe in series with a big valve amp?  Sound reasonable to me.

a) take your other DMM and hook it (safely) across the globe (or the amp socket) so you can get an idea of how your mains voltage is distributing between them (and by implication the load current.

b) I have a tong tester (1-3-10Amp) and a short lead with the wires individually broken out for it to clamp around and measure the current.


If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: ampeg vt-22 PI
« Reply #20 on: July 22, 2014, 02:53:08 PM »
k cool (kind of) -

new tubes are in power on no craziness -

i get about 200V+p-p off the PI plates (just before clipping) -

BUT i didnt get to power testing because i noticed that the power tubes flicker blue when i turn the amp off standby. like they are passing current, making normal good emission, but flickering around. a quick read with the DMM showed me that the voltages on the plates was jumping around from 530-550V, not staying steady. checking the PS i see ive got about a 20V p-p ripple - too big right?

furthermore, with my scope on the plates AND WITH the amp ON standby i have a ~60 cycle really weird looking thing (not typical ripple) - kind of like super crossover distorted sine wave with one of the peaks all squiggly - about 40-50V p-p. once i turn the amp on it gets filtered to the ~20V ripple....

seems like the smaller caps are to blame, perhaps, seeing as they are on the "hot" side of the standby switch and seem to be allowing weird line voltage to leak through even with the amp on standby....right??
« Last Edit: July 22, 2014, 03:26:06 PM by ilyaa »

ilyaa

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Re: ampeg vt-22 PI
« Reply #21 on: July 23, 2014, 11:15:16 AM »
EDIT: nevermind what i wrote here before -

opening the caps just showed me that the problem was coming upstream from the power tubes - new caps in that place and they are still flickering blue and there seems to be some unstable DC fluctuations on the plates etc

i guess probably bad power supply filtering/rectification, huh?

all the rectifiers test okay, though, so maybe the big caps

coming from upstream and getting magnified by preamp/phase inverter stuff and passed onto grids of power tubes....

only thing im not sure of is how to figure out where in the PS the issue is coming from

OR maybe its a NFB thing...it seems to appear at pin 6 of V3 and go from there through tthe PI to the power tubes...ill check that, too
« Last Edit: July 23, 2014, 08:55:49 PM by ilyaa »

Roly

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Re: ampeg vt-22 PI
« Reply #22 on: July 24, 2014, 02:35:31 PM »
Dead short the input of the power amp with a cliplead o.n.o., generally at the Master Volume wiper/PI input.

Still flickering?

If there is no signal going in to the PI, is there a signal coming out, and if so where is it coming from, and why?  Is it due to poor socket connections (wiggle and maybe Deoxit the valve sockets), is it due to a bottle failing/going microphonic (is the flickering sensitive to tapping the PI?).  Do you have a supersonic, VLF, or parasitic instability for some reason (failing bypass caps)?

Is the HT line fluctuating, and if so is it the cause or the symptom?

If you stick a suitable power resistor in series with the HT feed to the OP stage you can work out which.  If the current (voltage across this sense resistor) drops when the voltage drops, then the supply is at fault, but if the current rises when the voltage drops the fault is in the OP stage.

Does it have a bias supply?  Does that have an adjusting pot?  How clean and healthy is that?  cathode bias?  Look at the cathode bypass cap.


Overall the tech needs to develop a mental toolkit of tricks to sort the buoys from the gulls, the sheepses from the goats, and the fly$hit from the pepper.

"What test can I devise that will tell me if the problem is in this, or that?"

The shorted turn detector required some parts, but other tricks can come from just understanding how your instruments work, the important differences between a moving coil multimeter and a digital one, and how you can employ those differences.  You really need more than one meter so you can watch two variables at once, so you may as well have one of each.

One of my favorites is using my CROs in X-Y mode to plot one voltage against another.  Putting a small resistor in series with any speaker cab so you can see the current, and plotting that against applied voltage while you frequency sweep is a very powerful way of looking at cab reality.



If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: ampeg vt-22 PI
« Reply #23 on: July 27, 2014, 10:33:16 PM »
disconnected the C10/PI input - no flickering -

reconnected that and disconnected the NFB from the V3 cathode - no flickering -

took a closer look at the transformer/output wiring and realized that the impedance selector switch was not wired right!!! the 4 ohm output was not hooked up properly - rewired it according to the schematic and seems like problem(s) solved!

tested with the scope and i get just about 100 watts into 4 ohms - ill plug it in for real tomorrow and then we'll see what happens -

woo thanks again guys!

Roly

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Re: ampeg vt-22 PI
« Reply #24 on: July 28, 2014, 08:26:24 AM »
 :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.