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Author Topic: peavey classic B coupling caps  (Read 14236 times)

ilyaa

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Re: peavey classic B coupling caps
« Reply #30 on: July 07, 2014, 11:54:07 AM »
when i test the center tapped primary, does it matter which two leads of it i test across?

also, where i live (in the san francisco bay area) there are a few too many amp and music 'enthusiasts' for any amp to survive in a junkbin long enough for me to find it - im fixing this for a friend and hes okay to buy a new OT -

any suggestions where to get one online? ebay seems like a bit of a crapshoot for something like this as far as getting one with appropriate specs that does in fact work (no more shorted turns, please!) - is triode electronics reliable?
« Last Edit: July 07, 2014, 12:20:30 PM by ilyaa »

DrGonz78

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Re: peavey classic B coupling caps
« Reply #31 on: July 07, 2014, 06:34:16 PM »
I was really impressed with Classic Tone on how fast it arrived at my door (phoenix) and the quality of their transformers. I ordered the transformer on Monday morning and it was at my door Wednesday.

http://www.classictone.net/

Edit: Note that I am pretty sure I ordered from Amp Parts Direct and I think you were already referring to Triode USA. Both sell those brands.
« Last Edit: July 07, 2014, 11:54:11 PM by DrGonz78 »
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ilyaa

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Re: peavey classic B coupling caps
« Reply #32 on: July 07, 2014, 10:39:10 PM »

DrGonz78

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Re: peavey classic B coupling caps
« Reply #33 on: July 08, 2014, 06:45:41 AM »
That one looks good to me. Also curious if that OT is mounted by rivets? The Peavey Classic that I have has rivets mounting the OT in place. Pain in the butt that you have to drill them out or something. Anyone know good ways to get rivets out the best?
“A person who never made a mistake never tried anything new.” -Albert Einstein

ilyaa

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Re: peavey classic B coupling caps
« Reply #34 on: July 08, 2014, 11:37:22 AM »
yeah definitely rivets.....

J M Fahey

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Re: peavey classic B coupling caps
« Reply #35 on: July 08, 2014, 04:24:49 PM »
Same nominal diameter drill so in theory at least it drills out just the aluminum shaft without enlarging the mounting hole.

ilyaa

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Re: peavey classic B coupling caps
« Reply #36 on: July 12, 2014, 09:35:32 PM »
alright got the rivets out

got the new OT - tested it with my tester and it looks good

plugged it in, wired it up, etc.

all voltages look good

BUT when i turned the amp on and tried to bias the tubes up, something bad happened - as soon as i turned the grid bias positive enough to let a tiny bit of current pass, seems like the floodgates opend and the tubes started passing like 140mA right away! and started to glow blue....i tried a couple different sets of tubes and they all did it - whats happening?? seems like there is some kind of path for current that there shouldnt be but i cant imagine what it is - i didnt change anything in the amp - just took the old OT out and out the new one in.....

....(only thing that looks weird in the amp is that one of the death caps (this amp has a 3 prong cord but still has the old death caps in it) looks it exploded inside - it looks all bloated!)

J M Fahey

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Re: peavey classic B coupling caps
« Reply #37 on: July 12, 2014, 10:37:25 PM »
1) pull that death cap off ... NOW.

2) bias voltage is *never*  positive.
Set it, without the tubes, to what the schematic asks for or , say, to -52V just to go to a classic value as a starter.

3) 140mA idle (by the way, how do you know that?) .... strongly blue tubes ... all points to loss of bias.
Check that (bad trimmer, cracked connection, etc.)

ilyaa

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Re: peavey classic B coupling caps
« Reply #38 on: July 13, 2014, 04:04:30 AM »
woo!

1) pulled the death cap

2) i know bias is never positive - i meant as i made it more positive/less negative

3) im measuring across a homemade bias probe that interjects a 1ohm resistor between cathode to ground - but i think i figured it out!!

there did not seem to be any issues with the trimpot or any of that stuff. i scoped the power tube grids and turned on the amp and waited for the tubes to start pulling crazy current and lo and behold i got a huge AC signal on the grids with no input or anything! i opened the bias filter cap - no difference. i opened the caps coming from the PI and voila! once those were open the tubes biased up normally and current stayed where it should. i guess those were leaky! the high voltage on the PI collectors must have been leaking through those coupling caps and appearing as a large signal on the grids, pulling current through as if i was driving them!

so ill just some new caps for that spot pop them in and we'll see how that goes -

im curious why the tubes biased up properly with the old/bad OPT - any ideas? if those caps were leaky they must have been leaky before, too, but this problem arose only once i put the new OPT in....

Roly

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Re: peavey classic B coupling caps
« Reply #39 on: July 13, 2014, 09:32:15 AM »
"AC signal" ain't leakage!

The OPT phasing is incorrect so NFB is actually PFB and the OP stage is jumping into full power oscillation when the bias allows.  It stopped because you broke the signal loop. (strong blue glow means healthy emission BTW)

Reverse anode connections (or secondary tho the anodes are normally easier) and try again.

A shorted turn on the OPT is an AC condition and bias is a DC condition so the shorted turn can have no effect of the DC idle conditions.
If you say theory and practice don't agree you haven't applied enough theory.

ilyaa

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Re: peavey classic B coupling caps
« Reply #40 on: July 13, 2014, 01:57:27 PM »
oh, man!

doh!

you're right, of course - put the caps back, flipped the anode connections and we are all good! 49 watts into 4 ohms. and sounds good!

i learned something! i knew about transformer phasing but for whatever reason figured that because this was a new one and was made for exactly this kind of amp why should it not be in phase? but i guess its a crapshoot! should have tried that FIRST not last!

thanks, guys!

now on to the tremolo that isnt tremoloing......

Roly

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Re: peavey classic B coupling caps
« Reply #41 on: July 14, 2014, 07:03:09 AM »
I'm 65, built my first radio when I was about 12, and I'm still learning.

Technique tip: when replacing an OPT initially leave the NFB connection off (if it has one, not all guitar amps use NFB).  Pass a signal at low level through the amp, then touch the NFB connection onto where it should go on the secondary.  If the phasing is correct the level will drop, if incorrect the level will rise or even oscillate.  This is more reliable than just "no oscillation" because many valve guitar amps don't have enough feedback to cause oscillation if incorrect (and yes, I have been caught by this  :-[ ).

Lead colours on a specific OPT should be dependable, but as you have just discovered, not always.

Quote from: ilyaa
now on to the tremolo that isnt tremoloing......

First step, check that the LFO is actually oscillating, anode and cathode voltages.  These normally require the gain of a 12AX7 and a common problem is that somebody has replaced it with a 12AU7 or 12AT7 (because they look the same).
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: peavey classic B coupling caps
« Reply #42 on: July 14, 2014, 08:01:34 AM »
This particular amp has an SS tremolo, but standard troubleshooting still applies.

1) as suggested, check that you have (low frequency) oscillation .
Or start by checking DC voltages there, as shown on the schematic.
*If* you use a needle multimeter you´ll see it wiggle up/down.
The footswitch grounds oscillator bias to kill it, check that too.

2) that oscillation must reach the FET

3) the FET rhytmically grounds preamp signal through the depth pot

4) the bias pot adjusts the best tremolo effect

So you see that you have a lot of stuff to check. ???

ilyaa

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Re: peavey classic B coupling caps
« Reply #43 on: July 15, 2014, 05:33:48 PM »
k its all good -

i guess i just had to "tune" it using the tremolo bias pot -

i turned the pot with my scope on the preamp output -
at some settings there was no tremolo at all, until i hit a sweet spot where it looked great, after which if i kept turning it disappeared again - i was not able to find a setting that was 'perfect' - no matter where i set it the depth knob seemed to not really give me a full range of tremolo, only activating the effect once i hit 5 or 6 - but perhaps thats just the nature of this effect!

anyway, amps working great now, thanks, guys!!

ilyaa

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Re: peavey classic B coupling caps
« Reply #44 on: July 21, 2014, 01:37:38 PM »
one quick follow-up question which applies to this and many other amps:

whats the right way to interpret the two speaker jacks on the output?

the transformer needs 4 ohms for maximum power transfer - currently, there are two 8ohm speakers in parallel connected to one of the jacks. if i wanted to connect another cab, would it appear in parallel or in series with the first jack? looks like parallel.

so the only way to properly load the amp using both jacks would be two separate 8 ohm loads, is that right? or is there something im missing?