peavey amp theory and design

[g1]

Butcher tranny 31.2/71.3
???
And flyback spikes blasting out pcb between heater and plate. Id say that is a potential issue

I'll have to disagree with some here and say those OT primary readings are an issue. One side is more than double the other side. Unless the 71ohm is across the whole primary and the 31ohm is one side.
And you are getting arcing on the pc board between heater and plate traces? If there is any carbon traces it will continue to arc along that path. All carbon traces must be removed or cut out.

Edit: I see you dremeled out where it had arced. Any chance of carbon under the traces? Did you cut away the traces where arcing occurred?

[xjbear]

Yea I hollowed out all around the pins and flew leads to them for plate. No more traces to jump no more carbon tracking. I threw TS reissue str's in it and cranked a little over halfway today with the shop lights turned off and seemed alright although the two outside tubes started to barely red plate a hair. With no signal at all and the gain and master dimed it starts to oscillate in higher frequencies dependant on the presence control. Its on the right tap and all so maybe thats hurting the tubes

[xjbear]

I had c16 wrong at .01 instead of .001 and bias resistor r55 was 47k instead of 4k7. Its biased cooler now and no more oscillation at full tilt. Got this one off ebay 'not working for parts or repair' and it was full of abandoned shitty mods. Seems to be great now.
This is the one id asked about a voltage doubler before for running 6550's in it. Added a heater tranny and all that but I reverted my experiment back to stock and removed the prior owners hacked in bias mods and tonestack mods. Unfortunately I missed the c16 cap AnD I screwed up and stuck that 47k in r55 lol

[xjbear]

Ill prolly settle on shelving some of the presence controls ultra high frequencies and i put a mid sweep on it tonight which has helped dial it in for the stoner/doom stuff im into.
Think adding a snubber on the pi would be the best route for low passing the presence somewhat? It really sounds like its hitting 10khz when the presence is all up

[Enzo]

Yes, that C16 change will change tone.

R55 is not a bias part, it is a heater offset part. its value is not critical as all it does is provide a reference voltage. It makes your heaters float at about +26v.


Sounds to me like you might enjoy playing with an EQ in the FX loop. A 31 bander would let you fiddle with all manner of tone nuance. Alternatively, a parametric there would let you tune ii in set the Q, etc.

[xjbear]

Thx tons for your help for sure. That explains why it would have been acting weird in the bias dept when it showed up. There was a pot for bias and the wiper was going right to r55 empty hole and the other side to ground of r52 and the bias pot was toast. Replaced the pot with a 20k and it died too so back to stock. unfortunatly I glanced at the schem for the value but looked at r53 got the value but stuck it in the same wrong hole the other guy did. Glad I double cgecked it!
No fx loop on this one. Just gonna slam it hard with a fuzz face style clone and crank it with the highs rolled back into that meat churning low end

[Mick Bailey]

Quite a few transformers have a drastic difference in resistance readings between primary halves. As Enzo points out, it's how the transformer is wound. I've never found an OT where the difference in resistance alone is a fault. Measuring resistance and assuming there's a fault without checking any other factors is a sure way to misdiagnose a problem. Where a resistance difference can be an indication is where the OT is severely shorted. Note that a shorted turn will not show up on a meter check. You have to do a ring test with a specific test unit or use R.G.'s method which works reliably.

It's worthwhile understanding how the resistance between primary halves is different; the transformer 'transforms' through its turns ratio. With some transformers half the primary is wound on the bobbin and either a loop bought out or connection made to the CT. Then the other half is wound over the top. The turns on the first half are closer to the centre of the bobbin, so the circumference of each winding is smaller. As the windings progress more wire is needed for each turn. So the DC resistance of the second half is greater. This difference is emphasized if the core of the transformer is small and the bobbin narrow. The main point is to understand it's the turns and not the DC resistance that's important.

If you want to check your old OT, use the neon test to determine if there's a short. Then put (say) 1vac on the secondary and measure the voltage on each half of the primary. That will show you if there's a serious discrepancy in the primary halves.

[xjbear]

Thanks. Yea built the tester RG has on his geofex site and also have an ancient flyback/esm meter but im gonna leave the TSL 100 tranny in it. I will keep the old butcher tranny and fully test it at some point. mainly i didnt trust it because two of the speaker shunt jacks were open and not shunting to the next tap in series and I just dont trust that ot or have time right now to test it out. I did hardwire the output jacks for 1x16ohm, 2x8ohm, and 1x4ohm and ditched the factory shunt/doubleshunt switching crap. Thanks for the input man!

[g1]

Quite a few transformers have a drastic difference in resistance readings between primary halves. As Enzo points out, it's how the transformer is wound. I've never found an OT where the difference in resistance alone is a fault. Measuring resistance and assuming there's a fault without checking any other factors is a sure way to misdiagnose a problem. Where a resistance difference can be an indication is where the OT is severely shorted.

Thanks for the info. I've never seen an OT where one half of primary is more than double the resistance of the other half. I was hoping it would turn out that the 71 ohm reading was actually for the whole primary.
That being said, I don't routinely check every OT primary resistance so I don't have a lot of data, I've seen mismatches by up to 50% but no more than that.
If you guys have seen OT primaries with more than 100% resistance mismatch of primaries, then that's good enough for me and I'm pretty sure I'll remember next time .

[gui_tarzan]

Whoever thought putting tube sockets on an unsupported PCB was an idiot. The Hiwatt I fixed a few months ago was so weak the owner and I both considered putting some extra stand-offs in it.

[nashvillebill]

Could be worse...like having the tube pins directly soldered to the PCB without any socket whatsoever!

[xjbear]

Theyre pretty solid with nice thick pcbs on the old peavey stuff. The power tube sockets are riveted to the chassis but if you solder brass nuts under the flanges and put button head screws thru them its super solid and reliable. The worst are fender hrd's and blues deluxes. The sockets just soldered to the board and stand off'd from the chassis. Pure shit

[tedmich]

check out how Mako amps handles tube socket stress


turret PCB hybrid! And the FR4 boards are hugely thick too.

[xjbear]

Neato!

[Mick Bailey]

If you guys have seen OT primaries with more than 100% resistance mismatch of primaries, then that's good enough for me and I'm pretty sure I'll remember next time .

A very large difference like that could indicate another problem and I'd certainly investigate to see if the transformer had another fault, or whether a mistake is made in measurement. Other than winding techniques, the only other way the DC resistance could be seriously mismatched (that I can think of) would be a short through the winding layers, or where the CT connection passes across the rest of the windings. A short like that would need to take out a fair number of turns to make a significant difference in resistance. Usually a primary would go OC with that kind of trauma, but the construction would influence the failure mode.

There's always the possibility of a miswound OT, but that would be way down my list of possibilities. The point is that the resistance can be a smoking gun, but in itself is not a problem if there is no other issue with the transformer.