Randall Pro Tube 1000 head

[Slimtim]

Version 1 randall pt1000h, 2el34,5 12ax7 This amp should have all i will ever need if i can fix it up.i picked it up on craigslist for a great deal but it needs some help.the overdrive switch for the front has a cut wire that i dont know where it goes.so no overdrive on this randall right now.something is wrong with the reverb.havn't tested the effects loop yet.but the first issue is that after it warms up and gets to sounding really good,something fails in the circuit.power and note definition goto hell and sounds distorted or out of phase.The amp has a trim pot for the power tubes and i'm wanting to make sure they're biased where they need to be before i look at other stuff.Is anyone familiar with these amps?i'm not finding much luck with google.what should a pair of mesa el34 be biased at and what pin do i need to be on with my meter?

[Ritchie200]

Slimtim,
First, do you have a manual with hopefully a schematic? I did a quick goog and could not find anything.  I did see several posts where Randall is very customer friendly with providing manuals and schematics for older amps, you just need to call.  That would be a start.  Post the schematic to we can see what you should have.  See if you can identify what has been changed/cut.  The schematic may also show voltages.

Jim

[Slimtim]

the only manual is for a version 2 and i have version 1.that manual is 2 pages lol.1 of the main diffs in a 2 is a bias switch on the back that mine doesn't have.i have seen posts about a preamp schem but havn't found it.


in one pic i saw it looks like the cut wire from the overdrive switch goes to the guitar jack.but nothing happens when i put it there so i'm gonna jumper the footswitch jack and the front jack.worth a shot i guess.

[Slimtim]

well that didn't work.but when i was doing that i noticed 1 of the ic pins was out of the socket.i think its a 4558.fixed that now i have spring reverb.still no overdrive and its still warming up sounding great then crapping out some time after.but i just noticed another failure by a previous intruder.they jumped off a broken resisitor and kept the signal flowing but didnt solder to the hole that leg goes in.lololol just did it on top.and it bypasses a bunch of stuff like this.alot of user error happened to this amp i think.already fixed one like that in the power section but at least it was connected to the hole.well lets break out the iron again.btw,i need a bench.this sux.

[Slimtim]

looks like that res went to a 12v relay for the overdrive led indicator which is missing.i have one that fits the hole so it must be correct.i'll put it in tomorrow maybe.tested the loop,it works.lovin this amp,it's a keeper.

[Slimtim]

gutshot

[Slimtim]

Hell yeah!!!ya cant beat that.cleaned the normal ch volume pot and so far no more crappy distorted sound after warm up.if i can get the overdrive channel fixed it's all good.I emailed randall in hopes for schematic.maybe they will come through.

[Ritchie200]

Cool beans!  Sometimes it takes more elbow grease than analysis!  Nice job!

Jim

[Slimtim]

Removed a non original cap that was piggy backed on top of a res in the reverb circuit.No real noticeable diff i can tell.went ahead and put the chassis back in the head.played on it a good bit today on and off and no issues on the normal channel.Going to order an led,trs 2 button foot switch and some other little stuff next week.maybe by then i will have a schem or at least something to go by.

[Slimtim]

preamp schem down there

[Slimtim]

ok i'm not getting 12vdc to the relay from the power side of the board.also i believe they had the wires from the overdrive switch to the led and opto switcher reversed.

[Slimtim]

i think the problem might be this non original long zener diode on the supply side of the board.so this project is on hold until i get parts and a new iron.

[Slimtim]

Now 1 of the power tubes is popping.I wonder if it was going bad already or if the bias is set too high.

[Slimtim]

i picked up a set of used jj el34 II tubes and they do the same thing.very loud constant buzzing and one of the power tubes is arcing very badly.even if the volume is on 0 and no guitar cable in.the big caps have been changed i wonder if they're the right value.so does this amp look to be some kind of modded jcm type of circuit?

[shooter]

if the bias is set too high.

without the whole schemo it's kinda hard, you can see the -bias point on your schematic after the PI, that can be check without power tubes.  If you have a pot labeled bias, adjust it for max -, hopefully -50ish?

 

[Slimtim]

i have no idea where to probe.i see the bias adjust pot but no point to probe.i'm probably looking right at it and dont know its there.with all tubes out i do get 475v + on the plates.how much would that drop with the el34 in.i'm pretty sure i tested the plates at 465 + with tubes in before the issue started.that seems on the high side.also the big filter caps that are not original are 250v 220uf that i think are in series of each other right?

[Slimtim]

also i get 475 b+ no tubes

[shooter]

try here;

PA tubes out
run the pot to both ends, measure and post

[Slimtim]

thx so much.-50 is half way on the bias pot.which is where the pot has been the whole time.so im guessing my problem is somewhere else.i reflowed the solder and added some on the big filter caps.

[sluckey]

What voltage do you measure on pin 5 of every output tube socket with output tubes pulled out?

[Slimtim]

-49.6vdc

[shooter]

was the new tube arcing in the same socket as the old tube arcing

[Slimtim]

with the old tubes it seemed to follow tube but i didnt switch the newer ones around.but i did just now fire it up with jj el34IIs and had it sounded great.then i switched on the metel muff and OMG this things loud as hell.it was on 4 pushing 2 celestion g12k 85.sounded great till something sparked inside the chassis and tripped the reset.now im getting 480 going into the big 2 caps but only half or so coming out the other side i believe.shouldn't those be 500v each like a marshall.they're only 250v each and there is almost 500v going in.plus they are not original and maybe they just put whatever in.all the other work they did was pretty half assed.

[Slimtim]

3hrs later tubes back in and sounding good.no idea what sparked.i even have the mesa power tubes in that i thought were shot. i did not use the metal muff this time but i did get the volume up over half way with no issue.

[shooter]

no idea what sparked

at your B+ anything loose fitting, sockets, solder connections, etc.
I would swap your 250v caps for 450v caps.
an undersized cap is more likely to explode than arc.
you might wanna take the time and draw up your PS rail, and PA tube layout.
if this is "a keeper" you might also wanna install 1ohm R's on all the power tubes cathode to ground instead of cathode grounded.  Will be worth it when troubleshooting

[Slimtim]

i'll order new caps today.how much uf do i need?the 2 that are there are 220 each but i think they just threw in what they had that would fit physically.2 47uf at 500 volt will that do it.?

[shooter]

until we can see the schematic, I'd follow Marshalls 100W power rail, compare to yours, note what's different.

this is just an example of what I typically do when my rectified unloaded DC is in the 500vdc range;

[Slimtim]

oops i went by the 50 watt jcm since mine is only 50.already ordered a pair of 47uf 500v illinois.

[Slimtim]

Been playing it off and on without issue.new caps arrived and will go in this afternoon.damn these things look small compared to the old ones.is 2x 47uf 500v caps gonna be enough?

[Slimtim]

the new caps are installed and the amp sounds great even though there isnt any overdrive ch yet.Now i need to get the 1ohm resistors.After that i'll be able to properly bias the amp with tubes in right?

[Slimtim]

And 6 months later still not changed a thing internally or bought the parts lol.But it still sounds great and it's now in a custom 2 12 enclosure.still no overdrive channel.just been playing the clean channel and adding pedals as i can find them.also the effects loop send isn't working but the return is.does it get its signal through the overdrive channel?

[PRR]

> does it get its signal through the overdrive channel?

No.

If you want it totally A-OK, you need to go through everything and find the details the previous owners messed-up.

[Slimtim]

thx.yeah i'm gonna get all into it tomorrow and hope to order parts from doug by tomorrow night.already have a pretty good list going.

[Slimtim]

Well i didnt get all into it but i did check a few things.still dont know what the problem is.maybe a -voltage issue.i bought a few new pedals and been playing them.the ehx neo mistress i bought makes a very high pitched squeal when on.doesnt do it on my friends amp just my randall.i've switched wall plugs,rooms,cables,guitars,amps and power supplies to the pedal.even tried a different battery.that pedal and my amp squeal.ac heater issue?hell i dont even know what my heaters are running on lol.maybe ac.but i think that pedal is digital.

[Slimtim]

i pulled the optocoupler and checked it and i think its bad.on pins 1 and 2 i get a reading both ways with the meter on diode test.so that means the internal led is bad right?same thing for the 12v zener diode from the power supply side its also shot.my order for Doug is getting long,which i just placed.

[Slimtim]

order from Doug(thx a bunch man) arrived and i noticed right out the bag i ordered the wrong zener.one i need is twice the diameter of the one i ordered.installed new volume pot and that fixed the popping,smooth volume now.i cant read the zener's #s on the side and since it has been replaced before i dont really trust its the right one anyway.so how do i tell what zener i need for sure?there's about 78vdc before a 1.5k 5 watt resistor then the zener and a 2200uf 16volt cap.that circuit is suppose to supply the 12vdc to the rest of the switching circuit.

[sluckey]

Sounds like you may need a 12V Zener. But what wattage? Without a schematic or a reference number it's just a guess. My guess is get a 12V 5 watt Zener.

[Slimtim]

thank you sir.i'll get one on the way

[Slimtim]

5 for $3 w/ free shipping,deal.they're on the way fromFl and maybe here by the weekend if they get them out tomorrow.well if nothing else i'll have some spares.


oh btw the effects loop send and return are working fine now and i have no idea what was wrong if anything.

[Slimtim]

still havn't nailed this down yet.parts installed and i do now have 12vdc getting to the front.but when i switch the o.d. on the led flashes once and goes out.going to try a lower wattage zener in the supply side,from 5 watt down to 1.the one that was in there was a 1n4742a which was a replacement by aprevious owner.

[Slimtim]

FINALLY!!!zener came and wasnt having any luck .and i noticed there was a slight signal there when switched to o.d. ch even though the led wasnt lighting a bit.so i cranked the master and gain and could hear faint static when turning eq pots on o.d. ch.then i started just poking around with a screw driver and found the main problem that was right there the whole time.the 12v switching relay has a bad connection on the bottom side of the board or is starting to fail internally.push the relay down toward board and od led lights up fully and od is all there.so i just need to pull the input and all the pots off the front of the amp,flip the board solder all my new parts in and fix the relay.i hope it ends up being just that easy.

[EL34]

There are Randall schematics in the library.
link at the bottom of all forum pages

[Slimtim]

thx but there isnt 1 for my amp.that rgt100 is close though.

[Slimtim]

got the od channel working.but now i have an issue with v1.tube,socket or circuit idk.makes a staticy fizzing in and out sound then it just goes BURRRRRRGHHHHGHHH.AND WONT STOP TILL I TAP V1 HARD OR PULL IT OUT and put it back in.maybe mess with it tomorrow or next weekend.but it's getting closer to being fully working.o.d. sounds f'ing wicked.