FREE IS GOOD,OR IS IT?need advice with Crate Stealth GT50H

[dNi3L]

i dont want to hit everyone with a long,boring,drawn out story but here we go..on my way home from work a week ago,i noticed the unmistakeable shape of a head laying in someones trash..so i stopped and snatched it up..turns out to be a crate stealth gt50 head..so i get it home give it a brief cleaning,found schematic and did a little research,the covering was ripped off,missing both control pots for the reverb as well as any reverb tank and no tubes..so a fellow bandmate gave me some old but working 6v6's and 12ax7's to pop in her..turned it on watched the tubes light up..bam..loud pop and smoke..unplugged it,took it apart to find a 100uf 450v electro burst apart..f@$#%..so i order 2 new ones,and went out on a limb and replaced all the pots while i had it apart with alpha solder lugs and wire to the board..put it all togetherand it started to do the same thing again to the same cap..so i shut it off quickly..looked at the transformer wiring according to the schematic one of the wires were crossed,swapped em,fired it up..awesomeness,that is til i hooked it to my cab,loud humming,clean and dirt channels distorted heavily,then pop..another cap blew..1000 uf 35v and smoke..so now im in dire need of advice,assistance,direction,
insight from one of the many pro's who populate this forum.everything looks/seems to be in good condition and soldering looks pretty solid..ive built my share of dirtboxes and modded wahs and such,this is my first tube amp..id like to keep it and fix it but i dont want this thing to become an obsessive hole in my wallet..all i can learn i would appreciate..questions are..

why is it blowing these caps/causes?caps are c40/100uf 450v and c48/1000uf 35v

the power transformer has 2red,2blue,2green,2orange wires from it,i see where they go on the schematic,but are they the right ones,i mean do they carry different voltages?

could a blown output tranformer cause this?
sorry for the psychotic rant and i appreciate any insight

[Dave Mac Farlane]

Things wot are goin' to help diagnosis - replace C48 - take out all of the valves - take the voltage on say, V4 anode (plate, if you're American), and then the grid and tell us.
Check the voltage on the two rectifiers D7 to D10 and D12 to D15 - the transformer side will be AC and the other DC - take the voltages ref to ground.
Check any components in the recifier chains for discolouring, including the rectifiers.
Bit uncomfortable about 'old but working' valves, and next we can check bias voltages, but we need a running chassis to check the voltages, otherwise we start checking basic resistance.
Also uncomfortable about you 'then hooking up the cab' - valves and o/p trannies don't tolerate no load......might have to check the o/p tranny.
Back at 1400.
Dave

[Enzo]

First, you left out some wires. The transformer has two red wires, and it is important they go to push on terminals J21, J22. You mention two green wires, but the schematic says one green wire - and it is the center tap of the reds. It plugs onto J50.

HOWEVER, there ARE two Green/white wires. That is green with a white stripe. Those must go to J29, J30. Green and green/white are not interchangable.

Same with the blue wires, they go onto J26, J28. The blue/white wire is the center tap, and goes to J27.

It is crit5ical teh center tap wires go to their proper ground connections aqnd the color wires to the right places.


Now look at all those rectifier diodes, D7-D15. Check them all for shorted condition.

I am surprised a exploded C40 didn;t take out fuse F2, is it the proper type and value?

The output transformer won;t blow up caps.

[rjb]

..turned it on watched the tubes light up..bam..loud pop and smoke..
..put it all togetherand it started to do the same thing again...
...then pop... and smoke..

Anyone care to tell us about bulb limiters?

[dNi3L]

thank you for your replies and im sorry i didnt explain clearer,was frustrated with the darn thing,like i said,first tube amp,but i am really willing to learn..

reply to dave
im not too educated on checking semiconductors,how does one check a diode,arent they supposed to limit or cut off voltage to a degree?and the tubes came out of a working fender that my friend has,4 tungsol 6v6,2 mesa 12ax7,1 ruby 12ax7..so v4 would,assuming be one of the power tubes?what pin would that fall on,still new to this..and what i meant about hooking my cab up,is that i was able to hear the hum and knew something was still not working right..

reply to enzo
all of the transformer wires were left the way i got it,assuming someone played with it before,hence the first cap that blew i think was because the green wire was on j22 and red on j50,and after i swapped them is when but after about 5 minutes the second cap blew,i only mention the matching colors because i was curious to if they were the same,or had differences,and yes you are correct i meant green/white..and looking at the schem i noticed the power cord was wrong also..black was on j34 when it should have been j41,maybe thats why the fuses didnt blow? which yes i fond odd that niether blew,they are the correct fuses though..so i work this afternoon,and a radio shack in that town,they have a 1000uf 35v,but its regular leads and 80c,should work for a quick fix to figure this thing out til i can order the correct componants,im hoping anyway,but like i said ,im confused about the matching wires and wondering if it may have an effect on my problem..

reply to rjb
what is a bulb limiter?

and i thank you all very much for your insight..

[rjb]

what is a bulb limiter?

A gizmo that limits current to your device under test, used to prevent "pop and smoke".

I was hoping someone would tell us whether it would have been useful in this situation.

(I'm an acolyte, not a guru, and want to avoid disseminating bogus info.)

[52 Bill]

Having the black AC wire connected to J34 completely bypasses the main fuse and the power switch as well.

I sounds like someone worked on this amp that had no clue about what they were doing. For your own safety as well as saving any additional new components, I strongly suggest that you do not plug this in again until you check and verify that the AC primary wiring is correct and that you have the correct fuse installed.

Then verify that that all of the transformer secondary wires are connected to the correct locations. Remove all tubes and power up with a light bulb limiter until you know that the amp's basic power supplies are working correctly (not blowing up caps).

When you replaced the pots did you have to pull a lot of the wires off the pc board? Are you sure that they all went back to the correct terminals?

[dNi3L]

52 Bill
thank you and yes thats what i figured,it bypassed the fuses direct to the circuit,as far as the pots go,they were pc mounted and a few had cracked solder,i figured wire and solder lug pots would eleviate the stress of the two solid objects coming together,im sure years of use/abuse led to the cracked soldering..but it looks pretty neat and clean now and nothing seems to be crossed...looking the board over with a magnifying lamp i dont see any discoloration or distorted componants..thus far its only been those 2 caps..and yes i dont plan on turning it on til i have an idea of what may have caused this...as far as the bulb limiter,im sure a small desk lamp with the end of an extention cord spliced on one wire would work?thank you again

[dNi3L]

ive just noticed another problem,the ribbon connector for send/return j43/44..was off one pin to where,looking at the schematic it was a ground that was connected to pins 2 and 7 of v3 and grounding it out,would that account for the hellacious hum i was hearing before the second cap popped?

[dNi3L]

well afterputting the ribbon cable on right and replacing c48 1000uf 35v 105c with a quick fix radio shack cap same value only cheaper and 80c..took daves advice and checked all the diodes by desoldering and lifting one end..they were all good except for d13,nothing at all..and d15 which was opened..they were 1n4002,i had a bunch of 4003's laying around so i replaced them with those...then quadruple checked everything..put it together and bam......it works..it f@#$$##n works...but now i owe my friend a tube when i order some because when the ribbon shorted v3 it also took the tube with it..checked it in each position and nothing,but with 2 pre tubes working one in v1,dead one in v2 and one in v3 it doesnt sound too awfully bad,kind of fuzz facey touch responsive and farty at times,lack of that tube im guessingand the power tubes are iffy..not as loud as i had hoped..but plugged a processor into the power amp/aux jack and it sounded pretty darn good,alot warmer in that regard to my flexwave 120...with new tubes,biasing and a few more tweeks i may be on to something good after all...i thank all of you who contributed your time and insight,it is greatly appreciated,hope to keep learning from all of you,thank you

[Dave Mac Farlane]

I'm smiling from ear to ear - this forum is the best - you guys are un condescending and absolutely helpful - even the occasional jab of humour - long may this continue, thank you for a splendid, working forum!
Dave

[DrGonz78]

Free is good!!! Got to love the outcome here! Good Job!

[dNi3L]

well its been a couple of days,and so far so good,other than the burned out tube..which i am ordering tomorrow,i think a quad of jj 6v6's,but im unsure of what to get as far as 12ax7's..im not nailed down as far as a playing style,i could be playing the beatles one minute and playing pantera the next,so i need to find a setup where i can dial anything in pretty much,any suggestions? how good are the sovtek wc's?they are supposedly really high gain,ive heard the wa's in a friends amp but wasnt too impressed.ive heard tungsols are good in v3/pi position..and i have another question about this amp,it has j176,which are pnp,would putting j201's in it,swapping the legs of course because they are npn,would they give the amp more balls?or am i just gettin ahead of myself and leave well enough alone..just pondering that..and does anyone know if the mod reverb tanks are any good?thank you!

[soundguruman]

Crate amps make excellent boat anchors, or door stops.
But one thing you should not do with a crate amp:
Spend money on it.

[soundguruman]

Dave,
Your picture is so much more attractive these days, and I can't figure out "why?"

[Dave Mac Farlane]

Dave,
Your picture is so much more attractive these days, and I can't figure out "why?"

Actually, that isn't me in the picture, that's our lovely Amy MacDonald - if you haven't heard her, she deserves a listen.
Got her on vinyl last week. Cool!.
(I don't look less attractive - only different).
Dave.

[dNi3L]

soundguruman
boat anchor?doorstop?how about a yard ornament?well it was free,and like the title says,free isnt always good.and yes i should have heeded your advice and not sunk any money into it,replacement caps,diodes and tubes,along with some cosmetic changes..the stealth didnt look too stealthy with a hot pink stripe..sanded,repainted and waterslide decals took care of that,simple black and white..replaced the wooden face with black diamondplate grounded to chassis,and replaced the red led's with blue,even made a matching 2 button footswitch...but thats all superficial,looks cool,but sounds terrible..the gain channel,no complaints,i knew it wasnt super high gain,but what it does have suits many styles and sounds pretty decent and gain2 accessedby the footswitch is identical to gain one,but one set lower than the other gives it possibilities..but the clean channel...well in a few words..it sucks.it is too bright and twangy,even with the bass dimed and highs on 1..tube swapping has no effect on it.so im just wondering now ,what i can do to eliminate all the high end..would replacing c37/c6 .022uf with say a .033 or even up to.1uf,to give it more bass?or would upping the value of c3 220pf on the treble be a better choice?any advice from anyone on how to tone this down would be appreciated,other than beating it with a hammer..thanks

[soundguruman]

soundguruman
boat anchor?doorstop?how about a yard ornament?well it was free,and like the title says,free isnt always good.and yes i should have heeded your advice and not sunk any money into it,replacement caps,diodes and tubes,along with some cosmetic changes..the stealth didnt look too stealthy with a hot pink stripe..sanded,repainted and waterslide decals took care of that,simple black and white..replaced the wooden face with black diamondplate grounded to chassis,and replaced the red led's with blue,even made a matching 2 button footswitch...but thats all superficial,looks cool,but sounds terrible..the gain channel,no complaints,i knew it wasnt super high gain,but what it does have suits many styles and sounds pretty decent and gain2 accessedby the footswitch is identical to gain one,but one set lower than the other gives it possibilities..but the clean channel...well in a few words..it sucks.it is too bright and twangy,even with the bass dimed and highs on 1..tube swapping has no effect on it.so im just wondering now ,what i can do to eliminate all the high end..would replacing c37/c6 .022uf with say a .033 or even up to.1uf,to give it more bass?or would upping the value of c3 220pf on the treble be a better choice?any advice from anyone on how to tone this down would be appreciated,other than beating it with a hammer..thanks

You are right,
There was one guy who made his into a flower planter.
And another guy used his for a bird house.
So, I guess it does have more uses than I mentioned.
(when administering hammer, always wear safety goggles)

[dNi3L]

lol..im beginning to see you do have a very good point..

[Enzo]

The clean channel is very basic, V1a. It is between OC1 and R13. Most anything you do outside those boundaries will also affect the OD channel.

You could increase C6. My rule of thumb is to change a component value expecting it to sound different, go with at least a factor of two. SO not .033uf, but .047 or larger. Unless you only want subtle change. C74 over by the clean master, try lifting it from the circuit. ANy help? m Cathode cap C2, you might play with that. List if, increase it, decrease it, see what happens.

And have you tried different speakers for the clean channel?

You want to play with the tone stack? Go over to Duncan's Amp Pages and download the free tone stack calculator. It is a REALLY cool real time tone stack simulator. You can select various types of stack, move the controls and watch the freq response curve move. You can also change the component values, so if you wonder how a 100k slope resistor sounds compared to a 39k slope resistor, or a 470pf versus 220pf treble cap - or whatever - you can.

[dNi3L]

thank you for the suggestions Enzo,ill take it apart and try each one,maybe even socket those componants to swap different values..and yes ive tried different speakers,like i said,the gain sounds good to my ears,but nothing i do seems to tame that brightness and twang in the clean channel..sounds good picking individual notes,melody lines,but strumming chords is god awful sounding..i realize crate isnt the greatest amp in the land,but it was free and its my first amp project,something good to learn on anyway...thanks again

[olddawg]

You learned a lot and got some confidence for very little money. I would put a peddle eq in the effects loop and us an OD pedal and a Rat into the clean channel. It will probably be usable. Did you sort out the reverb? Trust your own ears and don't listen to cork sniffing fools opinions. Tone is in the hands my friend. Personally I love resurrecting things like this.

[dNi3L]

enzo..i thank you again for your suggestions..but i found the amp sounds beautiful when one of the screws holding the chassis in the head,isnt grounding out on the middle lug of the treble pot..when i replaced the pots with solder lugs,i spun them around facing the opposite direction they were originally in,and the mounting screw was just long enough to touch..seemed to bypass the bass and mids for some reason...sounds wonderful outside of the head,i guess ill spin that pot back around...could kick myself for not being more aware,but im learning,and i thank you all for your time..

olddawg..
thank you for your reply..yes i have learned a bit here and there,step up from wah mods and fuzzboxes,it grows more and more addicting and no,im not letting anything negative people say,influence me to the point of giving up,but people do have a right to thier opinions and beliefs,sometimes disagreeable,but i respect what people think and say even if its not what i wish to hear...and no on the reverb,i still need to find me a tank,im an avid used gear shopper at guitar center,may find one there pretty cheap...but over all it does have a nice sound to it..i have processors and such,but ive slowly been veering away from the digital effects,they have no real warmth or character..but however the eq idea is good,i have a yamaha natural sound eq that may work,give it a try...thank you all for your advice

[Enzo]

Well, I am glad you sorted it, but still, DO go over to duncanamps and check out the tone stack calculator, it is really cool, and free.

[dNi3L]

thank you enzo,yes i have,makes me think of making a few changes...besides the voicing on the tone stacks im not caring much for the gain section..i made the footswitch in order to access gain2,but 1 and 2 are identical,and the only way to notice any difference would be to turn one or the other down,but even then it just sounds like an identical gain turned down..so if possible id like to give gain 2 a completely different voice...so i got to looking at the scem,thinking maybe i could swap diodes or add a different array of them to that gain section,but it turns out they arent independent and run of the same circuit....from looking at it anything i do as far as diode deviation,will also affect the sound of the initial gain..correct me if im wrong,still learning,but it looks as if the only componants that i could change without hurting gain1 would be c15 1uf np,r28 10k and q5 j176?..just curiousity..would changing the cap or the fet change the way gain2 behaves without meddling up gain1? and while i was venturing into this territory i noticed something ive never seen before are transistors q15/16,being used as diodes here?ive built dist/fuzzes but have never seen transistors used as diodes in an ic gain circuit..like i said,just curiousity and trying to understand and learn and again i thank you for your insight

[Steve Conner]

Q15/16 are being used as "amplified diodes" for clipping.

The JFETs Q4 and Q5 are a switching circuit that select one or the other gain pot as the input to the clipping stage. Since they are being used as switches, they don't colour the tone so there is no point in changing them.

C14, C15, R28 and R29 are the only components not shared between the two "gain" voices. Right now they are identical, so as you discovered, Gain 1 and Gain 2 sound the same. As a very simple mod maybe you can try changing C14 for a much smaller value (10nF? 1nF?) to make your Gain 2 channel bright and piercing. Or add a "bright cap" to one of the gain pots.

Beyond that, the stage is very similar to the clipping amp in the Tube Screamer, so you might like to look at some Tube Screamer mods for inspiration.