5E3 Voltage problems

[Watto]

Hi All,
I've just finished a 5E3 build and I've got some voltage problems.

I'm thinking it's the filter caps..New Sprague atoms.. but i'm wonderong if you could look at the voltages for me.

I'm getting 317Vac from the P/T into the rectifier, and 4.86Vac across pins 2 & 7

The 1st Filter cap is 275vdc (very low) 2nd cap is 205vdc and 3rd is 151vdc.
It seems to start at the first cap,then move on down the line

The 12AY7 voltages are pin6 75v,pin 1 80v and cathode is 1.19v
the 12ax7 voltages are pin 1 98vdc, pin 6 125vdc, pin 8 26.6v and 0.8v on the cathode
The 6V6 voltages are pin 3 270v and 270v, pin 4 200v and 201v, pin 6 52mV and 45mv
I've checked the Amperage on the 6V6's from Centre tap to pin and I'm getting 51.2mA on both the blue and the brown

This is my 2nd deluxe build and I've had this problem before. I did buy the Spragues in a batch of 12. It seems weird that it's happened twice,Might be a bad batch
Any thoughts would be appreciated
Cheers
Watto

[MWJB]

Looks like there is a drain on the power supply somewhere.

If you suspect the filter caps, and you have spares, it won't take long to change them and eliminate them as the problem. Look for any blobs of solder that might be causing any accidental grounds from the power supply.

Pull all the tubes except for the rectifier. Then measure voltage at pin 8. Does it bounce back up? With 317VAC at each pin 4 & 6 of the recifier, I'd be looking for 350vDC-ish with a 5Y3. Note that with no tubes installed there won't be much voltage drop as you go further towards the front of the amp, so don't panic if you try and measure preamp pins and they all read high.

If the voltages are still low, you have a short in the power supply (this is where a variac or "lightbulb limiter" comes in handy when powering up for the first time).

If you're getting 350v-ish at pin 8, then fit V1. If everything is still OK, fit V2.

With both preamp tubes fitted voltages should be more in line, then fit "known to be good" power tubes and check again.

With a 250ohm cathode resistor, at 275v "plate to ground" volts, I'd be surprised (and a little alarmed) if your power tubes were drawing much beyond 30mA. Certainly shouldn't be drawing 50mA+.

[Enzo]

Recheck your work and make sure there is not a filter cap wired backwards.

[Watto]

Originally Posted by MWJB

Looks like there is a drain on the power supply somewhere.

If you suspect the filter caps, and you have spares, it won't take long to change them and eliminate them as the problem. Look for any blobs of solder that might be causing any accidental grounds from the power supply.

Pull all the tubes except for the rectifier. Then measure voltage at pin 8. Does it bounce back up? With 317VAC at each pin 4 & 6 of the recifier, I'd be looking for 350vDC-ish with a 5Y3. Note that with no tubes installed there won't be much voltage drop as you go further towards the front of the amp, so don't panic if you try and measure preamp pins and they all read high.

If the voltages are still low, you have a short in the power supply (this is where a variac or "lightbulb limiter" comes in handy when powering up for the first time).

If you're getting 350v-ish at pin 8, then fit V1. If everything is still OK, fit V2.

With both preamp tubes fitted voltages should be more in line, then fit "known to be good" power tubes and check again.

With a 250ohm cathode resistor, at 275v "plate to ground" volts, I'd be surprised (and a little alarmed) if your power tubes were drawing much beyond 30mA. Certainly shouldn't be drawing 50mA+.

Thanks for your reply,
Just to help you with where I'm at,I've checked all the circuit with continuity runing through everything,Nothing accidently running to ground,All caps wired with correct polarity.I did bring it up on a variac with the initial start up.2 hours at 50v then 20 volts per hour after that until i reached full power.I know it's a bit quick but was gradual all the same.It's got a tested military grade 5y3,2 matched/tested 6v6 JJ's, an NOS Sylvania 12ay7 and 12ax7 all Tested properly.(local amp tech has a proper tester)

I removed all tubes except the rectifier and got 420vdc on pin 8 of rectifier. Pretty much the same across the board.Then I added the 12ay7 and 12 ax7 and got 12ay7 pin1 = 149vdc, pin 6 = 142vdc, 12ax7 pin 1= 183vdc, pin6 = 234vdc..... All way too high

Filter cap 1 = 422vdc, FC 2 = 398vdc, FC 3 = 290vdc

I haven't changed the Filter caps. Just took readings to see what I had first

I didn't put the 6v6's in,didn't see much point with voltages like that.

Do you think it's the Filter caps ?

I also checked the voltage of the P/T without a rectifier.it was 322vac

Any thoughts
Cheers
Watto

[MWJB]

Well I'd say things actually look a lot more promising now...I'd expect voltages to be a little elevated without the power tubes drawing current, I wouldn't necessarily assume they are "way too high" until you've tried the 6V6s.

Just to eliminate anything else, you have built it to the stock circuit/parts/trannies etc.?

Try known good 6V6s and see where it gets you. Let us know plate, screen & cathode voltages.

Your amp is the best "tester" for the tubes, that's where they are going to have to do their job.

[Watto]

Cool,

I've built it to the same circuit,just changed the values of the couplling caps to defart it a bit and the value of the preamp cathode caps for less bass.Got these values from the 5e3 builders forum

The caps are from input to output...... 12ay7 .01uf (pin 1) .022uf (pin 6)
12ax7 .022 pin 1 to 7
.047 pin 6

last cap is .1uf, The cathode caps are 4.7uf

I'm a newby and trying to learn by reading books and asking questions.How much does these changes adversly effect the amp

Cheers
Watto

[Bruce / Mission Amps]

I buy my Sprague filter caps in lots of 100 to 300 and FREQUENTLY find bad ones in every lot.
However when they are bad, they always feel warmer to the touch then the other ones.

This still could be an an issue found with the use of too small of power tranny, especially since you are idling the power tubes at a total of over 100ma which is about 30ma to 40ma higher then they should be at the correct B+ level.

What is the high voltage current rating of the power tranny?
Yours running +100ma and 275vdc is only a little bit too high in current but it should not drop anywhere this much B+ unless the PT is too small or there is a problem with the power tube(s), socket wiring or a filter cap.

[Watto]

Hi Bruce,

The P/T is rated at 100mA, I have a 500ma fuse inline to the rectifier just in case.Hopefully the fuse will blow before the p/t.But I'm not using this amp until the problem is solved.

The only problem I had when assembling was It took a lot of effort to get the tubes in the tube sockets(NOS Fender ones) My thoughts are that this might be causing a bit of unwanted resistance in the system because the voltages are in the ballpark up until you add the 6v6's, then away it goes.I've got a known set of RCA's that I use for recording in my other 5e3.I put these in and had the same results.I also took the GE's out of my Victoria Deluxe and same results,

Could old corroded sockets be at fault here? They look ok but they are 30 years old NOS stock.Just curious.I'm going to change them today anyway.

I'm using Ceriatone P/T's only because he's closer to Australia but I'm thinking of ordering the TDPT-LDEX from Hoffmans as that's what's in the Victoria.

Cheers
Watto

[Enzo]

The power supply in our amps is not regulated, so the more tubes there are in it, the more the voltage drops. With power tubes removed, it is perfectly normal for the high voltage to get a lot higher. Stick the 6V6s in and the B+ voltage will drop. From your experiment of putting the tubes in one at a time, we see that it is the 6V6s that are dropping your voltage, but in your case more than you want.

They could be directly at fault, or it could be something else like a bad output transformer or lack of bias or whatever that only comes to light when the tubes are installed.

If you had a scope I'd also suggest looking for RF oscillation. You wouldn't hear it, but the amp would be working as hard as it could trying to deafen all the dogs in the area.

[Watto]

Thanks for your help guys,

I left it alone for a few days because it was annoying me,but today I'd thought I'd get to the bottom of it.I finally tracked it down to a dud Output transformer.It was oscilating at frequencies beyond my ears,Giving bizzare readings that wouldn't add up.I wondered why the dog ran out of the room when I fired it up.I replaced the O/T and all is well.

Thanks again
Watto

[Shea]

Originally Posted by Watto

Thanks for your help guys,

I left it alone for a few days because it was annoying me,but today I'd thought I'd get to the bottom of it.I finally tracked it down to a dud Output transformer.It was oscilating at frequencies beyond my ears,Giving bizzare readings that wouldn't add up.I wondered why the dog ran out of the room when I fired it up.I replaced the O/T and all is well.

Thanks again
Watto

You didn't add a negative feedback loop to the circuit, did you?

[Watto]

Originally Posted by Shea

You didn't add a negative feedback loop to the circuit, did you?

No It's the standard Deluxe layout.I only changed the value of some coupling caps to get rid of some flabbyness,Otherwise she's stock

The only negative feedback it has was when I swore at if for an hour while I was trying to find out the problem.But we're friends again now

Watto