Another 5E3 assembly help request

Lowerleftcoast

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feeding a 1khz test signal in and still getting a low level output when all of a sudden it went crackle and the volume jumped 10 fold, farted and drop back to the low level again.
From your description, I would suspect an intermittent connection. Closely inspect the soldering. Perhaps clean and tighten the sockets/pins.
 

Glowinthedark

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Just another question!
Can anyone tell me what the nominal voltage gain is for the V1/V2 stages (Ballpark figures are fine)
I could sit down with my scientific calculator and figure it out but since contracting C-19 and fighting it off for 6 weeks, I am having trouble co-ordinating a knife and fork!
 

Glowinthedark

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Yeah, unless the probe shorted two pins or wires together... not that I'd know anything about that. :)
Haha no didn't do it when I was probing! Was in the " scratch head and think of what to do next" mode!
I had turned the volume up to check the gain/tube stability when it went "loud, farted and went soft"
 

Lowerleftcoast

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Already ticked the box on that one thanks!
Have you checked with the o-scope for the oscillation @nbfanc suggested?

Here are some voltages on one of my 5E3 builds:
V1: 12AY7 (a 12AX7 runs 163 on pin1, 160 on pin6, 1.32 on pins3&8)
pin 1: 133
pin 3: 1.95
pin 6: 130
pin 8: 1.95

V2:
pin 1: 166
pin 3: 1.13
pin 6: 199
pin 8: 45

V3:
pin 3: 361 V DC
pin 4: 330 V DC
pin 8: 22 V DC

V4:
pin 3: 364 V DC
pin 4: 331 V DC
pin 8: 22 V DC

B+: 374, B+2: 331, B+3: 245
Heater: 6.51
Wall voltage: 120VAC
Bias resistor: 272 Ohms
I am running 13W Russian 6P6S power tubes. Rob's calc shows 100%MPD.
 

Glowinthedark

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Have you checked with the o-scope for the oscillation @nbfanc suggested?

Here are some voltages on one of my 5E3 builds:
V1: 12AY7 (a 12AX7 runs 163 on pin1, 160 on pin6, 1.32 on pins3&8)
pin 1: 133
pin 3: 1.95
pin 6: 130
pin 8: 1.95

V2:
pin 1: 166
pin 3: 1.13
pin 6: 199
pin 8: 45

V3:
pin 3: 361 V DC
pin 4: 330 V DC
pin 8: 22 V DC

V4:
pin 3: 364 V DC
pin 4: 331 V DC
pin 8: 22 V DC

B+: 374, B+2: 331, B+3: 245
Heater: 6.51
Wall voltage: 120VAC
Bias resistor: 272 Ohms
I am running 13W Russian 6P6S power tubes. Rob's calc shows 100%MPD.
Yep checked for oscillations- nothing detected except for 1Khz tone and 100hz noise.
243vac wall volts and a 428 B+ All my tube volts are way higher than yours but have new rectifier and O/P tubes on the way. Estimated delivery date from Auspost - whenever!
 

King Fan

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So you were sitting and pondering when it went oof bang whimper? Never do that! :)

Seriously, you had turned up the volume. Did a tube die, or partly die, or die a little more? Remind me of the state of your output tubes. We had another ShoBro™ last week turn up his volume and lose amp function. Point is *IIRC* his plate voltages and bias were just a bit too high. I admit I’m playing one-note Charlie, but I worry your high B+ may be a serial killer…
 

Glowinthedark

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So you were sitting and pondering when it went oof bang whimper? Never do that! :)

Seriously, you had turned up the volume. Did a tube die, or partly die, or die a little more? Remind me of the state of your output tubes. We had another ShoBro™ last week turn up his volume and lose amp function. Point is *IIRC* his plate voltages and bias were just a bit too high. I admit I’m playing one-note Charlie, but I worry your high B+ may be a serial killer…
Yes my thoughts too! Just kicking the O/P tubes in the guts with a max input to wring the last bit of life out of them. Not touching it until new tubes arrive.
 

Glowinthedark

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New tubes arrived!
JJ 5Y3GT dropped B+ to 382v - very happy with that number.
Another set of JJ 6V6's plugged in and same O/P result - still a low signal.
The story so far:
500mv @ 1Khz signal into Bright #1 input
Volume knobs at 10 o'clock
Signal at grids of V3 & V4 - 19vpp & 21vpp
V3 & V4 Cathode 21vdc, Plates 375vdc, Screens 331vdc
V3 & V4 Plate signal = just rubbish no discernible signal apart from 100hz.
O/P @ 8Ohm terminal (across a speaker) 35mvPP 1khz
Not what I was hoping for!
The 1st two valve stages (gain & PI)are working ok. I can drive them into distortion with 50mvPP.
The O/P tranny resistance readings are P = 148 & 158 Ohms, S = .48 ohms (8 Ohm tag)
So.....these readings are when the amp is not functioning correctly - very low output.
Normal/loud output is when something (component/joint/planet alignment) has a sudden/abrupt change of state and given the proper behaviour of the previous stages, I can point my finger at the output stage!
B+1 or B+2 Caps (F&T 16uf 475v) or cathode cap dropping the signal maybe. Not many components in the O/P stage: tubes are new & Apex matched, transformer, cathode cap!
Going to put a dummy load in place of the speaker, scope the output and inject a 100hz through to 10Khz signal and see what I get out.
Any thoughts?
 
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andrewRneumann

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I'm going to sound like a one hit wonder. Did you check the grounding on all the power supply caps? I wonder if there is something wrong with your screen supply cap.

Also, you say the power tube grids are "floating". I'm not sure what you mean by that. They shouldn't float. They should be pegged at 0VDC or slightly positive at idle conditions. Can you confirm this please?

You mention seeing 100Hz on the plates. Can you estimate the Vp-p of this ripple?
 

2L man

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If 19Vpp drives won't show about 300...400Vp on power tube anodes while operative voltages are like that I think OT is suspect although its resistances look fine.

Find a scrab Mains Cable and feed Mains to OT primary coil ends. I do this to all OTs I buy before installing them because it take only few minutes. If you remove tubes you can use tube anode socket pins 3. Or just tack solder Mains hot and neutral to bottom side lugs. When you have loudspeaker connected you should hear loud mains frequency. I think this is safe when HV and Mains switches are Off and amp mains detached. Just in case unsolder the OT B+1 feed wire.

Then measure mains and output voltage and you can calculate accurate winding ratios. There also should come exactly half the mains to OT CT against neutral and hot.

Before all this check that loudspeaker jack is connected right because sometimes 6,3mm jacks have switches on contacts. I killed power tube when I did solder OT wires to loudspeaker jack wrong and amp did oscillate without any load when I thought there was a 10 ohm 100W resistor on output.
 
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Glowinthedark

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I'm going to sound like a one hit wonder. Did you check the grounding on all the power supply caps? I wonder if there is something wrong with your screen supply cap.

Also, you say the power tube grids are "floating". I'm not sure what you mean by that. They shouldn't float. They should be pegged at 0VDC or slightly positive at idle conditions. Can you confirm this please?

You mention seeing 100Hz on the plates. Can you estimate the Vp-p of this ripple?
Grounds are good. What makes you say the screen supply cap is suspect?
I was driving the amp - it wasn't in "idle" condition and the drive signal (19VPP not DC) was on the grids but not driving the O/P tubes. Plate 100hz was 35mvPP on the 375VDC.
 

andrewRneumann

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Grounds are good. What makes you say the screen supply cap is suspect?
I was driving the amp - it wasn't in "idle" condition and the drive signal (19VPP not DC) was on the grids but not driving the O/P tubes. Plate 100hz was 35mvPP on the 375VDC.

A screen supply with no filter cap would provide negative feedback. I’m not sure how much in your case. It was just a shot in the dark.

My next guess is that you have a short in the OT secondary. Did you by chance install an external speaker jack with a shunt tip? Make sure the OT secondary has one and only one connection to ground.
 

andrewRneumann

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Are you using an RCA style speaker jack? I have heard those are notoriously finicky—easy to bungle the solder job and create a short. No stray wire whiskers accidentally touching the chassis? Unused taps, if any, separated and secured?
 

Glowinthedark

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If 19Vpp drives won't show about 300...400Vp on power tube anodes while operative voltages are like that I think OT is suspect although its resistances look fine.

Find a scrab Mains Cable and feed Mains to OT primary coil ends. I do this to all OTs I buy before installing them because it take only few minutes. If you remove tubes you can use tube anode socket pins 3. Or just tack solder Mains hot and neutral to bottom side lugs. When you have loudspeaker connected you should hear loud mains frequency. I think this is safe when HV and Mains switches are Off and amp mains detached. Just in case unsolder the OT B+1 feed wire.

Then measure mains and output voltage and you can calculate accurate winding ratios. There also should come exactly half the mains to OT CT against neutral and hot.

Before all this check that loudspeaker jack is connected right because sometimes 6,3mm jacks have switches on contacts. I killed power tube when I did solder OT wires to loudspeaker jack wrong and amp did oscillate without any load when I thought there was a 10 ohm 100W resistor on output.
Good test checkout of the O/P tranny and setting up for the mains test I removed the O/P connection to the speaker jack to substitute a resistive load for the inductive load (8 Ohm speaker but checking both) and BINGO a LOUD 50hz hum. Checked the Switchcraft jack and it appears that inserting the plug did not completely disconnect the ground.
750mVAC across the output with jack and plug in circuit 7.8VAC when they are out of circuit and resistive load or speaker connected.
A odd shape (bump) on the tip tang of the jack allowed the switch to open then close the ground contact again during plug insertion!
Just some hum to get rid of now although it is very quiet without an input. Must be my Telecaster antenna (and a switch mode power supply).
Thanks for the help, very much appreciated (and a transformer test to carry out on my Super Reverb build)
Cheers
Chris.
Time to take the horse for a ride, and the dog ( horse won't go without the dog and vice versa) then a rub down, a feed for both and some Texas Roadhouse tunes & a glass of Jack for me!
 
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Glowinthedark

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Problem solved! Was the speaker jack not completely disconnecting the ground when the speaker plug was inserted. O/P was 750mVAC at the output with the rest of the signal shunted to ground. 7.8VAC when terminated properly during the test 2L Man recommended.
I spent a lot of time measuring component values before building them into the circuit but overlooked the mechanical detail. (Palm slap to forehead) - something to be aware of during the Super Reverb build.
A big thanks to everyone for their help, very much appreciated.
Cheers
Chris
 

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