New 5E3 build - great sound; at unamplified guitar level...

[MWJB]

10VAC into V2 triode B?

You should have equal voltages at the 6V6 grids. Let's say you are averaging 13VAC into the 6V6 grids then you should be seeing around 150VAC at each end of the OT primary and 7-8VAC at the OT secondary.

Measure the values of the 56K resistors at PI plate & cathode make sure that they are matched (replace them if any doubt). Double check that your OT does not have an internal connection from one secondary to the frame of the OT as your green & black wires could be miscoloured and the black could be the hot secondary and the green might be internally grounded?

Also check dc resistance (amp powered down & unplugged from wall) from each end of the OT primary to the centre tap (e.g. from brown plate wire to red centre tap & from blue plate wire to red centre tap)?

[FreddieRay]

MWJB, first of all, thanks for all your great advice so far If it weren't for people like you on this forum, I would still have zero understanding of the things going on in this amp. Now, I still don't know all that much but at least I'm learning every day.

As I figured this could be quite important, I redid the AC measurements with a more sensitive multimeter and a steady signal source (sine wave playing back from my pc). I did the tests once with the normal and once with the bright channel, always having the unused volume & tone at 12 o'clock and the used channel volume at 3,5 on the dial. These are my final measurements:

- 104 mV at V1-A grid
- 104 mV at V1-B grid

- 300 mV at V2-A grid
- 10,7 V at V2-B grid

- 10 V at V3 grid
- 10 V at V4 grid

- 9 V at OT primary (both)

- 60 mV at OT secondary

Now what's this? I redid every measurement a couple of times, making very sure I was touching the correct point and making good contact with the probe..

Next, I checked the 56K resistors at the PI anode/cathode, and they were both only slightly off. I then unsoldered the OT wires and did the measurements there:

- OT frame to chassis: short
- OT frame to black: open
- OT frame to green: open
- Black to green: 1,1 ohms

- Red to brown: 106 ohms
- Red to blue: 111 ohms

[MWJB]

9VAC at the OT primary x2 = 18VAC end to end accross the primary, divided by 60mVAC = 150:1...you should be looking at more like 35:1. Something isn't right here.

Check the speaker cable, what do you read accross the speaker jack plug in ohms?

I don't know what OT you are using but 106-111ohms seems a low DCR for a 5E3 OT...the Hammond 5E3 replacement has a DCR of 150ohms per side. I suspect there may be some shorts in the primary winding.

Unplug the amp from the wall, pull out the power tubes.

Find a low voltage AC source, say 1VAC to 6.3VAC or 9VAC and apply it to the speaker jack socket (measure the voltage actually applied at the jack with your meter DO NOT ASSUME that the voltage stated on the device feeding it is what it says it is). Now measure the VAC from the blue primary to the brown primary, & for the sake of completeness from the red centre tap to brown, then blue primaries. What do you get?

[FreddieRay]

Back with more numbers:

- 6,4 ohms across speaker terminals (speaker unplugged)
- With a 3 VAC (measured) source into the speaker jack input, I get on the OT:

• 34 mVAC green to black (???)
• 74 mVAC blue to brown
• 32 mVAC brown to red
• 44 mVAC blue to red

[MWJB]

Your OT looks to be toast, replace it. When you do, don't fire up until you have a NOS 5Y3, or rebias with a larger 6V6 cathode resistor.

[FreddieRay]

I ordered a Hammond OT and an NOS 5Y3 today, expect them to arrive in about a week. Thanks to everyone who helped me looking into this problem - I feel like I have learned a lot from you! Will be reporting back with results.

[FreddieRay]

Haha it finally works Can't believe it. Thanks again and see you later!

[FreddieRay]

I'm back

Hello everyone!

Hate to say it but I'm back with more of the same. I was so happy the volume was no longer whisper quiet, that I assumed everything was OK. I left the amp alone as I went back to college in September.

Some time ago, I played a friend's 5W tube amp, which reminded me of how loud these things get. After comparing with my 15W 5E3, it was clear that I had to reopen it and take another good look.. Which is why I returned here!

The new Hammond 1750E OT works perfectly; I measured DCR at primary and secondary, did the test with a 3V source at the secondary and measuring the voltage at the primary, and everything checked out as marked on the spec sheet:
150Ohm BLU-RED/BRN-RED
300Ohm BRN-BLU
.450Ohm BLK-GRN
2.6V SEC -> 84.9V PRIM (32.65 : 1 - the sheet says 31.91 : 1)
I also put in an NOS 5Y3, plus the tubes in V1 and V2 now correspond to the types as specified on the Fender schematic.

Then I checked the voltages with the Fender schematic they supply on their website - here is the result. I followed the procedure as marked; input 100 mV sine wave @ 1kHz, 8 Ohm 50W resistor at speaker terminal, knobs at 12 o'clock (note: heater voltage is two times 3.3VAC; that's not marked in the supplied file). The result is attached to this post.

I can see there's stuff wrong, but my knowledge is too limited to be able to draw conclusions, let alone come up with solutions. In the process, I rechecked (that must have been the tenth time or so) every resistor value, checked every point for correct continuty and checked the entire circuit with the schematic to make sure every connection was correct (that must have been the thirtieth time or so).

Pictures:
https://picasaweb.google.com/100543801863979860787/Amp

Voltages:

[Jazz P Bass]

"I can see there's stuff wrong,"
That is a mild understatement.
I would start by pulling all the tubes.
Power off, supply drained.
Run some basic resistance checks.
Start at the unresisted B+ & check things.
Like from there to V1 A & B.
Does it agree with the schematic.
Do that wherever there is a resistor on the B+ supply.
Take readings to ground.
At this point the only connection is through caps.
What really stands out is the voltage drop across R26.
99 volts. Through a 2.2K 2 watt resistor.
If everything looks o/k, start putting back the tubes.
I would start with the preamp tubes.
Something has to give itself away.

[FreddieRay]

Hello Jazz P Bass, thanks for your reply!

I pulled the tubes and started by checking resistor values on the entire B+ line. This is what I get (referring to the Fender schematic);





R27 5.07 kOhm
R26 21.9 kOhm (specified 22 kOhm, not 2.2)
R18 58.2 kOhm
R14 107.9 kOhm
R9 113.7 kOhm
R8 106.2 kOhm

That seems to be ok, no? I should also mention that the only resistor on the board that is off by more than 20% is R16, which reads 2.01 kOhm instead of the specified 1.5 kOhm. Could that be a problem?

Now I'm not sure what you wanted me to do next; you meant putting in only the rectifier tube and measuring B+ along the line?

[MWJB]

Check R16 & R17 values.

You say "V1 & V2 match the schematic" note that the schematic shows a 12AX7 in both V1 & V2. Your voltage at V1 pins 3 & 8 seems on the high side? Sure you don't have a 12AY7 in there?

Your OT turns ratio is wrong on your figures posted, triple check the OT wiring.

Post pics of the actual circuit...if your amp matched the schem you posted then you wouldn't have a problem, we need to see what you actually have, we already know what you are "supposed" to have". ;-)

[FreddieRay]

Thanks guys for bearing with me.

R16 2.01 kOhm
R17 56.4 kOhm

I should have been more clear about the tube types; I was referring to the common 12AY7/12AX7 configuration.. So in V1 I got an unlabeled NOS 5965 (12AV7, if that's correct, that's not too different from a 12AY7 right?) and a JJ ECC83S in V2.

What I posted earlier:

(...) did the test with a 3V source at the secondary and measuring the voltage at the primary, and everything checked out as marked on the spec sheet: (...)
2.6V SEC -> 84.9V PRIM (32.65 : 1 - the sheet says 31.91 : 1)

That's correct, no? But then in the circuit, the (7.7 + 11.1 =) 18.8V : 293 mVAC, that's 64.16 : 1... What am I missing here?

I have updated my picture set of the amp with new ones. I tried to make the pictures as clear as possible, I hope anyone can find something there. I feel like I have stared at it for so many hours that there's really nothing left to find (of course that's not true or I would be having a working amp).

The pictures are here:
https://picasaweb.google.com/1005438...87/Amp_newpics

[MWJB]

Match R17 & R18 values EXACTLY. R16 is really drifted a little far at 33% over stock.

Preamp voltages seem fine regarding V1...a bit of a mismatch & low voltage at 12AX7 in V2 on the "B" side, so match the resistors mentioned above & try a new tube in V2 (any 12A#7 type will do to see if the PI balances up regarding AC voltages, 12AY7 dc plate voltages will be around 35% lower than 12AX7, if that's all you have to try).

Sorry, can't check the updated pics from this PC. Triple check the output jack wiring & speaker jack plug.

[FreddieRay]

I quintuple checked the output jack wiring & the speaker jack plug; maybe someone could check for me on the pictures just one more time?

With nothing plugged into the output, OT green to ground (right at the jack lug) reads .4 Ohm (guess that's some kind of baseline my meter won't go under?). While gently pushing open the switch contact with a chopstick, the reading goes up to .8 Ohm. I guess that reconfirms my previous reading of OT BLK-GRN .450 Ohm (measured when the OT was not in the amp yet). Resistance across speaker jack tip-sleeve is 6.4 Ohm; nothing out of the ordinary here.

I pulled all tubes except for the rectifier, and started measuring B+ at the filter caps - that gave me 480VDC. I powered down the amp, and wanted to plug in the preamp tubes for further checking (also comparing the current 12AX7 with another one later, as suggested by MWJB). Then I realized, with no other tubes in there, the voltage wouldn't be drained out of the caps as is normally the case in this circuit. I don't have any spare resistors lying around to drain the caps safely (I have to order everything via mail here), so I'll have to wait until tomorrow for the voltage to drop to a point where it's ok to short it out with pliers (what I usually do with the last 5V in those caps).

Which brings me to this - having to order anything that is to be replaced in this amp, I'd rather wait with the R17/R18 exact match until there's no other clues left, maybe pointing to larger problems. Could a 3% mismatch of those resistors alone, mess everything up so badly?

Back for more tomorrow.

[FreddieRay]

Let's recap everything we had on OT and B+, and combine it with my new measurements. I should note that I don't have the 470 Ohm screen grid resistors, other than that, my circuit is identical to the modern day Fender schematic I have already posted a few times.

OT DCR

BLU-RED/BRN-RED	150 Ohm
BRN-BLU	300 Ohm
BLK-GRN	.450 Ohm

Speaker jack DCR

DCR

6.4 Ohm

B+ line resistances

R8	106.2 kOhm
R9	113.7 kOhm
R14	107.9 kOhm
R15	1.01 MOhm
R16	2.01 kOhm
R17	56.4 kOhm
R18	58.2 kOhm
R26	21.9 kOhm
R27	5.07 kOhm

B+ with NOS 5Y3 only

Full B+

480 V

B+ with rectifier and 2x EHX 6V6

Full B+	352 V
V3 screen grid	312 V
V4 screen grid	312 V

B+ with all tubes; V1: NOS 5965, V2: JJ 12AX7 in column 1, EHX 12AY7 in column 2

Full B+	351 V	351 V
V3 screen grid	295 V	295 V
V4 screen grid	295 V	295 V
@R26	194 V	202 V
@R17	91 V	46 V
@R16	95 V	48 V
V2-B plate	100 V	153 V
V2-A plate	127 V	95 V
V1-B plate	69 V	73 V
V1-A plate	70 V	74 V

So basically, everything is still wrong. Anyone seeing a pattern here?